0000776867-24-000005 | 10-K | XBRL Viewer | White Mountains Insurance Group LTD

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SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (loss) (including realized and unrealized gains and losses)	$ 2,166.7	$ 1,157.9	$ 614.4
Expenses	1,601.3	1,307.3	888.0
Pre-tax income (loss) from continuing operations	565.4	(149.4)	(273.6)
Income tax (expense) benefit	15.5	(41.4)	(44.4)
Net income (loss) from continuing operations	580.9	(190.8)	(318.0)
Net income (loss) from discontinued operations, net of tax - NSM Group		902.5	(2.9)
Net income (loss) attributable to White Mountains’s common shareholders	509.2	792.8	(275.4)
Other comprehensive income (loss), net of tax	2.4	(3.8)	1.7
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	0.0	(5.2)	0.2
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	0.0	(2.9)	0.0
Comprehensive income (loss) attributable to White Mountains’s common shareholders	511.1	787.6	(273.3)
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (loss) (including realized and unrealized gains and losses)	81.7	(0.4)	(0.2)
Expenses	60.5	67.1	39.0
Pre-tax income (loss) from continuing operations	21.2	(67.5)	(39.2)
Income tax (expense) benefit	(1.2)	(0.9)	0.0
Net income (loss) from continuing operations	20.0	(68.4)	(39.2)
Equity in (earnings) losses from consolidated and unconsolidated subsidiaries, net of tax	489.2	(42.0)	(232.3)
Net income (loss) from discontinued operations, net of tax - NSM Group	0.0	16.4	(22.6)
Net income (loss) attributable to White Mountains’s common shareholders	509.2	792.8	(275.4)
Other comprehensive income (loss), net of tax	1.9	(2.9)	1.9
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	0.0	(5.2)	0.2
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	0.0	2.9	0.0
Comprehensive income (loss) attributable to White Mountains’s common shareholders	511.1	787.6	(273.3)
Sirius Group
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net gain (loss) from sale of discontinued operations, net of tax - Sirius Group	0.0	0.0	18.7
Loss contingency accrual, period increase (decrease)			18.7
Sirius Group \| White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net gain (loss) from sale of discontinued operations, net of tax - Sirius Group	0.0	0.0	18.7
NSM
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net gain (loss) from sale of discontinued operations, net of tax - Sirius Group	0.0	886.8	0.0
NSM \| White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net gain (loss) from sale of discontinued operations, net of tax - Sirius Group	$ 0.0	$ 886.8	$ 0.0

SCHEDULE I - SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Millions	Dec. 31, 2023 USD ($)
Summary of investments other than investments in related parties
Cost	$ 5,883.8
Carrying Value	6,388.7
Fair Value	6,388.7
Fixed maturity investments
Summary of investments other than investments in related parties
Cost	2,198.4
Carrying Value	2,109.3
Fair Value	2,109.3
U.S. Government and agency obligations
Summary of investments other than investments in related parties
Cost	209.0
Carrying Value	204.9
Fair Value	204.9
Debt securities issued by corporations
Summary of investments other than investments in related parties
Cost	1,085.9
Carrying Value	1,045.0
Fair Value	1,045.0
Municipal obligations
Summary of investments other than investments in related parties
Cost	275.1
Carrying Value	260.9
Fair Value	260.9
Mortgage and asset-backed securities
Summary of investments other than investments in related parties
Cost	417.2
Carrying Value	389.4
Fair Value	389.4
Collateralized loan obligations
Summary of investments other than investments in related parties
Cost	211.2
Carrying Value	209.1
Fair Value	209.1
Short-term investments
Summary of investments other than investments in related parties
Cost	1,487.0
Carrying Value	1,487.9
Fair Value	1,487.9
Equity Method Investments \| Investment in MediaAlpha
Summary of investments other than investments in related parties
Cost	59.2
Carrying Value	254.9
Fair Value	254.9
Common equity securities - Industrial, Miscellaneous, and Other
Summary of investments other than investments in related parties
Cost	370.2
Carrying Value	400.6
Fair Value	400.6
Exchange-traded funds
Summary of investments other than investments in related parties
Cost	113.3
Carrying Value	137.8
Fair Value	137.8
Total common equity securities
Summary of investments other than investments in related parties
Cost	483.5
Carrying Value	538.4
Fair Value	538.4
Other long-term investments
Summary of investments other than investments in related parties
Cost	1,655.7
Carrying Value	1,998.2
Fair Value	$ 1,998.2

Investment Securities - Common Equity Securities and Other Long Term Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Common equity securities
Cost or Amortized Cost	$ 483.5	$ 660.6
Gross Unrealized Gains	62.3	26.7
Gross Unrealized Losses	(1.2)	(8.4)
Net Foreign Currency Gains (Losses)	(6.2)	(10.5)
Carrying Value	538.4	668.4
Other long-term investments
Cost or Amortized Cost	1,655.7	1,340.8
Gross Unrealized Gains	446.3	271.1
Gross Unrealized Losses	(94.5)	(107.1)
Net Foreign Currency Gains (Losses)	(9.3)	(16.8)
Carrying Value	1,998.2	1,488.0
MediaAlpha
Investment in MediaAlpha
Cost or Amortized Cost	59.2	0.0
Gross Unrealized Gains	195.7	168.6
Gross Unrealized Losses	0.0	0.0
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	$ 254.9	$ 168.6

Commitments and Contingencies	12 Months Ended
Commitments and Contingencies	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Legal Contingencies White Mountains, and the insurance industry in general, is routinely subject to claims-related litigation and arbitration in the normal course of business, as well as litigation and arbitration that do not arise from, nor are directly related to, claims activity. White Mountains’s estimates of the costs of settling matters routinely encountered in claims activity are reflected in the reserves for unpaid loss and LAE. See Note 5 — “Loss and Loss Adjustment Expense Reserves.” White Mountains considers the requirements of ASC 450 when evaluating its exposure to non-claims-related litigation and arbitration. ASC 450 requires that accruals be established for litigation and arbitration if it is probable that a loss has been incurred and it can be reasonably estimated. ASC 450 also requires that litigation and arbitration be disclosed if it is probable that a loss has been incurred or if there is a reasonable possibility that a loss may have been incurred. White Mountains does not have any current non-claims-related litigation that may have a material adverse effect on White Mountains’s financial condition, results of operations or cash flows.

Significant Transactions

12 Months Ended

Dec. 31, 2023

Significant Transactions [Abstract]

Significant Transactions

Bamboo Transaction

On October 19, 2023, White Mountains entered into an agreement and plan of merger (the “Bamboo Merger Agreement”) with Bamboo Ide8 Insurance Services LLC (“Bamboo MGA”) and John Chu, as the unitholders’ representative. Under the terms of the Bamboo Merger Agreement, White Mountains’s wholly-owned subsidiary, WM Pierce Merger Sub LLC, agreed to merge with and into Bamboo MGA, with Bamboo MGA continuing as the surviving company (the “Bamboo Merger”). Concurrently with the execution of the Bamboo Merger Agreement, certain Bamboo management unitholders agreed to rollover the majority of their existing equity in Bamboo MGA into PM Holdings LLC (“Bamboo Holdings”) (collectively with Bamboo MGA and Ide8 Limited (“Bamboo Captive”), “Bamboo”). White Mountains also agreed to make an equity contribution to Bamboo immediately following the Bamboo Merger (together with the Bamboo Merger and the rollover transactions, the “Bamboo Transaction”).

On January 2, 2024, White Mountains closed the Bamboo Transaction in accordance with the terms of the Bamboo Merger Agreement, investing $296.7 million in equity into Bamboo, which included the contribution of $36.0 million to retire Bamboo’s legacy credit facility and the contribution of $20.0 million of primary capital. The consideration is subject to customary purchase price adjustments. At closing, White Mountains owned 72.8% of Bamboo on a basic shares outstanding basis (63.7% on a fully-diluted/fully-converted basis, taking account of management’s equity incentives). At closing, 16.1% of basic shares outstanding are owned by Bamboo management (26.6% on a fully-diluted/fully-converted basis).

Bamboo is a capital-light, tech- and data-enabled insurance distribution platform providing homeowners’ insurance and related products to the residential property market in California. Bamboo operates primarily through Bamboo MGA, its full-service MGA business, where the company manages all aspects of the placement process on behalf of its fronting and reinsurance partners, including product development, marketing, underwriting, policy issuance and claims oversight, and it earns commissions based on the volume and profitability of the insurance that it places. It offers both admitted and non-admitted insurance products. Bamboo also operates two separate but integrated business models: (i) a retail agency, within Bamboo MGA, offering ancillary products (e.g., flood, earthquake) on behalf of third parties and (ii) Bamboo Captive, a Bermuda-domiciled captive reinsurer that participates in the underwriting risk of Bamboo’s MGA programs to align interests with reinsurance partners. Bamboo will be consolidated and presented as a reportable segment for periods after December 31, 2023.

NSM

On August 1, 2022, the NSM Transaction closed. White Mountains received $1.4 billion in net cash proceeds at closing and recognized a net transaction gain of $875.7 million, which was comprised of $886.8 million of net gain from sale of discontinued operations and $2.9 million of comprehensive income related to the recognition of foreign currency translation gains (losses) from the sale, partially offset by $14.0 million of compensation and other costs related to the transaction recorded in Other Operations.

WM Outrigger Re

During the fourth quarter of 2022, Ark sponsored the formation of Outrigger Re Ltd., a Bermuda company registered as a special purpose insurer and segregated accounts company, to provide reinsurance capacity to Ark. On December 20, 2022, Outrigger Re Ltd. issued $250 million of non-voting redeemable preference shares on behalf of four segregated accounts to White Mountains and unrelated third-party investors. Upon issuance of the preference shares, Outrigger Re Ltd. entered into collateralized quota share agreements with GAIL to provide reinsurance protection on Ark’s Bermuda global property catastrophe excess of loss portfolio written in the 2023 underwriting year. The proceeds from the issuance of the preference shares were deposited into collateral trust accounts to fund any potential obligations under the reinsurance agreements with GAIL. Outrigger Re Ltd.’s obligations under the reinsurance agreements with GAIL are subject to an aggregate limit equal to the assets in the collateral trusts. The terms of the reinsurance agreements are renewable upon the mutual agreement of Ark and the applicable preference shareholder of Outrigger Re Ltd.

During the fourth quarter of 2022, White Mountains purchased 100% of the preference shares linked to its segregated account, WM Outrigger Re, for $205.0 million. White Mountains consolidates WM Outrigger Re’s results in its financial statements. WM Outrigger Re’s quota share reinsurance agreement with GAIL eliminates in White Mountains’s consolidated financial statements.

During the fourth quarter of 2023, Ark renewed Outrigger Re Ltd. to provide reinsurance protection on Ark’s Bermuda global property catastrophe excess of loss portfolio written in the 2024 underwriting year. Outrigger Re Ltd. was capitalized with $250.0 million for business written in the 2024 underwriting year, of which White Mountains rolled over $130.0 million from its commitment to the 2023 underwriting year. The remaining capital was provided by new and continuing unrelated third-party investors.

As of December 31, 2023 and 2022, short-term investments of $265.3 million and $203.7 million were held in a collateral trust, after expenses of $1.3 million.

During January 2024, White Mountains received an initial return of capital of $68.1 million from WM Outrigger Re for business written in the 2023 underwriting year. White Mountains expects to receive any additional return of capital and associated profits from business written in the 2023 underwriting year beginning in the second quarter of 2024.

Ark

On October 1, 2020, White Mountains entered into a subscription and purchase agreement (the “Ark SPA”) with Ark and certain selling shareholders (collectively with Ark, the “Ark Sellers”). Under the terms of the Ark SPA, White Mountains agreed to contribute $605.4 million of equity capital to Ark, at a pre-money valuation of $300.0 million, and to purchase $40.9 million of shares from the Ark Sellers. White Mountains also agreed to contribute up to an additional $200.0 million of equity capital to Ark in 2021. In accordance with the Ark SPA, in the fourth quarter of 2020 White Mountains pre-funded/placed in escrow a total of $646.3 million in preparation for closing the transaction, including $280.0 million funded directly to Lloyd’s on behalf of Ark under the terms of a credit facility agreement and $366.3 million placed in escrow, which is reflected on the balance sheet within Other Operations as of December 31, 2020.

On January 1, 2021, White Mountains completed the Ark Transaction in accordance with the terms of the Ark SPA. As of December 31, 2021, White Mountains owned 72.0% of Ark on a basic shares outstanding basis (63.0% after taking account of management’s equity incentives). The remaining shares are owned by employees. In the future, management rollover shareholders could earn additional shares in Ark if and to the extent that White Mountains achieves certain thresholds for its multiple of invested capital return. If fully earned, these additional shares would represent 12.5% of the shares outstanding at closing.

White Mountains recognized total assets acquired related to the Ark Transaction of $2.5 billion, including goodwill and other intangible assets of $292.5 million, and total liabilities of $1.7 billion, including contingent consideration of $22.5 million and noncontrolling interest of $220.2 million. Ark incurred transaction costs of $25.3 million in the first quarter of 2021.

In the third quarter of 2021, Ark issued $163.3 million of floating rate unsecured subordinated notes (the “Ark 2021 Subordinated Notes”) in three separate transactions. See Note 7 — “Debt”. In connection with the issuance of the Ark 2021 Subordinated Notes, White Mountains and Ark terminated White Mountains’s commitment to provide up to $200.0 million of additional equity capital to Ark in 2021.

The following presents additional details of the assets acquired and liabilities assumed as of the January 1, 2021 acquisition date:


Millions	As of January 1, 2021
Investments	$	594.3
Cash	52.0		(1)
Reinsurance recoverables	433.4
Insurance premiums receivable	236.7
Ceded unearned premiums	170.2
Value of in-force business acquired	71.7
Other assets	88.9
Loss and loss adjustment expense reserves	(696.0)
Unearned insurance premiums	(326.1)
Debt	(46.4)

Ceded reinsurance payable	(528.3)
Other liabilities	(25.9)
Net tangible assets acquired	24.5
Goodwill	116.8
Other intangible assets - syndicate underwriting capacity	175.7
Deferred tax liability on other intangible assets	(33.4)
Net assets acquired	$	283.6

(1) Cash excludes the White Mountains cash contribution of $605.4 as part of the Ark Transaction.

The values of net tangible assets acquired and the resulting goodwill, other intangible assets and contingent consideration were recorded at fair value using Level 3 inputs. The majority of the tangible assets acquired and liabilities assumed were recorded at their carrying values, as their carrying values approximated their fair values due to their short-term nature. The fair values of other intangible assets and the contingent consideration liability were estimated primarily based on the income approach. The income approach estimates fair value based on the present value of the cash flows that the assets are expected to generate in the future. White Mountains developed internal estimates for the expected future cash flows and discount rates used in the present value calculations.

The value of in-force business acquired represents the estimated profits relating to the unexpired contracts, net of related prepaid reinsurance, at the acquisition date through the expiration date of the contracts. During the years ended December 31, 2023, 2022 and 2021, Ark recognized $0.0 million, $7.5 million and $64.2 million of amortization expense on the value of in-force business acquired. The value of the syndicate underwriting capacity intangible asset was estimated using net cash flows attributable to Ark’s rights to write business in the Lloyd’s market. The value of the in-force business acquired and the syndicate underwriting capacity were estimated using a discounted cash flow method. Significant inputs to the valuation models include estimates of growth in premium revenues, investment returns, claims costs, expenses and discount rates based on a weighted average cost of capital.

In evaluating the fair value of Ark’s loss and loss adjustment expense reserves, White Mountains determined that the risk-free rate of interest was approximately equal to the risk factor reflecting the uncertainty within the reserves, and thus no adjustment was necessary. During the years ended December 31, 2023, 2022 and 2021, Ark recognized pre-tax expense of $48.7 million, $17.3 million and $5.5 million for the change in the fair value of its contingent consideration liabilities. Any future adjustments to contingent consideration liabilities will be recognized through pre-tax income (loss). As of December 31, 2023 and 2022, Ark recognized total contingent consideration liabilities of $94.0 million and $45.3 million.

Ark’s segment income and expenses for 2023, 2022 and 2021 are presented in Note 15 — “Segment Information.”

Kudu

On May 26, 2022, Kudu raised $114.5 million of equity capital (the “Kudu Transaction”) from Massachusetts Mutual Life Insurance Company (“Mass Mutual”), White Mountains and Kudu management. Mass Mutual, White Mountains and Kudu management contributed $64.1 million, $50.0 million and $0.4 million at a pre-money valuation of 1.3x book value, or $114.0 million, above the December 31, 2021 equity value of Kudu’s go-forward portfolio of Participation Contracts. The go-forward portfolio of Kudu’s Participation Contracts excluded $54.3 million of enterprise value as of December 31, 2021 relating to two portfolio companies that had announced sale transactions prior to the capital raise. As a result of the Kudu Transaction, White Mountains’s basic ownership of Kudu decreased from 99.1% to 89.3%.

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value	$ 564.6	$ 575.2
HG Global Senior Notes
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	158.7	155.7
Carrying Value	146.9	146.5
Ark 2007 Subordinated Notes
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	30.5	28.4
Carrying Value	30.0	30.0
Ark 2021 Subordinated Notes
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	171.8	163.1
Carrying Value	155.5	153.7
Kudu Credit Facility
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	225.6	223.9
Carrying Value	203.8	208.3
Other Operations debt
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	30.0	38.2
Carrying Value	$ 28.4	$ 36.7

- Definition

Amount, after deduction of unamortized premium (discount) and debt issuance cost, of long-term debt. Excludes lease obligation.

+ References + Details

Investment Securities - Net Realized and Unrealized Investment Gains and Losses - (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	$ 434.2	$ (191.5)	$ (246.1)
Amount attributable to TPC Providers	0.0	6.8	(7.7)
Investment in MediaAlpha
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	27.1	(93.0)	(380.3)
Net realized and unrealized investment (losses) gains
Debt Securities, Available-for-sale [Line Items]
Net realized investment gains (losses)	96.8	115.1	156.8
Net unrealized investment gains (losses)	337.4	(313.4)	(395.2)
Net realized and unrealized investment gains (losses)	434.2	(198.3)	(238.4)
Fixed maturity investments
Debt Securities, Available-for-sale [Line Items]
Net realized investment gains (losses)	(4.7)	(11.9)	3.9
Net unrealized investment gains (losses)	66.1	(168.4)	(42.2)
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Net realized investment gains (losses)	(0.2)	(1.4)	(0.1)
Net unrealized investment gains (losses)	1.9	(1.1)	0.0
Common equity securities
Debt Securities, Available-for-sale [Line Items]
Net realized investment gains (losses)	29.5	0.0	0.4
Net unrealized investment gains (losses)	47.1	(7.1)	14.9
Net realized and unrealized investment gains (losses)	76.6	(7.1)	15.3
Equity Method Investments \| Investment in MediaAlpha
Debt Securities, Available-for-sale [Line Items]
Net realized investment gains (losses)	0.0	0.0	160.3
Net unrealized investment gains (losses)	27.1	(93.0)	(540.6)
Net realized and unrealized investment gains (losses)	27.1	(93.0)	(380.3)
Other long-term investments
Debt Securities, Available-for-sale [Line Items]
Net realized investment gains (losses)	72.2	128.4	(7.7)
Net unrealized investment gains (losses)	195.2	(43.8)	172.7
Fixed Maturities And Other Short-Term Investments
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	63.1	(182.8)	(38.4)
Less: net realized and unrealized gains (losses) on investment securities sold during the period	4.7	(2.9)	(8.4)
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	58.4	(179.9)	(30.0)
Common Stock And Equity Method Investments
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	103.7	(100.1)	(365.0)
Less: net realized and unrealized gains (losses) on investment securities sold during the period	28.4	0.0	20.3
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	75.3	(100.1)	(385.3)
Total Investments After TPC Providers - Investments
Debt Securities, Available-for-sale [Line Items]
Gain (loss) on investments, exchange rate effect	$ 13.6	$ (29.3)	$ (7.7)

Cover Page - USD ($)	12 Months Ended
Cover Page - USD ($)	Dec. 31, 2023	Feb. 21, 2024	Jun. 30, 2023
Document Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2023
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	1-8993
Entity Registrant Name	WHITE MOUNTAINS INSURANCE GROUP, LTD.
Entity Incorporation, State or Country Code	D0
Entity Tax Identification Number	94-2708455
Entity Address, Address Line One	23 South Main Street, Suite 3B
Entity Address, City or Town	Hanover,
Entity Address, State or Province	NH
Entity Address, Postal Zip Code	03755-2053
City Area Code	603
Local Phone Number	640-2200
Title of 12(b) Security	Common Shares, par value $1.00 per share
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Document Financial Statement Error Correction [Flag]	false
Entity Shell Company	false
Entity Public Float			$ 3,443,635,676
Entity Common Stock, Shares Outstanding (in shares)		2,555,183
Documents Incorporated by Reference	Portions of the Registrant’s Definitive Proxy Statement to be filed with the Securities and Exchange Commission (“SEC”) pursuant to Regulation 14A under the Securities Exchange Act of 1934, as amended (the “Exchange Act”), relating to the Registrant’s Annual General Meeting of Members scheduled to be held May 23, 2024 are incorporated by reference into Part III of this Form 10-K. With the exception of the portions of the Proxy Statement specifically incorporated herein by reference, the Proxy Statement is not deemed to be filed as part of this Form 10-K.
Entity Central Index Key	0000776867
Amendment Flag	false
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	FY
New York Stock Exchange
Document Information [Line Items]
Trading Symbol	WTM
Security Exchange Name	NYSE

Debt - Narrative (Details) € in Millions			1 Months Ended				3 Months Ended	6 Months Ended		12 Months Ended
Debt - Narrative (Details) € in Millions	Apr. 29, 2022 USD ($)	Sep. 08, 2021 USD ($)	Sep. 30, 2021 USD ($)	Aug. 31, 2021	Jul. 31, 2021	Mar. 31, 2007 USD ($)	Sep. 30, 2021 USD ($)	Dec. 31, 2023 USD ($) credit_facility	Jun. 30, 2023	Dec. 31, 2023 USD ($) credit_facility	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($) letterOfCredit	Dec. 31, 2023 EUR (€) credit_facility	Sep. 30, 2022 USD ($)	Jun. 16, 2022 USD ($)	Aug. 11, 2021 USD ($)	Jul. 13, 2021 USD ($)	Jul. 13, 2021 EUR (€)
Debt Instrument
Outstanding balance								$ 578,900,000		$ 578,900,000
Interest expense										62,700,000	$ 40,300,000	$ 20,500,000
Interest paid										42,200,000	30,500,000	$ 13,900,000
Interest Rate Cap \| Subsidiary - HG Global
Debt Instrument
Notational amount															$ 150,000,000
HG Global Senior Notes \| Senior Notes
Debt Instrument
Long-term line of credit	$ 150,000,000
Interest rate	6.00%
Anniversary of funding date	5 years
Annual payments	$ 15,000,000
Interest accrued multiplier															8
Interest reserve account, maximum															$ 29,300,000
Interest reserve account								30,400,000		$ 30,400,000	31,200,000
Additional interest rate										1.00%
Outstanding balance								150,000,000.0		$ 150,000,000.0	150,000,000.0
HG Global Senior Notes \| Senior Notes \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Debt Instrument
Basis spread on variable rate	0.26%
Ark 2007 Subordinated Notes \| Junior Subordinated Debt
Debt Instrument
Long-term line of credit						$ 30,000,000
Outstanding balance								30,000,000		30,000,000
Ark 2007 Subordinated Notes \| Junior Subordinated Debt \| London Interbank Offered Rate (LIBOR)
Debt Instrument
Basis spread on variable rate						4.60%
Ark 2007 Subordinated Notes \| Subordinated Debt
Debt Instrument
Outstanding balance								30,000,000.0		30,000,000.0	30,000,000.0
Ark 2021 Subordinated Notes \| Subordinated Debt
Debt Instrument
Long-term line of credit			$ 163,300,000				$ 163,300,000
Proceeds from debt, net of debt issuance costs							$ 157,800,000
Ark 2021 Subordinated Notes, Tranche 1 \| Subordinated Debt
Debt Instrument
Long-term line of credit																	$ 46,300,000	€ 39.1
Outstanding balance								42,700,000		42,700,000	41,300,000		€ 39.1
Ark 2021 Subordinated Notes, Tranche 1 \| Subordinated Debt \| Euro Interbank Offered Rate (EURIBOR)
Debt Instrument
Basis spread on variable rate					5.75%
Ark 2021 Subordinated Notes, Tranche 2 \| Subordinated Debt
Debt Instrument
Long-term line of credit																$ 47,000,000
Outstanding balance								47,000,000.0		$ 47,000,000.0	47,000,000.0
Ark 2021 Subordinated Notes, Tranche 2 \| Subordinated Debt \| London Interbank Offered Rate (LIBOR)
Debt Instrument
Basis spread on variable rate				5.75%
Ark 2021 Subordinated Notes, Tranche 3 \| Subordinated Debt
Debt Instrument
Long-term line of credit		$ 70,000,000
Additional interest rate										1.00%
Outstanding balance								70,000,000.0		$ 70,000,000.0	70,000,000.0
Debt instrument, period for interest rate increase		10 years
Solvency capital requirement			120.00%
Debt to capital requirement			40.00%
Ark 2021 Subordinated Notes, Tranche 3 \| Subordinated Debt \| London Interbank Offered Rate (LIBOR)
Debt Instrument
Basis spread on variable rate		6.10%
ING LOC Facility \| Line of Credit \| Letter of Credit
Debt Instrument
Number of letter of credit agreements \| letterOfCredit												2
Total commitment under revolving credit facility												$ 50,000,000
Citibank LOC Facility \| Line of Credit \| Letter of Credit
Debt Instrument
Total commitment under revolving credit facility												$ 125,000,000
Letters of credit outstanding								106,500,000		106,500,000
Pledged collateral								134,800,000		134,800,000
Lloyds LOC Facility \| Line of Credit \| Letter of Credit
Debt Instrument
Total commitment under revolving credit facility														$ 100,000,000
Letters of credit outstanding								28,600,000		28,600,000
Pledged collateral								64,100,000		64,100,000
Kudu Credit Facility \| Secured Debt
Debt Instrument
Long-term line of credit								$ 300,000,000		$ 300,000,000
Interest rate								4.30%	4.30%	4.30%			4.30%
Interest reserve account								$ 14,900,000		$ 14,900,000
Outstanding balance								$ 210,300,000		$ 210,300,000	215,200,000
Restricted cash											12,200,000
Debt covenant, maximum loan to value percent								25.00%		25.00%			25.00%
Percentage of borrowing base								35.00%		35.00%			35.00%
Remaining borrowing capacity under revolving credit facility								$ 89,700,000		$ 89,700,000
Kudu Credit Facility \| Secured Debt \| Debt Covenant, Period 1
Debt Instrument
Debt covenant, loan to value percent								50.00%		50.00%			50.00%
Kudu Credit Facility \| Secured Debt \| Debt Covenant, Period 2
Debt Instrument
Debt covenant, loan to value percent								40.00%		40.00%			40.00%
Kudu Credit Facility \| Secured Debt \| Debt Covenant, Period 3
Debt Instrument
Debt covenant, loan to value percent								25.00%		25.00%			25.00%
Kudu Credit Facility \| Secured Debt \| Debt Covenant, Period 4
Debt Instrument
Debt covenant, loan to value percent								15.00%		15.00%			15.00%
Kudu Credit Facility \| Secured Debt \| Debt Covenant, Period 5
Debt Instrument
Debt covenant, loan to value percent								0.00%		0.00%			0.00%
Kudu Credit Facility \| Secured Debt \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Debt Instrument
Basis spread on variable rate								0.26%
Kudu Credit Facility \| Secured Debt \| London Interbank Offered Rate (LIBOR)
Debt Instrument
Basis spread on variable rate									0.25%
Other Operations debt \| Secured Debt
Debt Instrument
Outstanding balance								$ 28,900,000		$ 28,900,000	$ 37,400,000
Secured credit facilities \| credit_facility								4		4			4

Debt (Tables)

12 Months Ended

Dec. 31, 2023

Debt Disclosure [Abstract]

Schedule of debt outstanding

The following table presents White Mountains’s debt outstanding as of December 31, 2023 and 2022:


	December 31,		Effective	December 31,		Effective
$ in Millions	2023		Rate (1)	2022		Rate (1)



HG Global Senior Notes (2)	$	150.0	11.9%	$	150.0	8.9%
Unamortized discount and issuance cost	(3.1)			(3.5)
HG Global Senior Notes, carrying value	146.9			146.5
Ark 2007 Subordinated Notes, carrying value	30.0			30.0
Ark 2021 Subordinated Notes Tranche 1	42.7			41.3
Ark 2021 Subordinated Notes Tranche 2	47.0			47.0
Ark 2021 Subordinated Notes Tranche 3	70.0			70.0
Unamortized issuance cost	(4.2)			(4.6)
Ark 2021 Subordinated Notes, carrying value	155.5			153.7
Total Ark Subordinated Notes, carrying value	185.5		11.0%	183.7		7.6%




Kudu Credit Facility	210.3		10.1%	215.2		6.1%
Unamortized issuance cost	(6.5)			(6.9)
Kudu Credit Facility, carrying value	203.8			208.3






Other Operations debt	28.9		10.2%	37.4		6.6%
Unamortized issuance cost	(.5)			(.7)
Other Operations debt, carrying value	28.4			36.7
Total debt	$	564.6		$	575.2

(1) Effective rate includes the effect of the amortization of debt issuance costs and, where applicable, the original issue discount.

(2) Effective rate excludes the effect of the interest rate cap.

The following table presents the change in debt under the Credit Facility for the years ended December 31, 2023, 2022 and 2021:


	Year Ended December 31,
Millions	2023		2022		2021







Kudu Credit Facility
Beginning balance	$	215.2	$	225.4	$	—
Term loans
Borrowings	12.0		35.0		232.0
Repayments	(16.9)		(45.2)		(6.6)
Ending balance	$	210.3	$	215.2	$	225.4

Schedule of contractual repayments of debt

The following table presents a schedule of contractual repayments of White Mountains’s debt as of December 31, 2023:


Millions	December 31, 2023
Due in one year or less	$	3.3
Due in two to three years	11.2
Due in four to five years	40.6
Due after five years	523.8
Total	$	578.9

- Definition

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
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+ Details

Investment Securities - Schedule of Pre-Tax Net Investment Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Net Investment Income [Line Items]
Total investment income	$ 196.6	$ 126.5	$ 84.8
Third-party investment expenses	(2.4)	(2.1)	(2.3)
Net investment income, pre-tax	194.2	124.4	82.5
Fixed maturity investments
Net Investment Income [Line Items]
Total investment income	64.4	41.4	28.8
Short-term investments
Net Investment Income [Line Items]
Total investment income	52.0	16.4	0.6
Common equity securities
Net Investment Income [Line Items]
Total investment income	4.1	1.6	0.1
Other long-term investments
Net Investment Income [Line Items]
Total investment income	76.1	68.1	56.3
Amount attributable to TPC Providers
Net Investment Income [Line Items]
Amount attributable to TPC Providers	$ 0.0	$ (1.0)	$ (1.0)

Held for Sale and Discontinued Operations - Earnings Per Share for Discontinued Operations (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ 509.2	$ 792.8	$ (275.4)
Less: net income (loss) from continuing operations	$ 580.9	(190.8)	(318.0)
Less: net (income) loss from continuing operations attributable to noncontrolling interest		81.1	45.5
Total gain (loss) from discontinued operations attributable to White Mountains’s common shareholders		902.5	(2.9)
Allocation of earnings to participating restricted common shares		(11.4)	0.0
Basic earnings per share numerators		891.1	(2.9)
Diluted earnings per share numerators		$ 891.1	$ (2.9)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	2,563,300	2,862,400	3,079,000
Average unvested restricted common shares (in shares)	(36,000.0)	(36,200)	(36,500)
Basic earnings (losses) per share denominator (in shares)	2,527,300	2,826,200	3,042,500
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)	2,563,300	2,862,400	3,079,000
Average unvested restricted common shares (in shares)	(36,000.0)	(36,200)	(36,500)
Diluted earnings (loss) per share denominator (in shares)	2,527,300	2,826,200	3,042,500
Basic earnings (losses) per share (in dollars) - discontinued operations (in dollars per share)	$ 0	$ 315.30	$ (0.94)
Diluted earnings (losses) per share (in dollars) - discontinued operations (in dollars per share)	$ 0	$ 315.30	$ (0.94)

Third-Party Reinsurance - Reinsurance Recoverables (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Ark Insurance Holdings Limited/Outrigger
Segment reporting information
Reinsurance recoverable on unpaid losses	$ 340.8	$ 505.0
Reinsurance recoverables on paid losses	27.4	31.1
Ceded unearned premiums	73.8	59.2
Reinsurance recoverables	442.0	595.3
Ceded	55.7	278.5	$ 128.1
Ark Insurance Holdings Limited/Outrigger \| Ark
Segment reporting information
Ceded	(15.6)
Ceded unearned premiums	(5.7)
Ark
Segment reporting information
Reinsurance recoverable on unpaid losses	340.8	505.0	$ 428.9	$ 433.4
Ark \| Ark
Segment reporting information
Ceded	$ (15.6)
Amount attributable to TPC Providers
Segment reporting information
Reinsurance recoverable on unpaid losses		$ 145.4

Third-Party Reinsurance (Tables)

12 Months Ended

Dec. 31, 2023

Reinsurance Disclosures [Abstract]

Effects of reinsurance

The following table summarizes the effects of reinsurance on written and earned premiums and on losses and LAE for the Ark/WM Outrigger segment for the years ended December 31, 2023, 2022 and 2021.


	Year Ended December 31,
Millions	2023			2022		2021
Written premiums:
Direct	$	931.9		$	760.4	$	619.5
Assumed	966.5			691.6		439.2
Gross written premiums	1,898.4			1,452.0		1,058.7
Ceded	(377.5)		(1)	(256.8)		(199.6)
Net written premiums	$	1,520.9		$	1,195.2	$	859.1
Earned premiums:
Direct	$	848.1		$	655.5	$	556.0
Assumed	925.6			668.7		330.4
Gross earned premiums	1,773.7			1,324.2		886.4
Ceded	(364.0)		(2)	(280.8)		(249.1)
Net earned premiums	$	1,409.7		$	1,043.4	$	637.3
Losses and LAE:
Gross	$	782.5		$	814.9	$	442.9
Ceded	(55.7)		(3)	(278.5)		(128.1)
Net losses and LAE	$	726.8		$	536.4	$	314.8

(1) The ceded written premiums exclude $110.0 ceded by Ark to WM Outrigger Re for the year ended December 31, 2023, which eliminate in White Mountains’s consolidated financial statements.

(2) The ceded earned premiums exclude $104.3 ceded by Ark to WM Outrigger Re for the year ended December 31, 2023, which eliminate in White Mountains’s consolidated financial statements.

(3) The ceded loss and LAE exclude $15.6 ceded by Ark to WM Outrigger Re for the year ended December 31, 2023, which eliminate in White Mountains’s consolidated financial statements.

The following table presents the Ark/WM Outrigger segment’s reinsurance recoverables as of December 31, 2023 and December 31, 2022:


Millions	December 31, 2023			December 31, 2022
Reinsurance recoverables on unpaid losses	$	340.8	(1)	$	505.0	(3)
Reinsurance recoverables on paid losses	27.4			31.1
Ceded unearned premiums	73.8		(2)	59.2
Reinsurance recoverables	$	442.0		$	595.3

(1) The reinsurance recoverables on unpaid losses exclude $15.6 ceded by Ark to WM Outrigger Re as of December 31, 2023, which eliminate in White Mountains’s consolidated financial statements.

(2) The ceded unearned premiums exclude $5.7 ceded by Ark to WM Outrigger Re as of December 31, 2023, which eliminate in White

Mountains’s consolidated financial statements.

(3) The reinsurance recoverables on unpaid losses include $145.4 attributable to TPC Providers as of December 31, 2022, which are collateralized.

The following table presents a schedule of written premiums and earned premiums included in the HG Global/BAM segment for the years ended December 31, 2023, 2022 and 2021:


	December 31,
Millions	2023		2022		2021
Written premiums:
Direct	$	58.6	$	63.8	$	51.0
Assumed	—		1.3		4.6
Gross written premiums (1)	$	58.6	$	65.1	$	55.6
Earned premiums:
Direct	$	28.7	$	28.6	$	23.2
Assumed	2.5		4.7		3.7
Gross earned premiums (1)	$	31.2	$	33.3	$	26.9

(1) There are no ceded premium amounts in the periods presented, and gross earned premiums are equivalent to net written premiums and net earned premiums.

Gross and net reinsurance recoverables by reinsurers ratings

The following table presents the Ark/WM Outrigger segment’s gross and net reinsurance recoverables by each reinsurer’s A.M. Best Company, Inc (“A.M. Best”) rating as of December 31, 2023:


$ in Millions	As of December 31, 2023
A.M. Best Rating (1)	Gross			Collateral			Net		% of Total
A+ or better	$	204.0		$	—		$	204.0	71.0	%
A- to A	76.2			—			76.2		26.5
B++ or lower and not rated	88.0		(2)	80.7		(2)	7.3		2.5
Total	$	368.2		$	80.7		$	287.5	100.0	%

(1) A.M. Best ratings as detailed above are: “A+ or better” (Superior) “A- to A” (Excellent), “B++” (Good).

(2) Excludes $15.6 ceded by Ark to WM Outrigger Re as of December 31, 2023, which eliminates in White Mountains’s consolidated financial statements.

Equity Method Eligible Investments - Balance Sheet and Income Statement Tables (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Investments in unconsolidated affiliates
Total assets	$ 8,385.9	$ 7,389.3
Total liabilities	3,824.3	3,454.3
Total revenues	2,166.7	1,157.9	$ 614.4
Total expenses	1,601.3	1,307.3	888.0
Net income (loss)	580.9	712.4	(321.9)
MediaAlpha
Investments in unconsolidated affiliates
Total assets	153.9	170.1
Total liabilities	248.4	256.2
Total revenues	388.1	459.1	645.3
Total expenses	444.7	531.5	653.8
Net income (loss)	(56.6)	(72.4)	(8.5)
Equity Method Investment, Nonconsolidated Investee, Excluding MediaAlpha
Investments in unconsolidated affiliates
Total assets	3,049.3	2,252.5
Total liabilities	657.2	526.9
Total revenues	1,187.3	735.3	987.4
Total expenses	681.5	515.9	418.7
Net income (loss)	$ 505.8	$ 219.4	$ 568.7

- Definition

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

+ Details

Employee Share-Based Incentive Compensation Plans - Narrative (Details) $ in Millions	12 Months Ended			36 Months Ended
	Dec. 31, 2023 USD ($) shares	Dec. 31, 2022 USD ($) shares	Dec. 31, 2021 USD ($) shares	Dec. 31, 2021 USD ($) shares	Dec. 31, 2020 USD ($)
Share-based compensation arrangement by share-based payment award
Grants in period (in shares)	10,895	13,225	13,475
Performance Shares
Share-based compensation arrangement by share-based payment award
Vesting period	3 years
Performance cycle period	3 years
New grants (in shares)				750
Additional compensation cost that would be recognized if all outstanding performance shares vested \| $	$ 27.9
Performance Shares \| 2023 – 2025
Share-based compensation arrangement by share-based payment award
Targeted performance goal (as a percent)	11.00%
Payout maximum growth target percentage	200.00%
Performance Shares \| 2022 – 2024
Share-based compensation arrangement by share-based payment award
Targeted performance goal (as a percent)	9.00%
Payout maximum growth target percentage	200.00%
Performance Shares \| 2023 – 2025
Share-based compensation arrangement by share-based payment award
Targeted performance goal (as a percent)	8.00%
Payout maximum growth target percentage	200.00%
Performance Shares \| Minimum
Share-based compensation arrangement by share-based payment award
Share-based compensation arrangement by share-based payment award, multiplier	0
Performance Shares \| Minimum \| 2023 – 2025
Share-based compensation arrangement by share-based payment award
Payout range for growth target percentage	6.00%
Performance Shares \| Minimum \| 2022 – 2024
Share-based compensation arrangement by share-based payment award
Payout range for growth target percentage	4.00%
Performance Shares \| Minimum \| 2023 – 2025
Share-based compensation arrangement by share-based payment award
Payout range for growth target percentage	3.00%
Performance Shares \| Maximum
Share-based compensation arrangement by share-based payment award
Share-based compensation arrangement by share-based payment award, multiplier	2
Performance Shares \| Maximum \| 2023 – 2025
Share-based compensation arrangement by share-based payment award
Payout range for growth target percentage	16.00%
Performance Shares \| Maximum \| 2022 – 2024
Share-based compensation arrangement by share-based payment award
Payout range for growth target percentage	14.00%
Performance Shares \| Maximum \| 2023 – 2025
Share-based compensation arrangement by share-based payment award
Payout range for growth target percentage	13.00%
Restricted Stock
Share-based compensation arrangement by share-based payment award
Vesting period	34 months
Additional compensation cost that would be recognized if all outstanding performance shares vested \| $	$ 16.2	$ 15.5	$ 15.9	$ 15.9	$ 15.2
Issued (in shares)	10,895	13,225	13,475

SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - CONDENSED BALANCE SHEETS (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Assets
Common equity securities, at fair value	$ 538.4	$ 668.4
Other long-term investments	1,998.2	1,488.0
Total assets	8,385.9	7,389.3
Liabilities
Total liabilities	3,824.3	3,454.3
Total White Mountains’s common shareholders’ equity	4,240.5	3,746.9
Total liabilities and equity	8,385.9	7,389.3
White Mountains Insurance Group Ltd.
Assets
Cash	0.3	0.1
Fixed maturity investments, at fair value	0.0	49.2
Common equity securities, at fair value	147.8	342.8
Short-term investments, at fair value	198.8	105.2
Other long-term investments	160.5	49.3
Net receivable due from subsidiaries	89.1	125.2
Other assets	4.0	75.3
Investments in consolidated subsidiaries	3,666.9	3,027.2
Total assets	4,267.4	3,774.3
Liabilities
Other liabilities	26.9	27.4
Total liabilities	26.9	27.4
Total White Mountains’s common shareholders’ equity	4,240.5	3,746.9
Total liabilities and equity	$ 4,267.4	$ 3,774.3

Equity Method Eligible Investments - Carrying Value and Ownership Percentages (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Kudu’s Participation Contracts
Investments in and Advances to Affiliates [Line Items]
Carrying Value	$ 890.5	$ 695.9
Investment in MediaAlpha
Investments in and Advances to Affiliates [Line Items]
Ownership Interest	34.90%	27.10%
Carrying Value	$ 254.9	$ 168.6
PassportCard/DavidShield
Investments in and Advances to Affiliates [Line Items]
Ownership Interest	53.80%	53.80%
Carrying Value	$ 150.0	$ 135.0
Elementum Holdings, L.P.
Investments in and Advances to Affiliates [Line Items]
Ownership Interest	26.60%	29.70%
Carrying Value	$ 35.0	$ 30.0
Other equity method eligible investments, at fair value
Investments in and Advances to Affiliates [Line Items]
Carrying Value	291.7	84.4
Other equity method eligible investments, at fair value
Investments in and Advances to Affiliates [Line Items]
Carrying Value	$ 24.8	$ 0.0
Minimum \| Kudu’s Participation Contracts
Investments in and Advances to Affiliates [Line Items]
Ownership Interest	4.10%	4.10%
Maximum \| Kudu’s Participation Contracts
Investments in and Advances to Affiliates [Line Items]
Ownership Interest	30.00%	30.00%

Goodwill and Other Intangible Assets

12 Months Ended

Dec. 31, 2023

Goodwill and Intangible Assets Disclosure [Abstract]

Goodwill and Other Intangible Assets

White Mountains accounts for business combinations using the acquisition method. Under the acquisition method, White Mountains recognizes and measures the assets acquired, liabilities assumed and any noncontrolling interest in the acquired entities at their acquisition date fair values. Goodwill represents the excess of the amount paid to acquire businesses over the fair value of identifiable net assets at the date of acquisition. The estimated acquisition date fair values, generally consisting of intangible assets and liabilities for contingent consideration, may be recorded at provisional amounts in circumstances where the information necessary to complete the acquisition accounting is not available at the reporting date. Any such provisional amounts are finalized as measurement period adjustments within one year of the acquisition date.

The following table presents the economic lives, acquisition date fair values, accumulated amortization and net carrying value for other intangible assets and goodwill as of December 31, 2023 and 2022:


$ in Millions	Weighted Average Economic Life (in years)	December 31, 2023				December 31, 2022
		Acquisition Date Fair Value		Accumulated Amortization		Net Carrying Value		Acquisition Date Fair Value		Accumulated Amortization		Net Carrying Value
Goodwill:
Ark	N/A	$	116.8	$	—	$	116.8	$	116.8	$	—	$	116.8
Kudu	N/A	7.6		—		7.6		7.6		—		7.6


Other Operations	N/A	44.4		—		44.4		52.1		—		52.1
Total goodwill		168.8		—		168.8		176.5		—		176.5
Other intangible assets:
Ark
Underwriting capacity	N/A	175.7		—		175.7		175.7		—		175.7
Kudu
Trade names	7.0	2.2		1.5		.7		2.2		1.2		1.0





Other Operations
Trade names	13.3	13.3		4.1		9.2		17.9		3.0		14.9
Customer relationships	10.9	24.8		10.3		14.5		29.5		7.5		22.0

Other	12.1	2.8		1.1		1.7		2.8		.5		2.3
Subtotal		40.9		15.5		25.4		50.2		11.0		39.2
Total other intangible assets		218.8		17.0		201.8		228.1		12.2		215.9
Total goodwill and other intangible assets		$	387.6	$	17.0	370.6		$	404.6	12.2		392.4

Intangible Assets Valuation Methods

The goodwill recognized for the entities shown above is attributed to expected future cash flows. The acquisition date fair values of other intangible assets with finite lives are estimated using income approach techniques, which use future expected cash flows to develop a discounted present value amount.

The multi-period-excess-earnings method estimates fair value using the present value of the incremental after-tax cash flows attributable solely to the other intangible asset over its remaining life. This approach was used to estimate the fair value of other intangible assets associated with the underwriting capacity and customer relationships.

The relief-from-royalty method was used to estimate fair value for other intangible assets that relate to rights that could be obtained via a license from a third-party owner. Under this method, the fair value is estimated using the present value of license fees avoided by owning rather than leasing the asset. This technique was used to estimate the fair value of trade names, patents and certain information technology platforms.

The with-or-without method estimates the fair value of other intangible assets that provide an incremental benefit. Under this method, the fair value of the other intangible asset is calculated by comparing the value of the entity with and without the other intangible asset. This approach was used to estimate the fair value of non-compete agreements.

On at least an annual basis beginning no later than the interim period included in the one-year anniversary of an acquisition, White Mountains evaluates goodwill and other intangible assets for potential impairment. Between annual evaluations, White Mountains considers changes in circumstances or events subsequent to the most recent evaluation that may indicate that an impairment exists and, if necessary will perform an interim review for potential impairment.

During the year ended December 31, 2023 and 2022, White Mountains did not recognize any impairments to goodwill and other intangible assets.

Rollforward of Goodwill and Other Intangible Asset

The following table presents the change in goodwill and other intangible assets:


	December 31, 2023						December 31, 2022
Millions	Goodwill		Other Intangible Assets		Total Goodwill and Other Intangible Assets		Goodwill		Other Intangible Assets		Total Goodwill and Other Intangible Assets
Beginning balance	$	176.5	$	215.9	$	392.4	$	142.3	$	198.2	$	340.5
Acquisition of businesses (1)	—		—		—		59.5		—		59.5
Disposition of businesses (2)	(6.7)		(6.9)		(13.6)		—		—		—
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	—		—		—		(22.9)		22.9		—
Measurement period adjustments (3)	(1.0)		—		(1.0)		(2.4)		—		(2.4)
Amortization	—		(7.2)		(7.2)		—		(5.2)		(5.2)
Ending balance	$	168.8	$	201.8	$	370.6	$	176.5	$	215.9	$	392.4

(1) Relates to acquisitions within Other Operations.

(2) Relates to a disposition within Other Operations.

(3) Measurement period adjustments relate to updated information about acquisition date fair values of assets acquired and liabilities assumed. Adjustments relate to acquisitions within Other Operations.

During the years ended December 31, 2023 and 2022, White Mountains did not recognize any impairments to goodwill and other intangible assets.

Amortization of Other Intangible Assets

Amortization expense was $7.2 million, $5.2 million and $4.6 million for the years ended December 31, 2023, 2022 and 2021.

White Mountains expects to recognize amortization expense in each of the next five years as presented in the following table:


Millions	Amortization Expense
2024	$	5.5
2025	4.8
2026	4.0
2027	3.6
2028 and years after	8.2
Total (1)	$	26.1

(1) Excludes Ark’s indefinite-lived intangible assets of $175.7.

Goodwill and Other Intangible Assets - Summary of Goodwill and Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Goodwill [Line Items]
Goodwill	$ 168.8	$ 176.5	$ 142.3
Indefinite-lived intangible assets	175.7
Other intangible assets, accumulated amortization	17.0	12.2
Other intangible assets, finite-lived, net carrying value	26.1
Total other intangible assets	218.8	228.1
Total other intangible assets	201.8	215.9	198.2
Total goodwill and other intangible assets	387.6	404.6
Goodwill and other intangible assets	370.6	392.4	$ 340.5
Other Operations
Goodwill [Line Items]
Goodwill	44.4	52.1
Other intangible assets, finite-lived, acquisition date fair value	40.9	50.2
Other intangible assets, accumulated amortization	15.5	11.0
Other intangible assets, finite-lived, net carrying value	25.4	39.2
Goodwill and other intangible assets	69.8	91.3
Ark/WM Outrigger \| Operating Segments
Goodwill [Line Items]
Goodwill and other intangible assets	292.5	292.5
Ark/WM Outrigger \| Operating Segments \| Ark
Goodwill [Line Items]
Goodwill	116.8	116.8
Ark/WM Outrigger \| Operating Segments \| Ark \| Underwriting Capacity
Goodwill [Line Items]
Indefinite-lived intangible assets	175.7	175.7
Kudu \| Operating Segments
Goodwill [Line Items]
Goodwill	7.6	7.6
Goodwill and other intangible assets	$ 8.3	8.6
Trade names \| Other Operations
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life (in years)	13 years 3 months 18 days
Other intangible assets, finite-lived, acquisition date fair value	$ 13.3	17.9
Other intangible assets, accumulated amortization	4.1	3.0
Other intangible assets, finite-lived, net carrying value	$ 9.2	14.9
Trade names \| Kudu \| Operating Segments
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life (in years)	7 years
Other intangible assets, finite-lived, acquisition date fair value	$ 2.2	2.2
Other intangible assets, accumulated amortization	1.5	1.2
Other intangible assets, finite-lived, net carrying value	$ 0.7	1.0
Customer relationships \| Other Operations
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life (in years)	10 years 10 months 24 days
Other intangible assets, finite-lived, acquisition date fair value	$ 24.8	29.5
Other intangible assets, accumulated amortization	10.3	7.5
Other intangible assets, finite-lived, net carrying value	14.5	22.0
Total other intangible assets
Goodwill [Line Items]
Other intangible assets, accumulated amortization	17.0	12.2
Total other intangible assets	$ 201.8	215.9
Total other intangible assets \| Other Operations
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life (in years)	12 years 1 month 6 days
Other intangible assets, finite-lived, acquisition date fair value	$ 2.8	2.8
Other intangible assets, accumulated amortization	1.1	0.5
Other intangible assets, finite-lived, net carrying value	$ 1.7	$ 2.3

Debt

12 Months Ended

Dec. 31, 2023

Debt Disclosure [Abstract]

Debt

The following table presents White Mountains’s debt outstanding as of December 31, 2023 and 2022:


	December 31,		Effective	December 31,		Effective
$ in Millions	2023		Rate (1)	2022		Rate (1)



HG Global Senior Notes (2)	$	150.0	11.9%	$	150.0	8.9%
Unamortized discount and issuance cost	(3.1)			(3.5)
HG Global Senior Notes, carrying value	146.9			146.5
Ark 2007 Subordinated Notes, carrying value	30.0			30.0
Ark 2021 Subordinated Notes Tranche 1	42.7			41.3
Ark 2021 Subordinated Notes Tranche 2	47.0			47.0
Ark 2021 Subordinated Notes Tranche 3	70.0			70.0
Unamortized issuance cost	(4.2)			(4.6)
Ark 2021 Subordinated Notes, carrying value	155.5			153.7
Total Ark Subordinated Notes, carrying value	185.5		11.0%	183.7		7.6%




Kudu Credit Facility	210.3		10.1%	215.2		6.1%
Unamortized issuance cost	(6.5)			(6.9)
Kudu Credit Facility, carrying value	203.8			208.3






Other Operations debt	28.9		10.2%	37.4		6.6%
Unamortized issuance cost	(.5)			(.7)
Other Operations debt, carrying value	28.4			36.7
Total debt	$	564.6		$	575.2

(1) Effective rate includes the effect of the amortization of debt issuance costs and, where applicable, the original issue discount.

(2) Effective rate excludes the effect of the interest rate cap.

The following table presents a schedule of contractual repayments of White Mountains’s debt as of December 31, 2023:


Millions	December 31, 2023
Due in one year or less	$	3.3
Due in two to three years	11.2
Due in four to five years	40.6
Due after five years	523.8
Total	$	578.9

HG Global Senior Notes

On April 29, 2022, HG Global received the proceeds from the issuance of its $150.0 million face value floating rate secured senior notes (the “HG Global Senior Notes”). The HG Global Senior Notes, which mature in April 2032, accrue interest at a floating rate equal to the three-month Secured Overnight Financing Rate (“SOFR”) plus a credit adjustment spread of 0.26% and a stated margin of 6.0% per annum. Subsequent to the five-year anniversary of the funding date, absent the occurrence of an early amortization trigger event, HG Global will be required to make payments of principal on a quarterly basis totaling $15.0 million annually. Upon the occurrence of an early amortization trigger event, HG Global is required to use all available cash flow to repay the notes. Early amortization trigger events include scenarios in which HG Re is effectively in runoff. HG Global has the option to redeem, in whole or in part, the HG Global Senior Notes after the five-year anniversary of the funding date at the outstanding principal amount plus accrued interest.

On June 16, 2022, HG Global entered into an interest rate cap agreement, effective on July 25, 2022, to limit its exposure to the risk of interest rate increases on the HG Global Senior Notes. The notional amount of the interest rate cap is $150.0 million and the termination date is July 25, 2025. See Note 9 — “Derivatives.”

The HG Global Senior Notes require HG Global to maintain an interest reserve account of eight times the interest accrued for the most recent quarterly interest period. While the interest rate cap is in force, the interest reserve account is subject to a maximum required balance of $29.3 million. As of December 31, 2023, the fair value of the interest reserve account, which is included in short-term investments, was $30.4 million.

The HG Global Senior Notes are secured by the capital stock and other equity interests of HG Global’s subsidiaries, the interest reserve account, and all cash and non-cash proceeds from such collateral. The HG Global Senior Notes contain various affirmative and negative covenants that White Mountains considers to be customary for such borrowings.

If the payment of principal and interest under the HG Global Senior Notes becomes subject to tax withholding on behalf of a relevant governmental authority for certain indemnified taxes, the HG Global Senior Notes require the payment of additional amounts such that the amount received by the noteholders is the same as would have been received absent the tax withholding being imposed. The HG Global Senior Notes require the payment of additional interest of 1.0% per annum if the HG Global Senior Notes receive a non-investment grade rating or are no longer rated. As of December 31, 2023, the HG Global Senior Notes had an investment grade rating.

As of December 31, 2023, the HG Global Senior Notes had an outstanding balance of $150.0 million.

Ark Subordinated Notes

In March 2007, GAIL issued $30.0 million face value of floating rate unsecured junior subordinated deferrable interest notes to Alesco Preferred Funding XII Ltd., Alesco Preferred Funding XIII Ltd. and Alesco Preferred Funding XIV Ltd (the “Ark 2007 Subordinated Notes”). The Ark 2007 Subordinated Notes, which mature in June 2037, accrued interest at a floating rate equal to the three-month U.S. London Interbank Offered Rate (“LIBOR”) plus 4.6% per annum until September 2023. Effective September 2023, the Ark 2007 Subordinated Notes accrue interest a a floating rate equal to the three-month SOFR plus a credit adjustment spread of 0.26% and a stated margin of 4.6% per annum. As of December 31, 2023, the Ark 2007 Subordinated Notes had an outstanding balance of $30.0 million. In January 2024, Ark provided notice to the lenders that it intends to redeem the Ark 2007 Subordinated Notes in full on March 15, 2024.

In the third quarter of 2021, GAIL issued $163.3 million face value floating rate unsecured subordinated notes at par in three separate transactions for proceeds of $157.8 million, net of debt issuance costs. These Ark 2021 Subordinated Notes were issued in private placement offerings that were exempt from the registration requirements of the Securities Act of 1933.

On July 13, 2021, Ark issued €39.1 million ($46.3 million based upon the foreign exchange spot rate as of the date of the transaction) face value floating rate unsecured subordinated notes (“Ark 2021 Subordinated Notes Tranche 1”). The Ark 2021 Subordinated Notes Tranche 1, which mature in July 2041, accrue interest at a floating rate equal to the three-month Euro Interbank Offered Rate (EURIBOR) plus 5.75% per annum.

On August 11, 2021, Ark issued $47.0 million face value floating rate unsecured subordinated notes (“Ark 2021 Subordinated Notes Tranche 2”). The Ark 2021 Subordinated Notes Tranche 2, which mature in August 2041, accrued interest at a floating rate equal to the three-month U.S. LIBOR plus 5.75% per annum until August 2023. Effective August 2023, the Ark 2021 Subordinated Notes Tranche 2 accrue interest at a floating rate equal to the three-month SOFR plus a credit adjustment spread of 0.26% and a stated margin of 5.75% per annum.

On September 8, 2021, Ark issued $70.0 million face value floating rate unsecured subordinated notes (“Ark 2021 Subordinated Notes Tranche 3”). The Ark 2021 Subordinated Notes Tranche 3, which mature in September 2041, accrued interest at a floating rate equal to the three-month U.S. LIBOR plus 6.1% per annum until September 2023. Effective September 2023, the Ark 2021 Subordinated Notes Tranche 3 accrue interest at a floating rate equal to the three-month SOFR plus a credit adjustment spread of 0.26% and a stated margin of 6.1% per annum.

On the ten-year anniversary of the issue dates, the interest rate for the Ark 2021 Subordinated Notes will increase by 1.0% per annum. Ark has the option to redeem, in whole or in part, the Ark 2021 Subordinated Notes ahead of contractual maturity at the outstanding principal amounts plus accrued interest at the ten-year anniversary or any subsequent interest payment date.

All payments of principal and interest under the Ark 2021 Subordinated Notes are conditional upon GAIL’s solvency and compliance with the enhanced capital requirements of the Bermuda Monetary Authority (“BMA”). The deferral of payments of principal and interest under these conditions does not constitute a default by Ark and does not give the noteholders any rights to accelerate repayment of the Ark 2021 Subordinated Notes or take any enforcement action under the Ark 2021 Subordinated Notes.

If the payments of principal and interest under the Ark 2021 Subordinated Notes become subject to tax withholding on behalf of Bermuda or any political subdivision there, the Ark 2021 Subordinated Notes require the payment of additional amounts such that the amount received by the noteholders is the same as would have been received absent the tax withholding being imposed. The Ark 2021 Subordinated Notes Tranche 3 require the payment of additional interest of 1.0% per annum upon the occurrence of a premium load event until such event is remedied. Premium load events include the failure to meet payment obligations of the Ark 2021 Subordinated Notes Tranche 3 when due, failure of GAIL to maintain an investment grade credit rating, failure of GAIL to maintain 120% of its Bermuda solvency capital requirement, failure of GAIL to maintain a debt to capital ratio below 40%, late filing of GAIL’s or Ark’s financial information and making a restricted payment or distribution on GAIL’s common stock or other securities that rank junior or pari passu with the Ark 2021 Subordinated Notes Tranche 3 when a different premium load event exists or will be caused by the restricted payment. As of December 31, 2023, there were no premium load events.

As of December 31, 2023, the Ark 2021 Subordinated Notes Tranche 1 had an outstanding balance of €39.1 million ($42.7 million based upon the foreign exchange spot rate as of December 31, 2023), the Ark 2021 Subordinated Notes Tranche 2 had an outstanding balance of $47.0 million and the Ark 2021 Subordinated Notes Tranche 3 had an outstanding balance of $70.0 million.

The Ark Subordinated Notes contain various affirmative and negative covenants that White Mountains considers to be customary for such borrowings.

Ark Standby Letter of Credit Facilities

In December 2021, Ark entered into two uncommitted secured standby letter of credit facility agreements to support the continued growth and expansion of GAIL’s insurance and reinsurance operations. The standby letter of credit facility agreements were executed with ING Bank N.V., London Branch (the “ING LOC Facility”), with capacity of $50.0 million on an uncollateralized basis, and with Citibank Europe Plc (the “Citibank LOC Facility”), with capacity of $125.0 million on a collateralized basis. In September 2022, Ark entered into an additional uncommitted standby letter of credit facility agreement with Lloyds Bank Corporate Markets PLC (the “Lloyds LOC Facility”), with capacity of $100.0 million on a collateralized basis.

As of December 31, 2023, the ING LOC Facility was undrawn. As of December 31, 2023, the Citibank LOC Facility had an outstanding balance of $106.5 million and short-term investments pledged as collateral of $134.8 million. As of December 31, 2023, the Lloyds LOC Facility had an outstanding balance of $28.6 million and short-term investments pledged as collateral of $64.1 million. Ark’s uncommitted secured standby letter of credit facility agreements contain various representations, warranties and covenants that White Mountains considers to be customary for such borrowings.

Kudu Credit Facility

On March 23, 2021, Kudu entered into a secured revolving credit facility (the “Kudu Credit Facility”) with Mass Mutual to repay its prior credit facility and to fund new investments and related transaction expenses. The maximum borrowing capacity of the Kudu Credit Facility is $300.0 million. The Kudu Credit Facility matures on March 23, 2036.

Through June 2023, interest on the Kudu Credit Facility accrued at a floating interest rate equal to the greater of the three-month LIBOR or 0.25%, plus in each case, 4.30%. Effective July 2023, the Kudu Credit Facility accrue interest at a floating interest rate equal to the three-month SOFR plus a credit adjustment spread of 0.26% and a stated margin of 4.30%. The Kudu Credit Facility requires Kudu to maintain an interest reserve account. As of December 31, 2023, the interest reserve account of $14.9 million was held in short term investments. As of December 31, 2022, the interest reserve account of $12.2 million was held in restricted cash. The Kudu Credit Facility requires Kudu to maintain a ratio of the outstanding balance to the sum of the fair market value of Kudu’s Participation Contracts and cash held in certain accounts (the “LTV Percentage”) of less than 50% in years 0-3, 40% in years 4-6, 25% in years 7-8, 15% in years 9-10 and 0% thereafter. As of December 31, 2023, Kudu had a 25% LTV Percentage.

Kudu may borrow undrawn balances within the initial three-year availability period, subject to customary terms and conditions, to the extent the amount borrowed under the Kudu Credit Facility does not exceed the borrowing base, which is equal to 35% of the fair value of Kudu’s qualifying Participation Contracts. When considering the fair value of Kudu’s qualifying Participation Contracts as of December 31, 2023, the available undrawn balance was $89.7 million.

The following table presents the change in debt under the Credit Facility for the years ended December 31, 2023, 2022 and 2021:


	Year Ended December 31,
Millions	2023		2022		2021







Kudu Credit Facility
Beginning balance	$	215.2	$	225.4	$	—
Term loans
Borrowings	12.0		35.0		232.0
Repayments	(16.9)		(45.2)		(6.6)
Ending balance	$	210.3	$	215.2	$	225.4

The Kudu Credit Facility is secured by all property of the loan parties and contains various affirmative and negative covenants that White Mountains considers to be customary for such borrowings.

Other Operations Debt

As of December 31, 2023, White Mountains’s Other Operations had debt with an outstanding balance of $28.9 million, which consisted of four secured credit facilities (collectively, “Other Operations debt”).

Compliance

As of December 31, 2023, White Mountains was in compliance, in all material respects, with all of the covenants under its debt instruments.

Interest

Total interest expense incurred by White Mountains for its indebtedness was $62.7 million, $40.3 million and $20.5 million for the years ended December 31, 2023, 2022 and 2021. Total interest paid by White Mountains for its indebtedness was $42.2 million, $30.5 million and $13.9 million for the years ended December 31, 2023, 2022 and 2021.

- Definition

The entire disclosure for information about short-term and long-term debt arrangements, which includes amounts of borrowings under each line of credit, note payable, commercial paper issue, bonds indenture, debenture issue, own-share lending arrangements and any other contractual agreement to repay funds, and about the underlying arrangements, rationale for a classification as long-term, including repayment terms, interest rates, collateral provided, restrictions on use of assets and activities, whether or not in compliance with debt covenants, and other matters important to users of the financial statements, such as the effects of refinancing and noncompliance with debt covenants.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (h)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1B

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(c))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480678/235-10-S99-1

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 470
-Name Accounting Standards Codification
-Publisher FASB
-URI https://asc.fasb.org//470/tableOfContent

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1B

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (g)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1B

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (i)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1B

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1C
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1C

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1C
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1C

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1C
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1C

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1E
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1E

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1I
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1I

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1I
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1I

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1I
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1I

+ Details

Basis of Presentation and Significant Accounting Policies	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Basis of Presentation and Significant Accounting Policies	Basis of Presentation and Significant Accounting Policies Basis of Presentation White Mountains Insurance Group, Ltd. (the “Company” or the “Registrant”) is an exempted Bermuda limited liability company whose principal businesses are conducted through its subsidiaries and other affiliates. The Company’s headquarters is located at 26 Reid Street, Hamilton, Bermuda HM 11, its principal executive office is located at 23 South Main Street, Suite 3B, Hanover, New Hampshire 03755-2053 and its registered office is located at Clarendon House, 2 Church Street, Hamilton, Bermuda HM 11. The Company’s website is www.whitemountains.com. The information contained on White Mountains’s website is not incorporated by reference into, and is not a part of, this report. The accompanying consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) and include the accounts of the Company, its subsidiaries (collectively with the Company, “White Mountains”) and other entities required to be consolidated under GAAP. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. Consolidation Principles Under GAAP, the Company is required to consolidate any entity in which it holds a controlling financial interest. A controlling financial interest is usually in the form of an investment representing the majority of the subsidiary’s voting interests. However, a controlling financial interest may also arise from a financial interest in a variable interest entity (“VIE”) through arrangements that do not involve ownership of voting interests. The Company consolidates a VIE if it determines that it is the primary beneficiary. The primary beneficiary is defined as the entity who holds a variable interest that gives it both the power to direct the VIE’s activities that most significantly impact its economic performance and the obligation to absorb losses of, or the right to receive returns from, the VIE that could potentially be significant to the VIE. See Note 17 — “Variable Interest Entities”. Intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. Reportable Segments As of December 31, 2023, White Mountains conducted its operations through three reportable segments: (1) HG Global/BAM, (2) Ark/WM Outrigger, and (3) Kudu, with our remaining operating businesses, holding companies and other assets included in Other Operations. White Mountains has made its segment determination based on consideration of the following criteria: (i) the nature of the business activities of each of the Company’s subsidiaries and affiliates; (ii) the manner in which the Company’s subsidiaries and affiliates are organized; (iii) the existence of primary managers responsible for specific subsidiaries and affiliates; and (iv) the organization of information provided to the Company’s chief operating decision makers and its Board of Directors. See Note 15 — “Segment Information”. The HG Global/BAM segment consists of HG Global Ltd. and its wholly-owned subsidiaries (collectively, “HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”) (collectively with HG Global, “HG Global/BAM”). BAM is the first and only mutual municipal bond insurance company in the United States. By insuring the timely payment of principal and interest, BAM provides market access to, and lowers interest expense for, issuers of municipal bonds used to finance essential public purpose projects, such as schools, utilities and transportation facilities. BAM is owned by and operated for the benefit of its members, the municipalities that purchase BAM’s insurance for their debt issuances. HG Global was established to fund the startup of BAM and, through its reinsurance subsidiary HG Re Ltd. (“HG Re”), to provide up to 15%-of-par, first loss reinsurance protection for policies underwritten by BAM. HG Global, together with its subsidiaries, funded the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM (the “BAM Surplus Notes”). As of December 31, 2023 and 2022, White Mountains owned 96.9% of HG Global's preferred equity and 88.4% of its common equity. White Mountains does not have an ownership interest in BAM. However, White Mountains is required to consolidate BAM’s results in its financial statements because BAM is a VIE for which White Mountains is the primary beneficiary. BAM’s results are all attributed to noncontrolling interests. The Ark/WM Outrigger segment consists of Ark Insurance Holdings Limited and its subsidiaries (collectively, “Ark”) and Outrigger Re Ltd. Segregated Account 2023-1 (“WM Outrigger Re”) (collectively with Ark, “Ark/WM Outrigger”). Ark is a specialty property and casualty insurance and reinsurance company that offers a wide range of niche insurance and reinsurance products, including property, specialty, marine & energy, casualty and accident & health. Ark underwrites select coverages through Lloyd’s Syndicates 4020 and 3902 (the “Syndicates”) and its wholly-owned subsidiary Group Ark Insurance Limited (“GAIL”). White Mountains acquired a controlling ownership interest in Ark on January 1, 2021 (the “Ark Transaction”). See Note 2 — “Significant Transactions”. As of December 31, 2023 and 2022, White Mountains owned 72.0% of Ark on a basic shares outstanding basis (61.9% and 63.0% after taking account of management’s equity incentives). The remaining shares are owned by current and former employees. In the future, management rollover shareholders could earn additional shares in Ark if and to the extent that White Mountains achieves certain thresholds for its multiple of invested capital return. If fully earned, these additional shares would represent 12.5% of the shares outstanding at closing. For the years of account prior to the Ark Transaction, a significant proportion of the Syndicates’ underwriting capital was provided by third-party insurance and reinsurance groups (“TPC Providers”) using whole account reinsurance contracts with Ark’s corporate member. For the years of account subsequent to the Ark Transaction, Ark is no longer using TPC Providers to provide underwriting capital for the Syndicates. Captions within results of operations and other comprehensive income are shown net of amounts relating to the TPC Providers’ share of the Syndicates’ results, including investment results. During the fourth quarter of 2022, Ark sponsored the formation of Outrigger Re Ltd., a Bermuda company registered as a special purpose insurer and segregated accounts company, to provide collateralized reinsurance protection on Ark’s Bermuda global property catastrophe excess of loss portfolio written in the 2023 underwriting year. Outrigger Re Ltd. issued non-voting redeemable preference shares on behalf of four segregated accounts to White Mountains and unrelated third-party investors. White Mountains consolidates the results of its segregated account, WM Outrigger Re, in its financial statements. See Note 2 — “Significant Transactions.” As of December 31, 2023 and 2022, White Mountains owned 100.0% of WM Outrigger Re’s preferred equity. During the fourth quarter of 2023, Ark renewed Outrigger Re Ltd.in the 2024 underwriting year. The Kudu segment consists of Kudu Investment Management, LLC and its subsidiaries (collectively, “Kudu”). Kudu provides capital solutions for boutique asset and wealth managers for a variety of purposes including generational ownership transfers, management buyouts, acquisition and growth finance and legacy partner liquidity. Kudu also provides strategic assistance to investees from time to time. Kudu’s capital solutions generally are structured as minority preferred equity stakes with distribution rights, typically tied to gross revenues and designed to generate immediate cash yields. As of December 31, 2023 and 2022, White Mountains owned 89.6% and 89.3% of the basic units outstanding of Kudu (76.3% and 76.1% on a fully-diluted/fully-converted basis). White Mountains’s other operations consists of the Company and its wholly-owned subsidiary, White Mountains Capital, LLC (“WM Capital”), its other intermediate holding companies, its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), investment assets managed by WM Advisors, its interests in MediaAlpha, Inc. (“MediaAlpha”), PassportCard Limited (“PassportCard”) and DavidShield Life Insurance Agency (2000) Ltd. (“DavidShield”) (collectively, “PassportCard/DavidShield”), Elementum Holdings LP (“Elementum”), certain other consolidated and unconsolidated entities (“Other Operating Businesses”) and certain other assets (“Other Operations”). On October 25, 2023, White Mountains announced the launch of White Mountains Partners LLC (“WTM Partners”), which will acquire businesses in non-insurance, non-financial services sectors including essential services, light industrial and specialty consumer. White Mountains expects to deploy up to $500 million of equity capital through WTM Partners over time. WTM Partners is included within Other Operations. Held for Sale and Discontinued Operations White Mountains recognizes assets and liabilities classified as held for sale at the lower of carrying value on the date the asset is initially classified as held for sale or fair value less costs to sell. At the time of reclassification to held for sale, White Mountains ceases recognizing depreciation and amortization on assets held for sale. The results of operations of a business that has either been disposed of or are classified as held for sale are reported in discontinued operations if the disposal of business represents a strategic shift that has (or will have) a major effect on White Mountains’s operations and financial results. On August 1, 2022, White Mountains Holdings (Luxembourg) S.à r.l. (“WTM Holdings Seller”), an indirect wholly-owned subsidiary of White Mountains, completed the sale of White Mountains Catskill Holdings, Inc. and NSM Insurance HoldCo, LLC (“NSM”) (collectively with White Mountains Catskill Holdings, Inc., the “NSM Group”) to Riser Merger Sub, Inc., an affiliate of The Carlyle Group Inc. (the “NSM Transaction”), pursuant to the terms of the securities purchase agreement, dated as of May 9, 2022. See Note 2 — “Significant Transactions.” NSM is a full-service managing general agent (“MGA”) and program administrator with delegated binding authorities for specialty property and casualty insurance. As a result of the NSM Transaction, the assets and liabilities of NSM Group have been presented in the balance sheet as held for sale for periods prior to the closing of the transaction, and the results of operations for NSM Group have been classified as discontinued operations in the statements of operations and comprehensive income through the closing of the transaction. Prior period amounts have been reclassified to conform to the current period’s presentation. See Note 20 — “Held for Sale and Discontinued Operations.” Significant Accounting Policies Investment Securities As of December 31, 2023 and 2022, White Mountains’s invested assets consisted of securities and other investments held for general investment purposes. White Mountains’s portfolio of investment securities held for general investment purposes consists of fixed maturity investments, short-term investments, common equity securities, its investment in MediaAlpha and other long-term investments. Short-term investments consist of interest-bearing money market funds, certificates of deposit and other fixed maturity securities, which at the time of purchase, mature or become available for use within one year. Short-term investments are carried at fair value, which approximated amortized or accreted cost as of December 31, 2023 and 2022. Other long-term investments consist primarily of unconsolidated entities, including noncontrolling equity interests in the form of revenue and earnings participation contracts held at Kudu (“Participation Contracts”), private equity funds and hedge funds, a bank loan fund, Lloyd’s of London (“Lloyd’s”) trust deposits, insurance-linked securities (“ILS”) funds and private debt instruments. White Mountains’s portfolio of fixed maturity investments, including those within short-term investments, are classified as trading securities. Trading securities are reported at fair value as of the balance sheet date. White Mountains’s portfolio of common equity securities, its investment in MediaAlpha and other long-term investments are measured at fair value. White Mountains has taken the fair value option for its equity method eligible investments. Changes in net unrealized investment gains (losses) are reported pre-tax in revenues. Realized investment gains (losses) are accounted for using the specific identification method and are reported pre-tax in revenues. Premiums and discounts on all fixed maturity investments are amortized and accreted to income over the anticipated life of the investment. Income on mortgage and asset-backed securities is recognized using an effective yield based on anticipated prepayments and the estimated economic life of the securities. When actual prepayments differ significantly from anticipated prepayments, the estimated economic life is recalculated, and the remaining unamortized premium or discount is amortized prospectively over the remaining economic life. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants (an exit price) at a particular measurement date. Fair value measurements are categorized into a hierarchy that distinguishes between inputs based on market data from independent sources (observable inputs) and a reporting entity’s internal assumptions based upon the best information available when external market data is limited or unavailable (unobservable inputs). Quoted prices in active markets for identical assets have the highest priority (“Level 1”), followed by observable inputs other than quoted prices including prices for similar but not identical assets or liabilities (“Level 2”), and unobservable inputs, including the reporting entity’s estimates of the assumptions that market participants would use, having the lowest priority (“Level 3”). Assets and liabilities carried at fair value include White Mountains’s investment portfolio and derivative instruments. Valuation of assets and liabilities measured at fair value require management to make estimates and apply judgment to matters that may carry a significant degree of uncertainty. In determining its estimates of fair value, White Mountains uses a variety of valuation approaches and inputs. Whenever possible, White Mountains estimates fair value using valuation methods that maximize the use of quoted market prices or other observable inputs. Where appropriate, assets and liabilities measured at fair value have been adjusted for the effect of counterparty credit risk. White Mountains uses outside pricing services and brokers to assist in determining fair values. The outside pricing services White Mountains uses have indicated that they will only provide prices where observable inputs are available. Level 1 Measurements Investments valued using Level 1 inputs include White Mountains’s fixed maturity investments, primarily investments in U.S. Treasuries and short-term investments, which include U.S. Treasury Bills, common equity securities, and its investment in MediaAlpha following the initial public offering of MediaAlpha on October 30, 2020 (the “MediaAlpha IPO”). For investments in active markets, White Mountains uses the quoted market prices provided by outside pricing services to determine fair value. Level 2 Measurements Investments valued using Level 2 inputs include fixed maturity investments which have been disaggregated into classes, including debt securities issued by corporations, municipal obligations, mortgage and asset-backed securities and collateralized loan obligations. Investments valued using Level 2 inputs also include certain international listed equity funds, which White Mountains values using the fund manager’s published net asset value (“NAV”) to account for the difference in market exchange close times. In circumstances where quoted market prices are unavailable or are not considered reasonable, White Mountains estimates the fair value using industry standard pricing methodologies and observable inputs such as benchmark yields, reported trades, broker-dealer quotes, issuer spreads, benchmark securities, bids, offers, credit ratings, prepayment speeds, reference data including research publications and other relevant inputs. Given that many fixed maturity investments do not trade on a daily basis, the outside pricing services evaluate a wide range of fixed maturity investments by regularly drawing parallels from recent trades and quotes of comparable securities with similar features. The characteristics used to identify comparable fixed maturity investments vary by asset type and take into account market convention. White Mountains’s process to assess the reasonableness of the market prices obtained from the outside pricing sources covers substantially all of its fixed maturity investments and includes, but is not limited to, the evaluation of pricing methodologies and a review of the pricing services’ quality control procedures on at least an annual basis, a comparison of invested asset prices obtained from alternate independent pricing vendors on at least a semi-annual basis, monthly analytical reviews of certain prices and a review of the underlying assumptions utilized by the pricing services for select measurements on an ad hoc basis throughout the year. White Mountains also performs back-testing of selected investment sales activity to determine whether there are any significant differences between the market price used to value the security prior to sale and the actual sale price of the security on an ad-hoc basis throughout the year. Prices provided by the pricing services that vary by more than $0.5 million and 5% from the expected price based on these assessment procedures are considered outliers, as are prices that have not changed from period to period and prices that have trended unusually compared to market conditions. In circumstances where the results of White Mountains’s review process does not appear to support the market price provided by the pricing services, White Mountains challenges the vendor provided price. If White Mountains cannot gain satisfactory evidence to support the challenged price, White Mountains will rely upon its own internal pricing methodologies to estimate the fair value of the security in question. The valuation process described above is generally applicable to all of White Mountains’s fixed maturity investments. The techniques and inputs specific to asset classes within White Mountains’s fixed maturity investments for Level 2 securities that use observable inputs are as follows: Debt Securities Issued by Corporations The fair value of debt securities issued by corporations is determined from a pricing evaluation technique that uses information from market sources and integrates relative credit information, observed market movements, and sector news. Key inputs include benchmark yields, reported trades, broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including sector, coupon, credit quality ratings, duration, credit enhancements, early redemption features and market research publications. Municipal Obligations The fair value of municipal obligations is determined from a pricing evaluation technique that uses information from market makers, brokers-dealers, buy-side firms, and analysts along with general market information. Key inputs include benchmark yields, reported trades, issuer financial statements, material event notices and new issue data, as well as broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including type, coupon, credit quality ratings, duration, credit enhancements, geographic location and market research publications. Mortgage and Asset-Backed Securities and Collateralized Loan Obligations The fair value of mortgage and asset-backed securities and collateralized loan obligations is determined from a pricing evaluation technique that uses information from market sources and leveraging similar securities. Key inputs include benchmark yields, reported trades, underlying tranche cash flow data, collateral performance, plus new issue data, as well as broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including issuer, vintage, loan type, collateral attributes, prepayment speeds, default rates, recovery rates, cash flow stress testing, credit quality ratings and market research publications. Level 3 Measurements Fair value estimates for investments that trade infrequently and have few or no quoted market prices or other observable inputs are classified as Level 3 measurements. Investments valued using Level 3 fair value estimates are based upon unobservable inputs and include investments in certain fixed maturity investments, common equity securities and other long-term investments where quoted market prices or other observable inputs are unavailable or are not considered reliable or reasonable. Level 3 valuations are generated from techniques that use assumptions not observable in the market. These unobservable inputs reflect White Mountains’s assumptions of what market participants would use in valuing the investment. In certain circumstances, investment securities may start out as Level 3 when they are originally issued, but as observable inputs become available in the market, they may be reclassified to Level 2. Transfers of securities between levels are based on investments held as of the beginning of the period. Other Long-Term Investments As of December 31, 2023, $1,138.2 million of White Mountains’s other long-term investments, which consisted primarily of unconsolidated entities, including Kudu’s Participation Contracts and PassportCard/DavidShield were classified as Level 3 investments in the GAAP fair value hierarchy. The determination of the fair value of these securities involves significant management judgment, and the use of valuation analyses and assumptions that are inherently subjective and uncertain. White Mountains may use a variety of valuation techniques to determine fair value depending on the nature of the investment, including a discounted cash flow analysis, market multiple approach, cost approach and/or liquidation analysis. On an ongoing basis, White Mountains also considers qualitative changes in facts and circumstances, which may impact the valuation of its unconsolidated entities, including economic and market changes in relevant industries, changes to the entity’s capital structure, business strategy and key personnel, and any recent transactions. On a quarterly basis, White Mountains evaluates the most recent qualitative and quantitative information of the business and completes a fair valuation analysis for all other long-term investments classified as Level 3 investments. Periodically, and at least on an annual basis, White Mountains uses a third-party valuation firm to complete an independent valuation analysis of significant unconsolidated entities. Other Long-term Investments - NAV As of December 31, 2023, $833.8 million of White Mountains’s other long-term investments, which consisted of private equity funds and hedge funds, a bank loan fund, Lloyd’s trust deposits and ILS funds, were valued at fair value using NAV as a practical expedient. Investments for which fair value is measured using NAV as a practical expedient are not classified within the fair value hierarchy. White Mountains employs a number of procedures to assess the reasonableness of the fair value measurements for other long-term investments measured at NAV, including obtaining and reviewing interim unaudited and annual audited financial statements as well as periodically discussing the valuations and methodologies used to value the underlying investments of each fund with fund managers. However, since fund managers do not provide sufficient information to evaluate the pricing methods and inputs for each underlying investment, White Mountains considers the valuation inputs to be unobservable. The fair value of White Mountains’s other long-term investments measured at NAV are generally determined using the fund manager’s NAV. In the event that White Mountains believes the fair value of the fund differs from the NAV reported by the fund manager due to illiquidity or other factors, White Mountains will make an adjustment to the reported NAV to more appropriately represent the fair value of its other long-term investment. Cash and Restricted Cash Cash includes amounts on hand and demand deposits with banks and other financial institutions. Amounts presented in the statement of cash flows are shown net of balances acquired and sold in the purchase or sale of the Company’s consolidated subsidiaries. Cash balances that are not immediately available for general corporate and operating purposes are classified as restricted. Derivatives From time to time, White Mountains holds derivative financial instruments for risk management purposes. White Mountains recognizes all derivatives as either assets or liabilities, measured at fair value, on its consolidated balance sheet. Changes in the fair value of derivative instruments that meet the criteria for hedge accounting are recognized in other comprehensive income (loss) and reclassified into current period pre-tax income (loss) when the hedged items are recognized therein. Changes in the fair value of derivative instruments that do not meet the criteria for hedge accounting are recognized in current period pre-tax income (loss). As of December 31, 2023, White Mountains holds an interest rate cap derivative instrument that does not meet the criteria for hedge accounting. See Note 9 — “Derivatives”. Municipal Bond Guarantee Insurance All of the contracts issued by BAM are accounted for as insurance contracts under ASC 944-605, Financial Guarantee Insurance Contracts. For capital appreciation bonds, par is adjusted to the estimated equivalent par value for current interest paying bonds. See Note 10 — “Municipal Bond Guarantee Insurance”. Premiums are generally received upfront and an unearned premium revenue liability, equal to the amount of the premium received, is established at contract inception. Premium revenues are recognized in revenue over the period of the contracts in proportion to the amount of insurance protection provided using a constant rate. The constant rate is calculated based on the relationship between the par outstanding in a given reporting period compared with the sum of each of the par amounts outstanding for all periods. Insurance premiums receivable represents amounts due from customers for municipal bond insurance policies. Installment premiums are measured at the present value of contractual premiums, discounted at the risk-free rate, which is set at the inception of the insurance contract. Deferred acquisition costs represent commissions, premium taxes, excise taxes and other costs which are directly attributable to and vary with the production of business. These costs are deferred and amortized to the extent they relate to successful contract acquisitions over the applicable premium recognition period as acquisition expenses. Deferred acquisition costs are limited to the amount expected to be recovered from future earned premiums and anticipated investment income. A premium deficiency is recognized if the sum of expected loss and loss adjustment expenses (“LAE”), expected dividends to policyholders, unamortized acquisition costs, and maintenance costs exceeds related unearned premiums and anticipated investment income. A premium deficiency is recognized by charging any unamortized acquisition costs to expense to the extent required in order to eliminate the deficiency. If the premium deficiency exceeds unamortized acquisition costs then a liability is accrued for the excess deficiency. Loss reserves are recorded only to the extent that the present value of any payments projected to be made by BAM, net of any expected recoveries, exceeds the associated unearned premium reserve. As of December 31, 2023 and 2022, BAM did not have any loss or loss adjustment expense reserves. Property and Casualty Insurance and Reinsurance Ark writes a diversified portfolio of reinsurance and insurance products, including property, specialty, marine & energy, casualty and accident & health. Ark accounts for insurance and reinsurance policies that it writes in accordance with ASC 944, Financial Services - Insurance. Ark’s premiums written comprise premiums on insurance contracts incepted during the year as well as premium adjustments related to prior underwriting years. Insurance premiums are recognized as revenues over the loss exposure or coverage period. In most cases, premiums are earned ratably over the term of the contract, with unearned premiums calculated on a monthly pro-rata basis. Catastrophe premiums are earned in proportion to the insurance protection provided. Premiums earned are presented net of amounts ceded to reinsurers. Insurance premiums receivable, representing amounts due from insureds, are presented net of an allowance for uncollectible premiums, including expected credit losses. The allowance is based upon Ark’s ongoing review of amounts outstanding, historical loss data, including delinquencies and write-offs, current and forecasted economic conditions and other relevant factors. Credit risk is partially mitigated by Ark’s ability to cancel the policy if the policyholder does not pay the premium. Deferred acquisition costs comprise commission and brokerage fees and taxes which are directly attributable to and vary with the production of business. These costs are deferred and amortized to the extent they relate to successful contract acquisitions over the applicable premium recognition period as insurance and reinsurance acquisition expenses. Deferred acquisition costs are limited to the amount expected to be recovered from future earned premiums and anticipated investment income. A premium deficiency is recognized if the sum of expected loss and LAE, expected dividends to policyholders, unamortized acquisition costs and maintenance costs exceeds related unearned premiums and anticipated investment income. A premium deficiency is recognized by charging any unamortized acquisition costs to expense to the extent required in order to eliminate the deficiency. If the premium deficiency exceeds unamortized acquisition costs, then a liability is accrued for the excess deficiency. Losses and LAE are charged against income as incurred. Unpaid losses and LAE, including estimates for amounts incurred but not reported (“IBNR”), are based on estimates of the ultimate costs of settling claims, including the effects of inflation and other societal and economic factors. Unpaid loss and LAE reserves represent management’s best estimate of ultimate losses and LAE, net of estimated salvage and subrogation recoveries, if applicable. Such estimates are regularly reviewed and updated, and any resulting adjustments are reflected in current results of operations. The process of estimating loss and LAE involves a considerable degree of judgment by management, and the ultimate amount of expense to be incurred could be considerably greater than or less than the amounts currently reflected in the financial statements. See Note 5 — “Loss and Loss Adjustment Expense Reserves”. As part of its enterprise risk management function, Ark purchases reinsurance for risk mitigation purposes. Ark utilizes reinsurance and retrocessional agreements to reduce earnings volatility, protect capital, limit its exposure to risk concentration and accumulation of loss and manage within its overall internal risk tolerances or those set and agreed by regulators, ratings agencies and Lloyd’s. Ark also enters into reinsurance and retrocessional agreements to reduce its liability on individual risks and enable it to underwrite policies with higher limits where Ark believes this has a broader business benefit. Ark seeks to protect its downside risk from catastrophes and large loss events by purchasing reinsurance, including excess of loss protections, aggregate covers and industry loss warranties. Ark also considers alternative structures such as collateralized reinsurance, retrocessional reinsurance and catastrophe bonds. The purchase of reinsurance does not discharge Ark from its primary liability for the full value of its policies, and thus the collectability of balances due from Ark’s reinsurers is critical to its financial strength. Ark monitors the financial strength and ratings of its reinsurers on an ongoing basis. See Note 6 — “Third-Party Reinsurance”. For the years of account prior to the Ark Transaction, a significant proportion of the Syndicates’ underwriting capital was provided by TPC Providers using whole account reinsurance contracts with Ark’s corporate member. For the years of account subsequent to the Ark Transaction, Ark is no longer using TPC Providers to provide underwriting capital for the Syndicates. Captions within results of operations and other comprehensive income are shown net of amounts relating to the TPC Providers share of the Syndicates’ results, including investment results. Reinsurance recoverables represent paid losses and LAE, case reserves and IBNR reserves ceded to reinsurers under reinsurance treaties. Amounts recoverable from reinsurers are estimated in a manner consistent with the associated claims liability. Ark reports its reinsurance recoverables net of an allowance for estimated uncollectible reinsurance, including expected credit losses and coverage disputes. The allowance is based upon Ark’s ongoing review of amounts outstanding, length of collection periods, changes in reinsurer credit standing, disputes, applicable coverage defenses and other relevant factors. Reinsurance Contracts Accounted for as Deposits Reinsurance contracts that do not meet the risk transfer requirements necessary to be accounted for as reinsurance are accounted for using the deposit method. Under the deposit method, ceded premiums paid are not recognized through income but rather treated as a deposit. BAM entered into three ceded reinsurance agreements with Fidus Re Ltd. (“Fidus Re”), a Bermuda-based special purpose insurer created in 2018 solely to provide reinsurance protection to BAM. BAM also entered into an excess of loss reinsurance agreement (the “XOLT”) with HG Re. The financing expenses paid by BAM under these reinsurance agreements are recorded in general and administrative expenses. See Note 10 — “Municipal Bond Guarantee Insurance”. Ark has an aggregate excess of loss contract with SiriusPoint Ltd. (“SiriusPoint”), formerly Third Point Reinsurance Ltd., which is accounted for using the deposit method and recorded within other assets. Ark earns an annual crediting rate of 3.0%, which is recorded within other revenue. See Note 6 — “Third-Party Reinsurance”. Revenue Recognition Kudu’s revenues are primarily generated from Participation Contracts, which are noncontrolling equity interests in revenue and earnings participation contracts with asset and wealth management firms. Kudu’s Participation Contracts are measured at fair value with the change therein recognized within net realized and unrealized investment gains (losses). Distributions from Kudu’s clients are recognized through investment income when Kudu’s right to receive payment has been established and can be reliably measured, which generally occurs on a quarterly basis in accordance with the terms of the Participation Contracts. White Mountains’s Other Operations recognizes agent commissions and other revenues when it has satisfied its performance obligations. Deferred revenues associated with unsatisfied performance obligations are recognized within other liabilities. Cost of Sales White Mountains’s Other Operations’ cost of sales consists of salaries and related expenses, professional services and marketing and advertising expenses directly related to sales generation. These expenses are recognized as incurred. Goodwill and Other Intangible Assets Goodwill represents the excess of the amount paid to acquire subsidiaries over the fair value of identifiable net assets at the date of acquisition. Other intangible assets consist primarily of underwriting capacity, customer relationships and trade names. Goodwill and other intangible assets with indefinite lives are not amortized, but rather are evaluated for impairment on an annual basis, or whenever indications of potential impairment exist. In the absence of any indications of potential impairment, the evaluation of goodwill and indefinite-lived intangible assets is performed no later than the interim period in which the anniversary of the acquisition date falls. White Mountains initially evaluates goodwill using a qualitative approach (step zero) to determine whether it is more likely than not that the implied fair value of goodwill is greater than its carrying value. If White Mountains determines, based on this qualitative review, that it is more likely than not that an impairment may exist, then White Mountains performs a quantitative analysis to compare the fair value of a reporting unit with its carrying value. If the carrying value exceeds the estimated fair value, then an impairment charge is recognized through current period pre-tax income (loss). Other intangible assets with finite lives are initially measured at their acquisition date fair values and subsequently amortized over their economic lives. Other intangible assets with finite lives are presented net of accumulated amortization on the balance sheet. Other intangible assets with finite lives are reviewed for impairment when events occur or there are changes in circumstances indicating that their carrying value may exceed fair value. Impairment exists when the carrying value of other intangible assets exceeds fair value. See Note 4 — “Goodwill and Other Intangible Assets”. Incentive Compensation White Mountains’s Long-Term Incentive Plan (the “WTM Incentive Plan”) provides for grants of various types of share-based and non-share-based incentive awards to key employees of White Mountains. Non-share-based awards are recognized over the related service periods based on management’s best estimate of the amounts at which the awards are expected to be paid. Share-based compensation which is typically settled in cash, such as performance shares, is classified as a liability-type award. The compensation cost for liability-classified awards is measured initially at the grant date fair value and remeasured each reporting period until settlement. The compensation cost for equity-classified awards expected to be settled in shares, such as options and restricted shares, is measured at the original grant date fair value of the award. The compensation cost for all awards is recognized for the vested portion of the awards over the related service periods. See Note 12 — “Employee Share-Based Incentive Compensation Plans”. Income Taxes White Mountains has subsidiaries and branches that operate in various jurisdictions around the world and are subject to tax in the jurisdictions in which they operate. As of December 31, 2023, the primary jurisdictions in which the Company’s subsidiaries and branches operate and were subject to tax include Israel, Luxembourg, the United Kingdom and the United States. Income earned or losses generated by companies outside the United States are generally subject to an overall effective tax rate lower than that imposed by the United States. Deferred tax assets and liabilities are recorded when a difference between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts for tax purposes exists, and for other temporary differences. The deferred tax asset or liability is recorded based on tax rates expected to be in effect when the difference reverses. Deferred tax assets represent amounts available to reduce income taxes payable in future periods. White Mountains records a valuation allowance against deferred tax assets if it becomes more likely than not that all or a portion of a deferred tax asset will not be realized. Changes in valuation allowances from period to period are included in income tax expense in the period of change. See Note 8 — “Income Taxes”. Leases White Mountains has entered into lease agreements, primarily for office space and equipment. These leases are classified as operating leases, with lease expense recognized on a straight-line basis over the term of the lease. Lease assets and liabilities are not recorded for leases with a term at inception of one year or less. Lease incentives, such as free rent or landlord reimbursements for leasehold improvements, are recognized at lease inception and amortized on a straight-line basis over the term of the lease. Lease expense and the amortization of leasehold improvements are recognized within general and administrative expenses. Lease payments related to options to extend or renew the lease term are excluded from the calculation of lease liabilities unless White Mountains is reasonably certain of exercising those options. As of December 31, 2023 and 2022, the right-of-use (“ROU”) assets were $20.3 million and $25.2 million, and the lease liabilities were $22.0 million and $27.1 million. Noncontrolling Interests Noncontrolling interests consist of the ownership interests of noncontrolling shareholders in consolidated subsidiaries and are presented separately on the balance sheet. The portion of comprehensive income (loss) attributable to noncontrolling interests is presented net of related income taxes in the statement of operations and comprehensive income (loss). See Note 13 — “Common Shareholders’ Equity and Noncontrolling Interests”. Foreign Currency Exchange The functional currencies for White Mountains’s non-U.S. based subsidiaries are measured, in most instances, using currencies other than the U.S. dollar. Net foreign exchange gains and losses arising from the translation of functional currencies are generally reported in shareholders’ equity, in accumulated other comprehensive income (loss). White Mountains also invests in securities denominated in foreign currencies. Assets and liabilities recorded in these foreign currencies are translated into U.S. dollars at exchange rates in effect at the balance sheet date, and revenues and expenses are converted using the weighted average exchange rates for the period. As of December 31, 2023 and 2022, White Mountains had unrealized foreign currency translation losses of $1.6 million and $3.5 million recorded in accumulated other comprehensive income (loss) on its consolidated balance sheet. Fair Value Measurements Fair value measurements are categorized into a hierarchy that distinguishes between inputs based on market data from independent sources (observable inputs) and a reporting entity’s internal assumptions based upon the best information available when external market data is limited or unavailable (unobservable inputs). Quoted prices in active markets for identical assets or liabilities have the highest priority (Level 1), followed by observable inputs other than quoted prices, including prices for similar but not identical assets or liabilities (Level 2), and unobservable inputs, including the reporting entity’s estimates of the assumptions that market participants would use, having the lowest priority (Level 3). See Note 18 — “Fair Value of Financial Instruments”. Business Combinations White Mountains accounts for purchases of businesses using the acquisition method, which requires the measurement of assets acquired, including other intangible assets, and liabilities assumed, including contingent liabilities, at their estimated fair values as of the acquisition date. The acquisition date fair values represent management’s best estimates and are based upon established valuation techniques, reasonable assumptions and, where appropriate, valuations performed by independent third parties. In circumstances where additional information is required in order to determine the acquisition date fair value of balance sheet amounts, provisional amounts may be recorded as of the acquisition date and may be subject to subsequent adjustment throughout the measurement period, which is up to one year from the acquisition date. Measurement period adjustments are recognized in the period in which they are determined. The results of operations and cash flows of businesses acquired are included in the consolidated financial statements from the date of acquisition. White Mountains accounts for purchases of other intangible assets that do not meet the definition of a business as asset acquisitions. Asset acquisitions are recognized at the amount of consideration paid, which is deemed to equal fair value.

Statutory Capital and Surplus

12 Months Ended

Dec. 31, 2023

Insurance [Abstract]

Loss and Loss Adjustment Expense Reserves

Ark establishes loss and LAE reserves that are estimates of amounts needed to pay claims and related expenses in the future for insured events that have already occurred. Ark’s process of estimating reserves involves a considerable degree of judgment by management and, as of any given date, is inherently uncertain.

WM Outrigger Re entered into collateralized quota share agreements with GAIL to provide reinsurance protection on Ark’s Bermuda global property catastrophe excess of loss portfolio written in the 2023 underwriting year. WM Outrigger Re’s quota share reinsurance agreement with GAIL, including its loss and LAE reserves, eliminates in White Mountains’s consolidated financial statements.

HG Re and BAM do not have any outstanding loss and LAE reserves related to BAM’s municipal bond guarantee business.

Loss and LAE reserves are typically comprised of case reserves for claims reported and reserves for losses that have occurred but for which claims have not yet been reported, referred to as IBNR reserves. IBNR reserves include a provision for expected future development on case reserves. Case reserves are estimated based on the experience and knowledge of claims staff regarding the nature and potential cost of each claim and are adjusted as additional information becomes known or payments are made. IBNR reserves are derived by subtracting paid loss and LAE and case reserves from estimates of ultimate loss and LAE. Actuaries estimate ultimate loss and LAE using various generally accepted actuarial methods applied to known losses and other relevant information. Like case reserves, IBNR reserves are adjusted as additional information becomes known or payments are made.

Losses and LAE are categorized by the year in which the policy is underwritten (the year of account, or underwriting year) for purposes of Ark’s claims management and estimation of the ultimate loss and LAE reserves. For purposes of Ark’s reporting under GAAP, losses and LAE are categorized by the year in which the claim is incurred (the accident year).

Ultimate loss and LAE are generally determined by extrapolation of claims emergence and settlement patterns observed in the past that can reasonably be expected to persist into the future. In forecasting ultimate loss and LAE for any line of business, past experience with respect to that line of business is the primary resource, but cannot be relied upon in isolation. Ark’s own experience, particularly claims development experience, such as trends in case reserves, payments on and closings of claims, as well as changes in business mix and coverage limits, is the most important information for estimating its reserves. External data, available from organizations such as the Lloyd’s Market Association, consulting firms and other insurance and reinsurance companies, is used to supplement or corroborate Ark’s own experience. External data can be especially useful for estimating costs on newer lines of business. Ultimate loss and LAE for major losses and catastrophes are estimated based on the known and expected exposures to the loss event, rather than simply relying on the extrapolation of reported and settled claims.

For some lines of business, such as long-tail coverages discussed below, claims data reported in the most recent years of account are often too limited to provide a meaningful basis for analysis due to the typical delay in reporting and settling of claims. For this type of business, Ark uses an expected loss ratio method for the initial years of account. This is a standard and accepted actuarial reserve estimation method in these circumstances in which the loss ratio is selected based upon information used in pricing policies for that line of business, as well as any publicly available industry data, such as industry pricing, experience and trends.

Uncertainties in estimating ultimate loss and LAE are magnified by the time lag between when a claim actually occurs and when it is reported and eventually settled. This time lag is sometimes referred to as the “claim-tail”. The claim-tail for reinsurance and insurance obtained through brokers, MGAs and reinsurance intermediaries (collectively, the “insurance and reinsurance intermediaries”) is further extended because claims are first reported to either the original primary insurance company or the insurance and reinsurance intermediaries. The claim-tail for most property coverages is typically short (usually a few days up to a few months). Settlements for casualty/liability coverages can extend for long periods of time as claims are often reported and ultimately paid or settled years after the related loss events occur. During the long claims reporting and settlement period, additional facts regarding coverages written in prior years of account, as well as about actual claims and trends, may become known and, as a result, Ark may adjust its reserves. The inherent uncertainties of estimating reserves are increased by the diversity of loss development patterns among different types of reinsurance treaties, facultative contracts or direct insurance contracts, the necessary reliance on the ceding companies and insurance and reinsurance intermediaries for information regarding reported claims and the differing reserving practices among ceding companies and insurance and reinsurance intermediaries.

If management determines that an adjustment is appropriate, the adjustment is booked in the accounting period in which such determination is made. Accordingly, should reserves need to be increased or decreased in the future from amounts currently established, future results of operations would be negatively or positively impacted.

In determining ultimate loss and LAE, the cost to indemnify claimants, provide needed legal defense and other services for insureds and administer the investigation and adjustment of claims are considered. These claims costs are influenced by many factors that change over time, such as expanded coverage definitions as a result of new court decisions, inflation in costs to repair or replace damaged property, inflation in the cost of medical services, and legislated changes in statutory benefits, as well as by the unique facts that pertain to each claim. As a result, the rate at which claims arose in the past and the costs to settle them may not always be representative of what will occur in the future. The factors influencing changes in claims costs are often difficult to isolate or quantify and developments in paid and incurred losses from historical trends are frequently subject to multiple and conflicting interpretations. Changes in coverage terms or claims handling practices may also cause future experience and/or development patterns to vary from the past. A key objective of actuaries in developing estimates of ultimate loss and LAE and resulting IBNR reserves is to identify aberrations and systemic changes occurring within historical experience and accurately adjust for them so that the future can be projected more reliably. Because of the factors previously discussed, this process requires the use of informed judgment and is inherently uncertain.

Ark performs an actuarial review of its recorded loss and LAE reserves each quarter, using several generally accepted actuarial methods to evaluate its loss reserves, each of which has its own strengths and weaknesses. Management places more or less reliance on a particular method based on the facts and circumstances at the time the reserve estimates are made. These methods generally fall into one of the following categories or are hybrids of one or more of the following categories:

•Historical paid loss development methods: These methods use historical loss payments over discrete periods of time to estimate future losses. Historical paid loss development methods assume that the ratio of losses paid in one period to losses paid in an earlier period will remain constant. These methods necessarily assume that factors that have affected paid losses in the past, such as inflation or the effects of litigation, will remain constant in the future. Because historical paid loss development methods do not use case reserves to estimate ultimate losses, they can be more reliable than the other methods discussed below that look to case reserves (such as actuarial methods that use incurred losses) in situations where there are significant changes in how case reserves are established by a company’s claims adjusters. However, historical paid loss development methods are more leveraged, meaning that small changes in payments have a larger impact on estimates of ultimate losses, than actuarial methods that use incurred losses because cumulative loss payments take much longer to approach the expected ultimate losses than cumulative incurred amounts. In addition, and for similar reasons, historical paid loss development methods are often slow to react to situations when new or different factors arise than those that have affected paid losses in the past.

•Historical incurred loss development methods: These methods, like historical paid loss development methods, assume that the ratio of losses in one period to losses in an earlier period will remain constant in the future. However, instead of using paid losses, these methods use incurred losses (i.e., the sum of cumulative historical loss payments plus outstanding case reserves) over discrete periods of time to estimate future losses. Historical incurred loss development methods can be preferable to historical paid loss development methods because they explicitly take into account open cases and the claims adjusters’ evaluations of the cost to settle all known claims. However, historical incurred loss development methods necessarily assume that case reserving practices are consistently applied over time. Therefore, when there have been significant changes in how case reserves are established, using incurred loss data to project ultimate losses can be less reliable than other methods.

•Expected loss ratio methods: These methods are based on the assumption that ultimate losses vary proportionately with premiums. Expected loss ratios are typically developed based upon the information used in pricing and are multiplied by the total amount of premiums earned to calculate ultimate losses. Expected loss ratio methods are useful for estimating ultimate losses in the early years of long-tail lines of business, when little or no paid or incurred loss information is available.

•Bornhuetter-Ferguson methods: These methods are a blend of the expected loss ratio and loss development methods. The percent of incurred (or paid) loss to ultimate loss implied by the selected development pattern from the incurred (or paid) loss development method is used to determine the percentage of ultimate loss yet to be developed. Inception to date losses are added to losses yet to be developed, yielding an estimate of ultimate for each year of account.

•Adjusted historical paid and incurred loss development methods: These methods take traditional historical paid and incurred loss development methods and adjust them for the estimated impact of changes from the past in factors such as inflation, the speed of claims payment or the adequacy of case reserves. Adjusted historical paid and incurred loss development methods are often more reliable methods of predicting ultimate losses in periods of significant change, provided the actuaries can develop methods to reasonably quantify the impact of changes.

As part of Ark’s quarterly actuarial review, Ark compares the previous quarter’s projections of incurred, paid and case reserve activity, including amounts incurred but not reported, to actual amounts experienced in the quarter. Differences between previous estimates and actual experience are evaluated to determine whether a given actuarial method for estimating loss and LAE reserves should be relied upon to a greater or lesser extent than it has been in the past. While some variance is expected each quarter due to the inherent uncertainty in estimating loss and LAE reserves, persistent or large variances would indicate that prior assumptions and/or reliance on certain actuarial methods may need to be revised going forward.

Upon completion of each quarterly review, Ark selects indicated loss and LAE reserve levels based on the results of the actuarial methods described previously, which are the primary consideration in determining management's best estimate of required loss and LAE reserves. However, in making its best estimate, management also considers other qualitative factors that may lead to a difference between held reserves and the actuarial central estimate of reserves. Typically, these qualitative factors are considered when management and Ark’s actuaries conclude that there is insufficient historical incurred and paid loss information or that trends included in the historical incurred and paid loss information are not likely to repeat in the future. Such qualitative factors include, among others, recent entry into new markets or new products, improvements in the claims department that are expected to lessen future ultimate loss costs, legal and regulatory developments or other uncertainties that may arise.

Ark Reserve Estimation by Line of Business

The process of establishing loss and LAE reserves, including amounts incurred but not reported, is complex and imprecise, as it must consider many variables that are subject to the outcome of future events. As a result, informed subjective estimates and judgments as to the ultimate exposure to losses are an integral component of the loss and LAE reserving process. Ark categorizes and tracks insurance and reinsurance reserves by “reserving class of business” for each underwriting office, London and Bermuda, and then aggregates the reserving classes by line of business, which are summarized herein as property and accident & health, specialty, marine & energy, casualty-active and casualty-runoff.

Ark regularly reviews the appropriateness of its loss and LAE reserves at the reserving class of business level, considering a variety of trends that impact the ultimate settlement of claims for the subsets of claims in each particular reserving class.

For loss and LAE reserves as of December 31, 2023, Ark considers that the impact of the various reserving factors, as described below, on future paid losses would be similar to the impact of those factors on historical paid losses.

The major causes of material uncertainty (i.e., reserving factors) generally will vary for each line of business, as well as for each separately analyzed reserving class of business within the line of business. Also, reserving factors can have offsetting or compounding effects on estimated loss and LAE reserves. In most cases, it is not possible to measure the effect of a single reserving factor and construct a meaningful sensitivity expectation. Actual results will likely vary from expectations for each of these assumptions, resulting in an ultimate claims liability that is different from that being estimated currently.

Additional causes of material uncertainty exist in most product lines and may impact the types of claims that could occur within a particular line of business or reserving class of business. Examples where reserving factors, within a line of business or reserving class of business, are subject to change include changing types of insureds (e.g., size of account, industry insured, jurisdiction), changing underwriting standards, or changing policy provisions (e.g., deductibles, policy limits, endorsements).

Following is a detailed description of the reserve factors and consideration for each of Ark’s reserving lines of business.

Property and Accident & Health

Ark’s property and accident & health reserving line of business contains short-tail reserving classes. As such, reserving for these classes generally involves less uncertainty given the speed of settlement.

For property reserving classes, the reserve risk is driven primarily by occasional catastrophe events, though the financial effect of these is mitigated by reinsurance and retrocessional purchases. Ark writes property business on both an insurance and reinsurance basis. The insurance business primarily consists of direct and facultative contracts. However, some business is written through line slips and MGA binding authorities, which could have a longer-tail due to the increased exposure period caused by underlying policies attaching to the binder contract. The reinsurance business can also have a longer-tail due to timing delays resulting from attachment points on excess of loss contracts.

For accident & health reserving classes, the losses emanate from a wide range of personal accident, sickness, travel and medical insurance risks. The underlying business is a mix of direct and facultative contracts, as well as some MGA and reinsurance contracts, which are typically shorter-tail lines. Certain smaller components of the accident & health business can be longer-tail. The accident & health business is also exposed to occasional catastrophic events though not to the same degree as the property business.

Specialty

Ark’s specialty portfolio is comprised of a diverse portfolio of insurance and reinsurance subclasses of business including aviation, space, political and credit, cyber, terrorism and political violence, product defect and contamination, nuclear, fine art & specie, surety and mortgage. Certain subclasses of business are exposed to both catastrophe events and man-made loss events; for example, terrorism, war and war-like actions, political violence and space. Although these subclasses have different coverages and exposures, they are all short-tail in nature and have similar reserving features.

Marine & Energy

Ark’s marine & energy reserving line of business is underwritten on both an insurance and reinsurance basis and can be broken down into physical damage on marine risks, physical damage on upstream energy platforms and marine & energy liabilities.

The marine reserving classes consist primarily of marine hull, cargo and specie risks. These all generally have some element of transportability, which mitigates the catastrophe risk exposure; for example, having the ability to move out of the path of a hurricane if provided with sufficient notice. The marine reserving classes are generally shorter-tail.

The energy platform reserving classes cover risks that are less transportable and therefore are exposed to catastrophe events similar to property reserving classes. Other energy reserving classes cover construction contracts, which often have considerably protracted exposure periods with the bulk of the risk towards the end of the coverage period. This can have the effect of increasing the tail on an otherwise short-tail reserving class.

The marine & energy liability reserving classes, which represent a smaller portion of the marine & energy business, are typically longer-tail compared to physical damage reserving classes.

Casualty–Active and Casualty–Runoff

Ark’s casualty reserving lines of business, which include casualty–active and casualty–runoff, are long-tail classes of business. Consequently, the ultimate liability may not be known at the date of loss, which results in greater uncertainty when reserving for casualty lines.

The casualty–active line of business consists of U.S. reinsurance and insurance risks written on an excess of loss basis. The casualty–runoff line of business consists of international reinsurance risks and U.S. casualty insurance risks written through an MGA binding authority. The losses arising from these lines of business are primarily related to medical malpractice, professional liability and general liability coverages, which are long-tail lines of business.

Casualty policies are generally written on either a claims-made or occurrence basis. On a claims-made basis, the trigger of loss is based on the date that the loss is reported. On an occurrence basis, the trigger of loss is the date that the loss occurred. Due to delays between loss occurrence and loss reporting, business written on an occurrence basis can be longer-tail than business written on a claims-made basis.

There are a number of common reserving factors for casualty lines that can affect the estimated casualty reserves, including:

•Changes in claims-handling practices, both in-house and through third-party claims administrators,

•Changes in court interpretations of policy provisions, and

•Trends in litigation or jury awards.

Cumulative Number of Reported Claims

Ark counts a claim for each unique combination of individual claimant, loss event and risk. A claim is still counted if the claim is closed with no payment. Bulk-coded losses are counted as one claim, as underlying claim counts are not available.

Discounting

Ark does not discount loss and LAE reserves.

Impact of Third-Party Capital

For the years of account prior to the Ark Transaction, a significant proportion of the Syndicates’ underwriting capital was provided by TPC Providers using whole account reinsurance contracts with Ark’s corporate member. For the years of account subsequent to the Ark Transaction, Ark is no longer using TPC Providers to provide underwriting capital for the Syndicates.

A Reinsurance to Close (“RITC”) agreement is generally put in place after the third year of operations for a year of account such that the outstanding loss and LAE reserves, including future development thereon, are reinsured into the next year of account. As a result, and in combination with the changing participation provided by TPC Providers, Ark’s participation on outstanding loss and LAE reserves reinsured into the next year of account changes. The impact of the changes in TPC Providers’ participation are reflected in the table below. After 2023, Ark is no longer subject to changes in TPC Providers’ participation.

During 2023, an RITC agreement was executed such that the outstanding loss and LAE reserves for claims arising out of the 2020 year of account, for which the TPC Providers’ participation in the total net results of the Syndicates was 42.8%, were reinsured into the 2021 year of account, for which the TPC Providers’ participation in the total net results of the Syndicates was 0.0%.

During 2022, an RITC agreement was executed such that the outstanding loss and LAE reserves for claims arising out of the 2019 year of account, for which the TPC Providers’ participation in the total net results of the Syndicates was 58.3%, were reinsured into the 2020 year of account, for which the TPC Providers’ participation in the total net results of the Syndicates was 42.8%.

During 2021, an RITC was executed such that the outstanding loss and LAE reserves for claims arising out of the 2018 year of account, for which the TPC Providers’ participation in the total net results of the Syndicates was 57.6%, were reinsured into the 2019 year of account, for which the TPC Providers’ participation in the total net results of the Syndicates was 58.3%.

Loss and Loss Adjustment Expense Reserve Summary

The following table summarizes the loss and LAE reserve activity of the Ark/WM Outrigger segment for the years ended December 31, 2023, 2022 and 2021:


	Year Ended December 31,
Millions	2023		2022		2021
Gross beginning balance	$	1,296.5	$	894.7	$	696.0
Less: beginning reinsurance recoverable on unpaid losses (1)	(505.0)		(428.9)		(433.4)
Net loss and LAE reserves	791.5		465.8		262.6

Losses and LAE incurred relating to:
Current year losses	706.9		588.1		336.3
Prior year losses	19.9		(51.7)		(21.5)
Net incurred losses and LAE	726.8		536.4		314.8

Loss and LAE paid relating to:
Current year losses	(51.3)		(98.9)		(43.9)
Prior year losses	(356.6)		(158.6)		(61.6)
Net paid losses and LAE	(407.9)		(257.5)		(105.5)

Change in TPC Providers’ participation (2)	145.4		57.5		(2.2)
Foreign currency translation and other adjustments to loss and LAE reserves	8.5		(10.7)		(3.9)

Net ending balance	1,264.3		791.5		465.8
Plus: ending reinsurance recoverable on unpaid losses (3)	340.8		505.0		428.9
Gross ending balance	$	1,605.1	$	1,296.5	$	894.7

(1) The beginning reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $145.4, $276.8 and $319.2 as of December 31, 2022, 2021 and 2020.

(2) Amount represents the impact to net loss and LAE reserves due to a change in the TPC Providers’ participation related to the annual RITC process.

(3) The ending reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $0.0, $145.4 and $276.8 as of December 31, 2023, 2022 and 2021.

During the year ended December 31, 2023, the Ark/WM Outrigger segment experienced $19.9 million of net unfavorable prior year loss reserve development. The net unfavorable prior year loss reserve development was driven primarily by the property and accident & health ($41.7 million) reserving line of business, partially offset by net favorable prior year loss reserve development within the specialty ($11.9 million) and casualty–runoff ($6.0 million) reserving lines of business. The net unfavorable prior year loss reserve development in the property and accident & health reserving line of business was driven primarily by Hurricane Ian, Winter Storm Elliott and a power outage claim. The net favorable prior year loss reserve development in the specialty and casualty-runoff reserving lines of business was driven primarily by positive claims experience within the 2021 and 2020 accident years.

During the year ended December 31, 2022, The Ark/WM Outrigger segment experienced $51.7 million of net favorable prior year loss reserve development. The net favorable prior year loss reserve development was driven primarily by the property and accident & health ($20.8 million), marine & energy ($18.8 million) and specialty ($12.7 million) reserving lines of business. The favorable prior year loss reserve development in the property and accident & health, marine & energy and specialty reserving lines of business was driven primarily by positive claims experience within the 2021 accident year.

During the year ended December 31, 2021, The Ark/WM Outrigger segment experienced $21.5 million of net favorable prior year loss reserve development. The net favorable prior year loss reserve development was driven primarily by the property and accident & health ($8.9 million), casualty-active ($3.7 million), specialty ($3.3 million) and casualty–runoff ($3.3 million) reserving lines of business. The favorable prior year loss reserve development in the property and accident & health reserving line of business was driven primarily by positive claims experience within the 2018 and 2019 accident years.

The following table summarizes the unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses, for each of the Ark/WM Outrigger segment’s major reserving lines of business as of December 31, 2023 and 2022:


	Year Ended December 31,
Millions	2023		2022
Property and Accident & Health	$	358.7	$	258.2
Specialty	339.7		204.3
Marine & Energy	331.7		196.4

Casualty-Active	137.1		71.5
Casualty-Runoff	81.3		60.8
WM Outrigger and Other	15.8		.3
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	1,264.3		791.5
Plus: Reinsurance recoverables on unpaid losses (1)
Property and Accident & Health (2)	126.9		224.6
Specialty	77.6		97.2
Marine & Energy	59.1		79.8
Casualty-Active	74.8		49.9
Casualty-Runoff	2.4		53.5
Total Reinsurance recoverables on unpaid losses (1) (2)	340.8		505.0
Total unpaid loss and LAE reserves	$	1,605.1	$	1,296.5

(1) The reinsurance recoverables on unpaid losses include amounts attributable to TPC Providers of $0.0 and $145.4 as of December 31, 2023 and 2022.

(2) The reinsurance recoverables on unpaid losses exclude $15.6 ceded by Ark to WM Outrigger Re as of December 31, 2023, which eliminate in White Mountains’s consolidated financial statements.

The following five tables cover each of Ark’s property and accident & health, specialty, marine & energy, casualty-active and casualty-runoff reserving lines of business and are presented net of reinsurance, which includes the impact of whole-account quota-share reinsurance arrangements related to TPC Providers. Through the annual RITC process, and in combination with the changing participation provided by TPC Providers, Ark’s participation on outstanding loss and LAE reserves on prior years of account can fluctuate. Depending on the change in the TPC Providers’ participation from one year of account to the next, the impact could be significant and is reflected in the tables on a retrospective basis by accident year. That is, for the RITC executed in the current year that changes Ark’s participation for claims relating to prior accident years, the prior year columns are adjusted to include the impact of the RITC. After 2023, Ark is no longer subject to changes in TPC Providers’ participation.

The following table summarizes the participation of Ark’s TPC Providers by year of account:


	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023
TPC Providers’ Participation	66.2	%	70.0	%	59.6	%	60.0	%	57.6	%	58.3	%	42.8	%	—	%	—	%	—	%

Each of the five tables includes three sections.

The top section of the table presents, for each of the previous 10 accident years: (1) cumulative total undiscounted incurred loss and LAE as of each of the previous 10 year-end evaluations, (2) total IBNR plus expected development on reported claims as of December 31, 2023 and (3) the cumulative number of reported claims as of December 31, 2023.

The middle section of the table presents cumulative paid loss and LAE for each of the previous 10 accident years as of each of the previous 10 year-end evaluations. Also included in this section is a calculation of the loss and LAE reserves as of December 31, 2023, which is then included in the reconciliation to the consolidated balance sheet presented above. The total unpaid loss and LAE reserves as of December 31, 2023 are calculated as the cumulative incurred loss and LAE from the top section less the cumulative paid loss and LAE from the middle section, plus any outstanding liabilities from accident years prior to 2014.

The bottom section of the table is supplementary information about the average historical claims duration as of December 31, 2023. It shows the weighted average annual percentage payout of incurred loss and LAE by accident year as of each age. For example, the first column is calculated as the incremental paid loss and LAE in the first calendar year for each given accident year (e.g. calendar year 2023 for accident year 2023, calendar year 2022 for accident year 2022) divided by the cumulative incurred loss and LAE as of December 31, 2023 for that accident year. The resulting ratios are weighted using cumulative incurred loss and LAE as of December 31, 2023.


Property and Accident & Health
$ in Millions
	Incurred Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2023
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2014	$	32.8	$	29.5	$	29.4	$	28.7	$	28.5	$	28.6	$	28.6	$	28.6	$	28.6	$	28.5	$	.1	2,924
2015			19.3		18.3		17.3		16.3		16.1		16.2		15.9		15.8		16.1		.2		2,829
2016					22.5		17.7		18.5		18.6		18.6		18.8		18.8		18.7		.2		3,428
2017							31.3		38.1		45.6		44.6		43.2		42.7		44.2		15.3		4,613
2018									41.0		47.4		49.3		47.0		47.1		46.7		1.9		4,269
2019											34.2		31.4		27.2		24.0		23.4		.8		4,011
2020													77.6		75.8		75.1		78.4		12.4		4,617
2021															171.8		155.5		167.3		5.7		3,465
2022																	244.3		270.5		14.3		3,899
2023																			214.2		156.9		2,963
																	Total		$	908.0


Property and Accident & Health
Millions
	Cumulative Paid Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023
	Unaudited
2014	$	13.7	$	25.1	$	27.3	$	27.6	$	27.7	$	27.9	$	28.0	$	28.0	$	28.0	$	28.0
2015			6.9		12.3		13.5		14.7		14.7		14.9		15.1		15.1		15.5
2016					8.6		13.2		16.5		16.9		17.0		17.3		17.9		18.1
2017							16.9		26.0		31.8		33.0		29.8		27.5		25.7
2018									15.7		32.4		40.3		40.3		41.1		43.1
2019											6.8		16.8		18.5		18.6		19.4
2020													11.3		34.3		47.3		56.1
2021															30.9		87.1		131.3
2022																	70.9		193.9
2023																			20.1
																	Total		551.2
All outstanding liabilities before 2014, net of reinsurance																			1.9
Loss and LAE reserves, net of reinsurance																			$	358.7


Property and Accident & Health
	Average Annual Percentage Payout of Incurred Losses and LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	25.8%	35.5%	20.1%	6.1%	1.3%	1.1%	0.4%	0.3%	0.1%	—%


Specialty
$ in Millions
	Incurred Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2023
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2014	$	46.0	$	44.2	$	41.2	$	40.8	$	41.2	$	43.8	$	43.8	$	43.7	$	43.5	$	43.5	$	—	1,359
2015			17.4		14.8		12.4		10.8		11.1		11.3		11.3		9.0		8.2		(.1)		1,841
2016					18.4		14.4		11.0		11.3		11.9		11.9		9.0		8.6		.1		1,932
2017							18.1		13.0		12.1		11.6		11.7		10.7		10.4		.1		2,191
2018									14.6		16.4		16.8		16.1		14.9		15.8		.4		2,117
2019											21.8		19.6		18.7		25.7		30.1		.2		2,368
2020													24.2		23.2		19.0		19.9		1.7		1,994
2021															71.2		62.6		51.7		16.4		1,696
2022																	181.1		176.6		97.2		1,407
2023																			215.6		151.6		1,258
																	Total		$	580.4


Specialty
Millions
	Cumulative Paid Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023
	Unaudited
2014	$	26.5	$	39.3	$	40.1	$	40.5	$	41.1	$	42.4	$	43.2	$	43.1	$	43.4	$	43.7
2015			4.0		7.1		7.7		8.1		8.1		8.2		8.2		6.5		6.3
2016					3.2		8.0		9.1		9.9		10.4		10.4		8.6		8.4
2017							3.2		6.7		8.5		8.6		8.6		9.3		9.1
2018									2.8		8.3		10.1		10.5		11.9		13.1
2019											4.9		7.0		7.5		18.3		25.2
2020													5.3		10.7		13.2		18.3
2021															5.1		24.1		35.7
2022																	16.0		62.1
2023																			18.8
																	Total		240.7
All outstanding liabilities before 2014, net of reinsurance																			—
Loss and LAE reserves, net of reinsurance																			$	339.7


Specialty
	Average Annual Percentage Payout of Incurred Losses and LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	20.5%	30.6%	9.7%	6.0%	7.7%	6.0%	1.2%	1.7%	(3.1)%	(0.5)%


Marine & Energy
$ in Millions
	Incurred Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2023
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2014	$	34.4	$	20.2	$	17.2	$	16.3	$	14.1	$	13.7	$	14.0	$	13.8	$	13.9	$	13.6	$	(.3)	2,572
2015			21.8		17.5		16.3		13.4		12.8		13.0		12.8		13.1		12.9		.1		3,242
2016					23.6		19.7		15.8		14.7		14.4		15.0		14.2		13.8		.1		3,770
2017							26.2		19.5		17.7		17.1		16.8		15.9		16.1		.3		4,133
2018									25.8		20.3		17.8		18.1		17.7		18.1		.5		3,225
2019											23.9		21.8		21.8		21.6		22.1		.9		2,366
2020													30.0		27.3		28.7		27.6		1.6		1,561
2021															86.8		69.8		67.7		9.3		1,461
2022																	150.2		154.2		50.4		1,802
2023																			197.8		154.8		1,539
																	Total		$	543.9


Marine & Energy
Millions
	Cumulative Paid Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023
	Unaudited
2014	$	5.9	$	12.3	$	13.4	$	14.2	$	14.3	$	13.6	$	13.8	$	13.6	$	13.9	$	13.8
2015			4.0		7.9		9.7		11.1		10.5		10.6		11.0		11.6		11.7
2016					5.6		10.1		12.7		13.1		13.3		13.9		13.6		13.6
2017							5.1		11.2		12.9		14.2		14.2		14.2		14.1
2018									2.7		12.7		14.3		15.0		15.6		15.6
2019											3.4		10.7		12.7		14.5		15.5
2020													3.2		12.8		16.2		18.7
2021															6.3		24.5		38.5
2022																	12.3		66.4
2023																			10.6
																	Total		218.5
All outstanding liabilities before 2013, net of reinsurance																			6.3
Loss and LAE reserves, net of reinsurance																			$	331.7


Marine & Energy
	Average Annual Percentage Payout of Incurred Losses and LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	14.2%	34.9%	19.4%	6.1%	4.3%	6.5%	0.3%	0.3%	(0.3)%	0.1%


Casualty-Active
$ in Millions
	Incurred Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2023
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2014	$	13.3	$	9.4	$	8.4	$	8.2	$	8.1	$	7.7	$	7.8	$	7.6	$	7.8	$	7.7	$	.3	1,393
2015			9.6		9.8		8.2		8.1		7.4		7.2		7.0		7.3		7.5		.4		1,295
2016					8.8		8.3		8.9		9.0		9.1		9.2		9.2		10.1		.5		1,553
2017							11.6		11.7		10.8		9.4		9.1		10.5		10.7		1.1		1,632
2018									12.9		13.3		11.1		10.9		8.6		9.2		1.4		1,098
2019											14.8		13.7		12.4		10.6		11.4		2.6		954
2020													13.5		12.1		10.9		9.2		4.3		605
2021															21.4		22.4		16.6		12.4		842
2022																	33.0		38.1		34.4		1,237
2023																			61.0		57.9		1,065
																	Total		$	181.5


Casualty-Active
Millions
	Cumulative Paid Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023
	Unaudited
2014	$	1.3	$	3.5	$	4.2	$	4.7	$	5.2	$	5.5	$	5.9	$	6.0	$	6.2	$	6.7
2015			1.8		2.4		3.2		4.4		4.7		4.9		5.1		5.5		6.1
2016					.2		1.0		2.3		4.0		4.6		5.3		6.5		8.1
2017							.8		1.7		2.8		3.4		4.2		5.7		7.5
2018									.3		1.4		3.5		4.3		4.3		6.2
2019											.3		1.4		2.3		3.0		5.7
2020													.5		1.0		2.0		3.3
2021															.5		.9		3.1
2022																	.4		1.6
2023																			.9
																	Total		49.2
All outstanding liabilities before 2014, net of reinsurance																			4.8
Loss and LAE reserves, net of reinsurance																			$	137.1


Casualty-Active
	Average Annual Percentage Payout of Incurred Losses and LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	4.8%	8.8%	14.2%	11.3%	8.8%	10.9%	6.1%	4.6%	2.0%	3.2%


Casualty-Runoff
$ in Millions
	Incurred Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2023
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2014	$	49.2	$	48.4	$	51.0	$	54.0	$	57.6	$	59.1	$	59.1	$	58.9	$	58.7	$	59.7	$	1.9	1,947
2015			36.7		32.1		33.3		36.8		36.5		37.5		36.8		39.3		40.3		2.0		2,000
2016					32.6		32.2		40.4		38.6		38.8		38.6		37.7		37.5		2.3		2,158
2017							30.6		34.0		31.4		32.1		31.6		29.9		28.3		2.9		1,603
2018									33.7		28.2		27.3		26.6		26.2		28.0		4.1		1,277
2019											26.6		23.3		23.4		24.9		23.6		5.8		971
2020													15.9		12.3		13.9		10.9		3.9		566
2021															10.5		7.0		5.5		3.0		282
2022																	.8		2.6		1.8		77
2023																			2.7		1.6		40
																	Total		$	239.1


Casualty-Runoff
Millions
	Cumulative Paid Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023
	Unaudited
2014	$	6.5	$	23.2	$	29.6	$	36.5	$	43.2	$	47.0	$	48.7	$	49.4	$	51.9	$	54.9
2015			4.3		8.2		14.5		21.4		24.7		27.4		29.0		33.1		35.3
2016					3.9		10.2		17.7		22.7		25.4		27.8		28.7		31.0
2017							3.2		9.4		14.7		18.5		21.4		22.5		22.8
2018									3.4		7.4		12.6		14.9		16.3		18.2
2019											3.3		5.8		7.8		12.2		15.2
2020													.8		1.3		3.1		6.0
2021															.5		1.7		1.9
2022																	.3		.5
2023																			.9
																	Total		186.7
All outstanding liabilities before 2014, net of reinsurance																			28.9
Loss and LAE reserves, net of reinsurance																			$	81.3


Casualty-Runoff
	Average Annual Percentage Payout of Incurred Losses and LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	8.6%	14.0%	15.5%	14.8%	8.6%	7.0%	5.5%	4.2%	3.0%	2.1%

Statutory Capital and Surplus

White Mountains’s insurance operations are subject to regulation and supervision in each of the jurisdictions where they are domiciled and licensed to conduct business. Generally, regulatory authorities have broad supervisory and administrative powers over such matters as licenses, standards of solvency, premium rates, policy forms, investments, security deposits, methods of accounting, form and content of financial statements, minimum capital and surplus requirements, dividends and other distributions to shareholders, periodic examinations and annual and other report filings. In general, such regulation is for the protection of policyholders rather than shareholders.

The Insurance Act 1978 of Bermuda and related regulations, as amended (“Insurance Act”), regulates the insurance business of Bermuda-domiciled insurers. Under the Insurance Act, insurers are required to maintain available statutory capital and surplus at a level equal to or in excess of its enhanced capital requirement which is established by reference to either a Bermuda Solvency Capital Requirement (“BSCR”) model or an approved internal capital model. Generally, the BMA has broad supervisory and administrative powers over such matters as licenses, standards of solvency, investments, methods of accounting, form and content of financial statements, minimum capital and surplus requirements and annual and other report filings.

HG Global/BAM

HG Re is a special purpose insurer under Bermuda insurance regulations and is subject to regulation and supervision by the BMA. As of December 31, 2023, HG Re had statutory capital and surplus of $784.2 million. As a special purpose insurer, HG Re has a nominal minimum regulatory capital requirement of $1.

BAM is domiciled in New York and is subject to regulation by the NYDFS. New York financial guarantee insurance law establishes single risk and aggregate limits with respect to insured obligations insured by financial guarantee insurers. BAM’s statutory net loss for the years ended December 31, 2023, 2022 and 2021 was $51.1 million, $55.0 million and $49.3 million. BAM’s statutory surplus, as reported to regulatory authorities as of December 31, 2023, was $269.3 million, which exceeds the minimum statutory surplus of $66.0 million necessary for BAM to maintain its New York State financial guarantee insurance license.

Ark

Syndicates 4020 and 3902 are subject to oversight by the Council of Lloyd’s. Ark Syndicate Management Limited is authorized by the U.K.’s Prudential Regulation Authority (the “PRA”) and regulated by the Financial Conduct Authority under the Financial Services and Markets Act 2000. The underwriting capacity of a Member of Lloyd’s must be supported by providing a deposit in the form of cash, securities or letters of credit in an amount determined under the capital adequacy regime of the PRA. This amount is determined by Lloyd’s and is based on each syndicate’s solvency and capital requirement as calculated through its internal model. In addition, if the Funds at Lloyd’s are not sufficient to cover all losses, the Lloyd’s Central Fund provides an additional discretionary level of security for policyholders. As of December 31, 2023, Ark had provided Funds at Lloyd’s of $345.2 million.

GAIL is subject to regulation and supervision by the BMA. As of December 31, 2023, GAIL had statutory capital and surplus of $1,087.5 million. GAIL’s minimum statutory capital and surplus requirement established by the BMA was $508.4 million as of December 31, 2023.

WM Outrigger Re

WM Outrigger Re is a special purpose insurer under Bermuda insurance regulations and is subject to regulation and supervision by the BMA. As of December 31, 2023, WM Outrigger Re had statutory capital and surplus of $273.0 million. As a special purpose insurer, WM Outrigger Re has a nominal minimum regulatory capital requirement of $1.

Dividend Capacity

There are no restrictions under Bermuda law or the law of any other jurisdiction on the payment of dividends from retained earnings by White Mountains, provided that after the payment of any dividend, the Company would continue to be able to pay its liabilities as they become due and the realizable value of the Company’s assets would remain greater that its liabilities. Following is a description of the dividend capacity of White Mountains’s insurance and reinsurance subsidiaries:

HG Global/BAM

HG Re is a special purpose insurer subject to regulation and supervision by the BMA. HG Re does not require regulatory approval to pay dividends, however, its dividend capacity is limited to amounts held outside of the Collateral Trusts pursuant to the FLRT with BAM. As of December 31, 2023, HG Re had $3.0 million of net unrestricted cash. As of December 31, 2023, HG Re had $105.8 million of accrued interest on the BAM Surplus Notes held outside the Collateral Trusts. As of December 31, 2023, HG Re had $784.2 million of statutory capital and surplus and $948.5 million of assets held in the Collateral Trusts.

Under its agreements with HG Global, BAM is required to seek regulatory approval to pay principal and interest on the BAM Surplus Notes only to the extent that its remaining qualified statutory capital and other capital resources continue to support its outstanding obligations, its business plan and its “AA/stable” rating from Standard & Poor’s. No payment of principal or interest on the BAM Surplus Notes may be made without the approval of the NYDFS.

In December 2023, BAM made a $27.4 million cash payment of principal and interest on the BAM Surplus Notes held by HG Global. Of this payment, $17.8 million was a repayment of principal held in the Supplemental Trust, $2.0 million was a payment of accrued interest held in the Supplemental Trust and $7.6 million was a payment of accrued interest held outside the Supplemental Trust.

In December 2022, BAM made a $36.0 million cash payment of principal and interest on the BAM Surplus Notes held by HG Global. Of this payment, $24.6 million was a repayment of principal held in the Supplemental Trust, $1.0 million was a payment of accrued interest held in the Supplemental Trust and $10.4 million was a payment of accrued interest held outside the Supplemental Trust.

Ark

During any 12-month period, GAIL, a class 4 licensed Bermuda insurer, has the ability to (i) make capital distributions of up to 15% of its total statutory capital per the previous year’s statutory financial statements, or (ii) make dividend payments of up to 25% of its total statutory capital and surplus per the previous year’s statutory financial statements, without prior approval of Bermuda regulatory authorities. Accordingly, GAIL will have the ability to pay a dividend of up to $271.9 million during 2024, which is equal to 25% of its December 31, 2023 statutory capital and surplus of $1,087.5 million, subject to meeting all appropriate liquidity and solvency requirements and the filing of its December 31, 2023 statutory financial statements. During 2023, GAIL paid a $15 million dividend to its immediate parent.

Third-Party Reinsurance - Remaining Gross and Net Reinsurance Recoverables (Details) - Ark $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($)
Segment Reporting Information [Line Items]
Gross	$ 368.2
Net	287.5
Ark
Segment Reporting Information [Line Items]
Ceded	$ (15.6)
Reinsurance Recoverable Including Reinsurance Premium Paid \| Reinsurer Concentration Risk
Segment Reporting Information [Line Items]
% of Total	100.00%
Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 80.7
A+ or better
Segment Reporting Information [Line Items]
Gross	204.0
Net	$ 204.0
A+ or better \| Reinsurance Recoverable Including Reinsurance Premium Paid \| Reinsurer Concentration Risk
Segment Reporting Information [Line Items]
% of Total	71.00%
A+ or better \| Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 0.0
A- to A
Segment Reporting Information [Line Items]
Gross	76.2
Net	$ 76.2
A- to A \| Reinsurance Recoverable Including Reinsurance Premium Paid \| Reinsurer Concentration Risk
Segment Reporting Information [Line Items]
% of Total	26.50%
A- to A \| Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 0.0
B++ or lower and not rated
Segment Reporting Information [Line Items]
Gross	88.0
Net	$ 7.3
B++ or lower and not rated \| Reinsurance Recoverable Including Reinsurance Premium Paid \| Reinsurer Concentration Risk
Segment Reporting Information [Line Items]
% of Total	2.50%
B++ or lower and not rated \| Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 80.7

Segment Information - Assets (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Segment reporting information
Total investments	$ 6,388.7	$ 5,170.0
Total assets	8,385.9	7,389.3
Total liabilities	3,824.3	3,454.3
Total White Mountains’s common shareholders’ equity	4,240.5	3,746.9
Noncontrolling interests	321.1	188.1
Debt	564.6	575.2
BAM Surplus Notes \| Surplus Notes
Segment reporting information
Debt	322.2	340.0
Interest receivable	174.5	157.9
Dividends payable	399.8	341.4
Other Operations
Segment reporting information
Total investments	1,709.1	1,532.7
Total assets	1,875.9	1,813.5
Total liabilities	141.1	157.1
Total White Mountains’s common shareholders’ equity	1,725.4	1,636.0
Noncontrolling interests	9.4	20.4
Debt	28.4	36.7
HG Global/BAM - Segment \| Operating Segments
Segment reporting information
Total investments	1,082.9	975.8
Total assets	1,172.0	1,058.5
Total liabilities	531.7	501.8
Total White Mountains’s common shareholders’ equity	779.3	712.0
Noncontrolling interests	(139.0)	(155.3)
Debt	146.9	146.5
Ark \| Operating Segments
Segment reporting information
Total investments	2,671.1	1,965.6
Total assets	4,378.6	3,691.4
Total liabilities	2,876.1	2,522.1
Total White Mountains’s common shareholders’ equity	1,165.6	921.4
Noncontrolling interests	336.9	247.9
Debt	185.5	183.7
Kudu \| Operating Segments
Segment reporting information
Total investments	925.6	695.9
Total assets	959.4	825.9
Total liabilities	275.4	273.3
Total White Mountains’s common shareholders’ equity	570.2	477.5
Noncontrolling interests	113.8	75.1
Debt	$ 203.8	$ 208.3

Variable Interest Entities (Details) $ in Millions	12 Months Ended
Variable Interest Entities (Details) $ in Millions	Dec. 31, 2023 USD ($)
Subsidiary - HG Global
Variable Interest Entity [Line Items]
Purchase of surplus notes	$ 503.0
Subsidiary - HG Global \| Maximum
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured	15.00%
Subsidiary - BAM
Variable Interest Entity [Line Items]
BAM member surplus contributions	$ 545.2
Variable Interest Entity, Not Primary Beneficiary \| PassportCard/DavidShield
Variable Interest Entity [Line Items]
Ownership interest	53.80%
Partnership interest, carrying value	$ 159.6
Variable Interest Entity, Not Primary Beneficiary \| Elementum Holdings, L.P.
Variable Interest Entity [Line Items]
Ownership interest	26.60%
Partnership interest, carrying value	$ 35.0
Variable Interest Entity, Not Primary Beneficiary \| Limited Partnerships
Variable Interest Entity [Line Items]
Partnership interest, carrying value	$ 248.6

Statutory Capital and Surplus (Details) - USD ($)	1 Months Ended		12 Months Ended
Statutory Capital and Surplus (Details) - USD ($)	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Statutory Accounting Practices [Line Items]
Assets held-in-trust		$ 856,900,000		$ 856,900,000
HG Global/BAM \| HG Global
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	$ 784,200,000		$ 784,200,000
Minimum regulatory capital requirement	1		1
Cash and investments outside of collateral trusts	3,000,000		3,000,000
Accrued interest held in supplemental trust	105,800,000		105,800,000
HG Global/BAM \| BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	269,300,000		269,300,000
Minimum regulatory capital requirement	66,000,000		66,000,000
Statutory net loss			(51,100,000)	$ (55,000,000)	$ (49,300,000)
Ark/WM Outrigger \| WM Outrigger Re
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	273,000,000		273,000,000
Minimum regulatory capital requirement	$ 1		$ 1
Ark/WM Outrigger \| Ark
Statutory Accounting Practices [Line Items]
Percent of statutory capital balance	15.00%		15.00%
Percent of statutory capital and surplus balance	25.00%		25.00%
Percent of statutory capital balance	$ 271,900,000		$ 271,900,000
Statutory surplus balance	1,087,500,000		1,087,500,000
Payment of dividend to immediate parent			15,000,000
Ark
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	1,087,500,000		1,087,500,000
Minimum regulatory capital requirement	508,400,000		508,400,000
Funds provided	345,200,000		345,200,000
HG Re
Statutory Accounting Practices [Line Items]
Assets held-in-trust	948,500,000		$ 948,500,000
Surplus Notes \| BAM Surplus Notes
Statutory Accounting Practices [Line Items]
Cash repayments on notes	27,400,000	36,000,000
Repayment of principal	17,800,000	24,600,000
Accrued interest payment held in trust	2,000,000	1,000,000
Accrued interest payment held outside of trust	$ 7,600,000	$ 10,400,000

Significant Transactions - Outrigger Re (Details) $ in Millions		1 Months Ended	3 Months Ended		12 Months Ended
	Dec. 20, 2022 USD ($) account	Jan. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2023 USD ($)
Outrigger Re
Other Significant Transactions [Line Items]
Ceded unearned premiums			$ 250.0		$ 250.0
Rollover amount			130.0
Short-term investments			$ 265.3	$ 203.7	265.3
Outrigger Re \| Subsequent Event
Other Significant Transactions [Line Items]
Return of capital		$ 68.1
Outrigger Re
Other Significant Transactions [Line Items]
Business combination, consideration transferred				$ 205.0
Transaction costs					$ 1.3
Subsidiary - WM Outrigger Re \| Preferred stocks
Other Significant Transactions [Line Items]
Basic ownership			100.00%	100.00%	100.00%
Redeemable Preferred Stock \| Outrigger Re Ltd
Other Significant Transactions [Line Items]
Issuances of common shares	$ 250.0
Number of segregated accounts \| account	4

Goodwill and Other Intangible Assets (Tables)

12 Months Ended

Dec. 31, 2023

Goodwill and Intangible Assets Disclosure [Abstract]

Schedule of intangible assets and goodwill


$ in Millions	Weighted Average Economic Life (in years)	December 31, 2023				December 31, 2022
		Acquisition Date Fair Value		Accumulated Amortization		Net Carrying Value		Acquisition Date Fair Value		Accumulated Amortization		Net Carrying Value
Goodwill:
Ark	N/A	$	116.8	$	—	$	116.8	$	116.8	$	—	$	116.8
Kudu	N/A	7.6		—		7.6		7.6		—		7.6


Other Operations	N/A	44.4		—		44.4		52.1		—		52.1
Total goodwill		168.8		—		168.8		176.5		—		176.5
Other intangible assets:
Ark
Underwriting capacity	N/A	175.7		—		175.7		175.7		—		175.7
Kudu
Trade names	7.0	2.2		1.5		.7		2.2		1.2		1.0





Other Operations
Trade names	13.3	13.3		4.1		9.2		17.9		3.0		14.9
Customer relationships	10.9	24.8		10.3		14.5		29.5		7.5		22.0

Other	12.1	2.8		1.1		1.7		2.8		.5		2.3
Subtotal		40.9		15.5		25.4		50.2		11.0		39.2
Total other intangible assets		218.8		17.0		201.8		228.1		12.2		215.9
Total goodwill and other intangible assets		$	387.6	$	17.0	370.6		$	404.6	12.2		392.4

The following table presents the change in goodwill and other intangible assets:


	December 31, 2023						December 31, 2022
Millions	Goodwill		Other Intangible Assets		Total Goodwill and Other Intangible Assets		Goodwill		Other Intangible Assets		Total Goodwill and Other Intangible Assets
Beginning balance	$	176.5	$	215.9	$	392.4	$	142.3	$	198.2	$	340.5
Acquisition of businesses (1)	—		—		—		59.5		—		59.5
Disposition of businesses (2)	(6.7)		(6.9)		(13.6)		—		—		—
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	—		—		—		(22.9)		22.9		—
Measurement period adjustments (3)	(1.0)		—		(1.0)		(2.4)		—		(2.4)
Amortization	—		(7.2)		(7.2)		—		(5.2)		(5.2)
Ending balance	$	168.8	$	201.8	$	370.6	$	176.5	$	215.9	$	392.4

(1) Relates to acquisitions within Other Operations.

(2) Relates to a disposition within Other Operations.

Schedule of finite-lived intangible assets, future amortization expense

White Mountains expects to recognize amortization expense in each of the next five years as presented in the following table:


Millions	Amortization Expense
2024	$	5.5
2025	4.8
2026	4.0
2027	3.6
2028 and years after	8.2
Total (1)	$	26.1

(1) Excludes Ark’s indefinite-lived intangible assets of $175.7.

Investment Securities - Cost and Amortized Cost Maturity Schedule (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Fixed Maturities, Excluding Asset-Backed Securities And Collateralized Loan Obligations
Cost or Amortized Cost
Due in one year or less	$ 319.9
Due after one year through five years	845.0
Due after five years through ten years	303.4
Due after ten years	101.7
Carrying Value
Due in one year or less	315.0
Due after one year through five years	813.9
Due after five years through ten years	287.0
Due after ten years	94.9
Mortgage and asset-backed securities and collateralized loan obligations
Cost or Amortized Cost
Mortgage and asset-backed securities and collateralized loan obligations	628.4
Carrying Value
Mortgage and asset-backed securities and collateralized loan obligations	598.5
Fixed maturity investments
Cost or Amortized Cost
Total fixed maturity investments	2,198.4	$ 2,076.0
Carrying Value
Total fixed maturity investments	$ 2,109.3	$ 1,920.9

SCHEDULE IV - REINSURANCE (Details) - USD ($) $ in Millions	12 Months Ended
SCHEDULE IV - REINSURANCE (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Premiums earned
Net earned premiums	$ 1,440.9	$ 1,076.7	$ 664.2
HG Global/BAM
Premiums earned
Gross Amount	28.7	28.6	23.2
Ceded	0.0	0.0	0.0
Assumed	2.5	4.7	3.7
Net earned premiums	$ 31.2	$ 33.3	$ 26.9
Percentage of Amount Assumed to Net	8.00%	14.10%	13.80%
Ark/WM Outrigger
Premiums earned
Gross Amount	$ 848.1	$ 655.5	$ 556.0
Ceded	(364.0)	(280.8)	(249.1)
Assumed	925.6	668.7	330.4
Net earned premiums	$ 1,409.7	$ 1,043.4	$ 637.3
Percentage of Amount Assumed to Net	65.70%	64.10%	51.80%

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References + Details

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenues:
Earned insurance premiums	$ 1,440.9	$ 1,076.7	$ 664.2
Net investment income	194.2	124.4	82.5
Net realized and unrealized investment gains (losses)	434.2	(191.5)	(246.1)
Commission revenues	13.2	11.5	9.6
Other revenues	84.2	136.8	104.2
Total revenues	2,166.7	1,157.9	614.4
Expenses:
Loss and loss adjustment expenses	726.8	536.4	314.8
Acquisition expenses	290.1	250.6	186.3
Cost of sales	40.4	98.6	69.3
General and administrative expenses	432.6	364.1	291.6
Change in fair value of contingent consideration	48.7	17.3	5.5
Interest expense	62.7	40.3	20.5
Total expenses	1,601.3	1,307.3	888.0
Pre-tax income (loss) from continuing operations	565.4	(149.4)	(273.6)
Income tax (expense) benefit	15.5	(41.4)	(44.4)
Net income (loss) from continuing operations	580.9	(190.8)	(318.0)
Net income (loss) from discontinued operations, net of tax - NSM Group	0.0	16.4	(22.6)
Net income (loss)	580.9	712.4	(321.9)
Net (income) loss attributable to noncontrolling interests	(71.7)	80.4	46.5
Net income (loss) attributable to White Mountains’s common shareholders	509.2	792.8	(275.4)
NSM Group
Expenses:
Net gain (loss) from sale of discontinued operations, net of tax	0.0	886.8	0.0
Sirius Group
Expenses:
Net gain (loss) from sale of discontinued operations, net of tax	0.0	0.0	18.7
Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	407.1	(98.5)	134.2
MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	27.1	(93.0)	(380.3)
Other Operations
Revenues:
Earned insurance premiums	0.0	0.0	0.0
Net investment income	30.1	32.2	18.2
Commission revenues	13.2	11.5	9.6
Other revenues	80.5	127.2	90.7
Total revenues	339.4	76.3	(211.1)
Expenses:
Loss and loss adjustment expenses	0.0	0.0	0.0
Acquisition expenses	0.0	0.0	0.0
Cost of sales	40.4	98.6	69.3
General and administrative expenses	182.3	174.1	109.7
Change in fair value of contingent consideration	0.0	0.0	0.0
Interest expense	3.7	1.9	1.5
Total expenses	226.4	274.6	180.5
Pre-tax income (loss) from continuing operations	113.0	(198.3)	(391.6)
Other Operations \| Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	188.5	(1.6)	50.7
Other Operations \| MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	27.1	(93.0)	(380.3)
HG Global/BAM - Segment
Revenues:
Earned insurance premiums	31.2	33.3	26.9
HG Global/BAM - Segment \| Operating Segments
Revenues:
Earned insurance premiums	31.2	33.3	26.9
Net investment income	31.7	21.5	17.5
Net realized and unrealized investment gains (losses)	26.6	(105.8)	(22.9)
Other revenues	2.9	4.6	1.5
Total revenues	92.4	(46.4)	23.0
Expenses:
Acquisition expenses	8.6	11.2	8.3
General and administrative expenses	68.9	69.1	57.1
Interest expense	16.5	8.3	0.0
Total expenses	94.0	88.6	65.4
Ark/WM Outrigger
Revenues:
Earned insurance premiums	1,409.7	1,043.4	637.3
Expenses:
Loss and loss adjustment expenses	726.8	536.4	314.8
Ark/WM Outrigger \| Operating Segments
Revenues:
Earned insurance premiums	1,409.7	1,043.4	637.3
Net investment income	61.4	16.3	2.9
Net realized and unrealized investment gains (losses)	85.9	(55.2)	16.5
Other revenues	0.8	5.0	11.8
Total revenues	1,557.8	1,009.5	668.5
Expenses:
Loss and loss adjustment expenses	726.8	536.4	314.8
Acquisition expenses	281.5	239.4	178.0
General and administrative expenses	162.0	106.2	110.0
Change in fair value of contingent consideration	48.7	17.3	5.5
Interest expense	21.3	15.1	7.3
Total expenses	1,240.3	914.4	615.6
Kudu \| Operating Segments
Revenues:
Earned insurance premiums	0.0	0.0	0.0
Net investment income	71.0	54.4	43.9
Net realized and unrealized investment gains (losses)	106.1	64.1	89.9
Commission revenues	0.0	0.0	0.0
Other revenues	0.0	0.0	0.2
Total revenues	177.1	118.5	134.0
Expenses:
Loss and loss adjustment expenses	0.0	0.0	0.0
Acquisition expenses	0.0	0.0	0.0
Cost of sales	0.0	0.0	0.0
General and administrative expenses	19.4	14.7	14.8
Change in fair value of contingent consideration	0.0	0.0	0.0
Interest expense	21.2	15.0	11.7
Total expenses	40.6	29.7	26.5
Pre-tax income (loss) from continuing operations	136.5	88.8	107.5
Kudu \| Operating Segments \| Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	106.1	64.1	89.9
Kudu \| Operating Segments \| MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	$ 0.0	$ 0.0	$ 0.0

Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Deferred tax assets related to:
U.S. federal and state net operating and capital loss carryforwards	$ 87.6	$ 85.4
Non-U.S. net operating loss carryforwards	68.2	36.1
Intangible assets	51.5	0.0
Incentive compensation	25.4	21.4
Unearned premiums	18.2	0.0
Accrued interest	10.1	8.8
Loss reserves	8.4	0.0
Deferred acquisition costs	3.7	8.0
Tax credit carryforwards	3.3	1.8
Net unrealized investment losses	0.0	4.7
Other items	1.0	1.3
Total gross deferred tax assets	277.4	167.5
Less: valuation allowances	116.1	94.3
Total net deferred tax assets	161.3	73.2
Deferred tax liabilities related to:
Member’s surplus contributions	82.2	71.3
Purchase accounting	43.9	43.9
Investment basis difference	40.9	33.9
Deferred underwriting	18.3	9.7
Net unrealized investment gains (losses)	15.5	0.0
Other items	1.2	2.2
Total deferred tax liabilities	202.0	161.0
Net deferred tax asset (liability)	$ (40.7)	$ (87.8)

Equity Method Eligible Investments - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Equity Method Investments and Joint Ventures [Abstract]
Dividend and income distributions from equity method eligible investments	$ 67.0	$ 68.9	$ 56.2

Earnings Per Share - Schedule of Undistributed Net Earnings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 502.0	$ (108.4)	$ (269.3)
Dividends declared, net of restricted common share amounts	(2.5)	(3.0)	(3.1)
Total undistributed net earnings (losses), net of restricted common share amounts	$ 499.5	$ (111.4)	$ (272.4)

- Definition

Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483443/250-10-50-3

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483443/250-10-50-11

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483443/250-10-50-11

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483443/250-10-50-4

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 5
-Subparagraph (SAB Topic 6.B)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483621/220-10-S99-5

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 22
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-22

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 260
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482662/260-10-50-1

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 30
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-30

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-32

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 260
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482689/260-10-45-10

Reference 11: http://www.xbrl.org/2003/role/exampleRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 31
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-31

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-32

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 260
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482689/260-10-45-11

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Topic 260
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 60B
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482689/260-10-45-60B

+ Details

SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION

12 Months Ended

Dec. 31, 2023

SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Abstract]

SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION

WHITE MOUNTAINS INSURANCE GROUP, LTD.

SUPPLEMENTARY INSURANCE INFORMATION


	Column A	Column B		Column C		Column D		Column E		Column F		Column G		Column H		Column I		Column J		Column K
	Millions Segment	Deferred Acquisition Costs		Future Policy Benefits, Losses, Claims and Loss Expenses		Unearned Premiums		Other Policy Claims and Benefits Payable		Premiums Earned		Net Investment Income		Benefits, Claims, Losses and Settlement Expenses		Amortization of Deferred Acquisition Costs		Other Operating Expenses		Premiums Written
Years ended:

December 31, 2023


	HG Global/BAM	$	40.1	$	—	$	325.8	$	—	$	31.2	$	31.7	$	—	$	8.6	$	.4	$	58.6

	Ark/WM Outrigger	145.3		1,605.1		743.6		—		1,409.7		61.4		726.8		273.1		113.6		1,520.9
December 31, 2022


	HG Global/BAM	36.0		—		298.3		—		33.3		21.5		—		11.2		.4		65.1
	Ark/WM Outrigger	127.2		1,296.5		623.2		—		1,043.4		16.3		536.4		238.3		78.7		1,195.2

December 31, 2021

	HG Global/BAM	33.1		—		266.3		—		26.9		17.5		—		8.3		.4		55.8
	Ark/WM Outrigger	100.8		894.7		495.9		—		637.3		2.9		314.8		111.3		64.6		859.1

Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

Municipal Bond Guarantee Insurance - Narrative (Details) - USD ($) $ in Millions	1 Months Ended		12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2018	Jan. 01, 2025	Sep. 30, 2022	Jan. 31, 2021
Guarantor Obligations [Line Items]
Percentage of risk premium ceded	60.00%			60.00%
Assets held-in-trust		$ 856.9			$ 856.9
Collateral trusts, investments and cash	$ 588.6	503.3		$ 588.6	503.3
Surplus notes	322.2	340.0		322.2	340.0
Interest receivable	37.7	13.6		37.7	13.6
Carrying Value	564.6	575.2		564.6	575.2
Subsidiary - HG Global
Guarantor Obligations [Line Items]
Purchase of surplus notes				$ 503.0
Renegotiation period				5 years
Percent over a target trust balance				102.00%
Subsidiary - HG Global \| Regulation 114
Guarantor Obligations [Line Items]
Assets held-in-trust	341.6	288.6		$ 341.6	288.6
Supplement trust target	603.0			603.0
Subsidiary - HG Global \| Supplemental Trust
Guarantor Obligations [Line Items]
Assets held-in-trust	606.9	568.3		606.9	568.3
Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company
Guarantor Obligations [Line Items]
Guaranteed contracts, par value									$ 42.5	$ 805.5
Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company \| Fidus Re Ltd.
Guarantor Obligations [Line Items]
Issuance of insurance linked securities					$ 150.0	$ 150.0	$ 100.0
Insurance-linked securities, term					12 years	12 years	12 years
Insurance-linked securities, callable term					7 years	5 years	5 years
Percentage of par value of policy reinsured					90.00%	90.00%	90.00%
Reinsurance, limit on total reimbursement					$ 150.0	$ 150.0	$ 100.0
Reinsured amount				$ 125.0
2018 Covered Portfolio \| Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company \| Fidus Re Ltd.
Guarantor Obligations [Line Items]
Percentage of financial guaranty policies covered				23.00%
2021 Covered Portfolio \| Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company \| Fidus Re Ltd.
Guarantor Obligations [Line Items]
Percentage of financial guaranty policies covered				27.00%
2022 Covered Portfolio \| Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company \| Fidus Re Ltd.
Guarantor Obligations [Line Items]
Percentage of financial guaranty policies covered				29.00%
BAM Surplus Notes \| Surplus Notes
Guarantor Obligations [Line Items]
Cash repayments on notes	27.4	36.0
Accrued interest payment held in trust	2.0	1.0
Repayment of principal	17.8	24.6
Accrued interest payment held outside of trust	7.6	10.4
Carrying Value	322.2	340.0		$ 322.2	340.0
Interest receivable	$ 174.5	$ 157.9		$ 174.5	157.9
Forecast \| US Treasury (UST) Interest Rate \| BAM Surplus Notes \| Surplus Notes
Guarantor Obligations [Line Items]
Basis spread on variable rate			3.00%
Interest rate			8.20%					8.00%
Maximum \| Fidus Re Ltd.
Guarantor Obligations [Line Items]
Reinsurance, maximum amount reinsured					276.7	301.7	276.1
Maximum \| Subsidiary - HG Global
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured	15.00%			15.00%
Minimum \| Fidus Re Ltd.
Guarantor Obligations [Line Items]
Reinsurance obligation					$ 110.0	$ 135.0	$ 165.0

Loss and Loss Adjustment Expense Reserves (Tables)

12 Months Ended

Dec. 31, 2023

Insurance Loss Reserves [Abstract]

Summary of loss and LAE reserve activity

The following table summarizes the loss and LAE reserve activity of the Ark/WM Outrigger segment for the years ended December 31, 2023, 2022 and 2021:


	Year Ended December 31,
Millions	2023		2022		2021
Gross beginning balance	$	1,296.5	$	894.7	$	696.0
Less: beginning reinsurance recoverable on unpaid losses (1)	(505.0)		(428.9)		(433.4)
Net loss and LAE reserves	791.5		465.8		262.6

Losses and LAE incurred relating to:
Current year losses	706.9		588.1		336.3
Prior year losses	19.9		(51.7)		(21.5)
Net incurred losses and LAE	726.8		536.4		314.8

Loss and LAE paid relating to:
Current year losses	(51.3)		(98.9)		(43.9)
Prior year losses	(356.6)		(158.6)		(61.6)
Net paid losses and LAE	(407.9)		(257.5)		(105.5)

Change in TPC Providers’ participation (2)	145.4		57.5		(2.2)
Foreign currency translation and other adjustments to loss and LAE reserves	8.5		(10.7)		(3.9)

Net ending balance	1,264.3		791.5		465.8
Plus: ending reinsurance recoverable on unpaid losses (3)	340.8		505.0		428.9
Gross ending balance	$	1,605.1	$	1,296.5	$	894.7

(1) The beginning reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $145.4, $276.8 and $319.2 as of December 31, 2022, 2021 and 2020.

(2) Amount represents the impact to net loss and LAE reserves due to a change in the TPC Providers’ participation related to the annual RITC process.

(3) The ending reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $0.0, $145.4 and $276.8 as of December 31, 2023, 2022 and 2021.


	Year Ended December 31,
Millions	2023		2022
Property and Accident & Health	$	358.7	$	258.2
Specialty	339.7		204.3
Marine & Energy	331.7		196.4

Casualty-Active	137.1		71.5
Casualty-Runoff	81.3		60.8
WM Outrigger and Other	15.8		.3
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	1,264.3		791.5
Plus: Reinsurance recoverables on unpaid losses (1)
Property and Accident & Health (2)	126.9		224.6
Specialty	77.6		97.2
Marine & Energy	59.1		79.8
Casualty-Active	74.8		49.9
Casualty-Runoff	2.4		53.5
Total Reinsurance recoverables on unpaid losses (1) (2)	340.8		505.0
Total unpaid loss and LAE reserves	$	1,605.1	$	1,296.5

(1) The reinsurance recoverables on unpaid losses include amounts attributable to TPC Providers of $0.0 and $145.4 as of December 31, 2023 and 2022.

(2) The reinsurance recoverables on unpaid losses exclude $15.6 ceded by Ark to WM Outrigger Re as of December 31, 2023, which eliminate in White Mountains’s consolidated financial statements.

Schedule of third party participation of LAE reserves by year of account

The following table summarizes the participation of Ark’s TPC Providers by year of account:


	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023
TPC Providers’ Participation	66.2	%	70.0	%	59.6	%	60.0	%	57.6	%	58.3	%	42.8	%	—	%	—	%	—	%

Short-duration insurance contracts, claims development


Property and Accident & Health
$ in Millions
	Incurred Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2023
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2014	$	32.8	$	29.5	$	29.4	$	28.7	$	28.5	$	28.6	$	28.6	$	28.6	$	28.6	$	28.5	$	.1	2,924
2015			19.3		18.3		17.3		16.3		16.1		16.2		15.9		15.8		16.1		.2		2,829
2016					22.5		17.7		18.5		18.6		18.6		18.8		18.8		18.7		.2		3,428
2017							31.3		38.1		45.6		44.6		43.2		42.7		44.2		15.3		4,613
2018									41.0		47.4		49.3		47.0		47.1		46.7		1.9		4,269
2019											34.2		31.4		27.2		24.0		23.4		.8		4,011
2020													77.6		75.8		75.1		78.4		12.4		4,617
2021															171.8		155.5		167.3		5.7		3,465
2022																	244.3		270.5		14.3		3,899
2023																			214.2		156.9		2,963
																	Total		$	908.0


Property and Accident & Health
Millions
	Cumulative Paid Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023
	Unaudited
2014	$	13.7	$	25.1	$	27.3	$	27.6	$	27.7	$	27.9	$	28.0	$	28.0	$	28.0	$	28.0
2015			6.9		12.3		13.5		14.7		14.7		14.9		15.1		15.1		15.5
2016					8.6		13.2		16.5		16.9		17.0		17.3		17.9		18.1
2017							16.9		26.0		31.8		33.0		29.8		27.5		25.7
2018									15.7		32.4		40.3		40.3		41.1		43.1
2019											6.8		16.8		18.5		18.6		19.4
2020													11.3		34.3		47.3		56.1
2021															30.9		87.1		131.3
2022																	70.9		193.9
2023																			20.1
																	Total		551.2
All outstanding liabilities before 2014, net of reinsurance																			1.9
Loss and LAE reserves, net of reinsurance																			$	358.7


Specialty
$ in Millions
	Incurred Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2023
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2014	$	46.0	$	44.2	$	41.2	$	40.8	$	41.2	$	43.8	$	43.8	$	43.7	$	43.5	$	43.5	$	—	1,359
2015			17.4		14.8		12.4		10.8		11.1		11.3		11.3		9.0		8.2		(.1)		1,841
2016					18.4		14.4		11.0		11.3		11.9		11.9		9.0		8.6		.1		1,932
2017							18.1		13.0		12.1		11.6		11.7		10.7		10.4		.1		2,191
2018									14.6		16.4		16.8		16.1		14.9		15.8		.4		2,117
2019											21.8		19.6		18.7		25.7		30.1		.2		2,368
2020													24.2		23.2		19.0		19.9		1.7		1,994
2021															71.2		62.6		51.7		16.4		1,696
2022																	181.1		176.6		97.2		1,407
2023																			215.6		151.6		1,258
																	Total		$	580.4


Specialty
Millions
	Cumulative Paid Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023
	Unaudited
2014	$	26.5	$	39.3	$	40.1	$	40.5	$	41.1	$	42.4	$	43.2	$	43.1	$	43.4	$	43.7
2015			4.0		7.1		7.7		8.1		8.1		8.2		8.2		6.5		6.3
2016					3.2		8.0		9.1		9.9		10.4		10.4		8.6		8.4
2017							3.2		6.7		8.5		8.6		8.6		9.3		9.1
2018									2.8		8.3		10.1		10.5		11.9		13.1
2019											4.9		7.0		7.5		18.3		25.2
2020													5.3		10.7		13.2		18.3
2021															5.1		24.1		35.7
2022																	16.0		62.1
2023																			18.8
																	Total		240.7
All outstanding liabilities before 2014, net of reinsurance																			—
Loss and LAE reserves, net of reinsurance																			$	339.7


Specialty
	Average Annual Percentage Payout of Incurred Losses and LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	20.5%	30.6%	9.7%	6.0%	7.7%	6.0%	1.2%	1.7%	(3.1)%	(0.5)%


Marine & Energy
$ in Millions
	Incurred Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2023
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2014	$	34.4	$	20.2	$	17.2	$	16.3	$	14.1	$	13.7	$	14.0	$	13.8	$	13.9	$	13.6	$	(.3)	2,572
2015			21.8		17.5		16.3		13.4		12.8		13.0		12.8		13.1		12.9		.1		3,242
2016					23.6		19.7		15.8		14.7		14.4		15.0		14.2		13.8		.1		3,770
2017							26.2		19.5		17.7		17.1		16.8		15.9		16.1		.3		4,133
2018									25.8		20.3		17.8		18.1		17.7		18.1		.5		3,225
2019											23.9		21.8		21.8		21.6		22.1		.9		2,366
2020													30.0		27.3		28.7		27.6		1.6		1,561
2021															86.8		69.8		67.7		9.3		1,461
2022																	150.2		154.2		50.4		1,802
2023																			197.8		154.8		1,539
																	Total		$	543.9


Marine & Energy
Millions
	Cumulative Paid Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023
	Unaudited
2014	$	5.9	$	12.3	$	13.4	$	14.2	$	14.3	$	13.6	$	13.8	$	13.6	$	13.9	$	13.8
2015			4.0		7.9		9.7		11.1		10.5		10.6		11.0		11.6		11.7
2016					5.6		10.1		12.7		13.1		13.3		13.9		13.6		13.6
2017							5.1		11.2		12.9		14.2		14.2		14.2		14.1
2018									2.7		12.7		14.3		15.0		15.6		15.6
2019											3.4		10.7		12.7		14.5		15.5
2020													3.2		12.8		16.2		18.7
2021															6.3		24.5		38.5
2022																	12.3		66.4
2023																			10.6
																	Total		218.5
All outstanding liabilities before 2013, net of reinsurance																			6.3
Loss and LAE reserves, net of reinsurance																			$	331.7


Casualty-Active
$ in Millions
	Incurred Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2023
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2014	$	13.3	$	9.4	$	8.4	$	8.2	$	8.1	$	7.7	$	7.8	$	7.6	$	7.8	$	7.7	$	.3	1,393
2015			9.6		9.8		8.2		8.1		7.4		7.2		7.0		7.3		7.5		.4		1,295
2016					8.8		8.3		8.9		9.0		9.1		9.2		9.2		10.1		.5		1,553
2017							11.6		11.7		10.8		9.4		9.1		10.5		10.7		1.1		1,632
2018									12.9		13.3		11.1		10.9		8.6		9.2		1.4		1,098
2019											14.8		13.7		12.4		10.6		11.4		2.6		954
2020													13.5		12.1		10.9		9.2		4.3		605
2021															21.4		22.4		16.6		12.4		842
2022																	33.0		38.1		34.4		1,237
2023																			61.0		57.9		1,065
																	Total		$	181.5


Casualty-Active
Millions
	Cumulative Paid Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023
	Unaudited
2014	$	1.3	$	3.5	$	4.2	$	4.7	$	5.2	$	5.5	$	5.9	$	6.0	$	6.2	$	6.7
2015			1.8		2.4		3.2		4.4		4.7		4.9		5.1		5.5		6.1
2016					.2		1.0		2.3		4.0		4.6		5.3		6.5		8.1
2017							.8		1.7		2.8		3.4		4.2		5.7		7.5
2018									.3		1.4		3.5		4.3		4.3		6.2
2019											.3		1.4		2.3		3.0		5.7
2020													.5		1.0		2.0		3.3
2021															.5		.9		3.1
2022																	.4		1.6
2023																			.9
																	Total		49.2
All outstanding liabilities before 2014, net of reinsurance																			4.8
Loss and LAE reserves, net of reinsurance																			$	137.1


Casualty-Runoff
$ in Millions
	Incurred Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2023
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2014	$	49.2	$	48.4	$	51.0	$	54.0	$	57.6	$	59.1	$	59.1	$	58.9	$	58.7	$	59.7	$	1.9	1,947
2015			36.7		32.1		33.3		36.8		36.5		37.5		36.8		39.3		40.3		2.0		2,000
2016					32.6		32.2		40.4		38.6		38.8		38.6		37.7		37.5		2.3		2,158
2017							30.6		34.0		31.4		32.1		31.6		29.9		28.3		2.9		1,603
2018									33.7		28.2		27.3		26.6		26.2		28.0		4.1		1,277
2019											26.6		23.3		23.4		24.9		23.6		5.8		971
2020													15.9		12.3		13.9		10.9		3.9		566
2021															10.5		7.0		5.5		3.0		282
2022																	.8		2.6		1.8		77
2023																			2.7		1.6		40
																	Total		$	239.1


Casualty-Runoff
Millions
	Cumulative Paid Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023
	Unaudited
2014	$	6.5	$	23.2	$	29.6	$	36.5	$	43.2	$	47.0	$	48.7	$	49.4	$	51.9	$	54.9
2015			4.3		8.2		14.5		21.4		24.7		27.4		29.0		33.1		35.3
2016					3.9		10.2		17.7		22.7		25.4		27.8		28.7		31.0
2017							3.2		9.4		14.7		18.5		21.4		22.5		22.8
2018									3.4		7.4		12.6		14.9		16.3		18.2
2019											3.3		5.8		7.8		12.2		15.2
2020													.8		1.3		3.1		6.0
2021															.5		1.7		1.9
2022																	.3		.5
2023																			.9
																	Total		186.7
All outstanding liabilities before 2014, net of reinsurance																			28.9
Loss and LAE reserves, net of reinsurance																			$	81.3

Short-duration insurance contracts, schedule of historical claims duration


Property and Accident & Health
	Average Annual Percentage Payout of Incurred Losses and LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	25.8%	35.5%	20.1%	6.1%	1.3%	1.1%	0.4%	0.3%	0.1%	—%


Marine & Energy
	Average Annual Percentage Payout of Incurred Losses and LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	14.2%	34.9%	19.4%	6.1%	4.3%	6.5%	0.3%	0.3%	(0.3)%	0.1%


Casualty-Active
	Average Annual Percentage Payout of Incurred Losses and LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	4.8%	8.8%	14.2%	11.3%	8.8%	10.9%	6.1%	4.6%	2.0%	3.2%


Casualty-Runoff
	Average Annual Percentage Payout of Incurred Losses and LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	8.6%	14.0%	15.5%	14.8%	8.6%	7.0%	5.5%	4.2%	3.0%	2.1%

Income Taxes - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
Income Taxes - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Contingency [Line Items]
Net deferred tax benefit	$ 68.0
Non-U.S. pre-tax income (loss)	450.0	$ (94.4)	$ (319.0)
Net income tax payments to US	42.7	$ 10.3	$ (0.1)
Ark \| Ark
Income Tax Contingency [Line Items]
Net deferred tax benefit	51.0
HG Global/BAM - Segment \| HG Global
Income Tax Contingency [Line Items]
Net deferred tax benefit	17.0
Domestic Tax Authority
Income Tax Contingency [Line Items]
Separate return limitation year operating loss carryforwards	0.7
Foreign Tax Authority
Income Tax Contingency [Line Items]
Tax credit carryforwards	$ 3.3

Loss and Loss Adjustment Expense Reserves - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Ark
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development	$ 19.9	$ (51.7)	$ (21.5)
Syndicate - Ark Insurance Holdings Limited
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of underwriting capital provided by third parties	0.00%	0.00%	0.00%	42.80%	58.30%	57.60%	60.00%	59.60%	70.00%	66.20%
Property and Accident & Health \| Ark
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development	$ 41.7	$ (20.8)	$ 8.9
Specialty \| Ark
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development	(11.9)	(12.7)	3.3
Casualty-Runoff \| Ark
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development	$ (6.0)		3.3
Marine & Energy \| Ark
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development		$ (18.8)
Casualty-Active \| Ark
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development			$ 3.7

Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment reporting information
Earned insurance premiums	$ 1,440.9	$ 1,076.7	$ 664.2
Net investment income	194.2	124.4	82.5
Net investment income (expense) - BAM Surplus Note interest	0.0
Net realized and unrealized investment gains (losses)	434.2	(191.5)	(246.1)
Commission revenues	13.2	11.5	9.6
Other revenues	84.2	136.8	104.2
Total revenues	2,166.7	1,157.9	614.4
Loss and loss adjustment expenses	726.8	536.4	314.8
Acquisition expenses	290.1	250.6	186.3
Cost of sales	40.4	98.6	69.3
General and administrative expenses	432.6	364.1	291.6
Change in fair value of contingent consideration	48.7	17.3	5.5
Interest expense	62.7	40.3	20.5
Total expenses	1,601.3	1,307.3	888.0
Pre-tax income (loss) from continuing operations	565.4	(149.4)	(273.6)
Other Operations
Segment reporting information
Earned insurance premiums	0.0	0.0	0.0
Net investment income	30.1	32.2	18.2
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0
Commission revenues	13.2	11.5	9.6
Other revenues	80.5	127.2	90.7
Total revenues	339.4	76.3	(211.1)
Loss and loss adjustment expenses	0.0	0.0	0.0
Acquisition expenses	0.0	0.0	0.0
Cost of sales	40.4	98.6	69.3
General and administrative expenses	182.3	174.1	109.7
Change in fair value of contingent consideration	0.0	0.0	0.0
Interest expense	3.7	1.9	1.5
Total expenses	226.4	274.6	180.5
Pre-tax income (loss) from continuing operations	113.0	(198.3)	(391.6)
Investments, Excluding MediaAlpha
Segment reporting information
Net realized and unrealized investment gains (losses)	407.1	(98.5)	134.2
Investments, Excluding MediaAlpha \| Other Operations
Segment reporting information
Net realized and unrealized investment gains (losses)	188.5	(1.6)	50.7
MediaAlpha
Segment reporting information
Net realized and unrealized investment gains (losses)	27.1	(93.0)	(380.3)
MediaAlpha \| Other Operations
Segment reporting information
Net realized and unrealized investment gains (losses)	27.1	(93.0)	(380.3)
HG Global/BAM - Segment
Segment reporting information
Earned insurance premiums	31.2	33.3	26.9
HG Global/BAM - Segment \| Operating Segments
Segment reporting information
Earned insurance premiums	31.2	33.3	26.9
Net investment income	31.7	21.5	17.5
Net realized and unrealized investment gains (losses)	26.6	(105.8)	(22.9)
Other revenues	2.9	4.6	1.5
Total revenues	92.4	(46.4)	23.0
Acquisition expenses	8.6	11.2	8.3
General and administrative expenses	68.9	69.1	57.1
Interest expense	16.5	8.3	0.0
Total expenses	94.0	88.6	65.4
HG Global/BAM - Segment \| HG Global \| Operating Segments
Segment reporting information
Earned insurance premiums	26.0	27.5	22.2
Net investment income	17.1	10.3	7.2
Net investment income (expense) - BAM Surplus Note interest	26.2	11.7	12.0
Commission revenues	0.0	0.0	0.0
Other revenues	0.0	0.5	0.5
Total revenues	82.9	(2.5)	28.2
Loss and loss adjustment expenses	0.0	0.0	0.0
Acquisition expenses	7.4	9.3	5.7
Cost of sales	0.0	0.0	0.0
General and administrative expenses	2.8	2.8	2.0
Change in fair value of contingent consideration	0.0	0.0	0.0
Interest expense	16.5	8.3	0.0
Total expenses	26.7	20.4	7.7
Pre-tax income (loss) from continuing operations	56.2	(22.9)	20.5
HG Global/BAM - Segment \| HG Global \| Investments, Excluding MediaAlpha \| Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	13.6	(52.5)	(13.7)
HG Global/BAM - Segment \| HG Global \| MediaAlpha \| Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	0.0	0.0	0.0
HG Global/BAM - Segment \| BAM \| Operating Segments
Segment reporting information
Earned insurance premiums	5.2	5.8	4.7
Net investment income	14.6	11.2	10.3
Net investment income (expense) - BAM Surplus Note interest	(26.2)	(11.7)	(12.0)
Commission revenues	0.0	0.0	0.0
Other revenues	2.9	4.1	1.0
Total revenues	9.5	(43.9)	(5.2)
Loss and loss adjustment expenses	0.0	0.0	0.0
Acquisition expenses	1.2	1.9	2.6
Cost of sales	0.0	0.0	0.0
General and administrative expenses	66.1	66.3	55.1
Change in fair value of contingent consideration	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0
Total expenses	67.3	68.2	57.7
Pre-tax income (loss) from continuing operations	(57.8)	(112.1)	(62.9)
HG Global/BAM - Segment \| BAM \| Investments, Excluding MediaAlpha \| Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	13.0	(53.3)	(9.2)
HG Global/BAM - Segment \| BAM \| MediaAlpha \| Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	0.0	0.0	0.0
Ark/WM Outrigger
Segment reporting information
Earned insurance premiums	1,409.7	1,043.4	637.3
Loss and loss adjustment expenses	726.8	536.4	314.8
Ark/WM Outrigger \| Operating Segments
Segment reporting information
Earned insurance premiums	1,409.7	1,043.4	637.3
Net investment income	61.4	16.3	2.9
Net realized and unrealized investment gains (losses)	85.9	(55.2)	16.5
Other revenues	0.8	5.0	11.8
Total revenues	1,557.8	1,009.5	668.5
Loss and loss adjustment expenses	726.8	536.4	314.8
Acquisition expenses	281.5	239.4	178.0
General and administrative expenses	162.0	106.2	110.0
Change in fair value of contingent consideration	48.7	17.3	5.5
Interest expense	21.3	15.1	7.3
Total expenses	1,240.3	914.4	615.6
Ark/WM Outrigger \| Ark \| United Kingdom
Segment reporting information
Earned insurance premiums	791.7	638.5	459.3
Ark/WM Outrigger \| Ark \| Bermuda
Segment reporting information
Earned insurance premiums	513.7	404.9	178.0
Ark/WM Outrigger \| Ark \| Operating Segments
Segment reporting information
Earned insurance premiums	1,305.4	1,043.4	637.3
Net investment income	50.4	16.3	2.9
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0
Commission revenues	0.0	0.0	0.0
Other revenues	0.8	5.0	11.8
Total revenues	1,442.5	1,009.5	668.5
Loss and loss adjustment expenses	711.2	536.4	314.8
Acquisition expenses	251.0	239.4	178.0
Cost of sales	0.0	0.0	0.0
General and administrative expenses	161.7	106.2	110.0
Change in fair value of contingent consideration	48.7	17.3	5.5
Interest expense	21.3	15.1	7.3
Total expenses	1,193.9	914.4	615.6
Pre-tax income (loss) from continuing operations	248.6	95.1	52.9
Ark/WM Outrigger \| Ark \| Investments, Excluding MediaAlpha \| Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	85.9	(55.2)	16.5
Ark/WM Outrigger \| Ark \| MediaAlpha \| Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	0.0	0.0	0.0
Ark/WM Outrigger \| WM Outrigger Re \| Operating Segments
Segment reporting information
Earned insurance premiums	104.3
Net investment income	11.0
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0
Commission revenues	0.0
Other revenues	0.0
Total revenues	115.3
Loss and loss adjustment expenses	15.6
Acquisition expenses	30.5
Cost of sales	0.0
General and administrative expenses	0.3
Change in fair value of contingent consideration	0.0
Interest expense	0.0
Total expenses	46.4
Pre-tax income (loss) from continuing operations	68.9
Ark/WM Outrigger \| WM Outrigger Re \| Investments, Excluding MediaAlpha \| Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	0.0
Ark/WM Outrigger \| WM Outrigger Re \| MediaAlpha \| Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	0.0
Kudu \| Operating Segments
Segment reporting information
Earned insurance premiums	0.0	0.0	0.0
Net investment income	71.0	54.4	43.9
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	106.1	64.1	89.9
Commission revenues	0.0	0.0	0.0
Other revenues	0.0	0.0	0.2
Total revenues	177.1	118.5	134.0
Loss and loss adjustment expenses	0.0	0.0	0.0
Acquisition expenses	0.0	0.0	0.0
Cost of sales	0.0	0.0	0.0
General and administrative expenses	19.4	14.7	14.8
Change in fair value of contingent consideration	0.0	0.0	0.0
Interest expense	21.2	15.0	11.7
Total expenses	40.6	29.7	26.5
Pre-tax income (loss) from continuing operations	136.5	88.8	107.5
Kudu \| Investments, Excluding MediaAlpha \| Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	106.1	64.1	89.9
Kudu \| MediaAlpha \| Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	$ 0.0	$ 0.0	$ 0.0

Segment Information (Tables)

12 Months Ended

Dec. 31, 2023

Segment Reporting [Abstract]

Schedule of financial information for segments

The following tables present the financial information for White Mountains’s segments:


	HG Global/BAM				Ark/WM Outrigger
Millions	HG Global		BAM (1)		Ark		WM Outrigger Re		Kudu		Other Operations		Total
Year Ended December 31, 2023
Earned insurance premiums (2)	$	26.0	$	5.2	$	1,305.4	$	104.3	$	—	$	—	$	1,440.9
Net investment income	17.1		14.6		50.4		11.0		71.0		30.1		194.2
Net investment income (expense) - BAM Surplus Note interest	26.2		(26.2)		—		—		—		—		—
Net realized and unrealized investment gains (losses)	13.6		13.0		85.9		—		106.1		188.5		407.1
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	—		—		—		—		—		27.1		27.1
Commission revenues	—		—		—		—		—		13.2		13.2
Other revenues	—		2.9		.8		—		—		80.5		84.2
Total revenues	82.9		9.5		1,442.5		115.3		177.1		339.4		2,166.7

Loss and loss adjustment expenses	—		—		711.2		15.6		—		—		726.8
Acquisition expenses	7.4		1.2		251.0		30.5		—		—		290.1

Cost of sales	—		—		—		—		—		40.4		40.4
General and administrative expenses	2.8		66.1		161.7		.3		19.4		182.3		432.6

Change in fair value of contingent consideration	—		—		48.7		—		—		—		48.7


Interest expense	16.5		—		21.3		—		21.2		3.7		62.7
Total expenses	26.7		67.3		1,193.9		46.4		40.6		226.4		1,601.3
Pre-tax income (loss)	$	56.2	$	(57.8)	$	248.6	$	68.9	$	136.5	$	113.0	$	565.4

(1) BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the BAM Surplus Notes and is not reduced by accruals of interest expense on the BAM Surplus Notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the NYDFS.

(2) Ark’s earned insurance premiums based on the location of Ark’s underwriting offices in the United Kingdom and Bermuda are $791.7 and $513.7.


	HG Global/BAM				Ark/WM Outrigger				Other Operations
Millions	HG Global		BAM (1)		Ark		Kudu				Total
Year Ended December 31, 2022
Earned insurance premiums (2)	$	27.5	$	5.8	$	1,043.4	$	—	$	—	$	1,076.7
Net investment income	10.3		11.2		16.3		54.4		32.2		124.4
Net investment income (expense) - BAM Surplus Note interest	11.7		(11.7)		—		—		—		—
Net realized and unrealized investment gains (losses)	(52.5)		(53.3)		(55.2)		64.1		(1.6)		(98.5)
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	—		—		—		—		(93.0)		(93.0)
Commission revenues	—		—		—		—		11.5		11.5
Other revenues	.5		4.1		5.0		—		127.2		136.8
Total revenues	(2.5)		(43.9)		1,009.5		118.5		76.3		1,157.9
Losses and loss adjustment expenses	—		—		536.4		—		—		536.4
Acquisition expenses	9.3		1.9		239.4		—		—		250.6

Cost of sales	—		—		—		—		98.6		98.6
General and administrative expenses	2.8		66.3		106.2		14.7		174.1		364.1
Change in fair value of contingent consideration	—		—		17.3		—		—		17.3

Interest expense	8.3		—		15.1		15.0		1.9		40.3
Total expenses	20.4		68.2		914.4		29.7		274.6		1,307.3
Pre-tax income (loss)	$	(22.9)	$	(112.1)	$	95.1	$	88.8	$	(198.3)	$	(149.4)

(2) Ark’s earned insurance premiums based on the location of Ark’s underwriting offices in the United Kingdom and Bermuda are $638.5 and $404.9.


	HG Global/BAM				Ark/WM Outrigger				Other Operations
Millions	HG Global		BAM (1)		Ark		Kudu				Total
Year Ended December 31, 2021
Earned insurance premiums (2)	$	22.2	$	4.7	$	637.3	$	—	$	—	$	664.2
Net investment income	7.2		10.3		2.9		43.9		18.2		82.5
Net investment income (expense) - BAM Surplus Note interest	12.0		(12.0)		—		—		—		—
Net realized and unrealized investment gains (losses)	(13.7)		(9.2)		16.5		89.9		50.7		134.2
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	—		—		—		—		(380.3)		(380.3)

Commission revenues	—		—		—		—		9.6		9.6
Other revenues	.5		1.0		11.8		.2		90.7		104.2
Total revenues	28.2		(5.2)		668.5		134.0		(211.1)		614.4
Loss and loss adjustment expenses	—		—		314.8		—		—		314.8
Acquisition expenses	5.7		2.6		178.0		—		—		186.3
Cost of sales	—		—		—		—		69.3		69.3
General and administrative expenses	2.0		55.1		110.0		14.8		109.7		291.6
Change in fair value of contingent consideration	—		—		5.5		—		—		5.5
Interest expense	—		—		7.3		11.7		1.5		20.5
Total expenses	7.7		57.7		615.6		26.5		180.5		888.0
Pre-tax income (loss)	$	20.5	$	(62.9)	$	52.9	$	107.5	$	(391.6)	$	(273.6)

(2) Ark’s earned insurance premiums based on the location of Ark’s underwriting offices in the United Kingdom and Bermuda are $459.3 and $178.0.


Millions Selected Balance Sheet Data	HG Global/BAM			Ark		Kudu		Other Operations			Total
December 31, 2023:
Total investments	$	1,082.9		$	2,671.1	$	925.6	$	1,709.1		$	6,388.7


Total assets	$	1,172.0	(1)	$	4,378.6	$	959.4	$	1,875.9	(2)	$	8,385.9


Total liabilities	$	531.7	(2)	$	2,876.1	$	275.4	$	141.1		$	3,824.3
Total White Mountains’s common shareholders’ equity	$	779.3	(2)	$	1,165.6	$	570.2	$	1,725.4	(2)	$	4,240.5
Noncontrolling interest	$	(139.0)		$	336.9	$	113.8	$	9.4		$	321.1
December 31, 2022:
Total investments	$	975.8		$	1,965.6	$	695.9	$	1,532.7		$	5,170.0


Total assets	$	1,058.5	(1)	$	3,691.4	$	825.9	$	1,813.5	(2)	$	7,389.3


Total liabilities	$	501.8	(2)	$	2,522.1	$	273.3	$	157.1		$	3,454.3
Total White Mountains’s common shareholders’ equity	$	712.0	(2)	$	921.4	$	477.5	$	1,636.0	(2)	$	3,746.9
Noncontrolling interest	$	(155.3)		$	247.9	$	75.1	$	20.4		$	188.1

(1) As of December 31, 2023 and 2022, total assets in the HG Global/BAM segment reflected the elimination of $322.2 and $340.0 of BAM Surplus Notes issued to HG Global and its subsidiaries and $174.5 and $157.9 in accrued interest related to the BAM Surplus Notes.

(2) HG Global preferred dividends payable to White Mountains’s subsidiaries is eliminated in White Mountains’s consolidated financial statements. For segment reporting, the HG Global preferred dividends payable to White Mountains’s subsidiaries included within the HG Global/BAM segment are eliminated against the offsetting receivable included within Other Operations and therefore added back to White Mountains’s common shareholders’ equity within the HG Global/BAM segment. As of December 31, 2023 and 2022, the HG Global preferred dividends payable to White Mountains’s subsidiaries was $399.8 and $341.4.

Held for Sale and Discontinued Operations - Financial Statement Disclosure (Details) - USD ($) $ in Millions		12 Months Ended
	Aug. 01, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2018	Dec. 31, 2020
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal, Statement of Income or Comprehensive Income [Extensible Enumeration]			General and administrative expenses	General and administrative expenses
Net income (loss) from discontinued operations, net of tax - NSM Group		$ 0.0	$ 16.4	$ (22.6)
Net (income) loss from discontinued operations attributable to noncontrolling interests		0.0	(0.7)	1.0
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders		0.0	903.2	(3.9)
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group		0.0	(5.2)	0.2
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group		0.0	2.9	0.0
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest		(0.5)	0.9	0.2
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from (used for) operations		0.0	38.7	42.3
Net cash provided from (used for) investing activities		0.0	7.1	(56.5)
Net cash used from (used for) financing activities		0.0	(17.5)	(1.0)
Cash balances at beginning of period (includes restricted cash of $89.2 and $78.4 )		0.0	111.6	126.6
Cash balances at end of period (includes restricted cash of $0.0 and $89.2)		0.0	0.0	111.6
NSM
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain (loss) from sale of discontinued operations, net of tax		0.0	886.8	0.0
Sirius Group
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain (loss) from sale of discontinued operations, net of tax		0.0	0.0	18.7
Discontinued Operations, Disposed of by Sale \| NSM
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total revenues - NSM Group			225.0	330.4
General and administrative expenses	$ 14.0		126.8	190.4
Broker commission expenses			52.9	80.2
Change in fair value of contingent consideration			0.1	1.0
Amortization of other intangible assets			9.1	35.2
Loss on assets held for sale			0.0	28.7
Interest expense			12.1	23.3
Total expenses - NSM Group			201.0	358.8
Pre-tax income (loss) from discontinued operations - NSM Group			24.0	(28.4)
Income tax (expense) benefit			(7.6)	5.8
Net income (loss) from discontinued operations, net tax - NSM Group			16.4	(22.6)
Net gain (loss) from sale of discontinued operations, net of tax	886.8		886.8	0.0
Net income (loss) from discontinued operations, net of tax - NSM Group			903.2	(3.9)
Net (income) loss from discontinued operations attributable to noncontrolling interests			(0.7)	1.0
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders			902.5	(2.9)
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group			(5.2)	0.2
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	$ 2.9		2.9	0.0
Comprehensive income (loss) attributable to White Mountains’s common shareholders			900.2	(2.7)
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest			0.2	(0.1)
Comprehensive income (loss) attributable to White Mountains’s common shareholders			900.4	(2.8)
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from (used for) operations			38.7	42.3
Net cash provided from (used for) investing activities			7.1	(56.5)
Net cash used from (used for) financing activities			(17.5)	(1.0)
Effect of exchange rate changes on cash			4.0	0.2
Net change in cash during the period			32.3	(15.0)
Cash balances at beginning of period (includes restricted cash of $89.2 and $78.4 )		$ 0.0	111.6	126.6
Restricted cash balance			0.0	89.2		$ 78.4
Cash sold as part of the sale of NSM Group (includes restricted cash of $105.1 and $0.0)			(143.9)	0.0
Restricted cash			105.1	0.0
Cash balances at end of period (includes restricted cash of $0.0 and $89.2)			0.0	111.6
Interest paid			(12.0)	(16.6)
Net income tax payments			0.0	0.0
Discontinued Operations, Disposed of by Sale \| NSM \| Commission revenues
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total revenues - NSM Group			176.9	258.0
Discontinued Operations, Disposed of by Sale \| NSM \| Other revenues
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total revenues - NSM Group			48.1	72.4
Discontinued Operations, Disposed of by Sale \| Sirius Group
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain (loss) from sale of discontinued operations, net of tax				17.6	$ (17.3)
Net gain (loss) from sale of discontinued operations, net of tax			$ 0.0	$ 18.7

Employee Share-Based Incentive Compensation Plans - Performance Shares (Details) - USD ($) $ in Millions	1 Months Ended			12 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Target Performance Shares Outstanding
Beginning of period (in shares)				39,449	40,828	42,458
Shares paid or expired (in shares)				(13,350)	(14,625)	(14,336)
Grants in period (in shares)				10,895	13,225	13,475
Forfeitures and cancellations (in shares)				37	21	(769)
End of period (in shares)				37,031	39,449	40,828
Accrued Expense
Beginning of period				$ 67.5	$ 42.2	$ 56.3
Shares paid or expired				(40.8)	(26.4)	(35.2)
New grants				0.0	0.0	0.0
Forfeitures and cancellations				(0.1)	(0.4)	0.4
Expense recognized				42.8	52.1	20.7
End of period				$ 69.4	$ 67.5	$ 42.2
Performance Shares
Accrued Expense
Percentage of target payout - minimum actual	200.00%	172.00%	200.00%

Investment Securities - Significant Unobservable Inputs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	$ 1,998.2	$ 1,488.0
Level 3 \| Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	19.0	41.1
Level 3 \| Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	$ 15.5	$ 9.6
Level 3 \| Discounted cash flow \| Discount Rate
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate		11.00%
Level 3 \| Discounted cash flow \| Discount Rate \| Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	11.00%
Level 3 \| Discounted cash flow \| Discount Rate \| Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	12.00%
Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
New and existing contracts deployed	$ 163.8	$ 99.8
Kudu’s Participation Contracts \| Level 3 \| Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	$ 890.5	$ 695.9
Kudu’s Participation Contracts \| Level 3 \| Discounted cash flow \| Discount Rate \| Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	18.00%	18.00%
Kudu’s Participation Contracts \| Level 3 \| Discounted cash flow \| Discount Rate \| Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	25.00%	25.00%
Kudu’s Participation Contracts \| Level 3 \| Discounted cash flow \| Terminal Cash Flow Exit Multiple \| Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	700.00%	700.00%
Kudu’s Participation Contracts \| Level 3 \| Discounted cash flow \| Terminal Cash Flow Exit Multiple \| Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	2200.00%	1600.00%
Kudu’s Participation Contracts \| Level 3 \| Discounted cash flow \| Measurement Input, Pre-Tax Discount Rate
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	20.00%	21.00%
Kudu’s Participation Contracts \| Level 3 \| Discounted cash flow \| Measurement, Input, Pre-Tax Terminal Cash Flow Exit Multiple
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	1300.00%	1200.00%
Kudu’s Participation Contracts \| Level 3 \| Probability weighted expected return method
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	$ 69.1	$ 189.0
PassportCard/DavidShield \| Level 3 \| Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	$ 150.0	$ 135.0
PassportCard/DavidShield \| Level 3 \| Discounted cash flow \| Discount Rate
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	24.00%	24.00%
PassportCard/DavidShield \| Level 3 \| Discounted cash flow \| Terminal Revenue Growth Rate
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	4.00%	4.00%
Elementum Holdings, L.P. \| Level 3 \| Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	$ 35.0	$ 30.0
Elementum Holdings, L.P. \| Level 3 \| Discounted cash flow \| Discount Rate
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	21.00%	21.00%
Elementum Holdings, L.P. \| Level 3 \| Discounted cash flow \| Terminal Revenue Growth Rate
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	4.00%	4.00%
Preferred securities \| Level 3 \| Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	$ 28.2
Preferred securities \| Level 3 \| Discounted cash flow \| Discount Rate
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	8.00%

SCHEDULE VI - SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS (Details) - Ark/WM Outrigger - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Current Year	$ 706.9	$ 588.1	$ 336.3
Prior Year	19.9	(51.7)	(21.5)
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters, Consolidated Property and Casualty Insurance Entity
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Deferred acquisition costs	145.3	127.2	100.8
Reserves for Unpaid Claims and Claims Adjustment Expenses	1,605.1	1,296.5	894.7
Discount, if any, deducted in Column C	0.0	0.0	0.0
Unearned Premiums	743.6	623.2	495.9
Earned Premiums	1,409.7	1,043.4	637.3
Net investment income	61.4	16.3	2.9
Current Year	706.9	588.1	336.3
Prior Year	19.9	(51.7)	(21.5)
Amortization of deferred policy acquisition costs	273.1	238.3	111.3
Paid Claims and Claims Adjustment Expenses	407.9	257.5	105.5
Premiums written	$ 1,520.9	$ 1,195.2	$ 859.1

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows from operations:
Net income (loss)	$ 580.9	$ 712.4	$ (321.9)
Adjustments to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses	(434.2)	191.5	246.1
Deferred income tax expense (benefit)	(46.9)	12.9	34.7
Amortization of restricted share awards	15.3	14.0	14.7
Amortization (accretion) and depreciation	(17.8)	4.3	21.6
Change in fair value of contingent consideration - Ark	48.7	17.3	5.5
Net (income) loss from discontinued operation, net of tax - NSM Group	0.0	(16.4)	22.6
Other operating items:
Net change in reinsurance recoverables	153.3	(79.8)	88.0
Net change in insurance premiums receivable	(67.0)	(127.8)	(179.3)
Net change in deferred acquisition costs and value of in-force business acquired	(22.2)	(21.9)	(41.8)
Net change in loss and loss adjustment expense reserves	308.6	401.8	198.7
Net change in unearned insurance premiums	147.9	159.3	198.6
Net change in reinsurance payable	(170.0)	(173.0)	(104.2)
Contributions to Kudu’s Participation Contracts	(163.8)	(99.8)	(223.4)
Proceeds from Kudu’s Participation Contracts sold	75.2	137.5	44.4
Net other operating activities	(3.9)	80.8	10.7
Net cash provided from (used for) operations - continuing operations	404.1	326.3	(3.7)
Net cash provided from (used for) operations - NSM Group discontinued operations (Note 20)	0.0	38.7	42.3
Net cash provided from (used for) operations	404.1	365.0	38.6
Cash flows from investing activities:
Net change in short-term investments	(495.8)	(455.1)	59.8
Sales of fixed maturity investments	195.1	188.7	286.3
Maturities, calls and paydowns of fixed maturity investments	266.5	198.5	199.0
Sales of common equity securities and investment in MediaAlpha	261.6	0.0	176.8
Distributions and redemptions of other long-term investments	116.9	115.4	106.4
Proceeds from the sale of business, net of cash sold	17.3	1,392.0	0.0
Release of cash pre-funded/placed in escrow for Ark Transaction	0.0	0.0	646.3
Release of cash (pre-funding) for ILS funds managed by Elementum	70.0	(70.0)	0.0
Purchases of fixed maturity investments	(592.7)	(585.5)	(1,219.4)
Purchases of common equity securities	(114.2)	(424.5)	(203.4)
Purchases of other long-term investments	(265.2)	(178.8)	(218.0)
Purchases of consolidated subsidiaries, net of cash acquired of $0.0, $0.3, and $52.2	0.0	(67.9)	10.6
Net other investing activities	(3.4)	(4.4)	3.8
Net cash provided from (used for) investing activities - continuing operations	(543.9)	108.4	(151.8)
Net cash provided from (used for) investing activities - NSM Group discontinued operations (Note 20)	0.0	7.1	(56.5)
Net cash provided from (used for) investing activities	(543.9)	115.5	(208.3)
Cash flows from financing activities:
Draw down of debt and revolving lines of credit	12.4	213.8	401.7
Repayment of debt and revolving lines of credit	(25.7)	(56.7)	(120.1)
Cash dividends paid to common shareholders	(2.6)	(3.0)	(3.1)
Repurchases and retirements of common shares	(32.7)	(615.8)	(107.5)
Net contributions from (distributions to) other noncontrolling interests	(3.0)	(21.6)	0.7
Net (contributions to) distributions from discontinued operations	0.0	11.6	0.0
Issuances of shares to noncontrolling interests	0.0	74.6	6.5
BAM member surplus contributions	72.8	81.4	62.2
Fidus Re premium payments	(13.6)	(10.1)	(8.8)
Net other financing activities	(0.4)	(1.6)	(13.0)
Net cash provided from (used for) financing activities - continuing operations	7.2	(327.4)	218.6
Net cash provided from (used for) financing activities - NSM Group discontinued operations (Note 20)	0.0	(17.5)	(1.0)
Net cash provided from (used for) financing activities – continuing operations	7.2	(344.9)	217.6
Net change in cash during the period - continuing operations	(132.6)	107.3	63.1
Cash balance at beginning of year	255.0	147.7	84.6
Cash balance at end of year	122.4	255.0	147.7
NSM
Adjustments to reconcile net income to net cash provided from (used for) operations:
Net gain (loss) from sale of discontinued operations, net of tax	0.0	(886.8)	0.0
Sirius Group
Adjustments to reconcile net income to net cash provided from (used for) operations:
Net gain (loss) from sale of discontinued operations, net of tax	0.0	0.0	(18.7)
MediaAlpha
Adjustments to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses	(27.1)	93.0	380.3
Investments, Excluding MediaAlpha
Adjustments to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses	$ (407.1)	$ 98.5	$ (134.2)

Significant Transactions - NSM (Details) - USD ($) $ in Millions		12 Months Ended
	Aug. 01, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Other Significant Noncash Transactions [Line Items]
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group		$ 0.0	$ 2.9	$ 0.0
NSM
Other Significant Noncash Transactions [Line Items]
Net gain (loss) from sale of discontinued operations, net of tax		$ 0.0	886.8	0.0
Discontinued Operations, Disposed of by Sale \| NSM
Other Significant Noncash Transactions [Line Items]
Disposal group, consideration	$ 1,400.0
Net transaction gain	875.7
Net gain (loss) from sale of discontinued operations, net of tax	886.8		886.8	0.0
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	2.9		2.9	0.0
General and administrative expenses	$ 14.0		$ 126.8	$ 190.4

SCHEDULE V - VALUATION AND QUALIFYING ACCOUNTS (Details) - Reinsurance recoverables: Allowance for uncollectible balances - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
VALUATION AND QUALIFYING ACCOUNTS
Balance at beginning of period	$ 7.8	$ 3.6	$ 2.3
Charged to costs and expenses	(0.2)	4.6	1.3
Charged to other accounts	0.0	0.0	0.0
Deductions described	0.1	(0.4)	0.0
Balance at end of period	$ 7.7	$ 7.8	$ 3.6

Derivatives	12 Months Ended
Derivatives	Dec. 31, 2023
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives	Derivatives HG Global Interest Rate Cap On June 16, 2022, HG entered into an interest rate cap agreement, effective on July 25, 2022, to limit its exposure to the risk of interest rate increases on the HG Global Senior Notes. The notional amount of the interest rate cap is $150.0 million and the termination date is July 25, 2025. HG Global paid initial premiums of $3.3 million for the interest rate cap. Under the terms of the interest rate cap agreement, if the current three month SOFR at the measurement date exceeds 3.5%, HG Global will receive payments from the counterparty equal to the difference between the three-month SOFR on the determination date and 3.5%, multiplied by the notional amount of the cap based on the number of days in the quarter. As of December 31, 2023, the three-month SOFR was 5.3%. HG Global accounts for the interest rate cap as a derivative at fair value, with changes in fair value recognized in current period earnings within interest expense. For the years ended December 31, 2023 and 2022, White Mountains recognized a gain (loss) of $(1.2) million and $0.8 million related to the change in fair value on the interest rate cap within interest expense. For the year ended December 31, 2023, White Mountains received a payment of $2.0 million related to the periodic settlement of the interest rate cap. During 2022, there were no payments received related to the periodic settlement of the interest rate cap. As of December 31, 2023 and 2022, the estimated fair value of the interest rate cap recorded in other assets was $2.9 million and $4.1 million. White Mountains classifies the interest rate cap as a Level 2 measurement.

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Restricted cash balance	$ 0.7	$ 18.3
Common shares, par value per share (in dollars per share)	$ 1	$ 1
Common shares, authorized shares (in shares)	50,000,000	50,000,000
Common shares, issued shares (in shares)	2,560,452	2,572,156
Common shares, outstanding shares (in shares)	2,560,452	2,572,156
Operating Segments \| Ark
Restricted cash balance	$ 0.7	$ 6.1
Operating Segments \| Kudu
Restricted cash balance	$ 0.0	$ 12.2

SCHEDULE VI - SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS

12 Months Ended

Dec. 31, 2023

SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Abstract]

SCHEDULE VI - SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS

WHITE MOUNTAINS INSURANCE GROUP, LTD.

SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS

(Millions)


Column A	Column B		Column C		Column D	Column E		Column F		Column G		Column H				Column I		Column J		Column K
	Deferred acquisition costs		Reserves for Unpaid Claims and Claims Adjustment Expenses		Discount, if any, deducted in Column C	Unearned Premiums		Earned Premiums		Net investment income		Claims and Claims Adjustment Expenses Incurred Related to				Amortization of deferred policy acquisition costs		Paid Claims and Claims Adjustment Expenses		Premiums written
Affiliation with registrant												Current Year		Prior Year
Ark/WM Outrigger:
2023	$	145.3	$	1,605.1	—	$	743.6	$	1,409.7	$	61.4	$	706.9	$	19.9	$	273.1	$	407.9	$	1,520.9
2022	127.2		1,296.5		—	623.2		1,043.4		16.3		588.1		(51.7)		238.3		257.5		1,195.2
2021	100.8		894.7		—	495.9		637.3		2.9		336.3		(21.5)		111.3		105.5		859.1

Loss and Loss Adjustment Expense Reserves

12 Months Ended

Dec. 31, 2023

Insurance Loss Reserves [Abstract]

Loss and Loss Adjustment Expense Reserves

HG Re and BAM do not have any outstanding loss and LAE reserves related to BAM’s municipal bond guarantee business.

Ark Reserve Estimation by Line of Business

Following is a detailed description of the reserve factors and consideration for each of Ark’s reserving lines of business.

Property and Accident & Health

Specialty

Marine & Energy

The marine & energy liability reserving classes, which represent a smaller portion of the marine & energy business, are typically longer-tail compared to physical damage reserving classes.

Casualty–Active and Casualty–Runoff

There are a number of common reserving factors for casualty lines that can affect the estimated casualty reserves, including:

•Changes in claims-handling practices, both in-house and through third-party claims administrators,

•Changes in court interpretations of policy provisions, and

•Trends in litigation or jury awards.

Cumulative Number of Reported Claims

Discounting

Ark does not discount loss and LAE reserves.

Impact of Third-Party Capital

Loss and Loss Adjustment Expense Reserve Summary

The following table summarizes the loss and LAE reserve activity of the Ark/WM Outrigger segment for the years ended December 31, 2023, 2022 and 2021:


	Year Ended December 31,
Millions	2023		2022		2021
Gross beginning balance	$	1,296.5	$	894.7	$	696.0
Less: beginning reinsurance recoverable on unpaid losses (1)	(505.0)		(428.9)		(433.4)
Net loss and LAE reserves	791.5		465.8		262.6

Losses and LAE incurred relating to:
Current year losses	706.9		588.1		336.3
Prior year losses	19.9		(51.7)		(21.5)
Net incurred losses and LAE	726.8		536.4		314.8

Loss and LAE paid relating to:
Current year losses	(51.3)		(98.9)		(43.9)
Prior year losses	(356.6)		(158.6)		(61.6)
Net paid losses and LAE	(407.9)		(257.5)		(105.5)

Change in TPC Providers’ participation (2)	145.4		57.5		(2.2)
Foreign currency translation and other adjustments to loss and LAE reserves	8.5		(10.7)		(3.9)

Net ending balance	1,264.3		791.5		465.8
Plus: ending reinsurance recoverable on unpaid losses (3)	340.8		505.0		428.9
Gross ending balance	$	1,605.1	$	1,296.5	$	894.7

(1) The beginning reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $145.4, $276.8 and $319.2 as of December 31, 2022, 2021 and 2020.

(2) Amount represents the impact to net loss and LAE reserves due to a change in the TPC Providers’ participation related to the annual RITC process.

(3) The ending reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $0.0, $145.4 and $276.8 as of December 31, 2023, 2022 and 2021.


	Year Ended December 31,
Millions	2023		2022
Property and Accident & Health	$	358.7	$	258.2
Specialty	339.7		204.3
Marine & Energy	331.7		196.4

Casualty-Active	137.1		71.5
Casualty-Runoff	81.3		60.8
WM Outrigger and Other	15.8		.3
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	1,264.3		791.5
Plus: Reinsurance recoverables on unpaid losses (1)
Property and Accident & Health (2)	126.9		224.6
Specialty	77.6		97.2
Marine & Energy	59.1		79.8
Casualty-Active	74.8		49.9
Casualty-Runoff	2.4		53.5
Total Reinsurance recoverables on unpaid losses (1) (2)	340.8		505.0
Total unpaid loss and LAE reserves	$	1,605.1	$	1,296.5

(1) The reinsurance recoverables on unpaid losses include amounts attributable to TPC Providers of $0.0 and $145.4 as of December 31, 2023 and 2022.

(2) The reinsurance recoverables on unpaid losses exclude $15.6 ceded by Ark to WM Outrigger Re as of December 31, 2023, which eliminate in White Mountains’s consolidated financial statements.

The following table summarizes the participation of Ark’s TPC Providers by year of account:


	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023
TPC Providers’ Participation	66.2	%	70.0	%	59.6	%	60.0	%	57.6	%	58.3	%	42.8	%	—	%	—	%	—	%

Each of the five tables includes three sections.


Property and Accident & Health
$ in Millions
	Incurred Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2023
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2014	$	32.8	$	29.5	$	29.4	$	28.7	$	28.5	$	28.6	$	28.6	$	28.6	$	28.6	$	28.5	$	.1	2,924
2015			19.3		18.3		17.3		16.3		16.1		16.2		15.9		15.8		16.1		.2		2,829
2016					22.5		17.7		18.5		18.6		18.6		18.8		18.8		18.7		.2		3,428
2017							31.3		38.1		45.6		44.6		43.2		42.7		44.2		15.3		4,613
2018									41.0		47.4		49.3		47.0		47.1		46.7		1.9		4,269
2019											34.2		31.4		27.2		24.0		23.4		.8		4,011
2020													77.6		75.8		75.1		78.4		12.4		4,617
2021															171.8		155.5		167.3		5.7		3,465
2022																	244.3		270.5		14.3		3,899
2023																			214.2		156.9		2,963
																	Total		$	908.0


Property and Accident & Health
Millions
	Cumulative Paid Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023
	Unaudited
2014	$	13.7	$	25.1	$	27.3	$	27.6	$	27.7	$	27.9	$	28.0	$	28.0	$	28.0	$	28.0
2015			6.9		12.3		13.5		14.7		14.7		14.9		15.1		15.1		15.5
2016					8.6		13.2		16.5		16.9		17.0		17.3		17.9		18.1
2017							16.9		26.0		31.8		33.0		29.8		27.5		25.7
2018									15.7		32.4		40.3		40.3		41.1		43.1
2019											6.8		16.8		18.5		18.6		19.4
2020													11.3		34.3		47.3		56.1
2021															30.9		87.1		131.3
2022																	70.9		193.9
2023																			20.1
																	Total		551.2
All outstanding liabilities before 2014, net of reinsurance																			1.9
Loss and LAE reserves, net of reinsurance																			$	358.7


Property and Accident & Health
	Average Annual Percentage Payout of Incurred Losses and LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	25.8%	35.5%	20.1%	6.1%	1.3%	1.1%	0.4%	0.3%	0.1%	—%


Specialty
$ in Millions
	Incurred Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2023
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2014	$	46.0	$	44.2	$	41.2	$	40.8	$	41.2	$	43.8	$	43.8	$	43.7	$	43.5	$	43.5	$	—	1,359
2015			17.4		14.8		12.4		10.8		11.1		11.3		11.3		9.0		8.2		(.1)		1,841
2016					18.4		14.4		11.0		11.3		11.9		11.9		9.0		8.6		.1		1,932
2017							18.1		13.0		12.1		11.6		11.7		10.7		10.4		.1		2,191
2018									14.6		16.4		16.8		16.1		14.9		15.8		.4		2,117
2019											21.8		19.6		18.7		25.7		30.1		.2		2,368
2020													24.2		23.2		19.0		19.9		1.7		1,994
2021															71.2		62.6		51.7		16.4		1,696
2022																	181.1		176.6		97.2		1,407
2023																			215.6		151.6		1,258
																	Total		$	580.4


Specialty
Millions
	Cumulative Paid Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023
	Unaudited
2014	$	26.5	$	39.3	$	40.1	$	40.5	$	41.1	$	42.4	$	43.2	$	43.1	$	43.4	$	43.7
2015			4.0		7.1		7.7		8.1		8.1		8.2		8.2		6.5		6.3
2016					3.2		8.0		9.1		9.9		10.4		10.4		8.6		8.4
2017							3.2		6.7		8.5		8.6		8.6		9.3		9.1
2018									2.8		8.3		10.1		10.5		11.9		13.1
2019											4.9		7.0		7.5		18.3		25.2
2020													5.3		10.7		13.2		18.3
2021															5.1		24.1		35.7
2022																	16.0		62.1
2023																			18.8
																	Total		240.7
All outstanding liabilities before 2014, net of reinsurance																			—
Loss and LAE reserves, net of reinsurance																			$	339.7


Specialty
	Average Annual Percentage Payout of Incurred Losses and LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	20.5%	30.6%	9.7%	6.0%	7.7%	6.0%	1.2%	1.7%	(3.1)%	(0.5)%


Marine & Energy
$ in Millions
	Incurred Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2023
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2014	$	34.4	$	20.2	$	17.2	$	16.3	$	14.1	$	13.7	$	14.0	$	13.8	$	13.9	$	13.6	$	(.3)	2,572
2015			21.8		17.5		16.3		13.4		12.8		13.0		12.8		13.1		12.9		.1		3,242
2016					23.6		19.7		15.8		14.7		14.4		15.0		14.2		13.8		.1		3,770
2017							26.2		19.5		17.7		17.1		16.8		15.9		16.1		.3		4,133
2018									25.8		20.3		17.8		18.1		17.7		18.1		.5		3,225
2019											23.9		21.8		21.8		21.6		22.1		.9		2,366
2020													30.0		27.3		28.7		27.6		1.6		1,561
2021															86.8		69.8		67.7		9.3		1,461
2022																	150.2		154.2		50.4		1,802
2023																			197.8		154.8		1,539
																	Total		$	543.9


Marine & Energy
Millions
	Cumulative Paid Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023
	Unaudited
2014	$	5.9	$	12.3	$	13.4	$	14.2	$	14.3	$	13.6	$	13.8	$	13.6	$	13.9	$	13.8
2015			4.0		7.9		9.7		11.1		10.5		10.6		11.0		11.6		11.7
2016					5.6		10.1		12.7		13.1		13.3		13.9		13.6		13.6
2017							5.1		11.2		12.9		14.2		14.2		14.2		14.1
2018									2.7		12.7		14.3		15.0		15.6		15.6
2019											3.4		10.7		12.7		14.5		15.5
2020													3.2		12.8		16.2		18.7
2021															6.3		24.5		38.5
2022																	12.3		66.4
2023																			10.6
																	Total		218.5
All outstanding liabilities before 2013, net of reinsurance																			6.3
Loss and LAE reserves, net of reinsurance																			$	331.7


Marine & Energy
	Average Annual Percentage Payout of Incurred Losses and LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	14.2%	34.9%	19.4%	6.1%	4.3%	6.5%	0.3%	0.3%	(0.3)%	0.1%


Casualty-Active
$ in Millions
	Incurred Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2023
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2014	$	13.3	$	9.4	$	8.4	$	8.2	$	8.1	$	7.7	$	7.8	$	7.6	$	7.8	$	7.7	$	.3	1,393
2015			9.6		9.8		8.2		8.1		7.4		7.2		7.0		7.3		7.5		.4		1,295
2016					8.8		8.3		8.9		9.0		9.1		9.2		9.2		10.1		.5		1,553
2017							11.6		11.7		10.8		9.4		9.1		10.5		10.7		1.1		1,632
2018									12.9		13.3		11.1		10.9		8.6		9.2		1.4		1,098
2019											14.8		13.7		12.4		10.6		11.4		2.6		954
2020													13.5		12.1		10.9		9.2		4.3		605
2021															21.4		22.4		16.6		12.4		842
2022																	33.0		38.1		34.4		1,237
2023																			61.0		57.9		1,065
																	Total		$	181.5


Casualty-Active
Millions
	Cumulative Paid Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023
	Unaudited
2014	$	1.3	$	3.5	$	4.2	$	4.7	$	5.2	$	5.5	$	5.9	$	6.0	$	6.2	$	6.7
2015			1.8		2.4		3.2		4.4		4.7		4.9		5.1		5.5		6.1
2016					.2		1.0		2.3		4.0		4.6		5.3		6.5		8.1
2017							.8		1.7		2.8		3.4		4.2		5.7		7.5
2018									.3		1.4		3.5		4.3		4.3		6.2
2019											.3		1.4		2.3		3.0		5.7
2020													.5		1.0		2.0		3.3
2021															.5		.9		3.1
2022																	.4		1.6
2023																			.9
																	Total		49.2
All outstanding liabilities before 2014, net of reinsurance																			4.8
Loss and LAE reserves, net of reinsurance																			$	137.1


Casualty-Active
	Average Annual Percentage Payout of Incurred Losses and LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	4.8%	8.8%	14.2%	11.3%	8.8%	10.9%	6.1%	4.6%	2.0%	3.2%


Casualty-Runoff
$ in Millions
	Incurred Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2023
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2014	$	49.2	$	48.4	$	51.0	$	54.0	$	57.6	$	59.1	$	59.1	$	58.9	$	58.7	$	59.7	$	1.9	1,947
2015			36.7		32.1		33.3		36.8		36.5		37.5		36.8		39.3		40.3		2.0		2,000
2016					32.6		32.2		40.4		38.6		38.8		38.6		37.7		37.5		2.3		2,158
2017							30.6		34.0		31.4		32.1		31.6		29.9		28.3		2.9		1,603
2018									33.7		28.2		27.3		26.6		26.2		28.0		4.1		1,277
2019											26.6		23.3		23.4		24.9		23.6		5.8		971
2020													15.9		12.3		13.9		10.9		3.9		566
2021															10.5		7.0		5.5		3.0		282
2022																	.8		2.6		1.8		77
2023																			2.7		1.6		40
																	Total		$	239.1


Casualty-Runoff
Millions
	Cumulative Paid Loss and LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2014		2015		2016		2017		2018		2019		2020		2021		2022		2023
	Unaudited
2014	$	6.5	$	23.2	$	29.6	$	36.5	$	43.2	$	47.0	$	48.7	$	49.4	$	51.9	$	54.9
2015			4.3		8.2		14.5		21.4		24.7		27.4		29.0		33.1		35.3
2016					3.9		10.2		17.7		22.7		25.4		27.8		28.7		31.0
2017							3.2		9.4		14.7		18.5		21.4		22.5		22.8
2018									3.4		7.4		12.6		14.9		16.3		18.2
2019											3.3		5.8		7.8		12.2		15.2
2020													.8		1.3		3.1		6.0
2021															.5		1.7		1.9
2022																	.3		.5
2023																			.9
																	Total		186.7
All outstanding liabilities before 2014, net of reinsurance																			28.9
Loss and LAE reserves, net of reinsurance																			$	81.3


Casualty-Runoff
	Average Annual Percentage Payout of Incurred Losses and LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	8.6%	14.0%	15.5%	14.8%	8.6%	7.0%	5.5%	4.2%	3.0%	2.1%

Statutory Capital and Surplus

HG Global/BAM

Ark

WM Outrigger Re

Dividend Capacity

HG Global/BAM

Ark

Income Taxes

12 Months Ended

Dec. 31, 2023

Income Tax Disclosure [Abstract]

Income Taxes

The Company and its Bermuda-domiciled subsidiaries were not subject to income tax in Bermuda in 2023 and prior years. On December 27, 2023, Bermuda enacted a 15% corporate income tax that will generally become effective on January 1, 2025. The Bermuda legislation defers the effective date for five years for Bermuda companies in consolidated groups that meet certain requirements. White Mountains expects to meet the requirements to be exempt from the Bermuda corporate income tax until January 1, 2030. The Bermuda legislation also provides for an economic transition adjustment that will reduce future years’ taxable income. Under GAAP, this economic transition adjustment was required to be recognized as a net deferred tax asset as of December 31, 2023. Accordingly, White Mountains’s net income for 2023 included a net deferred tax benefit of $68 million, of which $51 million was recorded at Ark and $17 million was recorded at HG Global.

The Company has subsidiaries and branches that operate in various other jurisdictions around the world and are subject to tax in the jurisdictions in which they operate. As of December 31, 2023, the primary jurisdictions in which the Company’s subsidiaries and branches operate and were subject to tax include Israel, Luxembourg, the United Kingdom and the United States.

The following table presents the total income tax (expense) benefit for the years ended December 31, 2023, 2022 and 2021:


	Year Ended December 31,
Millions	2023		2022		2021
Current income tax (expense) benefit:
U.S. federal	$	(12.6)	$	(16.9)	$	(4.8)
State	(2.2)		(4.5)		(2.1)
Non-U.S.	(16.6)		(7.1)		(2.8)
Total current income tax (expense) benefit	(31.4)		(28.5)		(9.7)
Deferred income tax (expense) benefit:
U.S. federal	(13.2)		(7.3)		(13.9)
State	(3.8)		(1.8)		(7.0)
Non-U.S.	63.9		(3.8)		(13.8)
Total deferred income tax (expense) benefit	46.9		(12.9)		(34.7)
Total income tax (expense) benefit	$	15.5	$	(41.4)	$	(44.4)

Effective Rate Reconciliation

The following table presents a reconciliation of taxes calculated for 2023, 2022 and 2021 using the 21% U.S. federal statutory rate (the tax rate at which the majority of White Mountains’s worldwide operations are taxed) to the income tax (expense) benefit on pre-tax income (loss):


	Year Ended December 31,
Millions	2023		2022		2021
Tax (expense) benefit at the U.S. statutory rate	$	(118.7)	$	31.4	$	57.5
Differences in taxes resulting from:
Non-U.S. earnings, net of foreign taxes	103.7		(41.2)		(78.2)
Bermuda corporate income tax	68.0		—		—
Change in valuation allowance	(30.2)		(19.6)		(2.0)
State taxes	(8.2)		(2.8)		(7.3)
Noncontrolling interest	6.8		3.4		2.1
Member’s surplus contributions	(5.7)		(6.2)		(5.6)
Withholding tax	(4.1)		(3.1)		(.3)
Tax rate changes	(.6)		(.4)		(10.9)
Officer compensation	(.2)		(1.0)		(1.5)



Tax exempt interest and dividends	.2		.2		.2



Other, net	4.5		(2.1)		1.6


Total income tax (expense) benefit on pre-tax income (loss)	$	15.5	$	(41.4)	$	(44.4)

The non-U.S. component of pre-tax income (loss) was $450.0 million, $(94.4) million and $(319.0) million for the years ended December 31, 2023, 2022 and 2021. On June 10, 2021, the U.K. enacted an increase in its corporate tax rate from 19% to 25% for periods after April 1, 2023. During 2021, White Mountains increased its net U.K. deferred tax liability to reflect the higher tax rate.

Tax Payments and Receipts

Net income tax (refunds) payments totaled $42.7 million, $10.3 million and $(0.1) million for the years ended December 31, 2023, 2022 and 2021.

Deferred Tax Assets and Liabilities

Deferred taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts for tax purposes.

The following table presents an outline of the significant components of White Mountains’s U.S. federal, state and non-U.S. deferred tax assets and liabilities:


	December 31,
Millions	2023		2022
Deferred tax assets related to:
U.S. federal and state net operating and capital loss carryforwards	$	87.6	$	85.4
Non-U.S. net operating loss carryforwards	68.2		36.1
Intangible assets	51.5		—
Incentive compensation	25.4		21.4
Unearned premiums	18.2		—
Accrued interest	10.1		8.8
Loss reserves	8.4		—
Deferred acquisition costs	3.7		8.0
Tax credit carryforwards	3.3		1.8
Net unrealized investment losses	—		4.7







Other items	1.0		1.3
Total gross deferred tax assets	277.4		167.5
Less: valuation allowances	116.1		94.3
Total net deferred tax assets	161.3		73.2
Deferred tax liabilities related to:

Member’s surplus contributions	82.2		71.3
Purchase accounting	43.9		43.9
Investment basis difference	40.9		33.9


Deferred underwriting	18.3		9.7
Net unrealized investment gains (losses)	15.5		—

Other items	1.2		2.2
Total deferred tax liabilities	202.0		161.0
Net deferred tax asset (liability)	$	(40.7)	$	(87.8)

White Mountains’s deferred tax assets (liabilities) are net of U.S. federal, state and non-U.S. valuation allowances and, to the extent they relate to non-U.S. jurisdictions, are shown at year-end exchange rates.

Valuation Allowance

White Mountains records a valuation allowance against deferred tax assets if it becomes more likely than not that all or a portion of a deferred tax asset will not be realized. Changes in valuation allowances from period to period are included in income tax expense in the period of change. In determining whether or not a valuation allowance, or change therein, is warranted, White Mountains considers factors such as prior earnings history, expected future earnings, carryback and carryforward periods and strategies that if executed would result in the realization of a deferred tax asset. It is possible that certain planning strategies or projected earnings in certain subsidiaries may not be sufficient to utilize the entire deferred tax asset, which could result in material changes to White Mountains’s deferred tax assets and tax expense.

Of the $116.1 million valuation allowance as of December 31, 2023, $47.6 million related to deferred tax assets on net operating losses and net unrealized investment gains and losses in Luxembourg subsidiaries, $46.9 million related to deferred tax assets on net operating losses in U.S. subsidiaries and other federal and state deferred tax benefits, $20.8 million related to net operating losses and other deferred tax benefits in Israeli subsidiaries and $0.8 million related to net operating losses in U.K. subsidiaries. Of the $94.3 million valuation allowance as of December 31, 2022, $56.6 million related to deferred tax assets on net operating losses in U.S. subsidiaries and other federal and state deferred tax benefits, $20.4 million related to net operating losses and other deferred tax benefits in Israeli subsidiaries, $16.0 million related to deferred tax assets on net operating losses and net investment unrealized gains and losses in Luxembourg subsidiaries and $1.3 million related to net operating losses in U.K. subsidiaries.

United States

During 2023, White Mountains recorded income tax benefit of $7.2 million to reflect the decrease in the valuation allowance on the net deferred tax assets for certain U.S. operations within Other Operations. The decrease is due to realized gains during the year which reduced the company’s net operating loss carryforward and related deferred tax asset. As the deferred tax asset declined so did the corresponding valuation allowance. As of December 31, 2023, White Mountains management does not currently anticipate sufficient taxable income to utilize the remaining deferred tax assets. During 2022, White Mountains recorded income tax expense of $7.8 million to reflect the increase in the valuation allowance on the net deferred tax assets for certain U.S. operations within Other Operations, as White Mountains management did not anticipate sufficient taxable income to utilize the remaining deferred tax assets.

During 2023 and 2022, White Mountains recorded income tax expense of $6.3 million and $17.5 million to reflect the increase in the valuation allowance on net deferred tax assets of BAM. White Mountains records both the tax expense related to BAM’s member surplus contributions (“MSC”) and the related changes in valuation allowance on such taxes directly through noncontrolling interest equity. During 2023 and 2022, BAM had income included in equity due to MSC that was available to offset its loss from continuing operations. In 2023 and 2022, BAM recorded both the income tax benefit on MSC of $8.7 million and $10.9 million and the offsetting expense in paid-in surplus. During 2023 and 2022, BAM continued to have a full valuation allowance recorded against its net deferred tax assets, as White Mountains management is unsure it will generate sufficient taxable income to utilize the deferred tax assets.

During 2023, White Mountains recorded income tax benefit of $0.6 million to reflect the decrease in the valuation allowance against the deferred tax assets relating to its investment in certain partnership portfolios, including Elementum Holdings LLC at the U.S. branches of the White Mountains U.K. holdings companies. The decrease is due to partnership taxable income which reduced the company’s net operating loss carryforward and related deferred tax asset. As the deferred tax asset declined, so did the corresponding valuation allowance at the U.S. branches of the White Mountains U.K. holdings companies. White Mountains management is unsure it will generate sufficient taxable income to utilize the remaining deferred tax assets.

During 2022, White Mountains recorded income tax expense of $4.0 million to reflect the establishment of a valuation allowance against the deferred tax assets relating to its investment in certain partnership portfolios, including Elementum Holdings LLC, and to reflect the change in the valuation allowance on deferred tax assets relating to net operating losses at the U.S. branches of the White Mountains U.K. holding companies. White Mountains management is unsure it will generate sufficient taxable income to utilize the deferred tax assets.

During 2023, White Mountains recorded income tax expense of $0.1 million to reflect the increase in the valuation allowance on the net deferred tax assets for certain U.S. operations within Ark, as White Mountains management does not currently anticipate sufficient taxable income to utilize the remaining deferred tax assets.

Non-U.S. Jurisdictions

During 2023, White Mountains recorded income tax expense of $31.6 million to reflect the increase in the full valuation allowance against deferred tax assets which primarily relate to losses on the write-down of foreign subsidiaries and investments held in Luxembourg-domiciled subsidiaries. During 2022, White Mountains recorded income tax benefit of $9.2 million to reflect the decrease in the full valuation allowance against deferred tax assets at certain Luxembourg-domiciled subsidiaries. The decrease is due to a Luxembourg company with net operating loss carryforwards re-domiciling to Bermuda which reduced White Mountains net operating loss carryforwards in Luxembourg and the related deferred tax asset. As the deferred tax asset declined, so did the corresponding valuation allowance. White Mountains management does not currently anticipate sufficient taxable income to utilize the remaining deferred tax assets.

During 2023, White Mountains recorded income tax expense of $0.4 million to reflect the increase in the valuation allowance against the deferred tax assets at certain Israel-domiciled subsidiaries, as White Mountains management does not currently anticipate sufficient taxable income to utilize the deferred tax assets. During 2022, White Mountains recorded income tax benefit of $1.5 million to reflect the decrease in the valuation allowance against the deferred tax assets at certain Israel-domiciled subsidiaries. The decrease is due to movement of foreign currency exchange rate gains during the year which reduced the company’s net operating loss carryforward and related deferred tax asset. As the deferred tax asset declined so did the corresponding valuation allowance. White Mountains management does not currently anticipate sufficient income to utilize the deferred tax assets.

During 2023, White Mountains recorded income tax benefit of $0.4 million to reflect the decrease in the full valuation allowance against deferred tax assets at certain U.K. subsidiaries. The decrease is due to investment income gains during the year which reduced the company’s net operating loss carryforward and related deferred tax asset. As the deferred tax asset declined, so did the corresponding valuation allowance. White Mountains management does not currently anticipate sufficient taxable income to utilize the remaining deferred tax assets. During 2022, White Mountains recorded income tax expense of $1.0 million to reflect the increase in the full valuation allowance against deferred tax assets at certain U.K. subsidiaries, as White Mountains management does not currently anticipate sufficient taxable income to utilize the deferred tax assets.

Net Operating Loss and Capital Loss Carryforwards

The following table presents net operating loss and capital loss carryforwards as of December 31, 2023, the expiration dates and the deferred tax assets thereon:


	December 31, 2023
Millions	United States		Luxembourg		United Kingdom		Israel		Total

2024-2028	$	—	$	—	$	—	$	—	$	—
2029-2033	90.8		—		—		—		90.8
2034-2043	304.1		188.1		—		—		492.2
No expiration date	1.9		—		3.2		89.1		94.2
Total	$	396.8	$	188.1	$	3.2	$	89.1	$	677.2
Gross deferred tax asset	$	87.6	$	46.9	$	.8	$	20.5	$	155.8
Valuation allowance	(87.6)		(46.9)		(.8)		(20.5)		(155.8)
Net deferred tax asset	$	—	$	—	$	—	$	—	$	—

Included in the U.S. net operating loss carryforwards are losses of $0.7 million subject to limitation on utilization under the separate-return-limitation-year (SRLY) rules in the Internal Revenue Code. These loss carryforwards will begin to expire in 2028. As of December 31, 2023, there are U.K. foreign tax credit carryforwards available of $3.3 million, which do not have an expiration date.

Uncertain Tax Positions

Recognition of the benefit of a given tax position is based upon whether a company determines that it is more likely than not that a tax position will be sustained upon examination based upon the technical merits of the position. In evaluating the more likely than not recognition threshold, White Mountains must presume that the tax position will be subject to examination by a taxing authority with full knowledge of all relevant information. If the recognition threshold is met, then the tax position is measured at the largest amount of benefit that is more than 50% likely of being realized upon ultimate settlement.

As of December 31, 2023 and 2022, White Mountains did not have any unrecognized tax benefits.

White Mountains classifies all interest and penalties on unrecognized tax benefits as part of income tax expense. During the years ended December 31, 2023, 2022 and 2021, White Mountains did not recognize any net interest (income) expense. There was no accrued interest as of December 31, 2023, 2022 and 2021.

Tax Examinations

With few exceptions, White Mountains is no longer subject to U.S. federal, state, or non-U.S. income tax examinations by tax authorities for years before 2018.

- Definition

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480990/946-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(h)(2))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480678/235-10-S99-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-Name Accounting Standards Codification
-Publisher FASB
-URI https://asc.fasb.org//740/tableOfContent

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-14

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 21
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-21

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 270
-Name Accounting Standards Codification
-Section 50
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Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 17
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-17

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 6.I.5.Q1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479360/740-10-S99-1

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
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-Paragraph 2
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-Publisher FASB
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Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482603/740-30-50-2

+ Details

SCHEDULE I - SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES

12 Months Ended

Dec. 31, 2023

SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Abstract]

SCHEDULE I - SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES

WHITE MOUNTAINS INSURANCE GROUP, LTD.

SUMMARY OF INVESTMENTS—OTHER THAN

INVESTMENTS IN RELATED PARTIES

As of December 31, 2023


Millions	Cost		Carrying Value		Fair Value
Fixed maturity investments:
U.S. Government and agency obligations	$	209.0	$	204.9	$	204.9
Debt securities issued by corporations	1,085.9		1,045.0		1,045.0
Municipal obligations	275.1		260.9		260.9
Mortgage and asset-backed securities	417.2		389.4		389.4
Collateralized loan obligations	211.2		209.1		209.1

Total fixed maturity investments	2,198.4		2,109.3		2,109.3
Short-term investments	1,487.0		1,487.9		1,487.9



Investment in MediaAlpha	59.2		254.9		254.9
Common equity securities:
Common equity securities - Industrial, Miscellaneous, and Other	370.2		400.6		400.6
Common equity securities - Exchange traded funds	113.3		137.8		137.8
Total common equity securities	483.5		538.4		538.4
Other long-term investments	1,655.7		1,998.2		1,998.2
Total investments	$	5,883.8	$	6,388.7	$	6,388.7

Loss and Loss Adjustment Expense Reserves - Schedule of Ending Liabilities for Unpaid Loss and LAE, Net of Reinsurance (Details) - Ark - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	$ 1,264.3	$ 791.5	$ 465.8	$ 262.6
Reinsurance recoverable on unpaid losses	(340.8)	(505.0)	(428.9)	(433.4)
Loss and loss adjustment expense reserves	1,605.1	1,296.5	894.7	696.0
Ark
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Ceded	(15.6)
Amount attributable to TPC Providers
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Reinsurance recoverable on unpaid losses	(0.0)	(145.4)	$ (276.8)	$ (319.2)
Property and Accident & Health
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	358.7	258.2
Total Reinsurance recoverables on unpaid losses	126.9	224.6
Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	339.7	204.3
Total Reinsurance recoverables on unpaid losses	77.6	97.2
Marine & Energy
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	331.7	196.4
Total Reinsurance recoverables on unpaid losses	59.1	79.8
Casualty-Active
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	137.1	71.5
Total Reinsurance recoverables on unpaid losses	74.8	49.9
Casualty-Runoff
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	81.3	60.8
Total Reinsurance recoverables on unpaid losses	2.4	53.5
Other
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	$ 15.8	$ 0.3

Equity Method Eligible Investments

12 Months Ended

Dec. 31, 2023

Equity Method Investments and Joint Ventures [Abstract]

Equity Method Eligible Investments

White Mountains’s equity method eligible investments include Kudu’s Participation Contracts, White Mountains’s investment in MediaAlpha, PassportCard/DavidShield, Elementum Holdings, L.P. and certain other unconsolidated entities, private equity funds and hedge funds in which White Mountains has the ability to exert significant influence over the investee’s operating and financial policies.

The following table presents the ownership interests and carrying values of White Mountain’s equity method eligible investments as of December 31, 2023 and 2022:


	December 31, 2023			December 31, 2022
$ in Millions	Ownership Interest		Carrying Value	Ownership Interest		Carrying Value
Kudu’s Participation Contracts (1)	4.1% - 30.0%		890.5	4.1% - 30.0%		$	695.9
Investment in MediaAlpha	34.9	%	254.9	27.1	%	168.6
PassportCard/DavidShield	53.8	%	150.0	53.8	%	135.0
Elementum Holdings, L.P.	26.6	%	35.0	29.7	%	30.0
Other equity method eligible investments, at fair value	Under 50.0%		291.7	Under 50.0%		84.4
Other equity method eligible investments, at fair value	50.0% and over		24.8	50.0% and over		—

(1) Ownership interest generally references basic ownership interest with the exception of Kudu’s Participation Contracts, which are noncontrolling equity interests in the form of revenue and earnings participation contracts.

For the years ended December 31, 2023, 2022 and 2021, White Mountains received dividend and income distributions from equity method eligible investments of $67.0 million, $68.9 million and $56.2 million, which were recorded within net investment income in the consolidated statements of operations.

Subsequent to the MediaAlpha IPO, White Mountains’s investment in MediaAlpha is accounted for at fair value based on the publicly traded share price of MediaAlpha’s common stock and White Mountains presents its investment in MediaAlpha as a separate line item on the balance sheet. See Note 3 — “Investment Securities”.

For the year ended December 31, 2021, MediaAlpha was considered a significant subsidiary. The following tables present summarized financial information for MediaAlpha as of December 31, 2023 and 2022 and for the years ended December 31, 2023, 2022, and 2021:


	December 31,
Millions	2023		2022
Balance sheet data:
Total assets	$	153.9	$	170.1
Total liabilities	$	248.4	$	256.2


	Year Ended December 31,
Millions	2023		2022		2021
Income statement data:
Total revenues	$	388.1	$	459.1	$	645.3
Total expenses	$	444.7	$	531.5	$	653.8
Net income (loss)	$	(56.6)	$	(72.4)	$	(8.5)

The following tables present aggregated summarized financial information for White Mountains’s investments in equity method eligible unconsolidated entities, excluding MediaAlpha:


	December 31,
Millions	2023		2022
Balance sheet data(1):
Total assets	$	3,049.3	$	2,252.5
Total liabilities	$	657.2	$	526.9

(1) Financial data for White Mountains’s equity method eligible investees is generally reported on a one-quarter lag.


	Year Ended December 31,
Millions	2023		2022		2021
Income statement data(1):
Total revenues	$	1,187.3	$	735.3	$	987.4
Total expenses	$	681.5	$	515.9	$	418.7
Net income (loss)	$	505.8	$	219.4	$	568.7

(1) Financial data for White Mountains’s equity method eligible investees is generally reported on a one-quarter lag.

SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - CONDENSED STATEMENTS OF CASH FLOWS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income (loss)	$ 580.9	$ 712.4	$ (321.9)
Charges (credits) to reconcile net income to net cash from operations:
Net realized and unrealized investment (gains) losses	(434.2)	191.5	246.1
Net (income) loss from discontinued operation, net of tax - NSM Group		(902.5)	2.9
Net other operating activities	(3.9)	80.8	10.7
Net cash provided from (used for) operations	404.1	365.0	38.6
Cash flows from investing activities:
Net change in short-term investments	(495.8)	(455.1)	59.8
Release of cash (pre-funding) for ILS funds managed by Elementum	70.0	(70.0)	0.0
Proceeds from the sale of business, net of cash sold	17.3	1,392.0	0.0
Cash sold in subsidiary transaction	0.8	144.4	0.0
Net cash provided from (used for) investing activities	(543.9)	115.5	(208.3)
Cash flows from financing activities:
Repurchases and retirement of common shares	(32.7)	(615.8)	(107.5)
Cash dividends paid to common shareholders	(2.6)	(3.0)	(3.1)
Net cash provided from (used for) financing activities – continuing operations	7.2	(344.9)	217.6
Cash balance at beginning of year	255.0	147.7	84.6
Cash balance at end of year	122.4	255.0	147.7
Amortization of restricted share awards	15.3	14.0	14.7
Total White Mountains’s common shareholders’ equity	4,240.5	3,746.9
Common equity securities, at fair value	538.4	668.4
Other long-term investments	1,998.2	1,488.0
Net change in (receivables) payables to subsidiaries	8.4	3.3	(6.5)
WM Lincoln
Cash flows from financing activities:
Investments in consolidated subsidiaries			212.6
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income (loss)	509.2	792.8	(275.4)
Charges (credits) to reconcile net income to net cash from operations:
Net realized and unrealized investment (gains) losses	(78.9)	5.6	0.1
Equity in (earnings) losses from consolidated and unconsolidated subsidiaries, net of tax	(489.2)	42.0	232.3
Net (income) loss from discontinued operation, net of tax - NSM Group	0.0	(16.4)	22.6
Other non-cash reconciling items	12.7	7.0	16.6
Accumulated earnings distributed from subsidiary in cash	17.3	7.0	0.0
Net other operating activities	9.2	17.2	(5.7)
Net cash provided from (used for) operations	(19.7)	(31.6)	(28.2)
Cash flows from investing activities:
Net change in short-term investments	(63.8)	1,165.6	17.7
Purchases of investment securities	(125.0)	(359.2)	0.0
Sales and maturities of investment securities	335.4	15.7	0.0
Purchases of investment securities from subsidiaries	0.0	(14.0)	(26.4)
Sales of investment securities to subsidiaries	0.0	0.0	36.4
Release of cash (pre-funding) for ILS funds managed by Elementum	70.0	(70.0)	0.0
Net issuance of debt (to) from subsidiaries	(132.5)	(142.0)	94.0
Net repayment of debt to (from) subsidiaries	31.4	(15.0)	0.0
Net distributions from (contributions to) subsidiaries	(60.3)	49.3	17.0
Proceeds from the sale of business, net of cash sold	0.0	19.5	0.0
Cash sold in subsidiary transaction	0.0	0.5	0.0
Net cash provided from (used for) investing activities	55.2	649.9	138.7
Cash flows from financing activities:
Repurchases and retirement of common shares	(32.7)	(615.8)	(107.5)
Cash dividends paid to common shareholders	(2.6)	(3.0)	(3.1)
Net cash provided from (used for) financing activities – continuing operations	(35.3)	(618.8)	(110.6)
Net change in cash during the year	0.2	(0.5)	(0.1)
Cash balance at beginning of year	0.1	0.6	0.7
Cash balance at end of year	0.3	0.1	0.6
Total White Mountains’s common shareholders’ equity	4,240.5	3,746.9
Investments in consolidated subsidiaries	3,666.9	3,027.2
Fixed maturity investments, at fair value	0.0	49.2
Common equity securities, at fair value	147.8	342.8
Other long-term investments	160.5	49.3
Short-term investments, at fair value	198.8	105.2
Other assets	4.0	75.3
Noncash payment in short-term investments	10.0
Subsidiary - Bridge Holdings, Ltd.
Charges (credits) to reconcile net income to net cash from operations:
Accumulated earnings distributed from subsidiary in cash		7.0
Cash flows from financing activities:
Repayments of principal debt		15.0
Contributions			12.6
Contributions, short-term investments and cash			18.0
Contributions, short-term investments			17.9
Contributions, cash			0.1
Subsidiary - White Mountain Adams, Inc.
Cash flows from financing activities:
Distributions			42.7
Distributions, short-term investments		100.0	37.1
Distributions, cash			5.6
Subsidiary - White Mountains Investments Bermuda
Cash flows from financing activities:
Contributions			19.7
Contributions, short-term investments			19.1
Distributions, cash		51.0
Subsidiary - HG Global
Cash flows from financing activities:
Contributions, cash		116.3
Proceeds from borrowings		6.0
Subsidiary - Ark Insurance Holdings Limited
Cash flows from financing activities:
Contributions, cash		7.7
Subsidiary - PSC Holdings, Ltd.
Cash flows from financing activities:
Contributions, cash		1.3
Subsidiary - White Mountain Hinson Bermuda
Cash flows from financing activities:
Contributions, cash			0.6
Proceeds from borrowings		205.0
WM Birkdale
Cash flows from financing activities:
Repayments of principal debt	20.5
Repayments of debt in investments	73.8
Repayments of debt in fixed maturity investments	36.9
Repayments of debt in short-term investments	36.9
Bridge Holdings, Ltd.
Cash flows from financing activities:
Total White Mountains’s common shareholders’ equity		3,540.6
Intercompany note receivable		76.4
Investments in consolidated subsidiaries		2,003.6
Fixed maturity investments, at fair value		28.6
Common equity securities, at fair value		8.1
Other long-term investments		52.2
Short-term investments, at fair value		1,358.7
Other assets		6.0
Proceeds from borrowings		69.0
Bridge Holdings, Ltd. \| White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Repayments of principal debt			200.0
WM Hinson
Cash flows from financing activities:
Repayments of principal debt	42.4
Proceeds from borrowings	20.0
Repayments of debt in investments	107.6
Repayments of debt in fixed maturity investments	45.9
Repayments of debt in short-term investments	61.7
HG Global
Cash flows from financing activities:
Repayments of principal debt	9.5
Proceeds from borrowings	7.5
White Mountain Investments Luxembourg
Cash flows from financing activities:
Proceeds from borrowings	105.0
White Mountains Adams, Inc.
Cash flows from financing activities:
Distributions, cash		25.0
NSM
Charges (credits) to reconcile net income to net cash from operations:
Net (gain) loss from sale of discontinued operations, net of tax	0.0	(886.8)	0.0
NSM \| White Mountains Insurance Group Ltd.
Charges (credits) to reconcile net income to net cash from operations:
Net (gain) loss from sale of discontinued operations, net of tax	0.0	(886.8)	0.0
Sirius Group
Charges (credits) to reconcile net income to net cash from operations:
Net (gain) loss from sale of discontinued operations, net of tax	0.0	0.0	(18.7)
Sirius Group \| White Mountains Insurance Group Ltd.
Charges (credits) to reconcile net income to net cash from operations:
Net (gain) loss from sale of discontinued operations, net of tax	$ 0.0	$ 0.0	$ (18.7)

Debt - Debt Outstanding (Details) € in Millions, $ in Millions	Dec. 31, 2023 USD ($)	Dec. 31, 2023 EUR (€)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Debt Instrument
Long-term debt, gross	$ 578.9
Carrying Value	564.6		$ 575.2
HG Global Senior Notes
Debt Instrument
Carrying Value	146.9		146.5
Ark 2007 Subordinated Notes
Debt Instrument
Carrying Value	30.0		30.0
Ark 2021 Subordinated Notes
Debt Instrument
Carrying Value	155.5		153.7
Kudu Credit Facility
Debt Instrument
Carrying Value	203.8		208.3
Other Operations debt
Debt Instrument
Carrying Value	28.4		36.7
Senior Notes \| HG Global Senior Notes
Debt Instrument
Long-term debt, gross	150.0		150.0
Unamortized discount and issuance cost	(3.1)		(3.5)
Carrying Value	$ 146.9		$ 146.5
Effective yield (as a percent)	11.90%	11.90%	8.90%
Subordinated Debt \| Ark
Debt Instrument
Carrying Value	$ 185.5		$ 183.7
Effective yield (as a percent)	11.00%	11.00%	7.60%
Subordinated Debt \| Ark 2007 Subordinated Notes
Debt Instrument
Long-term debt, gross	$ 30.0		$ 30.0
Subordinated Debt \| Ark 2021 Subordinated Notes
Debt Instrument
Unamortized issuance cost	(4.2)		(4.6)
Carrying Value	155.5		153.7
Subordinated Debt \| Ark 2021 Subordinated Notes, Tranche 1
Debt Instrument
Long-term debt, gross	42.7	€ 39.1	41.3
Subordinated Debt \| Ark 2021 Subordinated Notes, Tranche 2
Debt Instrument
Long-term debt, gross	47.0		47.0
Subordinated Debt \| Ark 2021 Subordinated Notes, Tranche 3
Debt Instrument
Long-term debt, gross	70.0		70.0
Secured Debt \| Kudu Credit Facility
Debt Instrument
Long-term debt, gross	210.3		215.2
Unamortized issuance cost	(6.5)		(6.9)
Carrying Value	$ 210.3		$ 215.2	$ 225.4	$ 0.0
Effective yield (as a percent)	10.10%	10.10%	6.10%
Secured Debt \| Other Operations debt
Debt Instrument
Long-term debt, gross	$ 28.9		$ 37.4
Unamortized issuance cost	$ (0.5)		$ (0.7)
Effective yield (as a percent)	10.20%	10.20%	6.60%

Basis of Presentation and Significant Accounting Policies (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) segment	Oct. 25, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 20, 2022 account	May 26, 2022	May 25, 2022
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of reportable segments \| segment	3
Other long-term investments	$ 1,998.2		$ 1,488.0
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Other assets		Other assets
Operating lease, right-of-use asset	$ 20.3		$ 25.2
Operating Lease, Liability, Statement of Financial Position [Extensible List]	Other liabilities		Other liabilities
Lease liabilities	$ 22.0		$ 27.1
Net unrealized foreign currency translation gain (losses)	$ (1.6)		(3.5)
Third Point Re \| Ark
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Annual crediting rate	3.00%
Level 2
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Variance threshold for outliers	$ 0.5
Variance threshold for outliers (as a percent)	5.00%
Level 2 \| Fair value measured on a recurring basis \| Other long-term investments
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Other long-term investments	$ 26.2		14.8
Level 3 \| Fair value measured on a recurring basis \| Other long-term investments
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Other long-term investments	1,138.2		911.6
NAV \| Fair value measured on a recurring basis \| Other long-term investments
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Other long-term investments	$ 833.8		$ 561.6
Redeemable Preferred Stock \| Outrigger Re Ltd
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of segregated accounts \| account				4
Ark
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Percent of shares outstanding	12.50%
Subsidiary - HG Global \| Preferred stocks
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Basic ownership	96.90%		96.90%
Subsidiary - HG Global \| Common Stock
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Basic ownership	88.40%		88.40%
Subsidiary - Ark Insurance Holdings Limited
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Basic ownership	72.00%		72.00%
Ownership percentage, after equity incentives	61.90%		63.00%
Subsidiary - WM Outrigger Re \| Preferred stocks
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Basic ownership	100.00%		100.00%
Subsidiary - Kudu Investment Management, LLC
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Basic ownership			89.60%
Ownership percentage basic units outstanding			89.30%		89.30%	99.10%
Ownership percentage, fully diluted	76.30%
Ownership percentage, fully converted			76.10%
Subsidiary - HG Global
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Purchase of surplus notes	$ 503.0
Subsidiary - HG Global \| Maximum
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Percentage of par value of policy reinsured	15.00%
Subsidiary - WTM Partners
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Expected capital contribution		$ 500.0

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2023

Income Tax Disclosure [Abstract]

Schedule of total income tax benefit (expense)

December 31, 2023, 2022 and 2021:


	Year Ended December 31,
Millions	2023		2022		2021
Current income tax (expense) benefit:
U.S. federal	$	(12.6)	$	(16.9)	$	(4.8)
State	(2.2)		(4.5)		(2.1)
Non-U.S.	(16.6)		(7.1)		(2.8)
Total current income tax (expense) benefit	(31.4)		(28.5)		(9.7)
Deferred income tax (expense) benefit:
U.S. federal	(13.2)		(7.3)		(13.9)
State	(3.8)		(1.8)		(7.0)
Non-U.S.	63.9		(3.8)		(13.8)
Total deferred income tax (expense) benefit	46.9		(12.9)		(34.7)
Total income tax (expense) benefit	$	15.5	$	(41.4)	$	(44.4)

Schedule of U.S. federal statutory income tax rate and actual effective tax rate on pre-tax income


	Year Ended December 31,
Millions	2023		2022		2021
Tax (expense) benefit at the U.S. statutory rate	$	(118.7)	$	31.4	$	57.5
Differences in taxes resulting from:
Non-U.S. earnings, net of foreign taxes	103.7		(41.2)		(78.2)
Bermuda corporate income tax	68.0		—		—
Change in valuation allowance	(30.2)		(19.6)		(2.0)
State taxes	(8.2)		(2.8)		(7.3)
Noncontrolling interest	6.8		3.4		2.1
Member’s surplus contributions	(5.7)		(6.2)		(5.6)
Withholding tax	(4.1)		(3.1)		(.3)
Tax rate changes	(.6)		(.4)		(10.9)
Officer compensation	(.2)		(1.0)		(1.5)



Tax exempt interest and dividends	.2		.2		.2



Other, net	4.5		(2.1)		1.6


Total income tax (expense) benefit on pre-tax income (loss)	$	15.5	$	(41.4)	$	(44.4)

Schedule of components of deferred income tax assets and liabilities

The following table presents an outline of the significant components of White Mountains’s U.S. federal, state and non-U.S. deferred tax assets and liabilities:


	December 31,
Millions	2023		2022
Deferred tax assets related to:
U.S. federal and state net operating and capital loss carryforwards	$	87.6	$	85.4
Non-U.S. net operating loss carryforwards	68.2		36.1
Intangible assets	51.5		—
Incentive compensation	25.4		21.4
Unearned premiums	18.2		—
Accrued interest	10.1		8.8
Loss reserves	8.4		—
Deferred acquisition costs	3.7		8.0
Tax credit carryforwards	3.3		1.8
Net unrealized investment losses	—		4.7







Other items	1.0		1.3
Total gross deferred tax assets	277.4		167.5
Less: valuation allowances	116.1		94.3
Total net deferred tax assets	161.3		73.2
Deferred tax liabilities related to:

Member’s surplus contributions	82.2		71.3
Purchase accounting	43.9		43.9
Investment basis difference	40.9		33.9


Deferred underwriting	18.3		9.7
Net unrealized investment gains (losses)	15.5		—

Other items	1.2		2.2
Total deferred tax liabilities	202.0		161.0
Net deferred tax asset (liability)	$	(40.7)	$	(87.8)

Schedule of net operating and capital loss carryforwards

The following table presents net operating loss and capital loss carryforwards as of December 31, 2023, the expiration dates and the deferred tax assets thereon:


	December 31, 2023
Millions	United States		Luxembourg		United Kingdom		Israel		Total

2024-2028	$	—	$	—	$	—	$	—	$	—
2029-2033	90.8		—		—		—		90.8
2034-2043	304.1		188.1		—		—		492.2
No expiration date	1.9		—		3.2		89.1		94.2
Total	$	396.8	$	188.1	$	3.2	$	89.1	$	677.2
Gross deferred tax asset	$	87.6	$	46.9	$	.8	$	20.5	$	155.8
Valuation allowance	(87.6)		(46.9)		(.8)		(20.5)		(155.8)
Net deferred tax asset	$	—	$	—	$	—	$	—	$	—

Significant Transactions (Tables)

12 Months Ended

Dec. 31, 2023

Significant Transactions [Abstract]

Schedule of assets acquired and liabilities assumed

The following presents additional details of the assets acquired and liabilities assumed as of the January 1, 2021 acquisition date:


Millions	As of January 1, 2021
Investments	$	594.3
Cash	52.0		(1)
Reinsurance recoverables	433.4
Insurance premiums receivable	236.7
Ceded unearned premiums	170.2
Value of in-force business acquired	71.7
Other assets	88.9
Loss and loss adjustment expense reserves	(696.0)
Unearned insurance premiums	(326.1)
Debt	(46.4)

Ceded reinsurance payable	(528.3)
Other liabilities	(25.9)
Net tangible assets acquired	24.5
Goodwill	116.8
Other intangible assets - syndicate underwriting capacity	175.7
Deferred tax liability on other intangible assets	(33.4)
Net assets acquired	$	283.6

(1) Cash excludes the White Mountains cash contribution of $605.4 as part of the Ark Transaction.

Income Taxes - Valuation Allowance (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Valuation Allowance [Line Items]
Valuation allowance	$ 116.1	$ 94.3
HG Global/BAM - Segment \| BAM
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	6.3	17.5
Net Operating Losses in Luxembourg Subsidiaries
Valuation Allowance [Line Items]
Valuation allowance	47.6	16.0
U S Losses and Other Federal Deferred Tax Benefits
Valuation Allowance [Line Items]
Valuation allowance	46.9	56.6
Net Operating Losses in Israel Subsidiaries
Valuation Allowance [Line Items]
Valuation allowance	20.8	20.4
Net Operating Losses in United Kingdom Subsidiaries
Valuation Allowance [Line Items]
Valuation allowance	0.8	1.3
Net Deferred Tax Assets For Certain U.S. Operations - Other Operations
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	(7.2)	7.8
Net Deferred Tax Assets For BAM
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	(8.7)	(10.9)
Investments and Net Operating Losses At United States Branches Of United Kingdom Holding Companies
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	(0.6)	4.0
Net Deferred Tax Assets For Certain U.S. Operations - Ark
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	0.1
Investments Held in Luxembourg-Domiciled Subsidiaries
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	31.6	(9.2)
Investments Held in Israel-Domiciled Subsidiaries
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	0.4	(1.5)
Investments Held in U.K.-Domiciled Subsidiaries
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	$ (0.4)	$ 1.0

Held for Sale and Discontinued Operations (Tables)

12 Months Ended

Dec. 31, 2023

Discontinued Operations and Disposal Groups [Abstract]

Schedule of discontinued operations balance sheet, income statement, and cash flows

The following summarizes the results of operations, including related income taxes associated with the businesses classified as discontinued operations for the years ended December 31, 2022, and 2021:


	December 31,
Millions	2022 (1)		2021
Revenues
Commission revenues	$	176.9	$	258.0
Other revenues	48.1		72.4
Total revenues - NSM Group	225.0		330.4
Expenses
General and administrative expenses	126.8		190.4
Broker commission expenses	52.9		80.2
Change in fair value of contingent consideration	.1		1.0
Amortization of other intangible assets	9.1		35.2
Loss on assets held for sale	—		28.7
Interest expense	12.1		23.3
Total expenses - NSM Group	201.0		358.8
Pre-tax income (loss) from discontinued operations - NSM Group	24.0		(28.4)
Income tax (expense) benefit	(7.6)		5.8
Net income (loss) from discontinued operations, net tax - NSM Group	16.4		(22.6)
Net gain (loss) from sale of discontinued operations, net of tax - NSM Group	886.8		—
Net gain (loss) from sale of discontinued operations, net of tax - Sirius Group	—		18.7
Total income (loss) from discontinued operations, net of tax	903.2		(3.9)
Net (income) loss from discontinued operations attributable to noncontrolling interests	(.7)		1.0
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders	902.5		(2.9)
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	(5.2)		.2
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	2.9		—
Comprehensive income (loss) from discontinued operations	900.2		(2.7)
Other comprehensive (income) loss from discontinued operations attributable to noncontrolling interests	.2		(.1)
Comprehensive income (loss) from discontinued operations attributable to White Mountains’s common shareholders	$	900.4	$	(2.8)

(1) As a result of the NSM Transaction, the results of operations for NSM Group are presented for the period from January 1, 2022 to August 1, 2022.

Net Change in Cash from Discontinued Operations

The following summarizes the net change in cash associated with the businesses classified as discontinued operations for the years ended December 31, 2022 and 2021:


	Year Ended December 31,
Millions	2022		2021
Net cash provided from (used for) operations	$	38.7	$	42.3
Net cash provided from (used for) investing activities	7.1		(56.5)
Net cash used from (used for) financing activities	(17.5)		(1.0)
Effect of exchange rate changes on cash	4.0		.2
Net change in cash during the period	32.3		(15.0)
Cash balances at beginning of period (includes restricted cash of $89.2 and $78.4 )	111.6		126.6
Cash sold as part of the sale of NSM Group (includes restricted cash of $105.1 and $0.0)	(143.9)		—
Cash balances at end of period (includes restricted cash of $0.0 and $89.2)	—		111.6
Supplemental cash flows information:
Interest paid	$	(12.0)	$	(16.6)
Net income tax payments	$	—	$	—

Schedule of earnings per share from continuing operations

The following table presents the Company’s computation of earnings per share from continuing operations for the years ended December 31, 2023, 2022 and 2021. See Note 20 — “Held for Sale and Discontinued Operations”.


	Year Ended December 31,
	2023		2022		2021
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$	509.2	$	792.8	$	(275.4)
Less: total income (loss) from discontinued operations, net of tax (1)	—		903.2		(3.9)
Less: net (income) loss from discontinued operations attributable to noncontrolling interests	—		(.7)		1.0
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	509.2		(109.7)		(272.5)
Allocation of (earnings) losses to participating restricted common shares (2)	(7.2)		1.3		3.2


Basic and diluted earnings (losses) per share numerators	$	502.0	$	(108.4)	$	(269.3)




Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	2,563.3		2,862.4		3,079.0
Average unvested restricted common shares (3)	(36.0)		(36.2)		(36.5)
Basic earnings (losses) per share denominator	2,527.3		2,826.2		3,042.5
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	2,563.3		2,862.4		3,079.0
Average unvested restricted common shares (3)	(36.0)		(36.2)		(36.5)

Diluted earnings (losses) per share denominator	2,527.3		2,826.2		3,042.5
Basic and diluted earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid	$	1.00	$	1.00	$	1.00
Undistributed earnings (losses)	197.60		(39.34)		(89.52)
Basic and diluted earnings (losses) per share	$	198.60	$	(38.34)	$	(88.52)

(1) Includes net income (loss) from discontinued operations, net of tax - NSM Group, net gain (loss) from sale of discontinued operations, net of tax - NSM Group and net gain (loss) from sale of discontinued operations, net of tax - Sirius Group. See Note 20 — “Held for Sale and Discontinued Operations.”

(2) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.

(3) Restricted shares outstanding vest upon a stated date. See Note 12 — “Employee Share-Based Incentive Compensation Plans”.

The following table presents the Company’s computation of earnings per share for discontinued operations for the years ended December 31, 2022 and 2021:


	Year Ended December 31,
	2022		2021
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$	792.8	$	(275.4)
Less: net income (loss) from continuing operations	(190.8)		(318.0)
Less: net (income) loss from continuing operations attributable to noncontrolling interest	81.1		45.5
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders (1)	902.5		(2.9)
Allocation of (earnings) losses to participating restricted common shares (2)	(11.4)		—
Basic and diluted (loss) earnings per share numerators	$	891.1	$	(2.9)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	2,862.4		3,079.0
Average unvested restricted common shares (3)	(36.2)		(36.5)
Basic earnings (loss) per share denominator	2,826.2		3,042.5
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	2,862.4		3,079.0
Average unvested restricted common shares (3)	(36.2)		(36.5)

Diluted earnings (loss) per share denominator	2,826.2		3,042.5
Basic (loss) earnings per share (in dollars) - discontinued operations:	$	315.30	$	(.94)
Diluted (loss) earnings per share (in dollars) - discontinued operations:	$	315.30	$	(.94)

(1) Includes net income (loss) from discontinued operations, net of tax - NSM Group, net gain (loss) from sale of discontinued operations, net of tax - NSM Group, net gain (loss) from sale of discontinued operations, net of tax - Sirius Group and net (income) loss from discontinued operations attributable to noncontrolling interests.

(2) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities.

(3) Restricted shares outstanding vest upon a stated date. See Note 12 — “Employee Share-Based Incentive Compensation Plans.”

- Definition

Tabular disclosure of information related to a disposal group. Includes, but is not limited to, a discontinued operation, disposal classified as held-for-sale or disposed of by means other than sale or disposal of an individually significant component.

+ References

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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 8: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 9: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
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-URI https://asc.fasb.org//1943274/2147483499/205-20-50-7

Reference 11: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 360
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Reference 12: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 13: http://fasb.org/us-gaap/role/ref/legacyRef
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-URI https://asc.fasb.org//1943274/2147483499/205-20-50-3

+ Details

Variable Interest Entities	12 Months Ended
Variable Interest Entities	Dec. 31, 2023
Variable Interest Entity, Not Primary Beneficiary, Disclosures [Abstract]
Variable Interest Entities	Variable Interest Entities Under GAAP, White Mountains is required to consolidate any entity in which it holds a controlling financial interest. A controlling financial interest is usually in the form of an investment representing the majority of the subsidiary’s voting interests. However, a controlling financial interest may also arise from a financial interest in a VIE through arrangements that do not involve ownership of voting interests. A VIE is a legal entity that (i) does not have sufficient equity at risk to finance its activities without additional financial support; (ii) is structured such that equity investors, as a group, lack the power, through voting or similar rights, to direct the activities that most significantly impact the entity’s economic performance; (iii) is structured such that the equity investors lack the obligation to absorb losses of, or the right to receive returns from, the entity; or (iv) is structured with non-substantive voting rights. White Mountains determines whether an entity is a VIE at the inception of its variable interest in the entity and upon the occurrence of certain reconsideration events. White Mountains consolidates a VIE if it determines that it is the primary beneficiary. The primary beneficiary is defined as the entity that holds a variable interest that gives it both the power to direct the VIE’s activities that most significantly impact its economic performance and the obligation to absorb losses of, or the right to receive returns from, the VIE that could potentially be significant to the VIE. The identification of the primary beneficiary of a VIE may require significant assumptions and judgment. When White Mountains determines it has a variable interest in a VIE, it determines whether it is the primary beneficiary of that VIE by performing an analysis that principally considers: (i) the VIE’s purpose and design, including the risks the VIE was designed to create and pass through to its variable interest holders; (ii) the VIE’s capital structure; (iii) the identification of the activities that most significantly impact the VIE’s economic performance; (iv) the governance provisions and other contractual arrangements between the VIE and its variable interest holders and other parties involved with the VIE; and (v) related party relationships. At inception of its variable interest in the VIE as well as on an ongoing basis, White Mountains performs qualitative assessments of its VIEs to determine whether White Mountains is the primary beneficiary of a VIE. BAM BAM is the first and only mutual municipal bond insurance company in the United States. HG Global, together with its subsidiaries, funded the initial capitalization of BAM through the purchase of $503.0 million of the BAM Surplus Notes and, through its reinsurance subsidiary HG Re, provides up to 15%-of-par, first loss reinsurance protection for policies underwritten by BAM. As a mutual insurance company, BAM is owned by and operated for the benefit of its members, the municipalities that purchase BAM’s insurance for their debt issuances. As of December 31, 2023, BAM’s members have contributed equity capital at risk through MSC of $545.2 million. However, the BAM Surplus Notes remain a significant portion of BAM’s statutory capital. Accordingly, White Mountains has determined that BAM is a VIE. BAM’s underwriting process was determined to be the activity that most significantly impacts BAM’s economic performance. BAM’s underwriting guidelines define the types of credits that BAM may insure. Pursuant to the FLRT, BAM’s underwriting guidelines may only be amended with the consent of HG Re. As a result, White Mountains concluded it has the power to direct BAM’s activities that most significantly impact its economic performance and it is the primary beneficiary. Accordingly, White Mountains is required to consolidate BAM’s results in its financial statements. Since BAM is owned by its members, its equity and results of operations are included in noncontrolling interests. BAM’s assets can be used only to settle BAM’s obligations, and general creditors of BAM have no recourse to the Company or HG Global. HG Re’s obligations to BAM under the FLRT are subject to an aggregate limit equal to the assets in the Collateral Trusts at any point in time. WM Outrigger Re White Mountains has determined that Outrigger Re Ltd. and WM Outrigger Re are VIEs. White Mountains is not the primary beneficiary of Outrigger Re Ltd. or the other segregated accounts. White Mountains is the primary beneficiary of WM Outrigger Re, as it has both the power to direct the activities that most significantly impact WM Outrigger Re’s economic performance and the obligation to absorb losses and the right to receive returns that could potentially be significant to WM Outrigger Re. As a result, White Mountains consolidates WM Outrigger Re’s results in its financial statements. The assets of WM Outrigger Re can only be used to settle the liabilities of WM Outrigger Re, and there is no recourse to the Company for any creditors of WM Outrigger Re. PassportCard/DavidShield As of December 31, 2023, White Mountains’s ownership interest in PassportCard/DavidShield was 53.8%. White Mountains has determined that both PassportCard and DavidShield are VIEs but that White Mountains is not the primary beneficiary and therefore does not consolidate either PassportCard or DavidShield. The governance structures for both PassportCard and DavidShield were designed to give White Mountains and its co-investor equal power to make the decisions that most significantly impact operations. White Mountains does not have the unilateral power to direct the operations of PassportCard or DavidShield and does not hold a controlling financial interest. White Mountains’s ownership interest gives White Mountains the ability to exert significant influence over the significant financial and operating activities of PassportCard/DavidShield. Accordingly, White Mountains’s investment in PassportCard/DavidShield meets the criteria to be accounted for under the equity method. White Mountains has taken the fair value option for its investment in PassportCard/DavidShield. Changes in the fair value of PassportCard/DavidShield are recorded in net realized and unrealized investment gains (losses). As of December 31, 2023, White Mountains’s maximum exposure to loss on its equity investment in PassportCard/DavidShield and the non-interest-bearing loan to its partner is the total carrying value of $159.6 million. Elementum As of December 31, 2023, White Mountains’s ownership interest in Elementum was 26.6%. White Mountains has determined that Elementum is a VIE but that White Mountains is not the primary beneficiary and therefore does not consolidate Elementum. White Mountains’s ownership interest gives White Mountains the ability to exert significant influence over the significant financial and operating activities of Elementum. Accordingly, Elementum meets the criteria to be accounted for under the equity method. White Mountains has taken the fair value option for its investment in Elementum. Changes in the fair value of Elementum are recorded in net realized and unrealized investment gains (losses). As of December 31, 2023, White Mountains’s maximum exposure to loss on its limited partnership interest in Elementum is the carrying value of $35.0 million. Limited Partnerships White Mountains’s investments in limited partnerships are generally considered VIEs because the limited partnership interests do not have substantive kick-out rights or participating rights. White Mountains does not have the unilateral power to direct the operations of these limited partnerships, and therefore White Mountains is not the primary beneficiary and does not consolidate the limited partnerships. White Mountains has taken the fair value option for its investments in limited partnerships, which are generally measured at NAV as a practical expedient. As of December 31, 2023, White Mountains’s maximum exposure to loss on its investments in limited partnerships is the carrying value of $248.6 million.

Loss and Loss Adjustment Expense Reserves - TPC Participation (Details)	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Syndicate - Ark Insurance Holdings Limited
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of underwriting capital provided by third parties	0.00%	0.00%	0.00%	42.80%	58.30%	57.60%	60.00%	59.60%	70.00%	66.20%

Equity Method Eligible Investments (Tables)

12 Months Ended

Dec. 31, 2023

Equity Method Investments and Joint Ventures [Abstract]

Schedule of significant equity method eligible investments

The following table presents the ownership interests and carrying values of White Mountain’s equity method eligible investments as of December 31, 2023 and 2022:


	December 31, 2023			December 31, 2022
$ in Millions	Ownership Interest		Carrying Value	Ownership Interest		Carrying Value
Kudu’s Participation Contracts (1)	4.1% - 30.0%		890.5	4.1% - 30.0%		$	695.9
Investment in MediaAlpha	34.9	%	254.9	27.1	%	168.6
PassportCard/DavidShield	53.8	%	150.0	53.8	%	135.0
Elementum Holdings, L.P.	26.6	%	35.0	29.7	%	30.0
Other equity method eligible investments, at fair value	Under 50.0%		291.7	Under 50.0%		84.4
Other equity method eligible investments, at fair value	50.0% and over		24.8	50.0% and over		—

The following tables present summarized financial information for MediaAlpha as of December 31, 2023 and 2022 and for the years ended December 31, 2023, 2022, and 2021:


	December 31,
Millions	2023		2022
Balance sheet data:
Total assets	$	153.9	$	170.1
Total liabilities	$	248.4	$	256.2


	Year Ended December 31,
Millions	2023		2022		2021
Income statement data:
Total revenues	$	388.1	$	459.1	$	645.3
Total expenses	$	444.7	$	531.5	$	653.8
Net income (loss)	$	(56.6)	$	(72.4)	$	(8.5)

The following tables present aggregated summarized financial information for White Mountains’s investments in equity method eligible unconsolidated entities, excluding MediaAlpha:


	December 31,
Millions	2023		2022
Balance sheet data(1):
Total assets	$	3,049.3	$	2,252.5
Total liabilities	$	657.2	$	526.9

(1) Financial data for White Mountains’s equity method eligible investees is generally reported on a one-quarter lag.


	Year Ended December 31,
Millions	2023		2022		2021
Income statement data(1):
Total revenues	$	1,187.3	$	735.3	$	987.4
Total expenses	$	681.5	$	515.9	$	418.7
Net income (loss)	$	505.8	$	219.4	$	568.7

(1) Financial data for White Mountains’s equity method eligible investees is generally reported on a one-quarter lag.

Loss and Loss Adjustment Expense Reserves - LAE Reserve Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Losses and LAE incurred relating to:
Net incurred losses and LAE	$ 726.8	$ 536.4	$ 314.8
Ark
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance	1,296.5	894.7	696.0
Less: beginning reinsurance recoverable on unpaid losses	(505.0)	(428.9)	(433.4)
Net loss and LAE reserves	791.5	465.8	262.6
Losses and LAE incurred relating to:
Current Year	706.9	588.1	336.3
Prior Year	19.9	(51.7)	(21.5)
Net incurred losses and LAE	726.8	536.4	314.8
Loss and LAE paid relating to:
Current year losses	(51.3)	(98.9)	(43.9)
Prior year losses	(356.6)	(158.6)	(61.6)
Net paid losses and LAE	(407.9)	(257.5)	(105.5)
Change in TPC providers’ participation	145.4	57.5	(2.2)
Foreign currency translation and other adjustments to loss and LAE reserves	8.5	(10.7)	(3.9)
Net ending balance	1,264.3	791.5	465.8
Plus: ending reinsurance recoverable on unpaid losses	(340.8)	(505.0)	(428.9)
Gross ending balance	1,605.1	1,296.5	894.7
Ark \| Amount attributable to TPC Providers
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Less: beginning reinsurance recoverable on unpaid losses	(145.4)	(276.8)	(319.2)
Loss and LAE paid relating to:
Plus: ending reinsurance recoverable on unpaid losses	$ (0.0)	$ (145.4)	$ (276.8)

Common Shareholders’ Equity and Noncontrolling Interests - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	Sep. 26, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Common Shares Repurchased and Retired
Additional authorized repurchase of common shares (in shares)		301,014
Common shares repurchased and retired (in shares)	327,795	24,165	461,256	98,511
Value of common shares repurchased and retired	$ 460.8	$ 32.7	$ 615.8	$ 107.5
Value of each common share repurchased and retired (in usd per share)	$ 1,400	$ 1,354.88	$ 1,335.11	$ 1,091.29
Shares repurchased to satisfy employee income tax withholding		4,629	4,011	7,218
Common Shares Issued
Stock issued (in shares)		12,461	15,640	15,066
Dividends on Common Shares
Cash dividends declared and paid		$ 2.6	$ 3.0	$ 3.1
Dividends declared and paid (in usd per share)		$ 1.00	$ 1.00	$ 1.00
General Board Authorization
Common Shares Repurchased and Retired
Common shares repurchased and retired (in shares)		19,536	129,450	91,293
Value of common shares repurchased and retired		$ 26.2	$ 150.9	$ 100.0
Value of each common share repurchased and retired (in usd per share)		$ 1,340.79	$ 1,165.84	$ 1,095.37
Key Personnel
Common Shares Issued
Stock issued (in shares)		10,895	13,225	13,475
Directors
Common Shares Issued
Stock issued (in shares)		1,566	2,415	1,591

Municipal Bond Guarantee - Schedule of Municipal Guarantee Insurance Contracts, Premium Received Over Contract Period (Details) - Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company $ in Millions	Dec. 31, 2023 USD ($)
Guarantor Obligations [Line Items]
January 1, 2024 - March 31, 2024	$ 7.5
April 1, 2024 - June 30, 2024	7.5
July 1, 2024 - September 30, 2024	7.3
October 1, 2024 - December 31, 2024	7.2
Total	29.5
2025	27.7
2026	26.0
2027	24.4
2028	22.6
2029 and thereafter	195.6
Total gross unearned insurance premiums	$ 325.8

Held for Sale and Discontinued Operations

12 Months Ended

Dec. 31, 2023

Discontinued Operations and Disposal Groups [Abstract]

Held for Sale and Discontinued Operations

NSM

On August 1, 2022, White Mountains closed the NSM Transaction. See Note 2 — “Significant Transactions.” As a result of the NSM Transaction, the assets and liabilities of NSM Group have been presented in the balance sheet as held for sale for periods prior to the closing of the transaction, and the results of operations for NSM Group have been classified as discontinued operations in the statements of operations and comprehensive income through the closing of the transaction. Prior period amounts have been reclassified to conform to the current period’s presentation.

Sirius Group

On April 18, 2016, White Mountains completed the sale of Sirius International Insurance Group, Ltd. (“Sirius Group”) to CM International Pte. Ltd. and CM Bermuda Limited (collectively, “CMI”). In connection with the sale, White Mountains indemnified Sirius Group against the loss of certain interest deductions claimed by Sirius Group related to periods prior to the sale of Sirius Group to CMI that had been disputed by the Swedish Tax Agency (STA). In late October 2018, the Swedish Administrative Court ruled against Sirius Group on its appeal of the STA’s denial of these interest deductions. As a result, in 2018 White Mountains recorded a loss of $17.3 million within net (loss) gain on sale of discontinued operations reflecting the value of these interest deductions.

As of December 31, 2020, White Mountains’s liability related to the tax indemnification provided in connection with the

sale of Sirius Group in 2016 was $18.7 million. In April 2021, the STA informed the Swedish Administrative Court of Appeal that Sirius Group should prevail in its appeal and that the interest deductions should not be disallowed. In June 2021, the Swedish Administrative Court of Appeal ruled in Sirius Group’s favor. For the year ended December 31, 2021, White Mountains recorded a gain of $17.6 million in discontinued operations to reverse the accrued liability and a $1.1 million gain related to foreign currency translation.

Net Income (Loss) from Discontinued Operations


	December 31,
Millions	2022 (1)		2021
Revenues
Commission revenues	$	176.9	$	258.0
Other revenues	48.1		72.4
Total revenues - NSM Group	225.0		330.4
Expenses
General and administrative expenses	126.8		190.4
Broker commission expenses	52.9		80.2
Change in fair value of contingent consideration	.1		1.0
Amortization of other intangible assets	9.1		35.2
Loss on assets held for sale	—		28.7
Interest expense	12.1		23.3
Total expenses - NSM Group	201.0		358.8
Pre-tax income (loss) from discontinued operations - NSM Group	24.0		(28.4)
Income tax (expense) benefit	(7.6)		5.8
Net income (loss) from discontinued operations, net tax - NSM Group	16.4		(22.6)
Net gain (loss) from sale of discontinued operations, net of tax - NSM Group	886.8		—
Net gain (loss) from sale of discontinued operations, net of tax - Sirius Group	—		18.7
Total income (loss) from discontinued operations, net of tax	903.2		(3.9)
Net (income) loss from discontinued operations attributable to noncontrolling interests	(.7)		1.0
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders	902.5		(2.9)
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	(5.2)		.2
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	2.9		—
Comprehensive income (loss) from discontinued operations	900.2		(2.7)
Other comprehensive (income) loss from discontinued operations attributable to noncontrolling interests	.2		(.1)
Comprehensive income (loss) from discontinued operations attributable to White Mountains’s common shareholders	$	900.4	$	(2.8)

(1) As a result of the NSM Transaction, the results of operations for NSM Group are presented for the period from January 1, 2022 to August 1, 2022.

Net Change in Cash from Discontinued Operations

The following summarizes the net change in cash associated with the businesses classified as discontinued operations for the years ended December 31, 2022 and 2021:


	Year Ended December 31,
Millions	2022		2021
Net cash provided from (used for) operations	$	38.7	$	42.3
Net cash provided from (used for) investing activities	7.1		(56.5)
Net cash used from (used for) financing activities	(17.5)		(1.0)
Effect of exchange rate changes on cash	4.0		.2
Net change in cash during the period	32.3		(15.0)
Cash balances at beginning of period (includes restricted cash of $89.2 and $78.4 )	111.6		126.6
Cash sold as part of the sale of NSM Group (includes restricted cash of $105.1 and $0.0)	(143.9)		—
Cash balances at end of period (includes restricted cash of $0.0 and $89.2)	—		111.6
Supplemental cash flows information:
Interest paid	$	(12.0)	$	(16.6)
Net income tax payments	$	—	$	—

Earnings Per Share from Discontinued Operations

White Mountains calculates earnings per share using the two-class method, which allocates earnings between common and unvested restricted common shares. Both classes of shares participate equally in earnings on a per share basis. Basic earnings per share amounts are based on the weighted average number of common shares outstanding adjusted for unvested restricted common shares. Diluted earnings per share amounts are also impacted by the net effect of potentially dilutive common shares outstanding. The following table presents the Company’s computation of earnings per share for discontinued operations for the years ended December 31, 2022 and 2021:


	Year Ended December 31,
	2022		2021
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$	792.8	$	(275.4)
Less: net income (loss) from continuing operations	(190.8)		(318.0)
Less: net (income) loss from continuing operations attributable to noncontrolling interest	81.1		45.5
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders (1)	902.5		(2.9)
Allocation of (earnings) losses to participating restricted common shares (2)	(11.4)		—
Basic and diluted (loss) earnings per share numerators	$	891.1	$	(2.9)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	2,862.4		3,079.0
Average unvested restricted common shares (3)	(36.2)		(36.5)
Basic earnings (loss) per share denominator	2,826.2		3,042.5
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	2,862.4		3,079.0
Average unvested restricted common shares (3)	(36.2)		(36.5)

Diluted earnings (loss) per share denominator	2,826.2		3,042.5
Basic (loss) earnings per share (in dollars) - discontinued operations:	$	315.30	$	(.94)
Diluted (loss) earnings per share (in dollars) - discontinued operations:	$	315.30	$	(.94)

(2) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities.

(3) Restricted shares outstanding vest upon a stated date. See Note 12 — “Employee Share-Based Incentive Compensation Plans.”

Investment Securities - Investment Holdings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
U.S. Government and agency obligations
Investment securities
Cost or Amortized Cost	$ 209.0	$ 216.6
Gross Unrealized Gains	0.3	0.0
Gross Unrealized Losses	(4.4)	(10.2)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	204.9	206.4
Debt securities issued by corporations
Investment securities
Cost or Amortized Cost	1,085.9	1,098.3
Gross Unrealized Gains	5.6	0.6
Gross Unrealized Losses	(45.4)	(78.3)
Net Foreign Currency Gains (Losses)	(1.1)	(1.8)
Carrying Value	1,045.0	1,018.8
Municipal obligations
Investment securities
Cost or Amortized Cost	275.1	281.6
Gross Unrealized Gains	0.9	0.4
Gross Unrealized Losses	(15.1)	(23.4)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	260.9	258.6
Mortgage and asset-backed securities
Investment securities
Cost or Amortized Cost	417.2	288.7
Gross Unrealized Gains	2.0	0.0
Gross Unrealized Losses	(29.8)	(34.5)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	389.4	254.2
Collateralized loan obligations
Investment securities
Cost or Amortized Cost	211.2	190.8
Gross Unrealized Gains	0.4	0.1
Gross Unrealized Losses	(2.4)	(6.0)
Net Foreign Currency Gains (Losses)	(0.1)	(2.0)
Carrying Value	209.1	182.9
Fixed maturity investments
Investment securities
Cost or Amortized Cost	2,198.4	2,076.0
Gross Unrealized Gains	9.2	1.1
Gross Unrealized Losses	(97.1)	(152.4)
Net Foreign Currency Gains (Losses)	(1.2)	(3.8)
Carrying Value	$ 2,109.3	$ 1,920.9

Third-Party Reinsurance - Narrative (Details) - Third Point Re - Ark - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Annual crediting rate		3.00%
Decrease in deposit assets	$ 31.7
Carrying value of investment		$ 20.4	$ 20.4

Third-Party Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Earned premiums:
Net earned premiums	$ 1,440.9	$ 1,076.7	$ 664.2
Losses and LAE:
Net incurred losses and LAE	726.8	536.4	314.8
Ark Insurance Holdings Limited/Outrigger
Written premiums:
Direct	931.9	760.4	619.5
Assumed	966.5	691.6	439.2
Gross written premiums	1,898.4	1,452.0	1,058.7
Ceded	(377.5)	(256.8)	(199.6)
Net written premiums	1,520.9	1,195.2	859.1
Earned premiums:
Direct	848.1	655.5	556.0
Assumed	925.6	668.7	330.4
Gross earned premiums	1,773.7	1,324.2	886.4
Ceded	(364.0)	(280.8)	(249.1)
Net earned premiums	1,409.7	1,043.4	637.3
Losses and LAE:
Gross	782.5	814.9	442.9
Ceded	(55.7)	(278.5)	(128.1)
Net incurred losses and LAE	726.8	$ 536.4	$ 314.8
Ark Insurance Holdings Limited/Outrigger \| Ark
Written premiums:
Ceded	110.0
Earned premiums:
Ceded	104.3
Losses and LAE:
Ceded	$ 15.6

Debt - Schedule of Contractual Repayments (Details) $ in Millions	Dec. 31, 2023 USD ($)
Debt Disclosure [Abstract]
Due in one year or less	$ 3.3
Due in two to three years	11.2
Due in four to five years	40.6
Due after five years	523.8
Total	$ 578.9

SCHEDULE IV - REINSURANCE

12 Months Ended

Dec. 31, 2023

SEC Schedule, 12-17, Insurance Companies, Reinsurance [Abstract]

SCHEDULE IV - REINSURANCE

WHITE MOUNTAINS INSURANCE GROUP, LTD.

REINSURANCE


Column A	Column B		Column C		Column D		Column E		Column F
$ in Millions Premiums Earned	Gross Amount		Ceded to Other Companies		Assumed from Other Companies		Net Amount		Percentage of Amount Assumed to Net
Year ended:
December 31, 2023


HG Global/BAM	$	28.7	$	—	$	2.5	$	31.2	8.0	%

Ark/WM Outrigger	848.1		(364.0)		925.6		1,409.7		65.7
December 31, 2022
HG Global/BAM	28.6		—		4.7		33.3		14.1
Ark/WM Outrigger	655.5		(280.8)		668.7		1,043.4		64.1


December 31, 2021

HG Global/BAM	23.2		—		3.7		26.9		13.8
Ark/WM Outrigger	556.0		(249.1)		330.4		637.3		51.8

Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

Fair Value of Financial Instruments

12 Months Ended

Dec. 31, 2023

Fair Value Disclosures [Abstract]

Fair Value of Financial Instruments

White Mountains records its financial instruments at fair value with the exception of debt obligations which are recorded as debt at face value less unamortized original issue discount. See Note 7 — “Debt”.

The following table presents the fair value and carrying value of these financial instruments as of December 31, 2023 and 2022:


	December 31, 2023				December 31, 2022
Millions	Fair Value		Carrying Value		Fair Value		Carrying Value
HG Global Senior Notes	$	158.7	$	146.9	$	155.7	$	146.5
Ark 2007 Subordinated Notes	$	30.5	$	30.0	$	28.4	$	30.0
Ark 2021 Subordinated Notes	$	171.8	$	155.5	$	163.1	$	153.7
Kudu Credit Facility	$	225.6	$	203.8	$	223.9	$	208.3
Other Operations debt	$	30.0	$	28.4	$	38.2	$	36.7

The fair value estimates for the HG Global Senior Notes, Ark 2007 Subordinated Notes, Ark 2021 Subordinated Notes, Kudu Credit Facility and Other Operations debt have been determined based on a discounted cash flow approach and are considered to be Level 3 measurements.

For the fair value level measurements associated with White Mountains’s investment securities see Note 3 — “Investment Securities.” For the fair value level measurements associated with White Mountains’s derivative instruments see Note 9 — “Derivatives.”

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Common Shareholders’ Equity and Noncontrolling Interests

12 Months Ended

Dec. 31, 2023

Equity [Abstract]

Common Shareholders’ Equity and Noncontrolling Interests

Common Shares Repurchased and Retired

During the past several years, White Mountains’s board of directors authorized the Company to repurchase its common shares, from time to time, subject to market conditions. Shares may be repurchased on the open market or through privately negotiated transactions. The repurchase authorizations do not have a stated expiration date. As of December 31, 2023, White Mountains may repurchase an additional 301,014 shares under these board authorizations. In addition, from time to time White Mountains has also repurchased its common shares through tender offers that were separately authorized by its board of directors.

During 2023, the Company repurchased 24,165 common shares for $32.7 million at an average share price of $1,354.88, which included 19,536 common shares repurchased under the board authorizations for $26.2 million at an average share price of $1,340.79 and 4,629 common shares repurchased to satisfy employee income tax withholding pursuant to employee benefit plans.

During 2022, the Company repurchased 461,256 common shares for $615.8 million at an average share price of $1,335.11, which included 129,450 common shares repurchased under the board authorizations for $150.9 million at an average share price of $1,165.84 and 4,011 common shares repurchased to satisfy employee income tax withholding pursuant to employee benefit plans. In addition, on September 26, 2022, the Company completed a self-tender offer, through which it repurchased 327,795 common shares at a purchase price of $1,400.00 per share for a total cost of approximately $460.8 million, including expenses.

During 2021, the Company repurchased 98,511 common shares for $107.5 million at an average share price of $1,091.29, which was comprised of 91,293 common shares repurchased under the board authorizations for $100.0 million at an average share price of $1,095.37 and 7,218 common shares repurchased to satisfy employee income tax withholding pursuant to employee benefit plans.

Common Shares Issued

During 2023, the Company issued a total of 12,461 common shares, which consisted of 10,895 restricted shares issued to key personnel and 1,566 shares issued to directors of the Company.

During 2022, the Company issued a total of 15,640 common shares, which consisted of 13,225 restricted shares issued to key personnel and 2,415 shares issued to directors of the Company.

During 2021, the Company issued a total of 15,066 common shares, which consisted of 13,475 restricted shares issued to key personnel and 1,591 shares issued to directors of the Company.

Dividends on Common Shares

For the years ended December 31, 2023, 2022 and 2021, the Company declared and paid cash dividends totaling $2.6 million, $3.0 million and $3.1 million (or $1.00 per common share).

Noncontrolling Interests

Noncontrolling interests consist of the ownership interests of noncontrolling shareholders in consolidated entities and are presented separately on the balance sheet.

The following table presents the balance of noncontrolling interests included in White Mountains’s total equity and the related percentage of each consolidated entity’s total equity owned by noncontrolling shareholders as of December 31, 2023 and 2022:


	December 31, 2023				December 31, 2022
$ in Millions	Noncontrolling Percentage (1)		Noncontrolling Equity		Noncontrolling Percentage (1)		Noncontrolling Equity
Noncontrolling interests, excluding BAM
HG Global	3.1	%	$	.8	3.1	%	$	(.6)
Ark	28.0	%	336.9		28.0	%	247.9
Kudu	10.5	%	113.8		10.8	%	75.1



Other	various		9.4		various		20.4
Total, excluding BAM			460.9				342.8

BAM	100.0	%	(139.8)		100.0	%	(154.7)
Total noncontrolling interests			$	321.1			$	188.1

(1) The noncontrolling percentage represents the basic ownership interests held by noncontrolling shareholders with the exception of HG Global, for which the noncontrolling percentage represents the preferred share ownership held by noncontrolling shareholders.

- Definition

The entire disclosure for equity.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Section 50
-Paragraph 13
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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Reference 9: http://www.xbrl.org/2003/role/disclosureRef
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Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 11: http://www.xbrl.org/2003/role/disclosureRef
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-Subparagraph (c)
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-URI https://asc.fasb.org//1943274/2147481112/505-10-50-14

Reference 16: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-16

Reference 17: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-18

Reference 18: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-18

Reference 19: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-18

+ Details

Derivatives (Details) - Subsidiary - HG Global - Interest Rate Cap - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Jun. 16, 2022
Derivative [Line Items]
Notational amount			$ 150,000,000
Premiums paid			$ 3,300,000
Periodic settlement of interest cap	$ 2,000,000	$ 0
Derivative, fair value, net	2,900,000	4,100,000
Interest Expense
Derivative [Line Items]
Gain (loss) on fair value of derivative instruments	$ (1,200,000)	$ 800,000
Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Derivative [Line Items]
Derivative, cap interest rate			3.50%

Income Taxes - Income Tax (Benefit) Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Current income tax (expense) benefit:
U.S. federal	$ (12.6)	$ (16.9)	$ (4.8)
State	(2.2)	(4.5)	(2.1)
Non-U.S.	(16.6)	(7.1)	(2.8)
Total current income tax (expense) benefit	(31.4)	(28.5)	(9.7)
Deferred income tax (expense) benefit:
U.S. federal	(13.2)	(7.3)	(13.9)
State	(3.8)	(1.8)	(7.0)
Non-U.S.	63.9	(3.8)	(13.8)
Total deferred income tax (expense) benefit	46.9	(12.9)	(34.7)
Total income tax (expense) benefit	$ 15.5	$ (41.4)	$ (44.4)

Loss and Loss Adjustment Expense Reserves - Average Annual Percentage Payout of Incurred Losses and Allocated LAE by Age, Net of Reinsurance (Details) - Ark	Dec. 31, 2023
Property and Accident & Health
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
1	25.80%
2	35.50%
3	20.10%
4	6.10%
5	1.30%
6	1.10%
7	0.40%
8	0.30%
9	0.10%
10	0.00%
Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
1	20.50%
2	30.60%
3	9.70%
4	6.00%
5	7.70%
6	6.00%
7	1.20%
8	1.70%
9	3.10%
10	0.50%
Marine & Energy
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
1	14.20%
2	34.90%
3	19.40%
4	6.10%
5	4.30%
6	6.50%
7	0.30%
8	0.30%
9	0.30%
10	0.10%
Casualty-Active
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
1	4.80%
2	8.80%
3	14.20%
4	11.30%
5	8.80%
6	10.90%
7	6.10%
8	4.60%
9	2.00%
10	3.20%
Casualty-Runoff
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
1	8.60%
2	14.00%
3	15.50%
4	14.80%
5	8.60%
6	7.00%
7	5.50%
8	4.20%
9	3.00%
10	2.10%

Segment Information

12 Months Ended

Dec. 31, 2023

Segment Reporting [Abstract]

Segment Information

As of December 31, 2023, White Mountains conducted its operations through three reportable segments: (1) HG Global/BAM, (2) Ark/WM Outrigger, and (3) Kudu, with its remaining operating businesses, holding companies and other assets included in Other Operations. White Mountains has made its segment determination based on consideration of the following criteria: (i) the nature of the business activities of each of the Company’s subsidiaries and affiliates; (ii) the manner in which the Company’s subsidiaries and affiliates are organized; (iii) the existence of primary managers responsible for specific subsidiaries and affiliates; and (iv) the organization of information provided to the Company’s chief operating decision makers and its Board of Directors. Significant intercompany transactions among White Mountains’s segments have been eliminated herein.

During the fourth quarter of 2022, Ark sponsored the formation of Outrigger Re Ltd. to provide reinsurance protection on Ark’s Bermuda global property catastrophe excess of loss portfolio written in the 2023 underwriting year. During the fourth quarter of 2023, Ark renewed Outrigger Re Ltd. for the 2024 underwriting year. White Mountains consolidates its segregated account of Outrigger Re Ltd., WM Outrigger Re, in its financial statements. WM Outrigger Re’s quota share reinsurance agreement with GAIL eliminates in White Mountains’s consolidated financial statements. WM Outrigger Re exclusively provides reinsurance protection to Ark. Beginning in 2023, WM Outrigger Re was aggregated with Ark within the Ark/WM Outrigger segment. See Note 2 — “Significant Transactions.”

As a result of the NSM Transaction, the results of operations for NSM, previously reported as a segment, have been classified as discontinued operations in the statements of operations and comprehensive income through the closing of the transaction. See Note 20 — “Held for Sale and Discontinued Operations.”

Prior period amounts have been reclassified to conform to the current period’s presentation.

The following tables present the financial information for White Mountains’s segments:


	HG Global/BAM				Ark/WM Outrigger
Millions	HG Global		BAM (1)		Ark		WM Outrigger Re		Kudu		Other Operations		Total
Year Ended December 31, 2023
Earned insurance premiums (2)	$	26.0	$	5.2	$	1,305.4	$	104.3	$	—	$	—	$	1,440.9
Net investment income	17.1		14.6		50.4		11.0		71.0		30.1		194.2
Net investment income (expense) - BAM Surplus Note interest	26.2		(26.2)		—		—		—		—		—
Net realized and unrealized investment gains (losses)	13.6		13.0		85.9		—		106.1		188.5		407.1
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	—		—		—		—		—		27.1		27.1
Commission revenues	—		—		—		—		—		13.2		13.2
Other revenues	—		2.9		.8		—		—		80.5		84.2
Total revenues	82.9		9.5		1,442.5		115.3		177.1		339.4		2,166.7

Loss and loss adjustment expenses	—		—		711.2		15.6		—		—		726.8
Acquisition expenses	7.4		1.2		251.0		30.5		—		—		290.1

Cost of sales	—		—		—		—		—		40.4		40.4
General and administrative expenses	2.8		66.1		161.7		.3		19.4		182.3		432.6

Change in fair value of contingent consideration	—		—		48.7		—		—		—		48.7


Interest expense	16.5		—		21.3		—		21.2		3.7		62.7
Total expenses	26.7		67.3		1,193.9		46.4		40.6		226.4		1,601.3
Pre-tax income (loss)	$	56.2	$	(57.8)	$	248.6	$	68.9	$	136.5	$	113.0	$	565.4

(2) Ark’s earned insurance premiums based on the location of Ark’s underwriting offices in the United Kingdom and Bermuda are $791.7 and $513.7.


	HG Global/BAM				Ark/WM Outrigger				Other Operations
Millions	HG Global		BAM (1)		Ark		Kudu				Total
Year Ended December 31, 2022
Earned insurance premiums (2)	$	27.5	$	5.8	$	1,043.4	$	—	$	—	$	1,076.7
Net investment income	10.3		11.2		16.3		54.4		32.2		124.4
Net investment income (expense) - BAM Surplus Note interest	11.7		(11.7)		—		—		—		—
Net realized and unrealized investment gains (losses)	(52.5)		(53.3)		(55.2)		64.1		(1.6)		(98.5)
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	—		—		—		—		(93.0)		(93.0)
Commission revenues	—		—		—		—		11.5		11.5
Other revenues	.5		4.1		5.0		—		127.2		136.8
Total revenues	(2.5)		(43.9)		1,009.5		118.5		76.3		1,157.9
Losses and loss adjustment expenses	—		—		536.4		—		—		536.4
Acquisition expenses	9.3		1.9		239.4		—		—		250.6

Cost of sales	—		—		—		—		98.6		98.6
General and administrative expenses	2.8		66.3		106.2		14.7		174.1		364.1
Change in fair value of contingent consideration	—		—		17.3		—		—		17.3

Interest expense	8.3		—		15.1		15.0		1.9		40.3
Total expenses	20.4		68.2		914.4		29.7		274.6		1,307.3
Pre-tax income (loss)	$	(22.9)	$	(112.1)	$	95.1	$	88.8	$	(198.3)	$	(149.4)

(2) Ark’s earned insurance premiums based on the location of Ark’s underwriting offices in the United Kingdom and Bermuda are $638.5 and $404.9.


	HG Global/BAM				Ark/WM Outrigger				Other Operations
Millions	HG Global		BAM (1)		Ark		Kudu				Total
Year Ended December 31, 2021
Earned insurance premiums (2)	$	22.2	$	4.7	$	637.3	$	—	$	—	$	664.2
Net investment income	7.2		10.3		2.9		43.9		18.2		82.5
Net investment income (expense) - BAM Surplus Note interest	12.0		(12.0)		—		—		—		—
Net realized and unrealized investment gains (losses)	(13.7)		(9.2)		16.5		89.9		50.7		134.2
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	—		—		—		—		(380.3)		(380.3)

Commission revenues	—		—		—		—		9.6		9.6
Other revenues	.5		1.0		11.8		.2		90.7		104.2
Total revenues	28.2		(5.2)		668.5		134.0		(211.1)		614.4
Loss and loss adjustment expenses	—		—		314.8		—		—		314.8
Acquisition expenses	5.7		2.6		178.0		—		—		186.3
Cost of sales	—		—		—		—		69.3		69.3
General and administrative expenses	2.0		55.1		110.0		14.8		109.7		291.6
Change in fair value of contingent consideration	—		—		5.5		—		—		5.5
Interest expense	—		—		7.3		11.7		1.5		20.5
Total expenses	7.7		57.7		615.6		26.5		180.5		888.0
Pre-tax income (loss)	$	20.5	$	(62.9)	$	52.9	$	107.5	$	(391.6)	$	(273.6)

(2) Ark’s earned insurance premiums based on the location of Ark’s underwriting offices in the United Kingdom and Bermuda are $459.3 and $178.0.


Millions Selected Balance Sheet Data	HG Global/BAM			Ark		Kudu		Other Operations			Total
December 31, 2023:
Total investments	$	1,082.9		$	2,671.1	$	925.6	$	1,709.1		$	6,388.7


Total assets	$	1,172.0	(1)	$	4,378.6	$	959.4	$	1,875.9	(2)	$	8,385.9


Total liabilities	$	531.7	(2)	$	2,876.1	$	275.4	$	141.1		$	3,824.3
Total White Mountains’s common shareholders’ equity	$	779.3	(2)	$	1,165.6	$	570.2	$	1,725.4	(2)	$	4,240.5
Noncontrolling interest	$	(139.0)		$	336.9	$	113.8	$	9.4		$	321.1
December 31, 2022:
Total investments	$	975.8		$	1,965.6	$	695.9	$	1,532.7		$	5,170.0


Total assets	$	1,058.5	(1)	$	3,691.4	$	825.9	$	1,813.5	(2)	$	7,389.3


Total liabilities	$	501.8	(2)	$	2,522.1	$	273.3	$	157.1		$	3,454.3
Total White Mountains’s common shareholders’ equity	$	712.0	(2)	$	921.4	$	477.5	$	1,636.0	(2)	$	3,746.9
Noncontrolling interest	$	(155.3)		$	247.9	$	75.1	$	20.4		$	188.1

- Definition

The entire disclosure for reporting segments including data and tables. Reportable segments include those that meet any of the following quantitative thresholds a) it's reported revenue, including sales to external customers and intersegment sales or transfers is 10 percent or more of the combined revenue, internal and external, of all operating segments b) the absolute amount of its reported profit or loss is 10 percent or more of the greater, in absolute amount of 1) the combined reported profit of all operating segments that did not report a loss or 2) the combined reported loss of all operating segments that did report a loss c) its assets are 10 percent or more of the combined assets of all operating segments.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 22
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-22

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 15
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-15

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 31
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-31

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-32

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-42

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-32

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 40
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-40

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-Name Accounting Standards Codification
-Publisher FASB
-URI https://asc.fasb.org//280/tableOfContent

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 26
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-26

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-34

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 41
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-41

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 21
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-21

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 21
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-21

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-32

+ Details

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Statement of Comprehensive Income [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ 509.2	$ 792.8	$ (275.4)
Other comprehensive income (loss), net of tax	2.4	(3.8)	1.7
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	0.0	(5.2)	0.2
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	0.0	2.9	0.0
Comprehensive income (loss)	511.6	786.7	(273.5)
Other comprehensive (income) loss attributable to noncontrolling interests	(0.5)	0.9	0.2
Comprehensive income (loss) attributable to White Mountains’s common shareholders	$ 511.1	$ 787.6	$ (273.3)
Basic earnings (loss) per share
Continuing operations (usd per share)	$ 198.60	$ (38.34)	$ (88.52)
Discontinued operations (usd per share)	0	315.30	(0.94)
Total consolidated operations (usd per share)	198.60	276.96	(89.46)
Diluted earnings (loss) per share
Continuing operations (usd per share)	198.60	(38.34)	(88.52)
Discontinued operations (usd per share)	0	315.30	(0.94)
Total consolidated operations (usd per share)	198.60	276.96	(89.46)
Dividends declared per White Mountains’s common share (usd per share)	1.00	1.00	1.00
Dividends paid per White Mountains’s common share (usd per share)	$ 1.00	$ 1.00	$ 1.00

Investment Securities - Other Long-Term Investments (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Investment securities
Other long-term investments	$ 1,998.2	$ 1,488.0
Operating Segments \| Ark
Investment securities
Other long-term investments	440.9	373.6
Operating Segments \| Kudu
Investment securities
Other long-term investments	896.3	695.9
Other Operations
Investment securities
Other long-term investments	661.0	418.5
PassportCard/DavidShield
Investment securities
Other long-term investments	150.0	135.0
PassportCard/DavidShield \| Operating Segments \| Ark
Investment securities
Other long-term investments	0.0	0.0
PassportCard/DavidShield \| Operating Segments \| Kudu
Investment securities
Other long-term investments	0.0	0.0
PassportCard/DavidShield \| Other Operations
Investment securities
Other long-term investments	150.0	135.0
Elementum Holdings, L.P.
Investment securities
Other long-term investments	35.0	30.0
Elementum Holdings, L.P. \| Operating Segments \| Ark
Investment securities
Other long-term investments	0.0	0.0
Elementum Holdings, L.P. \| Operating Segments \| Kudu
Investment securities
Other long-term investments	0.0	0.0
Elementum Holdings, L.P. \| Other Operations
Investment securities
Other long-term investments	35.0	30.0
Other unconsolidated entities
Investment securities
Other long-term investments	48.1	37.2
Other unconsolidated entities \| Operating Segments \| Ark
Investment securities
Other long-term investments	0.0	0.0
Other unconsolidated entities \| Operating Segments \| Kudu
Investment securities
Other long-term investments	0.0	0.0
Other unconsolidated entities \| Other Operations
Investment securities
Other long-term investments	48.1	37.2
Private equity funds and hedge funds
Investment securities
Other long-term investments	312.9	197.8
Private equity funds and hedge funds \| Operating Segments \| Ark
Investment securities
Other long-term investments	55.5	40.4
Private equity funds and hedge funds \| Operating Segments \| Kudu
Investment securities
Other long-term investments	0.0	0.0
Private equity funds and hedge funds \| Other Operations
Investment securities
Other long-term investments	257.4	157.4
Bank loan fund
Investment securities
Other long-term investments	194.4	174.8
Bank loan fund \| Operating Segments \| Ark
Investment securities
Other long-term investments	194.4	174.8
Bank loan fund \| Operating Segments \| Kudu
Investment securities
Other long-term investments	0.0	0.0
Bank loan fund \| Other Operations
Investment securities
Other long-term investments	0.0	0.0
ILS funds
Investment securities
Other long-term investments	160.5	49.3
ILS funds \| Operating Segments \| Ark
Investment securities
Other long-term investments	0.0	0.0
ILS funds \| Operating Segments \| Kudu
Investment securities
Other long-term investments	0.0	0.0
ILS funds \| Other Operations
Investment securities
Other long-term investments	160.5	49.3
Lloyd’s trust deposits
Investment securities
Other long-term investments	158.0	137.4
Lloyd’s trust deposits \| Operating Segments \| Ark
Investment securities
Other long-term investments	158.0	137.4
Lloyd’s trust deposits \| Operating Segments \| Kudu
Investment securities
Other long-term investments	0.0	0.0
Lloyd’s trust deposits \| Other Operations
Investment securities
Other long-term investments	0.0	0.0
Private debt instruments
Investment securities
Other long-term investments	10.0	9.6
Private debt instruments \| Operating Segments \| Ark
Investment securities
Other long-term investments	0.0	0.0
Private debt instruments \| Operating Segments \| Kudu
Investment securities
Other long-term investments	0.0	0.0
Private debt instruments \| Other Operations
Investment securities
Other long-term investments	10.0	9.6
Other
Investment securities
Other long-term investments	38.8	21.0
Other \| Operating Segments \| Ark
Investment securities
Other long-term investments	33.0	21.0
Other \| Operating Segments \| Kudu
Investment securities
Other long-term investments	5.8	0.0
Other \| Other Operations
Investment securities
Other long-term investments	0.0	0.0
Kudu’s Participation Contracts \| Kudu
Investment securities
Other long-term investments	890.5	695.9
Kudu’s Participation Contracts \| Kudu \| Operating Segments \| Ark
Investment securities
Other long-term investments	0.0	0.0
Kudu’s Participation Contracts \| Kudu \| Operating Segments \| Kudu
Investment securities
Other long-term investments	890.5	695.9
Kudu’s Participation Contracts \| Kudu \| Other Operations
Investment securities
Other long-term investments	0.0	0.0
Investments, Excluding MediaAlpha \| Total unconsolidated entities
Investment securities
Other long-term investments	1,123.6	898.1
Investments, Excluding MediaAlpha \| Total unconsolidated entities \| Operating Segments \| Ark
Investment securities
Other long-term investments	0.0	0.0
Investments, Excluding MediaAlpha \| Total unconsolidated entities \| Operating Segments \| Kudu
Investment securities
Other long-term investments	890.5	695.9
Investments, Excluding MediaAlpha \| Total unconsolidated entities \| Other Operations
Investment securities
Other long-term investments	$ 233.1	$ 202.2

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Investments [Abstract]
Common equity securities, at fair value	$ 538.4	$ 668.4
Other long-term investments	1,998.2	1,488.0
Total investments	6,388.7	5,170.0
Goodwill and other intangible assets	370.6	392.4
Total assets	8,385.9	7,389.3
Liabilities
Debt	564.6	575.2
Total liabilities	3,824.3	3,454.3
White Mountains’s common shareholders’ equity
White Mountains’s common shares at $1 par value per share—authorized 50,000,000 shares; issued and outstanding 2,560,452 and 2,572,156 shares	2.6	2.6
Paid-in surplus	548.7	536.0
Retained earnings	3,690.8	3,211.8
Accumulated other comprehensive income (loss), after-tax:
Net unrealized gains (losses) from foreign currency translation and interest rate swap	(1.6)	(3.5)
Total White Mountains’s common shareholders’ equity	4,240.5	3,746.9
Noncontrolling interests	321.1	188.1
Total equity	4,561.6	3,935.0
Total liabilities and equity	8,385.9	7,389.3
Ark/WM Outrigger
Liabilities
Loss and loss adjustment expense reserves	1,605.1	1,296.5
Operating Segments \| HG Global/BAM - Segment
Investments [Abstract]
Fixed maturity investments, at fair value	1,012.3	909.9
Short-term investments, at fair value	70.6	65.9
Total investments	1,082.9	975.8
Cash	6.7	18.2
Insurance premiums receivable	5.5	6.6
Deferred acquisition costs	40.1	36.0
Other assets	36.8	21.9
Total assets	1,172.0	1,058.5
Liabilities
Unearned insurance premiums	325.8	298.3
Debt	146.9	146.5
Accrued incentive compensation	27.2	28.0
Other liabilities	31.8	29.0
Total liabilities	531.7	501.8
Accumulated other comprehensive income (loss), after-tax:
Total White Mountains’s common shareholders’ equity	779.3	712.0
Noncontrolling interests	(139.0)	(155.3)
Operating Segments \| Ark/WM Outrigger
Investments [Abstract]
Fixed maturity investments, at fair value	866.8	772.8
Common equity securities, at fair value	400.6	334.6
Short-term investments, at fair value	962.8	484.6
Other long-term investments	440.9	373.6
Total investments	2,671.1	1,965.6
Cash	90.5	101.5
Reinsurance recoverables	442.0	595.3
Insurance premiums receivable	612.2	544.1
Deferred acquisition costs	145.3	127.2
Goodwill and other intangible assets	292.5	292.5
Other assets	125.0	65.2
Total assets	4,378.6	3,691.4
Liabilities
Loss and loss adjustment expense reserves	1,605.1	1,296.5
Unearned insurance premiums	743.6	623.2
Debt	185.5	183.7
Reinsurance payable	81.1	251.1
Contingent consideration	94.0	45.3
Other liabilities	166.8	122.3
Total liabilities	2,876.1	2,522.1
Accumulated other comprehensive income (loss), after-tax:
Total White Mountains’s common shareholders’ equity	1,165.6	921.4
Noncontrolling interests	336.9	247.9
Operating Segments \| Kudu
Investments [Abstract]
Short-term investments, at fair value	29.3	0.0
Other long-term investments	896.3	695.9
Total investments	925.6	695.9
Cash	1.4	101.4
Accrued investment income	17.6	12.4
Goodwill and other intangible assets	8.3	8.6
Other assets	6.5	7.6
Total assets	959.4	825.9
Liabilities
Debt	203.8	208.3
Other liabilities	71.6	65.0
Total liabilities	275.4	273.3
Accumulated other comprehensive income (loss), after-tax:
Total White Mountains’s common shareholders’ equity	570.2	477.5
Noncontrolling interests	113.8	75.1
Other Operations
Investments [Abstract]
Fixed maturity investments, at fair value	230.2	238.2
Common equity securities, at fair value	137.8	333.8
Investment in MediaAlpha, at fair value	254.9	168.6
Short-term investments, at fair value	425.2	373.6
Other long-term investments	661.0	418.5
Total investments	1,709.1	1,532.7
Cash	23.8	33.9
Goodwill and other intangible assets	69.8	91.3
Other assets	73.2	155.6
Total assets	1,875.9	1,813.5
Liabilities
Debt	28.4	36.7
Accrued incentive compensation	87.7	86.1
Other liabilities	25.0	34.3
Total liabilities	141.1	157.1
Accumulated other comprehensive income (loss), after-tax:
Total White Mountains’s common shareholders’ equity	1,725.4	1,636.0
Noncontrolling interests	$ 9.4	$ 20.4

Municipal Bond Guarantee Insurance

12 Months Ended

Dec. 31, 2023

Insurance [Abstract]

Municipal Bond Guarantee Insurance

HG Global was established to fund the startup of BAM, a mutual municipal bond insurer. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of the BAM Surplus Notes.

Reinsurance Treaties

FLRT

BAM is a party to a first loss reinsurance treaty (“FLRT”) with HG Re under which HG Re provides first loss protection up to 15%-of-par outstanding on each municipal bond insured by BAM. For capital appreciation bonds, par is adjusted to the estimated equivalent par value for current interest paying bonds. In return, BAM cedes approximately 60% of the risk premium charged for insuring the municipal bond, which is net of a ceding commission. The FLRT is a perpetual agreement with terms that can be renegotiated after a specified period of time. During 2021, BAM and HG Re agreed that the terms may be renegotiated at the end of 2024 and each subsequent five-year period thereafter.

Fidus Re

BAM is party to a collateralized financial guarantee excess of loss reinsurance agreement that serves to increase BAM’s claims paying resources and is provided by Fidus Re.

In 2018, Fidus Re was initially capitalized by the issuance of $100.0 million of insurance-linked securities (the “Fidus Re 2018 Agreement”), which have an initial term of 12 years and are callable five years after the date of issuance. The proceeds from issuance were placed in a collateral trust supporting Fidus Re’s obligations to BAM. Under the Fidus Re 2018 Agreement, Fidus Re reinsures 90% of aggregate losses exceeding $165.0 million on a portion of BAM’s financial guarantee portfolio (the “2018 Covered Portfolio”) up to a total reimbursement of $100.0 million. The Fidus Re 2018 Agreement does not provide coverage for losses in excess of $276.1 million. The 2018 Covered Portfolio consists of approximately 23% of BAM’s gross par outstanding as of December 31, 2023.

In 2021, Fidus Re issued an additional $150.0 million of insurance linked securities (the “Fidus Re 2021 Agreement”) which have an initial term of 12 years and are callable five years after the date of issuance. The proceeds from issuance were placed in a collateral trust supporting Fidus Re’s obligations to BAM. Under the Fidus Re 2021 Agreement, Fidus Re reinsures 90% of aggregate losses exceeding $135.0 million on a portion of BAM’s financial guarantee portfolio (the “2021 Covered Portfolio”) up to a total reimbursement of $150.0 million. The Fidus Re 2021 Agreement does not provide coverage for losses in excess of $301.7 million. The 2021 Covered Portfolio consists of approximately 27% of BAM’s gross par outstanding as of December 31, 2023.

In 2022, Fidus Re issued an additional $150.0 million of insurance linked securities (the “Fidus Re 2022 Agreement”) which have an initial term of 12 years and are callable seven years after the date of issuance. The proceeds from issuance were placed in a collateral trust supporting Fidus Re’s obligations to BAM. Under the Fidus Re 2022 Agreement, Fidus Re reinsures 90% of aggregate losses exceeding $110.0 million on a portion of BAM’s financial guarantee portfolio (the “2022 Covered Portfolio”) up to a total reimbursement of $150.0 million. The Fidus Re 2022 Agreement does not provide coverage for losses in excess of $276.7 million. The 2022 Covered Portfolio consists of approximately 29% of BAM’s gross par outstanding as of December 31, 2023.

The Fidus Re agreements are accounted for using deposit accounting, and any related financing expenses are recorded in general and administrative expenses as they do not meet the risk transfer requirements necessary to be accounted for as reinsurance.

XOLT

In January 2020, BAM entered into the XOLT with HG Re. Under the XOLT, HG Re provides last dollar protection for exposures on municipal bonds insured by BAM in excess of the New York State Department of Financial Services (“NYDFS”) single issuer limits. As of December 31, 2023, the XOLT is subject to an aggregate limit equal to the lesser of $125.0 million or the assets held in the supplemental collateral trust (the “Supplemental Trust”) at any point in time. The agreement is accounted for using deposit accounting, and any related financing expenses are recorded in general and administrative expenses as the agreement does not meet the risk transfer requirements necessary to be accounted for as reinsurance.

Collateral Trusts

HG Re’s obligations under the FLRT are subject to an aggregate limit equal to the assets in two collateral trusts: the Supplemental Trust and the Regulation 114 Trust (together, the “Collateral Trusts”) at any point in time.

On a monthly basis, BAM deposits cash equal to ceded premiums, net of ceding commissions, due to HG Re under the FLRT directly into the Regulation 114 Trust. The Regulation 114 Trust target balance is equal to HG Re’s unearned premiums and unpaid loss and LAE reserves, if any. If, at the end of any quarter, the Regulation 114 Trust balance is below the target balance, funds will be withdrawn from the Supplemental Trust and deposited into the Regulation 114 Trust in an amount equal to the shortfall. If, at the end of any quarter, the Regulation 114 Trust balance is above 102% of the target balance, funds will be withdrawn from the Regulation 114 Trust and deposited into the Supplemental Trust. The Regulation 114 Trust balance as of December 31, 2023 and 2022 was $341.6 million and $288.6 million.

The Supplemental Trust target balance is $603.0 million, less the amount of cash and securities in the Regulation 114 Trust in excess of its target balance (the “Supplemental Trust Target Balance”). If, at the end of any quarter, the Supplemental Trust balance exceeds the Supplemental Trust Target Balance, such excess may be distributed to HG Re. The distribution will be made first as an assignment of accrued interest on the BAM Surplus Notes and second in cash and/or fixed income securities.

As the BAM Surplus Notes are repaid over time, the BAM Surplus Notes will be replaced in the Supplemental Trust by cash and fixed income securities. The Supplemental Trust balance as of December 31, 2023 and 2022 was $606.9 million and $568.3 million.

As of December 31, 2023 and 2022, the Collateral Trusts held assets of $948.5 million and $856.9 million, which included $588.6 million and $503.3 million of cash, investments and accrued investment income, $322.2 million and $340.0 million of BAM Surplus Notes and $37.7 million and $13.6 million of interest receivable on the BAM Surplus Notes.

BAM Surplus Notes

Through 2024, the interest rate on the BAM Surplus Notes is a variable rate equal to the one-year U.S. Treasury rate plus 300 basis points, set annually. Beginning in 2025, the rate will be fixed through maturity at the higher of the 2024 variable rate or 8.0%. Accordingly, beginning in 2024 and through maturity, the interest rate on the BAM Surplus Notes will be 8.2%. Under its agreements with HG Global, BAM is required to seek regulatory approval to pay principal and interest on the BAM Surplus Notes only to the extent that its remaining qualified statutory capital and other capital resources continue to support its outstanding obligations, its business plan and its “AA/stable” rating from Standard & Poor’s. No payment of principal or interest on the BAM Surplus Notes may be made without the approval of the NYDFS.

As of December 31, 2023 and 2022, the principal balance on the BAM Surplus Notes was $322.2 million and $340.0 million and total interest receivable on the BAM Surplus Notes was $174.5 million and $157.9 million.

Insured Obligations and Premiums

The following table presents a schedule of BAM’s insured obligations as of December 31, 2023 and 2022:


	December 31,
	2023		2022
Contracts outstanding	14,485		13,382
Remaining weighted average contract period (in years)	11.1		10.8
Contractual debt service outstanding (in millions):
Principal	$	109,673.8	$	99,996.9
Interest	56,012.2		48,880.6
Total debt service outstanding	$	165,686.0	$	148,877.5

Gross unearned insurance premiums (in millions)	$	325.8	$	298.3

The following table presents a schedule of BAM’s future premium revenues as of December 31, 2023:


Millions	December 31, 2023
January 1, 2024 - March 31, 2024	$	7.5
April 1, 2024 - June 30, 2024	7.5
July 1, 2024 - September 30, 2024	7.3
October 1, 2024 - December 31, 2024	7.2
	29.5
2025	27.7
2026	26.0
2027	24.4
2028	22.6
2029 and thereafter	195.6
Total gross unearned insurance premiums	$	325.8

The following table presents a schedule of written premiums and earned premiums included in the HG Global/BAM segment for the years ended December 31, 2023, 2022 and 2021:


	December 31,
Millions	2023		2022		2021
Written premiums:
Direct	$	58.6	$	63.8	$	51.0
Assumed	—		1.3		4.6
Gross written premiums (1)	$	58.6	$	65.1	$	55.6
Earned premiums:
Direct	$	28.7	$	28.6	$	23.2
Assumed	2.5		4.7		3.7
Gross earned premiums (1)	$	31.2	$	33.3	$	26.9

(1) There are no ceded premium amounts in the periods presented, and gross earned premiums are equivalent to net written premiums and net earned premiums.

In September 2022, BAM entered into a 100% facultative quota share reinsurance agreement under which it assumed a portfolio of municipal bond guarantee contracts with a par value of $42.5 million. During 2023, the contracts associated with this assumed reinsurance transaction were novated.

In January 2021, BAM entered into a 100% facultative quota share reinsurance agreement under which it assumed a portfolio of municipal bond guarantee contracts with a par value of $805.5 million.

None of the contracts assumed under these reinsurance agreements were non-performing, and no loss reserves have been established for any of the contracts, either as of the transaction dates or as of December 31, 2023. The agreements, which cover future claims exposure only, meet the risk transfer criteria under ASC 944-20, Insurance Activities and accordingly have been accounted for as reinsurance.

Employee Share-Based Incentive Compensation Plans - Performance Shares Granted Under the WTM Incentive Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	37,595
Accrued Expense	$ 70.5
Assumed forfeitures (in shares)	(564)
Assumed forfeitures	$ (1.1)
Total target performance shares outstanding (in shares)	37,031	39,449	40,828	42,458
Total	$ 69.4	$ 67.5	$ 42.2	$ 56.3
2023 – 2025
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	10,895
Accrued Expense	$ 6.0
2022 – 2024
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,225
Accrued Expense	$ 25.8
2023 – 2025
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,475
Accrued Expense	$ 38.7

Held for Sale and Discontinued Operations - Narrative (Details) - Sirius Group - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2018	Dec. 31, 2020
Discontinued Operations
Net gain (loss) from sale of discontinued operations, net of tax	$ 0.0	$ 0.0	$ 18.7
Discontinued Operations, Disposed of by Sale
Discontinued Operations
Net gain (loss) from sale of discontinued operations, net of tax			17.6	$ (17.3)
Liabilities held for sale - NSM Group					$ 18.7
Net (income) loss from discontinued operations attributable to non-controlling interests			$ 1.1

Debt - Schedule of the Change in Debt under the Kudu Facility (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Debt Instrument
Beginning balance	$ 575.2
Borrowings	12.4	$ 213.8	$ 401.7
Repayments	(25.7)	(56.7)	(120.1)
Ending balance	564.6	575.2
Kudu Credit Facility
Debt Instrument
Beginning balance	208.3
Ending balance	203.8	208.3
Kudu Credit Facility \| Secured Debt
Debt Instrument
Beginning balance	215.2	225.4	0.0
Borrowings	12.0	35.0	232.0
Repayments	(16.9)	(45.2)	(6.6)
Ending balance	$ 210.3	$ 215.2	$ 225.4

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 835
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-Name Accounting Standards Codification
-Section 50
-Paragraph 1
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-Paragraph 1
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

- Definition

Amount, after deduction of unamortized premium (discount) and debt issuance cost, of long-term debt. Excludes lease obligation.

+ References + Details

Earnings Per Share - Schedule of Earnings Per Share from Continuing Operations (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 509.2	$ 792.8	$ (275.4)
Less: total income (loss) from discontinued operations, net of tax	0.0	903.2	(3.9)
Less: net (income) loss from discontinued operations attributable to noncontrolling interests	0.0	(0.7)	1.0
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	509.2	(109.7)	(272.5)
Allocation of losses (earnings) to participating restricted common shares	(7.2)	1.3	3.2
Basic earnings (losses) per share numerators	502.0	(108.4)	(269.3)
Diluted earnings (losses) per share numerators	$ 502.0	$ (108.4)	$ (269.3)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	2,563,300	2,862,400	3,079,000
Average unvested restricted common shares (in shares)	(36,000.0)	(36,200)	(36,500)
Basic earnings (losses) per share denominator (in shares)	2,527,300	2,826,200	3,042,500
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)	2,563,300	2,862,400	3,079,000
Average unvested restricted common shares (in shares)	(36,000.0)	(36,200)	(36,500)
Diluted earnings (loss) per share denominator (in shares)	2,527,300	2,826,200	3,042,500
Basic and diluted earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid (in usd per share)	$ 1.00	$ 1.00	$ 1.00
Distributed earnings - dividends declared and paid (in usd per share)	1.00	1.00	1.00
Undistributed earnings (losses) (in usd per share)	197.60	(39.34)	(89.52)
Undistributed earnings (losses) (in usd per share)	197.60	(39.34)	(89.52)
Continuing operations (usd per share)	198.60	(38.34)	(88.52)
Continuing operations (usd per share)	$ 198.60	$ (38.34)	$ (88.52)

Third-Party Reinsurance

12 Months Ended

Dec. 31, 2023

Reinsurance Disclosures [Abstract]

Third-Party Reinsurance

P&C Insurance and Reinsurance

In the normal course of business, Ark may seek to limit losses that arise from catastrophes or other events by reinsuring certain risks with third-party reinsurers. Ark remains liable for risks reinsured in the event that the reinsurer does not honor its obligations under reinsurance contracts. The following table summarizes the effects of reinsurance on written and earned premiums and on losses and LAE for the Ark/WM Outrigger segment for the years ended December 31, 2023, 2022 and 2021.


	Year Ended December 31,
Millions	2023			2022		2021
Written premiums:
Direct	$	931.9		$	760.4	$	619.5
Assumed	966.5			691.6		439.2
Gross written premiums	1,898.4			1,452.0		1,058.7
Ceded	(377.5)		(1)	(256.8)		(199.6)
Net written premiums	$	1,520.9		$	1,195.2	$	859.1
Earned premiums:
Direct	$	848.1		$	655.5	$	556.0
Assumed	925.6			668.7		330.4
Gross earned premiums	1,773.7			1,324.2		886.4
Ceded	(364.0)		(2)	(280.8)		(249.1)
Net earned premiums	$	1,409.7		$	1,043.4	$	637.3
Losses and LAE:
Gross	$	782.5		$	814.9	$	442.9
Ceded	(55.7)		(3)	(278.5)		(128.1)
Net losses and LAE	$	726.8		$	536.4	$	314.8

(1) The ceded written premiums exclude $110.0 ceded by Ark to WM Outrigger Re for the year ended December 31, 2023, which eliminate in White Mountains’s consolidated financial statements.

(2) The ceded earned premiums exclude $104.3 ceded by Ark to WM Outrigger Re for the year ended December 31, 2023, which eliminate in White Mountains’s consolidated financial statements.

(3) The ceded loss and LAE exclude $15.6 ceded by Ark to WM Outrigger Re for the year ended December 31, 2023, which eliminate in White Mountains’s consolidated financial statements.

The following table presents the Ark/WM Outrigger segment’s reinsurance recoverables as of December 31, 2023 and December 31, 2022:


Millions	December 31, 2023			December 31, 2022
Reinsurance recoverables on unpaid losses	$	340.8	(1)	$	505.0	(3)
Reinsurance recoverables on paid losses	27.4			31.1
Ceded unearned premiums	73.8		(2)	59.2
Reinsurance recoverables	$	442.0		$	595.3

(1) The reinsurance recoverables on unpaid losses exclude $15.6 ceded by Ark to WM Outrigger Re as of December 31, 2023, which eliminate in White Mountains’s consolidated financial statements.

(2) The ceded unearned premiums exclude $5.7 ceded by Ark to WM Outrigger Re as of December 31, 2023, which eliminate in White

Mountains’s consolidated financial statements.

(3) The reinsurance recoverables on unpaid losses include $145.4 attributable to TPC Providers as of December 31, 2022, which are collateralized.

As reinsurance contracts do not relieve Ark of its obligation to its policyholders, Ark seeks to reduce the credit risk associated with reinsurance balances by avoiding over-reliance on specific reinsurers through the application of concentration limits and thresholds. Ark is selective with its reinsurers, placing reinsurance with only those reinsurers having a strong financial condition. Ark monitors the financial strength of its reinsurers on an ongoing basis.

The following table presents the Ark/WM Outrigger segment’s gross and net reinsurance recoverables by each reinsurer’s A.M. Best Company, Inc (“A.M. Best”) rating as of December 31, 2023:


$ in Millions	As of December 31, 2023
A.M. Best Rating (1)	Gross			Collateral			Net		% of Total
A+ or better	$	204.0		$	—		$	204.0	71.0	%
A- to A	76.2			—			76.2		26.5
B++ or lower and not rated	88.0		(2)	80.7		(2)	7.3		2.5
Total	$	368.2		$	80.7		$	287.5	100.0	%

(1) A.M. Best ratings as detailed above are: “A+ or better” (Superior) “A- to A” (Excellent), “B++” (Good).

(2) Excludes $15.6 ceded by Ark to WM Outrigger Re as of December 31, 2023, which eliminates in White Mountains’s consolidated financial statements.

See Note 10 — “Municipal Bond Guarantee Insurance” for third-party reinsurance balances related to White Mountains’s financial guarantee business.

Reinsurance Contracts Accounted for as Deposits

Ark has an aggregate excess of loss contract with SiriusPoint, which is accounted for using the deposit method and recorded within other assets. Ark earns an annual crediting rate of 3.0%, which is recorded within other revenue. During 2021, Ark negotiated a reduction of $31.7 million, including accrued interest, to the aggregate excess of loss contract with SiriusPoint. As of December 31, 2023 and December 31, 2022, the carrying value of Ark’s deposit in SiriusPoint, including accrued interest, was $20.4 million.

See Note 10 — “Municipal Bond Guarantee Insurance” for reinsurance contracts accounted for as deposits related to White Mountains’s financial guarantee business.

Audit Information	12 Months Ended
Audit Information	Dec. 31, 2023
Audit Information [Abstract]
Auditor Firm ID	238
Auditor Name	PricewaterhouseCoopers LLP
Auditor Location	Atlanta, Georgia

Investment Securities - Investments In Private Equity Funds Subject to Lock-Up Periods (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Investment securities
1 – 3 years	$ 4.5
3 – 5 years	71.0
5 – 10 years	155.4
>10 years	17.7
Total	1,998.2	$ 1,488.0
Private equity funds
Investment securities
Total	$ 248.6

Fair Value of Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2023

Fair Value Disclosures [Abstract]

Schedule of the fair value and carrying value of financial instruments

The following table presents the fair value and carrying value of these financial instruments as of December 31, 2023 and 2022:


	December 31, 2023				December 31, 2022
Millions	Fair Value		Carrying Value		Fair Value		Carrying Value
HG Global Senior Notes	$	158.7	$	146.9	$	155.7	$	146.5
Ark 2007 Subordinated Notes	$	30.5	$	30.0	$	28.4	$	30.0
Ark 2021 Subordinated Notes	$	171.8	$	155.5	$	163.1	$	153.7
Kudu Credit Facility	$	225.6	$	203.8	$	223.9	$	208.3
Other Operations debt	$	30.0	$	28.4	$	38.2	$	36.7

Significant Transactions - Ark (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
	Oct. 01, 2020	Mar. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jan. 01, 2021	Dec. 31, 2020
Other Significant Noncash Transactions [Line Items]
Cash pre-funded/placed in escrow for Ark Transaction								$ 646.3
Change in fair value of contingent consideration - Ark			$ 48.7	$ 17.3	$ 5.5
Other Operations
Other Significant Noncash Transactions [Line Items]
Cash pre-funded/placed in escrow for Ark Transaction								366.3
Change in fair value of contingent consideration - Ark			$ 0.0	0.0	0.0
Ark
Other Significant Noncash Transactions [Line Items]
Basic ownership							72.00%
Ownership percentage, after equity incentives							63.00%
Percent of shares outstanding			12.50%
Subordinated Debt \| Ark 2021 Subordinated Notes
Other Significant Noncash Transactions [Line Items]
Long-term line of credit						$ 163.3
Ark \| Lloyd
Other Significant Noncash Transactions [Line Items]
Cash pre-funded/placed in escrow for Ark Transaction								$ 280.0
Ark
Other Significant Noncash Transactions [Line Items]
Merger agreement, equity capital contributed	$ 605.4
Equity capital, pre money valuation amount	300.0
Additional equity purchase from shareholders	40.9
Investment, potential additional contribution of equity capital	$ 200.0						$ 200.0
Percent of shares outstanding			12.50%
Assets acquired							2,500.0
Goodwill and intangible assets acquired							292.5
Liabilities acquired							1,700.0
Contingent consideration			$ 94.0	45.3			22.5
Noncontrolling interest acquired							$ 220.2
Transaction costs		$ 25.3
Amortization			0.0	7.5	64.2
Change in fair value of contingent consideration - Ark			$ 48.7	$ 17.3	$ 5.5

Common Shareholders’ Equity and Noncontrolling Interests - Noncontrolling Interests (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Noncontrolling Interest [Line Items]
Noncontrolling Equity	$ 321.1	$ 188.1
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling Equity	0.8	(0.6)
Ark
Noncontrolling Interest [Line Items]
Noncontrolling Equity	336.9	247.9
Subsidiary - Kudu Investment Management, LLC
Noncontrolling Interest [Line Items]
Noncontrolling Equity	113.8	75.1
Other
Noncontrolling Interest [Line Items]
Noncontrolling Equity	9.4	20.4
Total, excluding BAM
Noncontrolling Interest [Line Items]
Noncontrolling Equity	460.9	342.8
Variable Interest Entity, Primary Beneficiary
Noncontrolling Interest [Line Items]
Noncontrolling Equity	$ (139.8)	$ (154.7)
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	3.10%	3.10%
Ark
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	28.00%	28.00%
Subsidiary - Kudu Investment Management, LLC
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	10.50%	10.80%
Build America Mutual Assurance Company
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	100.00%	100.00%

Goodwill and Other Intangible Assets - Amortization Expense (Details) $ in Millions	Dec. 31, 2023 USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
2024	$ 5.5
2025	4.8
2026	4.0
2027	3.6
2028 and years after	8.2
Other intangible assets, finite-lived, net carrying value	26.1
Indefinite-lived intangible assets	$ 175.7

Employee Share-Based Incentive Compensation Plans (Tables)

12 Months Ended

Dec. 31, 2023

Share-Based Payment Arrangement [Abstract]

Schedule of performance share activity

The following table presents performance share activity for the years ended December 31, 2023, 2022 and 2021 for performance shares granted under the WTM Incentive Plan:


	Year Ended December 31,
	2023			2022			2021
$ in Millions	Target Performance Shares Outstanding	Accrued Expense		Target Performance Shares Outstanding	Accrued Expense		Target Performance Shares Outstanding	Accrued Expense
Beginning of period	39,449	$	67.5	40,828	$	42.2	42,458	$	56.3
Shares paid or expired (1)	(13,350)	(40.8)		(14,625)	(26.4)		(14,336)	(35.2)
New grants	10,895	—		13,225	—		13,475	—
Forfeitures and cancellations (2)	37	(.1)		21	(.4)		(769)	.4

Expense recognized	—	42.8		—	52.1		—	20.7
End of period	37,031	$	69.4	39,449	$	67.5	40,828	$	42.2

(1)WTM performance share payments in 2023 for the 2020-2022 performance cycle, which were paid in March 2023 at 200% of target.

WTM performance share payments in 2022 for the 2019-2021 performance cycle, which were paid in March 2022 at 172% of target.

WTM performance share payments in 2021 for the 2018-2020 performance cycle, which were paid in March 2021 at 200% of target.

(2)Amounts include changes in assumed forfeitures, as required under GAAP.

Schedule of performance shares outstanding and accrued expense for performance shares awarded

The following table presents performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan as of December 31, 2023 for each performance cycle:


	December 31, 2023
$ in Millions	Target Performance Shares Outstanding	Accrued Expense
Performance cycle:
2023 – 2025	10,895	$	6.0
2022 – 2024	13,225	25.8
2021 – 2023	13,475	38.7
Sub-total	37,595	70.5
Assumed forfeitures	(564)	(1.1)
Total	37,031	$	69.4

Schedule of unrecognized compensation costs associated with outstanding restricted share awards

The following table presents the unrecognized compensation cost associated with the outstanding restricted share awards under the WTM Incentive Plan for the years ended December 31, 2023, 2022 and 2021:


	Year Ended December 31,
	2023			2022			2021
$ in Millions	Restricted Shares	Unamortized Issue Date Fair Value		Restricted Shares	Unamortized Issue Date Fair Value		Restricted Shares	Unamortized Issue Date Fair Value
Non-vested:
Beginning of period	38,350	$	15.5	37,850	$	15.9	43,105	$	15.2
Issued	10,895	16.0		13,225	13.8		13,475	16.1
Vested	(11,650)	—		(12,725)	—		(17,936)	—
Forfeited	—	—		—	—		(794)	(.8)

Expense recognized	—	(15.3)		—	(14.2)		—	(14.6)
End of period	37,595	$	16.2	38,350	$	15.5	37,850	$	15.9

Investment Securities

12 Months Ended

Dec. 31, 2023

Investments, Debt and Equity Securities [Abstract]

Investment Securities

White Mountains’s portfolio of investment securities held for general investment purposes consists of fixed maturity investments, short-term investments, common equity securities, its investment in MediaAlpha and other long-term investments. White Mountains’s portfolio of fixed maturity investments, including those within short-term investments, are classified as trading securities. Trading securities are reported at fair value as of the balance sheet date. Short-term investments also include interest-bearing money market funds and certificates of deposit that are carried at fair value. White Mountains’s portfolio of common equity securities, its investment in MediaAlpha and other long-term investments are measured at fair value. Other long-term investments consist primarily of unconsolidated entities, Kudu’s Participation Contracts, private equity funds and hedge funds, a bank loan fund, Lloyd’s trust deposits, ILS funds and private debt instruments. White Mountains has taken the fair value option for its equity method eligible investments. See Note 16 — “Equity Method Eligible Investments”. Net realized and unrealized investment gains (losses) are reported in pre-tax revenues.

Net Investment Income

White Mountains’s net investment income is comprised primarily of interest income associated with White Mountains’s fixed maturity investments and short-term investments, dividend income from common equity securities and distributions from other long-term investments.

The following table presents pre-tax net investment income for the years ended December 31, 2023, 2022 and 2021:


	Year Ended December 31,
Millions	2023		2022		2021
Fixed maturity investments	$	64.4	$	41.4	$	28.8
Short-term investments	52.0		16.4		.6
Common equity securities	4.1		1.6		.1

Other long-term investments	76.1		68.1		56.3
Amount attributable to TPC Providers	—		(1.0)		(1.0)
Total investment income	196.6		126.5		84.8
Third-party investment expenses	(2.4)		(2.1)		(2.3)
Net investment income, pre-tax	$	194.2	$	124.4	$	82.5

Net Realized and Unrealized Investment Gains (Losses)

The following table presents net realized and unrealized investment gains (losses) for the years ended December 31, 2023, 2022 and 2021:


	Year Ended December 31,
Millions	2023		2022		2021
Realized investment gains (losses)
Fixed maturity investments	$	(4.7)	$	(11.9)	$	3.9
Short-term investments	(.2)		(1.4)		(.1)
Common equity securities	29.5		—		.4
Investment in MediaAlpha	—		—		160.3
Other long-term investments	72.2		128.4		(7.7)
Net realized investment gains (losses)	96.8		115.1		156.8

Unrealized investment gains (losses)
Fixed maturity investments	66.1		(168.4)		(42.2)
Short-term investments	1.9		(1.1)		—
Common equity securities	47.1		(7.1)		14.9
Investment in MediaAlpha	27.1		(93.0)		(540.6)
Other long-term investments	195.2		(43.8)		172.7
Net unrealized investment gains (losses)	337.4		(313.4)		(395.2)
Net realized and unrealized investment gains (losses), before amount attributable to TPC providers (1)	434.2		(198.3)		(238.4)
Amount attributable to TPC Providers	—		6.8		(7.7)
Net realized and unrealized investment gains (losses)	$	434.2	$	(191.5)	$	(246.1)

Fixed maturity and short-term investments
Net realized and unrealized investment gains (losses)	$	63.1	$	(182.8)	$	(38.4)
Less: net realized and unrealized gains (losses) on investment securities sold during the period	4.7		(2.9)		(8.4)
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	$	58.4	$	(179.9)	$	(30.0)

Common equity securities and investment in MediaAlpha
Net realized and unrealized investment gains (losses) on common equity securities	$	76.6	$	(7.1)	$	15.3
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	27.1		(93.0)		(380.3)
Total net realized and unrealized investment gains (losses)	103.7		(100.1)		(365.0)
Less: net realized and unrealized gains (losses) on investment securities sold during the period	28.4		—		20.3
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	$	75.3	$	(100.1)	$	(385.3)

(1) For 2023, 2022 and 2021, includes $13.6, $(29.3) and $(7.7) of net realized and unrealized investment gains (losses) related to foreign currency exchange.

For the years ended December 31, 2023, 2022 and 2021, all of White Mountains’s net realized and unrealized investment gains (losses) were recorded in the consolidated statements of operations. There were no investment gains (losses) recorded in other comprehensive income.

White Mountains recognized gross realized investment gains of $113.7 million, $129.9 million and $212.3 million and gross realized investment losses of $16.9 million, $14.8 million and $55.5 million on sales of investment securities for the years ending December 31, 2023, 2022 and 2021.

The following table presents total net unrealized gains (losses) attributable to Level 3 investments for the years ended December 31, 2023, 2022 and 2021 for investments still held at the end of the period:


	Year Ended December 31,
Millions	2023		2022		2021

Total net unrealized investment gains on other long-term investments held at the end of period, pre-tax	$	133.9	$	56.5	$	98.9

Proceeds from the sales and maturities of investments, excluding short-term investments, totaled $0.8 billion, $0.5 billion and $0.8 billion for the years ended December 31, 2023, 2022 and 2021.

Investment Holdings

The following tables present the cost or amortized cost, gross unrealized investment gains (losses), net foreign currency gains (losses) and carrying value of White Mountains’s fixed maturity investments as of December 31, 2023 and 2022:


	December 31, 2023
Millions	Cost or Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Net Foreign Currency Gains (Losses)		Carrying Value
U.S. Government and agency obligations	$	209.0	$	.3	$	(4.4)	$	—	$	204.9
Debt securities issued by corporations	1,085.9		5.6		(45.4)		(1.1)		1,045.0
Municipal obligations	275.1		.9		(15.1)		—		260.9
Mortgage and asset-backed securities	417.2		2.0		(29.8)		—		389.4
Collateralized loan obligations	211.2		.4		(2.4)		(.1)		209.1
Total fixed maturity investments	$	2,198.4	$	9.2	$	(97.1)	$	(1.2)	$	2,109.3


	December 31, 2022
Millions	Cost or Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Net Foreign Currency Gains (Losses)		Carrying Value
U.S. Government and agency obligations	$	216.6	$	—	$	(10.2)	$	—	$	206.4
Debt securities issued by corporations	1,098.3		.6		(78.3)		(1.8)		1,018.8
Municipal obligations	281.6		.4		(23.4)		—		258.6
Mortgage and asset-backed securities	288.7		—		(34.5)		—		254.2
Collateralized loan obligations	190.8		.1		(6.0)		(2.0)		182.9


Total fixed maturity investments	$	2,076.0	$	1.1	$	(152.4)	$	(3.8)	$	1,920.9

The weighted average duration of White Mountains’s fixed income portfolio was 1.9 years, including short-term investments, and 3.3 years, excluding short-term investments, as of December 31, 2023.

The following table presents the cost or amortized cost and carrying value of White Mountains’s fixed maturity investments by contractual maturity as of December 31, 2023. Actual maturities could differ from contractual maturities because borrowers may have the right to call or prepay certain obligations with or without call or prepayment penalties.


	December 31, 2023
Millions	Cost or Amortized Cost		Carrying Value
Due in one year or less	$	319.9	$	315.0
Due after one year through five years	845.0		813.9
Due after five years through ten years	303.4		287.0
Due after ten years	101.7		94.9
Mortgage and asset-backed securities and collateralized loan obligations	628.4		598.5

Total fixed maturity investments	$	2,198.4	$	2,109.3

The following tables present the cost or amortized cost, gross unrealized investment gains (losses), net foreign currency gains (losses) and carrying value of common equity securities, White Mountains’s investment in MediaAlpha and other long-term investments as of December 31, 2023 and 2022:


	December 31, 2023
Millions	Cost or Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Net Foreign Currency Gains (Losses)		Carrying Value

Common equity securities	$	483.5	$	62.3	$	(1.2)	$	(6.2)	$	538.4
Investment in MediaAlpha	$	59.2	$	195.7	$	—	$	—	$	254.9


Other long-term investments	$	1,655.7	$	446.3	$	(94.5)	$	(9.3)	$	1,998.2


	December 31, 2022
Millions	Cost or Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Net Foreign Currency Gains (Losses)		Carrying Value
Common equity securities	$	660.6	$	26.7	$	(8.4)	$	(10.5)	$	668.4
Investment in MediaAlpha	$	—	$	168.6	$	—	$	—	$	168.6

Other long-term investments	$	1,340.8	$	271.1	$	(107.1)	$	(16.8)	$	1,488.0

Fair Value Measurements by Level

The following tables present White Mountains’s fair value measurements for investments as of December 31, 2023 and 2022 by level. See Note 1 — “Basis of Presentation and Significant Accounting Policies”. The major security types were based on the legal form of the securities. White Mountains has disaggregated its fixed maturity investments based on the issuing entity type, which impacts credit quality, with debt securities issued by U.S. government entities carrying minimal credit risk, while the credit and other risks associated with other issuers, such as corporations, municipalities or entities issuing mortgage and asset-backed securities vary depending on the nature of the issuing entity type. White Mountains further disaggregates debt securities issued by corporations by industry sector because investors often reference commonly used benchmarks and their subsectors to monitor risk and performance. Accordingly, White Mountains has further disaggregated this asset class into subclasses based on the similar sectors and industry classifications it uses to evaluate investment risk and performance against commonly used benchmarks, such as the Bloomberg Barclays U.S. Intermediate Aggregate.


	December 31, 2023
Millions	Fair Value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	204.9	$	204.9	$	—	$	—

Debt securities issued by corporations:
Financials	309.8		—		309.8		—
Consumer	214.6		—		214.6		—
Industrial	121.6		—		121.6		—
Healthcare	121.3		—		121.3		—
Technology	108.9		—		108.9		—
Utilities	70.2		—		70.2		—
Communications	46.3		—		46.3		—
Energy	28.9		—		28.9		—
Materials	23.4		—		23.4		—

Total debt securities issued by corporations	1,045.0		—		1,045.0		—

Municipal obligations	260.9		—		260.9		—
Mortgage and asset-backed securities	389.4		—		389.4		—

Collateralized loan obligations	209.1		—		209.1		—
Total fixed maturity investments	2,109.3		204.9		1,904.4		—

Short-term investments	1,487.9		1,457.6		30.3		—
Common equity securities:

Exchange-traded funds	137.8		137.8		—		—
Other (1)	400.6		—		400.6		—
Total common equity securities	538.4		137.8		400.6		—
Investment in MediaAlpha	254.9		254.9		—		—












Other long-term investments	1,164.4		—		26.2		1,138.2
Other long-term investments — NAV (2)	833.8		—		—		—
Total other long-term investments	1,998.2		—		26.2		1,138.2
Total investments	$	6,388.7	$	2,055.2	$	2,361.5	$	1,138.2

(1) Consists of investments in listed funds that predominantly invest in international equities.

(2) Consists of private equity funds and hedge funds, a bank loan fund, Lloyd’s trust deposits and ILS funds for which fair value is measured using NAV as a practical expedient. Investments for which fair value is measured at NAV are not classified within the fair value hierarchy.


	December 31, 2022
Millions	Fair Value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	206.4	$	206.4	$	—	$	—

Debt securities issued by corporations:
Financials	291.2		—		291.2		—
Consumer	191.9		—		191.9		—
Industrial	115.4		—		115.4		—
Healthcare	121.3		—		121.3		—
Technology	123.7		—		123.7		—
Utilities	73.8		—		73.8		—
Communications	47.9		—		47.9		—
Energy	33.9		—		33.9		—
Materials	19.7		—		19.7		—

Total debt securities issued by corporations	1,018.8		—		1,018.8		—

Municipal obligations	258.6		—		258.6		—
Mortgage and asset-backed securities	254.2		—		254.2		—
Collateralized loan obligations	182.9		—		182.9		—


Total fixed maturity investments	1,920.9		206.4		1,714.5		—

Short-term investments	924.1		924.1		—		—
Common equity securities:

Exchange-traded funds	333.8		333.8		—		—
Other (1)	334.6		—		334.6		—
Total common equity securities	668.4		333.8		334.6		—
Investment in MediaAlpha	168.6		168.6		—		—
















Other long-term investments	926.4		—		14.8		911.6
Other long-term investments — NAV (2)	561.6		—		—		—
Total other long-term investments	1,488.0		—		14.8		911.6
Total investments	$	5,170.0	$	1,632.9	$	2,063.9	$	911.6

(1) Consists of investments in listed funds that predominantly invest in international equities.

Investments Held on Deposit or as Collateral

As of December 31, 2023 and 2022, investments of $585.6 million and $500.5 million were held in trusts required to be maintained in relation to HG Global’s reinsurance agreements with BAM.

HG Global is required to maintain an interest reserve account in connection with its senior notes issued in 2022. As of December 31, 2023 and 2022, the fair value of the interest reserve account, which is included in short-term investments, was $30.4 million and $31.2 million. See Note 7 — “Debt”.

BAM is required to maintain deposits with certain insurance regulatory agencies in order to maintain its insurance licenses. The fair value of such deposits, which represent state deposits and are included within the investment portfolio, totaled $4.6 million as of both December 31, 2023 and 2022.

Lloyd’s trust deposits are generally required of Lloyd's syndicates to protect policyholders in non-U.K. markets and are pledged into Lloyd’s trust accounts to provide a portion of the capital needed to support obligations at Lloyd’s. As of December 31, 2023 and 2022, Ark held Lloyd’s trust deposits with a fair value of $158.0 million and $137.4 million.

The underwriting capacity of a member of Lloyd’s must be supported by providing a deposit (“Funds at Lloyd’s”) in the form of cash, securities or letters of credit in an amount determined by Lloyd’s. The amount of such deposit is calculated for each member through the completion of an annual capital adequacy exercise. These requirements allow Lloyd’s to evaluate that each member has sufficient assets to meet its underwriting liabilities plus a required solvency margin. As of December 31, 2023 and 2022, the fair value of Ark’s Funds at Lloyd’s investment deposits totaled $344.7 million and $319.2 million.

As of December 31, 2023 and 2022, Ark held additional investments on deposit or as collateral for insurance regulators and reinsurance counterparties of $244.5 million and $257.0 million.

As of December 31, 2023 and 2022, Ark has $198.9 million and $90.3 million of short-term investments pledged as collateral under uncommitted standby letters of credit. See Note 7 — “Debt”.

As of December 31, 2023 and 2022, short-term investments of $265.3 million and $203.7 million were held in a collateral trust account required to be maintained in relation to WM Outrigger Re’s reinsurance agreement with GAIL.

Kudu is required to maintain an interest reserve account in connection with its credit facility. As of December 31, 2023, the interest reserve account of $14.9 million was held in short term investments. As of December 31, 2022 the interest reserve account of $12.2 million was held in restricted cash. See Note 7 - “Debt.”

Debt Securities Issued by Corporations

The following table presents the credit ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of December 31, 2023 and 2022:


	Fair Value as of December 31,
Millions	2023		2022
AAA	$	11.5	$	11.3
AA	83.8		96.0
A	552.4		567.9
BBB	390.9		337.7


Other	6.4		5.9
Debt securities issued by corporations (1)	$	1,045.0	$	1,018.8

(1) Credit ratings are based upon issuer credit ratings provided by Standard & Poor’s Financial Services LLC (“Standard & Poor’s”), or if unrated by Standard & Poor’s, long-term obligation ratings provided by Moody’s Investor Service, Inc.

Mortgage and Asset-backed Securities and Collateralized Loan Obligations

The following table presents the fair value of White Mountains’s mortgage and asset-backed securities and collateralized loan obligations as of December 31, 2023 and 2022:


	December 31, 2023						December 31, 2022
Millions	Fair Value		Level 2		Level 3		Fair Value		Level 2		Level 3
Mortgage-backed securities:
Agency:
FNMA	$	198.8	$	198.8	$	—	$	124.5	$	124.5	$	—
FHLMC	137.2		137.2		—		78.8		78.8		—
GNMA	30.2		30.2		—		28.3		28.3		—
Total agency (1)	366.2		366.2		—		231.6		231.6		—
Non-agency: Residential	.2		.2		—		.3		.3		—

Total non-agency	.2		.2		—		.3		.3		—
Total mortgage-backed securities	366.4		366.4		—		231.9		231.9		—
Other asset-backed securities:
Credit card receivables	3.3		3.3		—		11.9		11.9		—
Vehicle receivables	17.2		17.2		—		10.4		10.4		—
Other	2.5		2.5		—		—		—		—
Total other asset-backed securities	23.0		23.0		—		22.3		22.3		—
Total mortgage and asset-backed securities	389.4		389.4		—		254.2		254.2		—
Collateralized loan obligations:	209.1		209.1		—		182.9		182.9		—
Total mortgage and asset-backed securities and collateralized loan obligations	$	598.5	$	598.5	$	—	$	437.1	$	437.1	$	—

(1) Represents publicly traded mortgage-backed securities which carry the full faith and credit guaranty of the U.S. Government (i.e., GNMA) or are guaranteed by a government sponsored entity (i.e., FNMA, FHLMC).

As of December 31, 2023, White Mountains’s investment portfolio included $209.1 million of collateralized loan obligations that are within the senior tranches of their respective fund securitization structures. All of White Mountains’s collateral loan obligations were rated AAA or AA as of December 31, 2023.

Investment in MediaAlpha

Following the MediaAlpha IPO, White Mountains’s investment in MediaAlpha is accounted for at fair value based on the publicly traded share price of MediaAlpha’s common stock and is presented as a separate line item on the balance sheet.

During the second quarter of 2023, White Mountains completed a tender offer to purchase 5.9 million additional shares of MediaAlpha at a purchase price of $10.00 per share. As of December 31, 2023, White Mountains owned 22.9 million shares, representing a 34.9% basic ownership interest (33.1% on a fully-diluted/fully-converted basis). See Note 16 — “Equity Method Eligible Investments.” At White Mountains’s current level of ownership, each $1.00 per share increase or decrease in the share price of MediaAlpha will result in an approximate $9.00 per share increase or decrease in White Mountains’s book value per share. At the December 31, 2023 closing price of $11.15 per share, the fair value of White Mountains’s investment in MediaAlpha was $254.9 million.

Other Long-Term Investments


	Fair Value as of December 31, 2023
Millions	Ark/WM Outrigger		Kudu		Other		Total
Kudu’s Participation Contracts	$	—	$	890.5	$	—	$	890.5
PassportCard/DavidShield	—		—		150.0		150.0
Elementum Holdings, L.P.	—		—		35.0		35.0
Other unconsolidated entities (1)	—		—		48.1		48.1





Total unconsolidated entities	—		890.5		233.1		1,123.6
Private equity funds and hedge funds	55.5		—		257.4		312.9
Bank loan fund	194.4		—		—		194.4
ILS funds	—		—		160.5		160.5
Lloyd’s trust deposits	158.0		—		—		158.0
Private debt instruments	—		—		10.0		10.0
Other	33.0		5.8		—		38.8
Total other long-term investments	$	440.9	$	896.3	$	661.0	$	1,998.2

(1) Includes White Mountains’s noncontrolling equity interests in certain preferred securities, private common equity securities, limited liability company units and Simple Agreement for Future Equity (“SAFE”) investments.


	Fair Value as of December 31, 2022
Millions	Ark/WM Outrigger		Kudu		Other		Total
Kudu’s Participation Contracts	$	—	$	695.9	$	—	$	695.9
PassportCard/DavidShield	—		—		135.0		135.0
Elementum Holdings, L.P.	—		—		30.0		30.0
Other unconsolidated entities (1)	—		—		37.2		37.2





Total unconsolidated entities	—		695.9		202.2		898.1
Private equity funds and hedge funds	40.4		—		157.4		197.8
Bank loan fund	174.8		—		—		174.8
ILS funds	—		—		49.3		49.3
Lloyd’s trust deposits	137.4		—		—		137.4
Private debt instruments	—		—		9.6		9.6
Other	21.0		—		—		21.0
Total other long-term investments	$	373.6	$	695.9	$	418.5	$	1,488.0

(1) Includes White Mountains’s noncontrolling equity interests in certain preferred securities, private common equity securities, limited liability company units and SAFE.

Private Equity Funds and Hedge Funds

White Mountains invests in private equity funds and hedge funds, which are included in other long-term investments. The fair value of these investments is generally estimated using the NAV of the funds. As of December 31, 2023, White Mountains held investments in 16 private equity funds and two hedge funds. The largest investment in a single private equity fund or hedge fund was $57.3 million and $49.0 million as of December 31, 2023 and December 31, 2022.

The following table presents the fair value of investments and unfunded commitments in private equity funds and hedge funds by investment objective and sector as of December 31, 2023 and 2022:


	December 31, 2023				December 31, 2022
Millions	Fair Value		Unfunded Commitments		Fair Value		Unfunded Commitments

Private equity funds
Aerospace/Defense/Government	$	157.2	$	17.4	$	59.4	$	37.5
Financial services	87.5		37.8		77.1		54.3
Real estate	3.9		2.5		4.1		2.5



Total private equity funds	248.6		57.7		140.6		94.3

Hedge funds
Long/short equity financials and business services	54.4		—		49.0		—
European small/mid cap	9.9		—		8.2		—
Total hedge funds	64.3		—		57.2		—
Total private equity funds and hedge funds included in other long-term investments	$	312.9	$	57.7	$	197.8	$	94.3

Investments in private equity funds are generally subject to a lock-up period during which investors may not request a redemption. Distributions prior to the expected termination date of the fund may be limited to dividends or proceeds arising from the liquidation of the fund’s underlying investments. In addition, certain private equity funds have the option to extend the lock-up period.

The following table presents investments in private equity funds that were subject to lock-up periods as of December 31, 2023:


Millions	1 – 3 years		3 – 5 years		5 – 10 years		>10 years		Total
Private equity funds — expected lock-up period remaining	$	4.5	$	71.0	$	155.4	$	17.7	$	248.6

Investors in private equity funds are generally subject to indemnification obligations outside of the capital commitment period and prior to the winding up of the fund. As of December 31, 2023 and 2022, White Mountains is not aware of any indemnification claims relating to its investments in private equity funds.

Redemption of investments in most hedge funds is subject to restrictions, including lock-up periods where no redemptions or withdrawals are allowed, restrictions on redemption frequency and advance notice periods for redemptions. Amounts requested for redemptions remain subject to market fluctuations until the redemption effective date, which generally falls at the end of the defined redemption period. Advance notice requirements for redemptions from White Mountains’s hedge fund investments range from 45 to 90 days. One of White Mountains’s hedge fund investments also limits redemptions to every second anniversary following the date of the initial investment.

Bank Loan Fund

White Mountains’s other long-term investments include a bank loan fund with a fair value of $194.4 million and $174.8 million as of December 31, 2023 and 2022. The fair value of this investment is estimated using the NAV of the fund. The bank loan fund’s investment objective is to provide, on an unleveraged basis, high current income consistent with preservation of capital and low duration. The bank loan fund primarily invests in a broad portfolio of U.S. dollar-denominated, non-investment grade, floating-rate senior secured loans and may invest in other financial instruments, such as secured and unsecured corporate debt, credit default swaps, reverse repurchase agreements, synthetic indices and cash and cash equivalents.

The investment in the bank loan fund is subject to restrictions on redemption frequency and advance notice periods for redemptions. Amounts requested for redemptions remain subject to market fluctuations until the redemption effective date, which generally falls at the end of the defined redemption period. White Mountains may redeem all or a portion of its bank loan fund investment as of any calendar month-end upon 15 calendar days advanced written notice.

Lloyd’s Trust Deposits

White Mountains’s other long-term investments include Lloyd’s trust deposits, which consist of non-U.K. deposits and Canadian comingled pooled funds. The Lloyd’s trust deposits invest primarily in short-term government securities, agency securities and corporate bonds held in trusts that are managed by Lloyd's of London. These investments are generally required of Lloyd's syndicates to protect policyholders in non-U.K. markets and are pledged into Lloyd’s trust accounts to provide a portion of the capital needed to support obligations at Lloyd’s. The fair value of the Lloyd’s trust deposits is generally estimated using the NAV of the funds. As of December 31, 2023 and 2022, White Mountains held Lloyd’s trust deposits with a fair value of $158.0 million and $137.4 million.

Insurance-Linked Securities Funds

White Mountains’s other long-term investments include ILS fund investments. The fair value of these investments is generally estimated using the NAV of the funds. As of December 31, 2023 and 2022, White Mountains held investments in ILS funds with a fair value of $160.5 million and $49.3 million.

Investments in ILS funds are generally subject to restrictions, including lock-up periods where no redemptions or withdrawals are allowed, non-renewal clauses, restrictions on redemption frequency and advance notice periods for redemptions. From time to time, natural catastrophe, liquidity, market or other events will occur that make the determination of fair value for underlying investments in ILS funds less certain due to the potential for loss development. In such circumstances, the impacted investments may be subject to additional lock-up provisions.

ILS funds are typically subject to monthly and annual restrictions on redemptions and advance redemption notice period requirements that range between 30 and 90 days. Amounts requested for redemption remain subject to market fluctuations until the redemption effective date, which is generally at the end of the defined redemption period or when the underlying investment has fully matured or been commuted.

Rollforward of Level 3 Investments

Level 3 measurements as of December 31, 2023 and 2022 consist of securities for which the estimated fair value has not been determined based upon quoted market price inputs for identical or similar securities. The following table presents the changes in White Mountains’s fair value measurements for Level 3 investments for the years ended December 31, 2023 and 2022:


Level 3 Investments
Millions	Other Long-term Investments			Other Long-term Investments
Balance as of December 31, 2022	$	911.6	Balance as of December 31, 2021	$	890.6
Net realized and unrealized gains	157.3		Net realized and unrealized gains	60.4

Purchases and contributions	256.6		Purchases and contributions	129.8
Sales and distributions	(187.3)		Sales and distributions	(169.2)

Transfers in	—		Transfers in	—
Transfers out	—		Transfers out	—
Balance as of December 31, 2023	$	1,138.2	Balance as of December 31, 2022	$	911.6

Fair Value Measurements — Transfers Between Levels - For Years Ended December 31, 2023 and 2022

Transfers between levels are recorded using the fair value measurement as of the end of the quarterly period in which the event or change in circumstance giving rise to the transfer occurred.

During 2023 and 2022, there were no fixed maturity investments or other long-term investments classified as Level 3 measurements in the prior period that were transferred to Level 2 measurements.

During 2023 and 2022, there were no fixed maturity investments or other long-term investments classified as Level 2 measurements in the prior period that were transferred to Level 3 measurements.

Significant Unobservable Inputs

The following tables present significant unobservable inputs used in estimating the fair value of White Mountains’s other long-term investments classified within Level 3 as of December 31, 2023 and 2022. The tables below exclude $19.0 million and $41.1 million of Level 3 other long-term investments generally valued based on recent or expected transaction prices. The fair value of investments in private equity funds and hedge funds, bank loan funds, Lloyd’s trust deposits and ILS funds are generally estimated using the NAV of the funds.

$ in Millions

December 31, 2023

Description

Valuation Technique(s) (1)

Fair Value (2)

Unobservable Inputs

Discount Rate (6)

Terminal Cash Flow Exit Multiple (x) or Terminal Revenue Growth Rate (%) (6)

Kudu’s Participation Contracts (3)(4)(5)

Discounted cash flow

$890.5

18% - 25%

7x - 22x

PassportCard/DavidShield

Discounted cash flow

$150.0

24%

Elementum Holdings, L.P.

Discounted cash flow

$35.0

21%

Preferred securities

Discounted cash flow

$28.2

N/A

Private debt instruments

Discounted cash flow

$15.5

11% - 12%

N/A

(1) Key inputs to the discounted cash flow analysis generally include projections of future revenue and earnings, discount rates and terminal exit multiples or growth rates.

(2) Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs.

(3) Since Kudu’s Participation Contracts are not subject to corporate taxes within Kudu Investment Management, LLC, pre-tax discount rates are applied to pre-tax cash flows in determining fair values. The weighted average discount rate and weighted average terminal cash flow exit multiple applied to Kudu’s Participation Contracts was 20% and 13x.

(4) In 2023, Kudu deployed a total of $163.8 into new and existing Participation Contracts.

(5) As of December 31, 2023, one of Kudu’s Participation Contracts with a total fair value of $69.1 was valued using a probability weighted expected return method, which takes into account factors such as a discounted cash flow analysis, the expected value to be received in a pending sales transaction and the likelihood that a sales transaction will take place.

(6) Increases (decreases) to the discount rates in isolation would result in lower (higher) fair value measurements, while increases (decreases) to the terminal cash flow exit multiples or terminal revenue growth rates in isolation would result in higher (lower) fair value measurements.

$ in Millions

December 31, 2022

Description

Valuation Technique(s) (1)

Fair Value (2)

Unobservable Inputs

Discount Rate (6)

Terminal Cash Flow Exit Multiple (x) or Terminal Revenue Growth Rate (%) (6)

Kudu’s Participation Contracts (3)(4)(5)

Discounted cash flow

$695.9

18% - 25%

7x - 16x

PassportCard/DavidShield

Discounted cash flow

$135.0

24%

Elementum Holdings, L.P.

Discounted cash flow

$30.0

21%

Private debt instruments

Discounted cash flow

$9.6

11%

N/A

(1) Key inputs to the discounted cash flow analysis generally include projections of future revenue and earnings, discount rates and terminal exit multiples or growth rates.

(2) Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs.

(4) In 2022, Kudu deployed a total of $99.8 into new and existing Participation Contracts.

(5) As of December 31, 2022, two of Kudu’s Participation Contracts with a total fair value of $189.0 were valued using a probability weighted expected return method, which takes into account factors such as a discounted cash flow analysis, the expected value to be received in a pending sales transaction and the likelihood that a sales transaction will take place.

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Statement of Cash Flows [Abstract]
Cash sold in subsidiary transaction	$ 0.8	$ 144.4	$ 0.0
Cash acquired from acquisition	0.0	0.3	52.2
Restricted cash balance	0.7	18.3	4.5
Discontinued operations cash balance	$ 0.0	$ 0.0	$ 111.6

Goodwill and Other Intangible Assets - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization of other intangible assets	$ 7.2	$ 5.2	$ 4.6

Employee Share-Based Incentive Compensation Plans

12 Months Ended

Dec. 31, 2023

Share-Based Payment Arrangement [Abstract]

Employee Share-Based Incentive Compensation Plans

White Mountains’s share-based incentive compensation plans are designed to incentivize key employees to maximize shareholder value over long periods of time. White Mountains believes that this is best pursued by utilizing a pay-for-performance program that closely aligns the financial interests of management with those of its shareholders while rewarding appropriate risk taking. White Mountains accomplishes this by emphasizing variable long-term compensation that is contingent on performance over a number of years rather than fixed entitlements. White Mountains expenses all its share-based compensation. As a result, White Mountains’s calculation of its owners’ returns includes the expense of all outstanding share-based compensation awards.

The WTM Incentive Plan provides for grants of various types of share-based and non-share-based incentive awards to key employees and directors of White Mountains. As of December 31, 2023 and 2022, White Mountains’ share-based incentive compensation awards consist of performance shares and restricted shares.

Performance Shares

Performance shares are designed to reward employees for meeting company-wide performance targets. Performance shares are conditional grants of a specified maximum number of common shares or an equivalent amount of cash. Awards generally vest at the end of a three-year service period, are subject to the attainment of pre-specified performance goals, and are valued based on the market value of common shares at the time awards are paid. Performance shares earned under the WTM Incentive Plan are typically paid in cash but may be paid in common shares. Compensation expense is recognized for the vested portion of the awards over the related service periods. The level of payout ranges from zero to two times the number of shares initially granted, depending on White Mountains’s financial performance. Performance shares become payable at the conclusion of a performance cycle (typically three years) if pre-defined financial targets are met. The performance measures used for determining performance share payouts are growth in White Mountains’s adjusted book value per share and intrinsic value per share. Intrinsic value per share is generally calculated by adjusting adjusted book value per share for differences between the adjusted book value of certain assets and liabilities and White Mountains’s estimate of their underlying intrinsic values.

The following table presents performance share activity for the years ended December 31, 2023, 2022 and 2021 for performance shares granted under the WTM Incentive Plan:


	Year Ended December 31,
	2023			2022			2021
$ in Millions	Target Performance Shares Outstanding	Accrued Expense		Target Performance Shares Outstanding	Accrued Expense		Target Performance Shares Outstanding	Accrued Expense
Beginning of period	39,449	$	67.5	40,828	$	42.2	42,458	$	56.3
Shares paid or expired (1)	(13,350)	(40.8)		(14,625)	(26.4)		(14,336)	(35.2)
New grants	10,895	—		13,225	—		13,475	—
Forfeitures and cancellations (2)	37	(.1)		21	(.4)		(769)	.4

Expense recognized	—	42.8		—	52.1		—	20.7
End of period	37,031	$	69.4	39,449	$	67.5	40,828	$	42.2

(1)WTM performance share payments in 2023 for the 2020-2022 performance cycle, which were paid in March 2023 at 200% of target.

WTM performance share payments in 2022 for the 2019-2021 performance cycle, which were paid in March 2022 at 172% of target.

WTM performance share payments in 2021 for the 2018-2020 performance cycle, which were paid in March 2021 at 200% of target.

(2)Amounts include changes in assumed forfeitures, as required under GAAP.

During 2023, White Mountains granted 10,895 performance shares for the 2023-2025 performance cycle. During 2022, White Mountains granted 13,225 performance shares for the 2022-2024 performance cycle. During 2021, White Mountains granted 13,475 performance shares for the 2021-2023 performance cycle.

For the 2020-2022 performance cycle, all performance shares earned were settled in cash. For the 2019-2021 performance cycle, the Company issued common shares for 750 performance shares earned, and all other performance shares earned were settled in cash. For the 2018-2020 performance cycle, all performance shares earned were settled in cash. If all outstanding performance shares had vested on December 31, 2023, the total additional compensation cost to be recognized would have been $27.9 million, based on accrual factors as of December 31, 2023 (common share price and payout assumptions).

The following table presents performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan as of December 31, 2023 for each performance cycle:


	December 31, 2023
$ in Millions	Target Performance Shares Outstanding	Accrued Expense
Performance cycle:
2023 – 2025	10,895	$	6.0
2022 – 2024	13,225	25.8
2021 – 2023	13,475	38.7
Sub-total	37,595	70.5
Assumed forfeitures	(564)	(1.1)
Total	37,031	$	69.4

For the 2023-2025 performance cycle, the targeted performance goal for full payment of outstanding performance shares granted under the WTM Incentive Plan is 11% average annual growth in adjusted book value per share and intrinsic value per share. Average annual growth of 6% or less would result in no payout, and average annual growth of 16% or more would result in a payout of 200%.

For the 2022-2024 performance cycle, the targeted performance goal for full payment of outstanding performance shares granted under the WTM Incentive Plan is 9% average annual growth in adjusted book value per share and intrinsic value per share. Average annual growth of 4% or less would result in no payout, and average annual growth of 14% or more would result in a payout of 200%.

For the 2021-2023 performance cycle, the targeted performance goal for full payment of outstanding performance shares granted under the WTM Incentive Plan is 8% average annual growth in adjusted book value per share and intrinsic value per share. Average annual growth of 3% or less would result in no payout, and average annual growth of 13% or more would result in a payout of 200%.

Restricted Shares

Restricted shares are grants of a specified number of common shares that generally vest at the end of a 34-month service period. The following table presents the unrecognized compensation cost associated with the outstanding restricted share awards under the WTM Incentive Plan for the years ended December 31, 2023, 2022 and 2021:


	Year Ended December 31,
	2023			2022			2021
$ in Millions	Restricted Shares	Unamortized Issue Date Fair Value		Restricted Shares	Unamortized Issue Date Fair Value		Restricted Shares	Unamortized Issue Date Fair Value
Non-vested:
Beginning of period	38,350	$	15.5	37,850	$	15.9	43,105	$	15.2
Issued	10,895	16.0		13,225	13.8		13,475	16.1
Vested	(11,650)	—		(12,725)	—		(17,936)	—
Forfeited	—	—		—	—		(794)	(.8)

Expense recognized	—	(15.3)		—	(14.2)		—	(14.6)
End of period	37,595	$	16.2	38,350	$	15.5	37,850	$	15.9

During 2023, White Mountains issued 10,895 restricted shares that vest on January 1, 2026. During 2022, White Mountains issued 13,225 restricted shares that vest on January 1, 2025. During 2021, White Mountains issued 13,475 restricted shares that vest on January 1, 2024. The unamortized issue date fair value as of December 31, 2023 is expected to be recognized ratably over the remaining vesting periods.

Municipal Bond Guarantee Insurance (Tables)

12 Months Ended

Dec. 31, 2023

Insurance [Abstract]

Schedule of insured obligations

The following table presents a schedule of BAM’s insured obligations as of December 31, 2023 and 2022:


	December 31,
	2023		2022
Contracts outstanding	14,485		13,382
Remaining weighted average contract period (in years)	11.1		10.8
Contractual debt service outstanding (in millions):
Principal	$	109,673.8	$	99,996.9
Interest	56,012.2		48,880.6
Total debt service outstanding	$	165,686.0	$	148,877.5

Gross unearned insurance premiums (in millions)	$	325.8	$	298.3

Schedule of BAM’s future premium revenues

The following table presents a schedule of BAM’s future premium revenues as of December 31, 2023:


Millions	December 31, 2023
January 1, 2024 - March 31, 2024	$	7.5
April 1, 2024 - June 30, 2024	7.5
July 1, 2024 - September 30, 2024	7.3
October 1, 2024 - December 31, 2024	7.2
	29.5
2025	27.7
2026	26.0
2027	24.4
2028	22.6
2029 and thereafter	195.6
Total gross unearned insurance premiums	$	325.8

Effects of reinsurance

The following table summarizes the effects of reinsurance on written and earned premiums and on losses and LAE for the Ark/WM Outrigger segment for the years ended December 31, 2023, 2022 and 2021.


	Year Ended December 31,
Millions	2023			2022		2021
Written premiums:
Direct	$	931.9		$	760.4	$	619.5
Assumed	966.5			691.6		439.2
Gross written premiums	1,898.4			1,452.0		1,058.7
Ceded	(377.5)		(1)	(256.8)		(199.6)
Net written premiums	$	1,520.9		$	1,195.2	$	859.1
Earned premiums:
Direct	$	848.1		$	655.5	$	556.0
Assumed	925.6			668.7		330.4
Gross earned premiums	1,773.7			1,324.2		886.4
Ceded	(364.0)		(2)	(280.8)		(249.1)
Net earned premiums	$	1,409.7		$	1,043.4	$	637.3
Losses and LAE:
Gross	$	782.5		$	814.9	$	442.9
Ceded	(55.7)		(3)	(278.5)		(128.1)
Net losses and LAE	$	726.8		$	536.4	$	314.8

(1) The ceded written premiums exclude $110.0 ceded by Ark to WM Outrigger Re for the year ended December 31, 2023, which eliminate in White Mountains’s consolidated financial statements.

(2) The ceded earned premiums exclude $104.3 ceded by Ark to WM Outrigger Re for the year ended December 31, 2023, which eliminate in White Mountains’s consolidated financial statements.

(3) The ceded loss and LAE exclude $15.6 ceded by Ark to WM Outrigger Re for the year ended December 31, 2023, which eliminate in White Mountains’s consolidated financial statements.

The following table presents the Ark/WM Outrigger segment’s reinsurance recoverables as of December 31, 2023 and December 31, 2022:


Millions	December 31, 2023			December 31, 2022
Reinsurance recoverables on unpaid losses	$	340.8	(1)	$	505.0	(3)
Reinsurance recoverables on paid losses	27.4			31.1
Ceded unearned premiums	73.8		(2)	59.2
Reinsurance recoverables	$	442.0		$	595.3

(1) The reinsurance recoverables on unpaid losses exclude $15.6 ceded by Ark to WM Outrigger Re as of December 31, 2023, which eliminate in White Mountains’s consolidated financial statements.

(2) The ceded unearned premiums exclude $5.7 ceded by Ark to WM Outrigger Re as of December 31, 2023, which eliminate in White

Mountains’s consolidated financial statements.

(3) The reinsurance recoverables on unpaid losses include $145.4 attributable to TPC Providers as of December 31, 2022, which are collateralized.

The following table presents a schedule of written premiums and earned premiums included in the HG Global/BAM segment for the years ended December 31, 2023, 2022 and 2021:


	December 31,
Millions	2023		2022		2021
Written premiums:
Direct	$	58.6	$	63.8	$	51.0
Assumed	—		1.3		4.6
Gross written premiums (1)	$	58.6	$	65.1	$	55.6
Earned premiums:
Direct	$	28.7	$	28.6	$	23.2
Assumed	2.5		4.7		3.7
Gross earned premiums (1)	$	31.2	$	33.3	$	26.9

(1) There are no ceded premium amounts in the periods presented, and gross earned premiums are equivalent to net written premiums and net earned premiums.

Common Shareholders’ Equity and Noncontrolling Interests (Tables)

12 Months Ended

Dec. 31, 2023

Equity [Abstract]

Schedule of noncontrolling interest


	December 31, 2023				December 31, 2022
$ in Millions	Noncontrolling Percentage (1)		Noncontrolling Equity		Noncontrolling Percentage (1)		Noncontrolling Equity
Noncontrolling interests, excluding BAM
HG Global	3.1	%	$	.8	3.1	%	$	(.6)
Ark	28.0	%	336.9		28.0	%	247.9
Kudu	10.5	%	113.8		10.8	%	75.1



Other	various		9.4		various		20.4
Total, excluding BAM			460.9				342.8

BAM	100.0	%	(139.8)		100.0	%	(154.7)
Total noncontrolling interests			$	321.1			$	188.1

Investment Securities - Rollforward of Fair Value Measurements by Level (Details) - Other long-term investments - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 911.6	$ 890.6
Net realized and unrealized gains	157.3	60.4
Purchases and contributions	256.6	129.8
Sales and distributions	(187.3)	(169.2)
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Ending balance	$ 1,138.2	$ 911.6

Income Taxes - Effective Rate Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Tax (expense) benefit at the U.S. statutory rate	$ (118.7)	$ 31.4	$ 57.5
Differences in taxes resulting from:
Non-U.S. earnings, net of foreign taxes	103.7	(41.2)	(78.2)
Bermuda corporate income tax	68.0	0.0	0.0
Change in valuation allowance	(30.2)	(19.6)	(2.0)
State taxes	(8.2)	(2.8)	(7.3)
Noncontrolling interest	6.8	3.4	2.1
Member’s surplus contributions	(5.7)	(6.2)	(5.6)
Withholding tax	(4.1)	(3.1)	(0.3)
Tax rate changes	(0.6)	(0.4)	(10.9)
Officer compensation	(0.2)	(1.0)	(1.5)
Tax exempt interest and dividends	0.2	0.2	0.2
Other, net	4.5	(2.1)	1.6
Total income tax (expense) benefit	$ 15.5	$ (41.4)	$ (44.4)

CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Millions	Total	Total Equity	Common Shares and Paid-in Surplus	Retained Earnings	AOCL, After-tax	Total	Noncontrolling Interests
Beginning balance at Dec. 31, 2020		$ 3,817.9	$ 595.2	$ 3,311.2	$ (0.4)	$ 3,906.0	$ (88.1)
Increase (Decrease) in Shareholders' Equity
Net income (loss)	$ (275.4)	(321.9)		(275.4)		(275.4)	(46.5)
Other comprehensive income (loss), net of tax	1.7	1.9			1.9	1.9
Other comprehensive income from discontinued operations, net of tax - NSM Group	0.2	0.2			0.2	0.2
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	0.0
Comprehensive income (loss)		(319.8)		(275.4)	2.1	(273.3)	(46.5)
Dividends declared on common shares		(3.1)		(3.1)		(3.1)
Dividends to noncontrolling interests		(2.3)					(2.3)
Issuances of common shares		1.9	1.9			1.9
Issuances of shares of noncontrolling interest		6.5					6.5
Repurchases and retirements of common shares	(107.5)	(107.5)	(18.9)	(88.6)		(107.5)
BAM member surplus contribution, net of tax		62.2					62.2
Amortization of restricted share awards		14.7	14.7			14.7
Recognition of equity-based compensation expense of subsidiaries		3.5	3.0			3.0	0.5
Net contributions (distributions) and dilution from other noncontrolling interests		0.4	(7.0)	13.4		6.4	(6.0)
Acquisition of noncontrolling interests		230.3					230.3
Ending balance at Dec. 31, 2021		3,704.7	588.9	2,957.5	1.7	3,548.1	156.6
Increase (Decrease) in Shareholders' Equity
Net income (loss)	792.8	712.4		792.8		792.8	(80.4)
Other comprehensive income (loss), net of tax	(3.8)	(3.8)			(2.9)	(2.9)	(0.9)
Other comprehensive income from discontinued operations, net of tax - NSM Group	(5.2)	(5.2)			(5.2)	(5.2)
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	2.9	2.9			2.9	2.9
Comprehensive income (loss)		706.3		792.8	(5.2)	787.6	(81.3)
Dividends declared on common shares		(3.0)		(3.0)		(3.0)
Dividends to noncontrolling interests		(8.0)					(8.0)
Issuances of common shares		3.0	3.0			3.0
Issuances of shares of noncontrolling interest		74.6					74.6
Repurchases and retirements of common shares	(615.8)	(615.8)	(90.0)	(525.8)		(615.8)
BAM member surplus contribution, net of tax		81.4					81.4
Amortization of restricted share awards		14.0	14.0			14.0
Recognition of equity-based compensation expense of subsidiaries		9.5	8.6			8.6	0.9
Net contributions (distributions) and dilution from other noncontrolling interests		(14.2)	14.1	(9.7)		4.4	(18.6)
Acquisition of noncontrolling interests		(17.5)					(17.5)
Ending balance at Dec. 31, 2022	3,935.0	3,935.0	538.6	3,211.8	(3.5)	3,746.9	188.1
Increase (Decrease) in Shareholders' Equity
Net income (loss)	509.2	580.9		509.2		509.2	71.7
Other comprehensive income (loss), net of tax	2.4	2.4			1.9	1.9	0.5
Other comprehensive income from discontinued operations, net of tax - NSM Group	0.0
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	0.0
Comprehensive income (loss)		583.3		509.2	1.9	511.1	72.2
Dividends declared on common shares		(2.6)		(2.6)		(2.6)
Dividends to noncontrolling interests		(8.9)					(8.9)
Issuances of common shares		2.1	2.1			2.1
Repurchases and retirements of common shares	(32.7)	(32.7)	(5.1)	(27.6)		(32.7)
BAM member surplus contribution, net of tax		72.8					72.8
Amortization of restricted share awards		15.3	15.3			15.3
Recognition of equity-based compensation expense of subsidiaries		2.4	1.7			1.7	0.7
Net contributions (distributions) and dilution from other noncontrolling interests		4.3	(1.3)			(1.3)	5.6
Acquisition of noncontrolling interests		(9.4)					(9.4)
Ending balance at Dec. 31, 2023	$ 4,561.6	$ 4,561.6	$ 551.3	$ 3,690.8	$ (1.6)	$ 4,240.5	$ 321.1

Municipal Bond Guarantee Insurance - Schedule of Municipal Bond Guarantee Insured Obligations (Details) - Build America Mutual Assurance Company $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2023	Dec. 31, 2023 USD ($) contract	Dec. 31, 2022 USD ($) contract
Guarantor Obligations [Line Items]
Contracts outstanding \| contract		14,485	13,382
Remaining weighted average contract period (in years)	10 years 9 months 18 days	11 years 1 month 6 days
Principal		$ 109,673.8	$ 99,996.9
Interest		56,012.2	48,880.6
Total debt service outstanding		165,686.0	148,877.5
Gross unearned insurance premiums (in millions)		$ 325.8	$ 298.3

SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT

12 Months Ended

Dec. 31, 2023

Condensed Financial Information Disclosure [Abstract]

SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT

CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT

CONDENSED BALANCE SHEETS (1)


	December 31,
Millions	2023		2022
Assets:
Cash	$	.3	$	.1
Fixed maturity investments, at fair value	—		49.2
Common equity securities, at fair value	147.8		342.8
Short-term investments, at fair value	198.8		105.2
Other long-term investments	160.5		49.3
Net receivable due from subsidiaries	89.1		125.2
Other assets	4.0		75.3
Investments in consolidated and unconsolidated subsidiaries	3,666.9		3,027.2
Total assets	$	4,267.4	$	3,774.3
Liabilities:

Other liabilities	$	26.9	$	27.4
Total liabilities	26.9		27.4
Equity:
Total White Mountains’s common shareholders’ equity	4,240.5		3,746.9

Total liabilities and equity	$	4,267.4	$	3,774.3

(1) These condensed unconsolidated financial statements reflect the results of operations, financial condition and cash flows for the Company. Investments in which White Mountains holds a controlling financial interest are accounted for using the equity method. Under the equity method, investments in subsidiaries are recorded on the condensed balance sheets at the amount of the Company’s ownership percentage of the subsidiary’s GAAP book value. The income from subsidiaries is reported on a net of tax basis as equity in earnings from consolidated and unconsolidated subsidiaries on the condensed statements of operations and comprehensive income (loss). Capital contributions to consolidated subsidiaries are presented within investing activities on the condensed statements of cash flows. Distributions from consolidated subsidiaries are presented within operating or investing activities on the condensed statements of cash flows based on the nature of the distribution. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation.

Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

SCHEDULE II (continued)

CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS) (1)


	Year Ended December 31,
Millions	2023		2022		2021
Revenues (loss) (including realized and unrealized gains and losses)	$	81.7	$	(.4)	$	(.2)
Expenses	60.5		67.1		39.0
Pre-tax income (loss)	21.2		(67.5)		(39.2)
Income tax (expense) benefit	(1.2)		(.9)		—
Net income (loss)	20.0		(68.4)		(39.2)
Equity in earnings (losses) from consolidated and unconsolidated subsidiaries, net of tax	489.2		(42.0)		(232.3)
Equity in earnings from discontinued operations, net of tax - NSM Group (2)	—		16.4		(22.6)
Net gain (loss) from sale of discontinued operations, net of tax - NSM Group (2)	—		886.8		—
Net gain (loss) from sale of discontinued operations, net of tax - Sirius Group (2)	—		—		18.7
Net income (loss) attributable to White Mountains’s common shareholders	509.2		792.8		(275.4)
Equity in other comprehensive income (loss) from consolidated and unconsolidated subsidiaries, net of tax	1.9		(2.9)		1.9
Equity in other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	—		(5.2)		.2
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	—		2.9		—
Comprehensive income (loss) attributable to White Mountains’s common shareholders	511.1		787.6		(273.3)

(2) On August 1, 2022, White Mountains closed the NSM Transaction. See Note 2 — “Significant Transactions.” As a result of the NSM Transaction, the assets and liabilities of NSM Group have been presented in the balance sheet as held for sale for periods prior to the closing of the transaction, and the results of operations for NSM Group have been classified as discontinued operations in the statements of operations and comprehensive income through the closing of the transaction. During 2021, net gain (loss) from sale of discontinued operations, net of tax includes $18.7 arising from the tax contingency on the sale of Sirius Group. See Note 20 — “Held for Sale and Discontinued Operations”.

Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

SCHEDULE II (continued)

CONDENSED STATEMENTS OF CASH FLOWS (1)(2)


	Year Ended December 31,
Millions	2023		2022		2021
Net income (loss) attributable to White Mountains’s common shareholders	$	509.2	$	792.8	$	(275.4)
Charges (credits) to reconcile net income to net cash from operations:
Net realized and unrealized investment (gains) losses	(78.9)		5.6		.1
Equity in (earnings) losses from consolidated and unconsolidated subsidiaries, net of tax	(489.2)		42.0		232.3
Equity in (earnings) losses from discontinued operation, net of tax - NSM Group (2)	—		(16.4)		22.6
Net (gain) loss from sale of discontinued operations, net of tax - NSM Group(2)	—		(886.8)		—
Net (gain) loss from sale of discontinued operations, net of tax - Sirius Group (2)	—		—		(18.7)
Net other non-cash reconciling items (3)	12.7		7.0		16.6


Distributions of accumulated earnings from consolidated subsidiaries (4)	17.3		7.0		—
Net other operating activities (4)(5)	9.2		17.2		(5.7)


Net cash provided from (used for) operations	(19.7)		(31.6)		(28.2)
Cash flows from investing activities:
Net change in short-term investments (4)(6)(7)(8)	(63.8)		1,165.6		17.7
Purchases of investment securities	(125.0)		(359.2)		—
Sales and maturities of investment securities	335.4		15.7		—
Purchases of investment securities from subsidiaries	—		(14.0)		(26.4)
Sales of investment securities to subsidiaries	—		—		36.4

Release of cash (pre-funding) for ILS funds managed by Elementum	70.0		(70.0)		—
Net issuance of debt (to) from subsidiaries (8)	(132.5)		(142.0)		94.0
Net repayment of debt (to) from subsidiaries (6)(8)(9)	31.4		(15.0)		—
Net distributions from (contributions to) subsidiaries (6)(7)(8)	(60.3)		49.3		17.0
Proceeds from the sale of Other Operating Businesses, net of cash sold of $0.0.0 $0.5 and $0.0	—		19.5		—


Net cash provided from (used for) investing activities	55.2		649.9		138.7
Cash flows from financing activities:



Repurchases and retirement of common shares	(32.7)		(615.8)		(107.5)
Cash dividends paid to common shareholders	(2.6)		(3.0)		(3.1)


Net cash provided from (used for) financing activities	(35.3)		(618.8)		(110.6)
Net change in cash during the year	.2		(.5)		(.1)
Cash balance at beginning of year	.1		.6		.7
Cash balance at end of year	$	.3	$	.1	$	.6

(2) See Note 20 — “Held for Sale and Discontinued Operations”.

(3) Includes amortization of restricted share awards of $15.3, $14.0 and $14.7 for the years ended December 31, 2023, 2022 and 2021.

(4) During 2022, Bridge Holdings, Ltd. (“Bridge”), a wholly-owned subsidiary of the Company, merged into the Company. The merger was treated as a liquidation for financial statement purposes. As part of the liquidation, Bridge transferred cash of $7.0 and non-cash balances to the Company including ending net equity of $3,540.6, intercompany note receivable of $76.4, investments in its subsidiaries of $2,003.6, fixed maturity investments of $28.6, common equity securities of $8.1, other long-term investments of $52.2, short-term investments of $1,358.7 and other assets of $6.0.

(5) For 2023, 2022 and 2021, net other operating activities also included $8.4, $3.3, and $6.5 of net changes in (receivables) payables to the Company’s subsidiaries. During 2023, the Company made a non-cash payment in fixed maturity investments of $10.0 for a payable due to its wholly-owned subsidiary, White Mountains Lafayette Holdings, Inc.

(6) During 2021, Bridge repaid $200.0 of outstanding intercompany debt to the Company by transferring shares of its wholly-owned subsidiary, White Mountains Lincoln Holdings, Inc., (“WM Lincoln”), which had carrying value of $212.6. The $12.6 in excess of the intercompany debt was a non-cash distribution to the Company. Also, as part of the transaction, the Company received a distribution of $18.0 from Bridge, including $17.9 of short-term investments and $0.1 of cash. Subsequent to that transaction, the Company contributed the shares of WM Lincoln, which had a carrying value of $212.6, to its wholly-owned subsidiary White Mountains Adams, Inc. (“WM Adams”). The Company also contributed an additional $42.7 to WM Adams, including $37.1 of short-term investments and $5.6 of cash.

(7) During 2022, the Company made cash contributions to its wholly-owned subsidiaries White Mountains Investments (Bermuda), Ltd. (“WMIB”) of $51.0 and WM Adams of $25.0. Also, during 2022, the Company made non-cash contributions of $100.0 in short term investments to WM Adams. During 2022, the Company received cash distributions of $116.3 from HG Global, $7.7 from Ark and $1.3 from its wholly-owned subsidiary, PSC Holdings, Ltd (“PSC Holdings”). During 2021, the Company received a distribution of $19.7, including $19.1 of short-term investments and $0.6 of cash, from WMIB.

(8) During 2023, the Company had issuances of debt of $20.0 to WM Hinson (Bermuda) Ltd. (“WM Hinson”), a wholly-owned subsidiaries of the Company and $7.5 to HG Global. During 2023, the Company received repayments of debt of $42.4 in cash and $107.6 in investments ($45.9 in fixed maturity investments and $61.7 short-term investments) from WM Hinson and $9.5 from HG Global. Also, during 2023, the Company made repayments of debt of $20.5 in cash and $73.8 in investments ($36.9 million in fixed maturity investments and $36.9 in short-term investments) to WM Birkdale Ltd. (“WM Birkdale”), a wholly-owned subsidiaries of the Company. Also, in 2023, the Company had issuances of debt of $105.0 million in cash to White Mountains Investments (Luxembourg) S.a.r.l. under a new promissory note. Subsequently, the Company contributed the promissory note to WM Birkdale. During 2022, prior to the merger of Bridge into the Company, Bridge had an issuance of debt of $69.0 to the Company. Also, during 2022, the Company had issuances of debt of $205.0 to WM Hinson and $6.0 to HG Global.

(9) During 2022, prior to the merger of Bridge into the Company, the Company had a repayment of debt to Bridge of $15.0.

Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

Segment Information - Narrative (Details)	12 Months Ended
Segment Information - Narrative (Details)	Dec. 31, 2023 segment
Segment Reporting [Abstract]
Number of reportable segments	3

Investment Securities - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2023 USD ($) $ / shares	Dec. 31, 2023 USD ($) fund $ / shares shares	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Concentration Risk [Line Items]
Gross realized investment gains		$ 113.7	$ 129.9	$ 212.3
Gross realized investment losses		(16.9)	(14.8)	(55.5)
Proceeds from sale and maturity of investments		$ 800.0	500.0	$ 800.0
Weighted average portfolio period		1 year 10 months 24 days
Weighted average portfolio period, excluding short-term invetments		3 years 3 months 18 days
Investments held in trusts		$ 585.6	500.5
Assets held by insurance regulators		4.6	4.6
Other long-term investments		1,998.2	1,488.0
Fair value investments		6,388.7	5,170.0
Private equity funds
Concentration Risk [Line Items]
Other long-term investments		$ 248.6
Number of investments \| fund		16
Hedge funds
Concentration Risk [Line Items]
Number of investments \| fund		2
Hedge funds \| Minimum
Concentration Risk [Line Items]
Notice period		45 days
Hedge funds \| Maximum
Concentration Risk [Line Items]
Notice period		90 days
Bank loan fund
Concentration Risk [Line Items]
Other long-term investments		$ 194.4	174.8
Other long-term investments, redemption, written notice period		15 days
Foreign Regulatory Deposits
Concentration Risk [Line Items]
Other long-term investments		$ 158.0	137.4
ILS funds
Concentration Risk [Line Items]
Other long-term investments		$ 160.5	49.3
ILS funds \| Minimum
Concentration Risk [Line Items]
Notice period		30 days
MediaAlpha \| IPO
Concentration Risk [Line Items]
Increase in book value per share (in dollars per share) \| $ / shares		$ 9.00
Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value		$ 209.1	182.9
Fair value measured on a recurring basis
Concentration Risk [Line Items]
Fair value investments		6,388.7	5,170.0
Fair value measured on a recurring basis \| MediaAlpha
Concentration Risk [Line Items]
Other long-term investments		254.9	168.6
Fair value measured on a recurring basis \| Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value		209.1	182.9
Asset Pledged as Collateral
Concentration Risk [Line Items]
Short-term investments		198.9	90.3
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments		158.0	137.4
Lloyds Bank
Concentration Risk [Line Items]
Fair value investments		344.7	319.2
Additional Investments, Insurance Regulators and Reinsurance Counterparties \| Asset Pledged as Collateral
Concentration Risk [Line Items]
Fair value investments		244.5	257.0
Outrigger Re
Concentration Risk [Line Items]
Short-term investments		$ 265.3	$ 203.7
MediaAlpha
Concentration Risk [Line Items]
Tender offer purchase price	$ 5.9
Purchase price (in dollars per share) \| $ / shares	$ 10.00	$ 11.15
Investment owned (in shares) \| shares		22.9
Ownership interest (as a percent)		34.90%	27.10%
Fully diluted ownership interest (as a percent)		33.10%
Increase in investment owned (in dollars per share) \| $ / shares		$ 1.00
Largest Investment \| Investments \| Private Equity and Hedge Fund Concentration Risk
Concentration Risk [Line Items]
Other long-term investments		$ 57.3	$ 49.0
HG Global Senior Notes \| Senior Notes
Concentration Risk [Line Items]
Interest reserve account		30.4	31.2
Kudu Credit Facility \| Secured Debt
Concentration Risk [Line Items]
Interest reserve account		$ 14.9
Restricted cash			$ 12.2

Municipal Bond Guarantee Insurance - Schedule of Net Written Premiums (Details) - HG Global/BAM - Segment - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Written premiums:
Direct	$ 58.6	$ 63.8	$ 51.0
Assumed	0.0	1.3	4.6
Gross written premiums	58.6	65.1	55.6
Earned premiums:
Direct	28.7	28.6	23.2
Assumed	2.5	4.7	3.7
Gross earned premiums	$ 31.2	$ 33.3	$ 26.9

Significant Transactions - Kudu (Details) $ in Millions	May 26, 2022 USD ($)	Dec. 31, 2022	May 25, 2022	Dec. 31, 2021 USD ($) portfolioCompany
Other Significant Noncash Transactions [Line Items]
Number of portfolio companies \| portfolioCompany				2
Subsidiary - Kudu Investment Management, LLC
Other Significant Noncash Transactions [Line Items]
Ownership percentage basic units outstanding	89.30%	89.30%	99.10%
Subsidiary - Kudu Investment Management, LLC
Other Significant Noncash Transactions [Line Items]
Equity capital raised	$ 114.5
Pre-money valuation ratio	1.3
Pre-money valuation amount	$ 114.0
Subsidiary - Kudu Investment Management, LLC \| Kudu’s Participation Contracts
Other Significant Noncash Transactions [Line Items]
Enterprise value excluded				$ 54.3
Subsidiary - Kudu Investment Management, LLC \| Massachusetts Mutual Life Insurance Company
Other Significant Noncash Transactions [Line Items]
Equity capital raised	64.1
Subsidiary - Kudu Investment Management, LLC \| White Mountains
Other Significant Noncash Transactions [Line Items]
Equity capital raised	50.0
Subsidiary - Kudu Investment Management, LLC \| Kudu
Other Significant Noncash Transactions [Line Items]
Equity capital raised	$ 0.4

Earnings Per Share

12 Months Ended

Dec. 31, 2023

Earnings Per Share [Abstract]

Earnings Per Share

White Mountains calculates earnings per share using the two-class method, which allocates earnings between common shares and unvested restricted common shares. Both classes of shares participate equally in dividends and earnings on a per share basis. Basic earnings per share amounts are based on the weighted average number of common shares outstanding adjusted for unvested restricted common shares.


	Year Ended December 31,
	2023		2022		2021
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$	509.2	$	792.8	$	(275.4)
Less: total income (loss) from discontinued operations, net of tax (1)	—		903.2		(3.9)
Less: net (income) loss from discontinued operations attributable to noncontrolling interests	—		(.7)		1.0
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	509.2		(109.7)		(272.5)
Allocation of (earnings) losses to participating restricted common shares (2)	(7.2)		1.3		3.2


Basic and diluted earnings (losses) per share numerators	$	502.0	$	(108.4)	$	(269.3)




Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	2,563.3		2,862.4		3,079.0
Average unvested restricted common shares (3)	(36.0)		(36.2)		(36.5)
Basic earnings (losses) per share denominator	2,527.3		2,826.2		3,042.5
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	2,563.3		2,862.4		3,079.0
Average unvested restricted common shares (3)	(36.0)		(36.2)		(36.5)

Diluted earnings (losses) per share denominator	2,527.3		2,826.2		3,042.5
Basic and diluted earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid	$	1.00	$	1.00	$	1.00
Undistributed earnings (losses)	197.60		(39.34)		(89.52)
Basic and diluted earnings (losses) per share	$	198.60	$	(38.34)	$	(88.52)

(2) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.

(3) Restricted shares outstanding vest upon a stated date. See Note 12 — “Employee Share-Based Incentive Compensation Plans”.

The following table presents the undistributed net earnings (losses) from continuing operations for the years ended December 31, 2023, 2022 and 2021. See Note 20 — “Held for Sale and Discontinued Operations”.


	Year Ended December 31,
Millions		2023		2022		2021
Undistributed net earnings - continuing operations:
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts		$	502.0	$	(108.4)	$	(269.3)
Dividends declared, net of restricted common share amounts (1)		(2.5)		(3.0)		(3.1)
Total undistributed net earnings (losses), net of restricted common share amounts		$	499.5	$	(111.4)	$	(272.4)

(1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.

Significant Transactions - Schedule of Assets Acquired and Liabilities Assumed, Ark (Details) - USD ($) $ in Millions	Jan. 01, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Other Significant Transactions [Line Items]
Goodwill		$ 168.8	$ 176.5	$ 142.3
Ark
Other Significant Transactions [Line Items]
Investments	$ 594.3
Cash	52.0
Reinsurance recoverables	433.4
Insurance premiums receivable	236.7
Ceded unearned premiums	170.2
Value of in-force business acquired	71.7
Other assets	88.9
Loss and loss adjustment expense reserves	(696.0)
Unearned insurance premiums	(326.1)
Debt	(46.4)
Ceded reinsurance payable	(528.3)
Other liabilities	(25.9)
Net tangible assets acquired	24.5
Goodwill	116.8
Other intangible assets - syndicate underwriting capacity	175.7
Deferred tax liability on other intangible assets	(33.4)
Net assets acquired	283.6
Merger agreement, equity capital contributed	$ 605.4

Investment Securities (Tables)

12 Months Ended

Dec. 31, 2023

Investments, Debt and Equity Securities [Abstract]

Schedule of pre-tax net investment income

The following table presents pre-tax net investment income for the years ended December 31, 2023, 2022 and 2021:


	Year Ended December 31,
Millions	2023		2022		2021
Fixed maturity investments	$	64.4	$	41.4	$	28.8
Short-term investments	52.0		16.4		.6
Common equity securities	4.1		1.6		.1

Other long-term investments	76.1		68.1		56.3
Amount attributable to TPC Providers	—		(1.0)		(1.0)
Total investment income	196.6		126.5		84.8
Third-party investment expenses	(2.4)		(2.1)		(2.3)
Net investment income, pre-tax	$	194.2	$	124.4	$	82.5

Schedule of net realized and unrealized investment gains and losses

The following table presents net realized and unrealized investment gains (losses) for the years ended December 31, 2023, 2022 and 2021:


	Year Ended December 31,
Millions	2023		2022		2021
Realized investment gains (losses)
Fixed maturity investments	$	(4.7)	$	(11.9)	$	3.9
Short-term investments	(.2)		(1.4)		(.1)
Common equity securities	29.5		—		.4
Investment in MediaAlpha	—		—		160.3
Other long-term investments	72.2		128.4		(7.7)
Net realized investment gains (losses)	96.8		115.1		156.8

Unrealized investment gains (losses)
Fixed maturity investments	66.1		(168.4)		(42.2)
Short-term investments	1.9		(1.1)		—
Common equity securities	47.1		(7.1)		14.9
Investment in MediaAlpha	27.1		(93.0)		(540.6)
Other long-term investments	195.2		(43.8)		172.7
Net unrealized investment gains (losses)	337.4		(313.4)		(395.2)
Net realized and unrealized investment gains (losses), before amount attributable to TPC providers (1)	434.2		(198.3)		(238.4)
Amount attributable to TPC Providers	—		6.8		(7.7)
Net realized and unrealized investment gains (losses)	$	434.2	$	(191.5)	$	(246.1)

Fixed maturity and short-term investments
Net realized and unrealized investment gains (losses)	$	63.1	$	(182.8)	$	(38.4)
Less: net realized and unrealized gains (losses) on investment securities sold during the period	4.7		(2.9)		(8.4)
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	$	58.4	$	(179.9)	$	(30.0)

Common equity securities and investment in MediaAlpha
Net realized and unrealized investment gains (losses) on common equity securities	$	76.6	$	(7.1)	$	15.3
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	27.1		(93.0)		(380.3)
Total net realized and unrealized investment gains (losses)	103.7		(100.1)		(365.0)
Less: net realized and unrealized gains (losses) on investment securities sold during the period	28.4		—		20.3
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	$	75.3	$	(100.1)	$	(385.3)

(1) For 2023, 2022 and 2021, includes $13.6, $(29.3) and $(7.7) of net realized and unrealized investment gains (losses) related to foreign currency exchange.


	Year Ended December 31,
Millions	2023		2022		2021

Total net unrealized investment gains on other long-term investments held at the end of period, pre-tax	$	133.9	$	56.5	$	98.9

Schedule of equity securities and other long-term investments


	December 31, 2023
Millions	Cost or Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Net Foreign Currency Gains (Losses)		Carrying Value
U.S. Government and agency obligations	$	209.0	$	.3	$	(4.4)	$	—	$	204.9
Debt securities issued by corporations	1,085.9		5.6		(45.4)		(1.1)		1,045.0
Municipal obligations	275.1		.9		(15.1)		—		260.9
Mortgage and asset-backed securities	417.2		2.0		(29.8)		—		389.4
Collateralized loan obligations	211.2		.4		(2.4)		(.1)		209.1
Total fixed maturity investments	$	2,198.4	$	9.2	$	(97.1)	$	(1.2)	$	2,109.3


	December 31, 2022
Millions	Cost or Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Net Foreign Currency Gains (Losses)		Carrying Value
U.S. Government and agency obligations	$	216.6	$	—	$	(10.2)	$	—	$	206.4
Debt securities issued by corporations	1,098.3		.6		(78.3)		(1.8)		1,018.8
Municipal obligations	281.6		.4		(23.4)		—		258.6
Mortgage and asset-backed securities	288.7		—		(34.5)		—		254.2
Collateralized loan obligations	190.8		.1		(6.0)		(2.0)		182.9


Total fixed maturity investments	$	2,076.0	$	1.1	$	(152.4)	$	(3.8)	$	1,920.9


	December 31, 2023
Millions	Cost or Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Net Foreign Currency Gains (Losses)		Carrying Value

Common equity securities	$	483.5	$	62.3	$	(1.2)	$	(6.2)	$	538.4
Investment in MediaAlpha	$	59.2	$	195.7	$	—	$	—	$	254.9


Other long-term investments	$	1,655.7	$	446.3	$	(94.5)	$	(9.3)	$	1,998.2


	December 31, 2022
Millions	Cost or Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Net Foreign Currency Gains (Losses)		Carrying Value
Common equity securities	$	660.6	$	26.7	$	(8.4)	$	(10.5)	$	668.4
Investment in MediaAlpha	$	—	$	168.6	$	—	$	—	$	168.6

Other long-term investments	$	1,340.8	$	271.1	$	(107.1)	$	(16.8)	$	1,488.0

Schedule of contractual maturities of fixed maturities


	December 31, 2023
Millions	Cost or Amortized Cost		Carrying Value
Due in one year or less	$	319.9	$	315.0
Due after one year through five years	845.0		813.9
Due after five years through ten years	303.4		287.0
Due after ten years	101.7		94.9
Mortgage and asset-backed securities and collateralized loan obligations	628.4		598.5

Total fixed maturity investments	$	2,198.4	$	2,109.3

Schedule of fair value measurements for investment securities by level


	December 31, 2023
Millions	Fair Value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	204.9	$	204.9	$	—	$	—

Debt securities issued by corporations:
Financials	309.8		—		309.8		—
Consumer	214.6		—		214.6		—
Industrial	121.6		—		121.6		—
Healthcare	121.3		—		121.3		—
Technology	108.9		—		108.9		—
Utilities	70.2		—		70.2		—
Communications	46.3		—		46.3		—
Energy	28.9		—		28.9		—
Materials	23.4		—		23.4		—

Total debt securities issued by corporations	1,045.0		—		1,045.0		—

Municipal obligations	260.9		—		260.9		—
Mortgage and asset-backed securities	389.4		—		389.4		—

Collateralized loan obligations	209.1		—		209.1		—
Total fixed maturity investments	2,109.3		204.9		1,904.4		—

Short-term investments	1,487.9		1,457.6		30.3		—
Common equity securities:

Exchange-traded funds	137.8		137.8		—		—
Other (1)	400.6		—		400.6		—
Total common equity securities	538.4		137.8		400.6		—
Investment in MediaAlpha	254.9		254.9		—		—












Other long-term investments	1,164.4		—		26.2		1,138.2
Other long-term investments — NAV (2)	833.8		—		—		—
Total other long-term investments	1,998.2		—		26.2		1,138.2
Total investments	$	6,388.7	$	2,055.2	$	2,361.5	$	1,138.2

(1) Consists of investments in listed funds that predominantly invest in international equities.


	December 31, 2022
Millions	Fair Value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	206.4	$	206.4	$	—	$	—

Debt securities issued by corporations:
Financials	291.2		—		291.2		—
Consumer	191.9		—		191.9		—
Industrial	115.4		—		115.4		—
Healthcare	121.3		—		121.3		—
Technology	123.7		—		123.7		—
Utilities	73.8		—		73.8		—
Communications	47.9		—		47.9		—
Energy	33.9		—		33.9		—
Materials	19.7		—		19.7		—

Total debt securities issued by corporations	1,018.8		—		1,018.8		—

Municipal obligations	258.6		—		258.6		—
Mortgage and asset-backed securities	254.2		—		254.2		—
Collateralized loan obligations	182.9		—		182.9		—


Total fixed maturity investments	1,920.9		206.4		1,714.5		—

Short-term investments	924.1		924.1		—		—
Common equity securities:

Exchange-traded funds	333.8		333.8		—		—
Other (1)	334.6		—		334.6		—
Total common equity securities	668.4		333.8		334.6		—
Investment in MediaAlpha	168.6		168.6		—		—
















Other long-term investments	926.4		—		14.8		911.6
Other long-term investments — NAV (2)	561.6		—		—		—
Total other long-term investments	1,488.0		—		14.8		911.6
Total investments	$	5,170.0	$	1,632.9	$	2,063.9	$	911.6

(1) Consists of investments in listed funds that predominantly invest in international equities.

Schedule of credit ratings of debt securities issued

The following table presents the credit ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of December 31, 2023 and 2022:


	Fair Value as of December 31,
Millions	2023		2022
AAA	$	11.5	$	11.3
AA	83.8		96.0
A	552.4		567.9
BBB	390.9		337.7


Other	6.4		5.9
Debt securities issued by corporations (1)	$	1,045.0	$	1,018.8

Schedule of mortgage and asset-backed securities

The following table presents the fair value of White Mountains’s mortgage and asset-backed securities and collateralized loan obligations as of December 31, 2023 and 2022:


	December 31, 2023						December 31, 2022
Millions	Fair Value		Level 2		Level 3		Fair Value		Level 2		Level 3
Mortgage-backed securities:
Agency:
FNMA	$	198.8	$	198.8	$	—	$	124.5	$	124.5	$	—
FHLMC	137.2		137.2		—		78.8		78.8		—
GNMA	30.2		30.2		—		28.3		28.3		—
Total agency (1)	366.2		366.2		—		231.6		231.6		—
Non-agency: Residential	.2		.2		—		.3		.3		—

Total non-agency	.2		.2		—		.3		.3		—
Total mortgage-backed securities	366.4		366.4		—		231.9		231.9		—
Other asset-backed securities:
Credit card receivables	3.3		3.3		—		11.9		11.9		—
Vehicle receivables	17.2		17.2		—		10.4		10.4		—
Other	2.5		2.5		—		—		—		—
Total other asset-backed securities	23.0		23.0		—		22.3		22.3		—
Total mortgage and asset-backed securities	389.4		389.4		—		254.2		254.2		—
Collateralized loan obligations:	209.1		209.1		—		182.9		182.9		—
Total mortgage and asset-backed securities and collateralized loan obligations	$	598.5	$	598.5	$	—	$	437.1	$	437.1	$	—

Schedule of carrying values for other long-term investments


	Fair Value as of December 31, 2023
Millions	Ark/WM Outrigger		Kudu		Other		Total
Kudu’s Participation Contracts	$	—	$	890.5	$	—	$	890.5
PassportCard/DavidShield	—		—		150.0		150.0
Elementum Holdings, L.P.	—		—		35.0		35.0
Other unconsolidated entities (1)	—		—		48.1		48.1





Total unconsolidated entities	—		890.5		233.1		1,123.6
Private equity funds and hedge funds	55.5		—		257.4		312.9
Bank loan fund	194.4		—		—		194.4
ILS funds	—		—		160.5		160.5
Lloyd’s trust deposits	158.0		—		—		158.0
Private debt instruments	—		—		10.0		10.0
Other	33.0		5.8		—		38.8
Total other long-term investments	$	440.9	$	896.3	$	661.0	$	1,998.2


	Fair Value as of December 31, 2022
Millions	Ark/WM Outrigger		Kudu		Other		Total
Kudu’s Participation Contracts	$	—	$	695.9	$	—	$	695.9
PassportCard/DavidShield	—		—		135.0		135.0
Elementum Holdings, L.P.	—		—		30.0		30.0
Other unconsolidated entities (1)	—		—		37.2		37.2





Total unconsolidated entities	—		695.9		202.2		898.1
Private equity funds and hedge funds	40.4		—		157.4		197.8
Bank loan fund	174.8		—		—		174.8
ILS funds	—		—		49.3		49.3
Lloyd’s trust deposits	137.4		—		—		137.4
Private debt instruments	—		—		9.6		9.6
Other	21.0		—		—		21.0
Total other long-term investments	$	373.6	$	695.9	$	418.5	$	1,488.0

(1) Includes White Mountains’s noncontrolling equity interests in certain preferred securities, private common equity securities, limited liability company units and SAFE.

Schedule of investments and unfunded commitments by investment objective and sector

The following table presents the fair value of investments and unfunded commitments in private equity funds and hedge funds by investment objective and sector as of December 31, 2023 and 2022:


	December 31, 2023				December 31, 2022
Millions	Fair Value		Unfunded Commitments		Fair Value		Unfunded Commitments

Private equity funds
Aerospace/Defense/Government	$	157.2	$	17.4	$	59.4	$	37.5
Financial services	87.5		37.8		77.1		54.3
Real estate	3.9		2.5		4.1		2.5



Total private equity funds	248.6		57.7		140.6		94.3

Hedge funds
Long/short equity financials and business services	54.4		—		49.0		—
European small/mid cap	9.9		—		8.2		—
Total hedge funds	64.3		—		57.2		—
Total private equity funds and hedge funds included in other long-term investments	$	312.9	$	57.7	$	197.8	$	94.3

Schedule investments in private equity funds subject to lock-up periods

The following table presents investments in private equity funds that were subject to lock-up periods as of December 31, 2023:


Millions	1 – 3 years		3 – 5 years		5 – 10 years		>10 years		Total
Private equity funds — expected lock-up period remaining	$	4.5	$	71.0	$	155.4	$	17.7	$	248.6

Schedule of fair value investments by level

The following table presents the changes in White Mountains’s fair value measurements for Level 3 investments for the years ended December 31, 2023 and 2022:


Level 3 Investments
Millions	Other Long-term Investments			Other Long-term Investments
Balance as of December 31, 2022	$	911.6	Balance as of December 31, 2021	$	890.6
Net realized and unrealized gains	157.3		Net realized and unrealized gains	60.4

Purchases and contributions	256.6		Purchases and contributions	129.8
Sales and distributions	(187.3)		Sales and distributions	(169.2)

Transfers in	—		Transfers in	—
Transfers out	—		Transfers out	—
Balance as of December 31, 2023	$	1,138.2	Balance as of December 31, 2022	$	911.6

Schedule of significant unobservable inputs used in estimating the fair value of other long-term investments

$ in Millions

December 31, 2023

Description

Valuation Technique(s) (1)

Fair Value (2)

Unobservable Inputs

Discount Rate (6)

Terminal Cash Flow Exit Multiple (x) or Terminal Revenue Growth Rate (%) (6)

Kudu’s Participation Contracts (3)(4)(5)

Discounted cash flow

$890.5

18% - 25%

7x - 22x

PassportCard/DavidShield

Discounted cash flow

$150.0

24%

Elementum Holdings, L.P.

Discounted cash flow

$35.0

21%

Preferred securities

Discounted cash flow

$28.2

N/A

Private debt instruments

Discounted cash flow

$15.5

11% - 12%

N/A

(1) Key inputs to the discounted cash flow analysis generally include projections of future revenue and earnings, discount rates and terminal exit multiples or growth rates.

(2) Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs.

(4) In 2023, Kudu deployed a total of $163.8 into new and existing Participation Contracts.

$ in Millions

December 31, 2022

Description

Valuation Technique(s) (1)

Fair Value (2)

Unobservable Inputs

Discount Rate (6)

Terminal Cash Flow Exit Multiple (x) or Terminal Revenue Growth Rate (%) (6)

Kudu’s Participation Contracts (3)(4)(5)

Discounted cash flow

$695.9

18% - 25%

7x - 16x

PassportCard/DavidShield

Discounted cash flow

$135.0

24%

Elementum Holdings, L.P.

Discounted cash flow

$30.0

21%

Private debt instruments

Discounted cash flow

$9.6

11%

N/A

(1) Key inputs to the discounted cash flow analysis generally include projections of future revenue and earnings, discount rates and terminal exit multiples or growth rates.

(2) Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs.

(4) In 2022, Kudu deployed a total of $99.8 into new and existing Participation Contracts.

Investment Securities - Investment Gains and Losses For Level 3 (Details) - Other long-term investments - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Investment securities
Net unrealized investment gains (losses)	$ 195.2	$ (43.8)	$ 172.7
Level 3
Investment securities
Net unrealized investment gains (losses)	$ 133.9	$ 56.5	$ 98.9

Investment Securities - Debt Securities Issued By Corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Investment securities
Fixed maturity investments, at fair value	$ 1,045.0	$ 1,018.8
AAA
Investment securities
Fixed maturity investments, at fair value	11.5	11.3
AA
Investment securities
Fixed maturity investments, at fair value	83.8	96.0
A
Investment securities
Fixed maturity investments, at fair value	552.4	567.9
BBB
Investment securities
Fixed maturity investments, at fair value	390.9	337.7
Other
Investment securities
Fixed maturity investments, at fair value	$ 6.4	$ 5.9

Basis of Presentation and Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation White Mountains Insurance Group, Ltd. (the “Company” or the “Registrant”) is an exempted Bermuda limited liability company whose principal businesses are conducted through its subsidiaries and other affiliates. The Company’s headquarters is located at 26 Reid Street, Hamilton, Bermuda HM 11, its principal executive office is located at 23 South Main Street, Suite 3B, Hanover, New Hampshire 03755-2053 and its registered office is located at Clarendon House, 2 Church Street, Hamilton, Bermuda HM 11. The Company’s website is www.whitemountains.com. The information contained on White Mountains’s website is not incorporated by reference into, and is not a part of, this report. The accompanying consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) and include the accounts of the Company, its subsidiaries (collectively with the Company, “White Mountains”) and other entities required to be consolidated under GAAP. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Consolidation Principles	Consolidation Principles Under GAAP, the Company is required to consolidate any entity in which it holds a controlling financial interest. A controlling financial interest is usually in the form of an investment representing the majority of the subsidiary’s voting interests. However, a controlling financial interest may also arise from a financial interest in a variable interest entity (“VIE”) through arrangements that do not involve ownership of voting interests. The Company consolidates a VIE if it determines that it is the primary beneficiary. The primary beneficiary is defined as the entity who holds a variable interest that gives it both the power to direct the VIE’s activities that most significantly impact its economic performance and the obligation to absorb losses of, or the right to receive returns from, the VIE that could potentially be significant to the VIE. See Note 17 — “Variable Interest Entities”. Intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Reporting Segments	Reportable Segments As of December 31, 2023, White Mountains conducted its operations through three reportable segments: (1) HG Global/BAM, (2) Ark/WM Outrigger, and (3) Kudu, with our remaining operating businesses, holding companies and other assets included in Other Operations. White Mountains has made its segment determination based on consideration of the following criteria: (i) the nature of the business activities of each of the Company’s subsidiaries and affiliates; (ii) the manner in which the Company’s subsidiaries and affiliates are organized; (iii) the existence of primary managers responsible for specific subsidiaries and affiliates; and (iv) the organization of information provided to the Company’s chief operating decision makers and its Board of Directors. See Note 15 — “Segment Information”. The HG Global/BAM segment consists of HG Global Ltd. and its wholly-owned subsidiaries (collectively, “HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”) (collectively with HG Global, “HG Global/BAM”). BAM is the first and only mutual municipal bond insurance company in the United States. By insuring the timely payment of principal and interest, BAM provides market access to, and lowers interest expense for, issuers of municipal bonds used to finance essential public purpose projects, such as schools, utilities and transportation facilities. BAM is owned by and operated for the benefit of its members, the municipalities that purchase BAM’s insurance for their debt issuances. HG Global was established to fund the startup of BAM and, through its reinsurance subsidiary HG Re Ltd. (“HG Re”), to provide up to 15%-of-par, first loss reinsurance protection for policies underwritten by BAM. HG Global, together with its subsidiaries, funded the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM (the “BAM Surplus Notes”). As of December 31, 2023 and 2022, White Mountains owned 96.9% of HG Global's preferred equity and 88.4% of its common equity. White Mountains does not have an ownership interest in BAM. However, White Mountains is required to consolidate BAM’s results in its financial statements because BAM is a VIE for which White Mountains is the primary beneficiary. BAM’s results are all attributed to noncontrolling interests. The Ark/WM Outrigger segment consists of Ark Insurance Holdings Limited and its subsidiaries (collectively, “Ark”) and Outrigger Re Ltd. Segregated Account 2023-1 (“WM Outrigger Re”) (collectively with Ark, “Ark/WM Outrigger”). Ark is a specialty property and casualty insurance and reinsurance company that offers a wide range of niche insurance and reinsurance products, including property, specialty, marine & energy, casualty and accident & health. Ark underwrites select coverages through Lloyd’s Syndicates 4020 and 3902 (the “Syndicates”) and its wholly-owned subsidiary Group Ark Insurance Limited (“GAIL”). White Mountains acquired a controlling ownership interest in Ark on January 1, 2021 (the “Ark Transaction”). See Note 2 — “Significant Transactions”. As of December 31, 2023 and 2022, White Mountains owned 72.0% of Ark on a basic shares outstanding basis (61.9% and 63.0% after taking account of management’s equity incentives). The remaining shares are owned by current and former employees. In the future, management rollover shareholders could earn additional shares in Ark if and to the extent that White Mountains achieves certain thresholds for its multiple of invested capital return. If fully earned, these additional shares would represent 12.5% of the shares outstanding at closing. For the years of account prior to the Ark Transaction, a significant proportion of the Syndicates’ underwriting capital was provided by third-party insurance and reinsurance groups (“TPC Providers”) using whole account reinsurance contracts with Ark’s corporate member. For the years of account subsequent to the Ark Transaction, Ark is no longer using TPC Providers to provide underwriting capital for the Syndicates. Captions within results of operations and other comprehensive income are shown net of amounts relating to the TPC Providers’ share of the Syndicates’ results, including investment results. During the fourth quarter of 2022, Ark sponsored the formation of Outrigger Re Ltd., a Bermuda company registered as a special purpose insurer and segregated accounts company, to provide collateralized reinsurance protection on Ark’s Bermuda global property catastrophe excess of loss portfolio written in the 2023 underwriting year. Outrigger Re Ltd. issued non-voting redeemable preference shares on behalf of four segregated accounts to White Mountains and unrelated third-party investors. White Mountains consolidates the results of its segregated account, WM Outrigger Re, in its financial statements. See Note 2 — “Significant Transactions.” As of December 31, 2023 and 2022, White Mountains owned 100.0% of WM Outrigger Re’s preferred equity. During the fourth quarter of 2023, Ark renewed Outrigger Re Ltd.in the 2024 underwriting year. The Kudu segment consists of Kudu Investment Management, LLC and its subsidiaries (collectively, “Kudu”). Kudu provides capital solutions for boutique asset and wealth managers for a variety of purposes including generational ownership transfers, management buyouts, acquisition and growth finance and legacy partner liquidity. Kudu also provides strategic assistance to investees from time to time. Kudu’s capital solutions generally are structured as minority preferred equity stakes with distribution rights, typically tied to gross revenues and designed to generate immediate cash yields. As of December 31, 2023 and 2022, White Mountains owned 89.6% and 89.3% of the basic units outstanding of Kudu (76.3% and 76.1% on a fully-diluted/fully-converted basis). White Mountains’s other operations consists of the Company and its wholly-owned subsidiary, White Mountains Capital, LLC (“WM Capital”), its other intermediate holding companies, its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), investment assets managed by WM Advisors, its interests in MediaAlpha, Inc. (“MediaAlpha”), PassportCard Limited (“PassportCard”) and DavidShield Life Insurance Agency (2000) Ltd. (“DavidShield”) (collectively, “PassportCard/DavidShield”), Elementum Holdings LP (“Elementum”), certain other consolidated and unconsolidated entities (“Other Operating Businesses”) and certain other assets (“Other Operations”). On October 25, 2023, White Mountains announced the launch of White Mountains Partners LLC (“WTM Partners”), which will acquire businesses in non-insurance, non-financial services sectors including essential services, light industrial and specialty consumer. White Mountains expects to deploy up to $500 million of equity capital through WTM Partners over time. WTM Partners is included within Other Operations.
Held for Sale and Discontinued Operations	Held for Sale and Discontinued Operations White Mountains recognizes assets and liabilities classified as held for sale at the lower of carrying value on the date the asset is initially classified as held for sale or fair value less costs to sell. At the time of reclassification to held for sale, White Mountains ceases recognizing depreciation and amortization on assets held for sale. The results of operations of a business that has either been disposed of or are classified as held for sale are reported in discontinued operations if the disposal of business represents a strategic shift that has (or will have) a major effect on White Mountains’s operations and financial results. On August 1, 2022, White Mountains Holdings (Luxembourg) S.à r.l. (“WTM Holdings Seller”), an indirect wholly-owned subsidiary of White Mountains, completed the sale of White Mountains Catskill Holdings, Inc. and NSM Insurance HoldCo, LLC (“NSM”) (collectively with White Mountains Catskill Holdings, Inc., the “NSM Group”) to Riser Merger Sub, Inc., an affiliate of The Carlyle Group Inc. (the “NSM Transaction”), pursuant to the terms of the securities purchase agreement, dated as of May 9, 2022. See Note 2 — “Significant Transactions.” NSM is a full-service managing general agent (“MGA”) and program administrator with delegated binding authorities for specialty property and casualty insurance. As a result of the NSM Transaction, the assets and liabilities of NSM Group have been presented in the balance sheet as held for sale for periods prior to the closing of the transaction, and the results of operations for NSM Group have been classified as discontinued operations in the statements of operations and comprehensive income through the closing of the transaction. Prior period amounts have been reclassified to conform to the current period’s presentation. See Note 20 — “Held for Sale and Discontinued Operations.”
Investment Securities and Other Long-Term Investments	Investment Securities As of December 31, 2023 and 2022, White Mountains’s invested assets consisted of securities and other investments held for general investment purposes. White Mountains’s portfolio of investment securities held for general investment purposes consists of fixed maturity investments, short-term investments, common equity securities, its investment in MediaAlpha and other long-term investments. Short-term investments consist of interest-bearing money market funds, certificates of deposit and other fixed maturity securities, which at the time of purchase, mature or become available for use within one year. Short-term investments are carried at fair value, which approximated amortized or accreted cost as of December 31, 2023 and 2022. Other long-term investments consist primarily of unconsolidated entities, including noncontrolling equity interests in the form of revenue and earnings participation contracts held at Kudu (“Participation Contracts”), private equity funds and hedge funds, a bank loan fund, Lloyd’s of London (“Lloyd’s”) trust deposits, insurance-linked securities (“ILS”) funds and private debt instruments. White Mountains’s portfolio of fixed maturity investments, including those within short-term investments, are classified as trading securities. Trading securities are reported at fair value as of the balance sheet date. White Mountains’s portfolio of common equity securities, its investment in MediaAlpha and other long-term investments are measured at fair value. White Mountains has taken the fair value option for its equity method eligible investments. Changes in net unrealized investment gains (losses) are reported pre-tax in revenues. Realized investment gains (losses) are accounted for using the specific identification method and are reported pre-tax in revenues. Premiums and discounts on all fixed maturity investments are amortized and accreted to income over the anticipated life of the investment. Income on mortgage and asset-backed securities is recognized using an effective yield based on anticipated prepayments and the estimated economic life of the securities. When actual prepayments differ significantly from anticipated prepayments, the estimated economic life is recalculated, and the remaining unamortized premium or discount is amortized prospectively over the remaining economic life. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants (an exit price) at a particular measurement date. Fair value measurements are categorized into a hierarchy that distinguishes between inputs based on market data from independent sources (observable inputs) and a reporting entity’s internal assumptions based upon the best information available when external market data is limited or unavailable (unobservable inputs). Quoted prices in active markets for identical assets have the highest priority (“Level 1”), followed by observable inputs other than quoted prices including prices for similar but not identical assets or liabilities (“Level 2”), and unobservable inputs, including the reporting entity’s estimates of the assumptions that market participants would use, having the lowest priority (“Level 3”). Assets and liabilities carried at fair value include White Mountains’s investment portfolio and derivative instruments. Valuation of assets and liabilities measured at fair value require management to make estimates and apply judgment to matters that may carry a significant degree of uncertainty. In determining its estimates of fair value, White Mountains uses a variety of valuation approaches and inputs. Whenever possible, White Mountains estimates fair value using valuation methods that maximize the use of quoted market prices or other observable inputs. Where appropriate, assets and liabilities measured at fair value have been adjusted for the effect of counterparty credit risk. White Mountains uses outside pricing services and brokers to assist in determining fair values. The outside pricing services White Mountains uses have indicated that they will only provide prices where observable inputs are available. Level 1 Measurements Investments valued using Level 1 inputs include White Mountains’s fixed maturity investments, primarily investments in U.S. Treasuries and short-term investments, which include U.S. Treasury Bills, common equity securities, and its investment in MediaAlpha following the initial public offering of MediaAlpha on October 30, 2020 (the “MediaAlpha IPO”). For investments in active markets, White Mountains uses the quoted market prices provided by outside pricing services to determine fair value. Level 2 Measurements Investments valued using Level 2 inputs include fixed maturity investments which have been disaggregated into classes, including debt securities issued by corporations, municipal obligations, mortgage and asset-backed securities and collateralized loan obligations. Investments valued using Level 2 inputs also include certain international listed equity funds, which White Mountains values using the fund manager’s published net asset value (“NAV”) to account for the difference in market exchange close times. In circumstances where quoted market prices are unavailable or are not considered reasonable, White Mountains estimates the fair value using industry standard pricing methodologies and observable inputs such as benchmark yields, reported trades, broker-dealer quotes, issuer spreads, benchmark securities, bids, offers, credit ratings, prepayment speeds, reference data including research publications and other relevant inputs. Given that many fixed maturity investments do not trade on a daily basis, the outside pricing services evaluate a wide range of fixed maturity investments by regularly drawing parallels from recent trades and quotes of comparable securities with similar features. The characteristics used to identify comparable fixed maturity investments vary by asset type and take into account market convention. White Mountains’s process to assess the reasonableness of the market prices obtained from the outside pricing sources covers substantially all of its fixed maturity investments and includes, but is not limited to, the evaluation of pricing methodologies and a review of the pricing services’ quality control procedures on at least an annual basis, a comparison of invested asset prices obtained from alternate independent pricing vendors on at least a semi-annual basis, monthly analytical reviews of certain prices and a review of the underlying assumptions utilized by the pricing services for select measurements on an ad hoc basis throughout the year. White Mountains also performs back-testing of selected investment sales activity to determine whether there are any significant differences between the market price used to value the security prior to sale and the actual sale price of the security on an ad-hoc basis throughout the year. Prices provided by the pricing services that vary by more than $0.5 million and 5% from the expected price based on these assessment procedures are considered outliers, as are prices that have not changed from period to period and prices that have trended unusually compared to market conditions. In circumstances where the results of White Mountains’s review process does not appear to support the market price provided by the pricing services, White Mountains challenges the vendor provided price. If White Mountains cannot gain satisfactory evidence to support the challenged price, White Mountains will rely upon its own internal pricing methodologies to estimate the fair value of the security in question. The valuation process described above is generally applicable to all of White Mountains’s fixed maturity investments. The techniques and inputs specific to asset classes within White Mountains’s fixed maturity investments for Level 2 securities that use observable inputs are as follows: Debt Securities Issued by Corporations The fair value of debt securities issued by corporations is determined from a pricing evaluation technique that uses information from market sources and integrates relative credit information, observed market movements, and sector news. Key inputs include benchmark yields, reported trades, broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including sector, coupon, credit quality ratings, duration, credit enhancements, early redemption features and market research publications. Municipal Obligations The fair value of municipal obligations is determined from a pricing evaluation technique that uses information from market makers, brokers-dealers, buy-side firms, and analysts along with general market information. Key inputs include benchmark yields, reported trades, issuer financial statements, material event notices and new issue data, as well as broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including type, coupon, credit quality ratings, duration, credit enhancements, geographic location and market research publications. Mortgage and Asset-Backed Securities and Collateralized Loan Obligations The fair value of mortgage and asset-backed securities and collateralized loan obligations is determined from a pricing evaluation technique that uses information from market sources and leveraging similar securities. Key inputs include benchmark yields, reported trades, underlying tranche cash flow data, collateral performance, plus new issue data, as well as broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including issuer, vintage, loan type, collateral attributes, prepayment speeds, default rates, recovery rates, cash flow stress testing, credit quality ratings and market research publications. Level 3 Measurements Fair value estimates for investments that trade infrequently and have few or no quoted market prices or other observable inputs are classified as Level 3 measurements. Investments valued using Level 3 fair value estimates are based upon unobservable inputs and include investments in certain fixed maturity investments, common equity securities and other long-term investments where quoted market prices or other observable inputs are unavailable or are not considered reliable or reasonable. Level 3 valuations are generated from techniques that use assumptions not observable in the market. These unobservable inputs reflect White Mountains’s assumptions of what market participants would use in valuing the investment. In certain circumstances, investment securities may start out as Level 3 when they are originally issued, but as observable inputs become available in the market, they may be reclassified to Level 2. Transfers of securities between levels are based on investments held as of the beginning of the period. Other Long-Term Investments As of December 31, 2023, $1,138.2 million of White Mountains’s other long-term investments, which consisted primarily of unconsolidated entities, including Kudu’s Participation Contracts and PassportCard/DavidShield were classified as Level 3 investments in the GAAP fair value hierarchy. The determination of the fair value of these securities involves significant management judgment, and the use of valuation analyses and assumptions that are inherently subjective and uncertain. White Mountains may use a variety of valuation techniques to determine fair value depending on the nature of the investment, including a discounted cash flow analysis, market multiple approach, cost approach and/or liquidation analysis. On an ongoing basis, White Mountains also considers qualitative changes in facts and circumstances, which may impact the valuation of its unconsolidated entities, including economic and market changes in relevant industries, changes to the entity’s capital structure, business strategy and key personnel, and any recent transactions. On a quarterly basis, White Mountains evaluates the most recent qualitative and quantitative information of the business and completes a fair valuation analysis for all other long-term investments classified as Level 3 investments. Periodically, and at least on an annual basis, White Mountains uses a third-party valuation firm to complete an independent valuation analysis of significant unconsolidated entities. Other Long-term Investments - NAV As of December 31, 2023, $833.8 million of White Mountains’s other long-term investments, which consisted of private equity funds and hedge funds, a bank loan fund, Lloyd’s trust deposits and ILS funds, were valued at fair value using NAV as a practical expedient. Investments for which fair value is measured using NAV as a practical expedient are not classified within the fair value hierarchy. White Mountains employs a number of procedures to assess the reasonableness of the fair value measurements for other long-term investments measured at NAV, including obtaining and reviewing interim unaudited and annual audited financial statements as well as periodically discussing the valuations and methodologies used to value the underlying investments of each fund with fund managers. However, since fund managers do not provide sufficient information to evaluate the pricing methods and inputs for each underlying investment, White Mountains considers the valuation inputs to be unobservable. The fair value of White Mountains’s other long-term investments measured at NAV are generally determined using the fund manager’s NAV. In the event that White Mountains believes the fair value of the fund differs from the NAV reported by the fund manager due to illiquidity or other factors, White Mountains will make an adjustment to the reported NAV to more appropriately represent the fair value of its other long-term investment.
Cash and Restricted Cash	Cash and Restricted Cash Cash includes amounts on hand and demand deposits with banks and other financial institutions. Amounts presented in the statement of cash flows are shown net of balances acquired and sold in the purchase or sale of the Company’s consolidated subsidiaries. Cash balances that are not immediately available for general corporate and operating purposes are classified as restricted.
Derivatives	Derivatives From time to time, White Mountains holds derivative financial instruments for risk management purposes. White Mountains recognizes all derivatives as either assets or liabilities, measured at fair value, on its consolidated balance sheet. Changes in the fair value of derivative instruments that meet the criteria for hedge accounting are recognized in other comprehensive income (loss) and reclassified into current period pre-tax income (loss) when the hedged items are recognized therein. Changes in the fair value of derivative instruments that do not meet the criteria for hedge accounting are recognized in current period pre-tax income (loss). As of December 31, 2023, White Mountains holds an interest rate cap derivative instrument that does not meet the criteria for hedge accounting. See Note 9 — “Derivatives”.
Municipal Bond Guarantee Insurance	Municipal Bond Guarantee Insurance All of the contracts issued by BAM are accounted for as insurance contracts under ASC 944-605, Financial Guarantee Insurance Contracts. For capital appreciation bonds, par is adjusted to the estimated equivalent par value for current interest paying bonds. See Note 10 — “Municipal Bond Guarantee Insurance”. Premiums are generally received upfront and an unearned premium revenue liability, equal to the amount of the premium received, is established at contract inception. Premium revenues are recognized in revenue over the period of the contracts in proportion to the amount of insurance protection provided using a constant rate. The constant rate is calculated based on the relationship between the par outstanding in a given reporting period compared with the sum of each of the par amounts outstanding for all periods. Insurance premiums receivable represents amounts due from customers for municipal bond insurance policies. Installment premiums are measured at the present value of contractual premiums, discounted at the risk-free rate, which is set at the inception of the insurance contract. Deferred acquisition costs represent commissions, premium taxes, excise taxes and other costs which are directly attributable to and vary with the production of business. These costs are deferred and amortized to the extent they relate to successful contract acquisitions over the applicable premium recognition period as acquisition expenses. Deferred acquisition costs are limited to the amount expected to be recovered from future earned premiums and anticipated investment income. A premium deficiency is recognized if the sum of expected loss and loss adjustment expenses (“LAE”), expected dividends to policyholders, unamortized acquisition costs, and maintenance costs exceeds related unearned premiums and anticipated investment income. A premium deficiency is recognized by charging any unamortized acquisition costs to expense to the extent required in order to eliminate the deficiency. If the premium deficiency exceeds unamortized acquisition costs then a liability is accrued for the excess deficiency. Loss reserves are recorded only to the extent that the present value of any payments projected to be made by BAM, net of any expected recoveries, exceeds the associated unearned premium reserve. As of December 31, 2023 and 2022, BAM did not have any loss or loss adjustment expense reserves.
Property and Casualty Insurance and Reinsurance	Property and Casualty Insurance and Reinsurance Ark writes a diversified portfolio of reinsurance and insurance products, including property, specialty, marine & energy, casualty and accident & health. Ark accounts for insurance and reinsurance policies that it writes in accordance with ASC 944, Financial Services - Insurance. Ark’s premiums written comprise premiums on insurance contracts incepted during the year as well as premium adjustments related to prior underwriting years. Insurance premiums are recognized as revenues over the loss exposure or coverage period. In most cases, premiums are earned ratably over the term of the contract, with unearned premiums calculated on a monthly pro-rata basis. Catastrophe premiums are earned in proportion to the insurance protection provided. Premiums earned are presented net of amounts ceded to reinsurers. Insurance premiums receivable, representing amounts due from insureds, are presented net of an allowance for uncollectible premiums, including expected credit losses. The allowance is based upon Ark’s ongoing review of amounts outstanding, historical loss data, including delinquencies and write-offs, current and forecasted economic conditions and other relevant factors. Credit risk is partially mitigated by Ark’s ability to cancel the policy if the policyholder does not pay the premium. Deferred acquisition costs comprise commission and brokerage fees and taxes which are directly attributable to and vary with the production of business. These costs are deferred and amortized to the extent they relate to successful contract acquisitions over the applicable premium recognition period as insurance and reinsurance acquisition expenses. Deferred acquisition costs are limited to the amount expected to be recovered from future earned premiums and anticipated investment income. A premium deficiency is recognized if the sum of expected loss and LAE, expected dividends to policyholders, unamortized acquisition costs and maintenance costs exceeds related unearned premiums and anticipated investment income. A premium deficiency is recognized by charging any unamortized acquisition costs to expense to the extent required in order to eliminate the deficiency. If the premium deficiency exceeds unamortized acquisition costs, then a liability is accrued for the excess deficiency. Losses and LAE are charged against income as incurred. Unpaid losses and LAE, including estimates for amounts incurred but not reported (“IBNR”), are based on estimates of the ultimate costs of settling claims, including the effects of inflation and other societal and economic factors. Unpaid loss and LAE reserves represent management’s best estimate of ultimate losses and LAE, net of estimated salvage and subrogation recoveries, if applicable. Such estimates are regularly reviewed and updated, and any resulting adjustments are reflected in current results of operations. The process of estimating loss and LAE involves a considerable degree of judgment by management, and the ultimate amount of expense to be incurred could be considerably greater than or less than the amounts currently reflected in the financial statements. See Note 5 — “Loss and Loss Adjustment Expense Reserves”. As part of its enterprise risk management function, Ark purchases reinsurance for risk mitigation purposes. Ark utilizes reinsurance and retrocessional agreements to reduce earnings volatility, protect capital, limit its exposure to risk concentration and accumulation of loss and manage within its overall internal risk tolerances or those set and agreed by regulators, ratings agencies and Lloyd’s. Ark also enters into reinsurance and retrocessional agreements to reduce its liability on individual risks and enable it to underwrite policies with higher limits where Ark believes this has a broader business benefit. Ark seeks to protect its downside risk from catastrophes and large loss events by purchasing reinsurance, including excess of loss protections, aggregate covers and industry loss warranties. Ark also considers alternative structures such as collateralized reinsurance, retrocessional reinsurance and catastrophe bonds. The purchase of reinsurance does not discharge Ark from its primary liability for the full value of its policies, and thus the collectability of balances due from Ark’s reinsurers is critical to its financial strength. Ark monitors the financial strength and ratings of its reinsurers on an ongoing basis. See Note 6 — “Third-Party Reinsurance”. For the years of account prior to the Ark Transaction, a significant proportion of the Syndicates’ underwriting capital was provided by TPC Providers using whole account reinsurance contracts with Ark’s corporate member. For the years of account subsequent to the Ark Transaction, Ark is no longer using TPC Providers to provide underwriting capital for the Syndicates. Captions within results of operations and other comprehensive income are shown net of amounts relating to the TPC Providers share of the Syndicates’ results, including investment results. Reinsurance recoverables represent paid losses and LAE, case reserves and IBNR reserves ceded to reinsurers under reinsurance treaties. Amounts recoverable from reinsurers are estimated in a manner consistent with the associated claims liability. Ark reports its reinsurance recoverables net of an allowance for estimated uncollectible reinsurance, including expected credit losses and coverage disputes. The allowance is based upon Ark’s ongoing review of amounts outstanding, length of collection periods, changes in reinsurer credit standing, disputes, applicable coverage defenses and other relevant factors.
Reinsurance Contracts Accounted for as Deposits	Reinsurance Contracts Accounted for as Deposits Reinsurance contracts that do not meet the risk transfer requirements necessary to be accounted for as reinsurance are accounted for using the deposit method. Under the deposit method, ceded premiums paid are not recognized through income but rather treated as a deposit. BAM entered into three ceded reinsurance agreements with Fidus Re Ltd. (“Fidus Re”), a Bermuda-based special purpose insurer created in 2018 solely to provide reinsurance protection to BAM. BAM also entered into an excess of loss reinsurance agreement (the “XOLT”) with HG Re. The financing expenses paid by BAM under these reinsurance agreements are recorded in general and administrative expenses. See Note 10 — “Municipal Bond Guarantee Insurance”. Ark has an aggregate excess of loss contract with SiriusPoint Ltd. (“SiriusPoint”), formerly Third Point Reinsurance Ltd., which is accounted for using the deposit method and recorded within other assets. Ark earns an annual crediting rate of 3.0%, which is recorded within other revenue. See Note 6 — “Third-Party Reinsurance”.
Revenue Recognition	Revenue Recognition Kudu’s revenues are primarily generated from Participation Contracts, which are noncontrolling equity interests in revenue and earnings participation contracts with asset and wealth management firms. Kudu’s Participation Contracts are measured at fair value with the change therein recognized within net realized and unrealized investment gains (losses). Distributions from Kudu’s clients are recognized through investment income when Kudu’s right to receive payment has been established and can be reliably measured, which generally occurs on a quarterly basis in accordance with the terms of the Participation Contracts. White Mountains’s Other Operations recognizes agent commissions and other revenues when it has satisfied its performance obligations. Deferred revenues associated with unsatisfied performance obligations are recognized within other liabilities.
Cost of Sales	Cost of Sales White Mountains’s Other Operations’ cost of sales consists of salaries and related expenses, professional services and marketing and advertising expenses directly related to sales generation. These expenses are recognized as incurred.
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets Goodwill represents the excess of the amount paid to acquire subsidiaries over the fair value of identifiable net assets at the date of acquisition. Other intangible assets consist primarily of underwriting capacity, customer relationships and trade names. Goodwill and other intangible assets with indefinite lives are not amortized, but rather are evaluated for impairment on an annual basis, or whenever indications of potential impairment exist. In the absence of any indications of potential impairment, the evaluation of goodwill and indefinite-lived intangible assets is performed no later than the interim period in which the anniversary of the acquisition date falls. White Mountains initially evaluates goodwill using a qualitative approach (step zero) to determine whether it is more likely than not that the implied fair value of goodwill is greater than its carrying value. If White Mountains determines, based on this qualitative review, that it is more likely than not that an impairment may exist, then White Mountains performs a quantitative analysis to compare the fair value of a reporting unit with its carrying value. If the carrying value exceeds the estimated fair value, then an impairment charge is recognized through current period pre-tax income (loss). Other intangible assets with finite lives are initially measured at their acquisition date fair values and subsequently amortized over their economic lives. Other intangible assets with finite lives are presented net of accumulated amortization on the balance sheet. Other intangible assets with finite lives are reviewed for impairment when events occur or there are changes in circumstances indicating that their carrying value may exceed fair value. Impairment exists when the carrying value of other intangible assets exceeds fair value. See Note 4 — “Goodwill and Other Intangible Assets”.
Incentive Compensation	Incentive Compensation White Mountains’s Long-Term Incentive Plan (the “WTM Incentive Plan”) provides for grants of various types of share-based and non-share-based incentive awards to key employees of White Mountains. Non-share-based awards are recognized over the related service periods based on management’s best estimate of the amounts at which the awards are expected to be paid. Share-based compensation which is typically settled in cash, such as performance shares, is classified as a liability-type award. The compensation cost for liability-classified awards is measured initially at the grant date fair value and remeasured each reporting period until settlement. The compensation cost for equity-classified awards expected to be settled in shares, such as options and restricted shares, is measured at the original grant date fair value of the award. The compensation cost for all awards is recognized for the vested portion of the awards over the related service periods. See Note 12 — “Employee Share-Based Incentive Compensation Plans”.
Income Taxes	Income Taxes White Mountains has subsidiaries and branches that operate in various jurisdictions around the world and are subject to tax in the jurisdictions in which they operate. As of December 31, 2023, the primary jurisdictions in which the Company’s subsidiaries and branches operate and were subject to tax include Israel, Luxembourg, the United Kingdom and the United States. Income earned or losses generated by companies outside the United States are generally subject to an overall effective tax rate lower than that imposed by the United States. Deferred tax assets and liabilities are recorded when a difference between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts for tax purposes exists, and for other temporary differences. The deferred tax asset or liability is recorded based on tax rates expected to be in effect when the difference reverses. Deferred tax assets represent amounts available to reduce income taxes payable in future periods. White Mountains records a valuation allowance against deferred tax assets if it becomes more likely than not that all or a portion of a deferred tax asset will not be realized. Changes in valuation allowances from period to period are included in income tax expense in the period of change. See Note 8 — “Income Taxes”.
Leases	Leases White Mountains has entered into lease agreements, primarily for office space and equipment. These leases are classified as operating leases, with lease expense recognized on a straight-line basis over the term of the lease. Lease assets and liabilities are not recorded for leases with a term at inception of one year or less. Lease incentives, such as free rent or landlord reimbursements for leasehold improvements, are recognized at lease inception and amortized on a straight-line basis over the term of the lease. Lease expense and the amortization of leasehold improvements are recognized within general and administrative expenses. Lease payments related to options to extend or renew the lease term are excluded from the calculation of lease liabilities unless White Mountains is reasonably certain of exercising those options. As of December 31, 2023 and 2022, the right-of-use (“ROU”) assets were $20.3 million and $25.2 million, and the lease liabilities were $22.0 million and $27.1 million.
Noncontrolling Interests	Noncontrolling Interests Noncontrolling interests consist of the ownership interests of noncontrolling shareholders in consolidated subsidiaries and are presented separately on the balance sheet. The portion of comprehensive income (loss) attributable to noncontrolling interests is presented net of related income taxes in the statement of operations and comprehensive income (loss). See Note 13 — “Common Shareholders’ Equity and Noncontrolling Interests”.
Foreign Currency Exchange	Foreign Currency Exchange The functional currencies for White Mountains’s non-U.S. based subsidiaries are measured, in most instances, using currencies other than the U.S. dollar. Net foreign exchange gains and losses arising from the translation of functional currencies are generally reported in shareholders’ equity, in accumulated other comprehensive income (loss). White Mountains also invests in securities denominated in foreign currencies. Assets and liabilities recorded in these foreign currencies are translated into U.S. dollars at exchange rates in effect at the balance sheet date, and revenues and expenses are converted using the weighted average exchange rates for the period. As of December 31, 2023 and 2022, White Mountains had unrealized foreign currency translation losses of $1.6 million and $3.5 million recorded in accumulated other comprehensive income (loss) on its consolidated balance sheet.
Fair Value Measurements	Fair Value Measurements Fair value measurements are categorized into a hierarchy that distinguishes between inputs based on market data from independent sources (observable inputs) and a reporting entity’s internal assumptions based upon the best information available when external market data is limited or unavailable (unobservable inputs). Quoted prices in active markets for identical assets or liabilities have the highest priority (Level 1), followed by observable inputs other than quoted prices, including prices for similar but not identical assets or liabilities (Level 2), and unobservable inputs, including the reporting entity’s estimates of the assumptions that market participants would use, having the lowest priority (Level 3). See Note 18 — “Fair Value of Financial Instruments”.
Business Combinations	Business Combinations White Mountains accounts for purchases of businesses using the acquisition method, which requires the measurement of assets acquired, including other intangible assets, and liabilities assumed, including contingent liabilities, at their estimated fair values as of the acquisition date. The acquisition date fair values represent management’s best estimates and are based upon established valuation techniques, reasonable assumptions and, where appropriate, valuations performed by independent third parties. In circumstances where additional information is required in order to determine the acquisition date fair value of balance sheet amounts, provisional amounts may be recorded as of the acquisition date and may be subject to subsequent adjustment throughout the measurement period, which is up to one year from the acquisition date. Measurement period adjustments are recognized in the period in which they are determined. The results of operations and cash flows of businesses acquired are included in the consolidated financial statements from the date of acquisition. White Mountains accounts for purchases of other intangible assets that do not meet the definition of a business as asset acquisitions. Asset acquisitions are recognized at the amount of consideration paid, which is deemed to equal fair value.

Income Taxes - Net Operating and Capital Loss Carryforwards (Details) $ in Millions	Dec. 31, 2023 USD ($)
Net deferred tax assets
No expiration date	$ 94.2
Total	677.2
Gross deferred tax asset	155.8
Valuation allowance	(155.8)
Total net deferred tax assets	0.0
2024-2028
Net deferred tax assets
2024-2043	0.0
2029-2033
Net deferred tax assets
2024-2043	90.8
2034-2043
Net deferred tax assets
2024-2043	492.2
United States
Net deferred tax assets
No expiration date	1.9
Total	396.8
Gross deferred tax asset	87.6
Valuation allowance	(87.6)
Total net deferred tax assets	0.0
United States \| 2024-2028
Net deferred tax assets
2024-2043	0.0
United States \| 2029-2033
Net deferred tax assets
2024-2043	90.8
United States \| 2034-2043
Net deferred tax assets
2024-2043	304.1
Luxembourg
Net deferred tax assets
No expiration date	0.0
Total	188.1
Gross deferred tax asset	46.9
Valuation allowance	(46.9)
Total net deferred tax assets	0.0
Luxembourg \| 2024-2028
Net deferred tax assets
2024-2043	0.0
Luxembourg \| 2029-2033
Net deferred tax assets
2024-2043	0.0
Luxembourg \| 2034-2043
Net deferred tax assets
2024-2043	188.1
United Kingdom
Net deferred tax assets
No expiration date	3.2
Total	3.2
Gross deferred tax asset	0.8
Valuation allowance	(0.8)
Total net deferred tax assets	0.0
United Kingdom \| 2024-2028
Net deferred tax assets
2024-2043	0.0
United Kingdom \| 2029-2033
Net deferred tax assets
2024-2043	0.0
United Kingdom \| 2034-2043
Net deferred tax assets
2024-2043	0.0
Israel
Net deferred tax assets
No expiration date	89.1
Total	89.1
Gross deferred tax asset	20.5
Valuation allowance	(20.5)
Total net deferred tax assets	0.0
Israel \| 2024-2028
Net deferred tax assets
2024-2043	0.0
Israel \| 2029-2033
Net deferred tax assets
2024-2043	0.0
Israel \| 2034-2043
Net deferred tax assets
2024-2043	$ 0.0

Investment Securities - Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Private equity funds
Investment securities
Fair Value	$ 248.6	$ 140.6
Unfunded Commitments	57.7	94.3
Hedge funds
Investment securities
Fair Value	64.3	57.2
Unfunded Commitments	0.0	0.0
Aerospace/Defense/Government \| Private equity funds
Investment securities
Fair Value	157.2	59.4
Unfunded Commitments	17.4	37.5
Financial services \| Private equity funds
Investment securities
Fair Value	87.5	77.1
Unfunded Commitments	37.8	54.3
Real estate \| Private equity funds
Investment securities
Fair Value	3.9	4.1
Unfunded Commitments	2.5	2.5
Long/short equity financials and business services \| Hedge funds
Investment securities
Fair Value	54.4	49.0
Unfunded Commitments	0.0	0.0
European small/mid cap \| Hedge funds
Investment securities
Fair Value	9.9	8.2
Unfunded Commitments	0.0	0.0
Private equity funds and hedge funds
Investment securities
Fair Value	312.9	197.8
Unfunded Commitments	$ 57.7	$ 94.3

Goodwill and Other Intangible Assets - Schedule of Goodwill and Other Intangible Assets Rollforward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Goodwill
Goodwill, beginning balance	$ 176.5	$ 142.3
Acquisitions of businesses	0.0	59.5
Dispositions of businesses	(6.7)	0.0
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	0.0	(22.9)
Measurement period adjustments	(1.0)	(2.4)
Goodwill, ending balance	168.8	176.5	$ 142.3
Other Intangible Assets
Other intangible assets (excluding goodwill), beginning balance	215.9	198.2
Acquisition of businesses	0.0	0.0
Dispositions of businesses	(6.9)	0.0
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	0.0	22.9
Amortization	(7.2)	(5.2)	(4.6)
Other intangible assets (excluding goodwill), ending balance	201.8	215.9	198.2
Total Goodwill and Other Intangible Assets
Total Goodwill and Other Intangible Assets, beginning balance	392.4	340.5
Acquisition of businesses	0.0	59.5
Disposition of businesses	(13.6)	0.0
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	0.0	0.0
Measurement period adjustments	(1.0)	(2.4)
Amortization	(7.2)	(5.2)	(4.6)
Total Goodwill and Other Intangible Assets, ending balance	$ 370.6	$ 392.4	$ 340.5

SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
HG Global/BAM
SUPPLEMENTARY INSURANCE INFORMATION
Deferred Acquisition Costs	$ 40.1	$ 36.0	$ 33.1
Future Policy Benefits, Losses, Claims and Loss Expenses	0.0	0.0	0.0
Unearned Premiums	325.8	298.3	266.3
Other Policy Claims and Benefits Payable	0.0	0.0	0.0
Premiums Earned	31.2	33.3	26.9
Net Investment Income	31.7	21.5	17.5
Benefits, Claims, Losses and Settlement Expenses	0.0	0.0	0.0
Amortization of Deferred Acquisition Costs	8.6	11.2	8.3
Other Operating Expenses	0.4	0.4	0.4
Premiums Written	58.6	65.1	55.8
Ark/WM Outrigger
SUPPLEMENTARY INSURANCE INFORMATION
Deferred Acquisition Costs	145.3	127.2	100.8
Future Policy Benefits, Losses, Claims and Loss Expenses	1,605.1	1,296.5	894.7
Unearned Premiums	743.6	623.2	495.9
Other Policy Claims and Benefits Payable	0.0	0.0	0.0
Premiums Earned	1,409.7	1,043.4	637.3
Net Investment Income	61.4	16.3	2.9
Benefits, Claims, Losses and Settlement Expenses	726.8	536.4	314.8
Amortization of Deferred Acquisition Costs	273.1	238.3	111.3
Other Operating Expenses	113.6	78.7	64.6
Premiums Written	$ 1,520.9	$ 1,195.2	$ 859.1

Employee Share-Based Incentive Compensation Plans - Restricted Shares (Details) - Restricted Stock - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Restricted Shares
Beginning of period (in shares)	38,350	37,850	43,105
Issued (in shares)	10,895	13,225	13,475
Vested (in shares)	(11,650)	(12,725)	(17,936)
Forfeited (in shares)	0	0	(794)
End of period (in shares)	37,595	38,350	37,850
Accrued Expense
Beginning of period	$ 15.5	$ 15.9	$ 15.2
Issued	16.0	13.8	16.1
Vested	0.0	0.0	0.0
Forfeited	0.0	0.0	(0.8)
Expense recognized	(15.3)	(14.2)	(14.6)
End of period	$ 16.2	$ 15.5	$ 15.9

Significant Transactions - Bamboo (Details) - Bamboo - Subsequent Event $ in Millions	Jan. 02, 2024 USD ($)
Other Significant Transactions [Line Items]
Contributions to subsidiaries	$ 296.7
Contribution of primary capital	20.0
Contributions to retire credit facility	$ 36.0
Basic ownership	72.80%
Ownership percentage, after equity incentives	63.70%
Bamboo Management
Other Significant Transactions [Line Items]
Basic ownership	16.10%
Ownership percentage, after equity incentives	26.60%

SCHEDULE V - VALUATION AND QUALIFYING ACCOUNTS

12 Months Ended

Dec. 31, 2023

SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract]

SCHEDULE V - VALUATION AND QUALIFYING ACCOUNTS

WHITE MOUNTAINS INSURANCE GROUP, LTD.

VALUATION AND QUALIFYING ACCOUNTS


Column A	Column B		Column C				Column D		Column E
			Additions (subtractions)
Millions	Balance at beginning of period		Charged to costs and expenses		Charged to other accounts		Deductions described (1)		Balance at end of period
Years ended:

December 31, 2023
Reinsurance recoverables:
Allowance for uncollectible balances	$	7.8	$	(.2)	$	—	$	.1	$	7.7




December 31, 2022
Reinsurance recoverables:
Allowance for uncollectible balances	3.6		4.6		—		(.4)		7.8
December 31, 2021
Reinsurance recoverables:	2.3		1.3		—		—		3.6
Allowance for uncollectible balances

(1)Represents net collections (charge-offs) of balances receivable and foreign currency translation.

Earnings Per Share (Tables)

12 Months Ended

Dec. 31, 2023

Earnings Per Share [Abstract]

Schedule of earnings per share from continuing operations


	Year Ended December 31,
	2023		2022		2021
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$	509.2	$	792.8	$	(275.4)
Less: total income (loss) from discontinued operations, net of tax (1)	—		903.2		(3.9)
Less: net (income) loss from discontinued operations attributable to noncontrolling interests	—		(.7)		1.0
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	509.2		(109.7)		(272.5)
Allocation of (earnings) losses to participating restricted common shares (2)	(7.2)		1.3		3.2


Basic and diluted earnings (losses) per share numerators	$	502.0	$	(108.4)	$	(269.3)




Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	2,563.3		2,862.4		3,079.0
Average unvested restricted common shares (3)	(36.0)		(36.2)		(36.5)
Basic earnings (losses) per share denominator	2,527.3		2,826.2		3,042.5
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	2,563.3		2,862.4		3,079.0
Average unvested restricted common shares (3)	(36.0)		(36.2)		(36.5)

Diluted earnings (losses) per share denominator	2,527.3		2,826.2		3,042.5
Basic and diluted earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid	$	1.00	$	1.00	$	1.00
Undistributed earnings (losses)	197.60		(39.34)		(89.52)
Basic and diluted earnings (losses) per share	$	198.60	$	(38.34)	$	(88.52)

(2) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.

(3) Restricted shares outstanding vest upon a stated date. See Note 12 — “Employee Share-Based Incentive Compensation Plans”.

The following table presents the Company’s computation of earnings per share for discontinued operations for the years ended December 31, 2022 and 2021:


	Year Ended December 31,
	2022		2021
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$	792.8	$	(275.4)
Less: net income (loss) from continuing operations	(190.8)		(318.0)
Less: net (income) loss from continuing operations attributable to noncontrolling interest	81.1		45.5
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders (1)	902.5		(2.9)
Allocation of (earnings) losses to participating restricted common shares (2)	(11.4)		—
Basic and diluted (loss) earnings per share numerators	$	891.1	$	(2.9)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	2,862.4		3,079.0
Average unvested restricted common shares (3)	(36.2)		(36.5)
Basic earnings (loss) per share denominator	2,826.2		3,042.5
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	2,862.4		3,079.0
Average unvested restricted common shares (3)	(36.2)		(36.5)

Diluted earnings (loss) per share denominator	2,826.2		3,042.5
Basic (loss) earnings per share (in dollars) - discontinued operations:	$	315.30	$	(.94)
Diluted (loss) earnings per share (in dollars) - discontinued operations:	$	315.30	$	(.94)

(2) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities.

(3) Restricted shares outstanding vest upon a stated date. See Note 12 — “Employee Share-Based Incentive Compensation Plans.”

Schedule of undistributed earnings


	Year Ended December 31,
Millions		2023		2022		2021
Undistributed net earnings - continuing operations:
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts		$	502.0	$	(108.4)	$	(269.3)
Dividends declared, net of restricted common share amounts (1)		(2.5)		(3.0)		(3.1)
Total undistributed net earnings (losses), net of restricted common share amounts		$	499.5	$	(111.4)	$	(272.4)

(1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.

Loss and Loss Adjustment Expense Reserves - Incurred Loss and Allocated LAE, Net of Reinsurance (Details) - Ark $ in Millions	Dec. 31, 2023 USD ($) claim	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	$ 908.0
Cumulative Paid Loss and LAE, Net of Reinsurance	551.2
All outstanding liabilities before 2014, net of reinsurance	1.9
Loss and LAE reserves, net of reinsurance	358.7
Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	580.4
Cumulative Paid Loss and LAE, Net of Reinsurance	240.7
All outstanding liabilities before 2014, net of reinsurance	0.0
Loss and LAE reserves, net of reinsurance	339.7
Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	543.9
Cumulative Paid Loss and LAE, Net of Reinsurance	218.5
All outstanding liabilities before 2014, net of reinsurance	6.3
Loss and LAE reserves, net of reinsurance	331.7
Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	181.5
Cumulative Paid Loss and LAE, Net of Reinsurance	49.2
All outstanding liabilities before 2014, net of reinsurance	4.8
Loss and LAE reserves, net of reinsurance	137.1
Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	239.1
Cumulative Paid Loss and LAE, Net of Reinsurance	186.7
All outstanding liabilities before 2014, net of reinsurance	28.9
Loss and LAE reserves, net of reinsurance	81.3
2014 \| Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	28.5	$ 28.6	$ 28.6	$ 28.6	$ 28.6	$ 28.5	$ 28.7	$ 29.4	$ 29.5	$ 32.8
Total IBNR plus expected development on reported claims	$ 0.1
Cumulative number of reported claims \| claim	2,924
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 28.0	28.0	28.0	28.0	27.9	27.7	27.6	27.3	25.1	13.7
2014 \| Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	43.5	43.5	43.7	43.8	43.8	41.2	40.8	41.2	44.2	46.0
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims \| claim	1,359
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 43.7	43.4	43.1	43.2	42.4	41.1	40.5	40.1	39.3	26.5
2014 \| Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	13.6	13.9	13.8	14.0	13.7	14.1	16.3	17.2	20.2	34.4
Total IBNR plus expected development on reported claims	$ (0.3)
Cumulative number of reported claims \| claim	2,572
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 13.8	13.9	13.6	13.8	13.6	14.3	14.2	13.4	12.3	5.9
2014 \| Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	7.7	7.8	7.6	7.8	7.7	8.1	8.2	8.4	9.4	13.3
Total IBNR plus expected development on reported claims	$ 0.3
Cumulative number of reported claims \| claim	1,393
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 6.7	6.2	6.0	5.9	5.5	5.2	4.7	4.2	3.5	1.3
2014 \| Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	59.7	58.7	58.9	59.1	59.1	57.6	54.0	51.0	48.4	49.2
Total IBNR plus expected development on reported claims	$ 1.9
Cumulative number of reported claims \| claim	1,947
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 54.9	51.9	49.4	48.7	47.0	43.2	36.5	29.6	23.2	$ 6.5
2015 \| Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	16.1	15.8	15.9	16.2	16.1	16.3	17.3	18.3	19.3
Total IBNR plus expected development on reported claims	$ 0.2
Cumulative number of reported claims \| claim	2,829
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 15.5	15.1	15.1	14.9	14.7	14.7	13.5	12.3	6.9
2015 \| Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	8.2	9.0	11.3	11.3	11.1	10.8	12.4	14.8	17.4
Total IBNR plus expected development on reported claims	$ (0.1)
Cumulative number of reported claims \| claim	1,841
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 6.3	6.5	8.2	8.2	8.1	8.1	7.7	7.1	4.0
2015 \| Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	12.9	13.1	12.8	13.0	12.8	13.4	16.3	17.5	21.8
Total IBNR plus expected development on reported claims	$ 0.1
Cumulative number of reported claims \| claim	3,242
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 11.7	11.6	11.0	10.6	10.5	11.1	9.7	7.9	4.0
2015 \| Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	7.5	7.3	7.0	7.2	7.4	8.1	8.2	9.8	9.6
Total IBNR plus expected development on reported claims	$ 0.4
Cumulative number of reported claims \| claim	1,295
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 6.1	5.5	5.1	4.9	4.7	4.4	3.2	2.4	1.8
2015 \| Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	40.3	39.3	36.8	37.5	36.5	36.8	33.3	32.1	36.7
Total IBNR plus expected development on reported claims	$ 2.0
Cumulative number of reported claims \| claim	2,000
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 35.3	33.1	29.0	27.4	24.7	21.4	14.5	8.2	$ 4.3
2016 \| Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	18.7	18.8	18.8	18.6	18.6	18.5	17.7	22.5
Total IBNR plus expected development on reported claims	$ 0.2
Cumulative number of reported claims \| claim	3,428
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 18.1	17.9	17.3	17.0	16.9	16.5	13.2	8.6
2016 \| Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	8.6	9.0	11.9	11.9	11.3	11.0	14.4	18.4
Total IBNR plus expected development on reported claims	$ 0.1
Cumulative number of reported claims \| claim	1,932
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 8.4	8.6	10.4	10.4	9.9	9.1	8.0	3.2
2016 \| Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	13.8	14.2	15.0	14.4	14.7	15.8	19.7	23.6
Total IBNR plus expected development on reported claims	$ 0.1
Cumulative number of reported claims \| claim	3,770
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 13.6	13.6	13.9	13.3	13.1	12.7	10.1	5.6
2016 \| Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	10.1	9.2	9.2	9.1	9.0	8.9	8.3	8.8
Total IBNR plus expected development on reported claims	$ 0.5
Cumulative number of reported claims \| claim	1,553
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 8.1	6.5	5.3	4.6	4.0	2.3	1.0	0.2
2016 \| Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	37.5	37.7	38.6	38.8	38.6	40.4	32.2	32.6
Total IBNR plus expected development on reported claims	$ 2.3
Cumulative number of reported claims \| claim	2,158
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 31.0	28.7	27.8	25.4	22.7	17.7	10.2	$ 3.9
2017 \| Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	44.2	42.7	43.2	44.6	45.6	38.1	31.3
Total IBNR plus expected development on reported claims	$ 15.3
Cumulative number of reported claims \| claim	4,613
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 25.7	27.5	29.8	33.0	31.8	26.0	16.9
2017 \| Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	10.4	10.7	11.7	11.6	12.1	13.0	18.1
Total IBNR plus expected development on reported claims	$ 0.1
Cumulative number of reported claims \| claim	2,191
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 9.1	9.3	8.6	8.6	8.5	6.7	3.2
2017 \| Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	16.1	15.9	16.8	17.1	17.7	19.5	26.2
Total IBNR plus expected development on reported claims	$ 0.3
Cumulative number of reported claims \| claim	4,133
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 14.1	14.2	14.2	14.2	12.9	11.2	5.1
2017 \| Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	10.7	10.5	9.1	9.4	10.8	11.7	11.6
Total IBNR plus expected development on reported claims	$ 1.1
Cumulative number of reported claims \| claim	1,632
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 7.5	5.7	4.2	3.4	2.8	1.7	0.8
2017 \| Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	28.3	29.9	31.6	32.1	31.4	34.0	30.6
Total IBNR plus expected development on reported claims	$ 2.9
Cumulative number of reported claims \| claim	1,603
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 22.8	22.5	21.4	18.5	14.7	9.4	$ 3.2
2018 \| Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	46.7	47.1	47.0	49.3	47.4	41.0
Total IBNR plus expected development on reported claims	$ 1.9
Cumulative number of reported claims \| claim	4,269
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 43.1	41.1	40.3	40.3	32.4	15.7
2018 \| Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	15.8	14.9	16.1	16.8	16.4	14.6
Total IBNR plus expected development on reported claims	$ 0.4
Cumulative number of reported claims \| claim	2,117
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 13.1	11.9	10.5	10.1	8.3	2.8
2018 \| Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	18.1	17.7	18.1	17.8	20.3	25.8
Total IBNR plus expected development on reported claims	$ 0.5
Cumulative number of reported claims \| claim	3,225
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 15.6	15.6	15.0	14.3	12.7	2.7
2018 \| Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	9.2	8.6	10.9	11.1	13.3	12.9
Total IBNR plus expected development on reported claims	$ 1.4
Cumulative number of reported claims \| claim	1,098
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 6.2	4.3	4.3	3.5	1.4	0.3
2018 \| Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	28.0	26.2	26.6	27.3	28.2	33.7
Total IBNR plus expected development on reported claims	$ 4.1
Cumulative number of reported claims \| claim	1,277
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 18.2	16.3	14.9	12.6	7.4	$ 3.4
2019 \| Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	23.4	24.0	27.2	31.4	34.2
Total IBNR plus expected development on reported claims	$ 0.8
Cumulative number of reported claims \| claim	4,011
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 19.4	18.6	18.5	16.8	6.8
2019 \| Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	30.1	25.7	18.7	19.6	21.8
Total IBNR plus expected development on reported claims	$ 0.2
Cumulative number of reported claims \| claim	2,368
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 25.2	18.3	7.5	7.0	4.9
2019 \| Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	22.1	21.6	21.8	21.8	23.9
Total IBNR plus expected development on reported claims	$ 0.9
Cumulative number of reported claims \| claim	2,366
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 15.5	14.5	12.7	10.7	3.4
2019 \| Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	11.4	10.6	12.4	13.7	14.8
Total IBNR plus expected development on reported claims	$ 2.6
Cumulative number of reported claims \| claim	954
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 5.7	3.0	2.3	1.4	0.3
2019 \| Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	23.6	24.9	23.4	23.3	26.6
Total IBNR plus expected development on reported claims	$ 5.8
Cumulative number of reported claims \| claim	971
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 15.2	12.2	7.8	5.8	$ 3.3
2020 \| Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	78.4	75.1	75.8	77.6
Total IBNR plus expected development on reported claims	$ 12.4
Cumulative number of reported claims \| claim	4,617
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 56.1	47.3	34.3	11.3
2020 \| Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	19.9	19.0	23.2	24.2
Total IBNR plus expected development on reported claims	$ 1.7
Cumulative number of reported claims \| claim	1,994
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 18.3	13.2	10.7	5.3
2020 \| Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	27.6	28.7	27.3	30.0
Total IBNR plus expected development on reported claims	$ 1.6
Cumulative number of reported claims \| claim	1,561
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 18.7	16.2	12.8	3.2
2020 \| Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	9.2	10.9	12.1	13.5
Total IBNR plus expected development on reported claims	$ 4.3
Cumulative number of reported claims \| claim	605
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 3.3	2.0	1.0	0.5
2020 \| Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	10.9	13.9	12.3	15.9
Total IBNR plus expected development on reported claims	$ 3.9
Cumulative number of reported claims \| claim	566
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 6.0	3.1	1.3	$ 0.8
2021 \| Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	167.3	155.5	171.8
Total IBNR plus expected development on reported claims	$ 5.7
Cumulative number of reported claims \| claim	3,465
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 131.3	87.1	30.9
2021 \| Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	51.7	62.6	71.2
Total IBNR plus expected development on reported claims	$ 16.4
Cumulative number of reported claims \| claim	1,696
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 35.7	24.1	5.1
2021 \| Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	67.7	69.8	86.8
Total IBNR plus expected development on reported claims	$ 9.3
Cumulative number of reported claims \| claim	1,461
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 38.5	24.5	6.3
2021 \| Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	16.6	22.4	21.4
Total IBNR plus expected development on reported claims	$ 12.4
Cumulative number of reported claims \| claim	842
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 3.1	0.9	0.5
2021 \| Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	5.5	7.0	10.5
Total IBNR plus expected development on reported claims	$ 3.0
Cumulative number of reported claims \| claim	282
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 1.9	1.7	$ 0.5
2022 \| Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	270.5	244.3
Total IBNR plus expected development on reported claims	$ 14.3
Cumulative number of reported claims \| claim	3,899
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 193.9	70.9
2022 \| Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	176.6	181.1
Total IBNR plus expected development on reported claims	$ 97.2
Cumulative number of reported claims \| claim	1,407
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 62.1	16.0
2022 \| Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	154.2	150.2
Total IBNR plus expected development on reported claims	$ 50.4
Cumulative number of reported claims \| claim	1,802
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 66.4	12.3
2022 \| Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	38.1	33.0
Total IBNR plus expected development on reported claims	$ 34.4
Cumulative number of reported claims \| claim	1,237
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 1.6	0.4
2022 \| Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	2.6	0.8
Total IBNR plus expected development on reported claims	$ 1.8
Cumulative number of reported claims \| claim	77
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 0.5	$ 0.3
2023 \| Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	214.2
Total IBNR plus expected development on reported claims	$ 156.9
Cumulative number of reported claims \| claim	2,963
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 20.1
2023 \| Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	215.6
Total IBNR plus expected development on reported claims	$ 151.6
Cumulative number of reported claims \| claim	1,258
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 18.8
2023 \| Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	197.8
Total IBNR plus expected development on reported claims	$ 154.8
Cumulative number of reported claims \| claim	1,539
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 10.6
2023 \| Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	61.0
Total IBNR plus expected development on reported claims	$ 57.9
Cumulative number of reported claims \| claim	1,065
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 0.9
2023 \| Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	2.7
Total IBNR plus expected development on reported claims	$ 1.6
Cumulative number of reported claims \| claim	40
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 0.9