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Document and Entity Information - $ / shares	3 Months Ended
Document and Entity Information - $ / shares	Mar. 31, 2018	May 01, 2018	Dec. 31, 2017
Document and Entity Information [Abstract]
Entity Registrant Name	WHITE MOUNTAINS INSURANCE GROUP LTD
Entity Central Index Key	0000776867
Document Type	10-Q
Document Period End Date	Mar. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		3,753,405
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1
Common shares, par value per share (in dollars per share)	$ 1.00		$ 1.00

CONSOLIDATED BALANCE SHEETS (Statement) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Assets
Goodwill and other intangible assets	$ 59.2	$ 62.1
Total assets	3,587.0	3,659.2
Liabilities
Debt	21.5	23.8
Total liabilities	291.2	298.4
White Mountains’s common shareholders’ equity
White Mountains’ common shares	3.8	3.8
Paid-in surplus	671.5	666.8
Retained earnings	2,765.0	2,823.2
Accumulated other comprehensive loss, after tax:
Net unrealized foreign currency translation losses	(1.3)	(1.3)
Total White Mountains’s common shareholders’ equity	3,439.0	3,492.5
Non-controlling interests
Non-controlling interests	(143.2)	(131.7)
Total equity	3,295.8	3,360.8
Total liabilities and equity	3,587.0	3,659.2
HG Global-BAM
Assets
Fixed maturity investments, at fair value	622.1	623.6
Short-term investments, at amortized cost (which approximates fair value)	88.0	69.8
Total investments	710.1	693.4
Cash	9.4	25.6
Insurance premiums receivable	5.2	4.5
Deferred acquisition costs	15.9	14.8
Accrued investment income	4.4	3.4
Accounts receivable on unsettled investment sales	0.0	0.1
Other assets	5.1	5.6
Total assets	750.1	747.4
Liabilities
Unearned insurance premiums	140.2	136.8
Accrued incentive compensation	8.4	18.2
Accounts payable on unsettled investment purchases	18.8	0.6
Other liabilities	12.2	11.4
Total liabilities	179.6	167.0
MediaAlpha
Assets
Cash	15.9	9.1
Goodwill and other intangible assets	50.8	53.7
Accounts receivable from publishers and advertisers	35.4	32.4
Other assets	1.6	1.3
Total assets	103.7	96.5
Liabilities
Debt	21.5	23.8
Amounts due to publishers and advertisers	37.4	31.6
Accrued incentive compensation	0.9	2.0
Other liabilities	0.9	2.4
Total liabilities	60.7	59.8
Non-controlling interests
Non-controlling interests	16.2	13.1
Other Entity
Assets
Fixed maturity investments, at fair value	711.7	1,506.1
Short-term investments, at amortized cost (which approximates fair value)	675.0	106.3
Common equity securities, at fair value	945.6	866.1
Other long-term investments	253.1	208.8
Total investments	2,585.4	2,687.3
Cash	94.9	62.4
Accrued investment income	10.7	13.9
Accounts receivable on unsettled investment sales	14.0	20.9
Goodwill and other intangible assets	8.4	8.4
Other assets	16.5	19.1
Assets held for sale	3.3	3.3
Total assets	2,733.2	2,815.3
Liabilities
Accrued incentive compensation	23.7	60.6
Accounts payable on unsettled investment purchases	17.7	0.0
Other liabilities	9.5	11.0
Total liabilities	$ 50.9	$ 71.6

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Cash	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Common shares, par value per share (in dollars per share)	$ 1.00	$ 1.00
Common shares, authorized shares	50,000,000	50,000,000
Common shares, issued shares	3,753,405	3,750,171
Common shares, outstanding shares	3,753,405	3,750,171

CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenues:
Earned insurance premiums	$ 3.0	$ 3.0
Net investment income	19.7	12.8
Net realized and unrealized investment gains	(53.7)	36.3
Advertising and commission revenues	71.0	33.8
Other revenue	2.1	2.9
Total revenues	42.1	88.8
Expenses:
Loss and loss adjustment expenses		1.1
Insurance acquisition expenses	1.4	1.3
Other underwriting expenses	0.1	0.1
General and administrative expenses	47.9	60.7
Amortization of other intangible assets	2.9	2.3
Interest expense	0.6	0.4
Total expenses	108.1	92.4
Pre-tax income (loss) from continuing operations	(66.0)	(3.6)
Income tax benefit	(0.7)	0.3
Net income (loss) from continuing operations	(66.7)	(3.3)
Gain from sale of other discontinued operations, net of tax	0.1	(1.0)
Net (loss) income from discontinued operations, net of tax	0.0	32.3
Net income	(66.6)	28.0
Net loss (income) attributable to non-controlling interests	18.6	1.0
Net income attributable to White Mountains’s common shareholders	(48.0)	29.0
HG Global-BAM
Revenues:
Earned insurance premiums	3.0	2.0
Net investment income	3.7	2.6
Net realized and unrealized investment gains	(7.9)	1.3
Other revenue	0.2	0.4
Total revenues	(1.0)	6.3
Expenses:
Insurance acquisition expenses	1.4	1.2
Other underwriting expenses	0.1	0.1
General and administrative expenses	11.8	10.6
Total expenses	13.3	11.9
MediaAlpha
Revenues:
Advertising and commission revenues	70.1	32.5
Other revenue	1.6	0.0
Total revenues	71.7	32.5
Expenses:
Cost of sales	57.4	27.7
General and administrative expenses	11.2	3.2
Amortization of other intangible assets	2.9	2.4
Interest expense	0.4	0.2
Total expenses	71.9	33.5
Other Entity
Revenues:
Earned insurance premiums	0.0	1.0
Net investment income	16.0	10.2
Net realized and unrealized investment gains	(45.8)	35.0
Advertising and commission revenues	0.9	1.3
Other revenue	0.3	2.5
Total revenues	(28.6)	50.0
Expenses:
Loss and loss adjustment expenses	0.0	1.1
Insurance acquisition expenses	0.0	0.1
Cost of sales	0.7	1.1
General and administrative expenses	22.0	44.5
Interest expense	0.2	0.2
Total expenses	$ 22.9	$ 47.0

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net (loss) income attributable to White Mountains’s common shareholders	$ (48.0)	$ 29.0
Other comprehensive income, net of tax:
Other comprehensive income, net of tax	0.0	0.1
Comprehensive income from discontinued operations, net of tax	0.0	0.1
Comprehensive income	(48.0)	29.2
Other comprehensive income attributable to non-controlling interests	0.0	0.0
Comprehensive income attributable to White Mountains’s common shareholders	$ (48.0)	$ 29.2
Basic (loss) earnings per share
Continuing operations (in usd per share)	$ (12.85)	$ (0.52)
Discontinued operations (in usd per share)	0.03	6.86
Total consolidated operations (in usd per share)	(12.82)	6.34
Diluted (loss) earnings per share
Continuing operations (in usd per share)	(12.85)	(0.52)
Discontinued operations (in usd per share)	0.03	6.86
Total consolidated operations (in usd per share)	(12.82)	6.34
Dividends declared per White Mountains’s common share	$ 1.00	$ 1.00

CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (Unaudited) - USD ($) $ in Millions	Total	Common shares and paid-in surplus	Retained earnings	AOCI, after tax	Total	Non-controlling interest	Total Equity	Foreign Currency Gain (Loss) Common shares and paid-in surplus	Foreign Currency Gain (Loss) Retained earnings	Foreign Currency Gain (Loss) AOCI, after tax	Foreign Currency Gain (Loss) Total	Foreign Currency Gain (Loss) Non-controlling interest	Foreign Currency Gain (Loss) Total Equity	OneBeacon Common shares and paid-in surplus	OneBeacon Retained earnings	OneBeacon AOCI, after tax	OneBeacon Total	OneBeacon Non-controlling interest	OneBeacon Total Equity	Sale of Tranzact Common shares and paid-in surplus	Sale of Tranzact Retained earnings	Sale of Tranzact AOCI, after tax	Sale of Tranzact Total	Sale of Tranzact Non-controlling interest	Sale of Tranzact Total Equity
Beginning Balances at Dec. 31, 2016		$ 810.7	$ 2,776.6	$ (4.6)	$ 3,582.7	$ 133.3	$ 3,716.0
Increase (Decrease) in Shareholders' Equity
Net loss (income)	$ 28.0	0.0	29.0	0.0	29.0	(1.0)	28.0
Net change in foreign currency translation and benefit plan assets and obligations								$ 0.0	$ 0.0	$ 0.2	$ 0.2	$ 0.0	$ 0.2
Total comprehensive income (loss)	29.2	0.0	29.0	0.2	29.2	(1.0)	28.2
Dividends declared on common shares		0.0	(4.6)	0.0	(4.6)	0.0	(4.6)
Dividends to non-controlling interests		0.0	0.0	0.0	0.0	(6.6)	(6.6)
Repurchases and retirements of common shares		(1.4)	(5.1)	0.0	(6.5)	(1.1)	(7.6)
Dilution from equity-based units of subsidiary														$ (4.1)	$ 0.0	$ 0.0	$ (4.1)	$ 4.1	$ 0.0
Capital contributions from BAM members, net of tax	(9.6)	0.0	0.0	0.0	0.0	7.0	7.0
Amortization of restricted share awards		2.6	0.0	0.0	2.6	0.2	2.8
Deconsolidation of non-controlling interests associated with the sale of Tranzact																				$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ (4.4)	$ (4.4)
Ending Balances at Mar. 31, 2017	3,730.8	807.8	2,795.9	(4.4)	3,599.3	131.5	3,730.8
Beginning Balances at Dec. 31, 2016		810.7	2,776.6	(4.6)	3,582.7	133.3	3,716.0
Increase (Decrease) in Shareholders' Equity
Recognition of equity-based units of subsidiary						(2.3)	(6.4)
Dilution from equity-based units of subsidiary						0.9	0.0
Ending Balances at Dec. 31, 2017	3,360.8	670.6	2,823.2	(1.3)	3,492.5	(131.7)	3,360.8
Increase (Decrease) in Shareholders' Equity
Net loss (income)	(66.6)	0.0	(48.0)	0.0	(48.0)	(18.6)	(66.6)
Net change in foreign currency translation and benefit plan assets and obligations								$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Total comprehensive income (loss)	(48.0)	0.0	(48.0)	0.0	(48.0)	(18.6)	(66.6)
Dividends declared on common shares		0.0	(3.8)	0.0	(3.8)	0.0	(3.8)
Dividends to non-controlling interests		0.0	0.0	0.0	0.0	(0.3)	(0.3)
Repurchases and retirements of common shares		(1.9)	(6.4)	0.0	(8.3)	0.0	(8.3)
Recognition of equity-based units of subsidiary		(4.1)	0.0	0.0	(4.1)
Dilution from equity-based units of subsidiary		(0.9)	0.0	0.0	(0.9)
Capital contributions from BAM members, net of tax	(4.9)	0.0	0.0	0.0	0.0	4.2	4.2
Amortization of restricted share awards		3.4	0.0	0.0	3.4	0.0	3.4
Ending Balances at Mar. 31, 2018	$ 3,295.8	$ 675.3	$ 2,765.0	$ (1.3)	$ 3,439.0	$ (143.2)	$ 3,295.8

CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Net Cash Provided by (Used in) Operating Activities, Continuing Operations [Abstract]
Net loss (income)	$ (66.6)		$ 28.0
Charges (credits) to reconcile net income to net cash used for operations:
Net realized and unrealized investment losses	53.7		(36.3)
Deferred income benefit	(0.8)		(3.0)
Net income from discontinued operations	0.0		(32.3)
Net (gain) loss from sale of discontinued operations, net of tax	(0.1)	$ (0.1)	1.0
Amortization of restricted share and option awards	3.2		1.8
Amortization and depreciation	4.8		5.8
Net change in unearned insurance premiums	3.4		17.4
Net change in deferred acquisition costs	(1.1)		(1.3)
Net change in other assets and liabilities, net	(41.3)		(35.9)
Net cash used for operations - continuing operations	(44.8)		(54.8)
Net cash provided from used for operations	(44.7)		(56.1)
Cash flows from investing activities:
Net change in short-term investments	(587.5)		36.1
Sales of fixed maturity and convertible investments	1,266.5		375.4
Maturities, calls and paydowns of fixed maturity and convertible investments	40.6		70.2
Sales of common equity securities	14.4		15.9
Distributions and redemptions of other long-term investments and settlements of forward contracts	(6.9)		3.2
Net settlement of investment cash flows and contributions with discontinued operations	0.1		0.0
Purchases of other long-term investments	(46.0)		(21.6)
Purchases of common equity securities	(109.4)		(111.9)
Purchases of fixed maturity and convertible investments	(537.1)		(363.4)
Net change in unsettled investment purchases and sales	42.9		3.1
Net acquisitions of property and equipment	(0.1)		0.0
Net cash provided from investing activities - continuing operations	77.5		7.0
Net cash provided from investing activities	77.4		39.3
Cash flows from financing activities:
Repayment of debt and revolving line of credit	(2.3)		(1.2)
Cash dividends paid to the Company’s common shareholders	(3.8)		(4.6)
Distribution to non-controlling interest shareholders	0.0		(0.3)
Contributions from discontinued operations	0.0		15.1
Capital contributions from BAM members	4.9		9.6
Restricted share statutory withholding tax payments	(8.4)		(6.5)
Net cash used for financing activities - continuing operations	(9.6)		12.1
Net cash used for financing activities	(9.6)		(8.9)
Net change in cash during the period - continuing operations	23.1		(35.7)
Cash balances at beginning of period	97.1		80.2	$ 80.2
Add: cash held for sale, excluding discontinued operations, at the beginning of period	0.0	0.0	0.0	0.0	$ 0.9
Less: cash held for sale, excluding discontinued operations, at the end of period	0.0	0.0	0.0	0.0	0.9
Cash balances at end of period	120.2	97.1	45.4	97.1	80.2
Supplemental cash flows information:
Interest paid	(0.3)		(0.2)
Net income tax refunds	0.2		0.0
Discontinued Operations
Charges (credits) to reconcile net income to net cash used for operations:
Net cash provided from (used for) operations - discontinued operations	0.1		(1.3)
Cash flows from investing activities:
Net cash (used for) provided from investing activities - discontinued operations	(0.1)		32.3
Cash flows from financing activities:
Net cash used for financing activities - discontinued operations	0.0		(21.0)
Cash balances at beginning of period	0.0		70.5	70.5
Cash balances at end of period	0.0	$ 0.0	79.6	$ 0.0	$ 70.5
Supplemental cash flows information:
Net income tax refunds	$ 0.0		$ 0.0

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Cash balance	$ 120.2	$ 97.1	$ 45.4	$ 80.2
Discontinued Operations
Cash balance	$ 0.0	$ 0.0	$ 79.6	$ 70.5

Basis of Presentation and Significant Accouting Policies	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation and Significant Accouting Policies	Basis of Presentation and Significant Accounting Policies Basis of Presentation The Company is an exempted Bermuda limited liability company whose principal businesses are conducted through its insurance 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 80 South Main Street, Hanover, New Hampshire 03755-2053 and its registered office is located at Clarendon House, 2 Church Street, Hamilton, Bermuda HM 11. 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 White Mountains Insurance Group, Ltd. (the “Company” or the “Registrant”), its subsidiaries (collectively with the Company, “White Mountains”) and other entities required to be consolidated under GAAP. 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. Intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. These interim financial statements include all adjustments considered necessary by management to fairly state the financial position, results of operations and cash flows of White Mountains. These interim financial statements may not be indicative of financial results for the full year and should be read in conjunction with the Company’s 2017 Annual Report on Form 10-K. 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 White Mountains has determined its reportable segments based on the nature of the underlying businesses, the manner in which the Company’s subsidiaries and affiliates are organized and managed and the organization of the financial information provided to the chief operating decision maker to assess performance and make decisions regarding allocation of resources. White Mountains’s reportable segments are HG Global/BAM, MediaAlpha and Other Operations. See Note 12 — “Segment Information”. The HG Global/BAM segment consists of HG Global Ltd. and its wholly-owned subsidiaries (“HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”) (collectively, “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 purposes 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 wholly-owned 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, provided the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM (the “BAM Surplus Notes”). As of March 31, 2018, $499.0 million of the surplus notes remain outstanding. As of March 31, 2018 and December 31, 2017, 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 attributed to non-controlling interests. The MediaAlpha segment consists of QL Holdings LLC and its wholly-owned subsidiary QuoteLab, LLC (collectively “MediaAlpha”). MediaAlpha is a leading marketing technology company that develops technology that enables the programmatic buying and selling of vertical-specific, performance-based media between advertisers (buyers of advertising inventory) and publishers (sellers of advertising inventory) through cost-per-click, cost-per-call and cost-per-lead pricing models. MediaAlpha's media buying platform enables advertisers to create and automate data-driven bidding strategies designed to improve the efficiency and enhance overall performance of their marketing campaigns that target high-intent consumers at the time and place they are ready to purchase. MediaAlpha’s publisher platform is used by publishers to sell their vertical-specific, performance-based media to advertisers through transparent, programmatic, auction-based marketplaces. MediaAlpha works with 550 advertisers and 325 publishers across a number of insurance (auto, motorcycle, home, renter, health and life) and non-insurance (travel, education, personal finance and home services) verticals. White Mountains’s Other Operations segment consists of the Company, its wholly-owned subsidiary, White Mountains Capital, Inc. (“WM Capital”), its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), and its other intermediate holding companies, as well as certain consolidated and unconsolidated private capital and other investments. The consolidated private capital investments include Wobi Insurance Agency Ltd. (“Wobi”) and Removal Stars Ltd. (“Buzzmove”). During 2017, White Mountains revised certain of its previously issued financial statements for amounts relating to Wobi. See Note 17 — “Financial Statement Revisions”. Discontinued Operations and Assets and Liabilities Held for Sale On September 28, 2017, Intact Financial Corporation completed its acquisition of OneBeacon Insurance Group, Ltd. (“OneBeacon”) in an all-cash transaction for $18.10 per share (the “OneBeacon Transaction”). On July 21, 2016, White Mountains completed its sale of Tranzact Holdings, LLC (“Tranzact”) to an affiliate of Clayton, Dubilier & Rice, LLC. On April 18, 2016, White Mountains completed its sale of Sirius International Insurance Group, Ltd. (“Sirius Group”) to CM International Pte. Ltd. and CM Bermuda Limited (collectively “CMI”), the Singapore-based investment arm of China Minsheng Investment Corp., Ltd. White Mountains has presented the results of OneBeacon, Tranzact and Sirius Group as discontinued operations in the statement of operations and comprehensive income and their assets and liabilities as held for sale in the balance sheet for all periods prior to the completion of each transaction. White Mountains has classified its Guilford, Connecticut property, which consists of an office building and adjacent land, as held for sale as of March 31, 2018 and December 31, 2017. See Note 16 — “Held for Sale and Discontinued Operations”. Significant Accounting Policies Refer to the Company’s 2017 Annual Report on Form 10-K for a complete discussion regarding White Mountains’s significant accounting policies. Recently Adopted Changes in Accounting Principles Revenue Recognition On January 1, 2018, White Mountains adopted ASU 2014-09, Revenue from Contracts with Customers (ASC 606), which modifies the guidance for revenue recognition. Under ASU 2014-09, revenue is recognized at an amount that reflects the consideration to which an entity expects to be entitled once it fulfills its performance obligations under the terms of its contract with the customer. The scope of the new guidance includes agent commissions and other non-insurance revenues. Adoption of ASU 2014-09 did not have any impact on White Mountains's financial statements. Share-Based Compensation On January 1, 2018, White Mountains adopted ASU 2017-09, Stock Compensation: Scope of Modification Accounting (ASC 718), which narrows the scope of transactions subject to modification accounting to changes in terms of an award that result in a change in the award’s fair value, vesting conditions or classification. Adoption of ASU 2017-09 did not have any impact on White Mountains’s financial statements. On January 1, 2017, White Mountains adopted ASU 2016-09, Improvements to Employee Share-Based Payment Accounting (ASC 718) which simplifies certain aspects of the accounting for share-based compensation. The new guidance provides an accounting policy election to account for forfeitures by either applying an assumption, as required under existing guidance, or by recognizing forfeitures when they actually occur. At adoption, White Mountains did not change its accounting policy for forfeitures, which is to apply an assumed forfeiture rate. The new guidance has also changed the threshold for partial cash settlement to settle statutory withholding requirements for equity classified awards, increasing the threshold up to the maximum statutory tax rate. As a result of adoption, White Mountains reported $8.4 million and $6.5 million of statutory withholding tax payments made in connection with the settlement of restricted shares as financing cash flows for the three-month periods ended March 31, 2018 and 2017. Such payments were classified as operating cash flows prior to adoption. In addition, the new guidance changed the treatment for excess tax benefits which arise from the difference between the deduction for tax purposes and the compensation costs recognized for financial reporting. Under the new guidance, a reporting entity recognizes excess tax benefits or expense in current period earnings, regardless of whether it is in a taxes payable position. Business Combinations On January 1, 2018, White Mountains adopted ASU 2017-01, Business Combinations: Clarifying the Definition of a Business (ASC 805), which clarifies the definition of a business and affects the determination of whether acquisitions or disposals are accounted for as assets or as a business. Under the new guidance, when substantially all of the fair value of the assets is concentrated in a single identifiable asset or group of similar assets, it is not a business. White Mountains has not had any transactions falling within the scope of ASU 2017-01 during the period ended March 31, 2018 and, accordingly, adoption did not have any impact on White Mountains's financial statements. Cash Flow Statement On January 1, 2018, White Mountains adopted ASU 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (ASC 230), which addresses the classification and presentation of certain items, including debt prepayment and extinguishment costs, contingent consideration payments made after a business combination and distributions received from equity method investees, for which there was diversity in practice prior to the issuance of ASU 2016-15. Also on January 1, 2018, White Mountains adopted ASU 2016-18, Statement of Cash Flows: Restricted Cash (ASC 230), which modifies the guidance for the treatment of restricted cash amounts in the cash flow statement. The new guidance requires restricted cash to be included in the reconciliation of beginning and end-of-period amounts presented on the statement of cash flows and requires a description of the nature of the changes in restricted cash during the periods presented. Adoption of ASU 2016-15 and ASU 2016-18 did not have any impact on White Mountains's statement of cash flows. Financial Instruments - Recognition and Measurement On January 1, 2018, White Mountains adopted ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities (ASC 825-10), which modifies the guidance for financial instruments, including investments in equity securities. Under the new guidance, all equity securities with readily determinable fair values are required to be measured at fair value with changes therein recognized through current period earnings. In addition, the new ASU requires a qualitative assessment for equity securities without readily determinable fair values to identify impairment, and for impaired equity securities to be measured at fair value. White Mountains measures its portfolio of investment securities at fair value with changes therein recognized through current period earnings and, accordingly, adoption of ASU 2016-01 did not have any impact on White Mountains's financial statements. Recently Issued Accounting Pronouncements Premium Amortization on Callable Debt Securities In March 2017, the FASB issued ASU 2017-08, Premium Amortization on Purchased Callable Debt Securities (ASC 310-20), which changes the amortization period for certain purchased callable debt securities. Under the new guidance, for investments in callable debt securities held at a premium, the premium will be amortized over the period to the earliest call date. The new guidance does not change the amortization period for callable debt securities held at a discount. ASU 2017-08 is not expected to have any impact on White Mountains's financial statements at adoption but may affect the amortization recognized in future periods. Credit Losses In June 2016, the FASB issued ASU 2016-13, Measurement of Credit Losses on Financial Instruments (ASC 326), which establishes new guidance for the recognition of credit losses for financial assets measured at amortized cost. The new ASU requires reporting entities to estimate the credit losses expected over the life of a credit exposure using historical information, current information and reasonable and supportable forecasts that affect the collectability of the financial asset. This differs from current GAAP, which delays recognition until it is probable a loss has been incurred. The new guidance is expected to accelerate recognition of credit losses. The types of assets within the scope of the new guidance include premium receivables, reinsurance recoverables and loans. ASU 2016-13 is effective for annual periods beginning after January 1, 2020, including interim periods. White Mountains measures its portfolio of investment securities at fair value with changes therein recognized through current period earnings and, accordingly, does not expect adoption to have any effect on its financial statements. Leases In February 2016, the FASB issued ASU 2016-02, Leases (ASC 842). The new guidance requires lessees to recognize lease assets and liabilities on the balance sheet for both operating and financing leases, with the exception of leases with an original term of 12 months or less. Under existing guidance recognition of lease assets and liabilities is not required for operating leases. The lease assets and liabilities to be recognized are both measured initially based on the present value of the lease payments. Under the new guidance, a sale-leaseback transaction must meet the recognition criteria under ASC 606, Revenues, in order to be accounted for as sale. The new guidance is effective for White Mountains for years beginning after December 15, 2018, including interim periods therein. White Mountains is evaluating the expected impact of this guidance and available adoption methods.

Significant Transactions	3 Months Ended
Significant Transactions	Mar. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Significant Transactions	Significant Transactions Dispositions OneBeacon On September 28, 2017, White Mountains received $1.3 billion in cash proceeds from the OneBeacon Transaction and recorded a gain of $554.6 million, net of transaction costs. As a result of the OneBeacon Transaction, OneBeacon’s results have been reported as discontinued operations within White Mountains’s GAAP financial statements. See Note 16 — “Held for Sale and Discontinued Operations”. Star & Shield On March 7, 2017, White Mountains completed the sale of Star & Shield Services LLC, Star & Shield Risk Management LLC, and Star & Shield Claims Services LLC (collectively “Star & Shield”) and its investment in Star & Shield Insurance Exchange (“SSIE”) surplus notes to K2 Insurance Services, LLC. White Mountains did not recognize any gain or loss on the sale. Through December 31, 2016, Star & Shield’s assets and liabilities are reported as held for sale within White Mountains’s GAAP financial statements. See Note 16 — “Held for Sale and Discontinued Operations”. Acquisitions The following acquisitions are included in White Mountains’s consolidated financial statements from the date of acquisition. The assets acquired and liabilities assumed have been measured at their acquisition date fair values. DavidShield On January 24, 2018, White Mountains acquired 50% of DavidShield Life Insurance Agency (2000) Ltd. (“DavidShield”), its joint venture partner in PassportCard Limited (“PassportCard”). DavidShield is a managing general agency that is the leading provider of expatriate medical insurance in Israel and uses the same card-based delivery system as PassportCard. As part of the transaction, White Mountains restructured its equity stake in PassportCard so that White Mountains and its partner in DavidShield would each own 50% of both businesses. To facilitate the transaction, White Mountains provided financing to its partner in the form of a non-interest bearing loan that is secured by the partner’s equity in PassportCard and DavidShield. The gross purchase price for the 50% of DavidShield was $41.8 million, or $28.3 million net of the financing provided for the restructuring. Kudu On February 5, 2018, White Mountains entered into an agreement to fund up to $127.5 million in Kudu Investment Management, LLC (“Kudu”), a leading capital provider to asset management and wealth management firms. Kudu specializes in providing capital solutions to asset managers and registered investment advisers, including generational ownership transfers, management buyouts, acquisition and growth finance, as well as liquidity for legacy partners. As of March 31, 2018, White Mountains has funded $1.8 million in Kudu. White Mountains has determined that Kudu is a VIE, however White Mountains is not the primary beneficiary. White Mountains has elected to take the fair value option for its investment in Kudu. MediaAlpha On October 5, 2017, MediaAlpha acquired certain assets associated with the Health, Life and Medicare insurance business of Healthplans.com for an aggregate purchase price of $28.0 million. The majority of assets acquired, which are included in other intangible assets, consist of customer relationships, a non-compete agreement from the seller and domain names. See Note 4 — “Goodwill and Other Intangibles Assets”. On October 5, 2017, White Mountains acquired 131,579 newly-issued Class A common units of MediaAlpha for $12.5 million. As of March 31, 2018 and December 31, 2017 White Mountains’s ownership share in MediaAlpha was 62.3% and 64.4%.

Investment Securities

3 Months Ended

Mar. 31, 2018

Investments, Debt and Equity Securities [Abstract]

Investment Securities

Investments Securities

White Mountains’s portfolio of investment securities held for general investment purposes consists of fixed maturity investments, short-term investments, common equity securities and other long-term investments, which are all classified as trading securities. Trading securities are reported at fair value as of the balance sheet date. Net realized and unrealized investment gains (losses) on trading securities are reported in pre-tax revenues.

White Mountains’s fixed maturity investments are generally valued using industry standard pricing methodologies. Key inputs include benchmark yields, benchmark securities, reported trades, issuer spreads, bids, offers, credit ratings and prepayment speeds. 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.

Realized investment gains (losses) resulting from sales of investment securities are accounted for using the specific identification method. Premiums and discounts on all fixed maturity investments are amortized or accreted to income over the anticipated life of the investment. Short-term investments consist of interest-bearing money market funds, certificates of deposit and other securities, which at the time of purchase, mature or become available for use within one year. Short-term investments are carried at amortized or accreted cost, which approximated fair value as of March 31, 2018 and December 31, 2017.

Other long-term investments consist primarily of hedge funds, private equity funds and unconsolidated private capital investments.

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 and dividend income from its common equity securities and other long-term investments.

The following table presents pre-tax net investment income for the three months ended March 31, 2018 and 2017.



	Three Months Ended
	March 31,
Millions	2018			2017
Investment income:
Fixed maturity investments	$	10.9		$	11.9
Short-term investments	2.6			.2
Common equity securities	5.0			1.3
Other long-term investments	1.9			(.1		)
Total investment income	20.4			13.3
Third-party investment expenses	(.7		)	(.5		)
Net investment income, pre-tax	$	19.7		$	12.8

Net Realized and Unrealized Investment Gains (Losses)

The following table presents net realized and unrealized investment gains (losses) for the three months ended March 31, 2018 and 2017:



	Three Months Ended
	March 31,
Millions	2018			2017
Net realized investment (losses) gains, pre-tax	$	(5.1	)	$	.6
Net unrealized investment (losses) gains, pre-tax	(48.6		)	35.7
Net realized and unrealized investment (losses) gains, pre-tax	(53.7		)	36.3
Income tax benefit (expense) attributable to net realized and unrealized investment (losses) gains	5.7			(3.9		)
Net realized and unrealized investment (losses) gains, after-tax	$	(48.0	)	$	32.4

Net Realized Investment Gains (Losses)

The following table presents net realized investment gains (losses) for the three months ended March 31, 2018 and 2017



	Three Months Ended									Three Months Ended
	March 31, 2018									March 31, 2017
Millions	Net realized (losses) gains			Net foreign exchange gains (losses)			Total net realized gains (losses) reflected in earnings			Net realized (losses) gains			Net foreign exchange gains		Total net realized (losses) gains reflected in earnings
Fixed maturity investments	$	(13.7	)	$	18.2		$	4.5		$	(1.0	)	$	.1	$	(.9	)
Short-term investments	(.1		)	—			(.1		)	—			—		—
Common equity securities	1.2			—			1.2			.8			.1		.9
Other long-term investments	(3.5		)	(7.2		)	(10.7		)	.6			—		.6
Net realized investment (losses) gains, pre-tax	(16.1		)	11.0			(5.1		)	.4			.2		.6
Income tax expense attributable to net realized investment (losses) gains	(.6		)	—			(.6		)	(.2		)	—		(.2		)
Net realized investment (losses) gains, after-tax	$	(16.7	)	$	11.0		$	(5.7	)	$	.2		$	.2	$	.4

Net Unrealized Investment Gains (Losses)

The following tables present net unrealized investment gains (losses) and changes in the carrying value of investments measured at fair value for the three months ended March 31, 2018 and 2017:



	Three Months Ended									Three Months Ended
	March 31, 2018									March 31, 2017
Millions	Net unrealized losses			Net foreign exchange (losses) gains			Total net unrealized (losses) gains reflected in earnings			Net unrealized gains			Net foreign exchange gains (losses)			Total net unrealized gains reflected in earnings
Fixed maturity investments	$	(18.6	)	$	(14.8	)	$	(33.4	)	$	10.2		$	1.7		$	11.9
Short-term investments	(.7		)	—			(.7		)	—			—			—
Common equity securities	(16.7		)	—			(16.7		)	19.1			.5			19.6
Other long-term investments	(1.9		)	4.1			2.2			6.9			(2.7		)	4.2
Net unrealized investment (losses) gains, pre-tax	(37.9		)	(10.7		)	(48.6		)	36.2			(.5		)	35.7
Income tax benefit (expense) attributable to net unrealized investment gains (losses)	6.3			—			6.3			(3.7		)	—			(3.7		)
Net unrealized investment (losses) gains, after-tax	$	(31.6	)	$	(10.7	)	$	(42.3	)	$	32.5		$	(.5	)	$	32.0

The following table presents total gains (losses) included in earnings attributable to net unrealized investment gains (losses) for Level 3 investments for the three months ended March 31, 2018 and 2017:



	Three Months Ended
	March 31,
Millions	2018			2017
Other long-term investments	$	(5.1	)	$	.2
Total net unrealized investment (losses) gains, pre-tax - Level 3 investments	$	(5.1	)	$	.2

Investment Holdings

The following tables present the cost or amortized cost, gross unrealized investment gains (losses), net foreign currency gains, and carrying values of White Mountains’s fixed maturity investments as of March 31, 2018 and December 31, 2017.



	March 31, 2018
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency gains		Carrying value
U.S. Government and agency obligations	$	199.9	$	—	$	(2.2	)	$	—	$	197.7
Debt securities issued by corporations	760.5		.7		(17.5		)	—		743.7
Mortgage and asset-backed securities	132.5		.1		(2.6		)	—		130.0
Municipal obligations	261.8		1.9		(1.3		)	—		262.4
Total fixed maturity investments	$	1,354.7	$	2.7	$	(23.6	)	$	—	$	1,333.8



	December 31, 2017
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency gains		Carrying value
U.S. Government and agency obligations	$	297.8	$	—	$	(1.3	)	$	—	$	296.5
Debt securities issued by corporations	867.6		2.9		(4.3		)	14.7		880.9
Mortgage and asset-backed securities	697.2		1.6		(4.1		)	—		694.7
Municipal obligations	252.0		3.7		(.8		)	—		254.9
Foreign government, agency and provincial obligations	2.6		—		—			.1		2.7
Total fixed maturity investments	$	2,117.2	$	8.2	$	(10.5	)	$	14.8	$	2,129.7

The following tables present the cost or amortized cost, gross unrealized investment gains (losses), net foreign currency losses, and carrying values of White Mountains’s common equity securities and other long-term investments as of March 31, 2018 and December 31, 2017:



	March 31, 2018
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency losses			Carrying value
Common equity securities	$	835.9	$	111.7	$	(2.0	)	$	—		$	945.6
Other long-term investments	$	288.8	$	10.0	$	(44.8	)	$	(.9	)	$	253.1



	December 31, 2017
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency losses			Carrying value
Common equity securities	$	739.7	$	129.4	$	(3.0	)	$	—		$	866.1
Other long-term investments	$	246.6	$	6.8	$	(39.7	)	$	(4.9	)	$	208.8

Other Long-Term Investments

The following table presents the carrying values of White Mountains’s other long-term investments as of March 31, 2018 and December 31, 2017:



	Carrying Value at
Millions	March 31, 2018		December 31, 2017
Hedge funds and private equity funds, at fair value	$	130.6	$	125.3
Private equity securities, at fair value ^(1)(2)(3)	108.4		83.2
Foreign currency forward contracts	—		(3.7		)
Other	14.1		4.0
Total other long-term investments	$	253.1	$	208.8

⁽¹⁾See Fair Value Measurements by Level table.

⁽²⁾ Includes non-controlling interests in common equity securities, limited liability companies and private convertible preferred securities.

⁽³⁾White Mountains holds a 20% ownership interest in OneTitle Holdings LLC (“OneTitle”) and has provided a $10.0 million surplus note facility under which OneTitle’s wholly-owned insurance subsidiary, OneTitle National Guaranty Company, Inc., may draw funds under certain circumstances. At March 31, 2018, no funds had been drawn on the surplus note facility.

Hedge Funds and Private Equity Funds

White Mountains invests in hedge funds and private equity 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 March 31, 2018, White Mountains held investments in one hedge fund and ten private equity funds. The largest investment in a single fund was $56.4 million as of March 31, 2018 and $54.9 million as of December 31, 2017.

The following table presents investments in hedge funds and private equity funds by investment objective and sector as of March 31, 2018 and December 31, 2017:



	March 31, 2018				December 31, 2017
Millions	Fair Value		Unfunded Commitments		Fair Value		Unfunded Commitments
Hedge funds
Long/short banks and financial	$	56.4	$	—	$	54.9	$	—
Total hedge funds	56.4		—		54.9		—

Private equity funds
Manufacturing/Industrial	44.5		10.4		43.3		10.4
Aerospace/Defense/Government	17.0		12.9		15.8		12.9
Direct lending	7.9		22.5		7.1		23.1
Financial services	4.8		11.1		4.2		11.7
Insurance	—		41.2		—		41.2
Real estate	—		50.0		—		—
Total private equity funds	74.2		148.1		70.4		99.3
Total hedge funds and private equity funds included in other long-term investments	$	130.6	$	148.1	$	125.3	$	99.3

Redemption of investments in certain 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. As of March 31, 2018, White Mountains held one active hedge fund with a fair value of $56.4 million. The hedge fund is subject to a lock-up period that expires on September 1, 2018, with a semi-annual restriction on redemption frequency thereafter and an advance notice period requirement of not less than 45 days.

White Mountains redeemed its one investment in a long/short equity REIT hedge fund having a fair value of $20.8 million as of December 31, 2017. The bulk of the redemption proceeds were received early in the first quarter of 2018 with the balance received in April of 2018.

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 provide an option to extend the lock-up period at either, the sole discretion of the fund manager or upon agreement between the fund and its investors.

The following table presents investments in private equity funds that were subject to lock-up periods as of March 31, 2018:



Millions	1 – 3 years	3 – 5 years	5 – 10 years	>10 years	Total
Private equity funds — expected lock-up period remaining	$6.1	$6.2	$49.2	$12.7	$74.2

Fair Value Measurements as of March 31, 2018

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”). As of March 31, 2018 and December 31, 2017, White Mountains used quoted market prices or other observable inputs to determine fair value for approximately 92% and 94% of the investment portfolio.

Fair Value Measurements by Level

The following tables present White Mountains’s fair value measurements for investments as of March 31, 2018 and December 31, 2017 by level. 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, foreign governments, 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 and common equity securities by industry sector because investors often reference commonly used benchmarks and their subsectors to monitor risk and performance. Accordingly, White Mountains has further disaggregated these asset classes 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 and S&P 500 indices.



	March 31, 2018
Millions	Fair Value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	197.7	$	197.7	$	—	$	—

Debt securities issued by corporations:
Financials	151.0		—		151.0		—
Consumer	144.8		—		144.8		—
Technology	91.6		—		91.6		—
Communications	84.9		—		84.9		—
Health care	80.3		—		80.3		—
Materials	76.1		—		76.1		—
Energy	66.1		—		66.1		—
Industrial	33.9		—		33.9		—
Utilities	15.0		—		15.0		—
Total debt securities issued by corporations:	743.7		—		743.7		—

Mortgage and asset-backed securities	130.0		—		130.0		—
Municipal obligations	262.4		—		262.4		—
Total fixed maturity investments	1,333.8		197.7		1,136.1		—

Short-term investments⁽¹⁾	763.0		737.3		25.7		—

Common equity securities:
Exchange traded funds ⁽²⁾	653.2		592.2		61.0		—
Technology	18.0		18.0		—		—
Health care	16.9		16.9		—		—
Financials	15.9		15.9		—		—
Industrial	13.3		13.3		—		—
Consumer	8.6		8.6		—		—
Energy	6.5		6.5		—		—
Communications	5.2		5.2		—		—
Other ⁽³⁾	208.0		—		208.0		—
Total common equity securities	945.6		676.6		269.0		—

Other long-term investments ⁽⁴⁾	122.5		—		—		122.5
Total investments	$	3,164.9	$	1,611.6	$	1,430.8	$	122.5

⁽¹⁾Short-term investments are measured at amortized cost, which approximates fair value.

⁽²⁾ETFs traded on foreign exchanges are priced using the fund's published NAV to account for the difference in market close times and are therefore designated a level 2 measurement.

⁽³⁾Consists of two investments in unit trusts that primarily invest in international equities.

⁽⁴⁾Excludes carrying value of $130.6 associated with hedge funds and private equity funds for which fair value is measured at NAV using the practical expedient.



	December 31, 2017
Millions	Fair Value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	296.5	$	296.5	$	—	$	—

Debt securities issued by corporations:
Consumer	185.1		—		185.1		—
Communications	127.8		—		127.8		—
Financials	114.8		—		114.8		—
Utilities	108.9		—		108.9		—
Materials	95.5		—		95.5		—
Health care	94.3		—		94.3		—
Technology	80.5		—		80.5		—
Energy	48.1		—		48.1		—
Industrial	25.9		—		25.9		—
Total debt securities issued by corporations:	880.9		—		880.9		—

Mortgage and asset-backed securities	694.7		—		694.7		—
Municipal obligations	254.9		—		254.9		—
Foreign government, agency and provincial obligations	2.7		—		2.7		—
Total fixed maturity investments	2,129.7		296.5		1,833.2		—

Short-term investments ⁽¹⁾	176.1		151.0		25.1		—

Common equity securities:
Exchange traded funds ⁽²⁾	569.7		508.1		61.6		—
Health care	17.1		17.1		—		—
Financials	16.3		16.3		—		—
Technology	15.1		15.1		—		—
Industrial	11.9		11.9		—		—
Communications	10.9		10.9		—		—
Consumer	10.7		10.7		—		—
Energy	3.8		3.8		—		—
Other ⁽³⁾	210.6		—		210.6		—
Total common equity securities	866.1		593.9		272.2		—

Other long-term investments⁽⁴⁾⁽⁵⁾	87.2		—		—		87.2
Total investments	$	3,259.1	$	1,041.4	$	2,130.5	$	87.2

⁽¹⁾Short-term investments are measured at amortized cost, which approximates fair value.

⁽²⁾ETFs traded on foreign exchanges are priced using the fund’s published NAV to account for the difference in market close times and are therefore designated a level 2 measurement.

⁽³⁾Consists of two investments in unit trusts that primarily invests in international equities.

⁽⁴⁾ Excludes carrying value of $(3.7) related to foreign currency forward contracts.

⁽⁵⁾Excludes carrying value of $125.3 associated with hedge funds and private equity funds for which fair value is measured at NAV using the practical expedient.

Debt Securities Issued by Corporations

The following table presents the ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of March 31, 2018 and December 31, 2017:



	Fair Value at
Millions	March 31, 2018		December 31, 2017
AAA	$	8.9	$	1.6
AA	79.1		42.6
A	274.6		192.5
BBB	211.5		465.2
BB	151.0		161.7
B	18.6		17.3
Debt securities issued by corporations ⁽¹⁾	$	743.7	$	880.9

⁽¹⁾Credit ratings are assigned based on the following hierarchy: (1) Standard & Poor’s Financial Services LLC (“S&P”) and (2) Moody's Investor Service, Inc. (“Moody’s”).

Mortgage and Asset-backed Securities

White Mountains purchases commercial mortgage-backed securities (“CMBS”) and residential mortgage-backed securities (“RMBS”) with the goal of maximizing risk adjusted returns in the context of a diversified portfolio.

White Mountains considers sub-prime mortgage-backed securities as those that have underlying loan pools that exhibit weak credit characteristics, or those that are issued from dedicated sub-prime shelves or dedicated second-lien shelf registrations (i.e., White Mountains considers investments backed primarily by second-liens to be sub-prime risks regardless of credit scores or other metrics). As of March 31, 2018 White Mountains did not hold any RMBS categorized as sub-prime.

White Mountains considers mortgage-backed securities as “non-prime” (also called “Alt A” or “A-”) if they are backed by collateral that has overall credit quality between prime and sub-prime based on White Mountains’s review of the characteristics of their underlying mortgage loan pools, such as credit scores and financial ratios. As of March 31, 2018, White Mountains did not hold any RMBS classified as non-prime.

The following table presents the carrying value of White Mountains’s mortgage and asset-backed securities as of March 31, 2018 and December 31, 2017:



	March 31, 2018						December 31, 2017
Millions	Fair Value		Level 2		Level 3		Fair Value		Level 2		Level 3
Mortgage-backed securities:
Agency:
GNMA	$	27.3	$	27.3	$	—	$	46.3	$	46.3	$	—
FNMA	53.9		53.9		—		84.5		84.5		—
FHLMC	36.1		36.1		—		62.0		62.0		—
Total agency ⁽¹⁾	117.3		117.3		—		192.8		192.8		—
Non-agency:
Commercial	—		—		—		70.5		70.5		—
Total non-agency	—		—		—		70.5		70.5		—

Total mortgage-backed securities	117.3		117.3		—		263.3		263.3		—
Other asset-backed securities:
Credit card receivables	8.9		8.9		—		206.0		206.0		—
Vehicle receivables	3.8		3.8		—		142.4		142.4		—
Other	—		—		—		83.0		83.0		—
Total other asset-backed securities	12.7		12.7		—		431.4		431.4		—
Total mortgage and asset-backed securities	$	130.0	$	130.0	$	—	$	694.7	$	694.7	$	—

⁽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).

Rollforward of Fair Value Measurements by Level

White Mountains uses quoted market prices where available as the inputs to estimate fair value for its investments in active markets. Such measurements are considered to be either Level 1 or Level 2 measurements, depending on whether the quoted market price inputs are for identical securities (Level 1) or similar securities (Level 2). Level 3 measurements for fixed maturity investments, common equity securities and other long-term investments as of March 31, 2018 and 2017 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 tables present the changes in White Mountains’s fair value measurements by level for the three months ended March 31, 2018 and 2017:



							Level 3 Investments
Millions	Level 1 investments			Level 2 investments			Fixed maturity investments		Other long-term investments			Hedge Funds and Private Equity Funds measured at NAV⁽³⁾			Total
Balance at January 1, 2018	$	890.4		$	2,105.4		$	—	$	87.2		$	125.3		$	3,208.3		⁽¹⁾⁽²⁾
Net realized and unrealized (losses) gains	(14.3		)	(26.0		)	—		(8.7		)	3.6			(45.4		)	⁽⁴⁾
Amortization/Accretion	—			(1.2		)	—		—			—			(1.2		)
Purchases	238.9			407.7			—		44.0			2.0			692.6
Sales	(240.7		)	(1,080.8		)	—		—			(.3		)	(1,321.8		)
Transfers in	—			—			—		—			—			—
Transfers out	—			—			—		—			—			—
Balance at March 31, 2018	$	874.3		$	1,405.1		$	—	$	122.5		$	130.6		$	2,532.5		⁽²⁾

⁽¹⁾Excludes carrying value of $(3.7) as of January 1, 2018 associated with foreign currency forward contracts.

⁽²⁾Excludes carrying value of $176.1 and $763.0 at January 1, 2018 and March 31, 2018 classified as short-term investments.

⁽³⁾Investments for which fair value is measured at NAV using the practical expedient are no longer classified within the fair value hierarchy. See Note 1 — “Basis of Presentation and Significant Accounting Policies”.

⁽⁴⁾ Excludes realized and unrealized losses associated with foreign currency forward contracts, foreign currency on cash and open trades and short-term investments of $3.5, $4.2 and $0.6 for the three months ended March 31, 2018.



							Level 3 Investments
Millions	Level 1 investments			Level 2 investments			Fixed maturity investments		Other long-term investments			Hedge Funds and Private Equity Funds measured at NAV⁽³⁾			Total
Balance at January 1, 2017	$	279.5		$	2,093.8		$	—	$	91.4		$	82.6		$	2,547.3		^(1)(2)(4)
Net realized and unrealized gains	13.1			18.4			.1		.2			7.3			39.1			⁽⁵⁾
Amortization/Accretion	—			(2.5		)	—		—			—			(2.5		)
Purchases	115.2			351.1			11.0		.2			21.4			498.9
Sales	(76.0		)	(387.6		)	—		(2.0		)	(1.2		)	(466.8		)
Deconsolidation of SSIE	—			(5.2		)	—		—			—			(5.2		)
Transfers in	—			—			—		—			—			—
Transfers out	—			—			—		—			—			—
Balance at March 31, 2017	$	331.8		$	2,068.0		$	11.1	$	89.8		$	110.1		$	2,610.8		⁽¹⁾⁽²⁾

⁽¹⁾Excludes carrying value of $175.0 and $138.2 at January 1, 2017 and March 31, 2017 classified as short-term investments, of which $0.1 is classified as held for sale at January 1, 2017.

⁽²⁾Excludes carrying value of $(1.2) and $(4.1) as of January 1, 2017 and March 31, 2017 associated with foreign currency forward contracts.

⁽³⁾ Investments for which fair value is measured at NAV using the practical expedient are no longer classified within the fair value hierarchy. See Note 1 — “Basis of Presentation and Significant Accounting Policies”.

⁽⁴⁾Includes carrying value of $6.6 of fixed maturity investments at January 1, 2017 that is classified as assets held for sale related to SSIE.

⁽⁵⁾ Excludes realized and unrealized losses associated with foreign currency forward contracts of $2.8 for the three months ended March 31, 2017.

Fair Value Measurements — Transfers Between Levels - Three-months ended March 31, 2018 and 2017

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 the first three months of 2018 and 2017, 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.

Significant Unobservable Inputs

The following tables present significant unobservable inputs used in estimating the fair value of investment securities, other than hedge funds and private equity funds, classified within Level 3 as of March 31, 2018 and December 31, 2017. The fair value of investments in hedge funds and private equity funds are generally estimated using the NAV of the funds.



$ in millions, except share price	March 31, 2018
Description	Valuation Technique(s)	Fair Value ⁽¹⁾	Unobservable Input
Private equity security	Discounted cash flow	$21.0	Discount rate	-	25.0%
Private equity security	Discounted cash flow	$22.1	Implied share price	-	$.68
Private convertible preferred security	Discounted cash flow	$14.5	Implied share price	-	$2.06
Private equity security	Discounted cash flow/ Option pricing method	$11.6	Discount rate	-	21.0%
			Time until expiration	-	4 years
		Volatility/Standard deviation		-	50.0%
		Risk free rate		-	1.77%
Private debt instrument	Discounted cash flow	$10.5	Discount rate	-	9.62%

⁽¹⁾Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs.



$ in millions, except share price	December 31, 2017
Description	Valuation Technique(s)	Fair Value ⁽¹⁾	Unobservable Input
Private equity security	Share price of most recent transaction	$21.0	Share price	-	$1.00
Private equity security	Discounted cash flow	$22.1	Implied share price	-	$.68
Private convertible preferred security	Discounted cash flow	$14.5	Implied share price	-	$2.06
Private equity security	Discounted cash flow/ Option pricing method	$11.3	Discount rate	-	21.0%
			Time until expiration	-	4 years
		Volatility/Standard deviation		-	50.0%
		Risk free rate		-	1.77%
Private equity security	Share price of most recent transaction	$3.6	Share price	-	$2.52
Private convertible preferred security	Multiple of EBITDA	$0.6	EBITDA multiple	-	6.00

⁽¹⁾Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs.

Goodwill and Other Intangible Assets

3 Months Ended

Mar. 31, 2018

Goodwill and Intangible Assets Disclosure [Abstract]

Goodwill and Other Intangible Assets

White Mountains has recognized goodwill and other intangible assets at the acquisition date fair values in connection with its purchases of subsidiaries.

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



	Three Months Ended March 31,
	2018								2017
Millions	Goodwill		Other intangible assets			Total			Goodwill		Other intangible assets			Total
Beginning balance	$	25.9	$	36.2		$	62.1		$	25.9	$	19.3		$	45.2
Amortization, including foreign currency translation	—		(2.9		)	(2.9		)	—		(2.3		)	(2.3		)
Ending balance	$	25.9	$	33.3		$	59.2		$	25.9	$	17.0		$	42.9

The following table presents the goodwill and other intangible assets as of March 31, 2018 and December 31, 2017:



Millions	March 31, 2018			December 31, 2017
Goodwill
MediaAlpha	$	18.3		$	18.3
Other	7.6			7.6
Total goodwill	25.9			25.9
Other intangible assets
MediaAlpha	32.5			35.4
Other	.8			.8
Total other intangible assets	33.3			36.2
Total goodwill and other intangible assets	59.2			62.1
Goodwill and other intangible assets attributed to non-controlling interests	(21.1		)	(21.1		)
Goodwill and other intangible assets included in White Mountains’s common shareholders’ equity	$	38.1		$	41.0

- Definition

The entire disclosure for the aggregate amount of goodwill and a description of intangible assets, which may include (a) for amortizable intangible assets (also referred to as finite-lived intangible assets), the carrying amount, the amount of any significant residual value, and the weighted-average amortization period, (b) for intangible assets not subject to amortization (also referred to as indefinite-lived intangible assets), the carrying amount, and (c) the amount of research and development assets acquired and written off in the period, including the line item in the income statement in which the amounts written off are aggregated, if not readily apparent from the income statement. Also discloses (a) for amortizable intangibles assets in total and by major class, the gross carrying amount and accumulated amortization, the total amortization expense for the period, and the estimated aggregate amortization expense for each of the five succeeding fiscal years, (b) for intangible assets not subject to amortization the carrying amount in total and by major class, and (c) for goodwill, in total and for each reportable segment, the changes in the carrying amount of goodwill during the period (including the aggregate amount of goodwill acquired, the aggregate amount of impairment losses recognized, and the amount of goodwill included in the gain (loss) on disposal of a reporting unit). If any part of goodwill has not been allocated to a reportable segment, discloses the unallocated amount and the reasons for not allocating. For each impairment loss recognized related to an intangible asset (excluding goodwill), discloses: (a) a description of the impaired intangible asset and the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method for determining fair value, (c) the caption in the income statement or the statement of activities in which the impairment loss is aggregated, and (d) the segment in which the impaired intangible asset is reported. For each goodwill impairment loss recognized, discloses: (a) a description of the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method of determining the fair value of the associated reporting unit, and (c) if a recognized impairment loss is an estimate not finalized and the reasons why the estimate is not final. May also disclose the nature and amount of any significant adjustments made to a previous estimate of an impairment loss.

+ References + Details

Debt

3 Months Ended

Mar. 31, 2018

Debt Disclosure [Abstract]

Debt

The following table presents White Mountains’s debt outstanding as of March 31, 2018 and December 31, 2017:



Millions	March 31, 2018			Effective Rate ⁽¹⁾	December 31, 2017			Effective Rate ⁽¹⁾
WTM Bank Facility	$	—		N/A	$	—		N/A
Unamortized issue costs	—				—
WTM Bank Facility, carrying value	—				—
MediaAlpha Bank Facility	21.6			6.2%	23.9			5.6%
Unamortized issuance cost	(.1		)		(.1		)
MediaAlpha Bank Facility, carrying value	21.5				23.8
Total debt	$	21.5			$	23.8

⁽¹⁾Effective rate considers the effect of the debt issuance costs.

WTM Bank Facility

On August 14, 2013, the Company entered into a revolving credit facility with a syndicate of lenders administered by Wells Fargo Bank, N.A., which has a total commitment of $425.0 million and has a maturity date of August 14, 2018 (the “WTM Bank Facility”). As of March 31, 2018, the WTM Bank Facility was undrawn.

The WTM Bank Facility contains various affirmative, negative and financial covenants which White Mountains considers to be customary for such borrowings, including certain minimum net worth and maximum debt to capitalization standards.

MediaAlpha Bank Facility

On May 12, 2017, MediaAlpha entered into a secured credit facility (the “MediaAlpha Bank Facility”) with Western Alliance Bank, which has a total commitment of $20.0 million and has a maturity date of May 12, 2020. On October 5, 2017, MediaAlpha refinanced the MediaAlpha Bank Facility in order to fund the acquisition of certain assets associated with the Health, Life and Medicare insurance business of Healthplans.com. The total commitment of the MediaAlpha Bank Facility was increased to $28.4 million and has a maturity date of October 6, 2020. The MediaAlpha Bank Facility consists of a $18.4 million term loan facility, which has an outstanding balance of $16.6 million as of March 31, 2018, and a revolving loan facility for $10.0 million, which has an outstanding balance of $5.0 million as of March 31, 2018.

The MediaAlpha Bank Facility carries a variable interest rate that is based on the Prime Rate, as published by the Wall Street Journal, plus a spread of 1.5% on the term loan facility and 0.25% on the revolving credit facility as of March 31, 2018.

During the three months ended March 31, 2018, MediaAlpha repaid $1.3 million on the term loan and $1.0 million on the revolving loan under the MediaAlpha Bank Facility.

The MediaAlpha Bank Facility is secured by intellectual property and the common stock of MediaAlpha’s subsidiaries, and contains various affirmative, negative and financial covenants that White Mountains considers to be customary for such borrowings, including a fixed charge coverage ratio and an asset coverage ratio.

Compliance

At March 31, 2018, White Mountains was in compliance with the covenants under all of its debt instruments.

Income Taxes	3 Months Ended
Income Taxes	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company and its Bermuda domiciled subsidiaries are not subject to Bermuda income tax under current Bermuda law. If there is a change in the current law such that taxes are imposed, the Company and its Bermuda domiciled subsidiaries would be exempt from such tax until March 31, 2035, pursuant to the Bermuda Exempted Undertakings Tax Protection Act of 1966. The Company has subsidiaries and branches that operate in various other jurisdictions around the world that are subject to tax in the jurisdictions in which they operate. The jurisdictions in which the Company’s subsidiaries and branches are subject to tax are Barbados, Gibraltar, Ireland, Israel, Luxembourg, the United Kingdom and the United States. White Mountains’s income tax expense related to pre-tax loss from continuing operations for the three months ended March 31, 2018 represented an effective tax rate of (1.1)%. The effective tax rate was different from the U.S. statutory rate of 21%, primarily due to a full valuation allowance on all net deferred tax assets at U.S. operations, withholding taxes and a tax benefit recorded at BAM. For BAM, member surplus contributions (“MSC”) and the related taxes thereon are recorded directly to non-controlling interest equity, while the valuation allowance on such taxes is recorded through the income statement. For the three months ended March 31, 2018, BAM recorded a tax benefit of $0.6 million associated with the valuation allowance on taxes related to MSC that is included in the effective tax rate. White Mountains’s income tax benefit related to pre-tax loss from continuing operations for the three months ended March 31, 2017 represented an effective tax rate of 8.3%. The effective tax rate was different from the U.S. statutory rate of 35%, primarily due to a full valuation allowance on all net deferred tax assets at U.S. operations and a tax benefit recorded at BAM. For BAM, MSC and the related taxes thereon are recorded directly to non-controlling interest equity, while the valuation allowance on such taxes is recorded through the income statement. For the three months ended March 31, 2017, BAM recorded a tax benefit of $2.7 million associated with the valuation allowance on taxes related to MSC that is included in the effective tax rate. In arriving at the effective tax rate for the three months ended March 31, 2018 and 2017, White Mountains forecasted all income and expense items including the change in unrealized investment gains (losses) and realized investment gains (losses) for the years ending December 31, 2018 and 2017. 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. 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 2013.

Derivatives	3 Months Ended
Derivatives	Mar. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives	Derivatives White Mountains’s investment portfolio includes investments denominated in Japanese Yen, Euros, GBP and other foreign currencies. White Mountains previously entered into foreign currency forward contracts to manage its foreign currency exposure related to these investments. In the first quarter of 2018, in conjunction with the liquidation of the GBP investment grade corporate bond mandate, White Mountains closed the associated foreign currency forward contract. White Mountains no longer has any open foreign currency forward contracts. As of December 31, 2017, White Mountains held $206.3 million total gross notional value of a foreign currency forward contract with a carrying value of $(3.7) million. The derivative (losses) gains recognized in net realized and unrealized investment gains (losses) for the three months ended March 31, 2018 and 2017 were $(3.5) million and $3.0 million.

Municipal Bond Guarantee

3 Months Ended

Mar. 31, 2018

Guarantees [Abstract]

Municipal Bond Guarantee Insurance

In 2012, HG Global was capitalized with $594.5 million from White Mountains and $14.5 million from non-controlling interests to fund BAM, a newly formed mutual municipal bond insurer. As of March 31, 2018, White Mountains owned 96.9% of HG Global’s preferred equity and 88.4% of its common equity. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of BAM Surplus Notes. At inception, BAM and HG Re also entered into a first loss reinsurance treaty (“FLRT”). HG Re provides first loss reinsurance protection up to 15% of par outstanding on each municipal bond insured by BAM. In return, BAM cedes 60% of the risk premium charged for insuring the municipal bond, net of a ceding commission. During 2017, HG Global and BAM made certain changes to the ceding commission arrangements under the FLRT. These changes serve to accelerate growth in BAM's statutory capital but do not impact the net risk premium ceded from BAM to HG Re. HG Re’s obligations to BAM are collateralized in trusts, and there is an aggregate loss limit that is equal to the total assets in the collateral trusts at any point in time.

The interest rate on the BAM Surplus Notes for the five years ending December 31, 2018 is a variable rate equal to the one-year U.S. treasury rate plus 300 basis points, set annually, which is 4.60% for 2018 and was 3.54% for 2017. Prior to the end of 2018, BAM has the option to extend the variable rate period for an additional three years. At the end of the variable rate period, the interest rate will be fixed at the higher of the then current variable rate or 8.0%. No payment of interest or principal on the BAM Surplus Notes may be made without the approval of the New York State Department of Financial Services (“NYDFS”). BAM has stated its intention to seek regulatory approval to pay interest and principal on its surplus notes only to the extent that its remaining qualified statutory capital and other capital resources continue to support its outstanding obligations, business plan and its AA stable rating from S&P. BAM repaid $4.0 million of the BAM Surplus Notes and $1.0 million of the related accrued interest during the year ended December 31, 2017. There were no repayments for the three months ended March 31, 2018.

In order to further support BAM’s long-term capital position and business prospects, in 2017 HG Global agreed to contribute the $203.0 million of Series A BAM Surplus Notes (“Series A Notes”) into the supplemental collateral trust (the “Supplemental Trust”) at HG Re. The Supplemental Trust already held the $300.0 million of Series B BAM Surplus Notes (“Series B Notes”). Assets held in the Supplemental Trust serve to collateralize HG Re’s obligations to BAM under the FLRT. HG Global and BAM also changed the payment terms of the Series B Notes, so that payments will reduce principal and accrued interest on a pro rata basis, consistent with the payment terms on the Series A Notes. The terms of the Series B Notes had previously stipulated that payments would first reduce interest owed, then reduce principal owed once all accrued interest had been paid. The Supplemental Trust target balance is equal to approximately $603.0 million. 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 collateral trust balances must be at target levels before capital can be distributed out of the Supplemental Trust. In connection with the contribution, the Series A Notes were merged with the Series B Notes.

As of March 31, 2018 and December 31, 2017, the collateral trusts held assets of $721.9 million and $715.1 million, which both included $499.0 million of BAM Surplus Notes. As of March 31, 2018 and December 31, 2017, HG Global has accrued $131.7 million and $126.0 million of interest receivable on the BAM Surplus Notes.

The following table presents a schedule of BAM’s insured obligations:



	March 31, 2018		December 31, 2017
Contracts outstanding	6,541		6,371
Remaining weighted average contract period outstanding (in years)	10.8		10.9
Contractual debt service outstanding (in millions):
Principal	$	43,139.2	$	42,090.6
Interest	21,420.6		21,057.1
Total debt service outstanding	$	64,559.8	$	63,147.7

Gross unearned insurance premiums	$	140.2	$	136.8

The following table presents a schedule of BAM’s future premium revenues as of March 31, 2018:



Millions	March 31, 2018
April 1, 2018 - December 31, 2018	$	8.9

January 1, 2019 - March 31, 2019	2.9
April 1, 2019 - June 30, 2019	2.9
July 1, 2019 - September 30, 2019	2.9
October 1, 2019 - December 31, 2019	2.9
	11.6

2020	11.1
2021	10.6
2022	10.1
2023 and thereafter	87.9
Total gross unearned insurance premiums	$	140.2

The following table presents a schedule of net written premiums included in White Mountains’s HG Global/BAM segment for the three months years ended March 31, 2018 and 2017:



	Three Months Ended March 31,
Millions	2018		2017
Gross written premiums	$	6.4	$	18.7
Assumed (ceded) written premiums	—		—
Net written premiums	$	6.4	$	18.7

Earnings Per Share

3 Months Ended

Mar. 31, 2018

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.

The following table presents the Company’s computation of earnings per share from continuing operations for the three months ended March 31, 2018 and 2017. See Note 16 — “Held for Sale and Discontinued Operations”.



	Three Months Ended
	March 31,
	2018			2017
Basic and diluted earnings per share numerators (in millions):
Net (loss) income attributable to White Mountains’s common shareholders	$	(48.0	)	$	29.0
Less: total income from discontinued operations, net of tax	.1			31.3
Net loss from continuing operations attributable to White Mountains’s common shareholders	$	(48.1	)	$	(2.3	)
Allocation of earnings to participating restricted common shares⁽¹⁾	.4			—
Basic and diluted earnings per share numerators	$	(47.7	)	$	(2.3	)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,746.1			4,564.6
Average unvested restricted common shares⁽²⁾	(35.9		)	(52.5		)
Basic earnings per share denominator	3,710.2			4,512.1
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,746.1			4,564.6
Average unvested restricted common shares⁽²⁾	(35.9		)	(52.5		)
Diluted earnings per share denominator	3,710.2			4,512.1
Basic and diluted earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid	$	1.00		$	1.00
Undistributed earnings (losses)	(13.85		)	(1.52		)
Basic and diluted earnings per share	$	(12.85	)	$	(.52	)

⁽¹⁾ Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.

⁽²⁾Restricted shares outstanding vest either in equal annual installments or upon a stated date. See Note 10 — “Employee Share-Based Incentive Compensation Plans”

The following table presents the undistributed net earnings (losses) from continuing operations for the three months ended March 31, 2018 and 2017. See Note 16 — “Held for Sale and Discontinued Operations”.



	Three Months Ended
	March 31,
Millions	2018			2017
Undistributed net losses - continuing operations:
Net loss attributable to White Mountains’s common shareholders, net of restricted common share amounts	$	(47.7	)	$	(2.3	)
Dividends declared net of restricted common share amounts ⁽¹⁾	(3.7		)	(4.5		)
Total undistributed net losses, net of restricted common share amounts	$	(51.4	)	$	(6.8	)

⁽¹⁾ Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.

Employee Share-Based Incentive Compensation Plans

3 Months Ended

Mar. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Employee Share-Based Incentive Compensation Plans

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. As of March 31, 2018, White Mountains’s share-based compensation incentive awards consist of performance shares and restricted shares.

Performance Shares

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 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 approved for payment.

The following table presents the performance share activity for the three months ended March 31, 2018 and 2017 for performance shares granted under the WTM Incentive Plan:



	Three Months Ended March 31,
	2018					2017
Millions, except share amounts	Target Performance Shares Outstanding		Accrued Expense			Target Performance Shares Outstanding		Accrued Expense
Beginning of period	50,515		$	45.8		80,353		$	42.4
Shares paid ⁽¹⁾	(23,186	)	(28.4		)	(30,167	)	(20.8		)
New grants	14,105		—			16,460		—
Forfeitures and cancellations⁽²⁾	(818	)	.3			(9,841	)	(5.7		)
Expense recognized	—		3.3			—		8.1
End of period⁽³⁾	40,616		$	21.0		56,805		$	24.0

⁽¹⁾WTM performance share payments in 2018 for the 2015-2017 performance cycle, which were paid in March 2018, ranged from 145% to 147% of target. WTM performance share payments in 2017 for the 2014-2016 performance cycle, which were paid in March 2017, ranged from 34% to 76% of target.

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

⁽³⁾Outstanding performance share awards as of March 31, 2017 excludes 2,195 performance share awards granted to employees of Sirius Group.

For performance shares earned in the 2015-2017 and 2014-2016 performance cycles, all performance shares earned were settled in cash. If all the outstanding WTM performance shares had vested on March 31, 2018, the total additional compensation cost to be recognized would have been $26.1 million, based on accrual factors (common share price and payout assumptions) at March 31, 2018.

The following table presents performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan at March 31, 2018 for each performance cycle:



	Three Months Ended March 31, 2018
Millions, except share amounts	Target Performance Shares Outstanding		Accrued Expense
Performance cycle:
2016 – 2018	13,715		$	12.7
2017 – 2019	14,070		7.7
2018 – 2020	13,450		.9
Sub-total	41,235		21.3
Assumed forfeitures	(619	)	(.3		)
March 31, 2018	40,616		$	21.0

Restricted Shares

The following table presents the unrecognized compensation cost associated with the outstanding restricted share awards for the three months ended March 31, 2018 and 2017:



	Three Months Ended March 31,
	2018					2017
Millions, except share amounts	Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value
Non-vested,
Beginning of period	53,755		$	14.3		70,620		$	19.7
Issued	14,105		11.4			16,735		15.8
Vested	(25,381	)	—			(22,015	)	—
Forfeited	(969	)	(.2		)	(5,200	)	(2.8		)
Expense recognized	—		(3.3		)	—		(3.6		)
End of period⁽¹⁾	41,510		$	22.2		60,140		$	29.1

⁽¹⁾Restricted share awards outstanding as of March 31, 2017 includes 2,195 restricted shares issued to employees of Sirius Group, which was accounted for as discontinued operations.

During the first quarter of 2018, White Mountains issued 13,450 restricted shares that vest on January 1, 2021, 290 restricted shares that vest on January 1, 2020 and 365 restricted shares that vest on January 1, 2019. During the first quarter of 2017, White Mountains issued 16,735 restricted shares that vest on January 1, 2020. The unamortized issue date fair value at March 31, 2018 is expected to be recognized ratably over the remaining vesting periods.

Non-Qualified Options

As of January 20, 2017, the 125,000 Non-Qualified options issued to the Company’s former Chairman and CEO were exercised. During the first quarter of 2017, 40,000 Non-Qualified Options, with an intrinsic value of $4.4 million, were exercised in exchange for 5,142 common shares with an equal total market value. The intrinsic value represents the difference between the market price of the Company’s common shares at the date of exercise and the fixed strike price of $742 per common share. The Non-Qualified Options were fully amortized as of 2011.

MediaAlpha Class B Unit Awards

MediaAlpha has issued Class B unit awards to certain employees. The units entitle the award recipient to participate in distributions from MediaAlpha, subject to a cumulative distribution threshold, which is a performance condition, and a service period. The grant date fair value of the awards is determined when it is deemed probable that the distribution threshold will be met. The service period ranges from 36 months to 48 months. For the three months ended March 31, 2018, MediaAlpha recognized $6.4 million of compensation expense for the vested portion of the awards for which achievement of the performance award is now probable, and $0.6 million of unearned compensation expense for unvested awards, which will be recognized over the remaining service periods of the awards.

Non-controlling Interest

3 Months Ended

Mar. 31, 2018

Noncontrolling Interest [Abstract]

Non-controlling Interests

Non-controlling interests consist of the ownership interests of non-controlling shareholders in consolidated entities and are presented separately on the balance sheet.

The following table presents the balance of non-controlling interests included in White Mountains’s total equity and the related percentage of each consolidated entity’s total equity owned by non-controlling shareholders as of March 31, 2018 and December 31, 2017:



	March 31, 2018					December 31, 2017
$ in millions	Non-controlling Percentage		Non-controlling Equity			Non-controlling Percentage		Non-controlling Equity
Other, excluding mutuals
HG Global	3.1	%	$	15.7		3.1	%	$	15.9
MediaAlpha	37.7		16.2			35.7		13.1
Buzzmove	22.9		2.4			22.9		2.5
Total other, excluding mutuals			34.3					31.5
Mutuals
BAM	100.0		(177.5		)	100.0		(163.2		)
Total non-controlling interests			$	(143.2	)			$	(131.7	)

Segment Information

3 Months Ended

Mar. 31, 2018

Segment Reporting [Abstract]

Segment Informartion

Segment Information

White Mountains has determined that its reportable segments are HG Global/BAM, MediaAlpha and Other Operations. As a result of OneBeacon Transaction, the results of operations for OneBeacon, previously reported in its own segment has been classified as discontinued operations and is now presented, net of related income taxes, as such in the statement of operations and comprehensive income. Prior year amounts have been reclassified to conform to the current period’s presentation. See Note 16 — “Held for Sale and Discontinued Operations”.

Beginning in the second quarter of 2017, MediaAlpha’s results have been presented as a separate segment within White Mountains’s consolidated financial statements. Amounts for MediaAlpha for the three months ended March 31, 2017 have been reclassified to conform to the current period’s presentation.

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 chief operating decision makers and the Board of Directors.

Significant intercompany transactions among White Mountains’s segments have been eliminated herein. The following table presents the financial information for White Mountains’s segments:



Millions	HG Global/BAM			MediaAlpha			Other Operations			Total
Three Months Ended March 31, 2018
Earned insurance premiums	$	3.0		$	—		$	—		$	3.0
Net investment income	3.7			—			16.0			19.7
Net realized and unrealized investment losses	(7.9		)	—			(45.8		)	(53.7		)
Advertising and commission revenues ⁽¹⁾	—			70.1			.9			71.0
Other revenue	.2			1.6			.3			2.1
Total revenues	(1.0		)	71.7			(28.6		)	42.1
Insurance acquisition expenses	1.4			—			—			1.4
Other underwriting expenses	.1			—			—			.1
Cost of sales	—			57.4			.7			58.1
General and administrative expenses	11.8			14.1			22.0			47.9
Interest expense	—			.4			.2			.6
Total expenses	13.3			71.9			22.9			108.1
Pre-tax loss	$	(14.3	)	$	(.2	)	$	(51.5	)	$	(66.0	)


⁽¹⁾	Approximately 31% of MediaAlpha’s advertising revenue was associated with one customer for the three months ended March 31, 2018.



Millions	HG Global/BAM			MediaAlpha			Other Operations		Total
Three Months Ended March 31, 2017
Earned insurance premiums	$	2.0		$	—		$	1.0	$	3.0
Net investment income	2.6			—			10.2		12.8
Net realized and unrealized investment gains	1.3			—			35.0		36.3
Advertising and commission revenues ⁽¹⁾	—			32.5			1.3		33.8
Other revenue	.4			—			2.5		2.9
Total revenues	6.3			32.5			50.0		88.8
Losses and loss adjustment expenses	—			—			1.1		1.1
Insurance acquisition expenses	1.2			—			.1		1.3
Other underwriting expenses	.1			—			—		.1
Cost of sales	—			27.7			1.1		28.8
General and administrative expenses	10.6			5.6			44.5		60.7
Interest expense	—			.2			.2		.4
Total expenses	11.9			33.5			47.0		92.4
Pre-tax (loss) income	$	(5.6	)	$	(1.0	)	$	3.0	$	(3.6	)


⁽¹⁾	Approximately 27% of MediaAlpha’s advertising revenue was associated with one customer for the three months ended March 31, 2017.

Investments in Unconsolidated Entities

3 Months Ended

Mar. 31, 2018

Equity Method Investments and Joint Ventures [Abstract]

Investments in Unconsolidated Entities

White Mountains’s investments in unconsolidated entities are included within other long-term investments and consist of investments in common equity securities or similar instruments, which give White Mountains the ability to exert significant influence over the investee’s operating and financial policies (“equity method eligible unconsolidated entities”). Such investments may be accounted for under either the equity method or, alternatively, White Mountains may elect to account for them under the fair value option.

The following table presents the carrying values of investments in equity method eligible unconsolidated entities recorded within other long-term investments:



Millions	March 31, 2018		December31, 2017
Equity method eligible private equity securities, at fair value	$	84.7	$	58.0
Investments, accounted for under the equity method	5.7		4.6
Total investments in equity method eligible unconsolidated entities	90.4		62.6
Other unconsolidated investments ⁽¹⁾	162.7		146.2
Total other long-term investments	$	253.1	$	208.8

⁽¹⁾Consists of other long-term investments that are not equity method eligible.

The following table presents White Mountains’s investments in equity method eligible unconsolidated entities as of March 31, 2018 and December 31, 2017:



Investee	Ownership Interest		Instrument Held
	March 31, 2018	December 31, 2017
Compare.com	22%	22%	Common shares
DavidShield ⁽¹⁾	50%	—	Common shares
durchblicker	45%	45%	Common shares
Kudu	50%	—	Units
OneTitle	20%	20%	Common shares
PassportCard ⁽¹⁾	50%	50%	Common shares
Tuckerman Capital Fund III	21%	21%	Units

⁽¹⁾ At March 31, 2018, White Mountains's ownership interest in PassportCard comprised a 25% direct ownership interest and a 25% indirect interest through DavidShield. At December 31, 2017, White Mountains's ownership interest was a 50% direct ownership interest. See Note 2 — “Significant Transactions”.

Fair Value of Financial Instruments

3 Months Ended

Mar. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value of Financial Instruments

White Mountains accounts for all its financial instruments at fair value with the exception of the WTM Bank Facility, which was undrawn at March 31, 2018 and December 31, 2017, and the MediaAlpha Bank Facility, which is recorded as debt at face value less unamortized original issue discount.

The following table presents the fair value and carrying value of this financial instrument as of March 31, 2018 and December 31, 2017:



	March 31, 2018				December 31, 2017
Millions	Fair Value		Carrying Value		Fair Value		Carrying Value
MediaAlpha Bank Facility	$	21.8	$	21.6	$	23.9	$	23.8

The fair value estimate for the MediaAlpha Bank Facility has been determined based on a discounted cash flows approach and is considered to be a Level 3 measurement.

- 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

Contingencies	3 Months Ended
Contingencies	Mar. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Contingencies	Contingencies Legal Contingencies White Mountains is subject to litigation and arbitration in the normal course of business. White Mountains considers the requirements of ASC 450 when evaluating its exposure to 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 litigation that may have a material adverse effect on White Mountains’s financial condition, results of operations or cash flows. The following description presents significant legal contingencies, ongoing non-claims related litigation or arbitration as of March 31, 2018: Sirius Tax Contingencies A subsidiary of Sirius Group, which was sold by White Mountains in 2016, has been denied interest deductions by the Swedish Tax Authority (“STA”) for tax years 2013-2016. The Swedish subsidiary has filed an appeal in the Swedish Administrative Court for tax year 2013 and will preserve its right to contest the STA’s decision for later years if needed. Sirius Group believes it is more likely than not that it will prevail in the court proceedings. However, if the ultimate decision in the Swedish courts is unfavorable, White Mountains would be required to indemnify Sirius Group for the tax effect of the interest deductions, which is approximately $18.5 million.

Held for Sale and Discontinued Operations

3 Months Ended

Mar. 31, 2018

Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]

Held for Sale and Discontinued Operations

OneBeacon

On September 28, 2017, Intact Financial Corporation completed its acquisition of OneBeacon in an all-cash transaction for $18.10 per share. White Mountains received total proceeds of $1.3 billion and recorded a gain of $554.6 million, net of transaction costs. Net income from discontinued operations related to OneBeacon was $32.3 million for the three months ended March 31, 2017.

Tranzact

On July 21, 2016, White Mountains completed its sale of Tranzact to Clayton, Dubilier & Rice, LLC and received net proceeds of $221.3 million at closing. On October 5, 2016, White Mountains received additional proceeds of $1.2 million following the release of the post-closing purchase price adjustment escrow. During 2016, White Mountains recorded a $51.9 million gain from the sale of Tranzact in discontinued operations, which included a $30.2 million tax expense for the reversal of a tax valuation allowance that is offset by a tax benefit recorded in continuing operations.

During the three months ended March 31, 2017, White Mountains recorded a $1.0 million reduction to the gain from sale of Tranzact in discontinued operations as a result of 2016 state tax payments.

Sirius Group

On April 18, 2016, White Mountains completed the sale of Sirius Group to CMI for $2.6 billion. During 2016, White Mountains recorded a $363.2 million gain from the sale of Sirius Group in discontinued operations in the statement of operations and $113.3 million in other comprehensive income from discontinued operations.

During the three months ended March 31, 2018, White Mountains recorded a $0.1 million gain from sale of Sirius Group as a result of a change to the valuation of the accrued incentive compensation payable to Sirius Group employees.

Other

As of December 31, 2017, White Mountains has classified its Guilford, Connecticut property, which consists of an office building and adjacent land, as held for sale. The property has been measured at its estimated fair value net of costs of disposal, of $3.3 million as of March 31, 2018 and December 31, 2017.

Discontinued Operations

Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]

Held for Sale and Discontinued Operations

Net Income (Loss) from Discontinued Operations

The following table presents the results of operations, including related income taxes, associated with the business classified as discontinued operations. For the three months ended March 31, 2018, the amounts presented relate to Sirius Group. For the three months ended March 31, 2017, the amounts presented relate to OneBeacon and Tranzact. The results of discontinued operations from Sirius Group and Tranzact up to the closing date of the transaction inured to White Mountains. Given the fixed price nature of the OneBeacon Transaction, OneBeacon’s results were economically transferred to the buyer at signing.



	Three Months Ended				Three Months Ended
	March 31, 2018				March 31, 2017
Millions	Sirius Group		Total		OneBeacon			Tranzact			Total
Revenues
Earned insurance premiums	$	—	$	—	$	261.8		$	—		$	261.8
Net investment income	—		—		12.2			—			12.2
Net realized and unrealized gains	—		—		15.0			—			15.0
Other revenue	—		—		3.4			—			3.4
Total revenues	—		—		292.4			—			292.4
Expenses
Loss and loss adjustment expenses	—		—		150.6			—			150.6
Insurance and reinsurance acquisition expenses	—		—		45.3			—			45.3
Other underwriting expenses	—		—		51.7			—			51.7
General and administrative expenses	—		—		5.0			—			5.0
Interest expense	—		—		3.3			—			3.3
Total expenses	—		—		255.9			—			255.9
Pre-tax income	—		—		36.5			—			36.5
Income tax expense	—		—		(4.2		)	—			(4.2		)
Net income from discontinued operations	—		—		32.3			—			32.3
Net gain (loss) from sale of discontinued operations	.1		.1		—			(1.0		)	(1.0		)
Total income (loss) from discontinued operations	.1		.1		32.3			(1.0		)	31.3
Change in foreign currency translation and other from discontinued operations	—		—		(.1		)	—			(.1		)
Comprehensive income (loss) from discontinued operations	$	.1	$	.1	$	32.2		$	(1.0	)	$	31.2

Net Change in Cash from Discontinued Operations

The following tables presents the net change in cash, including income tax payments to national governments and interest paid associated with the business classified as discontinued operations:



	Three Months Ended
	March 31,
Millions	2018			2017
Net cash provided from (used for) operations	$	.1		$	(1.3	)
Net cash (used for) provided from investing activities	(.1		)	32.3
Net cash used for financing activities	—			(21.0		)
Net change in cash during the period	—			10.0
Cash balances at beginning of period	—			70.5
Net change in cash held for sale, excluding discontinued operations	—			(.9		)
Cash balances at end of period	$	—		$	79.6
Supplemental cash flows information:
Interest paid	$	—		$	—
Net income tax payment to national governments	$	—		$	—

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 three months ended March 31, 2018 and 2017:



	Three Months Ended
	March 31,
	2018			2017
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$	(48.0	)	$	29.0
Less total loss from continuing operations, net of tax	48.1			2.3
Net income from discontinued operations attributable to White Mountains’s common shareholders	$	.1		$	31.3
Allocation of earnings to participating restricted common shares ⁽¹⁾	—			(.4		)
Basic and diluted earnings per share numerators	$	.1		$	30.9
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,746.1			4,564.6
Average unvested restricted common shares ⁽³⁾	(35.9		)	(52.5		)
Basic earnings per share denominator	3,710.2			4,512.1
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,746.1			4,564.6
Average unvested restricted common shares ⁽³⁾	(35.9		)	(52.5		)
Diluted earnings per share denominator	3,710.2			4,512.1
Basic earnings per share (in dollars) - discontinued operations:	$	.03		$	6.86
Diluted earnings per share (in dollars) - discontinued operations:	$	.03		$	6.86

⁽¹⁾Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities.

⁽²⁾Net earnings attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the three months ended March 31, 2018 and 2017.

⁽³⁾ Restricted common shares outstanding vest either in equal annual installments or upon a stated date. See Note 10 — “Employee Share-Based Incentive Compensation Plans”.

Financial Statement Revisions

3 Months Ended

Mar. 31, 2018

Accounting Changes and Error Corrections [Abstract]

Financial Statement Revisions

In October 2017, White Mountains discovered that the former CEO of Wobi, one of its overseas portfolio companies, had been reporting overstated commission revenues and related receivables to White Mountains. Upon discovery of the overstatements, White Mountains initiated an investigation, conducted by outside counsel, of the reporting of these overstatements by Wobi to White Mountains. As a result of this investigation, White Mountains has revised certain of its previously issued financial statements. The revisions resulted in reductions to commission revenues (included in advertising and commission revenues) and commissions receivable (included in other assets). In addition, the overstatements led White Mountains to write down the goodwill and other intangible assets related to Wobi to zero. Wobi also conducted a separate investigation with the assistance of Israeli counsel to support the preparation of Wobi’s standalone financial statements. That separate investigation has been completed and its results did not materially impact White Mountains’s financial statements.

White Mountains evaluated the impact of the misstatements resulting from the overstatements at Wobi on prior periods’ financial statements in accordance with SEC Staff Accounting Bulletin (“SAB”) Topic 1.M, Materiality, and concluded the misstatements were not material to any previously reported financial statements. However, while not material to any previously reported annual or quarterly period, the aggregate amount of prior period misstatements could be material to White Mountains's results for the full fiscal year ended December 31, 2017. White Mountains has therefore revised all periods impacted including its consolidated balance sheet as of March 31, 2017 and its consolidated statements of operations and comprehensive income, consolidated statements of changes in equity, cash flows and earnings per share for the three months March 31, 2017. The impact of these revisions to each of the previously reported consolidated statements are disclosed below.

Amounts previously reported reflect the reclassification of OneBeacon to discontinued operations for all applicable periods presented.

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (Unaudited)



	Three months ended March 31, 2017
Millions, except for per share amounts	As previously reported			Adjustments			As revised
Revenues:
Financial Guarantee (HG Global/BAM)
Financial Guarantee revenues	$	6.3		$	—		$	6.3
Marketing Technology (MediaAlpha)
Marketing Technology revenues	32.5			—			32.5
Other
Advertising & commission revenues	6.0			(4.7		)	1.3
Other revenues ⁽¹⁾	48.7			—			48.7
Total revenues	93.5			(4.7		)	88.8

Expenses:
Financial Guarantee (HG Global/BAM)
Financial Guarantee expenses	11.9			—			11.9
Marketing Technology (MediaAlpha)
Marketing Technology expenses	33.5			—			33.5
Other
Other expenses ⁽²⁾⁽³⁾	46.8			.2			47.0
Total expenses	92.2			.2			92.4
Pre-tax income (loss)	1.3			(4.9		)	(3.6		)
Income tax benefit	.3			—			.3
Net income (loss) from continuing operations	1.6			(4.9		)	(3.3		)
Loss on sale of discontinued operations	(1.0		)	—			(1.0		)
Net income from discontinued operations	32.3			—			32.3
Net income (loss) ⁽⁴⁾	32.9			(4.9		)	28.0
Net loss (income) attributable to non-controlling interests	1.3			(.3		)	1.0
Net income (loss) attributable to White Mountains’s common shareholders	34.2			(5.2		)	29.0
Other comprehensive income (loss), net of tax	.9			(.7		)	.2
Comprehensive income (loss)	35.1			(5.9		)	29.2
Comprehensive loss attributable to non-controlling interests	—			—			—
Comprehensive income (loss) attributable to White Mountains’s common shareholders	$	35.1		$	(5.9	)	$	29.2

Basic and diluted earnings per share - continuing operations	$	7.49		$	(1.15	)	$	6.34

⁽¹⁾Total other revenues include earned insurance premiums, net investment income, net realized and unrealized losses, and other revenues.

⁽²⁾Total other expenses include loss and loss adjustment expenses, insurance acquisition expenses, cost of sales, general and administrative expenses, and interest expense.

⁽³⁾The adjustments to other expenses is primarily related to the write-off of goodwill and intangible assets for Wobi.

⁽⁴⁾The adjustment to net income resulted in a corresponding adjustment in the statement of cash flows, with an offsetting adjustment to the change in other assets and liabilities within the operating cash flows section. There was no change to cash flows from operations, cash flows from investing activities or cash flows from financing activities.

CONSOLIDATED STATEMENTS OF SHAREHOLDERS’ EQUITY



	Three months ended March 31, 2017
Millions	As previously reported			Adjustments			As revised

Common shares and paid-in surplus	$	807.1		$	.7		$	807.8

Retained earnings, beginning of year	2,797.2			(20.6		)	2,776.6
Share repurchases	(5.1		)	—			(5.1		)
Net income (loss)	34.2			(5.2		)	29.0
Dividends	(4.6		)	—			(4.6		)
Retained earnings, end of period	2,821.7			(25.8		)	2,795.9

Accumulated other comprehensive loss, after tax, beginning of year	(4.6		)	—			(4.6		)
Net change in foreign currency translation	.9			(.7		)	.2
Accumulated other comprehensive income, after tax, end of period	(3.7		)	(.7		)	(4.4		)

White Mountains’s common shareholders’ equity	3,625.1			(25.8		)	3,599.3

Non-controlling interests, beginning of year	133.9			(.6		)	133.3
Net loss	(1.3		)	.3			(1.0		)
Other changes in non-controlling interests	(.2		)	(.6		)	(.8		)
Non-controlling interests, end of period	132.4			(.9		)	131.5
Total equity	$	3,757.5		$	(26.7	)	$	3,730.8

CONSOLIDATED BALANCE SHEETS (Unaudited)



	March 31, 2017
Millions	As previously reported			Adjustments			As revised
Assets
Financial Guarantee (HG Global/BAM)
Financial Guarantee assets	$	699.1		$	—		$	699.1
Marketing Technology (MediaAlpha)
Marketing Technology assets	53.2			—			53.2
Other
Goodwill	13.4			(5.8		)	7.6
Other intangible assets	4.5			(3.4		)	1.1
Other assets — commissions receivable	21.4			(17.2		)	4.2
Other assets	2,146.3			—			2,146.3
Assets held for sale	3,578.9			—			3,578.9
Total assets	$	6,516.8		$	(26.4	)	$	6,490.4
Liabilities
Financial Guarantee (HG Global/BAM)
Financial Guarantee liabilities	$	126.4		$	—		$	126.4
Marketing Technology (MediaAlpha)
Marketing Technology liabilities	25.6			—			25.6
Other
Liabilities held for sale	2,535.2			—			2,535.2
Other liabilities	72.1			.3			72.4
Total liabilities	2,759.3			.3			2,759.6

Equity
White Mountains’s common shares	4.6			—			4.6
Paid in surplus	802.5			.7			803.2
Retained earnings	2,821.7			(25.8		)	2,795.9
Accumulated other comprehensive income, net of tax	(3.7		)	(.7		)	(4.4		)
White Mountains’s common shareholders’ equity	3,625.1			(25.8		)	3,599.3
Non-controlling interests	132.4			(.9		)	131.5
Total equity	3,757.5			(26.7		)	3,730.8
Total liabilities and equity	$	6,516.8		$	(26.4	)	$	6,490.4

Subsequent Events	3 Months Ended
Subsequent Events	Mar. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events NSM On April 2, 2018, White Mountains announced it had entered into an agreement to acquire 95% of NSM Insurance Group (“NSM”) for cash in an amount equal to $368.3 million, subject to certain adjustments. NSM expects to borrow approximately $100.0 million of third party indebtedness in conjunction with the acquisition which will reduce White Mountains’s net investment to approximately $273.3 million. The transaction is subject to regulatory approvals and other customary closing conditions and is expected to close by the end of the second quarter of 2018. Fidus Re In April 2018, BAM expanded its claims paying resources by $100.0 million through a collateralized reinsurance agreement with Fidus Re Ltd., a special-purpose insurer created solely to provide collateralized reinsurance protection to BAM. Tender Offer On April 10, 2018, White Mountains commenced a modified Dutch tender offer for up to 500,000 White Mountains common shares that will expire on May 7, 2018.

Basis of Presentation and Significant Accouting Policies (Policies)	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The Company is an exempted Bermuda limited liability company whose principal businesses are conducted through its insurance 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 80 South Main Street, Hanover, New Hampshire 03755-2053 and its registered office is located at Clarendon House, 2 Church Street, Hamilton, Bermuda HM 11. 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 White Mountains Insurance Group, Ltd. (the “Company” or the “Registrant”), its subsidiaries (collectively with the Company, “White Mountains”) and other entities required to be consolidated under GAAP. 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. Intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. These interim financial statements include all adjustments considered necessary by management to fairly state the financial position, results of operations and cash flows of White Mountains. These interim financial statements may not be indicative of financial results for the full year and should be read in conjunction with the Company’s 2017 Annual Report on Form 10-K.
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.
Reportable Segments	Reportable Segments White Mountains has determined its reportable segments based on the nature of the underlying businesses, the manner in which the Company’s subsidiaries and affiliates are organized and managed and the organization of the financial information provided to the chief operating decision maker to assess performance and make decisions regarding allocation of resources. White Mountains’s reportable segments are HG Global/BAM, MediaAlpha and Other Operations. See Note 12 — “Segment Information”. The HG Global/BAM segment consists of HG Global Ltd. and its wholly-owned subsidiaries (“HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”) (collectively, “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 purposes 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 wholly-owned 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, provided the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM (the “BAM Surplus Notes”). As of March 31, 2018, $499.0 million of the surplus notes remain outstanding. As of March 31, 2018 and December 31, 2017, 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 attributed to non-controlling interests. The MediaAlpha segment consists of QL Holdings LLC and its wholly-owned subsidiary QuoteLab, LLC (collectively “MediaAlpha”). MediaAlpha is a leading marketing technology company that develops technology that enables the programmatic buying and selling of vertical-specific, performance-based media between advertisers (buyers of advertising inventory) and publishers (sellers of advertising inventory) through cost-per-click, cost-per-call and cost-per-lead pricing models. MediaAlpha's media buying platform enables advertisers to create and automate data-driven bidding strategies designed to improve the efficiency and enhance overall performance of their marketing campaigns that target high-intent consumers at the time and place they are ready to purchase. MediaAlpha’s publisher platform is used by publishers to sell their vertical-specific, performance-based media to advertisers through transparent, programmatic, auction-based marketplaces. MediaAlpha works with 550 advertisers and 325 publishers across a number of insurance (auto, motorcycle, home, renter, health and life) and non-insurance (travel, education, personal finance and home services) verticals. White Mountains’s Other Operations segment consists of the Company, its wholly-owned subsidiary, White Mountains Capital, Inc. (“WM Capital”), its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), and its other intermediate holding companies, as well as certain consolidated and unconsolidated private capital and other investments. The consolidated private capital investments include Wobi Insurance Agency Ltd. (“Wobi”) and Removal Stars Ltd. (“Buzzmove”). During 2017, White Mountains revised certain of its previously issued financial statements for amounts relating to Wobi. See Note 17 — “Financial Statement Revisions”.
Discontinued Operations and Assets and Liabilities Held for Sale	Discontinued Operations and Assets and Liabilities Held for Sale On September 28, 2017, Intact Financial Corporation completed its acquisition of OneBeacon Insurance Group, Ltd. (“OneBeacon”) in an all-cash transaction for $18.10 per share (the “OneBeacon Transaction”). On July 21, 2016, White Mountains completed its sale of Tranzact Holdings, LLC (“Tranzact”) to an affiliate of Clayton, Dubilier & Rice, LLC. On April 18, 2016, White Mountains completed its sale of Sirius International Insurance Group, Ltd. (“Sirius Group”) to CM International Pte. Ltd. and CM Bermuda Limited (collectively “CMI”), the Singapore-based investment arm of China Minsheng Investment Corp., Ltd. White Mountains has presented the results of OneBeacon, Tranzact and Sirius Group as discontinued operations in the statement of operations and comprehensive income and their assets and liabilities as held for sale in the balance sheet for all periods prior to the completion of each transaction. White Mountains has classified its Guilford, Connecticut property, which consists of an office building and adjacent land, as held for sale as of March 31, 2018 and December 31, 2017.
Recently Adopted Changes in Accounting Principles and Recently Issued Accounting Pronouncements	Recently Adopted Changes in Accounting Principles Revenue Recognition On January 1, 2018, White Mountains adopted ASU 2014-09, Revenue from Contracts with Customers (ASC 606), which modifies the guidance for revenue recognition. Under ASU 2014-09, revenue is recognized at an amount that reflects the consideration to which an entity expects to be entitled once it fulfills its performance obligations under the terms of its contract with the customer. The scope of the new guidance includes agent commissions and other non-insurance revenues. Adoption of ASU 2014-09 did not have any impact on White Mountains's financial statements. Share-Based Compensation On January 1, 2018, White Mountains adopted ASU 2017-09, Stock Compensation: Scope of Modification Accounting (ASC 718), which narrows the scope of transactions subject to modification accounting to changes in terms of an award that result in a change in the award’s fair value, vesting conditions or classification. Adoption of ASU 2017-09 did not have any impact on White Mountains’s financial statements. On January 1, 2017, White Mountains adopted ASU 2016-09, Improvements to Employee Share-Based Payment Accounting (ASC 718) which simplifies certain aspects of the accounting for share-based compensation. The new guidance provides an accounting policy election to account for forfeitures by either applying an assumption, as required under existing guidance, or by recognizing forfeitures when they actually occur. At adoption, White Mountains did not change its accounting policy for forfeitures, which is to apply an assumed forfeiture rate. The new guidance has also changed the threshold for partial cash settlement to settle statutory withholding requirements for equity classified awards, increasing the threshold up to the maximum statutory tax rate. As a result of adoption, White Mountains reported $8.4 million and $6.5 million of statutory withholding tax payments made in connection with the settlement of restricted shares as financing cash flows for the three-month periods ended March 31, 2018 and 2017. Such payments were classified as operating cash flows prior to adoption. In addition, the new guidance changed the treatment for excess tax benefits which arise from the difference between the deduction for tax purposes and the compensation costs recognized for financial reporting. Under the new guidance, a reporting entity recognizes excess tax benefits or expense in current period earnings, regardless of whether it is in a taxes payable position. Business Combinations On January 1, 2018, White Mountains adopted ASU 2017-01, Business Combinations: Clarifying the Definition of a Business (ASC 805), which clarifies the definition of a business and affects the determination of whether acquisitions or disposals are accounted for as assets or as a business. Under the new guidance, when substantially all of the fair value of the assets is concentrated in a single identifiable asset or group of similar assets, it is not a business. White Mountains has not had any transactions falling within the scope of ASU 2017-01 during the period ended March 31, 2018 and, accordingly, adoption did not have any impact on White Mountains's financial statements. Cash Flow Statement On January 1, 2018, White Mountains adopted ASU 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (ASC 230), which addresses the classification and presentation of certain items, including debt prepayment and extinguishment costs, contingent consideration payments made after a business combination and distributions received from equity method investees, for which there was diversity in practice prior to the issuance of ASU 2016-15. Also on January 1, 2018, White Mountains adopted ASU 2016-18, Statement of Cash Flows: Restricted Cash (ASC 230), which modifies the guidance for the treatment of restricted cash amounts in the cash flow statement. The new guidance requires restricted cash to be included in the reconciliation of beginning and end-of-period amounts presented on the statement of cash flows and requires a description of the nature of the changes in restricted cash during the periods presented. Adoption of ASU 2016-15 and ASU 2016-18 did not have any impact on White Mountains's statement of cash flows. Financial Instruments - Recognition and Measurement On January 1, 2018, White Mountains adopted ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities (ASC 825-10), which modifies the guidance for financial instruments, including investments in equity securities. Under the new guidance, all equity securities with readily determinable fair values are required to be measured at fair value with changes therein recognized through current period earnings. In addition, the new ASU requires a qualitative assessment for equity securities without readily determinable fair values to identify impairment, and for impaired equity securities to be measured at fair value. White Mountains measures its portfolio of investment securities at fair value with changes therein recognized through current period earnings and, accordingly, adoption of ASU 2016-01 did not have any impact on White Mountains's financial statements. Recently Issued Accounting Pronouncements Premium Amortization on Callable Debt Securities In March 2017, the FASB issued ASU 2017-08, Premium Amortization on Purchased Callable Debt Securities (ASC 310-20), which changes the amortization period for certain purchased callable debt securities. Under the new guidance, for investments in callable debt securities held at a premium, the premium will be amortized over the period to the earliest call date. The new guidance does not change the amortization period for callable debt securities held at a discount. ASU 2017-08 is not expected to have any impact on White Mountains's financial statements at adoption but may affect the amortization recognized in future periods. Credit Losses In June 2016, the FASB issued ASU 2016-13, Measurement of Credit Losses on Financial Instruments (ASC 326), which establishes new guidance for the recognition of credit losses for financial assets measured at amortized cost. The new ASU requires reporting entities to estimate the credit losses expected over the life of a credit exposure using historical information, current information and reasonable and supportable forecasts that affect the collectability of the financial asset. This differs from current GAAP, which delays recognition until it is probable a loss has been incurred. The new guidance is expected to accelerate recognition of credit losses. The types of assets within the scope of the new guidance include premium receivables, reinsurance recoverables and loans. ASU 2016-13 is effective for annual periods beginning after January 1, 2020, including interim periods. White Mountains measures its portfolio of investment securities at fair value with changes therein recognized through current period earnings and, accordingly, does not expect adoption to have any effect on its financial statements. Leases In February 2016, the FASB issued ASU 2016-02, Leases (ASC 842). The new guidance requires lessees to recognize lease assets and liabilities on the balance sheet for both operating and financing leases, with the exception of leases with an original term of 12 months or less. Under existing guidance recognition of lease assets and liabilities is not required for operating leases. The lease assets and liabilities to be recognized are both measured initially based on the present value of the lease payments. Under the new guidance, a sale-leaseback transaction must meet the recognition criteria under ASC 606, Revenues, in order to be accounted for as sale. The new guidance is effective for White Mountains for years beginning after December 15, 2018, including interim periods therein. White Mountains is evaluating the expected impact of this guidance and available adoption methods.

Investment Securities (Tables)

3 Months Ended

Mar. 31, 2018

Investments, Debt and Equity Securities [Abstract]

Pre-tax net investment income

The following table presents pre-tax net investment income for the three months ended March 31, 2018 and 2017.



	Three Months Ended
	March 31,
Millions	2018			2017
Investment income:
Fixed maturity investments	$	10.9		$	11.9
Short-term investments	2.6			.2
Common equity securities	5.0			1.3
Other long-term investments	1.9			(.1		)
Total investment income	20.4			13.3
Third-party investment expenses	(.7		)	(.5		)
Net investment income, pre-tax	$	19.7		$	12.8

Schedule of Realized and Unrealized Gain (Loss) on Investments

The following table presents net realized and unrealized investment gains (losses) for the three months ended March 31, 2018 and 2017:



	Three Months Ended
	March 31,
Millions	2018			2017
Net realized investment (losses) gains, pre-tax	$	(5.1	)	$	.6
Net unrealized investment (losses) gains, pre-tax	(48.6		)	35.7
Net realized and unrealized investment (losses) gains, pre-tax	(53.7		)	36.3
Income tax benefit (expense) attributable to net realized and unrealized investment (losses) gains	5.7			(3.9		)
Net realized and unrealized investment (losses) gains, after-tax	$	(48.0	)	$	32.4

Schedule of realized gain (loss) on investments

The following table presents net realized investment gains (losses) for the three months ended March 31, 2018 and 2017



	Three Months Ended									Three Months Ended
	March 31, 2018									March 31, 2017
Millions	Net realized (losses) gains			Net foreign exchange gains (losses)			Total net realized gains (losses) reflected in earnings			Net realized (losses) gains			Net foreign exchange gains		Total net realized (losses) gains reflected in earnings
Fixed maturity investments	$	(13.7	)	$	18.2		$	4.5		$	(1.0	)	$	.1	$	(.9	)
Short-term investments	(.1		)	—			(.1		)	—			—		—
Common equity securities	1.2			—			1.2			.8			.1		.9
Other long-term investments	(3.5		)	(7.2		)	(10.7		)	.6			—		.6
Net realized investment (losses) gains, pre-tax	(16.1		)	11.0			(5.1		)	.4			.2		.6
Income tax expense attributable to net realized investment (losses) gains	(.6		)	—			(.6		)	(.2		)	—		(.2		)
Net realized investment (losses) gains, after-tax	$	(16.7	)	$	11.0		$	(5.7	)	$	.2		$	.2	$	.4

Schedule of unrealized gain (loss) on investments

The following tables present net unrealized investment gains (losses) and changes in the carrying value of investments measured at fair value for the three months ended March 31, 2018 and 2017:



	Three Months Ended									Three Months Ended
	March 31, 2018									March 31, 2017
Millions	Net unrealized losses			Net foreign exchange (losses) gains			Total net unrealized (losses) gains reflected in earnings			Net unrealized gains			Net foreign exchange gains (losses)			Total net unrealized gains reflected in earnings
Fixed maturity investments	$	(18.6	)	$	(14.8	)	$	(33.4	)	$	10.2		$	1.7		$	11.9
Short-term investments	(.7		)	—			(.7		)	—			—			—
Common equity securities	(16.7		)	—			(16.7		)	19.1			.5			19.6
Other long-term investments	(1.9		)	4.1			2.2			6.9			(2.7		)	4.2
Net unrealized investment (losses) gains, pre-tax	(37.9		)	(10.7		)	(48.6		)	36.2			(.5		)	35.7
Income tax benefit (expense) attributable to net unrealized investment gains (losses)	6.3			—			6.3			(3.7		)	—			(3.7		)
Net unrealized investment (losses) gains, after-tax	$	(31.6	)	$	(10.7	)	$	(42.3	)	$	32.5		$	(.5	)	$	32.0

Net unrealized investment gains (losses) for Level 3 investments



	Three Months Ended
	March 31,
Millions	2018			2017
Other long-term investments	$	(5.1	)	$	.2
Total net unrealized investment (losses) gains, pre-tax - Level 3 investments	$	(5.1	)	$	.2

Investment holdings, equity securities, convertible fixed maturities and other long-term investments



	March 31, 2018
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency gains		Carrying value
U.S. Government and agency obligations	$	199.9	$	—	$	(2.2	)	$	—	$	197.7
Debt securities issued by corporations	760.5		.7		(17.5		)	—		743.7
Mortgage and asset-backed securities	132.5		.1		(2.6		)	—		130.0
Municipal obligations	261.8		1.9		(1.3		)	—		262.4
Total fixed maturity investments	$	1,354.7	$	2.7	$	(23.6	)	$	—	$	1,333.8



	December 31, 2017
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency gains		Carrying value
U.S. Government and agency obligations	$	297.8	$	—	$	(1.3	)	$	—	$	296.5
Debt securities issued by corporations	867.6		2.9		(4.3		)	14.7		880.9
Mortgage and asset-backed securities	697.2		1.6		(4.1		)	—		694.7
Municipal obligations	252.0		3.7		(.8		)	—		254.9
Foreign government, agency and provincial obligations	2.6		—		—			.1		2.7
Total fixed maturity investments	$	2,117.2	$	8.2	$	(10.5	)	$	14.8	$	2,129.7



	March 31, 2018
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency losses			Carrying value
Common equity securities	$	835.9	$	111.7	$	(2.0	)	$	—		$	945.6
Other long-term investments	$	288.8	$	10.0	$	(44.8	)	$	(.9	)	$	253.1



	December 31, 2017
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency losses			Carrying value
Common equity securities	$	739.7	$	129.4	$	(3.0	)	$	—		$	866.1
Other long-term investments	$	246.6	$	6.8	$	(39.7	)	$	(4.9	)	$	208.8

Other Long-Term Investments

The following table presents the carrying values of White Mountains’s other long-term investments as of March 31, 2018 and December 31, 2017:



	Carrying Value at
Millions	March 31, 2018		December 31, 2017
Hedge funds and private equity funds, at fair value	$	130.6	$	125.3
Private equity securities, at fair value ^(1)(2)(3)	108.4		83.2
Foreign currency forward contracts	—		(3.7		)
Other	14.1		4.0
Total other long-term investments	$	253.1	$	208.8

⁽¹⁾See Fair Value Measurements by Level table.

⁽²⁾ Includes non-controlling interests in common equity securities, limited liability companies and private convertible preferred securities.

Other long-term investments

The following table presents investments in hedge funds and private equity funds by investment objective and sector as of March 31, 2018 and December 31, 2017:



	March 31, 2018				December 31, 2017
Millions	Fair Value		Unfunded Commitments		Fair Value		Unfunded Commitments
Hedge funds
Long/short banks and financial	$	56.4	$	—	$	54.9	$	—
Total hedge funds	56.4		—		54.9		—

Private equity funds
Manufacturing/Industrial	44.5		10.4		43.3		10.4
Aerospace/Defense/Government	17.0		12.9		15.8		12.9
Direct lending	7.9		22.5		7.1		23.1
Financial services	4.8		11.1		4.2		11.7
Insurance	—		41.2		—		41.2
Real estate	—		50.0		—		—
Total private equity funds	74.2		148.1		70.4		99.3
Total hedge funds and private equity funds included in other long-term investments	$	130.6	$	148.1	$	125.3	$	99.3

Fair Value of 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 March 31, 2018:



Millions	1 – 3 years	3 – 5 years	5 – 10 years	>10 years	Total
Private equity funds — expected lock-up period remaining	$6.1	$6.2	$49.2	$12.7	$74.2

Fair value measurements by level, investment securities



	March 31, 2018
Millions	Fair Value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	197.7	$	197.7	$	—	$	—

Debt securities issued by corporations:
Financials	151.0		—		151.0		—
Consumer	144.8		—		144.8		—
Technology	91.6		—		91.6		—
Communications	84.9		—		84.9		—
Health care	80.3		—		80.3		—
Materials	76.1		—		76.1		—
Energy	66.1		—		66.1		—
Industrial	33.9		—		33.9		—
Utilities	15.0		—		15.0		—
Total debt securities issued by corporations:	743.7		—		743.7		—

Mortgage and asset-backed securities	130.0		—		130.0		—
Municipal obligations	262.4		—		262.4		—
Total fixed maturity investments	1,333.8		197.7		1,136.1		—

Short-term investments⁽¹⁾	763.0		737.3		25.7		—

Common equity securities:
Exchange traded funds ⁽²⁾	653.2		592.2		61.0		—
Technology	18.0		18.0		—		—
Health care	16.9		16.9		—		—
Financials	15.9		15.9		—		—
Industrial	13.3		13.3		—		—
Consumer	8.6		8.6		—		—
Energy	6.5		6.5		—		—
Communications	5.2		5.2		—		—
Other ⁽³⁾	208.0		—		208.0		—
Total common equity securities	945.6		676.6		269.0		—

Other long-term investments ⁽⁴⁾	122.5		—		—		122.5
Total investments	$	3,164.9	$	1,611.6	$	1,430.8	$	122.5

⁽¹⁾Short-term investments are measured at amortized cost, which approximates fair value.

⁽²⁾ETFs traded on foreign exchanges are priced using the fund's published NAV to account for the difference in market close times and are therefore designated a level 2 measurement.

⁽³⁾Consists of two investments in unit trusts that primarily invest in international equities.

⁽⁴⁾Excludes carrying value of $130.6 associated with hedge funds and private equity funds for which fair value is measured at NAV using the practical expedient.



	December 31, 2017
Millions	Fair Value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	296.5	$	296.5	$	—	$	—

Debt securities issued by corporations:
Consumer	185.1		—		185.1		—
Communications	127.8		—		127.8		—
Financials	114.8		—		114.8		—
Utilities	108.9		—		108.9		—
Materials	95.5		—		95.5		—
Health care	94.3		—		94.3		—
Technology	80.5		—		80.5		—
Energy	48.1		—		48.1		—
Industrial	25.9		—		25.9		—
Total debt securities issued by corporations:	880.9		—		880.9		—

Mortgage and asset-backed securities	694.7		—		694.7		—
Municipal obligations	254.9		—		254.9		—
Foreign government, agency and provincial obligations	2.7		—		2.7		—
Total fixed maturity investments	2,129.7		296.5		1,833.2		—

Short-term investments ⁽¹⁾	176.1		151.0		25.1		—

Common equity securities:
Exchange traded funds ⁽²⁾	569.7		508.1		61.6		—
Health care	17.1		17.1		—		—
Financials	16.3		16.3		—		—
Technology	15.1		15.1		—		—
Industrial	11.9		11.9		—		—
Communications	10.9		10.9		—		—
Consumer	10.7		10.7		—		—
Energy	3.8		3.8		—		—
Other ⁽³⁾	210.6		—		210.6		—
Total common equity securities	866.1		593.9		272.2		—

Other long-term investments⁽⁴⁾⁽⁵⁾	87.2		—		—		87.2
Total investments	$	3,259.1	$	1,041.4	$	2,130.5	$	87.2

⁽¹⁾Short-term investments are measured at amortized cost, which approximates fair value.

⁽²⁾ETFs traded on foreign exchanges are priced using the fund’s published NAV to account for the difference in market close times and are therefore designated a level 2 measurement.

⁽³⁾Consists of two investments in unit trusts that primarily invests in international equities.

⁽⁴⁾ Excludes carrying value of $(3.7) related to foreign currency forward contracts.

⁽⁵⁾Excludes carrying value of $125.3 associated with hedge funds and private equity funds for which fair value is measured at NAV using the practical expedient.

Debt securities issued by corporations, credit ratings

The following table presents the ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of March 31, 2018 and December 31, 2017:



	Fair Value at
Millions	March 31, 2018		December 31, 2017
AAA	$	8.9	$	1.6
AA	79.1		42.6
A	274.6		192.5
BBB	211.5		465.2
BB	151.0		161.7
B	18.6		17.3
Debt securities issued by corporations ⁽¹⁾	$	743.7	$	880.9

⁽¹⁾Credit ratings are assigned based on the following hierarchy: (1) Standard & Poor’s Financial Services LLC (“S&P”) and (2) Moody's Investor Service, Inc. (“Moody’s”).

Mortgage-backed, asset-backed securities

The following table presents the carrying value of White Mountains’s mortgage and asset-backed securities as of March 31, 2018 and December 31, 2017:



	March 31, 2018						December 31, 2017
Millions	Fair Value		Level 2		Level 3		Fair Value		Level 2		Level 3
Mortgage-backed securities:
Agency:
GNMA	$	27.3	$	27.3	$	—	$	46.3	$	46.3	$	—
FNMA	53.9		53.9		—		84.5		84.5		—
FHLMC	36.1		36.1		—		62.0		62.0		—
Total agency ⁽¹⁾	117.3		117.3		—		192.8		192.8		—
Non-agency:
Commercial	—		—		—		70.5		70.5		—
Total non-agency	—		—		—		70.5		70.5		—

Total mortgage-backed securities	117.3		117.3		—		263.3		263.3		—
Other asset-backed securities:
Credit card receivables	8.9		8.9		—		206.0		206.0		—
Vehicle receivables	3.8		3.8		—		142.4		142.4		—
Other	—		—		—		83.0		83.0		—
Total other asset-backed securities	12.7		12.7		—		431.4		431.4		—
Total mortgage and asset-backed securities	$	130.0	$	130.0	$	—	$	694.7	$	694.7	$	—

Non-agency commercial mortgage securities, type of interest rate and tranche levels

Rollforward of fair value investments by level

The following tables present the changes in White Mountains’s fair value measurements by level for the three months ended March 31, 2018 and 2017:



							Level 3 Investments
Millions	Level 1 investments			Level 2 investments			Fixed maturity investments		Other long-term investments			Hedge Funds and Private Equity Funds measured at NAV⁽³⁾			Total
Balance at January 1, 2018	$	890.4		$	2,105.4		$	—	$	87.2		$	125.3		$	3,208.3		⁽¹⁾⁽²⁾
Net realized and unrealized (losses) gains	(14.3		)	(26.0		)	—		(8.7		)	3.6			(45.4		)	⁽⁴⁾
Amortization/Accretion	—			(1.2		)	—		—			—			(1.2		)
Purchases	238.9			407.7			—		44.0			2.0			692.6
Sales	(240.7		)	(1,080.8		)	—		—			(.3		)	(1,321.8		)
Transfers in	—			—			—		—			—			—
Transfers out	—			—			—		—			—			—
Balance at March 31, 2018	$	874.3		$	1,405.1		$	—	$	122.5		$	130.6		$	2,532.5		⁽²⁾

⁽¹⁾Excludes carrying value of $(3.7) as of January 1, 2018 associated with foreign currency forward contracts.

⁽²⁾Excludes carrying value of $176.1 and $763.0 at January 1, 2018 and March 31, 2018 classified as short-term investments.



							Level 3 Investments
Millions	Level 1 investments			Level 2 investments			Fixed maturity investments		Other long-term investments			Hedge Funds and Private Equity Funds measured at NAV⁽³⁾			Total
Balance at January 1, 2017	$	279.5		$	2,093.8		$	—	$	91.4		$	82.6		$	2,547.3		^(1)(2)(4)
Net realized and unrealized gains	13.1			18.4			.1		.2			7.3			39.1			⁽⁵⁾
Amortization/Accretion	—			(2.5		)	—		—			—			(2.5		)
Purchases	115.2			351.1			11.0		.2			21.4			498.9
Sales	(76.0		)	(387.6		)	—		(2.0		)	(1.2		)	(466.8		)
Deconsolidation of SSIE	—			(5.2		)	—		—			—			(5.2		)
Transfers in	—			—			—		—			—			—
Transfers out	—			—			—		—			—			—
Balance at March 31, 2017	$	331.8		$	2,068.0		$	11.1	$	89.8		$	110.1		$	2,610.8		⁽¹⁾⁽²⁾

⁽¹⁾Excludes carrying value of $175.0 and $138.2 at January 1, 2017 and March 31, 2017 classified as short-term investments, of which $0.1 is classified as held for sale at January 1, 2017.

⁽²⁾Excludes carrying value of $(1.2) and $(4.1) as of January 1, 2017 and March 31, 2017 associated with foreign currency forward contracts.

⁽³⁾ Investments for which fair value is measured at NAV using the practical expedient are no longer classified within the fair value hierarchy. See Note 1 — “Basis of Presentation and Significant Accounting Policies”.

⁽⁴⁾Includes carrying value of $6.6 of fixed maturity investments at January 1, 2017 that is classified as assets held for sale related to SSIE.

⁽⁵⁾ Excludes realized and unrealized losses associated with foreign currency forward contracts of $2.8 for the three months ended March 31, 2017.

Schedule of significant unobservable inputs used in estimating the fair value of investment securities



$ in millions, except share price	March 31, 2018
Description	Valuation Technique(s)	Fair Value ⁽¹⁾	Unobservable Input
Private equity security	Discounted cash flow	$21.0	Discount rate	-	25.0%
Private equity security	Discounted cash flow	$22.1	Implied share price	-	$.68
Private convertible preferred security	Discounted cash flow	$14.5	Implied share price	-	$2.06
Private equity security	Discounted cash flow/ Option pricing method	$11.6	Discount rate	-	21.0%
			Time until expiration	-	4 years
		Volatility/Standard deviation		-	50.0%
		Risk free rate		-	1.77%
Private debt instrument	Discounted cash flow	$10.5	Discount rate	-	9.62%

⁽¹⁾Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs.



$ in millions, except share price	December 31, 2017
Description	Valuation Technique(s)	Fair Value ⁽¹⁾	Unobservable Input
Private equity security	Share price of most recent transaction	$21.0	Share price	-	$1.00
Private equity security	Discounted cash flow	$22.1	Implied share price	-	$.68
Private convertible preferred security	Discounted cash flow	$14.5	Implied share price	-	$2.06
Private equity security	Discounted cash flow/ Option pricing method	$11.3	Discount rate	-	21.0%
			Time until expiration	-	4 years
		Volatility/Standard deviation		-	50.0%
		Risk free rate		-	1.77%
Private equity security	Share price of most recent transaction	$3.6	Share price	-	$2.52
Private convertible preferred security	Multiple of EBITDA	$0.6	EBITDA multiple	-	6.00

⁽¹⁾Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs.

Goodwill and Other Intangible Assets (Tables)

3 Months Ended

Mar. 31, 2018

Goodwill and Intangible Assets Disclosure [Abstract]

Changes in Goodwill and Intangible Assets

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



	Three Months Ended March 31,
	2018								2017
Millions	Goodwill		Other intangible assets			Total			Goodwill		Other intangible assets			Total
Beginning balance	$	25.9	$	36.2		$	62.1		$	25.9	$	19.3		$	45.2
Amortization, including foreign currency translation	—		(2.9		)	(2.9		)	—		(2.3		)	(2.3		)
Ending balance	$	25.9	$	33.3		$	59.2		$	25.9	$	17.0		$	42.9

Schedule of Intangible Assets and Goodwill

The following table presents the goodwill and other intangible assets as of March 31, 2018 and December 31, 2017:



Millions	March 31, 2018			December 31, 2017
Goodwill
MediaAlpha	$	18.3		$	18.3
Other	7.6			7.6
Total goodwill	25.9			25.9
Other intangible assets
MediaAlpha	32.5			35.4
Other	.8			.8
Total other intangible assets	33.3			36.2
Total goodwill and other intangible assets	59.2			62.1
Goodwill and other intangible assets attributed to non-controlling interests	(21.1		)	(21.1		)
Goodwill and other intangible assets included in White Mountains’s common shareholders’ equity	$	38.1		$	41.0

Debt (Tables)

3 Months Ended

Mar. 31, 2018

Debt Disclosure [Abstract]

Schedule of debt outstanding

The following table presents White Mountains’s debt outstanding as of March 31, 2018 and December 31, 2017:



Millions	March 31, 2018			Effective Rate ⁽¹⁾	December 31, 2017			Effective Rate ⁽¹⁾
WTM Bank Facility	$	—		N/A	$	—		N/A
Unamortized issue costs	—				—
WTM Bank Facility, carrying value	—				—
MediaAlpha Bank Facility	21.6			6.2%	23.9			5.6%
Unamortized issuance cost	(.1		)		(.1		)
MediaAlpha Bank Facility, carrying value	21.5				23.8
Total debt	$	21.5			$	23.8

⁽¹⁾Effective rate considers the effect of the debt issuance costs.

- 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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Publisher FASB
-Name Accounting Standards Codification
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21521-112644

+ Details

Municipal Bond Guarantee (Tables)

3 Months Ended

Mar. 31, 2018

Guarantees [Abstract]

Schedule of Municipal Bond Guarantee Insured Obligations

The following table presents a schedule of BAM’s insured obligations:



	March 31, 2018		December 31, 2017
Contracts outstanding	6,541		6,371
Remaining weighted average contract period outstanding (in years)	10.8		10.9
Contractual debt service outstanding (in millions):
Principal	$	43,139.2	$	42,090.6
Interest	21,420.6		21,057.1
Total debt service outstanding	$	64,559.8	$	63,147.7

Gross unearned insurance premiums	$	140.2	$	136.8

Financial Guarantee Insurance Contracts, Premium Received over Contract Period

The following table presents a schedule of BAM’s future premium revenues as of March 31, 2018:



Millions	March 31, 2018
April 1, 2018 - December 31, 2018	$	8.9

January 1, 2019 - March 31, 2019	2.9
April 1, 2019 - June 30, 2019	2.9
July 1, 2019 - September 30, 2019	2.9
October 1, 2019 - December 31, 2019	2.9
	11.6

2020	11.1
2021	10.6
2022	10.1
2023 and thereafter	87.9
Total gross unearned insurance premiums	$	140.2

Schedule of Net Written Premiums

The following table presents a schedule of net written premiums included in White Mountains’s HG Global/BAM segment for the three months years ended March 31, 2018 and 2017:



	Three Months Ended March 31,
Millions	2018		2017
Gross written premiums	$	6.4	$	18.7
Assumed (ceded) written premiums	—		—
Net written premiums	$	6.4	$	18.7

Earnings Per Share (Tables)

3 Months Ended

Mar. 31, 2018

Earnings Per Share [Abstract]

Computation of earnings per share



	Three Months Ended
	March 31,
	2018			2017
Basic and diluted earnings per share numerators (in millions):
Net (loss) income attributable to White Mountains’s common shareholders	$	(48.0	)	$	29.0
Less: total income from discontinued operations, net of tax	.1			31.3
Net loss from continuing operations attributable to White Mountains’s common shareholders	$	(48.1	)	$	(2.3	)
Allocation of earnings to participating restricted common shares⁽¹⁾	.4			—
Basic and diluted earnings per share numerators	$	(47.7	)	$	(2.3	)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,746.1			4,564.6
Average unvested restricted common shares⁽²⁾	(35.9		)	(52.5		)
Basic earnings per share denominator	3,710.2			4,512.1
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,746.1			4,564.6
Average unvested restricted common shares⁽²⁾	(35.9		)	(52.5		)
Diluted earnings per share denominator	3,710.2			4,512.1
Basic and diluted earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid	$	1.00		$	1.00
Undistributed earnings (losses)	(13.85		)	(1.52		)
Basic and diluted earnings per share	$	(12.85	)	$	(.52	)

⁽¹⁾ Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.

⁽²⁾Restricted shares outstanding vest either in equal annual installments or upon a stated date. See Note 10 — “Employee Share-Based Incentive Compensation Plans”

Disposal Groups, Including Discontinued Operations



	Three Months Ended
	March 31,
Millions	2018			2017
Undistributed net losses - continuing operations:
Net loss attributable to White Mountains’s common shareholders, net of restricted common share amounts	$	(47.7	)	$	(2.3	)
Dividends declared net of restricted common share amounts ⁽¹⁾	(3.7		)	(4.5		)
Total undistributed net losses, net of restricted common share amounts	$	(51.4	)	$	(6.8	)

⁽¹⁾ Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.

Net Income (Loss) from Discontinued Operations



	Three Months Ended				Three Months Ended
	March 31, 2018				March 31, 2017
Millions	Sirius Group		Total		OneBeacon			Tranzact			Total
Revenues
Earned insurance premiums	$	—	$	—	$	261.8		$	—		$	261.8
Net investment income	—		—		12.2			—			12.2
Net realized and unrealized gains	—		—		15.0			—			15.0
Other revenue	—		—		3.4			—			3.4
Total revenues	—		—		292.4			—			292.4
Expenses
Loss and loss adjustment expenses	—		—		150.6			—			150.6
Insurance and reinsurance acquisition expenses	—		—		45.3			—			45.3
Other underwriting expenses	—		—		51.7			—			51.7
General and administrative expenses	—		—		5.0			—			5.0
Interest expense	—		—		3.3			—			3.3
Total expenses	—		—		255.9			—			255.9
Pre-tax income	—		—		36.5			—			36.5
Income tax expense	—		—		(4.2		)	—			(4.2		)
Net income from discontinued operations	—		—		32.3			—			32.3
Net gain (loss) from sale of discontinued operations	.1		.1		—			(1.0		)	(1.0		)
Total income (loss) from discontinued operations	.1		.1		32.3			(1.0		)	31.3
Change in foreign currency translation and other from discontinued operations	—		—		(.1		)	—			(.1		)
Comprehensive income (loss) from discontinued operations	$	.1	$	.1	$	32.2		$	(1.0	)	$	31.2

Net Change in Cash from Discontinued Operations

The following tables presents the net change in cash, including income tax payments to national governments and interest paid associated with the business classified as discontinued operations:



	Three Months Ended
	March 31,
Millions	2018			2017
Net cash provided from (used for) operations	$	.1		$	(1.3	)
Net cash (used for) provided from investing activities	(.1		)	32.3
Net cash used for financing activities	—			(21.0		)
Net change in cash during the period	—			10.0
Cash balances at beginning of period	—			70.5
Net change in cash held for sale, excluding discontinued operations	—			(.9		)
Cash balances at end of period	$	—		$	79.6
Supplemental cash flows information:
Interest paid	$	—		$	—
Net income tax payment to national governments	$	—		$	—

Earnings Per Share from Discontinued Operations

The following table presents the Company’s computation of earnings per share for discontinued operations for the three months ended March 31, 2018 and 2017:



	Three Months Ended
	March 31,
	2018			2017
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$	(48.0	)	$	29.0
Less total loss from continuing operations, net of tax	48.1			2.3
Net income from discontinued operations attributable to White Mountains’s common shareholders	$	.1		$	31.3
Allocation of earnings to participating restricted common shares ⁽¹⁾	—			(.4		)
Basic and diluted earnings per share numerators	$	.1		$	30.9
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,746.1			4,564.6
Average unvested restricted common shares ⁽³⁾	(35.9		)	(52.5		)
Basic earnings per share denominator	3,710.2			4,512.1
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,746.1			4,564.6
Average unvested restricted common shares ⁽³⁾	(35.9		)	(52.5		)
Diluted earnings per share denominator	3,710.2			4,512.1
Basic earnings per share (in dollars) - discontinued operations:	$	.03		$	6.86
Diluted earnings per share (in dollars) - discontinued operations:	$	.03		$	6.86

⁽¹⁾Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities.

⁽²⁾Net earnings attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the three months ended March 31, 2018 and 2017.

⁽³⁾ Restricted common shares outstanding vest either in equal annual installments or upon a stated date. See Note 10 — “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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
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-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=65878427&loc=SL51721533-107759

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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-URI http://asc.fasb.org/extlink&oid=84176398&loc=SL51724579-110230

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+ Details

Employee Share-Based Incentive Compensation Plans (Tables) - White Mountains

3 Months Ended

Mar. 31, 2018

Share-based compensation arrangement by share-based payment award

Summary of performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan

The following table presents the performance share activity for the three months ended March 31, 2018 and 2017 for performance shares granted under the WTM Incentive Plan:



	Three Months Ended March 31,
	2018					2017
Millions, except share amounts	Target Performance Shares Outstanding		Accrued Expense			Target Performance Shares Outstanding		Accrued Expense
Beginning of period	50,515		$	45.8		80,353		$	42.4
Shares paid ⁽¹⁾	(23,186	)	(28.4		)	(30,167	)	(20.8		)
New grants	14,105		—			16,460		—
Forfeitures and cancellations⁽²⁾	(818	)	.3			(9,841	)	(5.7		)
Expense recognized	—		3.3			—		8.1
End of period⁽³⁾	40,616		$	21.0		56,805		$	24.0

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

⁽³⁾Outstanding performance share awards as of March 31, 2017 excludes 2,195 performance share awards granted to employees of Sirius Group.

Share-based Compensation, Performance Shares Award Outstanding Activity

The following table presents performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan at March 31, 2018 for each performance cycle:



	Three Months Ended March 31, 2018
Millions, except share amounts	Target Performance Shares Outstanding		Accrued Expense
Performance cycle:
2016 – 2018	13,715		$	12.7
2017 – 2019	14,070		7.7
2018 – 2020	13,450		.9
Sub-total	41,235		21.3
Assumed forfeitures	(619	)	(.3		)
March 31, 2018	40,616		$	21.0

Summary of restricted shares activity

The following table presents the unrecognized compensation cost associated with the outstanding restricted share awards for the three months ended March 31, 2018 and 2017:



	Three Months Ended March 31,
	2018					2017
Millions, except share amounts	Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value
Non-vested,
Beginning of period	53,755		$	14.3		70,620		$	19.7
Issued	14,105		11.4			16,735		15.8
Vested	(25,381	)	—			(22,015	)	—
Forfeited	(969	)	(.2		)	(5,200	)	(2.8		)
Expense recognized	—		(3.3		)	—		(3.6		)
End of period⁽¹⁾	41,510		$	22.2		60,140		$	29.1

⁽¹⁾Restricted share awards outstanding as of March 31, 2017 includes 2,195 restricted shares issued to employees of Sirius Group, which was accounted for as discontinued operations.

Non-controlling Interest (Tables)

3 Months Ended

Mar. 31, 2018

Noncontrolling Interest [Abstract]

Non-controlling Interests



	March 31, 2018					December 31, 2017
$ in millions	Non-controlling Percentage		Non-controlling Equity			Non-controlling Percentage		Non-controlling Equity
Other, excluding mutuals
HG Global	3.1	%	$	15.7		3.1	%	$	15.9
MediaAlpha	37.7		16.2			35.7		13.1
Buzzmove	22.9		2.4			22.9		2.5
Total other, excluding mutuals			34.3					31.5
Mutuals
BAM	100.0		(177.5		)	100.0		(163.2		)
Total non-controlling interests			$	(143.2	)			$	(131.7	)

Segment Information (Tables)

3 Months Ended

Mar. 31, 2018

Segment Reporting [Abstract]

Financial information for White Mountains' segments

The following table presents the financial information for White Mountains’s segments:



Millions	HG Global/BAM			MediaAlpha			Other Operations			Total
Three Months Ended March 31, 2018
Earned insurance premiums	$	3.0		$	—		$	—		$	3.0
Net investment income	3.7			—			16.0			19.7
Net realized and unrealized investment losses	(7.9		)	—			(45.8		)	(53.7		)
Advertising and commission revenues ⁽¹⁾	—			70.1			.9			71.0
Other revenue	.2			1.6			.3			2.1
Total revenues	(1.0		)	71.7			(28.6		)	42.1
Insurance acquisition expenses	1.4			—			—			1.4
Other underwriting expenses	.1			—			—			.1
Cost of sales	—			57.4			.7			58.1
General and administrative expenses	11.8			14.1			22.0			47.9
Interest expense	—			.4			.2			.6
Total expenses	13.3			71.9			22.9			108.1
Pre-tax loss	$	(14.3	)	$	(.2	)	$	(51.5	)	$	(66.0	)


⁽¹⁾	Approximately 31% of MediaAlpha’s advertising revenue was associated with one customer for the three months ended March 31, 2018.



Millions	HG Global/BAM			MediaAlpha			Other Operations		Total
Three Months Ended March 31, 2017
Earned insurance premiums	$	2.0		$	—		$	1.0	$	3.0
Net investment income	2.6			—			10.2		12.8
Net realized and unrealized investment gains	1.3			—			35.0		36.3
Advertising and commission revenues ⁽¹⁾	—			32.5			1.3		33.8
Other revenue	.4			—			2.5		2.9
Total revenues	6.3			32.5			50.0		88.8
Losses and loss adjustment expenses	—			—			1.1		1.1
Insurance acquisition expenses	1.2			—			.1		1.3
Other underwriting expenses	.1			—			—		.1
Cost of sales	—			27.7			1.1		28.8
General and administrative expenses	10.6			5.6			44.5		60.7
Interest expense	—			.2			.2		.4
Total expenses	11.9			33.5			47.0		92.4
Pre-tax (loss) income	$	(5.6	)	$	(1.0	)	$	3.0	$	(3.6	)


⁽¹⁾	Approximately 27% of MediaAlpha’s advertising revenue was associated with one customer for the three months ended March 31, 2017.

Investments in Unconsolidated Entities (Tables)

3 Months Ended

Mar. 31, 2018

Schedule of Equity Method Investments [Line Items]

Investments in and Advances to Affiliates

The following table presents the carrying values of investments in equity method eligible unconsolidated entities recorded within other long-term investments:



Millions	March 31, 2018		December31, 2017
Equity method eligible private equity securities, at fair value	$	84.7	$	58.0
Investments, accounted for under the equity method	5.7		4.6
Total investments in equity method eligible unconsolidated entities	90.4		62.6
Other unconsolidated investments ⁽¹⁾	162.7		146.2
Total other long-term investments	$	253.1	$	208.8

⁽¹⁾Consists of other long-term investments that are not equity method eligible.

Affiliated Entity

Schedule of Equity Method Investments [Line Items]

Equity Method Investments

The following table presents White Mountains’s investments in equity method eligible unconsolidated entities as of March 31, 2018 and December 31, 2017:



Investee	Ownership Interest		Instrument Held
	March 31, 2018	December 31, 2017
Compare.com	22%	22%	Common shares
DavidShield ⁽¹⁾	50%	—	Common shares
durchblicker	45%	45%	Common shares
Kudu	50%	—	Units
OneTitle	20%	20%	Common shares
PassportCard ⁽¹⁾	50%	50%	Common shares
Tuckerman Capital Fund III	21%	21%	Units

Fair Value of Financial Instruments (Tables)

3 Months Ended

Mar. 31, 2018

Fair Value Disclosures [Abstract]

Summary of the fair value and carrying value of financial instruments

The following table presents the fair value and carrying value of this financial instrument as of March 31, 2018 and December 31, 2017:



	March 31, 2018				December 31, 2017
Millions	Fair Value		Carrying Value		Fair Value		Carrying Value
MediaAlpha Bank Facility	$	21.8	$	21.6	$	23.9	$	23.8

Held for Sale and Discontinued Operations (Tables)

3 Months Ended

Mar. 31, 2018

Discontinued Operations and Disposal Groups [Abstract]

Disposal Groups, Including Discontinued Operations



	Three Months Ended
	March 31,
Millions	2018			2017
Undistributed net losses - continuing operations:
Net loss attributable to White Mountains’s common shareholders, net of restricted common share amounts	$	(47.7	)	$	(2.3	)
Dividends declared net of restricted common share amounts ⁽¹⁾	(3.7		)	(4.5		)
Total undistributed net losses, net of restricted common share amounts	$	(51.4	)	$	(6.8	)

⁽¹⁾ Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.

Net Income (Loss) from Discontinued Operations



	Three Months Ended				Three Months Ended
	March 31, 2018				March 31, 2017
Millions	Sirius Group		Total		OneBeacon			Tranzact			Total
Revenues
Earned insurance premiums	$	—	$	—	$	261.8		$	—		$	261.8
Net investment income	—		—		12.2			—			12.2
Net realized and unrealized gains	—		—		15.0			—			15.0
Other revenue	—		—		3.4			—			3.4
Total revenues	—		—		292.4			—			292.4
Expenses
Loss and loss adjustment expenses	—		—		150.6			—			150.6
Insurance and reinsurance acquisition expenses	—		—		45.3			—			45.3
Other underwriting expenses	—		—		51.7			—			51.7
General and administrative expenses	—		—		5.0			—			5.0
Interest expense	—		—		3.3			—			3.3
Total expenses	—		—		255.9			—			255.9
Pre-tax income	—		—		36.5			—			36.5
Income tax expense	—		—		(4.2		)	—			(4.2		)
Net income from discontinued operations	—		—		32.3			—			32.3
Net gain (loss) from sale of discontinued operations	.1		.1		—			(1.0		)	(1.0		)
Total income (loss) from discontinued operations	.1		.1		32.3			(1.0		)	31.3
Change in foreign currency translation and other from discontinued operations	—		—		(.1		)	—			(.1		)
Comprehensive income (loss) from discontinued operations	$	.1	$	.1	$	32.2		$	(1.0	)	$	31.2

Net Change in Cash from Discontinued Operations

The following tables presents the net change in cash, including income tax payments to national governments and interest paid associated with the business classified as discontinued operations:



	Three Months Ended
	March 31,
Millions	2018			2017
Net cash provided from (used for) operations	$	.1		$	(1.3	)
Net cash (used for) provided from investing activities	(.1		)	32.3
Net cash used for financing activities	—			(21.0		)
Net change in cash during the period	—			10.0
Cash balances at beginning of period	—			70.5
Net change in cash held for sale, excluding discontinued operations	—			(.9		)
Cash balances at end of period	$	—		$	79.6
Supplemental cash flows information:
Interest paid	$	—		$	—
Net income tax payment to national governments	$	—		$	—

Earnings Per Share from Discontinued Operations

The following table presents the Company’s computation of earnings per share for discontinued operations for the three months ended March 31, 2018 and 2017:



	Three Months Ended
	March 31,
	2018			2017
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$	(48.0	)	$	29.0
Less total loss from continuing operations, net of tax	48.1			2.3
Net income from discontinued operations attributable to White Mountains’s common shareholders	$	.1		$	31.3
Allocation of earnings to participating restricted common shares ⁽¹⁾	—			(.4		)
Basic and diluted earnings per share numerators	$	.1		$	30.9
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,746.1			4,564.6
Average unvested restricted common shares ⁽³⁾	(35.9		)	(52.5		)
Basic earnings per share denominator	3,710.2			4,512.1
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,746.1			4,564.6
Average unvested restricted common shares ⁽³⁾	(35.9		)	(52.5		)
Diluted earnings per share denominator	3,710.2			4,512.1
Basic earnings per share (in dollars) - discontinued operations:	$	.03		$	6.86
Diluted earnings per share (in dollars) - discontinued operations:	$	.03		$	6.86

⁽¹⁾Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities.

⁽²⁾Net earnings attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the three months ended March 31, 2018 and 2017.

⁽³⁾ Restricted common shares outstanding vest either in equal annual installments or upon a stated date. See Note 10 — “Employee Share-Based Incentive Compensation Plans”.

- Definition

+ References

+ Details

Financial Statement Revisions (Tables)

3 Months Ended

Mar. 31, 2018

Accounting Changes and Error Corrections [Abstract]

Schedule of Error Corrections and Prior Period Adjustments

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (Unaudited)



	Three months ended March 31, 2017
Millions, except for per share amounts	As previously reported			Adjustments			As revised
Revenues:
Financial Guarantee (HG Global/BAM)
Financial Guarantee revenues	$	6.3		$	—		$	6.3
Marketing Technology (MediaAlpha)
Marketing Technology revenues	32.5			—			32.5
Other
Advertising & commission revenues	6.0			(4.7		)	1.3
Other revenues ⁽¹⁾	48.7			—			48.7
Total revenues	93.5			(4.7		)	88.8

Expenses:
Financial Guarantee (HG Global/BAM)
Financial Guarantee expenses	11.9			—			11.9
Marketing Technology (MediaAlpha)
Marketing Technology expenses	33.5			—			33.5
Other
Other expenses ⁽²⁾⁽³⁾	46.8			.2			47.0
Total expenses	92.2			.2			92.4
Pre-tax income (loss)	1.3			(4.9		)	(3.6		)
Income tax benefit	.3			—			.3
Net income (loss) from continuing operations	1.6			(4.9		)	(3.3		)
Loss on sale of discontinued operations	(1.0		)	—			(1.0		)
Net income from discontinued operations	32.3			—			32.3
Net income (loss) ⁽⁴⁾	32.9			(4.9		)	28.0
Net loss (income) attributable to non-controlling interests	1.3			(.3		)	1.0
Net income (loss) attributable to White Mountains’s common shareholders	34.2			(5.2		)	29.0
Other comprehensive income (loss), net of tax	.9			(.7		)	.2
Comprehensive income (loss)	35.1			(5.9		)	29.2
Comprehensive loss attributable to non-controlling interests	—			—			—
Comprehensive income (loss) attributable to White Mountains’s common shareholders	$	35.1		$	(5.9	)	$	29.2

Basic and diluted earnings per share - continuing operations	$	7.49		$	(1.15	)	$	6.34

⁽¹⁾Total other revenues include earned insurance premiums, net investment income, net realized and unrealized losses, and other revenues.

⁽²⁾Total other expenses include loss and loss adjustment expenses, insurance acquisition expenses, cost of sales, general and administrative expenses, and interest expense.

⁽³⁾The adjustments to other expenses is primarily related to the write-off of goodwill and intangible assets for Wobi.

CONSOLIDATED STATEMENTS OF SHAREHOLDERS’ EQUITY



	Three months ended March 31, 2017
Millions	As previously reported			Adjustments			As revised

Common shares and paid-in surplus	$	807.1		$	.7		$	807.8

Retained earnings, beginning of year	2,797.2			(20.6		)	2,776.6
Share repurchases	(5.1		)	—			(5.1		)
Net income (loss)	34.2			(5.2		)	29.0
Dividends	(4.6		)	—			(4.6		)
Retained earnings, end of period	2,821.7			(25.8		)	2,795.9

Accumulated other comprehensive loss, after tax, beginning of year	(4.6		)	—			(4.6		)
Net change in foreign currency translation	.9			(.7		)	.2
Accumulated other comprehensive income, after tax, end of period	(3.7		)	(.7		)	(4.4		)

White Mountains’s common shareholders’ equity	3,625.1			(25.8		)	3,599.3

Non-controlling interests, beginning of year	133.9			(.6		)	133.3
Net loss	(1.3		)	.3			(1.0		)
Other changes in non-controlling interests	(.2		)	(.6		)	(.8		)
Non-controlling interests, end of period	132.4			(.9		)	131.5
Total equity	$	3,757.5		$	(26.7	)	$	3,730.8

CONSOLIDATED BALANCE SHEETS (Unaudited)



	March 31, 2017
Millions	As previously reported			Adjustments			As revised
Assets
Financial Guarantee (HG Global/BAM)
Financial Guarantee assets	$	699.1		$	—		$	699.1
Marketing Technology (MediaAlpha)
Marketing Technology assets	53.2			—			53.2
Other
Goodwill	13.4			(5.8		)	7.6
Other intangible assets	4.5			(3.4		)	1.1
Other assets — commissions receivable	21.4			(17.2		)	4.2
Other assets	2,146.3			—			2,146.3
Assets held for sale	3,578.9			—			3,578.9
Total assets	$	6,516.8		$	(26.4	)	$	6,490.4
Liabilities
Financial Guarantee (HG Global/BAM)
Financial Guarantee liabilities	$	126.4		$	—		$	126.4
Marketing Technology (MediaAlpha)
Marketing Technology liabilities	25.6			—			25.6
Other
Liabilities held for sale	2,535.2			—			2,535.2
Other liabilities	72.1			.3			72.4
Total liabilities	2,759.3			.3			2,759.6

Equity
White Mountains’s common shares	4.6			—			4.6
Paid in surplus	802.5			.7			803.2
Retained earnings	2,821.7			(25.8		)	2,795.9
Accumulated other comprehensive income, net of tax	(3.7		)	(.7		)	(4.4		)
White Mountains’s common shareholders’ equity	3,625.1			(25.8		)	3,599.3
Non-controlling interests	132.4			(.9		)	131.5
Total equity	3,757.5			(26.7		)	3,730.8
Total liabilities and equity	$	6,516.8		$	(26.4	)	$	6,490.4

Basis of Presentation and Significant Accouting Policies (Basis of Presentation) (Details) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) publisher advertiser	Mar. 31, 2017 USD ($)	Dec. 31, 2017	Sep. 28, 2017 $ / shares	Jul. 31, 2012 USD ($)
HG Global
Basis of Presentation
Percentage of par value of policy reinsured	15.00%
Surplus notes	$ 499.0				$ 503.0
MediaAlpha
Basis of Presentation
Ownership interest (as a percent)	62.30%		64.40%
Number of advertisers \| advertiser	550
Number of publishers \| publisher	325
Preferred Stock \| HG Global
Basis of Presentation
Ownership interest (as a percent)	96.90%		96.90%
Common Stock \| HG Global
Basis of Presentation
Ownership interest (as a percent)	88.40%		88.40%
Restricted Stock
Basis of Presentation
Payments related to tax withholding for share-based compensation	$ 8.4	$ 6.5
OneBeacon
Basis of Presentation
Business acquisition, share price \| $ / shares				$ 18.10

Significant Transactions (Details) - USD ($) $ in Millions						3 Months Ended			12 Months Ended
Significant Transactions (Details) - USD ($) $ in Millions	Jan. 24, 2018	Oct. 05, 2017	Sep. 28, 2017	Oct. 05, 2016	Jul. 21, 2016	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Feb. 05, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain from sale of other discontinued operations, net of tax						$ 0.1	$ 0.1	$ (1.0)
Healthplans.com
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to acquire businesses and interest in affiliates		$ 28.0
MediaAlpha
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)						62.30%	64.40%
OneBeacon
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold			$ 1,300.0
Gain from sale of other discontinued operations, net of tax			$ 554.6
Tranzact
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold				$ 1.2	$ 221.3
Gain from sale of other discontinued operations, net of tax									$ 51.9
DavidShield & PassportCard
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity method investment, aggregate cost	$ 42.0
Payments to acquire business, net	$ 28.0
Ownership interest (as a percent)	50.00%
Kudu Investment Management, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity method investment, aggregate cost						$ 1.8				$ 127.5
Ownership interest (as a percent)						50.00%	0.00%
Capital Unit, Class A \| MediaAlpha
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Capital units, purchased		131,579
Capital units, value		$ 12.5

Investment Securities (Net Investment Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Investment income
Total investment income	$ 20.4	$ 13.3
Third-party investment expenses	(0.7)	(0.5)
Net investment income, pre-tax	19.7	12.8
Fixed maturity investments
Investment income
Total investment income	10.9	11.9
Short-term investments
Investment income
Total investment income	2.6	0.2
Common Stock
Investment income
Total investment income	5.0	1.3
Other long-term investments
Investment income
Total investment income	$ 1.9	$ (0.1)

Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Net realized investment (losses) gains, pre-tax	$ (5.1)	$ 0.6
Net unrealized investment (losses) gains, pre-tax	(48.6)	35.7
Net realized and unrealized investment (losses) gains, pre-tax	(53.7)	36.3
Income tax benefit (expense) attributable to net realized and unrealized investment (losses) gains	5.7	(3.9)
Net realized and unrealized investment (losses) gains, after-tax	$ (48.0)	$ 32.4

Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	$ (16.1)	$ 0.4
Trading securities, Tax on Realized Holding Gain (Loss) on Investments	(0.6)	(0.2)
Trading securities realized holding gain (loss) on investments after tax	(16.7)	0.2
Foreign Currency Transaction Gain (Loss), Realized	11.0	0.2
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0
Trading securities, realized foreign currency gain (loss) net of tax	11.0	0.2
Net realized investment (losses) gains, pre-tax	(5.1)	0.6
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(0.6)	(0.2)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	(5.7)	0.4
Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	(13.7)	(1.0)
Foreign Currency Transaction Gain (Loss), Realized	18.2	0.1
Net realized investment (losses) gains, pre-tax	4.5	(0.9)
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	(0.1)	0.0
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0
Net realized investment (losses) gains, pre-tax	(0.1)	0.0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	1.2	0.8
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.1
Net realized investment (losses) gains, pre-tax	1.2	0.9
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	(3.5)	0.6
Foreign Currency Transaction Gain (Loss), Realized	(7.2)	0.0
Net realized investment (losses) gains, pre-tax	$ (10.7)	$ 0.6

Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ (37.9)	$ 36.2
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	6.3	(3.7)
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	(31.6)	32.5
Foreign Currency Transaction Gain (Loss), Unrealized	(10.7)	(0.5)
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	(10.7)	(0.5)
Net unrealized investment (losses) gains, pre-tax	(48.6)	35.7
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	6.3	(3.7)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	(42.3)	32.0
Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(18.6)	10.2
Foreign Currency Transaction Gain (Loss), Unrealized	(14.8)	1.7
Net unrealized investment (losses) gains, pre-tax	(33.4)	11.9
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(0.7)	0.0
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	0.0
Net unrealized investment (losses) gains, pre-tax	(0.7)	0.0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(16.7)	19.1
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	0.5
Net unrealized investment (losses) gains, pre-tax	(16.7)	19.6
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(1.9)	6.9
Foreign Currency Transaction Gain (Loss), Unrealized	4.1	(2.7)
Net unrealized investment (losses) gains, pre-tax	$ 2.2	$ 4.2

Investment Securities (Investment gains (losses) for Level 3) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ (37.9)	$ 36.2
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(1.9)	6.9
Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(5.1)	0.2
Level 3 Inputs \| Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ (5.1)	$ 0.2

Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
U.S. Government and agency obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	$ 199.9	$ 297.8
Trading Securities, Unrealized Holding Gain	0.0	0.0
Trading Securities, Unrealized Holding Loss	(2.2)	(1.3)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	197.7	296.5
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	760.5	867.6
Trading Securities, Unrealized Holding Gain	0.7	2.9
Trading Securities, Unrealized Holding Loss	(17.5)	(4.3)
Net foreign currency gains (losses)	0.0	14.7
Fixed maturity investments, at fair value	743.7	880.9
Municipal obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	261.8	252.0
Trading Securities, Unrealized Holding Gain	1.9	3.7
Trading Securities, Unrealized Holding Loss	(1.3)	(0.8)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	262.4	254.9
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	132.5	697.2
Trading Securities, Unrealized Holding Gain	0.1	1.6
Trading Securities, Unrealized Holding Loss	(2.6)	(4.1)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	130.0	694.7
Foreign government, agency and provincial obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost		2.6
Trading Securities, Unrealized Holding Gain		0.0
Trading Securities, Unrealized Holding Loss		0.0
Net foreign currency gains (losses)		0.1
Fixed maturity investments, at fair value		2.7
Fixed Income Investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	1,354.7	2,117.2
Trading Securities, Unrealized Holding Gain	2.7	8.2
Trading Securities, Unrealized Holding Loss	(23.6)	(10.5)
Net foreign currency gains (losses)	0.0	14.8
Total fixed maturity investments including assets held for sale	$ 1,333.8	$ 2,129.7

Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	$ 835.9	$ 739.7
Trading Securities, Unrealized Holding Gain	111.7	129.4
Trading Securities, Unrealized Holding Loss	(2.0)	(3.0)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	945.6	866.1
Other long-term investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	288.8	246.6
Trading Securities, Unrealized Holding Gain	10.0	6.8
Trading Securities, Unrealized Holding Loss	(44.8)	(39.7)
Net foreign currency gains (losses)	(0.9)	(4.9)
Fixed maturity investments, at fair value	$ 253.1	$ 208.8

Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Hedge and private equity funds included in other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	$ 130.6	$ 125.3
Equity method eligible private equity securities, at fair value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	108.4	83.2
Forward Contracts
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	0.0	(3.7)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	14.1	4.0
White Mountains
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	253.1	208.8
White Mountains \| Equity method eligible private equity securities, at fair value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	$ 84.7	$ 58.0
OneTitle
Schedule of Trading Securities and Other Trading Assets [Line Items]
Ownership interest (as a percent)	20.00%
Surplus notes	$ 10.0

Investment Securities (Hedge Funds and Private Equity Funds) (Details) $ in Millions	Mar. 31, 2018 USD ($) fund	Dec. 31, 2017 USD ($)
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of investments	1
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of investments	10
Hedge and private equity funds included in other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Largest single fund investment \| $	$ 56.4	$ 54.9

Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 74.2
Hedge and private equity funds included in other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	130.6	$ 125.3
Unfunded Commitments	148.1	99.3
Hedge funds \| Hedge Fund, Long/ Short Equity Banks and Financial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	56.4	54.9
Unfunded Commitments	0.0	0.0
Hedge funds \| Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	56.4	54.9
Unfunded Commitments	0.0	0.0
Private equity funds \| Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	44.5	43.3
Unfunded Commitments	10.4	10.4
Private equity funds \| Aerospace Defense Government
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	17.0	15.8
Unfunded Commitments	12.9	12.9
Private equity funds \| Direct Lending
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	7.9	7.1
Unfunded Commitments	22.5	23.1
Private equity funds \| Financial Services Sector
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	4.8	4.2
Unfunded Commitments	11.1	11.7
Private equity funds \| Private Equity Insurance
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0	0.0
Unfunded Commitments	41.2	41.2
Private equity funds \| Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	74.2	70.4
Unfunded Commitments	148.1	99.3
Private equity funds \| Real estate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0	0.0
Unfunded Commitments	$ 50.0	$ 0.0

Investment Securities (Restrictions on Redemption Frequency and Advance Notice Requirements) (Details) $ in Millions	Mar. 31, 2018 USD ($) fund	Dec. 31, 2017 USD ($) fund
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of investments \| fund	1
Hedge Funds Equity R E I T
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments		$ 20.8
September 2018 \| Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of hedge funds	1
Other long-term investments	$ 56.4
Hedge funds \| Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	56.4	54.9
Hedge funds \| Hedge Fund, Long/ Short Equity Banks and Financial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 56.4	$ 54.9
Number of investments \| fund		1

Investment Securities (Investments In Private Equity Funds Subject to Lock-Up Periods) (Details) - Private equity funds $ in Millions	Mar. 31, 2018 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 74.2
1-3 years
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	6.1
3-5 years
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	6.2
5-10 years
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	49.2
Greater than 10 years
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 12.7

Investment Securities (Fair Value Measurements) (Details) (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Percentage of investments recorded at fair value	92.00%	94.00%

Investment Securities (Debt securities issued by corporation) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 2,532.5	$ 3,208.3	$ 2,610.8	$ 2,547.3
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	743.7	880.9
Fair value investments	743.7	880.9
AAA \| Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	8.9	1.6
AA \| Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	79.1	42.6
A \| Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	274.6	192.5
BBB \| Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	211.5	465.2
BB \| Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	151.0	161.7
B \| Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 18.6	$ 17.3

Investment Securities (Mortgage-backed, Asset-backed Securities) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Fair value investments	$ 2,532.5	$ 3,208.3	$ 2,610.8	$ 2,547.3

- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References + Details

Investment Securities (Mortgage-backed Asset Securities Table) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 2,532.5	$ 3,208.3	$ 2,610.8	$ 2,547.3
Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	27.3	46.3
Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	53.9	84.5
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	36.1	62.0
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	117.3	192.8
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	70.5
Private debt instrument
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	70.5
Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	117.3	263.3
Collateralized Credit Card Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	8.9	206.0
Collateralized Auto Loans
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	3.8	142.4
Collateralized Securities, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	83.0
Asset-backed Securities, Securitized Loans and Receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	12.7	431.4
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	130.0	694.7
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,405.1	2,105.4	$ 2,068.0	$ 2,093.8
Level 2 Inputs \| Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	27.3	46.3
Level 2 Inputs \| Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	53.9	84.5
Level 2 Inputs \| Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	36.1	62.0
Level 2 Inputs \| Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	117.3	192.8
Level 2 Inputs \| Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	70.5
Level 2 Inputs \| Private debt instrument
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	70.5
Level 2 Inputs \| Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	117.3	263.3
Level 2 Inputs \| Collateralized Credit Card Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	8.9	206.0
Level 2 Inputs \| Collateralized Auto Loans
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	3.8	142.4
Level 2 Inputs \| Collateralized Securities, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	83.0
Level 2 Inputs \| Asset-backed Securities, Securitized Loans and Receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	12.7	431.4
Level 2 Inputs \| Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	130.0	694.7
Level 3 Inputs \| Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs \| Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs \| Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs \| Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs \| Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs \| Private debt instrument
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs \| Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs \| Collateralized Credit Card Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs \| Collateralized Auto Loans
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs \| Collateralized Securities, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs \| Asset-backed Securities, Securitized Loans and Receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs \| Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 0.0	$ 0.0

Investment Securities (Rollforward of Fair Value Measurements by Level) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	$ 3,208,300,000	$ 2,547,300,000
Net realized and unrealized (losses) gains	(45,400,000)	39,100,000
Amortization/Accretion	(1,200,000)	(2,500,000)
Purchases	692,600,000	498,900,000
Sales	(1,321,800,000)	(466,800,000)
Transfers in	0	0
Transfers out	0	0
Ending balance	2,532,500,000	2,610,800,000
Net realized and unrealized investment (losses) gains	(53,700,000)	36,300,000
Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	890,400,000	279,500,000
Net realized and unrealized (losses) gains	(14,300,000)	13,100,000
Amortization/Accretion	0	0
Purchases	238,900,000	115,200,000
Sales	(240,700,000)	(76,000,000)
Transfers in	0	0
Transfers out	0	0
Ending balance	874,300,000	331,800,000
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	2,105,400,000	2,093,800,000
Net realized and unrealized (losses) gains	(26,000,000)	18,400,000
Amortization/Accretion	(1,200,000)	(2,500,000)
Purchases	407,700,000	351,100,000
Sales	(1,080,800,000)	(387,600,000)
Transfers in	0	0
Transfers out	0	0
Ending balance	1,405,100,000	2,068,000,000
Fixed maturity investments \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	0	0
Net realized and unrealized (losses) gains	0	100,000
Amortization/Accretion	0	0
Purchases	0	11,000,000
Sales	0
Transfers in	0	0
Transfers out	0	0
Ending balance	0	11,100,000
Other long-term investments \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	87,200,000	91,400,000
Net realized and unrealized (losses) gains	(8,700,000)	200,000
Amortization/Accretion	0	0
Purchases	44,000,000	200,000
Sales	0	(2,000,000)
Transfers in	0	0
Transfers out	0	0
Ending balance	122,500,000	89,800,000
Hedge and private equity funds included in other long-term investments \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	125,300,000	82,600,000
Net realized and unrealized (losses) gains	3,600,000	7,300,000
Amortization/Accretion	0	0
Purchases	2,000,000	21,400,000
Sales	(300,000)	(1,200,000)
Transfers in	0	0
Transfers out	0	0
Ending balance	130,600,000	110,100,000
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment (losses) gains	600,000.0
Foreign Exchange Forward
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment (losses) gains	3,500,000.0	2,800,000.0
Foreign Currency on Cash and Open Trades
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment (losses) gains	4,200,000.0
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	3,259,100,000
Ending balance	3,164,900,000
Fair value measured on a recurring basis \| Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	1,041,400,000
Ending balance	1,611,600,000
Fair value measured on a recurring basis \| Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	2,130,500,000
Ending balance	1,430,800,000
Fair value measured on a recurring basis \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	87,200,000
Ending balance	122,500,000
Fair value measured on a recurring basis \| Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	2,129,700,000
Ending balance	1,333,800,000
Fair value measured on a recurring basis \| Fixed maturity investments \| Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	296,500,000
Ending balance	197,700,000
Fair value measured on a recurring basis \| Fixed maturity investments \| Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	1,833,200,000
Ending balance	1,136,100,000
Fair value measured on a recurring basis \| Fixed maturity investments \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	0
Ending balance	0
Fair value measured on a recurring basis \| Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	87,200,000
Ending balance	122,500,000
Fair value measured on a recurring basis \| Other long-term investments \| Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	0
Ending balance	0
Fair value measured on a recurring basis \| Other long-term investments \| Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	0
Ending balance	0
Fair value measured on a recurring basis \| Other long-term investments \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	87,200,000
Ending balance	122,500,000
Fair value measured on a recurring basis \| Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	176,100,000
Short-term investments, at amortized cost (which approximates fair value)			$ 763,000,000
Fair value measured on a recurring basis \| Short-term investments \| Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	151,000,000
Ending balance	737,300,000
Short-term investments, at amortized cost (which approximates fair value)		138,200,000	176,100,000	$ 175,000,000
Fair value measured on a recurring basis \| Short-term investments \| Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	25,100,000
Ending balance	25,700,000
Fair value measured on a recurring basis \| Short-term investments \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	0
Ending balance	$ 0
Forward Contracts \| Fair value measured on a recurring basis \| Other long-term investments \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative, fair value, net		(4,100,000)	$ (3,700,000)	(1,200,000)
Star & Shield Insurance Exchange
Schedule of Trading Securities and Other Trading Assets [Line Items]
Deconsolidation of SSIE		5,200,000
Star & Shield Insurance Exchange \| Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Deconsolidation of SSIE		0
Star & Shield Insurance Exchange \| Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Deconsolidation of SSIE		5,200,000
Star & Shield Insurance Exchange \| Fixed maturity investments \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Deconsolidation of SSIE		0
Star & Shield Insurance Exchange \| Other long-term investments \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Deconsolidation of SSIE		0
Star & Shield Insurance Exchange \| Hedge and private equity funds included in other long-term investments \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Deconsolidation of SSIE		$ 0
Star & Shield Insurance Exchange
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities debt reclassified to assets held-for-sale				6,600,000
Star & Shield Insurance Exchange \| Fair value measured on a recurring basis \| Short-term investments \| Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Short-term investments, at amortized cost (which approximates fair value)				$ 100,000

Investment Securities (Significant Unobservable Inputs) (Details) - NR - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Private equity funds
Quantitative information for Level 3 Fair Value Measurements Assets
Time until expiration of security	4 years	4 years
Volatility rate	50.00%	50.00%
Risk free rate	1.77%	1.77%
Private equity funds \| Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 22.1
Valuation techniques	.68
Private equity funds \| Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure		$ 21.0
Valuation techniques		1
Private equity funds \| Option pricing method
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 11.6	$ 11.3
Valuation techniques	0.21	0.21
Private debt instrument \| Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 10.5
Valuation techniques	0.0962
Convertible Preferred Stock \| Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 14.5	$ 14.5
Valuation techniques	2.06	2.06
Convertible Preferred Stock \| Multiple of EBITDA
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure		$ 0.6
Valuation techniques		6
Investment B \| Private equity funds \| Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 21.0
Valuation techniques	0.25
Investment E \| Private equity funds \| Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure		$ 22.1
Valuation techniques		.68
Investment A \| Private equity funds \| Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure		$ 3.6
Valuation techniques		2.521

Goodwill and Other Intangible Assets - Goodwill Rollforward (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 25.9	$ 25.9
Intangible assets, net (including Goodwill), beginning balance	62.1	45.2
Amortization, including foreign currency translation	(2.9)	(2.3)
Goodwill, ending balance	25.9	25.9
Intangible assets, net (including Goodwill), ending balance	59.2	42.9
Other Intangible Assets
Goodwill [Roll Forward]
Other intangible assets, beginning balance	36.2	19.3
Amortization, including foreign currency translation	(2.9)	(2.3)
Other intangible assets, ending balance	$ 33.3	$ 17.0

Goodwill and Other Intangible Assets - Goodwill and Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 25.9	$ 25.9	$ 25.9	$ 25.9
Goodwill and other intangible assets	59.2	62.1	42.9	45.2
MediaAlpha
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	18.3	18.3
Other
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	7.6	7.6
Other Intangible Assets
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Net (Excluding Goodwill)	33.3	36.2	$ 17.0	$ 19.3
Other Intangible Assets \| MediaAlpha
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Net (Excluding Goodwill)	32.5	35.4
Other Intangible Assets \| Other
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Net (Excluding Goodwill)	0.8	0.8
Non-controlling interest
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets	(21.1)	(21.1)
Total
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets	$ 38.1	$ 41.0

Debt (Details) - USD ($) $ in Millions	3 Months Ended
Debt (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Oct. 05, 2017	May 12, 2017	Aug. 14, 2013
Debt Instrument
Total debt	$ 21.5		$ 23.8
Repayments of debt	2.3	$ 1.2
WTM Bank Facility
Debt Instrument
Total commitment under revolving credit facility						$ 425.0
White Mountains \| WTM Bank Facility
Debt Instrument
Total debt	0.0		0.0
Unamortized issuance cost	0.0		0.0
MediaAlpha
Debt Instrument
Total debt	21.5		23.8
MediaAlpha \| MediaAlpha Bank Facility
Debt Instrument
Debt Instrument, Face Amount	21.6		23.9
Unamortized issuance cost	$ (0.1)		$ (0.1)
Debt instrument, interest rate, effective percentage	6.20%		5.60%
Total commitment under revolving credit facility				$ 28.4	$ 20.0
Long-term line of credit	$ 21.5		$ 23.8
Repayment of lines of credit	$ 1.0
Term Loan \| MediaAlpha Bank Facility
Debt Instrument
Basis points	1.50%
Term Loan \| MediaAlpha \| MediaAlpha Bank Facility
Debt Instrument
Total commitment under revolving credit facility	$ 18.4
Long-term line of credit	16.6
Repayments of debt	1.3
Revolving Credit Facility \| MediaAlpha \| MediaAlpha Bank Facility
Debt Instrument
Total commitment under revolving credit facility	10.0
Long-term line of credit	$ 5.0
Line of Credit \| MediaAlpha Bank Facility
Debt Instrument
Basis points	0.25%

Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended
Income Taxes (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Income Tax Contingency [Line Items]
Effective income tax rate reconciliation, percent	(1.10%)	(8.30%)
U.S. statutory rate (as a percent)	21.00%	35.00%
Income tax expense (benefit)	$ 0.7	$ (0.3)
Continuing Operations \| BAM
Income Tax Contingency [Line Items]
Income tax expense (benefit)	$ (0.6)	$ (2.7)

Derivatives Forward Contracts (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Fair value investments	$ 2,532.5	$ 2,610.8	$ 3,208.3	$ 2,547.3
Forward Contracts
Derivative [Line Items]
Notional amount			206.3
Fair value investments			$ (3.7)
Realized and unrealized gains (losses)	$ (3.5)	$ 3.0

- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References + Details

Municipal Bond Guarantee (Details) $ in Millions	3 Months Ended		12 Months Ended
Municipal Bond Guarantee (Details) $ in Millions	Mar. 31, 2018 USD ($) Contract yr	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) Contract	Dec. 31, 2012 USD ($)	Jul. 31, 2012 USD ($)
Guarantor Obligations [Line Items]
Repayments of debt	$ 2.3	$ 1.2
Interest paid	0.3	$ 0.2
Total gross unearned insurance premiums	140.2		$ 136.8
HG Global
Guarantor Obligations [Line Items]
Partners capital contribution				$ 594.5
Surplus notes	$ 499.0				$ 503.0
Assets held-in-trust			$ 603.0
HG Global \| Preferred Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)	96.90%		96.90%
HG Global \| Common Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)	88.40%		88.40%
BAM
Guarantor Obligations [Line Items]
Percentage of Risk Premium Ceded	60.00%
Year of U.S. Treasury rate \| yr	1
Contracts outstanding \| Contract	6,541		6,371
Remaining weighted average contract period outstanding (in years)	10 years 10 months		10 years 11 months
Municipal bond outstanding principal	$ 43,139.2		$ 42,090.6
Municipal bond outstanding interest	21,420.6		21,057.1
Municipal bond debt service outstanding	64,559.8		63,147.7
April 1, 2018 - December 31, 2018	8.9
2019	11.6
2020	11.1
2021	10.6
2022	10.1
2023 and thereafter	87.9
Total gross unearned insurance premiums	140.2
HG Global
Guarantor Obligations [Line Items]
Collateral held in supplement trust	721.9		715.1
Interest receivable	$ 131.7		126.0
HG Re
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured	15.00%
BAM
Guarantor Obligations [Line Items]
Surplus notes	$ 499.0		$ 499.0
BAM Management
Guarantor Obligations [Line Items]
Partners capital contribution				$ 14.5
Surplus Note \| BAM
Guarantor Obligations [Line Items]
Derivative, fixed interest rate (as a percent)	4.60%		3.54%
Surplus Note \| BAM
Guarantor Obligations [Line Items]
Repayments of debt			$ 4.0
Interest paid			1.0
Combination One \| Surplus Note \| BAM
Guarantor Obligations [Line Items]
Number of years of change in interest rate \| yr	5
Combination Two \| Surplus Note \| BAM
Guarantor Obligations [Line Items]
Number of years of change in interest rate \| yr	3
US Treasury Rate \| Surplus Note \| BAM
Guarantor Obligations [Line Items]
Basis points	3.00%
Fixed interest rate \| Surplus Note \| BAM
Guarantor Obligations [Line Items]
Derivative, fixed interest rate (as a percent)	8.00%
First Quarter, year two \| BAM
Guarantor Obligations [Line Items]
2019	$ 2.9
Second Quarter, year two \| BAM
Guarantor Obligations [Line Items]
2019	2.9
Third Quarter, year two \| BAM
Guarantor Obligations [Line Items]
2019	2.9
Fourth Quarter, year two \| BAM
Guarantor Obligations [Line Items]
2019	$ 2.9
Series A BAM Surplus Note \| HG Global
Guarantor Obligations [Line Items]
Surplus notes			203.0
Series B BAM Surplus Note \| HG Global
Guarantor Obligations [Line Items]
Surplus notes			$ 300.0

Municipal Bond Guarantee - Schedule of Net Written Premiums (Details) - HG/BAM - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Gross written premiums	$ 6.4	$ 18.7
Assumed (ceded) written premiums	0.0	0.0
Net written premiums	$ 6.4	$ 18.7

Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Basic and diluted earnings per share numerators (in millions):
Net (loss) income attributable to White Mountains’s common shareholders	$ (48.0)	$ 29.0
Total income (loss) from discontinued operations	0.1	31.3
Net loss from continuing operations attributable to White Mountains’s common shareholders	(48.1)	(2.3)
Allocation of earnings to participating restricted common shares	0.4	0.0
Basic and diluted earnings per share numerators	$ (47.7)	$ (2.3)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,746,100	4,564,600
Average unvested restricted common shares	(35,900)	(52,500)
Basic earnings per share denominator	3,710,200	4,512,100
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,746,100	4,564,600
Average unvested restricted common shares	(35,900)	(52,500)
Diluted earnings per share denominator	3,710,200	4,512,100
Basic earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid	$ (1.00)	$ (1.00)
Undistributed earnings (losses)	(13.85)	(1.52)
Basic and diluted earnings per share	$ (12.85)	$ (0.52)

Earnings Per Share - Undistributed Net Earnings (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Undistributed net losses - continuing operations:	$ (47.7)	$ (2.3)
Dividends declared net of restricted common share amounts	(3.7)	(4.5)
Total undistributed net losses, net of restricted common share amounts	$ (51.4)	$ (6.8)

Employee Share-Based Incentive Compensation Plans (Performance Shares) (Details) - Performance Share - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Range Of Performance Cycle From 2014-2016
Accrued Expense
Percentage of target payout - minimum actual			34.00%
Percentage of target payout - maximum actual			76.00%
Performance Cycle 2015 to 2017
Accrued Expense
Percentage of target payout - minimum actual	145.00%
Percentage of target payout - maximum actual	147.00%
White Mountains
Share-based compensation arrangement by share-based payment award
Vesting Period	3 years
Target Performance Share Activity
Beginning of period (in shares)	50,515	80,353	80,353
Shares paid or expired (in shares)	(23,186)	(30,167)
New grants (in shares)	14,105	16,460
Assumed forfeitures and cancellations	(818)	(9,841)
Ending of period (in shares)	40,616	56,805	50,515
Accrued Expense
Accrued expense at the beginning of period	$ 45.8	$ 42.4	$ 42.4
Payments and deferrals	(28.4)	(20.8)
Assumed forfeitures and cancellations	0.3	(5.7)
Expense recognized	3.3	8.1
Accrued expense at the end of the period	21.0	$ 24.0	$ 45.8
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 26.1
Sirius Group
Target Performance Share Activity
Ending of period (in shares)		2,195

Employee Share-Based Incentive Compensation Plans (Performance Shares Granted Under the WTM Incentive Plan) (Details) - White Mountains - Performance Share - W T M Incentive Plan - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	41,235
Accrued Expense	$ 21.3
Target Performance Shares Outstanding, Assumed forfeitures	(619)
Accrued Expense, Assumed forfeitures	(0.3)
Total Target Performance Shares Outstanding	40,616	50,515	56,805	80,353
Total Accrued Expense	$ 21.0	$ 45.8	$ 24.0	$ 42.4
Performance Cycle 2016 to 2018
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	13,715
Accrued Expense	$ 12.7
Performance Cycle 2017 to 2019
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	14,070
Accrued Expense	$ 7.7
Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	13,450
Accrued Expense	$ 0.9

Employee Share-Based Incentive Compensation Plans (Restricted Shares) (Details) - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
White Mountains \| Restricted Share Cliff Vesting in January 2021
Target Restricted Share Activity
Issued (in shares)	13,450
White Mountains \| Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	53,755	70,620
Issued (in shares)	14,105	16,735
Vested (in shares)	(25,381)	(22,015)
Assumed forfeitures and cancellations	(969)	(5,200)
Ending of period (in shares)	41,510	60,140
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 14.3	$ 19.7
Issued	11.4	15.8
Assumed forfeitures and cancellations	(0.2)	(2.8)
Expense recognized	(3.3)	(3.6)
Unamortized grant date fair value, ending balance	$ 22.2	$ 29.1
White Mountains \| Restricted Share Cliff Vesting in January 2019
Target Restricted Share Activity
Issued (in shares)	365
White Mountains \| Restricted Share Cliff Vesting in January 2020
Target Restricted Share Activity
Issued (in shares)	290	16,735
Sirius Group \| Restricted Stock
Target Restricted Share Activity
Ending of period (in shares)		2,195

Employee Share-Based Incentive Compensation Plans (Non-Qualified Options & MediaAlpha Class B Share Awards) (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended
	Jan. 20, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2016
Share-based compensation arrangement by share-based payment award
Shares issued, price per share				$ 742
Common Stock
Share-based compensation arrangement by share-based payment award
Conversion of stock, shares converted			5,142
Chief Executive Officer \| Employee Stock Option
Share-based compensation arrangement by share-based payment award
Non-option equity instruments, exercised	125,000		40,000
Options, exercises in period, intrinsic value			$ 4.4
MediaAlpha \| Class B
Share-based compensation arrangement by share-based payment award
Expense recognized		$ 6.4
Compensation cost not yet recognized		$ 0.6
Minimum \| MediaAlpha \| Class B
Share-based compensation arrangement by share-based payment award
Service period		36 months
Maximum \| MediaAlpha \| Class B
Share-based compensation arrangement by share-based payment award
Service period		48 months

Non-controlling Interest (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ (143.2)	$ (131.7)	$ 131.5
Total Non-Controlling Excluding Reciprocals
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ 34.3	$ 31.5
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	3.10%	3.10%
Total non-controlling interests	$ 15.7	$ 15.9
MediaAlpha
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	37.70%	35.70%
Total non-controlling interests	$ 16.2	$ 13.1
Buzzmove
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	22.90%	22.90%
Total non-controlling interests	$ 2.4	$ 2.5
Build America Mutual
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	$ (177.5)	$ (163.2)

Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
Segment Information (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Segment reporting information
Earned insurance premiums	$ 3.0	$ 3.0
Net investment income	19.7	12.8
Net realized and unrealized investment gains	(53.7)	36.3
Advertising and commission revenues	71.0	33.8
Other revenue	2.1	2.9
Total revenues	42.1	88.8
Loss and loss adjustment expenses		1.1
Insurance acquisition expenses	1.4	1.3
Other underwriting expenses	0.1	0.1
Cost of sales	58.1	28.8
General and administrative expenses	47.9	60.7
Interest expense	0.6	0.4
Total expenses	108.1	92.4
Pre-tax income (loss)	(66.0)	(3.6)
HG Global-BAM
Segment reporting information
Advertising and commission revenues	0.0	0.0
Loss and loss adjustment expenses		0.0
Cost of sales	0.0	0.0
Interest expense	0.0	0.0
Pre-tax income (loss)	(14.3)	(5.6)
MediaAlpha
Segment reporting information
Earned insurance premiums	0.0	0.0
Net investment income	0.0	0.0
Net realized and unrealized investment gains	0.0	0.0
Advertising and commission revenues	70.1	32.5
Other revenue	1.6	0.0
Total revenues	71.7	32.5
Insurance acquisition expenses	0.0	0.0
Other underwriting expenses	0.0	0.0
Cost of sales	57.4	27.7
General and administrative expenses	14.1	5.6
Interest expense	0.4	0.2
Total expenses	71.9	33.5
Pre-tax income (loss)	$ (0.2)	$ (1.0)
Percentage of advertising revenue	31.00%	27.00%
Other Operations
Segment reporting information
Earned insurance premiums	$ 0.0	$ 1.0
Net investment income	16.0	10.2
Net realized and unrealized investment gains	(45.8)	35.0
Advertising and commission revenues	0.9	1.3
Other revenue	0.3	2.5
Total revenues	(28.6)	50.0
Loss and loss adjustment expenses		1.1
Insurance acquisition expenses	0.0	0.1
Other underwriting expenses	0.0	0.0
Cost of sales	0.7	1.1
General and administrative expenses	22.0	44.5
Interest expense	0.2	0.2
Total expenses	22.9	47.0
Pre-tax income (loss)	$ (51.5)	$ 3.0

Investments in Unconsolidated Entities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
White Mountains
Investments in unconsolidated affiliates
Total other long-term investments	$ 253.1	$ 208.8
Equity method eligible private equity securities, at fair value
Investments in unconsolidated affiliates
Total other long-term investments	108.4	83.2
Equity method eligible private equity securities, at fair value \| White Mountains
Investments in unconsolidated affiliates
Total other long-term investments	84.7	58.0
Investments, accounted for under the equity method \| White Mountains
Investments in unconsolidated affiliates
Total other long-term investments	5.7	4.6
Total investments in equity method eligible unconsolidated entities \| White Mountains
Investments in unconsolidated affiliates
Total other long-term investments	90.4	62.6
Other unconsolidated investments \| White Mountains
Investments in unconsolidated affiliates
Total other long-term investments	$ 162.7	$ 146.2

Investments in Unconsolidated Entities - White Mountain's Investments (Details)	Mar. 31, 2018	Dec. 31, 2017
Compare.com
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	22.00%	22.00%
DavidShield
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%	0.00%
durchblicker
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	45.00%	45.00%
Kudu
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%	0.00%
OneTitle
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	20.00%	20.00%
PassportCard
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%	50.00%
Tuckerman Capital Fund III
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	21.00%	21.00%
White Mountains \| PassportCard
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	25.00%
DavidShield \| PassportCard
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	25.00%

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	$ 21.5	$ 23.8
MediaAlpha
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	21.5	23.8
MediaAlpha \| Book Value \| MediaAlpha Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	21.6	23.8
MediaAlpha \| Estimate of Fair Value Measurement \| MediaAlpha Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt instrument, fair value disclosure	$ 21.8	$ 23.9

Contingencies (Details) $ in Millions	Mar. 31, 2018 USD ($)
Swedish Tax Authority \| Sirius Group
Loss Contingencies [Line Items]
Loss contingency, estimate of possible loss	$ 18.5

Held for Sale and Discontinued Operations - (Details) - USD ($) $ / shares in Units, $ in Millions					3 Months Ended			12 Months Ended
	Sep. 28, 2017	Oct. 05, 2016	Jul. 21, 2016	Apr. 18, 2016	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Discontinued Operations [Line Items}
Gain from sale of other discontinued operations, net of tax					$ 0.1	$ 0.1	$ (1.0)
Net (loss) income from discontinued operations					0.0		32.3
OneBeacon
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	$ 1,300.0
Gain from sale of other discontinued operations, net of tax	$ 554.6
Net (loss) income from discontinued operations					32.3
Tranzact
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold		$ 1.2	$ 221.3
Gain from sale of other discontinued operations, net of tax								$ 51.9
Net (loss) income from discontinued operations							0.0
Discontinued operation, tax effect of discontinued operation							0.0	30.2
Sirius Group
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold				$ 2,600.0
Gain from sale of other discontinued operations, net of tax								363.2
Sale of Sirius Group
Discontinued Operations [Line Items}
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of subsidiary								$ 113.3
Discontinued Operations \| OneBeacon
Discontinued Operations [Line Items}
Net (loss) income from discontinued operations							32.3
Discontinued operation, tax effect of discontinued operation							(4.2)
Discontinued Operations \| Tranzact
Discontinued Operations [Line Items}
Gain from sale of other discontinued operations, net of tax							$ (1.0)
Discontinued Operations \| Sirius Group
Discontinued Operations [Line Items}
Gain from sale of other discontinued operations, net of tax					0.1
Net (loss) income from discontinued operations					0.0
Discontinued operation, tax effect of discontinued operation					0.0
Intact Financial Corporation \| OneBeacon
Discontinued Operations [Line Items}
Sale of stock, price per share	$ 18.10
Connecticut \| Discontinued Operations, Held-for-sale
Discontinued Operations [Line Items}
Assets held for sale					$ 3.3	$ 3.3

Held for Sale and Discontinued Operations - Financial Statement Disclosures (Details) - USD ($) $ in Millions		3 Months Ended			12 Months Ended
	Sep. 28, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums		$ 3.0		$ 3.0
Net investment income		19.7		12.8
Net realized and unrealized investment gains		(53.7)		36.3
Total revenues		42.1		88.8
Loss and loss adjustment expenses				1.1
Insurance acquisition expenses		1.4		1.3
Other underwriting expenses		0.1		0.1
General and administrative expenses		47.9		60.7
Interest expense		0.6		0.4
Total expenses		108.1		92.4
Net (loss) income from discontinued operations, net of tax		0.0		32.3
Gain from sale of other discontinued operations, net of tax		0.1	$ 0.1	(1.0)
Total income (loss) from discontinued operations		0.1		31.3
Total comprehensive income (loss)		(48.0)		29.2
Sirius Group
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain from sale of other discontinued operations, net of tax					$ 363.2
OneBeacon
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net (loss) income from discontinued operations, net of tax		32.3
Gain from sale of other discontinued operations, net of tax	$ 554.6
Tranzact
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums				0.0
Net investment income				0.0
Net realized and unrealized investment gains				0.0
Other revenue				0.0
Total revenues				0.0
Loss and loss adjustment expenses				0.0
Insurance acquisition expenses				0.0
Other underwriting expenses				0.0
General and administrative expenses				0.0
Interest expense				0.0
Total expenses				0.0
Pre-tax income				0.0
Income tax expense				0.0	30.2
Net (loss) income from discontinued operations, net of tax				0.0
Gain from sale of other discontinued operations, net of tax					$ 51.9
Total comprehensive income (loss)				(1.0)
Discontinued Operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total income (loss) from discontinued operations		0.1		31.3
Discontinued Operations \| Sirius Group
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums		0.0
Net investment income		0.0
Net realized and unrealized investment gains		0.0
Other revenue		0.0
Total revenues		0.0
Loss and loss adjustment expenses		0.0
Insurance acquisition expenses		0.0
Other underwriting expenses		0.0
General and administrative expenses		0.0
Interest expense		0.0
Total expenses		0.0
Pre-tax income		0.0
Income tax expense		0.0
Net (loss) income from discontinued operations, net of tax		0.0
Gain from sale of other discontinued operations, net of tax		0.1
Total income (loss) from discontinued operations		0.1
Total comprehensive income (loss)		0.1
Discontinued Operations \| Sirius Group \| Net gain (loss) from sale of discontinued operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain from sale of other discontinued operations, net of tax		0.1
Discontinued Operations \| OneBeacon
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums				261.8
Net investment income				12.2
Net realized and unrealized investment gains				15.0
Other revenue				3.4
Total revenues				292.4
Loss and loss adjustment expenses				150.6
Insurance acquisition expenses				45.3
Other underwriting expenses				51.7
General and administrative expenses				5.0
Interest expense				3.3
Total expenses				255.9
Pre-tax income				36.5
Income tax expense				(4.2)
Net (loss) income from discontinued operations, net of tax				32.3
Total income (loss) from discontinued operations				32.3
Net change in foreign currency translation and benefit plan assets and obligations				(0.1)
Total comprehensive income (loss)				32.2
Discontinued Operations \| Total
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums		0.0		261.8
Net investment income		0.0		12.2
Net realized and unrealized investment gains		0.0		15.0
Other revenue		0.0		3.4
Total revenues		0.0		292.4
Loss and loss adjustment expenses		0.0		150.6
Insurance acquisition expenses		0.0		45.3
Other underwriting expenses		0.0		51.7
General and administrative expenses		0.0		5.0
Interest expense		0.0		3.3
Total expenses		0.0		255.9
Pre-tax income		0.0		36.5
Income tax expense		0.0		(4.2)
Net (loss) income from discontinued operations, net of tax		0.0		32.3
Total income (loss) from discontinued operations		0.1		31.3
Net change in foreign currency translation and benefit plan assets and obligations		0.0		(0.1)
Total comprehensive income (loss)		0.1		31.2
Discontinued Operations \| Total \| Net gain (loss) from sale of discontinued operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain from sale of other discontinued operations, net of tax		$ 0.1		(1.0)
Discontinued Operations \| Tranzact
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain from sale of other discontinued operations, net of tax				(1.0)
Total income (loss) from discontinued operations				(1.0)
Discontinued Operations \| Tranzact \| Net gain (loss) from sale of discontinued operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain from sale of other discontinued operations, net of tax				$ (1.0)

Held for Sale and Discontinued Operations - Net Change Cash From Discontinued Operations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Cash balances at beginning of period	$ 97.1	$ 80.2
Cash balances at end of period	120.2	45.4
Net income tax payment to national governments	(0.2)	0.0
Discontinued Operations
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from (used for) operations - discontinued operations	0.1	(1.3)
Net cash (used for) provided from investing activities - discontinued operations	(0.1)	32.3
Net cash used for financing activities - discontinued operations	0.0	(21.0)
Net change in cash during the period	0.0	10.0
Cash balances at beginning of period	0.0	70.5
Cash balances at end of period	0.0	79.6
Interest paid	0.0	0.0
Net income tax payment to national governments	0.0	0.0
Discontinued Operations, Held-for-sale
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Cash balances at beginning of period	0.0	70.5
Cash balances at end of period	0.0	79.6
Disposal Group, Held-for-sale, Not Discontinued Operations
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net change in cash held for sale, excluding discontinued operations	$ 0.0	$ (0.9)

Held for Sale and Discontinued Operations - EPS (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Basic and diluted earnings per share numerators (in millions):
Net (loss) income attributable to White Mountains’s common shareholders	$ (48.0)	$ 29.0
Less total loss from continuing operations, net of tax	(48.1)	(2.3)
Net income from discontinued operations attributable to White Mountains’s common shareholders	0.1	31.3
Basic and diluted earnings per share numerators	$ (47.7)	$ (2.3)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,746,100	4,564,600
Average unvested restricted common shares	(35,900)	(52,500)
Basic earnings per share denominator	3,710,200	4,512,100
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,746,100	4,564,600
Average unvested restricted common shares	(35,900)	(52,500)
Diluted earnings per share denominator	3,710,200	4,512,100
Basic earnings per share (in dollars)	$ (12.82)	$ 6.34
Diluted earnings per share (in dollars)	$ (12.82)	$ 6.34
Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Net (loss) income attributable to White Mountains’s common shareholders	$ (48.0)	$ 29.0
Less total loss from continuing operations, net of tax	48.1	2.3
Net income from discontinued operations attributable to White Mountains’s common shareholders	0.1	31.3
Allocation of income for participating unvested restricted common shares	0.0	(0.4)
Basic and diluted earnings per share numerators	$ 0.1	$ 30.9
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,746,100	4,564,600
Average unvested restricted common shares	(35,900)	(52,500)
Basic earnings per share denominator	3,710,200	4,512,100
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,746,100	4,564,600
Average unvested restricted common shares	(35,900)	(52,500)
Diluted earnings per share denominator	3,710,200	4,512,100
Basic earnings per share (in dollars)	$ 0.03	$ 6.86
Diluted earnings per share (in dollars)	$ 0.03	$ 6.86

Financial Statement Revisions - Statement of Operations and Comprehensive Income (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Revenues:
Advertising and commission revenues	$ 71.0		$ 33.8
Total revenues	42.1		88.8
Expenses:
Total expenses	108.1		92.4
Pre-tax income (loss) from continuing operations	(66.0)		(3.6)
Income tax benefit	(0.7)		0.3
Net income (loss) from continuing operations	(66.7)		(3.3)
Gain from sale of other discontinued operations, net of tax	0.1	$ 0.1	(1.0)
Net (loss) income from discontinued operations, net of tax	0.0		32.3
Net income	(66.6)		28.0
Net loss (income) attributable to non-controlling interests	18.6		1.0
Net income attributable to White Mountains’s common shareholders	(48.0)		29.0
Other comprehensive income, net of tax:
Other comprehensive income (loss), net of tax			0.2
Comprehensive income	(48.0)		29.2
Other comprehensive income attributable to non-controlling interests	0.0		0.0
Comprehensive income attributable to White Mountains’s common shareholders	(48.0)		$ 29.2
Continuing operations (in usd per share)			$ 6.34
As previously reported
Revenues:
Total revenues			$ 93.5
Expenses:
Total expenses			92.2
Pre-tax income (loss) from continuing operations			1.3
Income tax benefit			0.3
Net income (loss) from continuing operations			1.6
Gain from sale of other discontinued operations, net of tax			(1.0)
Net (loss) income from discontinued operations, net of tax			32.3
Net income			32.9
Net loss (income) attributable to non-controlling interests			1.3
Net income attributable to White Mountains’s common shareholders			34.2
Other comprehensive income, net of tax:
Other comprehensive income (loss), net of tax			0.9
Comprehensive income			35.1
Other comprehensive income attributable to non-controlling interests			0.0
Comprehensive income attributable to White Mountains’s common shareholders			$ 35.1
Continuing operations (in usd per share)			$ 7.49
Adjustments
Revenues:
Total revenues			$ (4.7)
Expenses:
Total expenses			0.2
Pre-tax income (loss) from continuing operations			(4.9)
Income tax benefit			0.0
Net income (loss) from continuing operations			(4.9)
Gain from sale of other discontinued operations, net of tax			0.0
Net (loss) income from discontinued operations, net of tax			0.0
Net income			(4.9)
Net loss (income) attributable to non-controlling interests			(0.3)
Net income attributable to White Mountains’s common shareholders			(5.2)	$ (5.2)
Other comprehensive income, net of tax:
Other comprehensive income (loss), net of tax			(0.7)
Comprehensive income			(5.9)
Other comprehensive income attributable to non-controlling interests			0.0
Comprehensive income attributable to White Mountains’s common shareholders			$ (5.9)
Continuing operations (in usd per share)			$ (1.15)
HG Global-BAM
Revenues:
Total revenues	(1.0)		$ 6.3
Expenses:
Total expenses	13.3		11.9
HG Global-BAM \| Adjustments
Revenues:
Total revenues			0.0
Expenses:
Total expenses			0.0
MediaAlpha
Revenues:
Advertising and commission revenues	70.1		32.5
Total revenues	71.7		32.5
Expenses:
Total expenses	71.9		33.5
MediaAlpha \| As previously reported
Revenues:
Total revenues			32.5
Expenses:
Total expenses			33.5
MediaAlpha \| Adjustments
Revenues:
Total revenues			0.0
Expenses:
Total expenses			0.0
Other Entity
Revenues:
Advertising and commission revenues	0.9		1.3
Other revenues			48.7
Total revenues	(28.6)		50.0
Expenses:
Total expenses	22.9		47.0
Other Entity \| As previously reported
Revenues:
Advertising and commission revenues			6.0
Expenses:
Total expenses			46.8
Other Entity \| Adjustments
Revenues:
Advertising and commission revenues			(4.7)
Other revenues			0.0
Expenses:
Total expenses			0.2
HG Global-BAM
Revenues:
Advertising and commission revenues	0.0		0.0
Expenses:
Pre-tax income (loss) from continuing operations	$ (14.3)		(5.6)
HG Global-BAM \| As previously reported
Revenues:
Total revenues			6.3
Expenses:
Total expenses			$ 11.9

Financial Statement Revisions - Statements of Changes in Equity (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2016
Increase (Decrease) in Shareholders' Equity
Beginning Balances	$ 3,360.8
Net loss (income)	(66.6)	$ 28.0
Net (loss) income attributable to White Mountains’s common shareholders	(48.0)	29.0
Ending Balances	3,295.8	3,730.8
As previously reported
Increase (Decrease) in Shareholders' Equity
Net loss (income)		32.9
Net (loss) income attributable to White Mountains’s common shareholders		34.2
Ending Balances		3,757.5
Adjustments
Increase (Decrease) in Shareholders' Equity
Net loss (income)		(4.9)
Net (loss) income attributable to White Mountains’s common shareholders		(5.2)	$ (5.2)
Ending Balances		(26.7)
Common shares and paid-in surplus
Increase (Decrease) in Shareholders' Equity
Beginning Balances	670.6	810.7
Repurchases and retirements of common shares	(1.9)	(1.4)
Net loss (income)	0.0	0.0
Dividends declared on common shares	0.0	0.0
Ending Balances	675.3	807.8	810.7
Common shares and paid-in surplus \| As previously reported
Increase (Decrease) in Shareholders' Equity
Ending Balances		807.1
Common shares and paid-in surplus \| Adjustments
Increase (Decrease) in Shareholders' Equity
Ending Balances		0.7
Retained earnings
Increase (Decrease) in Shareholders' Equity
Beginning Balances	2,823.2	2,776.6
Repurchases and retirements of common shares	(6.4)	(5.1)
Net loss (income)	(48.0)	29.0
Dividends declared on common shares	(3.8)	(4.6)
Ending Balances	2,765.0	2,795.9	2,776.6
Retained earnings \| As previously reported
Increase (Decrease) in Shareholders' Equity
Beginning Balances		2,797.2
Repurchases and retirements of common shares		(5.1)
Net loss (income)		34.2
Dividends declared on common shares		(4.6)
Ending Balances		2,821.7	2,797.2
Retained earnings \| Adjustments
Increase (Decrease) in Shareholders' Equity
Beginning Balances		(20.6)
Repurchases and retirements of common shares		0.0
Dividends declared on common shares		0.0
Ending Balances		(25.8)	(20.6)
AOCI, after tax
Increase (Decrease) in Shareholders' Equity
Beginning Balances	(1.3)	(4.6)
Repurchases and retirements of common shares	0.0	0.0
Net loss (income)	0.0	0.0
Dividends declared on common shares	0.0	0.0
Ending Balances	(1.3)	(4.4)	(4.6)
AOCI, after tax \| As previously reported
Increase (Decrease) in Shareholders' Equity
Beginning Balances		(4.6)
Ending Balances		(3.7)	(4.6)
AOCI, after tax \| Adjustments
Increase (Decrease) in Shareholders' Equity
Beginning Balances		0.0
Ending Balances		(0.7)	0.0
Total
Increase (Decrease) in Shareholders' Equity
Beginning Balances	3,492.5	3,582.7
Repurchases and retirements of common shares	(8.3)	(6.5)
Net loss (income)	(48.0)	29.0
Dividends declared on common shares	(3.8)	(4.6)
Ending Balances	3,439.0	3,599.3	3,582.7
Total \| As previously reported
Increase (Decrease) in Shareholders' Equity
Ending Balances		3,625.1
Total \| Adjustments
Increase (Decrease) in Shareholders' Equity
Ending Balances		(25.8)
Non-controlling interest
Increase (Decrease) in Shareholders' Equity
Beginning Balances	(131.7)	133.3
Repurchases and retirements of common shares	0.0	(1.1)
Net loss (income)	(18.6)	(1.0)
Dividends declared on common shares	0.0	0.0
Other changes in non-controlling interests		(0.8)
Ending Balances	(143.2)	131.5	133.3
Non-controlling interest \| As previously reported
Increase (Decrease) in Shareholders' Equity
Beginning Balances		133.9
Net loss (income)		(1.3)
Other changes in non-controlling interests		(0.2)
Ending Balances		132.4	133.9
Non-controlling interest \| Adjustments
Increase (Decrease) in Shareholders' Equity
Beginning Balances		(0.6)
Net loss (income)		0.3
Other changes in non-controlling interests		(0.6)
Ending Balances		(0.9)	(0.6)
Total Equity
Increase (Decrease) in Shareholders' Equity
Beginning Balances	3,360.8	3,716.0
Repurchases and retirements of common shares	(8.3)	(7.6)
Net loss (income)	(66.6)	28.0
Dividends declared on common shares	(3.8)	(4.6)
Ending Balances	3,295.8	3,730.8	$ 3,716.0
Total Equity \| As previously reported
Increase (Decrease) in Shareholders' Equity
Ending Balances		3,757.5
Total Equity \| Adjustments
Increase (Decrease) in Shareholders' Equity
Ending Balances		(26.7)
Foreign Currency Gain (Loss) \| Common shares and paid-in surplus
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax	0.0	0.0
Foreign Currency Gain (Loss) \| Retained earnings
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax	0.0	0.0
Foreign Currency Gain (Loss) \| AOCI, after tax
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax	0.0	0.2
Foreign Currency Gain (Loss) \| AOCI, after tax \| As previously reported
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax		0.9
Foreign Currency Gain (Loss) \| AOCI, after tax \| Adjustments
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax		(0.7)
Foreign Currency Gain (Loss) \| Total
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax	0.0	0.2
Foreign Currency Gain (Loss) \| Non-controlling interest
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax	0.0	0.0
Foreign Currency Gain (Loss) \| Total Equity
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax	$ 0.0	$ 0.2

Financial Statement Revisions - Balance Sheet (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Assets
Goodwill	$ 25.9	$ 25.9	$ 25.9	$ 25.9
Total assets	3,587.0	3,659.2	6,490.4
Liabilities
Total liabilities	291.2	298.4	2,759.6
White Mountains’s common shareholders’ equity
White Mountains’ common shares	3.8	3.8	4.6
Paid-in surplus	671.5	666.8	803.2
Retained earnings	2,765.0	2,823.2	2,795.9
Accumulated other comprehensive loss, after tax:
Accumulated other comprehensive loss from net change in benefit plan assets and obligations			(4.4)
Total White Mountains’s common shareholders’ equity	3,439.0	3,492.5	3,599.3
Non-controlling interests	(143.2)	(131.7)	131.5
Total equity	3,295.8	3,360.8	3,730.8
Total liabilities and equity	3,587.0	3,659.2	6,490.4
As previously reported
Assets
Total assets			6,516.8
Liabilities
Total liabilities			2,759.3
White Mountains’s common shareholders’ equity
White Mountains’ common shares			4.6
Paid-in surplus			802.5
Retained earnings			2,821.7
Accumulated other comprehensive loss, after tax:
Accumulated other comprehensive loss from net change in benefit plan assets and obligations			(3.7)
Total White Mountains’s common shareholders’ equity			3,625.1
Non-controlling interests			132.4
Total equity			3,757.5
Total liabilities and equity			6,516.8
Adjustments
Assets
Total assets			(26.4)
Liabilities
Total liabilities			0.3
White Mountains’s common shareholders’ equity
White Mountains’ common shares			0.0
Paid-in surplus			0.7
Retained earnings			(25.8)
Accumulated other comprehensive loss, after tax:
Accumulated other comprehensive loss from net change in benefit plan assets and obligations			(0.7)
Total White Mountains’s common shareholders’ equity			(25.8)
Non-controlling interests			(0.9)
Total equity			(26.7)
Total liabilities and equity			(26.4)
HG Global-BAM
Assets
Other assets	5.1	5.6
Total assets	750.1	747.4	699.1
Liabilities
Other liabilities	12.2	11.4
Total liabilities	179.6	167.0	126.4
HG Global-BAM \| As previously reported
Assets
Total assets			699.1
Liabilities
Total liabilities			126.4
HG Global-BAM \| Adjustments
Assets
Total assets			0.0
Liabilities
Total liabilities			0.0
MediaAlpha
Assets
Other assets	1.6	1.3
Total assets	103.7	96.5	53.2
Liabilities
Other liabilities	0.9	2.4
Total liabilities	60.7	59.8	25.6
Accumulated other comprehensive loss, after tax:
Non-controlling interests	16.2	13.1
MediaAlpha \| As previously reported
Assets
Total assets			53.2
Liabilities
Total liabilities			25.6
MediaAlpha \| Adjustments
Assets
Total assets			0.0
Liabilities
Total liabilities			0.0
Other Entity
Assets
Goodwill			7.6
Other intangible assets			1.1
Other assets — commissions receivable			4.2
Assets held for sale	3.3	3.3	3,578.9
Other assets	16.5	19.1	2,146.3
Total assets	2,733.2	2,815.3
Liabilities
Other liabilities			2,535.2
Other liabilities	9.5	11.0	72.4
Total liabilities	$ 50.9	$ 71.6
Other Entity \| As previously reported
Assets
Goodwill			13.4
Other intangible assets			4.5
Other assets — commissions receivable			21.4
Assets held for sale			3,578.9
Other assets			2,146.3
Liabilities
Other liabilities			2,535.2
Other liabilities			72.1
Other Entity \| Adjustments
Assets
Goodwill			(5.8)
Other intangible assets			(3.4)
Other assets — commissions receivable			(17.2)
Assets held for sale			0.0
Other assets			0.0
Liabilities
Other liabilities			0.0
Other liabilities			$ 0.3

Subsequent Events (Details) - Subsequent Event - USD ($) $ in Millions		1 Months Ended
	Apr. 02, 2018	Apr. 30, 2018	Apr. 16, 2018
Subsequent Event [Line Items]
Tender offer, shares (up to)		500,000
NSM Insurance Group
Subsequent Event [Line Items]
Business acquisition, percentage of voting interests acquired	95.00%
Business combination, consideration transferred	$ 368.3
Expected borrowings	100.0
Business combination, net consideration transferred	$ 273.3
Fidus Re Ltd.
Subsequent Event [Line Items]
Reinsurance payable			$ 100.0