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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2018	Feb. 25, 2019	Jun. 30, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	WHITE MOUNTAINS INSURANCE GROUP LTD
Entity Central Index Key	0000776867
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 2,745,786,244
Entity Common Stock, Shares Outstanding		3,167,436
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Assets
Total investments	$ 2,542.9	$ 3,380.7
Goodwill and other intangible assets	537.5	62.1
Total assets	3,362.6	3,659.2
Liabilities
Debt	192.7	23.8
Total liabilities	644.4	298.4
White Mountains’s common shareholders’ equity
White Mountains’s common shares at $1 par value per share—authorized 50,000,000 shares; issued and outstanding 3,173,115 and 3,750,171 shares	3.2	3.8
Paid-in surplus	580.8	666.8
Retained earnings	2,264.9	2,823.2
Accumulated other comprehensive income (loss), after-tax:
Net unrealized foreign currency translation losses and interest rate swap	(5.8)	(1.3)
Total White Mountains’s common shareholders’ equity	2,843.1	3,492.5
Non-controlling interests
Non-controlling interests	(124.9)	(131.7)
Total equity	2,718.2	3,360.8
Total liabilities and equity	3,362.6	3,659.2
HG Global-BAM
Assets
Fixed maturity investments, at fair value	701.4	623.6
Short-term investments, at fair value	66.9	69.8
Total investments	768.3	693.4
Cash	12.5	25.6
Premium receivable	6.4	4.5
Deferred acquisition costs	19.0	14.8
Accrued investment income	4.9	3.4
Accounts receivable on unsettled investment sales	0.0	0.0
Other assets	5.1	5.7
Total assets	816.2	747.4
Liabilities
Unearned insurance premiums	176.0	136.8
Accounts payable on unsettled investment purchases	2.2	0.6
Other liabilities	34.3	29.6
Total liabilities	212.5	167.0
NSM Holding Co LLC
Assets
Short-term investments, at fair value	1.7	0.0
Cash	66.2	0.0
Commission Receivable	44.0	0.0
Goodwill and other intangible assets	486.2	0.0
Other assets	28.9	0.0
Total assets	627.0	0.0
Liabilities
Debt	178.5	0.0
Premiums payable	77.2	0.0
Contingent consideration earnout liabilities	20.2	0.0
Other liabilities	38.9	0.0
Total liabilities	314.8	0.0
MediaAlpha
Assets
Cash	5.7	9.1
Goodwill and other intangible assets	43.4	53.7
Accounts receivable from publishers and advertisers	37.0	32.4
Other assets	2.3	1.3
Total assets	88.4	96.5
Liabilities
Debt	14.2	23.8
Amounts due to publishers and advertisers	27.0	31.6
Other liabilities	5.7	4.4
Total liabilities	46.9	59.8
Other
Assets
Fixed maturity investments, at fair value	376.1	1,506.1
Short-term investments, at fair value	145.6	106.3
Common equity securities, at fair value	925.6	866.1
Other long-term investments	325.6	208.8
Total investments	1,772.9	2,687.3
Cash	25.9	62.4
Accrued investment income	5.5	13.9
Accounts receivable on unsettled investment sales	0.0	20.9
Goodwill and other intangible assets	7.9	8.4
Other assets	15.5	19.1
Assets held for sale	3.3	3.3
Total assets	1,831.0	2,815.3
Liabilities
Accrued incentive compensation	38.9	60.6
Accounts payable on unsettled investment purchases	5.0	0.0
Other liabilities	26.3	11.0
Total liabilities	$ 70.2	$ 71.6

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Cash restricted	$ 50.0	$ 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	3,173,115	3,750,171
Common shares, outstanding shares	3,173,115	3,750,171
NSM Holding Co LLC
Cash restricted	$ 50.0

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions		3 Months Ended									9 Months Ended	12 Months Ended
	Sep. 28, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 28, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues
Earned insurance premiums												$ 13.9	$ 10.4	$ 13.4
Net investment income												59.0	56.0	32.1
Net realized and unrealized investment gains												(108.3)	133.3	(27.4)
Commission revenues												394.3	167.0	118.3
Other revenue												10.2	7.1	21.3
Total revenues		$ 6.0	$ 198.7	$ 122.3	$ 42.1	$ 114.0	$ 87.5	$ 83.5	$ 88.8			369.1	373.8	157.7
Expenses
Loss and loss adjustment expenses													1.1	8.0
Insurance acquisition expenses												5.3	4.1	5.6
Other underwriting expenses												0.4	0.4	0.4
Cost of sales												248.7	139.4	102.0
General and administrative expenses												235.7	208.0	175.5
Broker commission expense												28.9
Amortization of other intangible assets												18.8	10.7	10.5
Interest expense on debt												9.5	2.3	3.0
Total expenses		150.1	154.4	134.7	108.1	108.8	79.1	85.7	92.4			547.3	366.0	305.0
Pre-tax income (loss)		(144.1)	44.3	(12.4)	(66.0)	5.2	8.4	(2.2)	(3.6)			(178.2)	7.8	(147.3)
Income tax (expense) benefit		3.6	3.6	(2.5)	(0.7)	2.5	4.0	1.0	0.3			4.0	7.8	32.9
Net income (loss) from continuing operations		(140.5)	47.9	(14.9)	(66.7)	7.7	12.4	(1.2)	(3.3)			(174.2)	15.6	(114.4)
Gain (loss) on sale of other discontinued operations, net of tax												(17.2)	557.0	415.1
Net income from discontinued operations, net of tax		0.0	(17.3)	0.0	0.1	4.3	539.1	2.8	31.3			0.0	20.5	108.3
Net income												(191.4)	593.1	409.0
Net loss attributable to non-controlling interests		3.0	10.2	18.4	18.6	10.5	10.6	12.0	1.0			50.2	34.1	(7.2)
Net income attributable to White Mountains’s common shareholders		$ (137.5)	$ 40.8	$ 3.5	$ (48.0)	$ 22.5	$ 562.1	$ 13.6	$ 29.0			(141.2)	627.2	401.8
HG Global-BAM
Revenues
Earned insurance premiums												13.9	9.4	5.9
Net investment income												16.7	12.3	9.0
Net realized and unrealized investment gains												(7.5)	0.6	0.7
Commission revenues												0.0	0.0	0.0
Other revenue												1.2	1.0	1.1
Total revenues												24.3	23.3	16.7
Expenses
Loss and loss adjustment expenses													0.0	0.0
Insurance acquisition expenses												5.3	4.0	3.4
Other underwriting expenses												0.4	0.4	0.4
Cost of sales												0.0	0.0	0.0
General and administrative expenses												48.0	42.9	39.6
Broker commission expense												0.0
Amortization of other intangible assets												0.0	0.0	0.0
Interest expense on debt												0.0	0.0	0.0
Total expenses												53.7	47.3	43.4
Pre-tax income (loss)												(29.4)	(24.0)	(26.7)
NSM Holding Co LLC
Revenues
Earned insurance premiums												0.0
Net investment income												0.0
Net realized and unrealized investment gains												0.0
Commission revenues												94.7	0.0	0.0
Other revenue												6.9	0.0	0.0
Total revenues												101.6	0.0	0.0
Expenses
Insurance acquisition expenses												0.0
Other underwriting expenses												0.0
Cost of sales												0.0
General and administrative expenses												61.6	0.0	0.0
Broker commission expense												28.9	0.0	0.0
Amortization of other intangible assets												8.3	0.0	0.0
Interest expense on debt												8.0	0.0	0.0
Total expenses												106.8	0.0	0.0
Pre-tax income (loss)												(5.2)
MediaAlpha
Revenues
Earned insurance premiums												0.0	0.0	0.0
Net investment income												0.0	0.0	0.0
Net realized and unrealized investment gains												0.0	0.0	0.0
Commission revenues												295.5	163.2	116.5
Other revenue												1.6	0.0	0.0
Total revenues												297.1	163.2	116.5
Expenses
Loss and loss adjustment expenses													0.0	0.0
Insurance acquisition expenses												0.0	0.0	0.0
Other underwriting expenses												0.0	0.0	0.0
Cost of sales												245.0	135.9	97.8
General and administrative expenses												31.7	16.2	11.8
Broker commission expense												0.0
Amortization of other intangible assets												10.3	10.5	10.1
Interest expense on debt												1.2	1.0	0.9
Total expenses												288.2	163.6	120.6
Pre-tax income (loss)												8.9	(0.4)	(4.1)
Other
Revenues
Earned insurance premiums												0.0	1.0	7.5
Net investment income												42.3	43.7	23.1
Net realized and unrealized investment gains												(100.8)	132.7	(28.1)
Commission revenues												4.1	3.8	1.8
Other revenue												0.5	6.1	20.2
Total revenues												(53.9)	187.3	24.5
Expenses
Loss and loss adjustment expenses												0.0	1.1	8.0
Insurance acquisition expenses												0.0	0.1	2.2
Other underwriting expenses												0.0	0.0	0.0
Cost of sales												3.7	3.5	4.2
General and administrative expenses												94.4	148.9	124.1
Broker commission expense												0.0
Amortization of other intangible assets												0.2	0.2	0.4
Interest expense on debt												0.3	1.3	2.1
Total expenses												98.6	155.1	141.0
Pre-tax income (loss)												(152.5)	32.2	(116.5)
OneBeacon
Expenses
Gain (loss) on sale of other discontinued operations, net of tax	$ 554.6
Net income from discontinued operations, net of tax											$ 20.5			108.6
Tranzact
Expenses
Gain (loss) on sale of other discontinued operations, net of tax													3.2	51.9
Sirius Group
Revenues
Net realized and unrealized investment gains														3.7
Expenses
Gain (loss) on sale of other discontinued operations, net of tax										$ (4.0)				363.2
Discontinued Operations
Expenses
Net income attributable to White Mountains’s common shareholders												(141.2)	627.2	401.8
Discontinued Operations \| OneBeacon
Revenues
Earned insurance premiums													807.6	1,100.6
Net investment income													39.7	50.6
Net realized and unrealized investment gains													38.8	37.7
Other revenue													7.7	5.5
Total revenues													893.8	1,194.4
Expenses
Loss and loss adjustment expenses													546.0	656.0
Other underwriting expenses													156.2	209.0
General and administrative expenses													21.2	14.2
Interest expense on debt													10.0	13.1
Total expenses													879.0	1,098.3
Gain (loss) on sale of other discontinued operations, net of tax												0.0	554.5	0.0
Net income from discontinued operations, net of tax													20.5	108.6
Discontinued Operations \| Tranzact
Revenues
Earned insurance premiums													0.0	0.0
Net investment income													0.0	0.0
Net realized and unrealized investment gains													0.0	0.0
Other revenue													0.0	119.6
Total revenues													0.0	119.6
Expenses
Loss and loss adjustment expenses													0.0	0.0
Other underwriting expenses													0.0	0.0
General and administrative expenses													0.0	116.7
Interest expense on debt														3.2
Total expenses													0.0	119.9
Income tax (expense) benefit														21.4
Gain (loss) on sale of other discontinued operations, net of tax												0.0	3.2	51.9
Net income from discontinued operations, net of tax													0.0	6.1
Discontinued Operations \| Sirius Group
Revenues
Earned insurance premiums													0.0	240.1
Net investment income													0.0	14.4
Net realized and unrealized investment gains													0.0	(1.5)
Other revenue													0.0	0.6
Total revenues													0.0	253.6
Expenses
Loss and loss adjustment expenses													0.0	154.9
Other underwriting expenses													0.0	30.9
General and administrative expenses													0.0	10.4
Interest expense on debt														7.9
Total expenses													0.0	263.1
Gain (loss) on sale of other discontinued operations, net of tax												$ (17.2)	(0.7)	363.2
Net income from discontinued operations, net of tax													$ 0.0	$ (6.4)

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income attributable to White Mountains’s common shareholders	$ (141.2)	$ 627.2	$ 401.8
Other comprehensive income, net of tax:
Other comprehensive (loss) income, net of tax	(4.8)	0.3	(0.7)
Comprehensive income from discontinued operations, net of tax	0.0	3.2	146.3
Comprehensive income	(146.0)	630.7	547.4
Comprehensive loss (income) attributable to non-controlling interests	0.3	(0.2)	(0.3)
Comprehensive income attributable to White Mountains’s common shareholders	$ (145.7)	$ 630.5	$ 547.1
Basic earnings per share
Continuing operations	$ (36.67)	$ 11.56	$ (24.26)
Discontinued operations	(5.09)	134.50	104.37
Total consolidated operations	(41.76)	146.06	80.11
Diluted earnings per share
Continuing operations	(36.67)	11.56	(24.26)
Discontinued operations	(5.09)	134.50	104.32
Total consolidated operations	(41.76)	146.06	80.06
Dividends declared and paid per White Mountains’s common share	$ 1.00	$ 1.00	$ 1.00

CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Millions	Total	Common shares and paid-in surplus	Retained earnings	AOCL, after-tax	Parent	Non-controlling interests	Total equity	Sale of Sirius Group	Sale of Sirius Group Non-controlling interests	Sale of Sirius Group Total equity	Tranzact Non-controlling interests	Tranzact Total equity	OneBeacon Non-controlling interests	OneBeacon Total equity	Star & Shield LLC Non-controlling interests	Star & Shield LLC Total equity	BAM Non-controlling interests	BAM Total equity	WOBI Common shares and paid-in surplus	WOBI Parent	WOBI Total equity
Beginning balance at Dec. 31, 2015		$ 978.2	$ 3,075.0	$ (149.9)	$ 3,903.3	$ 454.3	$ 4,357.6
Increase (Decrease) in Shareholders' Equity
Net (loss) income	$ 409.0		401.8		401.8	7.2	409.0
Net change in foreign currency translation and interest rate swap				31.4	31.4	0.3	31.7
Other comprehensive (loss) income, net of tax	(0.7)			113.9	113.9		113.9
Net change in pension liability and other accumulated comprehensive items								$ 113.3
Comprehensive income (loss)	547.4		401.8	145.3	547.1	7.5	554.6
Dividends declared on common shares			(5.4)		(5.4)		(5.4)
Dividends to non-controlling interests						(22.7)	(22.7)
Issuances of common shares		9.1			9.1		9.1
Repurchases and retirements of common shares	(887.2)	(192.4)	(694.8)		(887.2)		(887.2)
Capital contributions from non-controlling interests						27.3	27.3
Deconsolidation of non-controlling interests									$ (250.0)	$ (250.0)	$ (78.4)	$ (78.4)
Acquisition of noncontrolling interest		(2.7)			(2.7)	(8.8)	(11.5)
Acquisition of subsidiary						3.3	3.3
Amortization of restricted share and option awards		18.5			18.5	0.8	19.3
Ending balance 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	593.1		627.2		627.2	(34.1)	593.1
Net change in foreign currency translation and interest rate swap				0.4	0.4	0.1	0.5
Other comprehensive (loss) income, net of tax	0.3			2.9	2.9		2.9
Comprehensive income (loss)	630.7		627.2	3.3	630.5	(34.0)	596.5
Dividends declared on common shares			(4.6)		(4.6)		(4.6)
Dividends to non-controlling interests						(19.3)	(19.3)
Issuances of common shares		1.7			1.7		1.7
Repurchases and retirements of common shares	(723.9)	(147.9)	(576.0)		(723.9)	(5.2)	(729.1)
Issuance of shares of non-controlling interests		(4.1)			(4.1)	5.2	1.1
Capital contributions from non-controlling interests		(4.6)			(4.6)	3.0	(1.6)										$ 27.2	$ 27.2
Deconsolidation of non-controlling interests													$ (238.3)	$ (238.3)	$ (4.4)	$ (4.4)
Amortization of restricted share and option awards		14.8			14.8	0.8	15.6
Ending balance at Dec. 31, 2017	3,492.5	670.6	2,823.2	(1.3)	3,492.5	(131.7)	3,360.8
Increase (Decrease) in Shareholders' Equity
Net (loss) income	(191.4)		(141.2)		(141.2)	(50.2)	(191.4)
Net change in foreign currency translation and interest rate swap				(4.5)	(4.5)	(0.3)	(4.8)
Other comprehensive (loss) income, net of tax	(4.8)
Comprehensive income (loss)	(146.0)		(141.2)	(4.5)	(145.7)	(50.5)	(196.2)
Dividends declared on common shares			(3.8)		(3.8)		(3.8)
Dividends to non-controlling interests						(8.0)	(8.0)
Issuances of common shares		2.0			2.0		2.0
Repurchases and retirements of common shares	(519.4)	(105.8)	(413.3)		(519.1)		(519.1)
Capital contributions from non-controlling interests																	$ 45.0	$ 45.0
Noncontrolling Interest, Recognition of Equity-Based Shares		7.4	0.0	0.0	7.4	4.3	11.7
Noncontrolling Interest, Dilution from Equity-based Shares		(1.5)	0.0	0.0	(1.5)	1.5	0.0
Noncontrolling Interest, Decrease from Purchase of Interests																			$ (1.7)	$ (1.7)	$ (1.7)
Acquisition of subsidiary						14.5	14.5
Amortization of restricted share and option awards		13.0			13.0		13.0
Ending balance at Dec. 31, 2018	$ 2,843.1	$ 584.0	$ 2,264.9	$ (5.8)	$ 2,843.1	$ (124.9)	$ 2,718.2

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operations:
Net (loss) income	$ (191.4)	$ 593.1	$ 409.0
Charges (credits) to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment losses (gains)	108.3	(133.3)	27.4
Amortization of restricted share and option awards	13.0	14.8	18.5
Amortization and depreciation	25.7	22.4	20.4
Deferred income tax benefit	(8.4)	(11.4)	(12.5)
Net income from discontinued operations	0.0	(20.5)	(108.3)
Net loss (gain) on sale of discontinued operations	17.2	(557.0)	(415.1)
Other operating items:
Net change in unearned insurance premiums	39.2	54.5	31.7
Net change in deferred acquisition costs	(4.2)	(4.2)	(3.7)
Net change in restricted cash	(3.4)	0.0	0.0
Net change in other assets and liabilities, net	(27.1)	(20.8)	(146.1)
Net cash used for continuing operations	(31.1)	(62.4)	(178.7)
Net cash provided from discontinued operations (Note 19)	0.0	157.0	23.6
Net cash (used for) provided from operations	(31.1)	94.6	(155.1)
Cash flows from investing activities:
Net change in short-term investments	(39.0)	(1.7)	(27.2)
Sales of fixed maturity and convertible investments	1,848.5	2,124.4	2,605.8
Maturities, calls and paydowns of fixed maturity and convertible investments	141.0	213.4	253.4
Sales of common equity securities	169.9	424.1	815.9
Distributions and redemptions of other long-term investments	5.0	29.4	17.3
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	0.0	1,131.0	2,646.2
Proceeds paid to non-controlling common shareholders from the sale of consolidated subsidiaries	0.0	0.0	(141.6)
Purchases of other long-term investments	(95.9)	(84.1)	(38.5)
Net settlement of investment cash flows and contributions with discontinued operations	0.0	167.7	(402.0)
Purchases of common equity securities	(328.3)	(881.2)	(278.3)
Purchases of fixed maturity and convertible investments	(970.2)	(2,365.2)	(4,407.0)
Purchases of consolidated subsidiaries, net of cash acquired of $90.9, including $53.4 of restricted cash	(295.2)	(27.6)	(13.4)
Net change in unsettled investment purchases and sales	27.6	0.0	0.0
Other investing activities, net	(4.2)	(14.7)	4.8
Net cash provided from investing activities — continuing operations	459.2	715.5	1,035.4
Net cash provided from investing activities — discontinued operations (Note 19)	0.0	3.0	241.4
Net cash provided from investing activities	459.2	718.5	1,276.8
Cash flows from financing activities:
Draw down of debt and revolving line of credit	84.1	376.0	352.5
Repayment of debt and revolving line of credit	(15.4)	(365.0)	(404.6)
Cash dividends paid to the Company’s common shareholders	(3.8)	(4.6)	(5.4)
Acquisitions of additional shares from non-controlling interest	(1.7)	(0.7)	0.0
Distributions from discontinued operations	0.0	45.2	57.2
Common shares repurchased	(511.9)	(714.6)	(881.3)
Proceeds from issuances of common shares	0.0	0.0	3.7
Capital contributions from non-controlling interest shareholders	1.3	0.5	0.0
Distributions to non-controlling interest shareholders	(6.0)	(2.0)	(1.1)
Payments to contingent considerations related to purchases of consolidated subsidiaries	(2.6)	0.0	(7.8)
Capital contributions from BAM members	53.8	37.4	38.0
Fidus Re premium payment	(3.7)	0.0	0.0
Other financing activities, net	(8.4)	(9.3)	(5.8)
Net cash used for financing activities — continuing operations	(414.3)	(637.1)	(854.6)
Net cash used for financing activities — discontinued operations (Note 19)	0.0	(61.9)	(93.8)
Net cash used for financing activities	(414.3)	(699.0)	(948.4)
Net change in cash during the period - continuing operations	13.2	16.0	2.1
Effect of exchange rate changes on cash	(0.6)	0.0	0.0
Cash balance at beginning of year	97.1	80.2	77.8
Add: cash held for sale at the beginning of period	0.0	0.9	1.2
Less: cash held for sale at the end of period	0.0	0.0	0.9
Cash balance at end of year	$ 110.3	$ 97.1	$ 80.2

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash acquired	$ 90.9
Restricted cash acquired	53.4
Cash restricted	50.0	$ 0.0	$ 0.0	$ 5.8
Sirius Group
Held for sale and discontinued operations cash balance	$ 0.0	$ 0.0	$ 70.5	$ 245.4

Summary of Significant Accounting Policies	12 Months Ended
Summary of Significant Accounting Policies	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	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. Under GAAP, the Company is required to consolidate any entity in which it holds a controlling financial interest. A controlling financial interest is usually in the form of an investment representing the majority of the subsidiary’s voting interests. However, a controlling financial interest may also arise from a financial interest in a variable interest entity (“VIE”) through arrangements that do not involve ownership of voting interests. The Company consolidates a VIE if it determines that it is the primary beneficiary. The primary beneficiary is defined as the entity who holds a variable interest that gives it both the power to direct the VIE’s activities that most significantly impact its economic performance and the obligation to absorb losses of, or the right to receive returns from, the VIE that could potentially be significant to the VIE. See Note 15 — “Variable Interest Entities”. Intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. Reportable Segments 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, NSM, MediaAlpha and Other Operations. See Note 13 — “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 reinsurance subsidiary HG Re Ltd. (“HG Re”), to provide up to 15%-of-par, first loss reinsurance protection for policies underwritten by BAM. For capital appreciation bonds, par is adjusted to the estimated equivalent par value for current interest paying bonds. HG Global, together with its subsidiaries, funded the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM, consisting of $203.0 million of Series A Notes and $300.0 million of Series B Notes (the “BAM Surplus Notes”). As of December 31, 2018 and 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 NSM segment consists of NSM Insurance HoldCo, LLC and its wholly-owned subsidiaries (collectively, “NSM”). NSM is a full-service managing general underwriting agency (“MGU”) and program administrator for specialty property and casualty insurance. The company places insurance in niche sectors such as specialty transportation, social services and real estate. On behalf of its insurance carrier partners, NSM manages all aspects of the placement process, including product development, marketing, underwriting, policy issuance and claims. NSM earns commissions based on the volume and profitability of the insurance that it places. NSM does not take insurance risk. As of December 31, 2018, White Mountains owned 95.5% of NSM. The NSM segment also includes White Mountains Catskill Holdings, Inc., the immediate holding company of NSM. See Note 2 — “Significant Transactions". 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 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 the overall performance of their marketing campaigns. MediaAlpha has developed distinctive platform solutions for a range of insurance verticals, including auto, motorcycle, home, renter, health and life, and non-insurance verticals, including travel, education and personal finance. White Mountains’s Other Operations segment consists of the Company and its wholly-owned subsidiary, White Mountains Capital, Inc. (“WM Capital”), its other intermediate holding companies, its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), investment assets managed by WM Advisors, its interests in PassportCard Limited (“PassportCard”) and DavidShield Life Insurance Agency (2000) Ltd. (“DavidShield”) (collectively, “PassportCard/DavidShield”) and Kudu Investment Management, LLC (“Kudu”), certain other consolidated and unconsolidated entities (“Other Operating Businesses”) and certain other strategic investments (“Strategic Investments”). The consolidated entities include Wobi Insurance Agency Ltd. (“Wobi”) and Removal Stars Ltd. (“Buzz”). White Mountains’s Other Operations segment also included its variable annuity reinsurance business, White Mountains Life Reinsurance (Bermuda) Ltd. (“Life Re Bermuda”), which completed its runoff with all of its contracts fully matured on June 30, 2016 and was liquidated in the third quarter of 2017, and its U.S.-based service provider, White Mountains Financial Services LLC, which was liquidated in the second quarter of 2017 (collectively, “WM Life Re”). Discontinued Operations and Assets and Liabilities Held for Sale On September 28, 2017, Intact Financial Corporation completed its previously announced 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. 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 was required to consolidate SSIE in its GAAP financial statements until White Mountains completed the sale. White Mountains has presented Star & Shield’s and SSIE’s assets and liabilities as held for sale as of December 31, 2016. See Note 19 — “Held for Sale and Discontinued Operations”. 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 disposal costs of $3.3 million. The related write down of $3.7 million was recorded within other expenses during 2017. Significant Accounting Policies Cash Cash includes amounts on hand and demand deposits with banks and other financial institutions. Amounts presented in the statement of cash flows are shown net of balances acquired and sold in the purchase or sale of the Company’s consolidated subsidiaries and exclude changes in amounts of restricted cash. See Note 7 — “Derivatives”. Short-Term Investments Short-term investments consist of interest-bearing money market funds 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 December 31, 2018 and 2017. Investment Securities As of December 31, 2018, White Mountains’s invested assets consisted of securities and other investments held for general investment purposes. White Mountains’s portfolio of fixed maturity investments, common equity securities and other long-term investments held for general investment purposes are classified as trading securities and are reported at fair value as of the balance sheet date. Changes in net unrealized investment gains (losses) are reported pre-tax in revenues. Realized investment gains (losses) are accounted for using the specific identification method and are reported pre-tax in revenues. Premiums and discounts on all fixed maturity investments are amortized and accreted to income over the anticipated life of the investment. White Mountains’s invested assets that are measured at fair value include fixed maturity investments, common equity securities and other long-term investments, including unconsolidated entities, private equity funds and hedge funds. In determining its estimates of fair value, White Mountains uses a variety of valuation approaches and inputs. Whenever possible, White Mountains estimates fair value using valuation methods that maximize the use of quoted prices and other observable inputs. Fair value measurements are categorized into a hierarchy that distinguishes between inputs based on market data from independent sources (“observable inputs”) and a reporting entity’s internal assumptions based upon the best information available when external market data is limited or unavailable (“unobservable inputs”). Quoted prices in active markets for identical assets have the highest priority (“Level 1”), followed by observable inputs other than quoted prices including prices for similar but not identical assets or liabilities (“Level 2”), and unobservable inputs, including the reporting entity’s estimates of the assumptions that market participants would use, having the lowest priority (“Level 3”). White Mountains uses brokers and outside pricing services to assist in determining fair values. The outside pricing services White Mountains uses have indicated that they will only provide prices where observable inputs are available. As of December 31, 2018, approximately 87% of the investment portfolio recorded at fair value as Level 1 or Level 2 measurements. Level 1 Measurements Investments valued using Level 1 inputs include fixed maturity investments, primarily investments in U.S. Treasuries and short-term investments, which include U.S. Treasury Bills and common equity securities. For investments in active markets, White Mountains uses the quoted market prices provided by outside pricing services to determine fair value. Level 2 Measurements Investments valued using Level 2 inputs include fixed maturity investments, which have been disaggregated into classes, including debt securities issued by corporations, mortgage and asset-backed securities, municipal obligations, and foreign government, agency and provincial obligations. Investments valued using Level 2 inputs also include certain passive exchange traded funds (“ETFs”) that track U.S. stock indices such as the S&P 500 but are traded on foreign exchanges, which management values using the fund manager’s published NAV to account for the difference in market close times. In circumstances where quoted market prices are unavailable or are not considered reasonable, White Mountains estimates the fair value using industry standard pricing methodologies and observable inputs such as benchmark yields, reported trades, broker-dealer quotes, issuer spreads, benchmark securities, bids, offers, credit ratings, prepayment speeds, reference data including research publications and other relevant inputs. Given that many fixed maturity investments do not trade on a daily basis, the outside pricing services evaluate a wide range of fixed maturity investments by regularly drawing parallels from recent trades and quotes of comparable securities with similar features. The characteristics used to identify comparable fixed maturity investments vary by asset type and take into account market convention. White Mountains’s process to assess the reasonableness of the market prices obtained from the outside pricing sources covers substantially all of its fixed maturity investments and includes, but is not limited to, the evaluation of pricing methodologies and a review of the pricing services’ quality control procedures on at least an annual basis, a comparison of its invested asset prices obtained from alternate independent pricing vendors on at least a semi-annual basis, monthly analytical reviews of certain prices and a review of the underlying assumptions utilized by the pricing services for select measurements on an ad hoc basis throughout the year. White Mountains also performs back-testing of selected sales activity to determine whether there are any significant differences between the market price used to value the security prior to sale and the actual sale price on an ad-hoc basis throughout the year. Prices provided by the pricing services that vary by more than 5% and $0.5 million from the expected price based on these assessment procedures are considered outliers, as are prices that have not changed from period to period and prices that have trended unusually compared to market conditions. In circumstances where the results of White Mountains’s review process does not appear to support the market price provided by the pricing services, White Mountains challenges the vendor provided price. If White Mountains cannot gain satisfactory evidence to support the challenged price, White Mountains will rely upon its own pricing methodologies to estimate the fair value of the security in question. The valuation process described above is generally applicable to all of White Mountains’s fixed maturity investments. The techniques and inputs specific to asset classes within White Mountains’s fixed maturity investments for Level 2 securities that use observable inputs are as follows: Debt Securities Issued by Corporations The fair value of debt securities issued by corporations is determined from a pricing evaluation technique that uses information from market sources and integrates relative credit information, observed market movements, and sector news. Key inputs include benchmark yields, reported trades, broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including sector, coupon, credit quality ratings, duration, credit enhancements, early redemption features and market research publications. Mortgage and Asset-Backed Securities The fair value of mortgage and asset-backed securities is determined from a pricing evaluation technique that uses information from market sources and leveraging similar securities. Key inputs include benchmark yields, reported trades, underlying tranche cash flow data, collateral performance, plus new issue data, as well as broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including issuer, vintage, loan type, collateral attributes, prepayment speeds, default rates, recovery rates, cash flow stress testing, credit quality ratings and market research publications. Municipal Obligations The fair value of municipal obligations is determined from a pricing evaluation technique that uses information from market makers, brokers-dealers, buy-side firms, and analysts along with general market information. Key inputs include benchmark yields, reported trades, issuer financial statements, material event notices and new issue data, as well as broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including type, coupon, credit quality ratings, duration, credit enhancements, geographic location and market research publications. Foreign Government, Agency and Provincial Obligations The fair value of foreign government, agency and provincial obligations is determined from a pricing evaluation technique that uses feeds from data sources in each respective country, including active market makers and inter-dealer brokers. Key inputs include benchmark yields, reported trades, broker-dealer quotes, two-sided markets, benchmark securities, bids, offers, local exchange prices, foreign exchange rates and reference data including coupon, credit quality ratings, duration and market research publications. Level 3 Measurements Fair value estimates for investments that trade infrequently and have few or no observable market prices are classified as Level 3 measurements. Investments valued using Level 3 fair value estimates are based upon unobservable inputs and include investments in certain fixed maturity investments, equity securities and other long-term investments where quoted market prices are unavailable or are not considered reasonable. Level 3 valuations are generated from techniques that use assumptions not observable in the market. These unobservable assumptions reflect White Mountains’s assumptions that market participants would use in valuing the investment. Generally, certain securities may start out as Level 3 when they are originally issued but as observable inputs become available in the market, they may be reclassified to Level 2. Transfers between levels are based on investments held as of the beginning of the period. Other Long-Term Investments White Mountains maintains a portfolio of other long-term investments that primarily consists of unconsolidated entities, private equity funds and hedge funds. White Mountains’s portfolio of other long-term investments are generally valued at fair value, using level 3 measurements or net asset value “NAV” as a practical expedient. Investments for which fair value is measured at NAV using the practical expedient are not classified within the fair value hierarchy. Unconsolidated Entities White Mountains’s portfolio of other long-term investments includes certain unconsolidated entities, including non-controlling interests in certain private common equity securities, limited liability companies and convertible preferred securities. White Mountains portfolio of unconsolidated entities are generally valued using level 3 inputs. The determination of the fair value of unconsolidated entities may involve significant management judgment, the use of valuation models and assumptions that are inherently subjective. On an ongoing basis, White Mountains considers qualitative changes in facts and circumstances which may impact the valuation of unconsolidated entities, including economic changes of relevant industries and changes to the investee capital structure, business strategy and significant changes in key personnel. On an annual basis, or when facts and circumstances suggest a quantitative valuation analysis is necessary, White Mountains, with the assistance of a third-party valuation firm, completes a valuation analysis of significant unconsolidated entities. One of the following fair value approaches is applied: Income Approach: The income approach converts future amounts to a single current discounted amount, based on expected future cash flows that a company will generate, along with expected proceeds from disposition. The cash flows are discounted to their present value using a discount rate that incorporates the risk-free rate for the use of funds, the expected rate of inflation, and risks associated with the particular investment. Market Approach: The market approach uses prices and other relevant information generated by market transactions involving identical or comparable businesses, such as recent sales or offerings of comparable assets. Cost Approach: The cost approach reflects the amount that would be required currently to replace the service capacity of an asset, or the current replacement cost. When applied to the valuation of equity interests in businesses, value is based on the net aggregate fair market value of the entity’s underlying individual assets. Private Equity Funds and Hedge Funds White Mountains’s portfolio of other long-term investments includes investments in private equity funds and hedge funds. White Mountains employs a number of procedures to assess the reasonableness of the fair value measurements for its private equity funds and hedge funds, including obtaining and reviewing periodic and audited annual financial statements as well as discussing each fund’s pricing with the fund manager throughout the year. However, since the fund managers do not provide sufficient information to evaluate the pricing methods and inputs for each underlying investment, White Mountains considers the inputs to be unobservable. The fair value of White Mountains’s private equity fund and hedge fund investments has generally been determined using the fund manager’s NAV. In the event that White Mountains believes the fair value of a private equity fund or hedge fund differs from the NAV reported by the fund manager due to illiquidity or other factors, White Mountains will adjust the reported NAV to more appropriately represent the fair value of its investment in the private equity fund or hedge fund. As of December 31, 2018 and 2017, White Mountains did not adjust the reported NAV of its investments in private equity funds and hedge funds. Derivatives Financial Instruments White Mountains holds from time to time a variety of derivative financial instruments for risk management purposes. White Mountains recognizes all derivatives as either other assets or other liabilities, aside from the foreign currency forward contracts which are recognized within other long-term investments, measured at fair value, in the consolidated balance sheets. Changes in the fair value of derivative instruments are recognized in current period pre-tax income. From time to time, White Mountains holds warrants that it has received in the restructuring of certain of its common equity securities and fixed maturity investments. White Mountains accounts for its investments in warrants as derivatives. Variable Annuity Reinsurance In 2016, White Mountains completed the run-off of WM Life Re as all of its contracts matured as of June 30, 2016. WM Life Re entered into agreements to reinsure death and living benefit guarantees associated with certain variable annuities in Japan. The accounting for benefit guarantees differs depending on whether or not the guarantee is classified as a derivative or an insurance liability. The liability for guaranteed minimum death benefits was classified as a derivative and measured at fair value. The liability for guaranteed minimum accumulation benefits was classified as an insurance liability, and was measured using assumptions for interest rates, equity markets, foreign exchange rates and market volatilities at the valuation date, as well as annuitant-related actuarial assumptions, including surrender and mortality rates. WM Life Re entered into derivative contracts that were designed to economically hedge against changes in the fair value of living and death benefit liabilities associated with its variable annuity reinsurance arrangements. All WM Life Re’s derivative financial instruments were recorded as assets or liabilities at fair value on the balance sheet within other assets. These derivative financial instruments did not meet the criteria for hedge accounting treatment, and accordingly, changes in fair value were recognized in the appropriate period as gains or losses in the income statement within other revenues. Receivables BAM’s receivables consist primarily of premiums receivable from customers for municipal bond insurance policies. NSM’s receivables consist of insurance premiums receivable from customers and commissions receivable from insurance carriers, net of a provision for amounts estimated to be uncollectible. MediaAlpha receivables consist of advertising fee receivables from publishers and advertisers. Incentive Compensation White Mountains’s Long-Term Incentive Plan (the “WTM Incentive Plan”) provides for grants of various types of share-based and non-share-based incentive awards to key employees of White Mountains. Non-share based awards are recognized over the related service periods based on management’s best estimate of the amounts at which the awards are expected to be paid. Share-based compensation which is typically settled in cash, such as performance shares or performance units, is classified as a liability-type award. The compensation cost for liability-classified awards is measured initially at the grant date fair value and remeasured each reporting period until settlement. The compensation cost for equity-classified awards expected to be settled in shares, such as options and restricted shares, is measured at the original grant date fair value of the award. The compensation cost for all awards is recognized for the vested portion of the awards over the related service periods. See Note 10 — “Employee Share-Based Incentive Compensation Plans”. Goodwill and Other Intangible Assets Goodwill represents the excess of the amount paid to acquire subsidiaries over the fair value of identifiable net assets at the date of acquisition. Other intangible assets consist primarily of trademarks, URL and online names, customer relationships, information technology and insurance licenses. Goodwill is not amortized, but rather is evaluated for impairment on an annual basis, or whenever indications of potential impairment exist. In the absence of any indications of potential impairment, the evaluation of goodwill is performed during the fourth quarter of each year. White Mountains initially evaluates goodwill using a qualitative approach (step zero) to determine whether it is more likely than not that the fair value of goodwill is greater than its carrying value. If the results of the qualitative evaluation indicate that it is more likely than not that the carrying value of goodwill exceeds its fair value, White Mountains performs the two-step quantitative test for impairment. Other intangible assets with finite lives are measured at their acquisition date fair values, are amortized over their economic lives and presented net of accumulated amortization on the balance sheet. Other intangible assets with finite lives are evaluated for impairment at least annually and when events or changes in circumstances indicate that it is more likely than not that the asset is impaired. A significant portion of the goodwill and other identifiable intangible assets at December 31, 2018 arose from White Mountains’s acquisition of NSM in May 2018 and NSM’s subsequent acquisitions of Fresh Insurance and KBK. White Mountains believes that all of the goodwill and intangible assets recorded in respect of these acquisitions is fully recoverable. Because of the timing of these acquisitions, all of which are still within the one-year measurement period following the acquisition date, these amounts have not yet been subject to White Mountains’s annual review for potential impairment. White Mountains evaluated the recoverability of goodwill and other intangible assets other than the portion associated with the NSM acquisitions and did not recognize any impairment losses for any of the years ended December 31, 2018, 2017 and 2016. See Note 4 — “Goodwill and Other Intangible Assets”. Municipal Bond Guarantee Insurance All of the contracts issued by BAM are accounted for as insurance contracts under ASC 944-605, Financial Guarantee Insurance Contracts. Premiums are generally received upfront and an unearned premium revenue liability, equal to the amount of the premium received, is established at contract inception. Installment premiums are measured at the present value of contractual premiums, discounted at the risk free rate, which is set at the inception of the insurance contract. Premium revenues are recognized in revenue over the period of the contracts in proportion to the amount of insurance protection provided using a constant rate. The constant rate is calculated based on the relationship between the par outstanding in a given reporting period compared with the sum of each of the par amounts outstanding for all periods. Deferred acquisition costs represent commissions, premium taxes, excise taxes and other costs which are directly attributable to and vary with the production of business. These costs are deferred and amortized to the extent they relate to successful contract acquisitions over the applicable premium recognition period as acquisition expenses. Deferred acquisition costs are limited to the amount expected to be recovered from future earned premiums and anticipated investment income. BAM’s obligation for outstanding contracts consists of the unearned premium reserve and any loss reserves. Loss reserves are recorded only to the extent that the present value of the expected amount of any losses to be paid, net of any expected recoveries, exceeds the associated unearned premium reserve. As of December 31, 2018 and 2017, BAM did not have any loss or LAE reserves. Revenue Recognition MediaAlpha recognizes advertising and publishing fee revenues based on the contractual amount of the fees, adjusted for any amounts expected to be refunded or uncollectible, when it has satisfied its contractual performance obligations, which is generally at the time each transaction is executed. For transactions where MediaAlpha acts as the principal, such as the Open exchange, revenue amounts are reported gross. For transactions where MediaAlpha acts as an agent facilitating transactions between third parties, revenue amounts are reported at the net fee billed. Agent and commission revenues are measured based on the contractual rates with insurance carriers, net of any amounts expected to be uncollectible and any amounts associated with expected policy cancellations adjustments, and are recognized when contractual performance obligations have been fulfilled. Cost of Sales and Broker Commission Expense MediaAlpha’s cost of sales consists primarily of revenue sharing payments to publisher partners and traffic acquisition costs to top tier search engines. Cost of sales are measured based on contract terms and recognized when the related revenue transactions are executed. NSM’s broker commission expense consists of commissions paid to sub-agents and brokers. Broker commission expense is measured in accordance with contractual terms and recognized when incurred, which is generally at the policy issuance date. Other Operations’s cost of sales consists of salaries and related expenses, professional services and marketing and advertising expenses directly related to sales generation. These expenses are recognized as incurred. Federal and Foreign Income Taxes A number of White Mountains’s subsidiaries file consolidated tax returns in the United States. Income earned or losses generated by companies outside the United States are generally subject to an overall effective tax rate lower than that imposed by the United States. Deferred tax assets and liabilities are recorded when a difference between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts for tax purposes exists, and for other temporary differences. The deferred tax asset or liability is recorded based on tax rates expected to be in effect when the difference reverses. The deferred tax asset is recognized when it is more likely than not that it will be realized. Foreign Currency Exchange The functional currency for White Mountains’s non-U.S. based subsidiaries are measured, in most instances, using functional currencies other than the U.S. dollar. Net foreign exchange gains and losses arising from the translation of functional currencies are generally reported in shareholders’ equity, in accumulated other comprehensive income or loss. White Mountains also invests in securities denominated in foreign currencies. Assets and liabilities recorded in these foreign currencies are translated into U.S. dollars at exchange rates in effect at the balance sheet date, and revenues and expenses are converted using the weighted average exchange rates for the period. As of December 31, 2018 and 2017, White Mountains had unrealized foreign currency translation losses of $3.9 million and $1.3 million recorded in accumulated other comprehensive income on its consolidated balance sheet. Non-controlling Interests Non-controlling interests consist of the ownership interests of non-controlling shareholders in consolidated subsidiaries, and are presented separately on the balance sheet. The portion of comprehensive income attributable to non-controlling interests is presented net of related income taxes in the statement of operations and comprehensive income. See Note 11 — “Common Shareholders’ Equity and Non-controlling Interests”. 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 the 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 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 $9.2 million of statutory withholding tax payments made in connection with the settlement of restricted shares as financing cash flows for the year ended December 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 that 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. 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. Adoption of ASU 2017-01 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 Financial Accounting Standards Board (“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. The new guidance is effective for annual periods beginning after December 15, 2018. White Mountains does not expect adoption to have any effect on its financial statements. Goodwill In January 2017, the FASB issue ASU 2017-04, Simplifying the Test for Goodwill Impairment (ASC 350), which changes the guidance on goodwill impairment testing. Under the new guidance, the qualitative assessment of the recoverability of goodwill remains the same. However, the second step required under the existing guidance has been eliminated. Goodwill is considered impaired if the carrying value exceeds the estimated fair value. The new guidance is effective for fiscal years beginning after December 15, 2019. 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 loans and trade receivables such as premium receivables and reinsurance recoverables on paid losses. 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 liabilities will be measured initially based on the present value of the lease payments. The lease asset will be measured as the sum of the lease liabilities, adjusted for initial direct costs. White Mountains will be adopting the new guidance effective January 1, 2019, and will use the optional transition method that permits prospective adoption with recognition of a cumulative effect adjustment to the opening balance of retained earnings. White Mountains will not apply the new guidance to comparative periods presented in the year of adoption. White Mountains plans to elect available practical expedients permitted under ASC 842, which will allow White Mountains to carry forward its historical lease classification and not reassess leases for the definition of a lease under the new guidance. White Mountains has estimated that it will recognize lease right-of-use assets and lease liabilities of approximately $24.5 million. White Mountains does not expect the cumulative effect adjustment to opening retained earnings to be material. Financial Instruments - Recognition and Measurement In January 2016, the FASB issued ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities (ASC 825-10). The new ASU 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. ASU 2016-01 is effective for fiscal years beginning after December 15, 2017, with early adoption permitted. 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.

Significant Transactions

12 Months Ended

Dec. 31, 2018

Significant Transactions [Abstract]

Significant Transactions

Acquisitions

NSM

On May 11, 2018, White Mountains closed its acquisition of 95% of NSM’s equity for cash consideration of $274.2 million. During the third quarter of 2018, White Mountains recorded a purchase price adjustment of $2.1 million. White Mountains recognized total assets acquired related to NSM of $495.2 million, including $383.0 million of goodwill and other intangible assets, total liabilities assumed of $204.6 million, including contingent consideration earnout liabilities related to NSM’s previous acquisitions of its U.K.-based operations, of $10.2 million, and non-controlling interest of $14.4 million reflecting acquisition date fair values. In connection with the acquisition, White Mountains incurred transaction costs of $6.3 million, which were expensed in 2018.

On May 18, 2018, NSM acquired 100% of Fresh Insurance Group Limited (“Fresh Insurance”) for upfront cash consideration of $49.6 million. During the third quarter of 2018, NSM recorded a purchase price adjustment of $0.7 million. The purchase price is subject to additional adjustments based upon growth in EBITDA during two earnout periods ending in February 2020 and February 2022. NSM recognized total assets acquired related to Fresh Insurance of $72.6 million, including $54.6 million of goodwill and other intangible assets, and total liabilities assumed of $22.3 million, reflecting acquisition date fair values. In connection with the acquisition, NSM recorded a contingent consideration earnout liability of $7.5 million.

On December 3, 2018, NSM acquired all the net assets of KBK for upfront cash consideration of $60.0 million and recognized $59.4 million of goodwill and other intangible assets, reflecting acquisition date fair values. The relative fair values of goodwill and other intangible assets recognized in connection with the acquisitions of KBK had not yet been determined at December 31, 2018. The purchase price is subject to additional adjustments based upon growth in EBITDA during three earnout periods ending in December 2019, December 2020 and December 2021. In connection to the KBK acquisition, NSM expects to record a contingent consideration earnout liability in the first quarter of 2019. White Mountains acquired 16,694,869 additional newly-issued units of NSM for $29 million in connection with NSM’s acquisition of KBK.

The contingent consideration earnout liabilities related to these acquisitions are subject to adjustment based upon EBITDA, EBITDA projections, and present value factors for acquired entities. For the period from May 11, 2018 through December 31, 2018, NSM recognized pre-tax expense of $2.7 million for the change in the fair value of its contingent consideration earnout liabilities for Fresh Insurance and its other U.K.-based operations. Any future adjustments to contingent consideration earnout liabilities under the agreements will also be recognized through pre-tax income. As of December 31, 2018, NSM recorded contingent consideration earnout liabilities of $20.2 million.

DavidShield

On January 24, 2018, White Mountains acquired 50% of DavidShield, its joint venture partner in 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 reorganized 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 $125.0 million in Kudu, a capital provider to asset management and wealth management firms. As of December 31, 2018, White Mountains owned 49.5% of Kudu. Kudu specializes in providing capital solutions to asset managers and registered investment advisers for purposes including generational ownership transfers, management buyouts, acquisition and growth finance and legacy partner liquidity. Kudu also provides strategic assistance to investees from time to time. Kudu’s capital solutions typically are structured as long-term or permanent revenue shares.

Kudu closed its first three transactions in 2018 deploying $63.0 million of capital in total, of which $31.5 million was from White Mountains. White Mountains has determined that Kudu is a VIE, but that White Mountains is not the primary beneficiary. White Mountains has elected to take the fair value option for its investment in Kudu. As of December 31, 2018, White Mountains had funded $31.5 million of its $125 million commitment, plus an additional $3.8 million for working capital.

On February 14, 2019, White Mountains entered into a definitive agreement to buy all of the interests in Kudu held by certain funds managed by Oaktree Capital Management, L.P. (“Oaktree”). See Note 20 — “Subsequent Events”.

Buzz

On August 4, 2016, White Mountains acquired a 70.9% ownership share in Buzz for a purchase price of British Pound Sterling (“GBP”) 6.1 million (approximately $8.1 million based upon the foreign exchange spot rate at the date of acquisition). White Mountains recognized total assets acquired related to Buzz of $11.5 million, including $7.6 million of goodwill and $1.1 million of other intangible assets, and total liabilities assumed of $0.1 million, reflecting acquisition date fair values.

On August 1, 2017, White Mountains acquired 37,409 newly-issued preferred shares of Buzz for GBP 4.0 million (approximately $5.0 million based upon the foreign exchange spot rate at the date of acquisition) and 5,808 common shares from the company founders for GBP 0.5 million (approximately $0.7 million based upon the spot rate at the date of acquisition). As of December 31, 2018 and 2017, White Mountains’s ownership share in Buzz was 77.1%.

MediaAlpha

On January 15, 2016, MediaAlpha acquired certain assets from Oversee.net for an aggregate purchase price of $3.9 million. The majority of assets acquired, which are included in other intangible assets, consists of customer relationships, a customer contract, a non-compete agreement from the seller, domain names and technology.

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, consists 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 December 31, 2018 and 2017, White Mountains’s ownership share in MediaAlpha was 61.0% and 64.4%.

Dispositions

OneBeacon

On September 28, 2017, White Mountains received $1.3 billion in cash proceeds from the OneBeacon Transaction and recorded a net gain of $554.6 million, net of transaction costs, and other comprehensive income of $2.9 million. 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 19 — “Held for Sale and Discontinued Operations”.

Tranzact

On July 21, 2016, White Mountains completed the sale of Tranzact to an affiliate of Clayton, Dubilier & Rice, LLC and received net proceeds of $221.3 million. In connection with the sale of Tranzact, the purchaser directly repaid $56.3 million for the portion of Tranzact’s debt attributable to White Mountains’s common shareholders. On October 5, 2016, White Mountains received additional proceeds of $1.2 million following the release of the post-closing purchase price adjustment escrow.

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. See Note 6 — “Income Taxes”. The increase to White Mountains’s book value from the sale of Tranzact was $82.1 million.

The following table presents a reconciliation of the gain reported in discontinued operations to the impact to White Mountains’s book value:



Millions	Year ended December 31, 2016
Gain from sale of Tranzact reported in discontinued operations	$	51.9
Add back reclassification from continuing operations for the release of a tax valuation allowance	30.2
Increase to White Mountains’s book value from sale of Tranzact	$	82.1

Through July 21, 2016, Tranzact’s results of operations are reported as discontinued operations and assets and liabilities held for sale within White Mountains’s GAAP financial statements. See Note 19 — “Held for Sale and Discontinued Operations”.

During 2017, White Mountains recorded a $3.2 million increase to the gain from sale of Tranzact in discontinued operations as a result of a change in state tax expense.

Sirius Group

On April 18, 2016, White Mountains completed the sale of Sirius Group to CMI for approximately $2.6 billion. Of this amount, $161.8 million was used to purchase certain assets retained by White Mountains out of Sirius Group, including shares of OneBeacon. The amount paid at closing was based on an estimate of Sirius Group’s closing date tangible common shareholder’s equity. During the third quarter of 2016, there was a final true-up to Sirius Group’s tangible common shareholder’s equity that resulted in a $4.0 million reduction to the gain. During 2016, White Mountains recorded $363.2 million of gain from sale of Sirius Group in discontinued operations and $113.3 million in other comprehensive income from discontinued operations from Sirius Group.

During 2017, White Mountains recorded a $0.7 million reduction to the 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.

During 2018, White Mountains recorded a loss of $17.3 million within net (loss) gain on sale of discontinued operations for a contingency related to the sale of Sirius Group. See Note 18 — “Commitments and Contingencies”.

Through April 18, 2016, Sirius Group’s results are reported as discontinued operations and assets and liabilities held for sale within White Mountains’s GAAP financial statements. See Note 19 — “Held for Sale and Discontinued Operations”.

Investment Securities

12 Months Ended

Dec. 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 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 December 31, 2018 and 2017.

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

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 2018, 2017 and 2016:



	Year Ended December 31,
Millions	2018			2017			2016
Fixed maturity investments	$	35.1		$	44.9		$	28.5
Short-term investments	8.0			1.8			.9
Common equity securities	15.1			10.6			4.0
Other long-term investments	3.8			1.2			1.1
Total investment income	62.0			58.5			34.5
Third-party investment expenses	(3.0		)	(2.5		)	(2.4		)
Net investment income, pre-tax	$	59.0		$	56.0		$	32.1

Net Realized and Unrealized Investment Gains (Losses)

The following table presents net realized and unrealized investment gains (losses) for 2018, 2017 and 2016:



	Year Ended December 31,
Millions	2018			2017			2016
Net realized investment (losses) gains, pre-tax	$	(12.9	)	$	24.1		$	270.0
Net unrealized investment (losses) gains, pre-tax	(95.4		)	109.2			(297.4		)
Net realized and unrealized investment (losses) gains, pre-tax	(108.3		)	133.3			(27.4		)
Income tax benefit (expense) attributable to net realized and unrealized investment (losses) gains	18.2			(12.9		)	2.7
Net realized and unrealized investment (losses) gains, after-tax	$	(90.1	)	$	120.4		$	(24.7	)

For the years ended December 31, 2018, 2017 and 2016, all of White Mountains’s net realized and unrealized investment gains (losses) were recorded in the statements of operations. There were no investments gains (losses) recorded in other comprehensive income.

Net Realized Investment Gains (Losses)

The following tables present net realized investment gains (losses) for 2018, 2017 and 2016:



	Year Ended December 31, 2018
Millions	Net Realized (Losses) Gains			Net Foreign Exchange Gains (Losses)			Total Net Realized (Losses) Gains Reflected in Earnings
Fixed maturity investments	$	(29.8	)	$	18.2		$	(11.6	)
Short-term investments	(.8		)	—			(.8		)
Common equity securities	6.6			—			6.6
Other long-term investments	.1			(7.2		)	(7.1		)
Net realized investment (losses) gains, pre-tax	(23.9		)	11.0			(12.9		)
Income tax benefit attributable to net realized investment (losses) gains	9.1			—			9.1
Net realized investment (losses) gains, after-tax	$	(14.8	)	$	11.0		$	(3.8	)



	Year Ended December 31, 2017
Millions	Net Realized (Losses) Gains			Net Foreign Exchange Gains (Losses)			Total Net Realized Gains (Losses) Reflected in Earnings
Fixed maturity investments	$	(1.6	)	$	4.1		$	2.5
Short-term investments	(.3		)	—			(.3		)
Common equity securities	18.1			6.0			24.1
Other long-term investments	19.1			(21.3		)	(2.2		)
Net realized investment gains (losses), pre-tax	35.3			(11.2		)	24.1
Income tax expense attributable to net realized investment gains (losses)	(8.9		)	—			(8.9		)
Net realized investment gains (losses), after-tax	$	26.4		$	(11.2	)	$	15.2



	Year Ended December 31, 2016
Millions	Net Realized (Losses) Gains			Net Foreign Exchange Gains		Total Net Realized (Losses) Gains Reflected in Earnings
Fixed maturity investments	$	(1.9	)	$	.3	$	(1.6	)
Short-term investments	.4			—		.4
Common equity securities	268.5			—		268.5
Other long-term investments	2.7			—		2.7
Net realized investment gains, pre-tax	269.7			.3		270.0
Income tax expense attributable to net realized investment gains	(45.6		)	—		(45.6		)
Net realized investment gains, after-tax	$	224.1		$	.3	$	224.4

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 years ended 2018, 2017 and 2016:



	Year Ended December 31, 2018
Millions	Net Unrealized (Losses) Gains			Net Foreign Exchange (Losses) Gains			Total Net Unrealized (Losses) Gains Reflected in Earnings
Fixed maturity investments	$	(8.3	)	$	(14.8	)	$	(23.1	)
Common equity securities	(105.5		)	—			(105.5		)
Other long-term investments	30.2			3.0			33.2
Net unrealized investment losses, pre-tax	(83.6		)	(11.8		)	(95.4		)
Income tax benefit attributable to net unrealized investment losses	9.1			—			9.1
Net unrealized investment losses, after-tax	$	(74.5	)	$	(11.8	)	$	(86.3	)



	Year Ended December 31, 2017
Millions	Net Unrealized Gains (Losses)			Net Foreign Exchange Gains (Losses)			Total Net Unrealized Gains (Losses) Reflected in Earnings
Fixed maturity investments	$	13.8		$	12.7		$	26.5
Common equity securities	99.3			—			99.3
Other long-term investments	(15.6		)	(1.0		)	(16.6		)
Net unrealized investment gains, pre-tax	97.5			11.7			109.2
Income tax expense attributable to net unrealized investment gains	(4.0		)	—			(4.0		)
Net unrealized investment gains, after-tax	$	93.5		$	11.7		$	105.2



	Year Ended December 31, 2016
Millions	Net Unrealized Losses			Net Foreign Exchange Gains (Losses)			Total Net Unrealized Losses Reflected in Earnings
Fixed maturity investments	$	(14.6	)	$	2.1		$	(12.5	)
Common equity securities	(257.4		)	(3.3		)	(260.7		)
Other long-term investments	(22.7		)	(1.5		)	(24.2		)
Net unrealized investment losses, pre-tax	(294.7		)	(2.7		)	(297.4		)
Income tax benefit attributable to net unrealized investment losses	48.3			—			48.3
Net unrealized investment losses, after-tax	$	(246.4	)	$	(2.7	)	$	(249.1	)

White Mountains recognized gross realized investment gains of $49.4 million, $61.5 million and $283.7 million and gross realized investment losses of $62.3 million, $37.4 million and $13.7 million on sales of investment securities during 2018, 2017 and 2016.

The following table presents total gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments for the years ended December 31, 2018, 2017 and 2016:



	Year Ended December 31,
Millions	2018		2017			2016
Fixed maturity investments	$	—	$	—		$	0.1
Other long-term investments	22.6		(15.4		)	(14.3		)
Total net unrealized investment gains (losses), pre-tax - Level 3 investments	$	22.6	$	(15.4	)	$	(14.2	)

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 December 31, 2018 and 2017.



	December 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	$	154.0	$	.1	$	(.9	)	$	—	$	153.2
Debt securities issued by corporations	519.0		1.0		(9.5		)	—		510.5
Municipal obligations	279.0		2.4		(1.1		)	—		280.3
Mortgage and asset-backed securities	136.1		.1		(2.7		)	—		133.5
Total fixed maturity investments	$	1,088.1	$	3.6	$	(14.2	)	$	—	$	1,077.5



	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 weighted average duration of White Mountains’s fixed income portfolio was approximately 3.4 years when including short-term investments and approximately 4.0 years when excluding short-term investments as of December 31, 2018.

The following table presents the cost or amortized cost and carrying value of White Mountains’s fixed maturity investments by contractual maturity as of December 31, 2018. Actual maturities could differ from contractual maturities because borrowers may have the right to call or prepay certain obligations with or without call or prepayment penalties.



	December 31, 2018
Millions	Cost or Amortized Cost		Carrying Value
Due in one year or less	$	84.8	$	84.4
Due after one year through five years	500.8		496.2
Due after five years through ten years	222.4		218.0
Due after ten years	144.0		145.4
Mortgage and asset-backed securities	136.1		133.5
Total	$	1,088.1	$	1,077.5

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 December 31, 2018 and 2017:



	December 31, 2018
Millions	Cost or Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Net Foreign Currency Losses			Carrying Value
Common equity securities	$	904.7	$	51.0	$	(30.1	)	$	—		$	925.6
Other long-term investments	$	330.3	$	52.2	$	(54.9	)	$	(2.0	)	$	325.6



	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

Proceeds from the sales and maturities of investments, excluding short-term investments, totaled $2.2 billion, $2.8 billion and $3.7 billion for the years ended December 31, 2018, 2017 and 2016.

Investments Held on Deposit or as Collateral

As of December 31, 2018 and 2017, investments of $254.3 million and $204.6 million, were held in trusts required to be maintained in relation to HG Global’s reinsurance agreements with BAM. White Mountains’s insurance subsidiaries are required to maintain deposits with certain insurance regulatory agencies in order to maintain their insurance licenses. The fair value of such deposits, which represent BAM’s state deposits and are included within the investment portfolio, totaled $6.1 million and $6.0 million as of December 31, 2018 and 2017.

Fair Value Measurements as of December 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 December 31, 2018 and 2017, White Mountains used quoted market prices or other observable inputs to determine fair value for approximately 87% and 94% of the investment portfolio. See Note 1 — “Basis of Presentation and Significant Accounting Policies”.

Fair Value Measurements by Level

The following tables present White Mountains’s fair value measurements for investments as of December 31, 2018 and 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.



	December 31, 2018
Millions	Fair Value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	153.2	$	153.2	$	—	$	—
Debt securities issued by corporations:
Financials	143.4		—		143.4		—
Consumer	68.5		—		68.5		—
Technology	60.5		—		60.5		—
Energy	57.6		—		57.6		—
Healthcare	55.0		—		55.0		—
Industrial	47.6		—		47.6		—
Communications	31.8		—		31.8		—
Materials	26.3		—		26.3		—
Utilities	19.8		—		19.8		—
Total debt securities issued by corporations	510.5		—		510.5		—

Mortgage and asset-backed securities	133.5		—		133.5		—
Municipal obligations	280.3		—		280.3		—
Total fixed maturity investments	1,077.5		153.2		924.3		—

Short-term investments ⁽¹⁾	214.2		204.4		9.8		—

Common equity securities:

Exchange traded funds ⁽²⁾	675.3		617.0		58.3		—
Healthcare	14.0		14.0		—		—
Financials	13.5		13.5		—		—
Communications	12.7		12.7		—		—
Industrial	11.4		11.4		—		—
Technology	7.4		7.4		—		—
Consumer	6.2		6.2		—		—
Energy	4.1		4.1		—		—
Materials	3.1		3.1		—		—
Other ⁽³⁾	177.9		—		177.9		—
Total common equity securities	925.6		689.4		236.2		—
Other long-term investments	138.7		—		—		138.7
Other long-term investments — NAV⁽⁴⁾	186.9		—		—		—
Total investments	$	2,542.9	$	1,047.0	$	1,170.3	$	138.7

⁽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⁾ Consists of unconsolidated entities, private equity funds and one hedge fund for which fair value is measured at NAV using the practical expedient. Investments for which fair value is measured at NAV are not classified within the fair value hierarchy.



	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		—
Healthcare	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		—
Healthcare	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⁾	77.2		—		—		77.2
Other long-term investments — NAV⁽⁵⁾	135.3		—		—		—
Total investments	$	3,384.4	$	1,041.4	$	2,130.5	$	77.2

⁽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⁾Consists of unconsolidated entities, private equity funds and one hedge funds for which fair value is measured at NAV using the practical expedient. Investments for which fair value is measured at NAV are not classified within the fair value hierarchy.

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 December 31, 2018 and 2017:



	Fair Value at December 31,
Millions	2018		2017
AAA	$	8.9	$	1.6
AA	88.7		42.6
A	270.5		192.5
BBB	142.4		465.2
BB	—		161.7
B	—		17.3
Debt securities issued by corporations ⁽¹⁾	$	510.5	$	880.9


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

Mortgage and Asset-backed Securities

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



	December 31, 2018						December 31, 2017
Millions	Fair Value		Level 2		Level 3		Fair Value		Level 2		Level 3
Mortgage-backed securities:
Agency:
FNMA	$	53.6	$	53.6	$	—	$	84.5	$	84.5	$	—
FHLMC	38.1		38.1		—		62.0		62.0		—
GNMA	23.7		23.7		—		46.3		46.3		—
Total agency ⁽¹⁾	115.4		115.4		—		192.8		192.8		—
Non-agency:
Commercial	—		—		—		70.5		70.5		—
Total non-agency	—		—		—		70.5		70.5		—

Total mortgage-backed securities	115.4		115.4		—		263.3		263.3		—
Other asset-backed securities:
Vehicle receivables	9.2		9.2		—		142.4		142.4		—
Credit card receivables	8.9		8.9		—		206.0		206.0		—
Other	—		—		—		83.0		83.0		—
Total other asset-backed securities	18.1		18.1		—		431.4		431.4		—
Total mortgage and asset-backed securities	$	133.5	$	133.5	$	—	$	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).

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 December 31, 2018, White Mountains did not hold any mortgage-backed securities 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 December 31, 2018, White Mountains did not hold any mortgage-backed securities classified as non-prime.

Other Long-Term Investments

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



	Carrying Value at December 31,
Millions	2018		2017
PassportCard/DavidShield ⁽¹⁾	$	75.0	$	21.0
Kudu	30.7		—
Other unconsolidated entities ⁽¹⁾⁽²⁾	60.0		62.2
Total unconsolidated entities⁽¹⁾⁽²⁾	165.7		83.2
Private equity funds and hedge funds	146.1		125.3
Foreign currency forward contracts	—		(3.7		)
Other	13.8		4.0
Total other long-term investments	$	325.6	$	208.8

⁽¹⁾See Fair Value Measurements by Level table.

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

Private Equity Funds and Hedge Funds

White Mountains invests in private equity funds and hedge funds, which are included in other long-term investments. The fair value of these investments is generally estimated using the NAV of the funds. As of December 31, 2018, White Mountains held investments in 12 private equity funds and one hedge fund. The largest investment in a single fund was $54.3 million as of December 31, 2018 and $54.9 million as of December 31, 2017.

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



	December 31, 2018				December 31, 2017
Millions	Fair Value		Unfunded Commitments		Fair Value		Unfunded Commitments

Private equity funds
Manufacturing/Industrial	$	42.9	$	10.5	$	43.3	$	10.4
Aerospace/Defense/Government	27.6		34.9		15.8		12.9
Direct lending	13.0		17.7		7.1		23.1
Financial services	8.3		13.6		4.2		11.7
Total private equity funds	91.8		76.7		70.4		58.1

Hedge funds
Long/short banks and financial	54.3		—		54.9		—
Total hedge funds	54.3		—		54.9		—
Total private equity funds and hedge funds included in other long-term investments	$	146.1	$	76.7	$	125.3	$	58.1

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

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



Millions	1 – 3 years		3 – 5 years		5 – 10 years		>10 years		Total
Private equity funds — expected lock-up period remaining	$	1.8	$	5.5	$	63.2	$	21.3	$	91.8

Investors in private equity funds are generally subject to indemnification obligations outside of the capital commitment period and prior to the winding up of the fund. As of December 31, 2018 and 2017, White Mountains is not aware of any indemnification claims relating to its investments in private equity funds.

Redemption of investments in most hedge funds is subject to restrictions including lock-up periods where no redemptions or withdrawals are allowed, restrictions on redemption frequency and advance notice periods for redemptions. Amounts requested for redemptions remain subject to market fluctuations until the redemption effective date, which generally falls at the end of the defined redemption period. As of December 31, 2018, White Mountains held one active hedge fund with a fair value of $54.3 million. The hedge fund is subject to a semi-annual restriction on redemptions and an advance notice period requirement of 45 days.

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 December 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 years ended December 31, 2018 and 2017:



							Level 3 Investments		Unconsolidated Entities, Private Equity Funds and Hedge Funds Measured at NAV ⁽³⁾
Millions	Level 1 Investments			Level 2 Investments			Other Long-term Investments					Total
Balance at December 31, 2017	$	890.4		$	2,105.4		$	77.2	$	135.3		$	3,208.3		⁽¹⁾⁽²⁾
Net realized and unrealized (losses) gains	(64.9		)	(64.4		)	16.2		13.3			(99.8		)	⁽⁴⁾
Amortization/accretion	.2			(2.7		)	—		—			(2.5		)
Purchases	514.7			783.8			45.3		50.5			1,394.3
Sales	(497.8		)	(1,661.6		)	—		(12.2		)	(2,171.6		)
Transfers in	—			—			—		—			—
Transfers out	—			—			—		—			—
Balance at December 31, 2018	$	842.6		$	1,160.5		$	138.7	$	186.9		$	2,328.7		⁽²⁾


⁽¹⁾	Excludes carrying value of $(3.7) as of December 31, 2017 associated with foreign currency forward contracts.


⁽²⁾	Excludes carrying value of $214.2 and $176.1 as of December 31, 2018 and 2017 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 $0.8 for the year ended December 31, 2018.



							Level 3 Investments						Unconsolidated Entities, Private Equity Funds and Hedge Funds Measured at NAV (3)
Millions	Level 1 Investments			Level 2 Investments			Fixed Maturity Investments			Other Long- term Investments						Total
Balance at December 31, 2016	$	279.5		$	2,093.8		$	—		$	91.4		$	82.6		$	2,547.3		^(1)(2)(5)
Net realized and unrealized gains (losses)	82.7			69.6			—			(15.3		)	20.4			157.4			⁽⁴⁾
Amortization/accretion	—			(9.1		)	—			—			—			(9.1		)
Purchases	1,209.3			2,007.9			31.2			3.1			81.0			3,332.5
Sales	(681.1		)	(2,070.3		)	(12.5		)	(2.0		)	(48.7		)	(2,814.6		)
Deconsolidation of SSIE	—			(5.2		)	—			—			—			(5.2		)
Transfers in	—			18.7			—			—			—			18.7
Transfers out	—			—			(18.7		)	—			—			(18.7		)
Balance at December 31, 2017	$	890.4		$	2,105.4		$	—		$	77.2		$	135.3		$	3,208.3		⁽¹⁾⁽²⁾

⁽¹⁾Excludes carrying value of $(3.7) and $(1.2) as of December 31, 2017 and 2016 associated with foreign currency forward contracts.

⁽²⁾Excludes carrying value of $176.1 and $175.0 as of December 31, 2017 and 2016 classified as short-term investments, of which $0.1 is classified as held for sale at December 31, 2016.


⁽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 and short-term investments of $23.8 and $0.3 for the year ended December 31, 2017.

⁽⁵⁾ Includes carrying value of $6.6 of fixed maturity investments at December 31, 2016 that is classified as assets held for sale related to SSIE.

Fair Value Measurements — Transfers Between Levels - For Years Ended December 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 2018, there were no fixed maturity investments or other long-term investments classified as Level 3 measurements in the prior period that were transferred to Level 2 measurements.

During 2017, three fixed maturity investments classified as a Level 3 measurement in the prior period were transferred to Level 2 measurement because quoted market prices for similar securities that were considered reliable and could be validated against an alternative source were available as of December 31, 2017. These measurements comprise “Transfers out” of Level 3 and “Transfers in” to Level 2 of $18.7 million for the period ended December 31, 2017.

Significant Unobservable Inputs

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



$ in Millions, Except Share Price	December 31, 2018
Description	Valuation Technique(s)	Fair Value ⁽¹⁾	Unobservable Input
PassportCard/DavidShield	Discounted cash flow	$75.0	Discount rate	-	18.0%
			Exit multiple	-	1.00
Compare.com	Discounted cash flow	$16.9	Discount rate	-	22.0%
			Exit multiple	-	2.75
YOUSURE Tarifvergleich GmbH (“durchblicker”)	Discounted cash flow	$15.5	Discount rate	-	23.0%
			Exit multiple	-	2.25
Captricity, Inc.	Discounted cash flow	$14.5	Discount rate	-	23.0%
			Exit multiple	-	3.75
Galvanic Applied Sciences	Multiple of EBITDA	$3.1	EBITDA multiple	-	6.00
Private debt instrument	Discounted cash flow	$10.0	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
PassportCard	Discounted cash flow	$21.0	Discount rate	-	25.0%
			Exit multiple	-	1.00
Compare.com	Discounted cash flow	$22.1	Discount rate	-	35.0%
			Exit multiple	-	1.75
durchblicker	Discounted cash flow	$11.3	Discount rate	-	21.0%
			Exit multiple	-	1.75
Captricity, Inc.	Discounted cash flow	$14.5	Discount rate	-	30.0%
			Exit multiple	-	3.50
Galvanic Applied Sciences	Multiple of EBITDA	$0.6	EBITDA multiple	-	6.00
OneTitle Holdings LLC	Share price of most recent transaction	$3.6	Share price	-	$2.52

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

Goodwill and Other Intangible Assets

12 Months Ended

Dec. 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 economic lives, acquisition date values, accumulated amortization and net carrying values for other intangible assets and goodwill, by company:



$ in Millions	Weighted Average Economic Life (in Years)	December 31, 2018							December 31, 2017
$ in Millions	Weighted Average Economic Life (in Years)	Acquisition Date Fair Value		Accumulated Amortization		Net Carrying Value			Acquisition Date Fair Value		Accumulated Amortization		Net Carrying Value
Goodwill:
NSM ⁽¹⁾⁽²⁾	N/A	$	354.3	$	—	$	354.3		$	—	$	—	$	—
MediaAlpha	N/A	18.3		—		18.3			18.3		—		18.3
Buzz ⁽³⁾	N/A	7.3		—		7.3			7.6		—		7.6
Total goodwill		379.9		—		379.9			25.9		—		25.9

Other intangible assets:
NSM ⁽¹⁾
Customer relationships	9	85.3		6.0		79.3			—		—		—
Trade names	20	51.2		1.8		49.4			—		—		—
Information technology	5	3.7		.5		3.2			—		—		—
Subtotal		140.2		8.3		131.9			—		—		—

MediaAlpha
Customer relationships	9	26.8		4.9		21.9			26.8		2.4		24.4
Information technology	5	33.3		30.9		2.4			33.3		24.3		9.0
Other	3	9.8		9.0		.8			9.8		7.8		2.0
Subtotal		69.9		44.8		25.1			69.9		34.5		35.4

Buzz
Trademark	7	.6		.2		.4			.6		.1		.5
Information technology	5	.5		.3		.2			.5		.2		.3
Subtotal		1.1		.5		.6			1.1		.3		.8

Total other intangible assets		211.2		53.6		157.6			71.0		34.8		36.2
Total goodwill and other intangible assets		$	591.1	$	53.6	$	537.5		$	96.9	$	34.8	$	62.1

Goodwill and other intangible assets attributed to non-controlling interests						(40.6		)					(21.1		)
Goodwill and other intangible assets included in White Mountains’s common shareholders’ equity						$	496.9						$	41.0

⁽¹⁾Includes the effect of foreign currency translation from the date of acquisition of $(2.2) for goodwill and $(0.7) for other intangible assets.

⁽²⁾The relative fair values of goodwill and other intangible assets recognized in connection with the acquisition of KBK had not yet been determined at December 31, 2018. See Note 2 — “Significant Transactions”.

⁽³⁾Includes the effect of foreign currency translation from the date of acquisition of $(0.3) for goodwill.

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

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

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

The with-or-without method estimates the fair value of another intangible asset that provides an incremental benefit. Under this method, the fair value of the other intangible asset is calculated by comparing the value of the entity with and without the other intangible asset. This approach was used to estimate the fair value of favorable lease terms.

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



	December 31,
	2018						2017
Millions	Goodwill			Other Intangible Assets			Goodwill		Other Intangible Assets
Beginning balance	$	25.9		$	36.2		$	25.9	$	19.3
Acquisitions of businesses ⁽¹⁾	356.5			140.9			—		—
Acquisitions of asset groups⁽²⁾	—			—			—		27.6
Foreign currency translation	(2.5		)	(.7		)	—		—
Acquisitions of other intangible assets	—			—			—		—
Amortization	—			(18.8		)	—		(10.7		)
Ending balance	$	379.9		$	157.6		$	25.9	$	36.2

⁽¹⁾During 2018, amounts include acquisitions related to NSM, Fresh Insurance and KBK. See Note 2 — “Significant Transactions”.

⁽²⁾During 2017, amounts include certain assets associated with the Health, Life and Medicare insurance business of Healthplans.com for an aggregate purchase price of $28.0. See Note 2 — “Significant Transactions”.

Amortization expense was $18.8 million, $10.7 million and $10.5 million for the years ended December 31, 2018, 2017 and 2016.

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



Millions	Amortization Expense
2019	$	18.1
2020	16.0
2021	16.0
2022	15.8
2023 and years after	89.5
Total	$	155.4

Debt

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Debt

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



	December 31,			Effective	December 31,			Effective
Millions	2018			Rate ⁽¹⁾	2017			Rate ⁽¹⁾
WTM Bank Facility	$	—		N/A	$	—		N/A
NSM Bank Facility	180.4			7.4%	—
Unamortized issuance cost	(3.8		)		—
NSM Bank Facility, carrying value	176.6			—	—
MediaAlpha Bank Facility	14.3			7.1%	23.9			5.6%
Unamortized issuance cost	(.1		)		(.1		)
MediaAlpha Bank Facility, carrying value	14.2				23.8
Other NSM debt, carrying value	1.9				—
Total debt	$	192.7			$	23.8

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

The following table presents a schedule of contractual repayments of White Mountains’s debt as of December 31, 2018:



Millions	December 31, 2018
Due in one year or less	$	5.2
Due in two to three years	10.4
Due in four to five years	9.4
Due after five years	171.9
Total	$	196.9

WTM Bank Facility

White Mountains had a revolving credit facility with a syndicate of lenders administered by Wells Fargo Bank, N.A., which had a total commitment of $425.0 million (the “WTM Bank Facility”). White Mountains terminated the WTM Bank Facility on May 8, 2018.

White Mountains recorded $0.3 million, $0.6 million, and $1.2 million of interest expense on the WTM Bank Facility for the years ended December 31, 2018, 2017 and 2016.

NSM Bank Facility

On May 11, 2018, NSM entered into a secured credit facility (the “NSM Bank Facility”) with Ares Capital Corporation in order to refinance NSM’s existing debt and to fund the acquisitions of subsidiaries. The NSM Bank Facility is comprised of a term loan of $100.0 million, two delayed-draw term loans of $51.0 million, to fund the Fresh Insurance acquisition, and $30.1 million, to fund the KBK acquisition, and a revolving credit loan commitment of $10.0 million, under which NSM initially borrowed $2.0 million. The term loans under the NSM Bank Facility mature on May 11, 2024, and the revolving loan under the NSM Bank Facility matures on May 11, 2023. During the period from May 11, 2018 through December 31, 2018, NSM repaid $0.8 million on the term loans and $2.0 million on the revolving credit loan. As of December 31, 2018, $180.4 million of the term loans were outstanding and the revolving credit loan was undrawn.

Interest on the NSM Bank Facility accrues at a floating interest rate equal to the three month LIBOR or the Prime Rate, as published by the Wall Street Journal plus, in each case, an applicable margin. The margin over LIBOR may vary between 4.25% and 4.75%, and the margin over the Prime Rate may vary between 3.25% and 3.75%, in each case, depending on the consolidated total leverage ratio of the borrower.

On June 15, 2018, NSM entered into an interest rate swap agreement to hedge its exposure to interest rate risk on $151.0 million of its variable rate term loans. Under the terms of the swap agreement, NSM pays a fixed rate of 2.97% and receives a variable rate, which is reset monthly, based on the then-current LIBOR. As of December 31, 2018, the variable rate received by NSM under the swap agreement was 2.52%. As of December 31, 2018, the effective interest rate for the outstanding term loans of $150.2 million that are hedged by the swap was 7.47%. The effective interest on the outstanding term loans of $30.0 million that are unhedged was 6.85%. The effective interest rate on the total outstanding term loans under the NSM Bank Facility of $180.4 million was 7.42%. See Note 7 — “Derivatives — NSM Interest Rate Swap”.

NSM recorded $8.0 million of interest expense on the NSM Bank Facility for the 2018 ownership period.

The NSM Bank Facility is secured by all property of the loan parties and contains various affirmative, negative and financial covenants that White Mountains considers to be customary for such borrowings, including a maximum consolidated total leverage ratio covenant.

MediaAlpha Bank Facility

On May 12, 2017, MediaAlpha entered into a secured credit facility (the “MediaAlpha Bank Facility”) with Western Alliance Bank, which had a total commitment of $20.0 million and had 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 $14.3 million as of December 31, 2018, and a revolving loan facility for $10.0 million, which was undrawn as of December 31, 2018. The MediaAlpha Bank Facility replaced MediaAlpha’s previous credit facility with Opus Bank, which had a total commitment of $20.0 million.

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 December 31, 2018.

During 2018, under the MediaAlpha Bank Facility, MediaAlpha repaid $3.6 million on the term loan and borrowed $3.0 million and repaid $9.0 million on the revolving loan. In 2017, under the MediaAlpha Bank Facility, MediaAlpha borrowed $20.0 million and repaid $2.1 million on the term loan and borrowed $6.0 million on the revolving loan. During 2017, under the previous MediaAlpha Bank Facility, MediaAlpha repaid $12.9 million.

MediaAlpha recorded $1.2 million, $1.0 million, $0.9 million of interest expense on the MediaAlpha Bank Facility for the years ended December 31, 2018, 2017 and 2016.

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.

Other NSM Debt

On December 12, 2016, in connection with the acquisition of a wholly-owned subsidiary, NSM assumed a secured term loan facility with Ageas Insurance Limited, which has a maturity date of May 11, 2024. As of December 31, 2018, the secured term loan facility has an outstanding balance of $2.2 million. The carrying value of the debt includes a purchase accounting adjustment of $(0.3) million to reflect the debt at its acquisition date fair value. The $(0.3) million is being amortized over the remaining term of the loan.

Debt Covenants

As of December 31, 2018, White Mountains was in compliance with all of the covenants under all of its debt facilities.

Interest

Total interest expense incurred by White Mountains for its indebtedness was $9.5 million, $2.3 million and $3.0 million for the periods ended December 31, 2018, 2017 and 2016. Total interest paid by White Mountains for its indebtedness was $8.8 million, $1.4 million, and $2.1 million for the periods ended December 31, 2018, 2017 and 2016.

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

The Company and its Bermuda domiciled subsidiaries are not subject to Bermuda income tax under current Bermuda law. In the event 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, Ireland, Israel, Luxembourg, the United Kingdom and the United States.

The following table presents the total income tax benefit for the years ended December 31, 2018, 2017 and 2016:



	Year Ended December 31,
Millions	2018			2017			2016
Current income tax (expense) benefit:
U.S. federal	$	(.1	)	$	(.3	)	$	21.4
State	(1.4		)	(1.3		)	(.7		)
Non-U.S.	(2.9		)	(2.0		)	(.3		)
Total current income tax (expense) benefit	(4.4		)	(3.6		)	20.4
Deferred income tax benefit:
U.S. federal	8.3			11.4			12.5
Non-U.S.	.1			—			—
Total deferred income tax benefit	8.4			11.4			12.5
Total income tax benefit	$	4.0		$	7.8		$	32.9

Effective Rate Reconciliation

The following table presents a reconciliation of taxes calculated for 2018 using the 21% U.S. federal statutory rate and for 2017 and 2016 using the 35% U.S. federal statutory rate (the tax rate at which the majority of White Mountains’s worldwide operations are taxed) to the income tax benefit (expense) on pre-tax (loss) income:



	Year Ended December 31,
Millions	2018			2017			2016
Tax benefit (expense) at the U.S. statutory rate	$	37.4		$	(2.7	)	$	51.6
Differences in taxes resulting from:
Change in valuation allowance	(31.0		)	42.6			6.9
State taxes	4.0			.6			(1.2		)
Non-U.S. earnings, net of foreign taxes	(2.9		)	21.5			(19.2		)
Withholding tax	(2.7		)	(2.0		)	(.2		)
Member’s surplus contributions (“MSC”)	(2.6		)	(3.0		)	(2.3		)
Tax rate changes	1.7			(44.3		)	(3.9		)
Tax reserve adjustments	(.8		)	(.3		)	—
Tax exempt interest and dividends	.6			.5			.1
Officer compensation	—			(4.1		)	—
Other, net	.3			(1.0		)	1.1
Total income tax benefit on pre-tax (loss) income	$	4.0		$	7.8		$	32.9

The non-U.S. component of pre-tax (loss) income was $(30.1) million, $71.3 million and $(66.3) million for the years ended December 31, 2018, 2017 and 2016.

Tax Payments and Receipts

Net income tax payments to national governments (primarily the United States) totaled $3.5 million, $2.0 million, and $0.3 million for the years ended December 31, 2018, 2017 and 2016.

Deferred Tax Assets and Liabilities

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

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



	December 31,
Millions	2018		2017
Deferred tax assets related to:
U.S. federal and state net operating and capital loss carryforwards	$	95.5	$	73.0
Non-U.S. net operating loss carryforwards	36.6		33.9
Incentive compensation	14.1		20.4
Investment basis difference	11.1		4.9
Net unrealized investment losses	9.5		—
Tax credit carryforwards	4.2		1.3
Deferred acquisition costs	3.5		2.0
Other items	5.8		1.6
Total gross deferred tax assets	180.3		137.1
Less: valuation allowances	139.9		109.6
Total net deferred tax assets	40.4		27.5
Deferred tax liabilities related to:
MSC	32.8		24.1
Purchase accounting	4.2		.2
Net unrealized investment gains	—		1.0
Other items	1.2		.9
Total deferred tax liabilities	38.2		26.2
Net deferred tax asset	$	2.2	$	1.3

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

Tax Cuts and Jobs Act of 2017

On December 22, 2017, the Tax Cuts and Jobs Act of 2017 (the “TCJA”) was enacted. Substantially all of the provisions of the TCJA are effective for taxable years beginning after December 31, 2017. The TCJA includes significant changes to the Internal Revenue Code of 1986 (the “Code”), including amendments which significantly change the taxation of individuals and business entities. The more significant changes in the TCJA that impact White Mountains are reductions in the corporate federal income tax rate from 35% to 21% and several technical provisions including, among others, limiting the utilization of net operating losses arising after December 31, 2017 to 80% of taxable income with an indefinite carryforward.

The TCJA did not have a material impact on White Mountains’s financial statements in 2017 due to a full valuation allowance previously having been recorded against its U.S. deferred tax assets. Under U.S. GAAP, specifically ASC Topic 740, Income Taxes, the tax effects of changes in tax laws must be recognized in the period in which the law is enacted, or December 22, 2017 for the TCJA. ASC 740 also requires deferred tax assets and liabilities to be measured at the enacted tax rate expected to apply when temporary differences are to be realized or settled. Thus, at the date of enactment, White Mountains’s deferred taxes were re-measured based upon the new tax rate. For White Mountains, a change in deferred taxes was recorded as an adjustment to our deferred tax provision for $43.1 million of federal tax expense, which was offset by a change in the valuation allowance.

The staff of the U.S. Securities and Exchange Commission has recognized the complexity of reflecting the impacts of the TCJA and on December 22, 2017 issued guidance in Staff Accounting Bulletin 118 (“SAB 118”) which clarifies accounting for income taxes under ASC 740 if information is not yet available or complete and provides for up to a one-year period in which to complete the required analyses and accounting (the measurement period). SAB 118 describes three scenarios associated with a company’s status of accounting for income tax reform: (1) a company is complete with its accounting for certain effects of tax reform, (2) a company is able to determine a reasonable estimate for certain effects of tax reform and records that estimate as a provisional amount, or (3) a company is not able to determine a reasonable estimate and therefore continues to apply ASC 740, based on the provisions of the tax laws that were in effect immediately prior to the TCJA being enacted. White Mountains completed its accounting for the effects of the TCJA, which have been reflected in the December 31, 2017 financial statements.

Valuation Allowance

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

Of the $139.9 million valuation allowance as of December 31, 2018, $102.6 million related to deferred tax assets on net operating losses in U.S. subsidiaries and other federal and state deferred tax benefits, $21.1 million related to deferred tax assets on net operating losses and net investment unrealized gains and losses in Luxembourg subsidiaries, $14.5 million related to net operating losses and other deferred tax benefits in Israeli subsidiaries and $1.7 million related to net operating losses and other deferred tax benefits in U.K. subsidiaries. Of the $109.6 million valuation allowance as of December 31, 2017, $74.8 million related to deferred tax assets on net operating losses in U.S. subsidiaries and other federal and state deferred tax benefits, $20.3 million related to deferred tax assets on net operating losses and net investment unrealized gains and losses in Luxembourg subsidiaries, $13.5 million related to net operating losses in Israeli subsidiaries and $1.0 million related to net operating losses in a U.K. subsidiary.

United States

During 2018 and 2017, White Mountains recorded income tax expense (benefit) of $22.7 million to establish and $(21.5) million to release a valuation allowance against deferred tax assets of Guilford Holdings, Inc. and subsidiaries (“Guilford”). Guilford consists of MediaAlpha, various service companies and certain investments and other assets that are included in the Other Operations segment. The TCJA reduced the U.S. federal income tax rate from 35% to 21%, which reduced the deferred tax assets of Guilford by $20.4 million. In 2017, White Mountains recorded a $20.4 million tax expense for the reduction, which was offset with a tax benefit due to the release in the valuation allowance against the deferred tax assets. During 2018 and 2017, Guilford continued to have a full valuation allowance recorded against its deferred tax assets as White Mountains management is unsure it will generate sufficient taxable income to utilize the deferred tax assets.

During 2018 and 2017, White Mountains recorded income tax expense (benefit) of $1.5 million to establish and $(18.4) million to release a valuation allowance against deferred tax assets of BAM. The reduction in the U.S. federal income tax rate under the TCJA reduced the deferred tax assets of BAM by $22.7 million. In 2017, White Mountains recorded a $22.7 million income tax expense for the reduction, which was offset with an income tax benefit due to the release in the valuation allowance against the deferred tax assets. Also during 2018 and 2017, BAM had income in equity that was available to offset its loss from continuing operations. As a result, BAM recorded an income tax benefit of $8.7 million and $10.1 million, in continuing operations, with an offsetting tax expense in paid-in surplus. During 2018 and 2017, BAM continued to have a full valuation allowance recorded against its deferred tax assets as White Mountains management is unsure it will generate sufficient taxable income to utilize the deferred tax assets.

During 2018, White Mountains recorded income tax expense of $2.8 million to establish a valuation allowance against a deferred tax asset related to foreign tax credits at White Mountains Catskill Holdings, Inc. as White Mountains management is unsure it will generate sufficient taxable income to utilize the deferred tax asset.

Non-U.S. Jurisdictions

During 2018, White Mountains recorded income tax expense of $1.2 million to establish a full valuation allowance against deferred tax assets which primarily related to unrealized losses on investments held in Luxembourg-domiciled subsidiaries.

During 2017, White Mountains recorded an income tax benefit of $6.4 million in Luxembourg to reduce a full valuation allowance against deferred tax assets due to the recapture of previously deducted losses offset by a loss on the write down of Wobi.

During 2018 and 2017, White Mountains recorded income tax expense of $2.1 million and $3.0 million to establish a full valuation allowance against deferred tax assets at certain Israel-domiciled subsidiaries, as White Mountains management does not currently anticipate sufficient taxable income to utilize the deferred tax assets.

During 2018 and 2017, White Mountains recorded income tax expense of $0.7 million and $0.7 million to establish a full valuation allowance against deferred tax assets at certain U.K.-domiciled subsidiaries, as White Mountains management does not currently anticipate sufficient taxable income to utilize the deferred tax assets.

Net Operating Loss and Capital Loss Carryforwards

The following table presents net operating loss and capital loss carryforwards as of December 31, 2018, the expiration dates and the deferred tax assets thereon:



	December 31, 2018
Millions	United States			Luxembourg			United Kingdom			Israel			Total
2019-2023	$	.4		$	—		$	—		$	—		$	.4
2024-2028	—			—			—			—			—
2029-2038	342.8			47.0			—			—			389.8
No expiration date	81.5			29.8			14.9			61.1			187.3
Total	$	424.7		$	76.8		$	14.9		$	61.1		$	577.5
Gross deferred tax asset	89.7			20.0			2.6			14.0			126.3
Valuation allowance	(87.3		)	(20.0		)	(1.8		)	(14.0		)	(123.1		)
Net deferred tax asset	$	2.4		$	—		$	.8		$	—		$	3.2

Included in the U.S. net operating loss carryforwards are losses of $3.7 million subject to an annual limitation on utilization under Internal Revenue Code Section 382. These loss carryforwards will begin to expire in 2032. Also included in the U.S. net operating loss carryforwards are losses of $6.8 million due to additional deductions related to equity compensation. These loss carryforwards will begin to expire in 2032. As of December 31, 2018, there are U.S. alternative minimum tax credit carryforwards of $0.7 million, which are refundable under provisions of the TCJA.

Uncertain Tax Positions

Recognition of the benefit of a given tax position is based upon whether a company determines that it is more likely than not that a tax position will be sustained upon examination based upon the technical merits of the position. In evaluating the more-likely-than-not recognition threshold, White Mountains must presume that the tax position will be subject to examination by a taxing authority with full knowledge of all relevant information. If the recognition threshold is met, then the tax position is measured at the largest amount of benefit that is more than 50% likely of being realized upon ultimate settlement. There were no uncertain tax positions for the year ended December 31, 2016.

The following table presents a reconciliation of the beginning and ending amount of unrecognized tax benefits for 2018 and 2017:



Millions	Permanent Differences ⁽¹⁾		Temporary Differences ⁽²⁾		Interest and Penalties ⁽³⁾		Total
Balance at January 1, 2017	$	—	$	—	$	—	$	—
Changes in prior year tax positions	.1		—		—		.1
Tax positions taken during the current year	.2		—		—		.2
Balance at December 31, 2017	.3		—		—		.3
Changes in prior year tax positions	.8		—		—		.8
Balance at December 31, 2018	$	1.1	$	—	$	—	$	1.1


⁽¹⁾	Represents the amount of unrecognized tax benefits that, if recognized, would impact the effective tax rate.


⁽²⁾	Represents the amount of unrecognized tax benefits that, if recognized, would create a temporary difference between the reported amount of an item in White Mountains’s Consolidated Balance Sheet and its tax basis.


⁽³⁾	Net of tax benefit.

White Mountains classifies all interest and penalties on unrecognized tax benefits as part of income tax expense. During the years ended December 31, 2018, 2017 and 2016, White Mountains did not recognize any net interest (income) expense. There was no accrued interest as of December 31, 2018 and December 31, 2017.

Tax Examinations

With few exceptions, White Mountains is no longer subject to U.S. federal, state, or non-U.S. income tax examinations by tax authorities for years before 2013.

In the second quarter of 2016, White Mountains recorded an increase in deferred tax assets of $0.6 million and a corresponding increase in valuation allowance of $0.6 million related to the settlement of the IRS audit of Guilford for tax year 2012.

In the first quarter of 2018, the Israeli Tax Authority commenced an examination of the 2013 to 2016 income tax returns for Wobi. White Mountains does not expect the resolution of this examination to result in a material change to its financial condition, results of operations and cash flows.

Derivatives

12 Months Ended

Dec. 31, 2018

Derivative [Line Items]

Derivatives

Forward Contracts

Derivative [Line Items]

Derivatives

Foreign Currency Forward Contracts

White Mountains’s investment portfolio includes investments denominated in GBP, Euros, Japanese Yen and other foreign currencies. White Mountains previously entered into foreign currency forward contracts to manage its foreign currency exposure related to certain of these investments. The foreign currency forward contracts did not meet the criteria to be accounted for as a hedge. Mismatches between currency driven movements in foreign denominated investments and foreign currency forward contracts may result in net foreign currency positions being outside pre-defined ranges and/or may result in net foreign currency gains (losses). White Mountains’s foreign currency forward contracts were traded over-the-counter. The fair value of the foreign currency forward contracts were estimated using OTC quotes for similar instruments and accordingly, the measurements were classified as Level 2 measurements.

During the fourth quarter of 2017, White Mountains closed the foreign currency forward contracts associated with certain non-U.S. common equity securities. In conjunction with the liquidation of the GBP investment grade corporate bond mandate in the first quarter of 2018, White Mountains closed the associated foreign currency forward contract.

As of December 31, 2018, White Mountains no longer has any open foreign currency forward contracts. As of December 31, 2017, White Mountains held $206.3 million (GBP 152.0 million) total gross notional value of a foreign currency forward contract.

The derivative (losses) recognized in net realized and unrealized investment gains (losses) for the years ended

December 31, 2018, 2017 and 2016 were $(3.5) million, $(23.8) million and $(1.2) million. White Mountains’s foreign currency forward contracts were subject to a master netting agreement. As of December 31, 2017 and 2016, the gross liability amount offset under the master netting agreement and the net amount recognized in other long-term investments was $(3.7) million and $(1.2) million.

White Mountains did not hold or provide any collateral under its foreign currency forward contract.

The following table presents the gross notional amounts and carrying values associated with the foreign currency forward contracts as of December 31, 2017:



	December 31, 2017
Millions	Notional Amount		Carrying Value			Standard & Poor’s Rating ⁽¹⁾
Barclays Bank PLC	$	206.3	$	(3.7	)	A

⁽¹⁾“A” is the sixth highest of 23 credit ratings assigned by Standard & Poor’s.

Variable Annuity Reinsurance

Derivative [Line Items]

Derivatives

Variable Annuity Reinsurance

White Mountains entered into agreements to reinsure death and living benefit guarantees associated with certain variable annuities in Japan. During the third quarter of 2015, the variable annuity contracts reinsured by WM Life Re began to mature and were fully runoff by June 30, 2016. The reinsurance agreement was commuted in December 2016. WM Life Re was liquidated in the third quarter of 2017.

The following table presents the pre-tax operating results of WM Life Re for the year ended December 31, 2016:



Millions	Year Ended December 31, 2016
Fees, included in other revenue	$	1.2
Change in fair value of variable annuity liability, included in other revenue	(.3		)
Change in fair value of derivatives, included in other revenue	(2.0		)
Foreign exchange, included in other revenue	1.3
Total revenues	.2
Death benefit claims paid, included in general and administrative expenses	(.3		)
General and administrative expenses	(2.6		)
Pre-tax loss	$	(2.7	)

The following table presents realized and unrealized derivative gains (losses) recognized in other revenue for the year ended December 31, 2016 by type of instrument:



Millions	Gains (Losses) Year Ended December 31, 2016
Fixed income/interest rate	$	1.8
Foreign exchange	(4.8		)
Equity	1.0
Total	$	(2.0	)

The following tables present the changes in White Mountains’s variable annuity reinsurance liabilities and derivative instruments for the year ended December 31, 2016:



	Variable Annuity Liabilities			Derivative Instruments
Millions	Level 3			Level 3 ⁽¹⁾			Level 2 ⁽¹⁾⁽²⁾			Level 1 ⁽³⁾			Total
Balance at January 1, 2016	$	.3		$	2.7		$	16.5		$	.9		$	20.1
Purchases	—			—			—			—			—
Realized and unrealized (losses) gains	(.3		)	2.9			(.7		)	(4.2		)	(2.0		)
Transfers in	—			—			—			—			—
Sales/settlements	—			(5.6		)	(15.8		)	3.3			(18.1		)
Balance at December 31, 2016	$	—		$	—		$	—		$	—		$	—


⁽¹⁾	Consists of over-the-counter instruments.


⁽²⁾	Consists of interest rate swaps, total return swaps, foreign currency forward contracts, and bond forwards. Fair value measurement based upon bid/ask pricing quotes for similar instruments that are actively traded, where available. Swaps for which an active market does not exist have been priced using observable inputs including the swap curve and the underlying bond index.


⁽³⁾	Consists of exchange traded equity index, foreign currency and interest rate futures. Fair value measurements based upon quoted prices for identical instruments that are actively traded.

All of White Mountains’s variable annuity reinsurance liabilities were classified as Level 3 measurements. The fair value of White Mountains’s variable annuity reinsurance liabilities were estimated using actuarial and capital market assumptions related to the projected discounted cash flows over the term of the reinsurance agreement. Actuarial assumptions regarding future policyholder behavior, including surrender and lapse rates, were generally unobservable inputs and significantly impacted the fair value estimates. White Mountains used derivative instruments to mitigate the risks associated with changes in the fair value of the reinsured variable annuity guarantees. The types of inputs used to estimate the fair value of these derivative instruments, with the exception of actuarial assumptions regarding policyholder behavior and risk margins, were generally the same as those used to estimate the fair value of variable annuity liabilities.

Municipal Bond Guarantee Insurance

12 Months Ended

Dec. 31, 2018

Insurance [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 the initial capitalization of BAM, a newly formed mutual municipal bond insurer. As of December 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 protection up to 15%-of-par outstanding on each municipal bond insured by BAM. For capital appreciation bonds, par is adjusted to the estimated equivalent par value for current interest paying bonds. In return, BAM cedes 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 under the FLRT are limited to the assets in two collateral trusts: a Regulation 114 Trust and a supplemental collateral trust (the “Supplemental Trust” and together with the Regulation 114 Trust, the “Collateral Trusts”). Losses required to be reimbursed under the FLRT are subject to an aggregate limit equal to the assets held in the Collateral Trusts at any point in time.

Effective January 1, 2014, HG Global and BAM agreed to change the interest rate on the BAM Surplus Notes for the five years ending December 31, 2018 from a fixed rate of 8.0% to a variable rate equal to the one-year U.S. treasury rate plus 300 basis points, set annually, which was 3.78%, 4.60% and 5.70% for 2017, 2018 and 2019. In 2018, BAM exercised its 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 to the extent that its remaining qualified statutory capital and other capital resources continue to support its outstanding obligations, its business plan and its “AA/stable” rating from Standard & Poor’s. BAM repaid $17.7 million of the BAM Surplus Notes and $5.3 million of accrued interest during the year ended December 31, 2018. BAM repaid $4.0 million of the BAM Surplus Notes and $1.0 million of accrued interest during the year ended December 31, 2017.

At inception, the Supplemental Trust contained the original $300 million of Series B Notes and $100 million of cash and fixed income securities. During 2017, in order to further support BAM’s long-term capital position and business prospects, HG Global agreed to contribute the original $203.0 million of Series A Notes into the Supplemental Trust. At the same time 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 NYDFS approved the change during 2017. In connection with the contribution and change in payment terms of the Series B Notes, the Series A Notes were merged into the Series B Notes to become the Series S-1 Surplus Notes.

On December 3, 2018, the Series S-1 Surplus Notes were exchanged for Series S-2 Surplus Notes, which reflect all of the unpaid principal and accrued interest from the Series S-1 Surplus Notes. The Series S-2 Surplus Notes are held in an HG Re sponsored vehicle within the Supplemental Trust.

The Regulation 114 Trust target balance is equal to gross ceded unearned premiums and unpaid ceded loss and LAE expenses, if any. The Supplemental Trust target balance is equal to $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 excess funds can be distributed out of the Supplemental Trust.

Under GAAP, if the terms of a debt instrument are amended, unless there is a greater than 10% change in the expected discounted future cash flows of such instrument, a change in the instrument’s carrying value is not permitted. White Mountains has determined that the impact of the changes made during 2017 to the terms of the BAM Surplus Notes on the expected discounted future cash flows was not greater than 10%.

As of December 31, 2018 and 2017, the Collateral Trusts held assets of $757.4 million and $715.1 million, which included $481.3 million and $499.0 million of BAM Surplus Notes. As of December 31, 2018 and 2017, HG Global has accrued $143.7 million and $126.0 million of interest receivable on the BAM Surplus Notes.

The following table presents a schedule of BAM’s insured obligations as of December 31, 2018 and 2017:



	December 31, 2018		December 31, 2017
Contracts outstanding	7,525		6,371
Remaining weighted average contract period (in years)	10.7		10.9
Contractual debt service outstanding (in millions):
Principal	$	52,201.6	$	42,090.6
Interest and capital appreciation	26,560.3		21,057.1
Total debt service outstanding	$	78,761.9	$	63,147.7

Gross unearned insurance premiums	$	176.0	$	136.8

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



Millions	December 31, 2018
January 1, 2019 - March 31, 2019	$	4.1
April 1, 2019 - June 30, 2019	4.0
July 1, 2019 - September 30, 2019	4.0
October 1, 2019 - December 31, 2019	3.8
	15.9
2020	15.0
2021	14.0
2022	13.2
2023	12.3
2024 and thereafter	105.6
Total gross unearned insurance premiums	$	176.0

The following table presents a schedule of net written premiums and net earned premiums included in White Mountains’s HG Global/BAM segment for the years ended December 31, 2018, 2017 and 2016:



Millions	December 31, 2018		December 31, 2017		December 31, 2016
Written premiums:
Direct	$	44.8	$	63.2	$	38.6
Assumed	8.1		—		—
Net written premiums	$	52.9	$	63.2	$	38.6
Earned premiums:
Direct	$	13.6	$	9.4	$	5.9
Assumed	.3		—		—
Net earned premiums	$	13.9	$	9.4	$	5.9

In April 2018, BAM entered into a collateralized financial guarantee excess of loss reinsurance agreement with Fidus Re, Ltd. (“Fidus Re”), a Bermuda based special purpose insurer created solely to provide reinsurance protection to BAM. Fidus Re was capitalized by the issuance of $100.0 million of insurance linked securities. The proceeds from issuance were placed in a collateral trust supporting Fidus Re’s obligations to BAM. The insurance linked securities were issued by Fidus Re with an initial term of twelve years and are callable five years after the date of issuance. Under the agreement, BAM retains the first $165.0 million of aggregate losses, before giving effect to HG’s reinsurance coverage, on the ceded business. Fidus Re reinsures 90% of aggregate losses exceeding $165.0 million on a portion of BAM’s financial guarantee portfolio up to a total reimbursement of $100 million. The aggregate loss limit under the agreement is $276.1 million. The agreement is accounted for using deposit accounting and any related financing expenses are recorded in general and administrative expenses as the agreement does not meet the risk transfer requirements necessary to be accounted for as reinsurance.

In November 2018, BAM entered into a 100% quota share facultative reinsurance agreement under which it assumed a portfolio of municipal bond guarantee contracts with a par value of $2.2 billion. None of the contracts assumed were non-performing and no loss reserves have been established for any of the contracts, either at the transaction date or subsequent thereto. The agreement, which covers future claims exposure only, meets the risk transfer criteria under ASC 944-20, Insurance Activities and accordingly has been accounted for as reinsurance.

Earnings Per Share

12 Months Ended

Dec. 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 years ended December 31, 2018, 2017 and 2016. See Note 19 — “Held for Sale and Discontinued Operations”.



	Year Ended December 31,
	2018			2017			2016
Basic and diluted earnings per share numerators (in millions):
Net (loss) income attributable to White Mountains’s common shareholders	$	(141.2	)	$	627.2		$	401.8
Less: total (loss) income from discontinued operations, net of tax	(17.2		)	577.5			523.4
Net (loss) income from continuing operations attributable to White Mountains’s common shareholders	(124.0		)	49.7			(121.6		)
Allocation of earnings (losses) to participating restricted common shares ⁽¹⁾	1.4			(.7		)	1.5
Basic and diluted (losses) earnings per share numerators	$	(122.6	)	$	49.0		$	(120.1	)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	$	3,382.5		$	4,293.8		$	5,014.9
Average unvested restricted common shares ⁽²⁾	(40.1		)	(54.3		)	(64.8		)
Basic (losses) earnings per share denominator	$	3,342.4		$	4,239.5		$	4,950.1
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	$	3,382.5		$	4,293.8		$	5,018.1
Average unvested restricted common shares ⁽²⁾	(40.1		)	(54.3		)	(64.8		)
Diluted (losses) earnings per share denominator ⁽³⁾	$	3,342.4		$	4,239.5		$	4,953.3
Basic and diluted earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid	$	1.00		$	1.00		$	1.00
Undistributed (losses) earnings	$	(37.67	)	$	10.56		$	(25.26	)
Basic and diluted (losses) earnings per share	$	(36.67	)	$	11.56		$	(24.26	)


⁽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”.


⁽³⁾	The diluted earnings (loss) per share denominator for the year ended December 31, 2016, includes the impact of 40,000 common shares issuable upon exercise of the non-qualified options outstanding, which resulted in 3,217 incremental shares outstanding over the period.

The following table presents the undistributed net earnings (losses) from continuing operations for the years ended December 31, 2018, 2017 and 2016. See Note 19 — “Held for Sale and Discontinued Operations”.



	Year Ended December 31,
Millions		2018			2017			2016
Undistributed net earnings - continuing operations:
Net (loss) income attributable to White Mountains’s common shareholders, net of restricted common share amounts		$	(122.6	)	$	49.0		$	(120.1	)
Dividends declared, net of restricted common share amounts ⁽¹⁾		(3.7		)	(4.5		)	(5.4		)
Total undistributed net (losses) earnings, net of restricted common share amounts		$	(126.3	)	$	44.5		$	(125.5	)

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

Employee Share-Based Incentive Compensation Plans

12 Months Ended

Dec. 31, 2018

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

Employee Share-Based Incentive Compensation Plans

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

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 and directors of White Mountains. The WTM Incentive Plan was adopted by the Board, was approved by the Company’s sole shareholder in 1985 and was subsequently amended by its shareholders in 1995, 2001, 2003, 2005, 2010 and 2013. Share-based incentive awards that may be granted under the plan include performance shares, restricted shares, incentive stock options and non-qualified stock options (“Non-Qualified Options”).

Performance Shares

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

The following table presents performance share activity for the years ended December 31, 2018, 2017 and 2016 for performance shares granted under the WTM Incentive Plan:



	Year Ended December 31,
	2018					2017					2016
$ in Millions	Target Performance Shares Outstanding		Accrued Expense			Target Performance Shares Outstanding		Accrued Expense			Target Performance Shares Outstanding		Accrued Expense
Beginning of period	50,515		$	45.8		80,353		$	42.4		93,654		$	57.7
Shares paid or expired ⁽¹⁾	(23,186	)	(28.4		)	(30,838	)	(21.9		)	(36,294	)	(41.0		)
New grants	14,105		—			17,710		—			22,615		—
Forfeitures ⁽²⁾	(818	)	.1			(16,710	)	(9.3		)	378		.5
Expense recognized	—		14.2			—		34.6			—		25.2
End of period ⁽³⁾	40,616		$	31.7		50,515		$	45.8		80,353		$	42.4


⁽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. WTM performance shares payments in 2016 for the 2013-2015 performance cycle ranged from 140% to 142% of target.


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


⁽³⁾	Outstanding performance share awards as of December 31, 2018, 2017 and 2016 exclude 0, 2,195 and 7,315 unvested performance shares awards for employees of discontinued operations.

For the 2015-2017 and 2014-2016 performance cycle, all performance shares earned were settled in cash. For the performance shares earned in the 2013-2015 performance cycle, the Company issued 5,000 common shares and settled the remainder in cash. If the outstanding performance shares had vested on December 31, 2018, the total additional compensation cost to be recognized would have been $13.8 million, based on accrual factors as of December 31, 2018 (common share price and payout assumptions).

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



$ in Millions	Target Performance Shares Outstanding		Accrued Expense
Performance cycle:
2018 – 2020	13,450		$	3.8
2017 – 2019	14,070		11.9
2016 – 2018	13,715		16.5
Sub-total	41,235		32.2
Assumed forfeitures	(619	)	(.5		)
Total	40,616		$	31.7

For the 2018-2020 performance cycle, the targeted performance goal for full payment of outstanding performance shares granted under the WTM Incentive Plan is 6% average growth in adjusted book value per share and intrinsic value per share. Average growth of 2% or less would result in no payout and average growth of 10% or more would result in a payout of 200%.

For the 2017-2019 performance cycle, the targeted performance goal for full payment of outstanding performance shares granted under the WTM Incentive Plan to non-investment personnel is 5% average growth in adjusted book value per share and intrinsic value per share. Average growth of 1% or less would result in no payout and average growth of 9% or more would result in a payout of 200%.

For the 2016-2018 performance cycle, the targeted performance goal for full payment of outstanding performance shares granted under the WTM Incentive Plan to non-investment personnel is 4% average growth in adjusted book value per share and intrinsic value per share. Average growth of 0% or less would result in no payout and average growth of 8% or more would result in a payout of 200%.

For investment personnel, for the periods ended December 31, 2016 and 2017, the targeted performance goals for full payment of outstanding performance shares granted under the WTM Incentive Plan are based one-third on average growth in adjusted book value per share and intrinsic value per share (as described above) and two-thirds on achieving a total return on invested assets as measured against metrics based on the 10-year U.S. Treasury Note. For periods after December 31, 2017, the targeted performance goals for full payment of outstanding performance shares granted under the WTM Incentive Plan are based on the same performance goals described above for non-investment personnel.

Restricted Shares

Restricted shares are grants of a specified number of common shares that generally vest at the end of a three-year service period. The following table presents the unrecognized compensation cost associated with the outstanding restricted share awards under the WTM Incentive Plan for the years ended December 31, 2018, 2017 and 2016:



	Year Ended December 31,
	2018					2017					2016
$ in Millions	Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value
Non-vested,
Beginning of period	53,755		$	14.3		70,620		$	19.7		70,675		$	15.7
Issued	14,105		11.4			17,985		16.3			25,365		20.2
Vested	(25,381	)	—			(28,846	)	—			(24,620	)	—
Forfeited	(969	)	(.2		)	(6,004	)	(3.5		)	(800	)	(.3		)
Expense recognized	—		(13.0		)	—		(18.2		)	—		(15.9		)
End of period ⁽¹⁾	41,510		$	12.5		53,755		$	14.3		70,620		$	19.7


⁽¹⁾	Outstanding restricted share awards as of December 31, 2018, 2017 and 2016 include 0, 2,195, and 5,235 unvested restricted shares for employees of Sirius Group.

During 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 2017, White Mountains issued 17,485 restricted shares that vest on January 1, 2020, 250 restricted shares that vest on January 1, 2019 and 250 restricted shares that vest on January 1, 2018. During 2016, White Mountains issued 24,615 restricted shares that vest on January 1, 2019 and 750 restricted shares that vest on January 1, 2018. The unrecognized compensation cost as of December 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 had been 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. During 2016, 5,000 Non-Qualified Options, with an intrinsic value of $0.4 million, were exercised at $742 per common share and 80,000 Non-Qualified Options, with an intrinsic value of $8.4 million, were exercised in exchange for 9,930 common shares with an equal total market value. 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 2018, MediaAlpha recognized $11.7 million of compensation expense for the vested portion of the awards for which achievement of the performance award was deemed probable, and $3.3 million of unearned compensation expense for unvested awards, which will be recognized over the remaining service periods of the awards.

Common Shareholders’ Equity and Non-controlling Interests

12 Months Ended

Dec. 31, 2018

Stockholders' Equity Note [Abstract]

Common Shareholder's Equity

Common Shareholders’ Equity and Non-controlling Interests

Common Shares Repurchased and Retired

During the past several years, White Mountains’s board of directors authorized the Company to repurchase its common shares, from time to time, subject to market conditions. Shares may be repurchased on the open market or through privately negotiated transactions. The repurchase authorizations do not have a stated expiration date. As of December 31, 2018, White Mountains may repurchase an additional 635,705 shares under these board authorizations. In addition, from time to time White Mountains has also repurchased its common shares through tender offers that were separately authorized by its board of directors.

During 2018, the Company repurchased 592,458 common shares for $519.4 million at an average share price of $877, which were comprised of 582,493 common shares repurchased under the board authorizations for $511.0 million at an average share price of $877 and 9,965 common shares repurchased pursuant to employee benefit plans. Shares repurchased pursuant to employee benefit plans do not fall under the board authorizations referred to above.

During 2017, the Company repurchased 832,725 common shares for $723.9 million at an average share price of $869, which were comprised of 821,732 common shares repurchased under the board authorizations for $713.1 million at an average share price of $870 and 10,993 common shares repurchased pursuant to employee benefit plans.

During 2016, the Company repurchased 1,106,145 common shares for $887.2 million at an average share price of $802, which were comprised of 1,098,123 common shares repurchased under the board authorizations for $881.4 million at an average share price of $803 and 8,022 common shares repurchased pursuant to employee benefit plans.

Common Shares Issued

During 2018, the Company issued a total of 16,377 common shares, which consisted of 14,105 restricted shares to key personnel and 2,272 shares issued to directors of the Company.

During 2017, the Company issued a total of 25,086 common shares, which consisted of 17,985 restricted shares to key personnel, 5,142 shares issued to the Company’s former Chairman and CEO as a result of exercised options, and 1,959 shares issued to directors of the Company.

During 2016, the Company issued a total of 47,030 common shares, which consisted of 25,365 restricted shares issued to key personnel, 14,930 shares issued to the Company’s former Chairman and CEO as a result of exercised options, 5,000 shares issued in satisfaction of performance shares and 1,735 shares issued to directors of the Company.

Dividends on Common Shares

For the years ended December 31, 2018, 2017 and 2016, the Company declared and paid cash dividends totaling $3.8 million, $4.6 million and $5.4 million (or $1.00 per common share).

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 December 31, 2018 and 2017:



	December 31, 2018					December 31, 2017
$ in Millions	Non-controlling Percentage		Non-controlling Equity			Non-controlling Percentage		Non-controlling Equity
Other, excluding BAM
HG Global	3.1	%	$	14.5		3.1	%	$	15.9
NSM	4.5		13.6			—		—
MediaAlpha	39.0		16.2			35.7		13.1
Buzz	22.9		1.1			22.9		2.5
Other NSM	13.4		.3			—		—
Total other, excluding BAM			45.7					31.5

BAM	100.0		(170.6		)	100.0		(163.2		)
Total non-controlling interests			$	(124.9	)			$	(131.7	)

Statutory Capital and Surplus	12 Months Ended
Statutory Capital and Surplus	Dec. 31, 2018
Insurance [Abstract]
Statutory Capital and Surplus	Statutory Capital and Surplus White Mountains’s insurance operations are subject to regulation and supervision in each of the jurisdictions where they are domiciled and licensed to conduct business. Generally, regulatory authorities have broad supervisory and administrative powers over such matters as licenses, standards of solvency, premium rates, policy forms, investments, security deposits, methods of accounting, form and content of financial statements, minimum capital and surplus requirements, dividends and other distributions to shareholders, periodic examinations and annual and other report filings. In general, such regulation is for the protection of policyholders rather than shareholders. The Insurance Act 1978 of Bermuda and related regulations, as amended (“Insurance Act”), regulates the insurance business of Bermuda-domiciled insurers. Under the Insurance Act, insurers are required to maintain available statutory capital and surplus at a level equal to or in excess of its enhanced capital requirement which is established by reference to either a Bermuda Solvency Capital Requirement (“BSCR”) model or an approved internal capital model. Generally, the Bermuda Monetary Authority (“BMA”) has broad supervisory and administrative powers over such matters as licenses, standards of solvency, investments, methods of accounting, form and content of financial statements, minimum capital and surplus requirements, and annual and other report filings. HG Global/BAM HG Re is a Special Purpose Insurer under Bermuda insurance regulations and is subject to regulation and supervision by the BMA. As of December 31, 2018, HG Re had statutory capital and surplus of $698.9 million. As a Special Purpose Insurer, HG Re has a nominal minimum regulatory capital requirement of $1. BAM is domiciled in New York and is subject to regulation by the NYDFS. New York financial guarantee insurance law establishes single risk and aggregate limits with respect to insured obligations insured by financial guarantee insurers. BAM’s statutory net loss for the years ended December 31, 2018, 2017 and 2016 was $34.6 million, $25.4 million and $32.7 million. BAM’s members’ surplus, as reported to regulatory authorities as of December 31, 2018, was $413.7 million, which exceeds the minimum members’ surplus necessary for BAM to maintain its New York State financial guarantee insurance license of $66.0 million. Dividend Capacity There are no restrictions under Bermuda law or the law of any other jurisdiction on the payment of dividends from retained earnings by White Mountains, provided that after the payment of any dividend, the Company would continue to be able to pay its liabilities as they become due and the realizable value of the Company’s assets would remain greater that its liabilities. Following is a description of the dividend capacity of White Mountains’s reinsurance and other operating subsidiaries: HG Global/BAM As of December 31, 2018, HG Global had $619.0 million face value of preferred shares outstanding, of which White Mountains owned 96.9%. Holders of the HG Global preferred shares receive cumulative dividends at a fixed annual rate of 6.0% on a quarterly basis, when and if declared by HG Global. HG Global did not declare or pay any preferred dividends in 2018. As of December 31, 2018, HG Global has accrued $288.1 million of dividends payable to holders of its preferred shares, $278.5 million of which is payable to White Mountains and eliminated in consolidation. As of December 31, 2018, HG Global and its subsidiaries had $2.2 million of cash outside of HG Re. HG Re is a Special Purpose Insurer subject to regulation and supervision by the BMA, but does not require regulatory approval to pay dividends. However, HG Re’s dividend capacity is limited to amounts held outside of the Collateral Trusts pursuant to the FLRT with BAM. As of December 31, 2018, HG Re had statutory capital and surplus of $698.9 million, $757.4 million of assets held in the Collateral Trusts pursuant to the FLRT with BAM and $3.0 million of cash and investments outside the Collateral Trusts. Effective January 1, 2014, HG Global and BAM agreed to change the interest rate on the BAM Surplus Notes for the five years ending December 31, 2018 from a fixed rate of 8.0% to a variable rate equal to the one-year U.S. treasury rate plus 300 basis points, set annually, which was 4.6% for 2018 and is 5.7% for 2019. During 2018, BAM exercised its option to extend the variable rate period on the BAM Surplus Notes for three years to December 31, 2021. 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%. BAM is required to seek regulatory approval to pay interest and principal on the BAM Surplus Notes only to the extent that its capital resources continue to support its outstanding obligations, business plan and rating. 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”). During 2017, HG Global and BAM agreed to change 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 NYDFS approved the change during the third quarter of 2017. During 2018 and 2017, BAM repaid $17.7 million and $4.0 million of the BAM Surplus Notes and $5.3 million and $1.0 million of accrued interest. NSM During the period from May 11, 2018, the date of White Mountains’s acquisition of NSM, through December 31, 2018, NSM did not pay any dividends to its shareholders. As of December 31, 2018, NSM had $16.2 million of net unrestricted cash. MediaAlpha During 2018, MediaAlpha paid $15.9 million of dividends, of which $9.8 million was paid to White Mountains. As of December 31, 2018, MediaAlpha had $5.7 million of net unrestricted cash. Other Operations During 2018, White Mountains paid a $3.8 million common share dividend. As of December 31, 2018, the Company and its intermediate holding companies held $536.0 million of net unrestricted cash, short-term investments and fixed maturity investments, $925.6 million of common equity securities and $67.3 million of other long-term investments included in its Other Operations segment.

Segment Information

12 Months Ended

Dec. 31, 2018

Segment Reporting [Abstract]

Segment Information

White Mountains has determined that its reportable segments are HG Global/BAM, NSM, MediaAlpha and Other Operations.

As a result of the OneBeacon, Sirius Group and Tranzact transactions, the results of operations for OneBeacon and Sirius Group, previously reported in their own respective segments, and Tranzact, previously reported in the Other Operations segment, have been classified as discontinued operations and are now presented, net of related income taxes, as such in the statement of operations and comprehensive income. See Note 19 — “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. Prior year amounts 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.

The HG Global/BAM segment consists of White Mountains’s investment in HG Global and the consolidated results of BAM. BAM is a municipal bond insurer domiciled in New York that was established to provide insurance on municipal bonds issued to support essential U.S. public purposes such as schools, utilities, core governmental functions and existing transportation facilities. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of BAM Surplus Notes. HG Global also provides up to 15%-of-par, first loss reinsurance protection for policies underwritten by BAM. For capital appreciation bonds, par is adjusted to the estimated equivalent par value for current interest paying bonds. BAM's results are attributed to non-controlling interests.

NSM is a full-service MGU and program administrator for specialty property and casualty insurance. The company places insurance in niche sectors such as specialty transportation, social services and real estate. On behalf of its insurance carrier partners, NSM manages all aspects of the placement process, including product development, marketing, underwriting, policy issuance and claims. NSM earns commissions based on the volume and profitability of the insurance that it places. NSM does not take insurance risk.

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.

White Mountains’s Other Operations segment consists of the Company and its wholly-owned subsidiary, WM Capital, its other intermediate holding companies, its investment management subsidiary, WM Advisors, investment assets managed by WM Advisors, its interests in PassportCard/DavidShield and Kudu, certain other consolidated and unconsolidated entities and certain other strategic investments. The consolidated entities consist of Wobi and Buzz. White Mountains’s Other Operations segment also includes its variable annuity reinsurance business, WM Life Re.

Significant intercompany transactions among White Mountains’s segments have been eliminated herein.

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



Millions	HG Global/BAM ⁽¹⁾			NSM			MediaAlpha		Other Operations			Total
Year Ended December 31, 2018
Earned insurance premiums	$	13.9		$	—		$	—	$	—		$	13.9
Net investment income	16.7			—			—		42.3			59.0
Net realized and unrealized investment losses	(7.5		)	—			—		(100.8		)	(108.3		)
Advertising and commission revenues ⁽²⁾	—			94.7			295.5		4.1			394.3
Other revenues	1.2			6.9			1.6		.5			10.2
Total revenues	24.3			101.6			297.1		(53.9		)	369.1
Insurance acquisition expenses	5.3			—			—		—			5.3
Other underwriting expenses	.4			—			—		—			.4
Cost of sales	—			—			245.0		3.7			248.7
General and administrative expenses	48.0			61.6			31.7		94.4			235.7
Broker commission expense	—			28.9			—		—			28.9
Amortization of other intangible assets	—			8.3			10.3		.2			18.8
Interest expense	—			8.0			1.2		.3			9.5
Total expenses	53.7			106.8			288.2		98.6			547.3
Pre-tax (loss) income	$	(29.4	)	$	(5.2	)	$	8.9	$	(152.5	)	$	(178.2	)


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


⁽²⁾	As of December 31, 2018, approximately 29% of MediaAlpha’s advertising revenue was associated with one customer.As of December 31, 2018, approximately 33% of NSM’s commission revenue was associated with one single carrier.



Millions	HG Global/BAM ⁽¹⁾			MediaAlpha			Other Operations		Total
Year Ended December 31, 2017
Earned insurance premiums	$	9.4		$	—		$	1.0	$	10.4
Net investment income	12.3			—			43.7		56.0
Net realized and unrealized investment gains	.6			—			132.7		133.3
Advertising and commission revenues⁽²⁾	—			163.2			3.8		167.0
Other revenues	1.0			—			6.1		7.1
Total revenues	23.3			163.2			187.3		373.8
Losses and LAE	—			—			1.1		1.1
Insurance acquisition expenses	4.0			—			.1		4.1
Other underwriting expenses	.4			—			—		.4
Cost of sales	—			135.9			3.5		139.4
General and administrative expenses	42.9			16.2			148.9		208.0
Amortization of other intangible assets	—			10.5			.2		10.7
Interest expense	—			1.0			1.3		2.3
Total expenses	47.3			163.6			155.1		366.0
Pre-tax (loss) income	$	(24.0	)	$	(.4	)	$	32.2	$	7.8


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


⁽²⁾	As of December 31, 2017, approximately 27% of MediaAlpha’s advertising revenue was associated with one customer.



Millions	HG Global/BAM ⁽¹⁾			MediaAlpha			Other Operations			Total
Year Ended December 31, 2016
Earned insurance premiums	$	5.9		$	—		$	7.5		$	13.4
Net investment income	9.0			—			23.1			32.1
Net realized and unrealized investment gains (losses)	.7			—			(28.1		)	(27.4		)
Advertising and commission revenues ⁽²⁾	—			116.5			1.8			118.3
Other revenues	1.1			—			20.2			21.3
Total revenues	16.7			116.5			24.5			157.7
Losses and LAE	—			—			8.0			8.0
Insurance acquisition expenses	3.4			—			2.2			5.6
Other underwriting expenses	.4			—			—			.4
Cost of sales	—			97.8			4.2			102.0
General and administrative expenses	39.6			11.8			124.1			175.5
Amortization of other intangible assets	—			10.1			.4			10.5
Interest expense	—			.9			2.1			3.0
Total expenses	43.4			120.6			141.0			305.0
Pre-tax loss	$	(26.7	)	$	(4.1	)	$	(116.5	)	$	(147.3	)


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


⁽²⁾	As of December 31, 2016, approximately 24% of MediaAlpha’s advertising revenue was associated with one customer.



Millions Selected Balance Sheet Data	HG Global/BAM				NSM		MediaAlpha		Other Operations			Held for Sale		Total
December 31, 2018:
Total investments	$	768.3			$	1.7	$	—	$	1,772.9		$	—	$	2,542.9
Total assets	$	816.2		⁽¹⁾	$	627.0	$	88.4	$	1,827.7	⁽²⁾	$	3.3	$	3,362.6
Total liabilities	$	212.5		⁽²⁾	$	314.8	$	46.9	$	70.2		$	—	$	644.4
Total White Mountains’s common shareholders’ equity	$	759.8		⁽²⁾	$	298.3	$	25.3	$	1,756.4	⁽²⁾	$	3.3	$	2,843.1
Non-controlling interest	$	(156.1	)		$	13.9	$	16.2	$	1.1		$	—	$	(124.9	)
December 31, 2017:
Total investments	$	693.4			$	—	$	—	$	2,687.3		$	—	$	3,380.7
Total assets	$	747.4		⁽¹⁾	$	—	$	96.5	$	2,812.0	⁽²⁾	$	3.3	$	3,659.2
Total liabilities	$	167.0		⁽²⁾	$	—	$	59.8	$	71.6		$	—	$	298.4
Total White Mountains’s common shareholders’ equity	$	727.7		⁽²⁾	$	—	$	23.6	$	2,737.9	⁽²⁾	$	3.3	$	3,492.5
Non-controlling interest	$	(147.3	)		$	—	$	13.1	$	2.5		$	—	$	(131.7	)


⁽¹⁾	As of December 2018 and 2017,BAM’s total assets reflected the elimination of $481.3 and $499.0 of BAM Surplus Notes issued to HG Global and its subsidiaries, and $143.7 and $126.0 in accrued interest related to the BAM Surplus Notes.

⁽²⁾

HG Global preferred dividends payable to White Mountains’s subsidiaries is eliminated in White Mountains’s consolidated financial statements. For segment reporting, the HG Global preferred dividends payable to White Mountains’s subsidiaries included within the HG Global/BAM segment are eliminated against the offsetting receivable included within the Other Operations segment and therefore added back to White Mountains’s common shareholders’ equity within the HG Global/BAM segment. As of December 31, 2018 and 2017, the HG Global preferred dividends payable to White Mountains’s subsidiaries was $278.5 and $227.9.

Investments in Unconsolidated Affiliates

12 Months Ended

Dec. 31, 2018

Equity Method Investments and Joint Ventures [Abstract]

Investments in Unconsolidated Affiliates

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:



	December 31,
Millions	2018		2017
Equity method eligible unconsolidated entities, at fair value	$	138.1	$	58.0
Investments accounted for under the equity method	1.3		4.6
Total investments in equity method eligible unconsolidated entities	139.4		62.6
Other unconsolidated investments ⁽¹⁾	186.2		146.2
Total other long-term investments	$	325.6	$	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 December 31, 2018 and 2017:



	Ownership Interest
Investee	December 31, 2018	December 31, 2017	Instrument Held
PassportCard/DavidShield ⁽¹⁾	50.0%	50% / 0%	Common shares
Kudu	49.5%	—	Units
durchblicker	45.0%	45.0%	Common shares
Tuckerman Capital Fund III, L.P.	18.5%	21.3%	Limited partnership interest
Compare.com	18.4%	22.1%	Common shares

⁽¹⁾ As of December 31, 2018, White Mountains’s ownership interest in DavidShield comprised a 50% direct interest and White Mountains’s ownership interest in PassportCard comprised a 25% direct ownership interest and a 25% indirect interest through DavidShield. As of December 31, 2017, White Mountains had no ownership in DavidShield and White Mountains’s ownership interest in PassportCard comprised a 50% direct interest. See Note 2 — “Significant Transactions”.

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



	December 31,
Millions	2018		2017
Balance sheet data ⁽¹⁾:
Total assets	$	218.8	$	75.4
Total liabilities	$	46.7	$	24.2

⁽¹⁾Financial data for durchblicker, Compare.com and Tuckerman Capital Fund III, L.P. is on a one-quarter lag.



	Year Ended December 31,
Millions	2018			2017			2016
Income statement data ⁽¹⁾:
Revenues	$	134.1		$	60.0		$	32.9
Expenses	$	(110.1	)	$	(66.8	)	$	(76.4	)

⁽¹⁾Financial data for durchblicker, Compare.com and Tuckerman Capital Fund III, L.P. is on a one-quarter lag.

Variable Interest Entities	12 Months Ended
Variable Interest Entities	Dec. 31, 2018
Variable Interest Entity, Not Primary Beneficiary, Disclosures [Abstract]
Variable Interest Entities	Variable Interest Entities BAM As a mutual insurance company, BAM is owned by its members. BAM charges an insurance premium on each municipal bond insurance policy it writes. A portion of the premium is a MSC and the remainder is a risk premium. In the event of a municipal bond refunding, the MSC from the original issuance can be reutilized, in effect serving as a credit against the total insurance premium on the refunding of the municipal bond. Issuers of debt insured by BAM are members of BAM so long as any of their BAM-insured debt is outstanding, and as members they have certain interests in BAM, including the right to vote for BAM’s directors and to receive dividends in the future, if declared. The equity at risk funded by BAM’s members is not sufficient to fund its operations without the additional financial support provided by the BAM Surplus Notes and accordingly, BAM is considered to be a VIE. At inception, BAM and HG Re also entered into the FLRT. HG Re provides first loss protection up to 15%-of-par outstanding on each municipal bond insured by BAM. For capital appreciation bonds, par is adjusted to the estimated equivalent par value for current interest paying bonds. In return, BAM cedes 60% of the risk premium charged for insuring the municipal bond, net of a ceding commission. HG Re’s obligations under the FLRT are limited to the assets in the Regulation 114 Trust and the Supplemental Trust. Losses required to be reimbursed under the FLRT are subject to an aggregate limit equal to the assets held in the Collateral Trusts at any point in time. In addition, under the FLRT, HG Holdings Ltd, a subsidiary of HG Global, has the right to designate two directors for election to BAM’s board of directors. Since BAM is owned by its members, its equity and results of operations are included in non-controlling interests. 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. Kudu On February 5, 2018, White Mountains entered into an agreement to fund up to $125.0 million in Kudu Investment Management, LLC (“Kudu”), a capital provider to asset management and wealth management firms. Kudu specializes in providing capital solutions to asset managers and registered investment advisers, for purposes including generational ownership transfers, management buyouts, acquisition and growth finance, and legacy partner liquidity. White Mountains has determined that Kudu is a VIE but that White Mountains is not the primary beneficiary. White Mountains’s ownership interest gives White Mountains the opportunity to exert significant influence over the significant financial and operating activities of Kudu. Accordingly, Kudu meets the criteria to be accounted for under the equity method. White Mountains has taken the fair value option for its investment in Kudu. White Mountains’s investment in Kudu is measured at fair value using NAV as a practical expedient. Changes in the fair value of Kudu have been recorded in realized and unrealized investment gains. White Mountains’s maximum loss in Kudu is limited to the amount invested. As of December 31, 2018, Kudu is recorded within other long-term investments at a carrying amount of $30.7 million.

Fair Value of Financial Instruments

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value of Financial Instruments

White Mountains records its financial instruments at fair value with the exception of the NSM Bank Facility and the MediaAlpha Bank Facility, which are recorded as debt at face value less unamortized original issue discount.

The following tables presents the fair value and carrying value of these financial instruments as of December 31, 2018 and December 31, 2017:



	December 31, 2018				December 31, 2017
Millions	Fair Value		Carrying Value		Fair Value		Carrying Value
NSM Bank Facility	$	176.1	$	176.6	$	—	$	—
MediaAlpha Bank Facility	$	14.6	$	14.2	$	23.9	$	23.8

The fair value estimates for the NSM Bank Facility and the MediaAlpha Bank Facility have been determined based on a discounted cash flows approach and are considered to be Level 3 measurements.

- 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

Transactions with Related Persons	12 Months Ended
Transactions with Related Persons	Dec. 31, 2018
Related Party Transactions [Abstract]
Transactions with Related Persons	Transactions with Related Persons During 2017, the Company repurchased shares from Franklin Mutual Advisers, a beneficial owner of the Company. On July 13, 2017, the Company repurchased 235,000 White Mountains common shares for $850.00 per share, the market price at the time the agreement was reached. During 2016, the Company repurchased shares from Franklin Mutual Advisers in two transactions. On April 19, 2016, the Company repurchased 325,000 White Mountains common shares for $807.00 per share, the market price at the time the agreement was reached. On September 15, 2016, the Company repurchased 305,000 White Mountains common shares for $820.00 per share, the market price at the time the agreement was reached.

Commitments and Contingencies	12 Months Ended
Commitments and Contingencies	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies White Mountains leases certain office spaces under non-cancellable operating leases that expire on various dates through 2022. Rental expense for all of White Mountains’s locations was $5.5 million, $3.5 million and $3.4 million for the years ended December 31, 2018, 2017 and 2016. White Mountains also has various other lease obligations that are immaterial in the aggregate. White Mountains’s future annual minimum rental payments required under non-cancellable leases, which are primarily for office space, are $6.6 million, $4.9 million, $4.2 million, and $12.0 million for the years 2019, 2020, 2021 and 2022 and thereafter. White Mountains also has future binding commitments to fund certain other long-term investments. These commitments, which totaled $170.2 million as of December 31, 2018, do not have fixed funding dates. 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 December 31, 2018: Esurance On October 7, 2011, the Company completed the sale of its Esurance Holdings, Inc. and its subsidiaries and Answer Financial Inc. and its subsidiaries (collectively, “Esurance”) to The Allstate Corporation (“Allstate”) pursuant to a Stock Purchase Agreement dated as of May 17, 2011. Subject to specified thresholds and limits, the Company remains contingently liable to Allstate for specified matters related to the pre-closing period, including (a) losses of Esurance arising from extra-contractual claims and claims in excess of policy limits, (b) certain corporate reorganizations effected to remove entities from Esurance that were not being sold in the transaction, and (c) certain tax matters, including certain net operating losses being less than stated levels. Sirius Tax Contingency 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. In October 2018, the Swedish Administrative Court ruled against Sirius Group on its appeal of the Swedish Tax Agency’s denial of certain interest deductions relating to periods prior to the sale of Sirius Group to CMI. In connection with the sale, White Mountains indemnified Sirius Group against the loss of certain tax attributes, including those related to these interest deductions. As a result, for the year ended December 31, 2018, White Mountains recorded a loss of $17.3 million within net (loss) gain on sale of discontinued operations reflecting the value of these interest deductions. Sirius Group has appealed the decision to the Swedish Administrative Court of Appeal. NSM Contingent Liability In connection with White Mountains’s acquisition of NSM, White Mountains and NSM entered into an agreement with American International Group, Inc. (“AIG”) to facilitate a sale of NSM’s U.S. collector car renewal rights owned by AIG to a third party by December 31, 2019. Under the terms of the agreement, if White Mountains and NSM are unable to facilitate a sale by December 31, 2019, AIG has the right to require NSM to purchase the renewal rights for $82.5 million. The Company has guaranteed NSM’s obligations under the agreement with AIG. The manner in which these obligations are ultimately discharged depends on a number of factors, including the market value of the renewal rights, the number of potential buyers and the current and prospective environment for U.S. collector car insurance. White Mountains believes that the estimated fair value of the renewal rights is equal to or greater than $82.5 million and, accordingly, no accrual of a liability is necessary at December 31, 2018.

Held for Sale and Discontinued Operations

12 Months Ended

Dec. 31, 2018

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

Held for Sale and Discontinued Operations

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. For 2017 through the closing date of the transaction, net income from discontinued operations related to OneBeacon was $20.5 million. Net income from discontinued operations related to OneBeacon was $108.6 million for the year ended December 31, 2016.

Star & Shield

On March 7, 2017, White Mountains completed the sale of Star & Shield and its investment in SSIE surplus notes to K2 Insurances LLC. White Mountains did not recognize any gain or loss on the sale.

Tranzact

On July 21, 2016, White Mountains completed the sale of Tranzact to an affiliate of 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. For the year ended December 31, 2016, White Mountains recorded $51.9 million of gain from the sale of Tranzact in discontinued operations in the statement of operations.

Through July 21, 2016, Tranzact's results of operations are reported as discontinued operations and assets and liabilities held for sale within White Mountains's GAAP financial statements. For the year ended December 31, 2016, White Mountains recorded net income from discontinued operations of $6.1 million from Tranzact. White Mountains recognized a $21.4 million income tax benefit in continuing operations related to the reversal of a valuation allowance that resulted from the gain on the sale of Tranzact recognized within discontinued operations. This income tax benefit was recorded in continuing operations with an offsetting amount of net income tax expense recorded in discontinued operations, including $30.2 million of income tax expense recorded to gain from sale of Tranzact in discontinued operations and $8.8 million of income tax benefit recorded to net income from discontinued operations.

During 2017, White Mountains recorded a $3.2 million increase to the gain from sale of Tranzact in discontinued operations as a result of state income tax expense.

Sirius Group

On April 18, 2016, White Mountains completed the sale of Sirius Group to CMI for approximately $2.6 billion. Of this amount, $161.8 million of this amount was used to purchase certain assets to be retained by White Mountains out of Sirius Group, including shares of OneBeacon. The amount paid at closing was based on an estimate of Sirius Group’s closing date tangible common shareholder’s equity. During 2016, White Mountains recorded $363.2 million of gain from sale of Sirius Group in discontinued operations in the statement of operations and $113.3 million in other comprehensive income from discontinued operations. During 2017, White Mountains recorded a $0.7 million reduction to the 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.

Through April 18, 2016, Sirius Group’s results are reported as discontinued operations within White Mountains’s GAAP financial statements. Net income (loss) from discontinued operations did not include investment gains (losses) from White Mountains’s investment in OneBeacon and certain other investments that were held in the Sirius Group legal entities. For the year ended December 31, 2016, $3.7 million of net investment income and net realized and unrealized investment gains (losses), net of related tax effects, that were included in the Sirius Group legal entities have been excluded from net income (loss) from discontinued operations. For the years ended December 31, 2016, White Mountains recorded $4.3 million of net loss from discontinued operations and $32.0 million of other comprehensive income from Sirius Group.

In October 2018, the Swedish Administrative Court ruled against Sirius Group on its appeal of the Swedish Tax Agency’s denial of certain interest deductions relating to periods prior to the sale of Sirius Group to CMI. In connection with the sale, White Mountains indemnified Sirius Group against the loss of certain tax attributes, including those related to these interest deductions. As a result, for the year ended December 31, 2018, White Mountains recorded a $17.3 million loss on sale of discontinued operations reflecting the value of these interest deductions. See Note 18 — “Commitments and Contingencies”.

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. As of December 31, 2018 and 2017, the property has been measured at its estimated fair value net of disposal costs of $3.3 million. The related write down of $3.7 million was recorded within other expenses during 2017.

Net Income (Loss) from Discontinued Operations

The following tables present the results of operations, including related income taxes associated with the business classified as discontinued operations. For the year ended December 31, 2017 and 2016, the amounts presented relate to OneBeacon, Sirius Group and Tranzact. The results of discontinued operations from Sirius Group and Tranzact up to the closing date of the transaction inure to White Mountains.



	Year Ended December 31, 2017
Millions	OneBeacon		Sirius Group			Tranzact		Total
Revenues
Earned insurance premiums	$	807.6	$	—		$	—	$	807.6
Net investment income	39.7		—			—		39.7
Net realized and unrealized investment gains	38.8		—			—		38.8
Other revenues	7.7		—			—		7.7
Total revenues	893.8		—			—		893.8
Expenses
Loss and loss adjustment expenses	546.0		—			—		546.0
Insurance and reinsurance acquisition expenses	145.6		—			—		145.6
Other underwriting expenses	156.2		—			—		156.2
General and administrative expenses	21.2		—			—		21.2
Interest expense	10.0							10.0
Total expenses	879.0		—			—		879.0
Pre-tax income	14.8		—			—		14.8
Income tax benefit	5.7		—			—		5.7
Net income from discontinued operations	20.5		—			—		20.5
Gain (loss) from sale of discontinued operations, net of tax	554.5		(.7		)	3.2		557.0
Total income (loss) from discontinued operations	575.0		(.7		)	3.2		577.5
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax	.3		—			—		.3
Recognition of benefit plan assets and obligations from the sale of OneBeacon, net of tax	2.9		—			—		2.9
Comprehensive income (loss) from discontinued operations	$	578.2	$	(.7	)	$	3.2	$	580.7



	Year Ended December 31, 2016
Millions	OneBeacon		Sirius Group			Tranzact			Total
Revenues
Earned insurance premiums	$	1,100.6	$	240.1		$	—		$	1,340.7
Net investment income	50.6		14.4			—			65.0
Net realized and unrealized investment gains (losses)	37.7		(1.5		)	—			36.2
Other revenues	5.5		.6			119.6			125.7
Total revenues	1,194.4		253.6			119.6			1,567.6
Expenses
Loss and loss adjustment expenses	656.0		154.9			—			810.9
Insurance and reinsurance acquisition expenses	206.0		59.0			—			265.0
Other underwriting expenses	209.0		30.9			—			239.9
General and administrative expenses	14.2		10.4			116.7			141.3
Interest expense	13.1		7.9			3.2			24.2
Total expenses	1,098.3		263.1			119.9			1,481.3
Pre-tax income (loss)	96.1		(9.5		)	(.3		)	86.3
Income tax benefit	12.5		3.1			6.4			22.0
Net income (loss) from discontinued operations	108.6		(6.4		)	6.1			108.3
Gain from sale of discontinued operations, net of tax	—		363.2			51.9			415.1
Total income from discontinued operations	108.6		356.8			58.0			523.4
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax	1.0		32.0			—			33.0
Recognition of foreign currency translation from sale of Sirius Group, net of tax	—		113.3			—			113.3
Comprehensive income from discontinued operations	$	109.6	$	502.1		$	58.0		$	669.7

Net Change in Cash from Discontinued Operations

The transactions to purchase the investments in OneBeacon and other investments held by Sirius Group prior to the closing are presented in the statement of cash flows as net settlement of investment cash flows with discontinued operations. The following table presents the net change in cash associated with the businesses classified as discontinued operations:



	Year Ended December 31,
Millions	2017			2016
Net cash provided from operations	$	157.0		$	23.6
Net cash provided from (used for) investing activities	3.0			241.4
Net cash used for financing activities	(61.9		)	(93.8		)
Net change in cash during the period	98.1			171.2
Cash balances at beginning of period	70.5			245.4
Net change in cash held for sale	(.9		)	(.3		)
Cash sold as part of sale of consolidated subsidiaries	(167.7		)	(345.8		)
Cash balances at end of period	$	—		$	70.5

Earnings Per Share from Discontinued Operations

White Mountains calculates earnings per share using the two-class method, which allocates earnings between common and unvested restricted common shares. Both classes of shares participate equally in earnings on a per share basis. Basic earnings per share amounts are based on the weighted average number of common shares outstanding adjusted for unvested restricted common shares. Diluted earnings per share amounts are also impacted by the net effect of potentially dilutive common shares outstanding.

The following table presents the Company’s computation of earnings per share for discontinued operations for the years ended December 31, 2018, 2017 and 2016:



	Year Ended December 31,
	2018			2017			2016
Basic and diluted earnings per share numerators (in millions):
Net (loss) income attributable to White Mountains’s common shareholders	$	(141.2	)	$	627.2		$	401.8
Less: total (loss) income from continuing operations, net of tax	(124.0		)	49.7			(121.6		)
Net (loss) income from discontinued operations attributable to White Mountains’s common shareholders	(17.2		)	577.5			523.4
Allocation of earnings (losses) to participating restricted common shares ⁽¹⁾	0.2			(7.3		)	(6.8		)
Basic and diluted (losses) earnings per share numerators	$	(17.0	)	$	570.2		$	516.6
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	$	3,382.5		$	4,293.8		$	5,014.9
Average unvested restricted common shares ⁽³⁾	(40.1		)	(54.3		)	(64.8		)
Basic earnings (losses) per share denominator	$	3,342.4		$	4,239.5		$	4,950.1
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	$	3,382.5		$	4,293.8		$	5,018.1
Average unvested restricted common shares ⁽³⁾	(40.1		)	(54.3		)	(64.8		)
Diluted earnings (losses) per share denominator ⁽⁴⁾	$	3,342.4		$	4,239.5		$	4,953.3
Basic (losses) earnings per share (in dollars) - discontinued operations:	$	(5.09	)	$	134.50		$	104.37
Diluted (losses) earnings per share (in dollars) - discontinued operations:	$	(5.09	)	$	134.50		$	104.32


⁽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 years ended December 31, 2018, 2017 and 2016.


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


⁽⁴⁾	The diluted earnings per share denominator for the year ended December 31, 2016 includes the impact of 40,000 common shares issuable upon exercise of the non-qualified options outstanding, which resulted in 3,217 incremental shares outstanding over the period.

Subsequent Event	12 Months Ended
Subsequent Event	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Event	Subsequent Events MediaAlpha On February 26, 2019, MediaAlpha completed the sale of a significant minority stake to Insignia Capital Group in connection with a recapitalization and cash distribution to existing equityholders. MediaAlpha also repurchased a portion of the holdings of existing equityholders. The transaction valued MediaAlpha at approximately $350.0 million. White Mountains retained a 42% ownership interest in MediaAlpha on a fully-diluted basis, and received a net cash distribution of approximately $88.0 million. As a result of the transaction, White Mountains also expects that it will no longer consolidate MediaAlpha in its financial statements and will mark its interest in MediaAlpha to fair value at the transaction closing date and in subsequent periods. Kudu On February 14, 2019, White Mountains entered into a definitive agreement to buy all of the interests in Kudu held by certain funds managed by Oaktree Capital Management, L.P. (“Oaktree”) for approximately $50.0 million. In connection with the transaction, White Mountains will assume all of Oaktree’s unfunded capital commitments to Kudu, increasing White Mountains’s total unfunded Kudu capital commitment to approximately $167.0 million. If Kudu calls additional capital from Oaktree prior to the closing of the transaction, the purchase price would increase by the amount of the capital called, and White Mountains’s assumed unfunded capital commitment would decrease by an equal amount. As a result of the transaction, White Mountains’s ownership of Kudu will increase from 49.5% to 99.0%, and it expects to consolidate Kudu in its financial statements after closing.

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Selected Quartely Financial Data (Unaudited)

SELECTED QUARTERLY FINANCIAL DATA

(Unaudited)

The following table presents selected quarterly financial data for 2018 and 2017. The quarterly financial data includes, in the opinion of management, all recurring adjustments necessary for a fair presentation of the results of operations for the interim periods. As a result of the OneBeacon, Sirius Group and Tranzact transactions, the results of operations for OneBeacon, Tranzact and Sirius Group have been classified as discontinued operations and are now presented, net of related income taxes, as such in the statement of comprehensive income. See Note 19 — “Held for Sale and Discontinued Operations”.

Prior year amounts have been reclassified to conform to the current period’s presentation. Prior year amounts have also been adjusted for the impact of White Mountains’s financial statement revisions.



	2018 Three Months Ended												2017 Three Months Ended
Millions, Except Per Share Amounts	Dec. 31			Sept. 30			June 30			Mar. 31			Dec. 31		Sept. 30		June 30			Mar. 31
Revenues	$	6.0		$	198.7		$	122.3		$	42.1		$	114.0	$	87.5	$	83.5		$	88.8
Expenses	150.1			154.4			134.7			108.1			108.8		79.1		85.7			92.4
Pre-tax income (loss)	(144.1		)	44.3			(12.4		)	(66.0		)	5.2		8.4		(2.2		)	(3.6		)
Tax benefit (expense)	3.6			3.6			(2.5		)	(.7		)	2.5		4.0		1.0			0.3
Income (loss) from continuing operations	(140.5		)	47.9			(14.9		)	(66.7		)	7.7		12.4		(1.2		)	(3.3		)
Income (loss) from discontinued operations, net of tax	—			(17.3		)	—			.1			4.3		539.1		2.8			31.3
Non-controlling interest in consolidated subsidiaries	3.0			10.2			18.4			18.6			10.5		10.6		12.0			1.0
Income (loss) attributable to White Mountains’s common shareholders	$	(137.5	)	$	40.8		$	3.5		$	(48.0	)	$	22.5	$	562.1	$	13.6		$	29.0
Income (loss) attributable to White Mountains’s common shareholders per share:
Basic
Continuing operations	$	(43.24	)	$	18.27		$	1.02		$	(12.85	)	$	4.85	$	5.36	$	2.36		$	(0.52	)
Discontinued operations	—			(5.44		)	—			.03			1.15		125.45		.61			6.86
Total consolidated operations	$	(43.24	)	$	12.83		$	1.02		$	(12.82	)	$	6.00	$	130.81	$	2.97		$	6.34
Diluted
Continuing operations	$	(43.24	)	$	18.27		$	1.02		$	(12.85	)	$	4.85	$	5.36	$	2.36		$	(0.52	)
Discontinued operations	—			(5.44		)	—			.03			1.15		125.45		.61			6.86
Total consolidated operations	$	(43.24	)	$	12.83		$	1.02		$	(12.82	)	$	6.00	$	130.81	$	2.97		$	6.34

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

12 Months Ended

Dec. 31, 2018

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

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

WHITE MOUNTAINS INSURANCE GROUP, LTD.

SUMMARY OF INVESTMENTS—OTHER THAN

INVESTMENTS IN RELATED PARTIES

At December 31, 2018



Millions	Cost		Carrying Value		Fair Value
Fixed maturity investments:
U.S. Government and government agencies and authorities	$	154.0	$	153.2	$	153.2
Debt securities issued by corporations	519.0		510.5		510.5
States, municipalities and political subdivisions	279.0		280.3		280.3
Mortgage and asset-backed securities	136.1		133.5		133.5
Total fixed maturity investments	1,088.1		1,077.5		1,077.5
Short-term investments	214.2		214.2		214.2
Common equity securities:
Exchange traded funds	681.8		675.3		675.3
Banks, trust and insurance companies	11.8		13.5		13.5
Industrial, miscellaneous and other	211.1		236.8		236.8
Total common equity securities	904.7		925.6		925.6
Other long-term investments	330.3		325.6		325.6
Total investments	$	2,537.3	$	2,542.9	$	2,542.9

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT

12 Months Ended

Dec. 31, 2018

Condensed Financial Information Disclosure [Abstract]

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT

CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT

CONDENSED BALANCE SHEETS⁽¹⁾



	December 31,
Millions	2018		2017
Assets:
Cash	$	.7	$	14.9
Fixed maturity investments, at fair value	—		869.6
Common equity securities, at fair value	335.6		641.8
Other long-term investments ⁽²⁾	—		(3.7		)
Short-term investments, at amortized cost	28.0		57.2
Other assets	1.8		30.9
Investments in consolidated subsidiaries	2,533.2		1,914.8
Total assets	$	2,899.3	$	3,525.5
Liabilities:
Payable to subsidiary	$	15.8	$	11.8
Other liabilities	25.9		21.2
Total liabilities⁽³⁾	41.7		33.0
White Mountains’s common shareholders’ equity	2,843.1		3,492.5
Non-controlling interests	14.5		—
Total liabilities and equity	$	2,899.3	$	3,525.5

⁽¹⁾ These condensed unconsolidated financial statements reflect the results of operations, financial condition and cash flows for the Company. Investments in controlling subsidiaries are accounted for using the equity method. Under the equity method, investments in subsidiaries are recorded on the condensed balance sheets at the amount of the Company’s ownership percentage of the subsidiary’s GAAP book value. The income from subsidiaries is reported on a net of tax basis as equity in earnings from consolidated and unconsolidated subsidiaries on the condensed statements of operations and comprehensive income. Capital contributions to and distributions from subsidiaries are presented within investing activities on the condensed statements of cash flows.

⁽²⁾As of December 31, 2017, other long-term investments includes $(3.7) related to foreign currency forward contracts. See Note 7 — “Derivatives—Foreign Currency Forward Contracts”.

⁽³⁾ As of December 31, 2018, White Mountains other liabilities includes $17.3 related to the Sirius tax contingency. See Note 18 — “Commitments and Contingencies”.

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

CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME⁽¹⁾



	Year Ended December 31,
Millions	2018				2017				2016
Revenues (loss) (including realized and unrealized gains and losses)	$	(47.7	)		$	27.3			$	(1.0	)
Expenses	45.9				99.7				68.2
Pre-tax loss	(93.6		)		(72.4		)		(69.2		)
Income tax expense	(2.5		)		(1.4		)		(.5		)
Net loss	(96.1		)		(73.8		)		(69.7		)
Net loss from discontinued operations, net of tax ⁽²⁾	(17.2		)	⁽²⁾	—			⁽³⁾	—			⁽⁴⁾
Equity in earnings from consolidated and unconsolidated subsidiaries, net of tax	(27.4		)		701.0			⁽³⁾	471.5			⁽⁴⁾
Net loss (income) attributable to non-controlling interests	(.5		)		—				—
Net (loss) income attributable to White Mountains’s common shareholders	(141.2		)		627.2				401.8
Other comprehensive (loss) income items, net of tax	(4.5		)		3.3				145.3
Comprehensive (loss) income attributable to White Mountains’s common shareholders	$	(145.7	)		$	630.5			$	547.1

⁽¹⁾ These condensed unconsolidated financial statements reflect the results of operations, financial condition and cash flows for the Company. Investments in which White Mountains holds a controlling financial interest are accounted for using the equity method. Under the equity method, investments in subsidiaries are recorded on the condensed balance sheets at the amount of the Company’s ownership percentage of the subsidiary’s GAAP book value. The income from subsidiaries is reported on a net of tax basis as equity in earnings of subsidiaries on the condensed statements of operations and comprehensive income. Capital contributions to and distributions from subsidiaries are presented within investing activities on the condensed statements of cash flows.

⁽²⁾ During 2018, net loss from discontinued operations includes $17.3 arising from the tax contingency on the sale of Sirius Group. See Note 18 — “Commitments and Contingencies”.

⁽³⁾ Equity in earnings from consolidated subsidiaries for the year ended December 31, 2017 includes $577.5 of income from discontinued operations associated primarily with the dispositions of OneBeacon, Sirius Group and Tranzact. See Note 19 — “Held for Sale and Discontinued Operations”.

⁽⁴⁾ Equity in earnings from consolidated subsidiaries for the year ended December 31, 2016 includes $523.4 of income from discontinued operations associated primarily with the dispositions of OneBeacon, Sirius Group and Tranzact. See Note 19 — “Held for Sale and Discontinued Operations”.

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

SCHEDULE II (continued)

CONDENSED STATEMENTS OF CASH FLOWS ⁽¹⁾⁽²⁾



	Year Ended December 31,
Millions	2018			2017			2016
Net income attributable to White Mountains’s common shareholders	$	(141.2	)	$	627.2		$	401.8
Charges (credits) to reconcile net income to net cash from operations:
Net realized and unrealized investment (gains) losses on sales of investments	57.8			(18.5		)	1.1
Undistributed earnings from subsidiaries	27.4			(701.0		)	(471.5		)
Net loss on sale of other discontinued operations ⁽³⁾	17.2			—			—
Other non-cash reconciling items, primarily amortization of restricted share and option awards ⁽⁴⁾	34.6			31.1			17.9
Accumulated earnings distributed from subsidiary in cash ⁽⁵⁾	—			1,256.7			—
Net change in other assets and liabilities ⁽⁶⁾	16.7			(4.9		)	(5.6		)
Net cash provided from (used for) operations	12.5			1,190.6			(56.3		)
Cash flows from investing activities:
Net change in short-term investments⁽⁷⁾	134.0			(24.7		)	10.9
Purchases of investment securities ⁽⁸⁾	(321.2		)	(474.7		)	—
Sales and maturities of investment securities⁽⁹⁾	967.6			367.1			—
Issuance of debt (to) from subsidiaries ⁽¹⁰⁾	(55.2		)	382.0			992.0
Repayment of debt to (from) subsidiaries⁽¹¹⁾	31.0			—			(5.0		)
(Contributions to) distributions from subsidiaries ^(12)(13)	(258.2		)	(700.0		)	—
Net cash provided from (used for) investing activities	498.0			(450.3		)	997.9
Cash flows from financing activities:
Draw down of revolving line of credit ⁽¹⁴⁾	—			350.0			350.0
Repayment of revolving line of credit ⁽¹⁴⁾	—			(350.0		)	(400.0		)
Proceeds from issuances of common shares	—			—			3.7
Repurchases and retirement of common shares ⁽¹⁰⁾	(510.9		)	(714.6		)	(881.3		)
Dividends paid on common shares	(3.8		)	(4.6		)	(5.4		)
Payments of restricted shares withholding taxes	(10.0		)	(9.3		)	(5.8		)
Net cash used for financing activities	(524.7		)	(728.5		)	(938.8		)
Net (decrease) increase in cash during the year	(14.2		)	11.8			2.8
Cash balance at beginning of year	14.9			3.1			.3
Cash balance at end of year	$	.7		$	14.9		$	3.1
Supplemental cash flow information: interest paid	$	—		$	(.6	)	$	(1.2	)

⁽¹⁾

These condensed unconsolidated financial statements reflect the results of operations, financial condition and cash flows for the Company. Investments in which White Mountains holds a controlling financial interest are accounted for using the equity method. Under the equity method, investments in subsidiaries are recorded on the condensed balance sheets at the amount of the Company’s ownership percentage of the subsidiary’s GAAP book value. The income from subsidiaries is reported on a net of tax basis as equity in earnings of subsidiaries on the condensed statements of operations and comprehensive income. Capital contributions to and distributions from subsidiaries are presented within investing activities on the condensed statements of cash flows.

⁽²⁾

During 2017, Lone Tree Holdings, Ltd. (“LTH”), a wholly-owned subsidiary of the Company, merged into the Company. The merger was treated as a liquidation for financial statement purposes. As part of the liquidation, significant non-cash balances that were transferred from LTH to the Company included ending net equity of $2,810.4, intercompany balances of $1,863.1, investments in its subsidiaries of $964.4, short-term investments of $13.0 and other liabilities of $14.1.


⁽³⁾	During 2018, Net loss from discontinued operations includes $17.3 arising from the tax contingency on the sale of Sirius Group. See Note 18 — “Commitments and Contingencies”.


⁽⁴⁾	For the years ended December 31, 2018, 2017 and 2016, amortization of restricted share and option awards was $13.0, $14.8 and $18.5.


⁽⁵⁾	During 2017, as part of its liquidation into the Company, LTH transferred $1,256.7 of cash, which included $1,037.6 of the proceeds from the sale of OneBeacon, to the Company.


⁽⁶⁾	For 2018, 2017 and 2016, net change in other assets and liabilities also included a $4.0, $11.6, and $0.2 net change in payables to the Company’s subsidiaries.


⁽⁷⁾	During 2018, the Company had non-cash (purchases) and sales of short-term investments of ($284.6) and $179.2.


⁽⁸⁾	During 2018, the Company had non-cash purchases of investment securities of $603.9, which included $170.5 of fixed maturity securities, $148.8 of common equity securities and $22.7 of other long term investments.


⁽⁹⁾	During 2018, the Company had non-cash sales of investment securities of $1,065.4, which included $373.4 of fixed maturity securities, $490.1 of common equity securities and $22.7 of other long term investments.

⁽¹⁰⁾

During 2018, the Company had non-cash issuance of debt of $349.5 to its wholly-owned subsidiary, Guilford Holding, Inc. (“GHI”). Proceeds of the debt, which included $170.4 of fixed maturity securities and $179.2 of short-term investments, were transferred to GHI. During 2017, the Company had non-cash issuance of debt from LTH of $94.2. During 2017 and 2016, the Company used cash proceeds received from the issuance of debt from LTH, primarily to fund repurchases of its common shares.

⁽¹¹⁾During 2018, the Company received non-cash repayments of $22.7 from its wholly-owned subsidiary, Bridge Holdings (“Bridge”) in the form of other long term investments.

⁽¹²⁾During 2018, the Company made non-cash contributions of $350.0 by transferring intercompany debt receivable from GHI to Bridge. Also during 2018, the Company made a non-cash contribution of $1.0 by transferring intercompany debt receivable from White Mountains Investment (Luxembourg), a wholly-owned subsidiary of Bridge, to Bridge. During 2018, the Company made cash contributions of $255.3 and $2.9 to its wholly-owned subsidiaries, Bridge and White Mountains Investment, Ltd. During 2017, the Company contributed $700.0 to its wholly-owned subsidiary, GHI.


⁽¹³⁾	During 2017, the Company received non-cash distributions of $1,238.9 from LTH, prior to its liquidation. The distribution was completed through the transfer of fixed maturity investments and common equity securities. During 2016, the Company received a non-cash distribution of $80.0 from LTH. The distribution was completed through the transfer of fixed maturity investments.


⁽¹⁴⁾	The WTM Bank Facility was a direct obligation of the Registrant as of December 31, 2017 and was terminated on May 8, 2018. See Note 5 — “Debt”.

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

SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION

12 Months Ended

Dec. 31, 2018

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

SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION

WHITE MOUNTAINS INSURANCE GROUP, LTD.

SUPPLEMENTARY INSURANCE INFORMATION ⁽¹⁾



	Column A	Column B		Column C		Column D		Column E		Column F		Column G		Column H		Column I		Column J		Column K
	Millions Segment	Deferred Acqui- sition Costs		Future Policy Benefits, Losses, Claims and Loss Expenses		Unearned Premiums		Other Policy Claims and Benefits Payable		Premiums Earned		Net Investment Income⁽²⁾		Benefits, Claims, Losses and Settlement Expenses		Amortization of Deferred Policy Acquisition Costs		Other Operating Expenses		Premiums Written
Years ended:

December 31, 2018
	HG Global/ BAM	$	19.0	$	—	$	176.0	$	—	$	13.9	$	16.7	$	—	$	5.3	$	.4	$	52.9
	Other Operations	—		—		—		—		—		—		—		—		—		—
December 31, 2017
	HG Global/ BAM	14.8		—		136.8		—		9.4		12.3		—		4.0		.4		63.2
	Other Operations⁽³⁾	—		—		—		—		1.0		—		1.1		.1		—		.9
December 31, 2016
	HG Global/ BAM	10.6		—		82.9		—		5.9		9.0		—		3.4		.4		38.6
	Other Operations⁽³⁾	—		—		—		—		7.5		.2		8.0		2.2		.1		6.5

⁽¹⁾Schedule excludes activity related to OneBeacon and Sirius Group for the years ended December 31, 2017 and 2016. See Note 19 — “Held for Sale and Discontinued Operations”.

⁽²⁾ The amounts shown exclude net investment income relating to non-insurance operations of $0.0, $43.7 and $22.9 for the twelve months ended December 31, 2018, 2017 and 2016, respectively.

⁽³⁾The Other Operations amounts shown relate to SSIE. White Mountains completed the sale of SSIE on March 7, 2017. See Note 19 — “Held for Sale and Discontinued Operations”.

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

SCHEDULE IV REINSURANCE

12 Months Ended

Dec. 31, 2018

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

SCHEDULE IV REINSURANCE

WHITE MOUNTAINS INSURANCE GROUP, LTD.

REINSURANCE ⁽¹⁾



Column A	Column B		Column C			Column D		Column E		Column F
$ in Millions Premiums Earned	Gross Amount		Ceded to Other Companies			Assumed from Other Companies		Net Amount		Percentage of Amount Assumed to Net
Year ended:
December 31, 2018
HG Global/BAM	$	13.6	$	—		$	.3	$	13.9	2.2	%
Other Operations	—		—			—		—		—
December 31, 2017
HG Global/BAM	9.4		—			—		9.4		—
Other Operations ⁽²⁾	1.0		—			—		1.0		—
December 31, 2016
HG Global/BAM	5.9		—			—		5.9		—
Other Operations ⁽²⁾	15.2		(7.7		)	—		7.5		—


⁽¹⁾	Schedule excludes activity related to OneBeacon and Sirius Group for the years ended December 31, 2017 and 2016. See Note 19 — “Held for Sale and Discontinued Operations”.


⁽²⁾	The Other Operations amounts shown relate to SSIE. White Mountains completed the sale of SSIE on March 7, 2017. See Note 19 — “Held for Sale and Discontinued Operations”.

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

Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2018	Dec. 31, 2016
Accounting Policies [Abstract]
Nature of Operations and 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. Under GAAP, the Company is required to consolidate any entity in which it holds a controlling financial interest. A controlling financial interest is usually in the form of an investment representing the majority of the subsidiary’s voting interests. However, a controlling financial interest may also arise from a financial interest in a variable interest entity (“VIE”) through arrangements that do not involve ownership of voting interests. The Company consolidates a VIE if it determines that it is the primary beneficiary. The primary beneficiary is defined as the entity who holds a variable interest that gives it both the power to direct the VIE’s activities that most significantly impact its economic performance and the obligation to absorb losses of, or the right to receive returns from, the VIE that could potentially be significant to the VIE. See Note 15 — “Variable Interest Entities”. Intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Reporting Segments	Reportable Segments 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, NSM, MediaAlpha and Other Operations. See Note 13 — “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 reinsurance subsidiary HG Re Ltd. (“HG Re”), to provide up to 15%-of-par, first loss reinsurance protection for policies underwritten by BAM. For capital appreciation bonds, par is adjusted to the estimated equivalent par value for current interest paying bonds. HG Global, together with its subsidiaries, funded the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM, consisting of $203.0 million of Series A Notes and $300.0 million of Series B Notes (the “BAM Surplus Notes”). As of December 31, 2018 and 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 NSM segment consists of NSM Insurance HoldCo, LLC and its wholly-owned subsidiaries (collectively, “NSM”). NSM is a full-service managing general underwriting agency (“MGU”) and program administrator for specialty property and casualty insurance. The company places insurance in niche sectors such as specialty transportation, social services and real estate. On behalf of its insurance carrier partners, NSM manages all aspects of the placement process, including product development, marketing, underwriting, policy issuance and claims. NSM earns commissions based on the volume and profitability of the insurance that it places. NSM does not take insurance risk. As of December 31, 2018, White Mountains owned 95.5% of NSM. The NSM segment also includes White Mountains Catskill Holdings, Inc., the immediate holding company of NSM. See Note 2 — “Significant Transactions". 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 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 the overall performance of their marketing campaigns. MediaAlpha has developed distinctive platform solutions for a range of insurance verticals, including auto, motorcycle, home, renter, health and life, and non-insurance verticals, including travel, education and personal finance. White Mountains’s Other Operations segment consists of the Company and its wholly-owned subsidiary, White Mountains Capital, Inc. (“WM Capital”), its other intermediate holding companies, its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), investment assets managed by WM Advisors, its interests in PassportCard Limited (“PassportCard”) and DavidShield Life Insurance Agency (2000) Ltd. (“DavidShield”) (collectively, “PassportCard/DavidShield”) and Kudu Investment Management, LLC (“Kudu”), certain other consolidated and unconsolidated entities (“Other Operating Businesses”) and certain other strategic investments (“Strategic Investments”). The consolidated entities include Wobi Insurance Agency Ltd. (“Wobi”) and Removal Stars Ltd. (“Buzz”). White Mountains’s Other Operations segment also included its variable annuity reinsurance business, White Mountains Life Reinsurance (Bermuda) Ltd. (“Life Re Bermuda”), which completed its runoff with all of its contracts fully matured on June 30, 2016 and was liquidated in the third quarter of 2017, and its U.S.-based service provider, White Mountains Financial Services LLC, which was liquidated in the second quarter of 2017 (collectively, “WM Life Re”).
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 previously announced 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. 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 was required to consolidate SSIE in its GAAP financial statements until White Mountains completed the sale. White Mountains has presented Star & Shield’s and SSIE’s assets and liabilities as held for sale as of December 31, 2016. See Note 19 — “Held for Sale and Discontinued Operations”. 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 disposal costs of $3.3 million. The related write down of $3.7 million
Cash	Cash Cash includes amounts on hand and demand deposits with banks and other financial institutions. Amounts presented in the statement of cash flows are shown net of balances acquired and sold in the purchase or sale of the Company’s consolidated subsidiaries and exclude changes in amounts of restricted cash. See Note 7 — “Derivatives”.
Short-term Investments	Short-Term Investments Short-term investments consist of interest-bearing money market funds 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 December 31, 2018 and 2017.
Investment securities	Investment Securities As of December 31, 2018, White Mountains’s invested assets consisted of securities and other investments held for general investment purposes. White Mountains’s portfolio of fixed maturity investments, common equity securities and other long-term investments held for general investment purposes are classified as trading securities and are reported at fair value as of the balance sheet date. Changes in net unrealized investment gains (losses) are reported pre-tax in revenues. Realized investment gains (losses) are accounted for using the specific identification method and are reported pre-tax in revenues. Premiums and discounts on all fixed maturity investments are amortized and accreted to income over the anticipated life of the investment. White Mountains’s invested assets that are measured at fair value include fixed maturity investments, common equity securities and other long-term investments, including unconsolidated entities, private equity funds and hedge funds. In determining its estimates of fair value, White Mountains uses a variety of valuation approaches and inputs. Whenever possible, White Mountains estimates fair value using valuation methods that maximize the use of quoted prices and other observable inputs. Fair value measurements are categorized into a hierarchy that distinguishes between inputs based on market data from independent sources (“observable inputs”) and a reporting entity’s internal assumptions based upon the best information available when external market data is limited or unavailable (“unobservable inputs”). Quoted prices in active markets for identical assets have the highest priority (“Level 1”), followed by observable inputs other than quoted prices including prices for similar but not identical assets or liabilities (“Level 2”), and unobservable inputs, including the reporting entity’s estimates of the assumptions that market participants would use, having the lowest priority (“Level 3”). White Mountains uses brokers and outside pricing services to assist in determining fair values. The outside pricing services White Mountains uses have indicated that they will only provide prices where observable inputs are available. As of December 31, 2018, approximately 87% of the investment portfolio recorded at fair value as Level 1 or Level 2 measurements. Level 1 Measurements Investments valued using Level 1 inputs include fixed maturity investments, primarily investments in U.S. Treasuries and short-term investments, which include U.S. Treasury Bills and common equity securities. For investments in active markets, White Mountains uses the quoted market prices provided by outside pricing services to determine fair value. Level 2 Measurements Investments valued using Level 2 inputs include fixed maturity investments, which have been disaggregated into classes, including debt securities issued by corporations, mortgage and asset-backed securities, municipal obligations, and foreign government, agency and provincial obligations. Investments valued using Level 2 inputs also include certain passive exchange traded funds (“ETFs”) that track U.S. stock indices such as the S&P 500 but are traded on foreign exchanges, which management values using the fund manager’s published NAV to account for the difference in market close times. In circumstances where quoted market prices are unavailable or are not considered reasonable, White Mountains estimates the fair value using industry standard pricing methodologies and observable inputs such as benchmark yields, reported trades, broker-dealer quotes, issuer spreads, benchmark securities, bids, offers, credit ratings, prepayment speeds, reference data including research publications and other relevant inputs. Given that many fixed maturity investments do not trade on a daily basis, the outside pricing services evaluate a wide range of fixed maturity investments by regularly drawing parallels from recent trades and quotes of comparable securities with similar features. The characteristics used to identify comparable fixed maturity investments vary by asset type and take into account market convention. White Mountains’s process to assess the reasonableness of the market prices obtained from the outside pricing sources covers substantially all of its fixed maturity investments and includes, but is not limited to, the evaluation of pricing methodologies and a review of the pricing services’ quality control procedures on at least an annual basis, a comparison of its invested asset prices obtained from alternate independent pricing vendors on at least a semi-annual basis, monthly analytical reviews of certain prices and a review of the underlying assumptions utilized by the pricing services for select measurements on an ad hoc basis throughout the year. White Mountains also performs back-testing of selected sales activity to determine whether there are any significant differences between the market price used to value the security prior to sale and the actual sale price on an ad-hoc basis throughout the year. Prices provided by the pricing services that vary by more than 5% and $0.5 million from the expected price based on these assessment procedures are considered outliers, as are prices that have not changed from period to period and prices that have trended unusually compared to market conditions. In circumstances where the results of White Mountains’s review process does not appear to support the market price provided by the pricing services, White Mountains challenges the vendor provided price. If White Mountains cannot gain satisfactory evidence to support the challenged price, White Mountains will rely upon its own pricing methodologies to estimate the fair value of the security in question. The valuation process described above is generally applicable to all of White Mountains’s fixed maturity investments. The techniques and inputs specific to asset classes within White Mountains’s fixed maturity investments for Level 2 securities that use observable inputs are as follows: Debt Securities Issued by Corporations The fair value of debt securities issued by corporations is determined from a pricing evaluation technique that uses information from market sources and integrates relative credit information, observed market movements, and sector news. Key inputs include benchmark yields, reported trades, broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including sector, coupon, credit quality ratings, duration, credit enhancements, early redemption features and market research publications. Mortgage and Asset-Backed Securities The fair value of mortgage and asset-backed securities is determined from a pricing evaluation technique that uses information from market sources and leveraging similar securities. Key inputs include benchmark yields, reported trades, underlying tranche cash flow data, collateral performance, plus new issue data, as well as broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including issuer, vintage, loan type, collateral attributes, prepayment speeds, default rates, recovery rates, cash flow stress testing, credit quality ratings and market research publications. Municipal Obligations The fair value of municipal obligations is determined from a pricing evaluation technique that uses information from market makers, brokers-dealers, buy-side firms, and analysts along with general market information. Key inputs include benchmark yields, reported trades, issuer financial statements, material event notices and new issue data, as well as broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including type, coupon, credit quality ratings, duration, credit enhancements, geographic location and market research publications. Foreign Government, Agency and Provincial Obligations The fair value of foreign government, agency and provincial obligations is determined from a pricing evaluation technique that uses feeds from data sources in each respective country, including active market makers and inter-dealer brokers. Key inputs include benchmark yields, reported trades, broker-dealer quotes, two-sided markets, benchmark securities, bids, offers, local exchange prices, foreign exchange rates and reference data including coupon, credit quality ratings, duration and market research publications. Level 3 Measurements Fair value estimates for investments that trade infrequently and have few or no observable market prices are classified as Level 3 measurements. Investments valued using Level 3 fair value estimates are based upon unobservable inputs and include investments in certain fixed maturity investments, equity securities and other long-term investments where quoted market prices are unavailable or are not considered reasonable. Level 3 valuations are generated from techniques that use assumptions not observable in the market. These unobservable assumptions reflect White Mountains’s assumptions that market participants would use in valuing the investment. Generally, certain securities may start out as Level 3 when they are originally issued but as observable inputs become available in the market, they may be reclassified to Level 2. Transfers between levels are based on investments held as of the beginning of the period.
Derivatives	Derivatives Financial Instruments White Mountains holds from time to time a variety of derivative financial instruments for risk management purposes. White Mountains recognizes all derivatives as either other assets or other liabilities, aside from the foreign currency forward contracts which are recognized within other long-term investments, measured at fair value, in the consolidated balance sheets. Changes in the fair value of derivative instruments are recognized in current period pre-tax income.
Warrants	From time to time, White Mountains holds warrants that it has received in the restructuring of certain of its common equity securities and fixed maturity investments. White Mountains accounts for its investments in warrants as derivatives.
Derivatives - Variable annuity reinsurance		Variable Annuity Reinsurance In 2016, White Mountains completed the run-off of WM Life Re as all of its contracts matured as of June 30, 2016. WM Life Re entered into agreements to reinsure death and living benefit guarantees associated with certain variable annuities in Japan. The accounting for benefit guarantees differs depending on whether or not the guarantee is classified as a derivative or an insurance liability. The liability for guaranteed minimum death benefits was classified as a derivative and measured at fair value. The liability for guaranteed minimum accumulation benefits was classified as an insurance liability, and was measured using assumptions for interest rates, equity markets, foreign exchange rates and market volatilities at the valuation date, as well as annuitant-related actuarial assumptions, including surrender and mortality rates. WM Life Re entered into derivative contracts that were designed to economically hedge against changes in the fair value of living and death benefit liabilities associated with its variable annuity reinsurance arrangements. All WM Life Re’s derivative financial instruments were recorded as assets or liabilities at fair value on the balance sheet within other assets. These derivative financial instruments did not meet the criteria for hedge accounting treatment, and accordingly, changes in fair value were recognized in the appropriate period as gains or losses in the income statement within other revenues.
Receivables	Receivables BAM’s receivables consist primarily of premiums receivable from customers for municipal bond insurance policies. NSM’s receivables consist of insurance premiums receivable from customers and commissions receivable from insurance carriers, net of a provision for amounts estimated to be uncollectible. MediaAlpha receivables consist of advertising fee receivables from publishers and advertisers.
Incentive Compensation	Incentive Compensation White Mountains’s Long-Term Incentive Plan (the “WTM Incentive Plan”) provides for grants of various types of share-based and non-share-based incentive awards to key employees of White Mountains. Non-share based awards are recognized over the related service periods based on management’s best estimate of the amounts at which the awards are expected to be paid. Share-based compensation which is typically settled in cash, such as performance shares or performance units, is classified as a liability-type award. The compensation cost for liability-classified awards is measured initially at the grant date fair value and remeasured each reporting period until settlement. The compensation cost for equity-classified awards expected to be settled in shares, such as options and restricted shares, is measured at the original grant date fair value of the award. The compensation cost for all awards is recognized for the vested portion of the awards over the related service periods. See Note 10 — “Employee Share-Based Incentive Compensation Plans”.
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets Goodwill represents the excess of the amount paid to acquire subsidiaries over the fair value of identifiable net assets at the date of acquisition. Other intangible assets consist primarily of trademarks, URL and online names, customer relationships, information technology and insurance licenses. Goodwill is not amortized, but rather is evaluated for impairment on an annual basis, or whenever indications of potential impairment exist. In the absence of any indications of potential impairment, the evaluation of goodwill is performed during the fourth quarter of each year. White Mountains initially evaluates goodwill using a qualitative approach (step zero) to determine whether it is more likely than not that the fair value of goodwill is greater than its carrying value. If the results of the qualitative evaluation indicate that it is more likely than not that the carrying value of goodwill exceeds its fair value, White Mountains performs the two-step quantitative test for impairment. Other intangible assets with finite lives are measured at their acquisition date fair values, are amortized over their economic lives and presented net of accumulated amortization on the balance sheet. Other intangible assets with finite lives are evaluated for impairment at least annually and when events or changes in circumstances indicate that it is more likely than not that the asset is impaired. A significant portion of the goodwill and other identifiable intangible assets at December 31, 2018 arose from White Mountains’s acquisition of NSM in May 2018 and NSM’s subsequent acquisitions of Fresh Insurance and KBK. White Mountains believes that all of the goodwill and intangible assets recorded in respect of these acquisitions is fully recoverable. Because of the timing of these acquisitions, all of which are still within the one-year measurement period following the acquisition date, these amounts have not yet been subject to White Mountains’s annual review for potential impairment. White Mountains evaluated the recoverability of goodwill and other intangible assets other than the portion associated with the NSM acquisitions and did not recognize any impairment losses for any of the years ended December 31, 2018, 2017 and 2016. See Note 4 — “Goodwill and Other Intangible Assets”.
Municipal Bond Guarantee Insurance	Municipal Bond Guarantee Insurance All of the contracts issued by BAM are accounted for as insurance contracts under ASC 944-605, Financial Guarantee Insurance Contracts. Premiums are generally received upfront and an unearned premium revenue liability, equal to the amount of the premium received, is established at contract inception. Installment premiums are measured at the present value of contractual premiums, discounted at the risk free rate, which is set at the inception of the insurance contract. Premium revenues are recognized in revenue over the period of the contracts in proportion to the amount of insurance protection provided using a constant rate. The constant rate is calculated based on the relationship between the par outstanding in a given reporting period compared with the sum of each of the par amounts outstanding for all periods. Deferred acquisition costs represent commissions, premium taxes, excise taxes and other costs which are directly attributable to and vary with the production of business. These costs are deferred and amortized to the extent they relate to successful contract acquisitions over the applicable premium recognition period as acquisition expenses. Deferred acquisition costs are limited to the amount expected to be recovered from future earned premiums and anticipated investment income. BAM’s obligation for outstanding contracts consists of the unearned premium reserve and any loss reserves. Loss reserves are recorded only to the extent that the present value of the expected amount of any losses to be paid, net of any expected recoveries, exceeds the associated unearned premium reserve. As of December 31, 2018 and 2017, BAM did not have any loss or LAE reserves.
Revenue Recognition	Revenue Recognition MediaAlpha recognizes advertising and publishing fee revenues based on the contractual amount of the fees, adjusted for any amounts expected to be refunded or uncollectible, when it has satisfied its contractual performance obligations, which is generally at the time each transaction is executed. For transactions where MediaAlpha acts as the principal, such as the Open exchange, revenue amounts are reported gross. For transactions where MediaAlpha acts as an agent facilitating transactions between third parties, revenue amounts are reported at the net fee billed. Agent and commission revenues are measured based on the contractual rates with insurance carriers, net of any amounts expected to be uncollectible and any amounts associated with expected policy cancellations adjustments, and are recognized when contractual performance obligations have been fulfilled.
Cost of Sales and Broker Commission Expense	Cost of Sales and Broker Commission Expense MediaAlpha’s cost of sales consists primarily of revenue sharing payments to publisher partners and traffic acquisition costs to top tier search engines. Cost of sales are measured based on contract terms and recognized when the related revenue transactions are executed. NSM’s broker commission expense consists of commissions paid to sub-agents and brokers. Broker commission expense is measured in accordance with contractual terms and recognized when incurred, which is generally at the policy issuance date. Other Operations’s cost of sales consists of salaries and related expenses, professional services and marketing and advertising expenses directly related to sales generation. These expenses are recognized as incurred.
Federal and foreign income taxes	Federal and Foreign Income Taxes A number of White Mountains’s subsidiaries file consolidated tax returns in the United States. Income earned or losses generated by companies outside the United States are generally subject to an overall effective tax rate lower than that imposed by the United States. Deferred tax assets and liabilities are recorded when a difference between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts for tax purposes exists, and for other temporary differences. The deferred tax asset or liability is recorded based on tax rates expected to be in effect when the difference reverses. The deferred tax asset is recognized when it is more likely than not that it will be realized.
Foreign currency exchange	Foreign Currency Exchange The functional currency for White Mountains’s non-U.S. based subsidiaries are measured, in most instances, using functional currencies other than the U.S. dollar. Net foreign exchange gains and losses arising from the translation of functional currencies are generally reported in shareholders’ equity, in accumulated other comprehensive income or loss. White Mountains also invests in securities denominated in foreign currencies. Assets and liabilities recorded in these foreign currencies are translated into U.S. dollars at exchange rates in effect at the balance sheet date, and revenues and expenses are converted using the weighted average exchange rates for the period.
Noncontrolling Interest	Non-controlling Interests Non-controlling interests consist of the ownership interests of non-controlling shareholders in consolidated subsidiaries, and are presented separately on the balance sheet. The portion of comprehensive income attributable to non-controlling interests is presented net of related income taxes in the statement of operations and comprehensive income. See Note 11 — “Common Shareholders’ Equity and Non-controlling Interests”.
Recent 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 the 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 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 $9.2 million of statutory withholding tax payments made in connection with the settlement of restricted shares as financing cash flows for the year ended December 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 that 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. 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. Adoption of ASU 2017-01 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 Financial Accounting Standards Board (“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. The new guidance is effective for annual periods beginning after December 15, 2018. White Mountains does not expect adoption to have any effect on its financial statements. Goodwill In January 2017, the FASB issue ASU 2017-04, Simplifying the Test for Goodwill Impairment (ASC 350), which changes the guidance on goodwill impairment testing. Under the new guidance, the qualitative assessment of the recoverability of goodwill remains the same. However, the second step required under the existing guidance has been eliminated. Goodwill is considered impaired if the carrying value exceeds the estimated fair value. The new guidance is effective for fiscal years beginning after December 15, 2019. 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 loans and trade receivables such as premium receivables and reinsurance recoverables on paid losses. 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 liabilities will be measured initially based on the present value of the lease payments. The lease asset will be measured as the sum of the lease liabilities, adjusted for initial direct costs. White Mountains will be adopting the new guidance effective January 1, 2019, and will use the optional transition method that permits prospective adoption with recognition of a cumulative effect adjustment to the opening balance of retained earnings. White Mountains will not apply the new guidance to comparative periods presented in the year of adoption. White Mountains plans to elect available practical expedients permitted under ASC 842, which will allow White Mountains to carry forward its historical lease classification and not reassess leases for the definition of a lease under the new guidance. White Mountains has estimated that it will recognize lease right-of-use assets and lease liabilities of approximately $24.5 million. White Mountains does not expect the cumulative effect adjustment to opening retained earnings to be material. Financial Instruments - Recognition and Measurement In January 2016, the FASB issued ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities (ASC 825-10). The new ASU 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. ASU 2016-01 is effective for fiscal years beginning after December 15, 2017, with early adoption permitted. 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.

Significant Transactions Significant Transactions (Tables)

12 Months Ended

Dec. 31, 2016

Significant Transactions [Abstract]

Reconciliation to the increase in book value

The following table presents a reconciliation of the gain reported in discontinued operations to the impact to White Mountains’s book value:



Millions	Year ended December 31, 2016
Gain from sale of Tranzact reported in discontinued operations	$	51.9
Add back reclassification from continuing operations for the release of a tax valuation allowance	30.2
Increase to White Mountains’s book value from sale of Tranzact	$	82.1

Investment Securities (Tables)

12 Months Ended

Dec. 31, 2018

Investments, Debt and Equity Securities [Abstract]

Pre-tax net investment income

The following table presents pre-tax net investment income for 2018, 2017 and 2016:



	Year Ended December 31,
Millions	2018			2017			2016
Fixed maturity investments	$	35.1		$	44.9		$	28.5
Short-term investments	8.0			1.8			.9
Common equity securities	15.1			10.6			4.0
Other long-term investments	3.8			1.2			1.1
Total investment income	62.0			58.5			34.5
Third-party investment expenses	(3.0		)	(2.5		)	(2.4		)
Net investment income, pre-tax	$	59.0		$	56.0		$	32.1

Net Realized and Unrealized Investment Gains and Losses

The following table presents net realized and unrealized investment gains (losses) for 2018, 2017 and 2016:



	Year Ended December 31,
Millions	2018			2017			2016
Net realized investment (losses) gains, pre-tax	$	(12.9	)	$	24.1		$	270.0
Net unrealized investment (losses) gains, pre-tax	(95.4		)	109.2			(297.4		)
Net realized and unrealized investment (losses) gains, pre-tax	(108.3		)	133.3			(27.4		)
Income tax benefit (expense) attributable to net realized and unrealized investment (losses) gains	18.2			(12.9		)	2.7
Net realized and unrealized investment (losses) gains, after-tax	$	(90.1	)	$	120.4		$	(24.7	)

Realized Gain (Loss) on Investments

The following tables present net realized investment gains (losses) for 2018, 2017 and 2016:



	Year Ended December 31, 2018
Millions	Net Realized (Losses) Gains			Net Foreign Exchange Gains (Losses)			Total Net Realized (Losses) Gains Reflected in Earnings
Fixed maturity investments	$	(29.8	)	$	18.2		$	(11.6	)
Short-term investments	(.8		)	—			(.8		)
Common equity securities	6.6			—			6.6
Other long-term investments	.1			(7.2		)	(7.1		)
Net realized investment (losses) gains, pre-tax	(23.9		)	11.0			(12.9		)
Income tax benefit attributable to net realized investment (losses) gains	9.1			—			9.1
Net realized investment (losses) gains, after-tax	$	(14.8	)	$	11.0		$	(3.8	)



	Year Ended December 31, 2017
Millions	Net Realized (Losses) Gains			Net Foreign Exchange Gains (Losses)			Total Net Realized Gains (Losses) Reflected in Earnings
Fixed maturity investments	$	(1.6	)	$	4.1		$	2.5
Short-term investments	(.3		)	—			(.3		)
Common equity securities	18.1			6.0			24.1
Other long-term investments	19.1			(21.3		)	(2.2		)
Net realized investment gains (losses), pre-tax	35.3			(11.2		)	24.1
Income tax expense attributable to net realized investment gains (losses)	(8.9		)	—			(8.9		)
Net realized investment gains (losses), after-tax	$	26.4		$	(11.2	)	$	15.2



	Year Ended December 31, 2016
Millions	Net Realized (Losses) Gains			Net Foreign Exchange Gains		Total Net Realized (Losses) Gains Reflected in Earnings
Fixed maturity investments	$	(1.9	)	$	.3	$	(1.6	)
Short-term investments	.4			—		.4
Common equity securities	268.5			—		268.5
Other long-term investments	2.7			—		2.7
Net realized investment gains, pre-tax	269.7			.3		270.0
Income tax expense attributable to net realized investment gains	(45.6		)	—		(45.6		)
Net realized investment gains, after-tax	$	224.1		$	.3	$	224.4

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 years ended 2018, 2017 and 2016:



	Year Ended December 31, 2018
Millions	Net Unrealized (Losses) Gains			Net Foreign Exchange (Losses) Gains			Total Net Unrealized (Losses) Gains Reflected in Earnings
Fixed maturity investments	$	(8.3	)	$	(14.8	)	$	(23.1	)
Common equity securities	(105.5		)	—			(105.5		)
Other long-term investments	30.2			3.0			33.2
Net unrealized investment losses, pre-tax	(83.6		)	(11.8		)	(95.4		)
Income tax benefit attributable to net unrealized investment losses	9.1			—			9.1
Net unrealized investment losses, after-tax	$	(74.5	)	$	(11.8	)	$	(86.3	)



	Year Ended December 31, 2017
Millions	Net Unrealized Gains (Losses)			Net Foreign Exchange Gains (Losses)			Total Net Unrealized Gains (Losses) Reflected in Earnings
Fixed maturity investments	$	13.8		$	12.7		$	26.5
Common equity securities	99.3			—			99.3
Other long-term investments	(15.6		)	(1.0		)	(16.6		)
Net unrealized investment gains, pre-tax	97.5			11.7			109.2
Income tax expense attributable to net unrealized investment gains	(4.0		)	—			(4.0		)
Net unrealized investment gains, after-tax	$	93.5		$	11.7		$	105.2



	Year Ended December 31, 2016
Millions	Net Unrealized Losses			Net Foreign Exchange Gains (Losses)			Total Net Unrealized Losses Reflected in Earnings
Fixed maturity investments	$	(14.6	)	$	2.1		$	(12.5	)
Common equity securities	(257.4		)	(3.3		)	(260.7		)
Other long-term investments	(22.7		)	(1.5		)	(24.2		)
Net unrealized investment losses, pre-tax	(294.7		)	(2.7		)	(297.4		)
Income tax benefit attributable to net unrealized investment losses	48.3			—			48.3
Net unrealized investment losses, after-tax	$	(246.4	)	$	(2.7	)	$	(249.1	)

Net unrealized investment gains (losses) for Level 3 investments



	Year Ended December 31,
Millions	2018		2017			2016
Fixed maturity investments	$	—	$	—		$	0.1
Other long-term investments	22.6		(15.4		)	(14.3		)
Total net unrealized investment gains (losses), pre-tax - Level 3 investments	$	22.6	$	(15.4	)	$	(14.2	)

Investment holdings, fixed maturity investments



	December 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	$	154.0	$	.1	$	(.9	)	$	—	$	153.2
Debt securities issued by corporations	519.0		1.0		(9.5		)	—		510.5
Municipal obligations	279.0		2.4		(1.1		)	—		280.3
Mortgage and asset-backed securities	136.1		.1		(2.7		)	—		133.5
Total fixed maturity investments	$	1,088.1	$	3.6	$	(14.2	)	$	—	$	1,077.5



	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

Schedule of contractual maturities of fixed maturities



	December 31, 2018
Millions	Cost or Amortized Cost		Carrying Value
Due in one year or less	$	84.8	$	84.4
Due after one year through five years	500.8		496.2
Due after five years through ten years	222.4		218.0
Due after ten years	144.0		145.4
Mortgage and asset-backed securities	136.1		133.5
Total	$	1,088.1	$	1,077.5

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



	December 31, 2018
Millions	Cost or Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Net Foreign Currency Losses			Carrying Value
Common equity securities	$	904.7	$	51.0	$	(30.1	)	$	—		$	925.6
Other long-term investments	$	330.3	$	52.2	$	(54.9	)	$	(2.0	)	$	325.6



	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

Fair value measurements by level, investment securities



	December 31, 2018
Millions	Fair Value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	153.2	$	153.2	$	—	$	—
Debt securities issued by corporations:
Financials	143.4		—		143.4		—
Consumer	68.5		—		68.5		—
Technology	60.5		—		60.5		—
Energy	57.6		—		57.6		—
Healthcare	55.0		—		55.0		—
Industrial	47.6		—		47.6		—
Communications	31.8		—		31.8		—
Materials	26.3		—		26.3		—
Utilities	19.8		—		19.8		—
Total debt securities issued by corporations	510.5		—		510.5		—

Mortgage and asset-backed securities	133.5		—		133.5		—
Municipal obligations	280.3		—		280.3		—
Total fixed maturity investments	1,077.5		153.2		924.3		—

Short-term investments ⁽¹⁾	214.2		204.4		9.8		—

Common equity securities:

Exchange traded funds ⁽²⁾	675.3		617.0		58.3		—
Healthcare	14.0		14.0		—		—
Financials	13.5		13.5		—		—
Communications	12.7		12.7		—		—
Industrial	11.4		11.4		—		—
Technology	7.4		7.4		—		—
Consumer	6.2		6.2		—		—
Energy	4.1		4.1		—		—
Materials	3.1		3.1		—		—
Other ⁽³⁾	177.9		—		177.9		—
Total common equity securities	925.6		689.4		236.2		—
Other long-term investments	138.7		—		—		138.7
Other long-term investments — NAV⁽⁴⁾	186.9		—		—		—
Total investments	$	2,542.9	$	1,047.0	$	1,170.3	$	138.7

⁽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.



	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		—
Healthcare	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		—
Healthcare	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⁾	77.2		—		—		77.2
Other long-term investments — NAV⁽⁵⁾	135.3		—		—		—
Total investments	$	3,384.4	$	1,041.4	$	2,130.5	$	77.2

⁽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.

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 December 31, 2018 and 2017:



	Fair Value at December 31,
Millions	2018		2017
AAA	$	8.9	$	1.6
AA	88.7		42.6
A	270.5		192.5
BBB	142.4		465.2
BB	—		161.7
B	—		17.3
Debt securities issued by corporations ⁽¹⁾	$	510.5	$	880.9


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

Mortgage-backed, asset-backed securities

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



	December 31, 2018						December 31, 2017
Millions	Fair Value		Level 2		Level 3		Fair Value		Level 2		Level 3
Mortgage-backed securities:
Agency:
FNMA	$	53.6	$	53.6	$	—	$	84.5	$	84.5	$	—
FHLMC	38.1		38.1		—		62.0		62.0		—
GNMA	23.7		23.7		—		46.3		46.3		—
Total agency ⁽¹⁾	115.4		115.4		—		192.8		192.8		—
Non-agency:
Commercial	—		—		—		70.5		70.5		—
Total non-agency	—		—		—		70.5		70.5		—

Total mortgage-backed securities	115.4		115.4		—		263.3		263.3		—
Other asset-backed securities:
Vehicle receivables	9.2		9.2		—		142.4		142.4		—
Credit card receivables	8.9		8.9		—		206.0		206.0		—
Other	—		—		—		83.0		83.0		—
Total other asset-backed securities	18.1		18.1		—		431.4		431.4		—
Total mortgage and asset-backed securities	$	133.5	$	133.5	$	—	$	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).

Other Investments Not Readily Marketable

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



	Carrying Value at December 31,
Millions	2018		2017
PassportCard/DavidShield ⁽¹⁾	$	75.0	$	21.0
Kudu	30.7		—
Other unconsolidated entities ⁽¹⁾⁽²⁾	60.0		62.2
Total unconsolidated entities⁽¹⁾⁽²⁾	165.7		83.2
Private equity funds and hedge funds	146.1		125.3
Foreign currency forward contracts	—		(3.7		)
Other	13.8		4.0
Total other long-term investments	$	325.6	$	208.8

⁽¹⁾See Fair Value Measurements by Level table.

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

Other long-term investments

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



	December 31, 2018				December 31, 2017
Millions	Fair Value		Unfunded Commitments		Fair Value		Unfunded Commitments

Private equity funds
Manufacturing/Industrial	$	42.9	$	10.5	$	43.3	$	10.4
Aerospace/Defense/Government	27.6		34.9		15.8		12.9
Direct lending	13.0		17.7		7.1		23.1
Financial services	8.3		13.6		4.2		11.7
Total private equity funds	91.8		76.7		70.4		58.1

Hedge funds
Long/short banks and financial	54.3		—		54.9		—
Total hedge funds	54.3		—		54.9		—
Total private equity funds and hedge funds included in other long-term investments	$	146.1	$	76.7	$	125.3	$	58.1

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 December 31, 2018:



Millions	1 – 3 years		3 – 5 years		5 – 10 years		>10 years		Total
Private equity funds — expected lock-up period remaining	$	1.8	$	5.5	$	63.2	$	21.3	$	91.8

Rollforward of fair value investments by level

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



							Level 3 Investments		Unconsolidated Entities, Private Equity Funds and Hedge Funds Measured at NAV ⁽³⁾
Millions	Level 1 Investments			Level 2 Investments			Other Long-term Investments					Total
Balance at December 31, 2017	$	890.4		$	2,105.4		$	77.2	$	135.3		$	3,208.3		⁽¹⁾⁽²⁾
Net realized and unrealized (losses) gains	(64.9		)	(64.4		)	16.2		13.3			(99.8		)	⁽⁴⁾
Amortization/accretion	.2			(2.7		)	—		—			(2.5		)
Purchases	514.7			783.8			45.3		50.5			1,394.3
Sales	(497.8		)	(1,661.6		)	—		(12.2		)	(2,171.6		)
Transfers in	—			—			—		—			—
Transfers out	—			—			—		—			—
Balance at December 31, 2018	$	842.6		$	1,160.5		$	138.7	$	186.9		$	2,328.7		⁽²⁾


⁽¹⁾	Excludes carrying value of $(3.7) as of December 31, 2017 associated with foreign currency forward contracts.


⁽²⁾	Excludes carrying value of $214.2 and $176.1 as of December 31, 2018 and 2017 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”.



							Level 3 Investments						Unconsolidated Entities, Private Equity Funds and Hedge Funds Measured at NAV (3)
Millions	Level 1 Investments			Level 2 Investments			Fixed Maturity Investments			Other Long- term Investments						Total
Balance at December 31, 2016	$	279.5		$	2,093.8		$	—		$	91.4		$	82.6		$	2,547.3		^(1)(2)(5)
Net realized and unrealized gains (losses)	82.7			69.6			—			(15.3		)	20.4			157.4			⁽⁴⁾
Amortization/accretion	—			(9.1		)	—			—			—			(9.1		)
Purchases	1,209.3			2,007.9			31.2			3.1			81.0			3,332.5
Sales	(681.1		)	(2,070.3		)	(12.5		)	(2.0		)	(48.7		)	(2,814.6		)
Deconsolidation of SSIE	—			(5.2		)	—			—			—			(5.2		)
Transfers in	—			18.7			—			—			—			18.7
Transfers out	—			—			(18.7		)	—			—			(18.7		)
Balance at December 31, 2017	$	890.4		$	2,105.4		$	—		$	77.2		$	135.3		$	3,208.3		⁽¹⁾⁽²⁾

⁽¹⁾Excludes carrying value of $(3.7) and $(1.2) as of December 31, 2017 and 2016 associated with foreign currency forward contracts.

⁽²⁾Excludes carrying value of $176.1 and $175.0 as of December 31, 2017 and 2016 classified as short-term investments, of which $0.1 is classified as held for sale at December 31, 2016.


⁽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 and short-term investments of $23.8 and $0.3 for the year ended December 31, 2017.

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



$ in Millions, Except Share Price	December 31, 2018
Description	Valuation Technique(s)	Fair Value ⁽¹⁾	Unobservable Input
PassportCard/DavidShield	Discounted cash flow	$75.0	Discount rate	-	18.0%
			Exit multiple	-	1.00
Compare.com	Discounted cash flow	$16.9	Discount rate	-	22.0%
			Exit multiple	-	2.75
YOUSURE Tarifvergleich GmbH (“durchblicker”)	Discounted cash flow	$15.5	Discount rate	-	23.0%
			Exit multiple	-	2.25
Captricity, Inc.	Discounted cash flow	$14.5	Discount rate	-	23.0%
			Exit multiple	-	3.75
Galvanic Applied Sciences	Multiple of EBITDA	$3.1	EBITDA multiple	-	6.00
Private debt instrument	Discounted cash flow	$10.0	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
PassportCard	Discounted cash flow	$21.0	Discount rate	-	25.0%
			Exit multiple	-	1.00
Compare.com	Discounted cash flow	$22.1	Discount rate	-	35.0%
			Exit multiple	-	1.75
durchblicker	Discounted cash flow	$11.3	Discount rate	-	21.0%
			Exit multiple	-	1.75
Captricity, Inc.	Discounted cash flow	$14.5	Discount rate	-	30.0%
			Exit multiple	-	3.50
Galvanic Applied Sciences	Multiple of EBITDA	$0.6	EBITDA multiple	-	6.00
OneTitle Holdings LLC	Share price of most recent transaction	$3.6	Share price	-	$2.52

⁽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)

12 Months Ended

Dec. 31, 2018

Goodwill and Intangible Assets Disclosure [Abstract]

Schedule of Intangible Assets and Goodwill

The following table presents the economic lives, acquisition date values, accumulated amortization and net carrying values for other intangible assets and goodwill, by company:



$ in Millions	Weighted Average Economic Life (in Years)	December 31, 2018							December 31, 2017
$ in Millions	Weighted Average Economic Life (in Years)	Acquisition Date Fair Value		Accumulated Amortization		Net Carrying Value			Acquisition Date Fair Value		Accumulated Amortization		Net Carrying Value
Goodwill:
NSM ⁽¹⁾⁽²⁾	N/A	$	354.3	$	—	$	354.3		$	—	$	—	$	—
MediaAlpha	N/A	18.3		—		18.3			18.3		—		18.3
Buzz ⁽³⁾	N/A	7.3		—		7.3			7.6		—		7.6
Total goodwill		379.9		—		379.9			25.9		—		25.9

Other intangible assets:
NSM ⁽¹⁾
Customer relationships	9	85.3		6.0		79.3			—		—		—
Trade names	20	51.2		1.8		49.4			—		—		—
Information technology	5	3.7		.5		3.2			—		—		—
Subtotal		140.2		8.3		131.9			—		—		—

MediaAlpha
Customer relationships	9	26.8		4.9		21.9			26.8		2.4		24.4
Information technology	5	33.3		30.9		2.4			33.3		24.3		9.0
Other	3	9.8		9.0		.8			9.8		7.8		2.0
Subtotal		69.9		44.8		25.1			69.9		34.5		35.4

Buzz
Trademark	7	.6		.2		.4			.6		.1		.5
Information technology	5	.5		.3		.2			.5		.2		.3
Subtotal		1.1		.5		.6			1.1		.3		.8

Total other intangible assets		211.2		53.6		157.6			71.0		34.8		36.2
Total goodwill and other intangible assets		$	591.1	$	53.6	$	537.5		$	96.9	$	34.8	$	62.1

Goodwill and other intangible assets attributed to non-controlling interests						(40.6		)					(21.1		)
Goodwill and other intangible assets included in White Mountains’s common shareholders’ equity						$	496.9						$	41.0

⁽¹⁾Includes the effect of foreign currency translation from the date of acquisition of $(2.2) for goodwill and $(0.7) for other intangible assets.

⁽³⁾Includes the effect of foreign currency translation from the date of acquisition of $(0.3) for goodwill.

Schedule of Goodwill and Intangible Assets Rollforward

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



	December 31,
	2018						2017
Millions	Goodwill			Other Intangible Assets			Goodwill		Other Intangible Assets
Beginning balance	$	25.9		$	36.2		$	25.9	$	19.3
Acquisitions of businesses ⁽¹⁾	356.5			140.9			—		—
Acquisitions of asset groups⁽²⁾	—			—			—		27.6
Foreign currency translation	(2.5		)	(.7		)	—		—
Acquisitions of other intangible assets	—			—			—		—
Amortization	—			(18.8		)	—		(10.7		)
Ending balance	$	379.9		$	157.6		$	25.9	$	36.2

⁽¹⁾During 2018, amounts include acquisitions related to NSM, Fresh Insurance and KBK. See Note 2 — “Significant Transactions”.

Schedule of Finite-Lived Intangible Assets, Future Amortization Expense

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



Millions	Amortization Expense
2019	$	18.1
2020	16.0
2021	16.0
2022	15.8
2023 and years after	89.5
Total	$	155.4

Debt (Tables)

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Schedule of debt outstanding

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



	December 31,			Effective	December 31,			Effective
Millions	2018			Rate ⁽¹⁾	2017			Rate ⁽¹⁾
WTM Bank Facility	$	—		N/A	$	—		N/A
NSM Bank Facility	180.4			7.4%	—
Unamortized issuance cost	(3.8		)		—
NSM Bank Facility, carrying value	176.6			—	—
MediaAlpha Bank Facility	14.3			7.1%	23.9			5.6%
Unamortized issuance cost	(.1		)		(.1		)
MediaAlpha Bank Facility, carrying value	14.2				23.8
Other NSM debt, carrying value	1.9				—
Total debt	$	192.7			$	23.8

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

Schedule of contractual repayments of debt

schedule of contractual repayments of White Mountains’s debt as of December 31, 2018:



Millions	December 31, 2018
Due in one year or less	$	5.2
Due in two to three years	10.4
Due in four to five years	9.4
Due after five years	171.9
Total	$	196.9

- Definition

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

+ References

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+ Details

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Schedule of total income tax benefit (expense)

The following table presents the total income tax benefit for the years ended December 31, 2018, 2017 and 2016:



	Year Ended December 31,
Millions	2018			2017			2016
Current income tax (expense) benefit:
U.S. federal	$	(.1	)	$	(.3	)	$	21.4
State	(1.4		)	(1.3		)	(.7		)
Non-U.S.	(2.9		)	(2.0		)	(.3		)
Total current income tax (expense) benefit	(4.4		)	(3.6		)	20.4
Deferred income tax benefit:
U.S. federal	8.3			11.4			12.5
Non-U.S.	.1			—			—
Total deferred income tax benefit	8.4			11.4			12.5
Total income tax benefit	$	4.0		$	7.8		$	32.9

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



	Year Ended December 31,
Millions	2018			2017			2016
Tax benefit (expense) at the U.S. statutory rate	$	37.4		$	(2.7	)	$	51.6
Differences in taxes resulting from:
Change in valuation allowance	(31.0		)	42.6			6.9
State taxes	4.0			.6			(1.2		)
Non-U.S. earnings, net of foreign taxes	(2.9		)	21.5			(19.2		)
Withholding tax	(2.7		)	(2.0		)	(.2		)
Member’s surplus contributions (“MSC”)	(2.6		)	(3.0		)	(2.3		)
Tax rate changes	1.7			(44.3		)	(3.9		)
Tax reserve adjustments	(.8		)	(.3		)	—
Tax exempt interest and dividends	.6			.5			.1
Officer compensation	—			(4.1		)	—
Other, net	.3			(1.0		)	1.1
Total income tax benefit on pre-tax (loss) income	$	4.0		$	7.8		$	32.9

Schedule of components of deferred income tax assets and liabilities

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



	December 31,
Millions	2018		2017
Deferred tax assets related to:
U.S. federal and state net operating and capital loss carryforwards	$	95.5	$	73.0
Non-U.S. net operating loss carryforwards	36.6		33.9
Incentive compensation	14.1		20.4
Investment basis difference	11.1		4.9
Net unrealized investment losses	9.5		—
Tax credit carryforwards	4.2		1.3
Deferred acquisition costs	3.5		2.0
Other items	5.8		1.6
Total gross deferred tax assets	180.3		137.1
Less: valuation allowances	139.9		109.6
Total net deferred tax assets	40.4		27.5
Deferred tax liabilities related to:
MSC	32.8		24.1
Purchase accounting	4.2		.2
Net unrealized investment gains	—		1.0
Other items	1.2		.9
Total deferred tax liabilities	38.2		26.2
Net deferred tax asset	$	2.2	$	1.3

Schedule of net operating and capital loss carryforwards by expiration dates and the deferred tax assets thereon

The following table presents net operating loss and capital loss carryforwards as of December 31, 2018, the expiration dates and the deferred tax assets thereon:



	December 31, 2018
Millions	United States			Luxembourg			United Kingdom			Israel			Total
2019-2023	$	.4		$	—		$	—		$	—		$	.4
2024-2028	—			—			—			—			—
2029-2038	342.8			47.0			—			—			389.8
No expiration date	81.5			29.8			14.9			61.1			187.3
Total	$	424.7		$	76.8		$	14.9		$	61.1		$	577.5
Gross deferred tax asset	89.7			20.0			2.6			14.0			126.3
Valuation allowance	(87.3		)	(20.0		)	(1.8		)	(14.0		)	(123.1		)
Net deferred tax asset	$	2.4		$	—		$	.8		$	—		$	3.2

Reconciliation of changes in the amount of unrecognized tax benefits

The following table presents a reconciliation of the beginning and ending amount of unrecognized tax benefits for 2018 and 2017:



Millions	Permanent Differences ⁽¹⁾		Temporary Differences ⁽²⁾		Interest and Penalties ⁽³⁾		Total
Balance at January 1, 2017	$	—	$	—	$	—	$	—
Changes in prior year tax positions	.1		—		—		.1
Tax positions taken during the current year	.2		—		—		.2
Balance at December 31, 2017	.3		—		—		.3
Changes in prior year tax positions	.8		—		—		.8
Balance at December 31, 2018	$	1.1	$	—	$	—	$	1.1


⁽¹⁾	Represents the amount of unrecognized tax benefits that, if recognized, would impact the effective tax rate.


⁽²⁾	Represents the amount of unrecognized tax benefits that, if recognized, would create a temporary difference between the reported amount of an item in White Mountains’s Consolidated Balance Sheet and its tax basis.


⁽³⁾	Net of tax benefit.

Derivatives (Tables)

12 Months Ended

Dec. 31, 2018

Derivative [Line Items]

Schedule of collateral

The following table presents the gross notional amounts and carrying values associated with the foreign currency forward contracts as of December 31, 2017:



	December 31, 2017
Millions	Notional Amount		Carrying Value			Standard & Poor’s Rating ⁽¹⁾
Barclays Bank PLC	$	206.3	$	(3.7	)	A

⁽¹⁾

Variable Annuity Reinsurance

Derivative [Line Items]

Pre-tax operating results

The following table presents the pre-tax operating results of WM Life Re for the year ended December 31, 2016:



Millions	Year Ended December 31, 2016
Fees, included in other revenue	$	1.2
Change in fair value of variable annuity liability, included in other revenue	(.3		)
Change in fair value of derivatives, included in other revenue	(2.0		)
Foreign exchange, included in other revenue	1.3
Total revenues	.2
Death benefit claims paid, included in general and administrative expenses	(.3		)
General and administrative expenses	(2.6		)
Pre-tax loss	$	(2.7	)

Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values, by the type of instrument

The following table presents realized and unrealized derivative gains (losses) recognized in other revenue for the year ended December 31, 2016 by type of instrument:



Millions	Gains (Losses) Year Ended December 31, 2016
Fixed income/interest rate	$	1.8
Foreign exchange	(4.8		)
Equity	1.0
Total	$	(2.0	)

Fair Value of Assets and Liabilities Measured on Recurring and Non-recurring Basis Table

The following tables present the changes in White Mountains’s variable annuity reinsurance liabilities and derivative instruments for the year ended December 31, 2016:



	Variable Annuity Liabilities			Derivative Instruments
Millions	Level 3			Level 3 ⁽¹⁾			Level 2 ⁽¹⁾⁽²⁾			Level 1 ⁽³⁾			Total
Balance at January 1, 2016	$	.3		$	2.7		$	16.5		$	.9		$	20.1
Purchases	—			—			—			—			—
Realized and unrealized (losses) gains	(.3		)	2.9			(.7		)	(4.2		)	(2.0		)
Transfers in	—			—			—			—			—
Sales/settlements	—			(5.6		)	(15.8		)	3.3			(18.1		)
Balance at December 31, 2016	$	—		$	—		$	—		$	—		$	—


⁽¹⁾	Consists of over-the-counter instruments.


⁽²⁾	Consists of interest rate swaps, total return swaps, foreign currency forward contracts, and bond forwards. Fair value measurement based upon bid/ask pricing quotes for similar instruments that are actively traded, where available. Swaps for which an active market does not exist have been priced using observable inputs including the swap curve and the underlying bond index.


⁽³⁾	Consists of exchange traded equity index, foreign currency and interest rate futures. Fair value measurements based upon quoted prices for identical instruments that are actively traded.

Municipal Bond Guarantee Insurance (Tables)

12 Months Ended

Dec. 31, 2018

Insurance [Abstract]

Financial Guarantee Insurance Contracts, Premium Received over Contract Period [Table Text Block]

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



Millions	December 31, 2018
January 1, 2019 - March 31, 2019	$	4.1
April 1, 2019 - June 30, 2019	4.0
July 1, 2019 - September 30, 2019	4.0
October 1, 2019 - December 31, 2019	3.8
	15.9
2020	15.0
2021	14.0
2022	13.2
2023	12.3
2024 and thereafter	105.6
Total gross unearned insurance premiums	$	176.0

Schedule of Insured Obligations

The following table presents a schedule of BAM’s insured obligations as of December 31, 2018 and 2017:



	December 31, 2018		December 31, 2017
Contracts outstanding	7,525		6,371
Remaining weighted average contract period (in years)	10.7		10.9
Contractual debt service outstanding (in millions):
Principal	$	52,201.6	$	42,090.6
Interest and capital appreciation	26,560.3		21,057.1
Total debt service outstanding	$	78,761.9	$	63,147.7

Gross unearned insurance premiums	$	176.0	$	136.8

Schedule of Net Written Premiums

The following table presents a schedule of net written premiums and net earned premiums included in White Mountains’s HG Global/BAM segment for the years ended December 31, 2018, 2017 and 2016:



Millions	December 31, 2018		December 31, 2017		December 31, 2016
Written premiums:
Direct	$	44.8	$	63.2	$	38.6
Assumed	8.1		—		—
Net written premiums	$	52.9	$	63.2	$	38.6
Earned premiums:
Direct	$	13.6	$	9.4	$	5.9
Assumed	.3		—		—
Net earned premiums	$	13.9	$	9.4	$	5.9

Earnings Per Share (Tables)

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Computation of earnings per share



	Year Ended December 31,
	2018			2017			2016
Basic and diluted earnings per share numerators (in millions):
Net (loss) income attributable to White Mountains’s common shareholders	$	(141.2	)	$	627.2		$	401.8
Less: total (loss) income from discontinued operations, net of tax	(17.2		)	577.5			523.4
Net (loss) income from continuing operations attributable to White Mountains’s common shareholders	(124.0		)	49.7			(121.6		)
Allocation of earnings (losses) to participating restricted common shares ⁽¹⁾	1.4			(.7		)	1.5
Basic and diluted (losses) earnings per share numerators	$	(122.6	)	$	49.0		$	(120.1	)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	$	3,382.5		$	4,293.8		$	5,014.9
Average unvested restricted common shares ⁽²⁾	(40.1		)	(54.3		)	(64.8		)
Basic (losses) earnings per share denominator	$	3,342.4		$	4,239.5		$	4,950.1
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	$	3,382.5		$	4,293.8		$	5,018.1
Average unvested restricted common shares ⁽²⁾	(40.1		)	(54.3		)	(64.8		)
Diluted (losses) earnings per share denominator ⁽³⁾	$	3,342.4		$	4,239.5		$	4,953.3
Basic and diluted earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid	$	1.00		$	1.00		$	1.00
Undistributed (losses) earnings	$	(37.67	)	$	10.56		$	(25.26	)
Basic and diluted (losses) earnings per share	$	(36.67	)	$	11.56		$	(24.26	)


⁽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”.


⁽³⁾	The diluted earnings (loss) per share denominator for the year ended December 31, 2016, includes the impact of 40,000 common shares issuable upon exercise of the non-qualified options outstanding, which resulted in 3,217 incremental shares outstanding over the period.

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



	Year Ended December 31,
Millions		2018			2017			2016
Undistributed net earnings - continuing operations:
Net (loss) income attributable to White Mountains’s common shareholders, net of restricted common share amounts		$	(122.6	)	$	49.0		$	(120.1	)
Dividends declared, net of restricted common share amounts ⁽¹⁾		(3.7		)	(4.5		)	(5.4		)
Total undistributed net (losses) earnings, net of restricted common share amounts		$	(126.3	)	$	44.5		$	(125.5	)

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

Net Income (Loss) from Discontinued Operations



	Year Ended December 31, 2017
Millions	OneBeacon		Sirius Group			Tranzact		Total
Revenues
Earned insurance premiums	$	807.6	$	—		$	—	$	807.6
Net investment income	39.7		—			—		39.7
Net realized and unrealized investment gains	38.8		—			—		38.8
Other revenues	7.7		—			—		7.7
Total revenues	893.8		—			—		893.8
Expenses
Loss and loss adjustment expenses	546.0		—			—		546.0
Insurance and reinsurance acquisition expenses	145.6		—			—		145.6
Other underwriting expenses	156.2		—			—		156.2
General and administrative expenses	21.2		—			—		21.2
Interest expense	10.0							10.0
Total expenses	879.0		—			—		879.0
Pre-tax income	14.8		—			—		14.8
Income tax benefit	5.7		—			—		5.7
Net income from discontinued operations	20.5		—			—		20.5
Gain (loss) from sale of discontinued operations, net of tax	554.5		(.7		)	3.2		557.0
Total income (loss) from discontinued operations	575.0		(.7		)	3.2		577.5
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax	.3		—			—		.3
Recognition of benefit plan assets and obligations from the sale of OneBeacon, net of tax	2.9		—			—		2.9
Comprehensive income (loss) from discontinued operations	$	578.2	$	(.7	)	$	3.2	$	580.7



	Year Ended December 31, 2016
Millions	OneBeacon		Sirius Group			Tranzact			Total
Revenues
Earned insurance premiums	$	1,100.6	$	240.1		$	—		$	1,340.7
Net investment income	50.6		14.4			—			65.0
Net realized and unrealized investment gains (losses)	37.7		(1.5		)	—			36.2
Other revenues	5.5		.6			119.6			125.7
Total revenues	1,194.4		253.6			119.6			1,567.6
Expenses
Loss and loss adjustment expenses	656.0		154.9			—			810.9
Insurance and reinsurance acquisition expenses	206.0		59.0			—			265.0
Other underwriting expenses	209.0		30.9			—			239.9
General and administrative expenses	14.2		10.4			116.7			141.3
Interest expense	13.1		7.9			3.2			24.2
Total expenses	1,098.3		263.1			119.9			1,481.3
Pre-tax income (loss)	96.1		(9.5		)	(.3		)	86.3
Income tax benefit	12.5		3.1			6.4			22.0
Net income (loss) from discontinued operations	108.6		(6.4		)	6.1			108.3
Gain from sale of discontinued operations, net of tax	—		363.2			51.9			415.1
Total income from discontinued operations	108.6		356.8			58.0			523.4
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax	1.0		32.0			—			33.0
Recognition of foreign currency translation from sale of Sirius Group, net of tax	—		113.3			—			113.3
Comprehensive income from discontinued operations	$	109.6	$	502.1		$	58.0		$	669.7

Net Change in Cash from Discontinued Operations



	Year Ended December 31,
Millions	2017			2016
Net cash provided from operations	$	157.0		$	23.6
Net cash provided from (used for) investing activities	3.0			241.4
Net cash used for financing activities	(61.9		)	(93.8		)
Net change in cash during the period	98.1			171.2
Cash balances at beginning of period	70.5			245.4
Net change in cash held for sale	(.9		)	(.3		)
Cash sold as part of sale of consolidated subsidiaries	(167.7		)	(345.8		)
Cash balances at end of period	$	—		$	70.5

- 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.

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+ Details

Employee Share-Based Incentive Compensation Plans (Tables)

12 Months Ended

Dec. 31, 2018

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

Summary of performance share activity

The following table presents performance share activity for the years ended December 31, 2018, 2017 and 2016 for performance shares granted under the WTM Incentive Plan:



	Year Ended December 31,
	2018					2017					2016
$ in Millions	Target Performance Shares Outstanding		Accrued Expense			Target Performance Shares Outstanding		Accrued Expense			Target Performance Shares Outstanding		Accrued Expense
Beginning of period	50,515		$	45.8		80,353		$	42.4		93,654		$	57.7
Shares paid or expired ⁽¹⁾	(23,186	)	(28.4		)	(30,838	)	(21.9		)	(36,294	)	(41.0		)
New grants	14,105		—			17,710		—			22,615		—
Forfeitures ⁽²⁾	(818	)	.1			(16,710	)	(9.3		)	378		.5
Expense recognized	—		14.2			—		34.6			—		25.2
End of period ⁽³⁾	40,616		$	31.7		50,515		$	45.8		80,353		$	42.4


⁽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. WTM performance shares payments in 2016 for the 2013-2015 performance cycle ranged from 140% to 142% of target.


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


⁽³⁾	Outstanding performance share awards as of December 31, 2018, 2017 and 2016 exclude 0, 2,195 and 7,315 unvested performance shares awards for employees of discontinued operations.

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

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



$ in Millions	Target Performance Shares Outstanding		Accrued Expense
Performance cycle:
2018 – 2020	13,450		$	3.8
2017 – 2019	14,070		11.9
2016 – 2018	13,715		16.5
Sub-total	41,235		32.2
Assumed forfeitures	(619	)	(.5		)
Total	40,616		$	31.7

Schedule of nonvested restricted stock activity

The following table presents the unrecognized compensation cost associated with the outstanding restricted share awards under the WTM Incentive Plan for the years ended December 31, 2018, 2017 and 2016:



	Year Ended December 31,
	2018					2017					2016
$ in Millions	Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value
Non-vested,
Beginning of period	53,755		$	14.3		70,620		$	19.7		70,675		$	15.7
Issued	14,105		11.4			17,985		16.3			25,365		20.2
Vested	(25,381	)	—			(28,846	)	—			(24,620	)	—
Forfeited	(969	)	(.2		)	(6,004	)	(3.5		)	(800	)	(.3		)
Expense recognized	—		(13.0		)	—		(18.2		)	—		(15.9		)
End of period ⁽¹⁾	41,510		$	12.5		53,755		$	14.3		70,620		$	19.7


⁽¹⁾	Outstanding restricted share awards as of December 31, 2018, 2017 and 2016 include 0, 2,195, and 5,235 unvested restricted shares for employees of Sirius Group

Common Shareholders’ Equity and Non-controlling Interests Common Shareholders’ Equity and Non-controlling Interests (Tables)

12 Months Ended

Dec. 31, 2018

Noncontrolling Interest [Abstract]

Noncontrolling Interest



	December 31, 2018					December 31, 2017
$ in Millions	Non-controlling Percentage		Non-controlling Equity			Non-controlling Percentage		Non-controlling Equity
Other, excluding BAM
HG Global	3.1	%	$	14.5		3.1	%	$	15.9
NSM	4.5		13.6			—		—
MediaAlpha	39.0		16.2			35.7		13.1
Buzz	22.9		1.1			22.9		2.5
Other NSM	13.4		.3			—		—
Total other, excluding BAM			45.7					31.5

BAM	100.0		(170.6		)	100.0		(163.2		)
Total non-controlling interests			$	(124.9	)			$	(131.7	)

Segment Information (Tables)

12 Months Ended

Dec. 31, 2018

Segment Reporting [Abstract]

Financial information for White Mountains' segments

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



Millions	HG Global/BAM ⁽¹⁾			NSM			MediaAlpha		Other Operations			Total
Year Ended December 31, 2018
Earned insurance premiums	$	13.9		$	—		$	—	$	—		$	13.9
Net investment income	16.7			—			—		42.3			59.0
Net realized and unrealized investment losses	(7.5		)	—			—		(100.8		)	(108.3		)
Advertising and commission revenues ⁽²⁾	—			94.7			295.5		4.1			394.3
Other revenues	1.2			6.9			1.6		.5			10.2
Total revenues	24.3			101.6			297.1		(53.9		)	369.1
Insurance acquisition expenses	5.3			—			—		—			5.3
Other underwriting expenses	.4			—			—		—			.4
Cost of sales	—			—			245.0		3.7			248.7
General and administrative expenses	48.0			61.6			31.7		94.4			235.7
Broker commission expense	—			28.9			—		—			28.9
Amortization of other intangible assets	—			8.3			10.3		.2			18.8
Interest expense	—			8.0			1.2		.3			9.5
Total expenses	53.7			106.8			288.2		98.6			547.3
Pre-tax (loss) income	$	(29.4	)	$	(5.2	)	$	8.9	$	(152.5	)	$	(178.2	)


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


⁽²⁾	As of December 31, 2018, approximately 29% of MediaAlpha’s advertising revenue was associated with one customer.As of December 31, 2018, approximately 33% of NSM’s commission revenue was associated with one single carrier.



Millions	HG Global/BAM ⁽¹⁾			MediaAlpha			Other Operations		Total
Year Ended December 31, 2017
Earned insurance premiums	$	9.4		$	—		$	1.0	$	10.4
Net investment income	12.3			—			43.7		56.0
Net realized and unrealized investment gains	.6			—			132.7		133.3
Advertising and commission revenues⁽²⁾	—			163.2			3.8		167.0
Other revenues	1.0			—			6.1		7.1
Total revenues	23.3			163.2			187.3		373.8
Losses and LAE	—			—			1.1		1.1
Insurance acquisition expenses	4.0			—			.1		4.1
Other underwriting expenses	.4			—			—		.4
Cost of sales	—			135.9			3.5		139.4
General and administrative expenses	42.9			16.2			148.9		208.0
Amortization of other intangible assets	—			10.5			.2		10.7
Interest expense	—			1.0			1.3		2.3
Total expenses	47.3			163.6			155.1		366.0
Pre-tax (loss) income	$	(24.0	)	$	(.4	)	$	32.2	$	7.8


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


⁽²⁾	As of December 31, 2017, approximately 27% of MediaAlpha’s advertising revenue was associated with one customer.



Millions	HG Global/BAM ⁽¹⁾			MediaAlpha			Other Operations			Total
Year Ended December 31, 2016
Earned insurance premiums	$	5.9		$	—		$	7.5		$	13.4
Net investment income	9.0			—			23.1			32.1
Net realized and unrealized investment gains (losses)	.7			—			(28.1		)	(27.4		)
Advertising and commission revenues ⁽²⁾	—			116.5			1.8			118.3
Other revenues	1.1			—			20.2			21.3
Total revenues	16.7			116.5			24.5			157.7
Losses and LAE	—			—			8.0			8.0
Insurance acquisition expenses	3.4			—			2.2			5.6
Other underwriting expenses	.4			—			—			.4
Cost of sales	—			97.8			4.2			102.0
General and administrative expenses	39.6			11.8			124.1			175.5
Amortization of other intangible assets	—			10.1			.4			10.5
Interest expense	—			.9			2.1			3.0
Total expenses	43.4			120.6			141.0			305.0
Pre-tax loss	$	(26.7	)	$	(4.1	)	$	(116.5	)	$	(147.3	)


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


⁽²⁾	As of December 31, 2016, approximately 24% of MediaAlpha’s advertising revenue was associated with one customer.



Millions Selected Balance Sheet Data	HG Global/BAM				NSM		MediaAlpha		Other Operations			Held for Sale		Total
December 31, 2018:
Total investments	$	768.3			$	1.7	$	—	$	1,772.9		$	—	$	2,542.9
Total assets	$	816.2		⁽¹⁾	$	627.0	$	88.4	$	1,827.7	⁽²⁾	$	3.3	$	3,362.6
Total liabilities	$	212.5		⁽²⁾	$	314.8	$	46.9	$	70.2		$	—	$	644.4
Total White Mountains’s common shareholders’ equity	$	759.8		⁽²⁾	$	298.3	$	25.3	$	1,756.4	⁽²⁾	$	3.3	$	2,843.1
Non-controlling interest	$	(156.1	)		$	13.9	$	16.2	$	1.1		$	—	$	(124.9	)
December 31, 2017:
Total investments	$	693.4			$	—	$	—	$	2,687.3		$	—	$	3,380.7
Total assets	$	747.4		⁽¹⁾	$	—	$	96.5	$	2,812.0	⁽²⁾	$	3.3	$	3,659.2
Total liabilities	$	167.0		⁽²⁾	$	—	$	59.8	$	71.6		$	—	$	298.4
Total White Mountains’s common shareholders’ equity	$	727.7		⁽²⁾	$	—	$	23.6	$	2,737.9	⁽²⁾	$	3.3	$	3,492.5
Non-controlling interest	$	(147.3	)		$	—	$	13.1	$	2.5		$	—	$	(131.7	)

Schedule of selected balance sheet data by segment



Millions Selected Balance Sheet Data	HG Global/BAM				NSM		MediaAlpha		Other Operations			Held for Sale		Total
December 31, 2018:
Total investments	$	768.3			$	1.7	$	—	$	1,772.9		$	—	$	2,542.9
Total assets	$	816.2		⁽¹⁾	$	627.0	$	88.4	$	1,827.7	⁽²⁾	$	3.3	$	3,362.6
Total liabilities	$	212.5		⁽²⁾	$	314.8	$	46.9	$	70.2		$	—	$	644.4
Total White Mountains’s common shareholders’ equity	$	759.8		⁽²⁾	$	298.3	$	25.3	$	1,756.4	⁽²⁾	$	3.3	$	2,843.1
Non-controlling interest	$	(156.1	)		$	13.9	$	16.2	$	1.1		$	—	$	(124.9	)
December 31, 2017:
Total investments	$	693.4			$	—	$	—	$	2,687.3		$	—	$	3,380.7
Total assets	$	747.4		⁽¹⁾	$	—	$	96.5	$	2,812.0	⁽²⁾	$	3.3	$	3,659.2
Total liabilities	$	167.0		⁽²⁾	$	—	$	59.8	$	71.6		$	—	$	298.4
Total White Mountains’s common shareholders’ equity	$	727.7		⁽²⁾	$	—	$	23.6	$	2,737.9	⁽²⁾	$	3.3	$	3,492.5
Non-controlling interest	$	(147.3	)		$	—	$	13.1	$	2.5		$	—	$	(131.7	)


⁽¹⁾	As of December 2018 and 2017,BAM’s total assets reflected the elimination of $481.3 and $499.0 of BAM Surplus Notes issued to HG Global and its subsidiaries, and $143.7 and $126.0 in accrued interest related to the BAM Surplus Notes.

⁽²⁾

Investments in Unconsolidated Affiliates (Tables)

12 Months Ended

Dec. 31, 2018

Schedule of Equity Method Investments [Line Items]

Schedule of changes in unconsolidated affiliates

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



	December 31,
Millions	2018		2017
Equity method eligible unconsolidated entities, at fair value	$	138.1	$	58.0
Investments accounted for under the equity method	1.3		4.6
Total investments in equity method eligible unconsolidated entities	139.4		62.6
Other unconsolidated investments ⁽¹⁾	186.2		146.2
Total other long-term investments	$	325.6	$	208.8

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

Summarized statement of financial position

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



	Ownership Interest
Investee	December 31, 2018	December 31, 2017	Instrument Held
PassportCard/DavidShield ⁽¹⁾	50.0%	50% / 0%	Common shares
Kudu	49.5%	—	Units
durchblicker	45.0%	45.0%	Common shares
Tuckerman Capital Fund III, L.P.	18.5%	21.3%	Limited partnership interest
Compare.com	18.4%	22.1%	Common shares

Financials

Schedule of Equity Method Investments [Line Items]

Summarized statement of financial position

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



	December 31,
Millions	2018		2017
Balance sheet data ⁽¹⁾:
Total assets	$	218.8	$	75.4
Total liabilities	$	46.7	$	24.2

⁽¹⁾Financial data for durchblicker, Compare.com and Tuckerman Capital Fund III, L.P. is on a one-quarter lag.



	Year Ended December 31,
Millions	2018			2017			2016
Income statement data ⁽¹⁾:
Revenues	$	134.1		$	60.0		$	32.9
Expenses	$	(110.1	)	$	(66.8	)	$	(76.4	)

⁽¹⁾Financial data for durchblicker, Compare.com and Tuckerman Capital Fund III, L.P. is on a one-quarter lag.

Fair Value of Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Summary of the fair value and carrying value of financial instruments

The following tables presents the fair value and carrying value of these financial instruments as of December 31, 2018 and December 31, 2017:



	December 31, 2018				December 31, 2017
Millions	Fair Value		Carrying Value		Fair Value		Carrying Value
NSM Bank Facility	$	176.1	$	176.6	$	—	$	—
MediaAlpha Bank Facility	$	14.6	$	14.2	$	23.9	$	23.8

Held for Sale and Discontinued Operations (Tables)

12 Months Ended

Dec. 31, 2018

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

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



	Year Ended December 31,
Millions		2018			2017			2016
Undistributed net earnings - continuing operations:
Net (loss) income attributable to White Mountains’s common shareholders, net of restricted common share amounts		$	(122.6	)	$	49.0		$	(120.1	)
Dividends declared, net of restricted common share amounts ⁽¹⁾		(3.7		)	(4.5		)	(5.4		)
Total undistributed net (losses) earnings, net of restricted common share amounts		$	(126.3	)	$	44.5		$	(125.5	)

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

Net Income (Loss) from Discontinued Operations



	Year Ended December 31, 2017
Millions	OneBeacon		Sirius Group			Tranzact		Total
Revenues
Earned insurance premiums	$	807.6	$	—		$	—	$	807.6
Net investment income	39.7		—			—		39.7
Net realized and unrealized investment gains	38.8		—			—		38.8
Other revenues	7.7		—			—		7.7
Total revenues	893.8		—			—		893.8
Expenses
Loss and loss adjustment expenses	546.0		—			—		546.0
Insurance and reinsurance acquisition expenses	145.6		—			—		145.6
Other underwriting expenses	156.2		—			—		156.2
General and administrative expenses	21.2		—			—		21.2
Interest expense	10.0							10.0
Total expenses	879.0		—			—		879.0
Pre-tax income	14.8		—			—		14.8
Income tax benefit	5.7		—			—		5.7
Net income from discontinued operations	20.5		—			—		20.5
Gain (loss) from sale of discontinued operations, net of tax	554.5		(.7		)	3.2		557.0
Total income (loss) from discontinued operations	575.0		(.7		)	3.2		577.5
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax	.3		—			—		.3
Recognition of benefit plan assets and obligations from the sale of OneBeacon, net of tax	2.9		—			—		2.9
Comprehensive income (loss) from discontinued operations	$	578.2	$	(.7	)	$	3.2	$	580.7



	Year Ended December 31, 2016
Millions	OneBeacon		Sirius Group			Tranzact			Total
Revenues
Earned insurance premiums	$	1,100.6	$	240.1		$	—		$	1,340.7
Net investment income	50.6		14.4			—			65.0
Net realized and unrealized investment gains (losses)	37.7		(1.5		)	—			36.2
Other revenues	5.5		.6			119.6			125.7
Total revenues	1,194.4		253.6			119.6			1,567.6
Expenses
Loss and loss adjustment expenses	656.0		154.9			—			810.9
Insurance and reinsurance acquisition expenses	206.0		59.0			—			265.0
Other underwriting expenses	209.0		30.9			—			239.9
General and administrative expenses	14.2		10.4			116.7			141.3
Interest expense	13.1		7.9			3.2			24.2
Total expenses	1,098.3		263.1			119.9			1,481.3
Pre-tax income (loss)	96.1		(9.5		)	(.3		)	86.3
Income tax benefit	12.5		3.1			6.4			22.0
Net income (loss) from discontinued operations	108.6		(6.4		)	6.1			108.3
Gain from sale of discontinued operations, net of tax	—		363.2			51.9			415.1
Total income from discontinued operations	108.6		356.8			58.0			523.4
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax	1.0		32.0			—			33.0
Recognition of foreign currency translation from sale of Sirius Group, net of tax	—		113.3			—			113.3
Comprehensive income from discontinued operations	$	109.6	$	502.1		$	58.0		$	669.7

Net Change in Cash from Discontinued Operations



	Year Ended December 31,
Millions	2017			2016
Net cash provided from operations	$	157.0		$	23.6
Net cash provided from (used for) investing activities	3.0			241.4
Net cash used for financing activities	(61.9		)	(93.8		)
Net change in cash during the period	98.1			171.2
Cash balances at beginning of period	70.5			245.4
Net change in cash held for sale	(.9		)	(.3		)
Cash sold as part of sale of consolidated subsidiaries	(167.7		)	(345.8		)
Cash balances at end of period	$	—		$	70.5

Discontinued operations, computation of earnings per share

The following table presents the Company’s computation of earnings per share for discontinued operations for the years ended December 31, 2018, 2017 and 2016:



	Year Ended December 31,
	2018			2017			2016
Basic and diluted earnings per share numerators (in millions):
Net (loss) income attributable to White Mountains’s common shareholders	$	(141.2	)	$	627.2		$	401.8
Less: total (loss) income from continuing operations, net of tax	(124.0		)	49.7			(121.6		)
Net (loss) income from discontinued operations attributable to White Mountains’s common shareholders	(17.2		)	577.5			523.4
Allocation of earnings (losses) to participating restricted common shares ⁽¹⁾	0.2			(7.3		)	(6.8		)
Basic and diluted (losses) earnings per share numerators	$	(17.0	)	$	570.2		$	516.6
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	$	3,382.5		$	4,293.8		$	5,014.9
Average unvested restricted common shares ⁽³⁾	(40.1		)	(54.3		)	(64.8		)
Basic earnings (losses) per share denominator	$	3,342.4		$	4,239.5		$	4,950.1
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	$	3,382.5		$	4,293.8		$	5,018.1
Average unvested restricted common shares ⁽³⁾	(40.1		)	(54.3		)	(64.8		)
Diluted earnings (losses) per share denominator ⁽⁴⁾	$	3,342.4		$	4,239.5		$	4,953.3
Basic (losses) earnings per share (in dollars) - discontinued operations:	$	(5.09	)	$	134.50		$	104.37
Diluted (losses) earnings per share (in dollars) - discontinued operations:	$	(5.09	)	$	134.50		$	104.32


⁽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 years ended December 31, 2018, 2017 and 2016.


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


⁽⁴⁾	The diluted earnings per share denominator for the year ended December 31, 2016 includes the impact of 40,000 common shares issuable upon exercise of the non-qualified options outstanding, which resulted in 3,217 incremental shares outstanding over the period.

Summary of the fair value and carrying value of financial instruments

The following tables presents the fair value and carrying value of these financial instruments as of December 31, 2018 and December 31, 2017:



	December 31, 2018				December 31, 2017
Millions	Fair Value		Carrying Value		Fair Value		Carrying Value
NSM Bank Facility	$	176.1	$	176.6	$	—	$	—
MediaAlpha Bank Facility	$	14.6	$	14.2	$	23.9	$	23.8

- Definition

+ References

+ Details

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Schedule of Quarterly Financial Information



	2018 Three Months Ended												2017 Three Months Ended
Millions, Except Per Share Amounts	Dec. 31			Sept. 30			June 30			Mar. 31			Dec. 31		Sept. 30		June 30			Mar. 31
Revenues	$	6.0		$	198.7		$	122.3		$	42.1		$	114.0	$	87.5	$	83.5		$	88.8
Expenses	150.1			154.4			134.7			108.1			108.8		79.1		85.7			92.4
Pre-tax income (loss)	(144.1		)	44.3			(12.4		)	(66.0		)	5.2		8.4		(2.2		)	(3.6		)
Tax benefit (expense)	3.6			3.6			(2.5		)	(.7		)	2.5		4.0		1.0			0.3
Income (loss) from continuing operations	(140.5		)	47.9			(14.9		)	(66.7		)	7.7		12.4		(1.2		)	(3.3		)
Income (loss) from discontinued operations, net of tax	—			(17.3		)	—			.1			4.3		539.1		2.8			31.3
Non-controlling interest in consolidated subsidiaries	3.0			10.2			18.4			18.6			10.5		10.6		12.0			1.0
Income (loss) attributable to White Mountains’s common shareholders	$	(137.5	)	$	40.8		$	3.5		$	(48.0	)	$	22.5	$	562.1	$	13.6		$	29.0
Income (loss) attributable to White Mountains’s common shareholders per share:
Basic
Continuing operations	$	(43.24	)	$	18.27		$	1.02		$	(12.85	)	$	4.85	$	5.36	$	2.36		$	(0.52	)
Discontinued operations	—			(5.44		)	—			.03			1.15		125.45		.61			6.86
Total consolidated operations	$	(43.24	)	$	12.83		$	1.02		$	(12.82	)	$	6.00	$	130.81	$	2.97		$	6.34
Diluted
Continuing operations	$	(43.24	)	$	18.27		$	1.02		$	(12.85	)	$	4.85	$	5.36	$	2.36		$	(0.52	)
Discontinued operations	—			(5.44		)	—			.03			1.15		125.45		.61			6.86
Total consolidated operations	$	(43.24	)	$	12.83		$	1.02		$	(12.82	)	$	6.00	$	130.81	$	2.97		$	6.34

Summary of Significant Accounting Policies (Reportable Segments and Discontinued Operations and Assets and Liabilities Held for Sale) (Details) - USD ($) $ / shares in Units, $ in Millions	Dec. 31, 2018	May 11, 2018	Dec. 31, 2017	Sep. 28, 2017	Dec. 31, 2012
Basis of Presentation
Percentage of par value of policy reinsured	15.00%		15.00%
HG Global
Basis of Presentation
Surplus notes					$ 503.0
Preferred stocks \| 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%
Intact Financial Corporation \| OneBeacon
Basis of Presentation
Sale of Stock, price per share				$ 18.10
NSM Insurance HoldCo, LLC
Basis of Presentation
Ownership interest (as a percent)	95.50%	95.00%
Series A BAM Surplus Note \| HG Global
Basis of Presentation
Surplus notes					203.0
Series B BAM Surplus Note \| HG Global
Basis of Presentation
Surplus notes					$ 300.0

Summary of Significant Accounting Policies (Investment Securities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Minimum Percentage of Variation from Expected Price Required to Treat Prices of Investments Provided by Pricing Services as Outliers	5.00%
Minimum Variation from Expected Price Required to Treat Prices of Investments Provided by Pricing Services as Outliers	$ 0.5
Percentage of investments recorded at fair value	87.00%	94.00%

Summary of Significant Accounting Policies (Foreign Currency Exchange) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Net unrealized foreign currency translation losses	$ (3.9)	$ (1.3)

Summary of Significant Accounting Policies (Recently Adopted Accounting Principles) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Accounting Standards Update 2016-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Employee service share-based compensation, tax benefit from exercise of stock options	$ 8.4	$ 9.2
Subsequent Event \| Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use asset			$ 24.5
Operating lease, liability			$ 24.5

Significant Transactions (Acquisitions) (Details)								3 Months Ended	8 Months Ended	12 Months Ended
Significant Transactions (Acquisitions) (Details)	Dec. 03, 2018 USD ($) shares	May 11, 2018 USD ($)	Jan. 24, 2018 USD ($)	Oct. 05, 2017 USD ($) shares	Aug. 01, 2017 USD ($) shares	Aug. 04, 2016 USD ($)	Jan. 15, 2016 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) earnout_period	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	May 18, 2018 USD ($)	Feb. 05, 2018 USD ($)
Unusual or Infrequent Item, or Both [Line Items]
Purchase of consolidated subsidiaries, net of cash acquired										$ 295,200,000	$ 27,600,000	$ 13,400,000
Assets									$ 3,362,600,000	3,362,600,000	3,659,200,000
Goodwill									379,900,000	379,900,000	25,900,000	$ 25,900,000
Liabilities									644,400,000	644,400,000	298,400,000
NSM Holding Co LLC
Unusual or Infrequent Item, or Both [Line Items]
Contingent consideration									2,700,000	2,700,000
Contingent consideration earnout liabilities									20,200,000	20,200,000	0
Assets									627,000,000	627,000,000	0
Liabilities									$ 314,800,000	$ 314,800,000	$ 0
Buzzmove
Unusual or Infrequent Item, or Both [Line Items]
Ownership interest (as a percent)						70.90%			77.10%	77.10%	77.10%
Assets						$ 11,500,000
Goodwill						7,600,000
Intangible assets, net (excluding goodwill)						1,100,000
Liabilities						100,000
LowFares.com
Unusual or Infrequent Item, or Both [Line Items]
Purchase of consolidated subsidiaries, net of cash acquired							$ 3,900,000
Healthplans.com
Unusual or Infrequent Item, or Both [Line Items]
Purchase of consolidated subsidiaries, net of cash acquired				$ 28,000,000
MediaAlpha
Unusual or Infrequent Item, or Both [Line Items]
Ownership interest (as a percent)									61.00%	61.00%	64.40%
Assets									$ 88,400,000	$ 88,400,000	$ 96,500,000
Liabilities									$ 46,900,000	$ 46,900,000	$ 59,800,000
Preferred stocks \| Buzzmove
Unusual or Infrequent Item, or Both [Line Items]
Stock issued during period, shares, acquisitions \| shares					37,409
Common Stock \| Buzzmove
Unusual or Infrequent Item, or Both [Line Items]
Stock issued during period, shares, acquisitions \| shares					5,808
GBP \| Buzzmove
Unusual or Infrequent Item, or Both [Line Items]
Payments to acquire interest in subsidiary						6,100,000
GBP \| Preferred stocks \| Buzzmove
Unusual or Infrequent Item, or Both [Line Items]
Payments to acquire interest in subsidiary					$ 4,000,000
GBP \| Common Stock \| Buzzmove
Unusual or Infrequent Item, or Both [Line Items]
Payments to acquire interest in subsidiary					500,000
United States of America, Dollars \| Buzzmove
Unusual or Infrequent Item, or Both [Line Items]
Payments to acquire interest in subsidiary						$ 8,100,000
United States of America, Dollars \| Preferred stocks \| Buzzmove
Unusual or Infrequent Item, or Both [Line Items]
Payments to acquire interest in subsidiary					5,000,000
United States of America, Dollars \| Common Stock \| Buzzmove
Unusual or Infrequent Item, or Both [Line Items]
Payments to acquire interest in subsidiary					$ 700,000
NSM Insurance HoldCo, LLC
Unusual or Infrequent Item, or Both [Line Items]
Ownership interest (as a percent)		95.00%							95.50%	95.50%
Purchase of consolidated subsidiaries, net of cash acquired		$ 274,200,000.0
Purchase price adjustment								$ 2,100,000
Assets acquired		495,200,000
Goodwill and intangible assets acquired		383,000,000
Liabilities acquired		204,600,000
Contingent consideration		10,200,000
Noncontrolling interest acquired		$ 14,400,000
Transaction costs									$ 6,300,000
Fresh Insurance Services Group Limited
Unusual or Infrequent Item, or Both [Line Items]
Ownership interest (as a percent)													100.00%
Purchase price adjustment								$ 700,000
Assets acquired													$ 72,600,000
Goodwill and intangible assets acquired													54,600,000
Liabilities acquired													22,300,000
Contingent consideration													7,500,000
Equity method investment, aggregate cost													$ 49,600,000
Number of earnout periods \| earnout_period										2
KBK Insurance Group, Inc. & KBK Premium Services, Inc.
Unusual or Infrequent Item, or Both [Line Items]
Purchase of consolidated subsidiaries, net of cash acquired	$ 60,000,000.0
Goodwill and intangible assets acquired	$ 59,400,000.0
Stock issued during period, shares, acquisitions \| shares	16,694,869
Stock issued during period, value, acquisitions	$ 29,000,000
DavidShield
Unusual or Infrequent Item, or Both [Line Items]
Ownership interest (as a percent)			50.00%						50.00%	50.00%
DavidShield \| White Mountains
Unusual or Infrequent Item, or Both [Line Items]
Ownership interest (as a percent)									25.00%	25.00%
PassportCard
Unusual or Infrequent Item, or Both [Line Items]
Ownership interest (as a percent)			50.00%						25.00%	25.00%	50.00%
DavidShield & PassportCard
Unusual or Infrequent Item, or Both [Line Items]
Ownership interest (as a percent)									50.00%	50.00%
Payments to acquire businesses, gross			$ 41,800,000
Payments to acquire businesses, net			$ 28,300,000
Kudu
Unusual or Infrequent Item, or Both [Line Items]
Ownership interest (as a percent)									49.50%	49.50%	0.00%
Equity method investment, aggregate cost														$ 125,000,000
Working capital funded									$ 3,800,000	$ 3,800,000
Additional funding									63,000,000	63,000,000
Kudu \| White Mountains
Unusual or Infrequent Item, or Both [Line Items]
Equity method investment, aggregate cost									31,500,000	31,500,000
Additional funding									$ 31,500,000	$ 31,500,000
Capital Unit, Class A \| MediaAlpha
Unusual or Infrequent Item, or Both [Line Items]
Capital units, purchased \| shares				131,579
Capital units, value				$ 12,500,000

Significant Transactions (Dispositions) (Details) - USD ($) $ in Millions					3 Months Ended									9 Months Ended	12 Months Ended
	Sep. 28, 2017	Oct. 05, 2016	Jul. 21, 2016	Apr. 18, 2016	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 28, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Significant Transactions
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold															$ 0.0	$ 1,131.0	$ 2,646.2
Gain (loss) on sale of other discontinued operations, net of tax															(17.2)	557.0	415.1
Other comprehensive (loss) income, net of tax															(4.8)	0.3	(0.7)
Repayments of debt, principal															15.4	365.0	404.6
Payments to acquire other investments															95.9	84.1	38.5
Net income from discontinued operations, net of tax					$ 0.0	$ (17.3)	$ 0.0	$ 0.1	$ 4.3	$ 539.1	$ 2.8	$ 31.3			0.0	20.5	108.3
OneBeacon
Significant Transactions
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	$ 1,300.0
Gain (loss) on sale of other discontinued operations, net of tax	554.6
Other comprehensive (loss) income, net of tax	$ 2.9
Net income from discontinued operations, net of tax														$ 20.5			108.6
Tranzact
Significant Transactions
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold		$ 1.2	$ 221.3
Gain (loss) on sale of other discontinued operations, net of tax																3.2	51.9
Repayments of debt, principal			56.3
Tax effect of discontinued operation																	30.2
Increase of book value			$ 82.1														82.1
Sirius Group
Significant Transactions
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold				$ 2,600.0
Gain (loss) on sale of other discontinued operations, net of tax													$ (4.0)				363.2
Payments to acquire other investments				$ 161.8
Net change in pension liability and other accumulated comprehensive items																	113.3
Sale of Sirius Group
Significant Transactions
Net change in pension liability and other accumulated comprehensive items																	113.3
Discontinued Operations \| OneBeacon
Significant Transactions
Gain (loss) on sale of other discontinued operations, net of tax															0.0	554.5	0.0
Tax effect of discontinued operation																(5.7)	(12.5)
Net change in pension liability and other accumulated comprehensive items																2.9	0.0
Net income from discontinued operations, net of tax																20.5	108.6
Discontinued Operations \| Tranzact
Significant Transactions
Gain (loss) on sale of other discontinued operations, net of tax															0.0	3.2	51.9
Tax effect of discontinued operation																0.0	(6.4)
Net change in pension liability and other accumulated comprehensive items																0.0	0.0
Net income from discontinued operations, net of tax																0.0	6.1
Discontinued Operations \| Sirius Group
Significant Transactions
Gain (loss) on sale of other discontinued operations, net of tax															(17.2)	(0.7)	363.2
Tax effect of discontinued operation																0.0	(3.1)
Net change in pension liability and other accumulated comprehensive items																0.0	113.3
Net income from discontinued operations, net of tax																$ 0.0	(6.4)
Discontinued Operations \| Sale of Sirius Group
Significant Transactions
Net income from discontinued operations, net of tax																	$ (4.3)
Swedish Tax Authority \| Sirius Group \| Sirius Group
Significant Transactions
Net income from discontinued operations, net of tax															$ (17.3)

Investment Securities (Net Investment Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment income:
Total investment income	$ 62.0	$ 58.5	$ 34.5
Third-party investment expenses	(3.0)	(2.5)	(2.4)
Net investment income, pre-tax	59.0	56.0	32.1
Fixed maturity investments
Investment income:
Total investment income	35.1	44.9	28.5
Short-term investments
Investment income:
Total investment income	8.0	1.8	0.9
Common Stock
Investment income:
Total investment income	15.1	10.6	4.0
Other long-term investments
Investment income:
Total investment income	$ 3.8	$ 1.2	$ 1.1

Investment Securities (Net Realized and Unrealized Investement Gains and Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Net realized investment (losses) gains, pre-tax	$ (12.9)	$ 24.1	$ 270.0
Net unrealized investment (losses) gains, pre-tax	(95.4)	109.2	(297.4)
Net realized and unrealized investment (losses) gains, pre-tax	(108.3)	133.3	(27.4)
Income tax benefit (expense) attributable to net realized and unrealized investment (losses) gains	18.2	(12.9)	2.7
Net realized and unrealized investment (losses) gains, after-tax	$ (90.1)	$ 120.4	$ (24.7)

Investment Securities (Net Realized Investment Gain (losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment securities
Net realized investment gains (losses), pre-tax	$ (23.9)	$ 35.3	$ 269.7
Net realized investment gains (losses), foreign, pre-tax	11.0	(11.2)	0.3
Net realized investment gains (losses), pre-tax, total	(12.9)	24.1	270.0
Income tax expense attributable to net realized investment gains (losses)	9.1	(8.9)	(45.6)
Income tax expense attributable to net realized investment gains (losses), foreign	0.0	0.0	0.0
Income tax expense attributable to net realized investment gains, total	9.1	(8.9)	(45.6)
Net realized investment gains (losses), after-tax	(14.8)	26.4	224.1
Net realized investment gains (losses), foreign, after-tax	11.0	(11.2)	0.3
Net realized investment gains (losses), total, after-tax	(3.8)	15.2	224.4
Fixed maturity investments
Investment securities
Net realized investment gains (losses), pre-tax	(29.8)	(1.6)	(1.9)
Net realized investment gains (losses), foreign, pre-tax	18.2	4.1	0.3
Net realized investment gains (losses), pre-tax, total	(11.6)	2.5	(1.6)
Short-term investments
Investment securities
Net realized investment gains (losses), pre-tax	(0.8)	(0.3)	0.4
Net realized investment gains (losses), foreign, pre-tax	0.0	0.0	0.0
Net realized investment gains (losses), pre-tax, total	(0.8)	(0.3)	0.4
Common equity securities
Investment securities
Net realized investment gains (losses), pre-tax	6.6	18.1	268.5
Net realized investment gains (losses), foreign, pre-tax	0.0	6.0	0.0
Net realized investment gains (losses), pre-tax, total	6.6	24.1	268.5
Other long-term investments
Investment securities
Net realized investment gains (losses), pre-tax	0.1	19.1	2.7
Net realized investment gains (losses), foreign, pre-tax	(7.2)	(21.3)	0.0
Net realized investment gains (losses), pre-tax, total	$ (7.1)	$ (2.2)	$ 2.7

Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment securities
Unrealized investment gains	$ (83.6)	$ 97.5	$ (294.7)
Foreign Currency Transaction Gain (Loss), Unrealized	(11.8)	11.7	(2.7)
Net unrealized investment gains (losses), pre-tax	(95.4)	109.2	(297.4)
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	9.1	(4.0)	48.3
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0	0.0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	9.1	(4.0)	48.3
Unrealized investment gains after tax	(74.5)	93.5	(246.4)
Trading Securities, Unrealized Foreign Currenc Gain (Loss) Net of Tax	(11.8)	11.7	(2.7)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	(86.3)	105.2	(249.1)
Fixed maturity investments
Investment securities
Unrealized investment gains	(8.3)	13.8	(14.6)
Foreign Currency Transaction Gain (Loss), Unrealized	(14.8)	12.7	2.1
Net unrealized investment gains (losses), pre-tax	(23.1)	26.5	(12.5)
Common equity securities
Investment securities
Unrealized investment gains	(105.5)	99.3	(257.4)
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	0.0	(3.3)
Net unrealized investment gains (losses), pre-tax	(105.5)	99.3	(260.7)
Other long-term investments
Investment securities
Unrealized investment gains	30.2	(15.6)	(22.7)
Foreign Currency Transaction Gain (Loss), Unrealized	3.0	(1.0)	(1.5)
Net unrealized investment gains (losses), pre-tax	$ 33.2	$ (16.6)	$ (24.2)

Investment Securities (Investment Gains (Losses) For Level 3) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment securities
Realized gains	$ 49.4	$ 61.5	$ 283.7
Realized losses	62.3	37.4	13.7
Unrealized investment gains	(83.6)	97.5	(294.7)
Level 3 Inputs
Investment securities
Unrealized investment gains	22.6	(15.4)	(14.2)
Fixed maturity investments
Investment securities
Unrealized investment gains	(8.3)	13.8	(14.6)
Fixed maturity investments \| Level 3 Inputs
Investment securities
Unrealized investment gains	0.0	0.0	0.1
Common equity securities
Investment securities
Unrealized investment gains	(105.5)	99.3	(257.4)
Other long-term investments
Investment securities
Unrealized investment gains	30.2	(15.6)	(22.7)
Other long-term investments \| Level 3 Inputs
Investment securities
Unrealized investment gains	$ 22.6	$ (15.4)	$ (14.3)

Investment Securities (Investment Holdings) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
U.S. Government and agency obligations
Investment securities
Cost or amortized cost	$ 154.0	$ 297.8
Gross unrealized gains	0.1	0.0
Gross unrealized losses	(0.9)	(1.3)
Net foreign currency gains	0.0	0.0
Fixed maturity investments, at fair value	153.2	296.5
Debt securities issued by corporations
Investment securities
Cost or amortized cost	519.0	867.6
Gross unrealized gains	1.0	2.9
Gross unrealized losses	(9.5)	(4.3)
Net foreign currency gains	0.0	14.7
Fixed maturity investments, at fair value	510.5	880.9
Mortgage and asset-backed securities
Investment securities
Cost or amortized cost	136.1	697.2
Gross unrealized gains	0.1	1.6
Gross unrealized losses	(2.7)	(4.1)
Net foreign currency gains	0.0	0.0
Fixed maturity investments, at fair value	133.5	694.7
Municipal obligations
Investment securities
Cost or amortized cost	279.0	252.0
Gross unrealized gains	2.4	3.7
Gross unrealized losses	(1.1)	(0.8)
Net foreign currency gains	0.0	0.0
Fixed maturity investments, at fair value	280.3	254.9
Foreign government, agency and provincial obligations
Investment securities
Cost or amortized cost		2.6
Gross unrealized gains		0.0
Gross unrealized losses		0.0
Net foreign currency gains		0.1
Fixed maturity investments, at fair value		2.7
Total fixed maturity investments
Investment securities
Cost or amortized cost	1,088.1	2,117.2
Gross unrealized gains	3.6	8.2
Gross unrealized losses	(14.2)	(10.5)
Net foreign currency gains	0.0	14.8
Total fixed maturity investments	$ 1,077.5	$ 2,129.7
Including Short Term Fixed
Investment securities
Duration of fixed maturities (years)	3 years 5 months
Excluding Short Term Investments
Investment securities
Duration of fixed maturities (years)	4 years

Investment Securities (Cost and Amortized Cost Maturity Schedule) (Details) $ in Millions	Dec. 31, 2018 USD ($)
Trading Securities by Maturity Cost or Amortized Cost
Cost of Amortized Cost	$ 1,088.1
Trading Securities by Maturity, Fair Value
Common equity securities	1,077.5
Fixed Maturities and Convertible Fixed Maturities Excluding A B S M B S and Preferred Stock
Trading Securities by Maturity Cost or Amortized Cost
Due in one year or less	84.8
Due after one year through five years	500.8
Due after five years through ten years	222.4
Due after ten years	144.0
Trading Securities by Maturity, Fair Value
Due in one year or less	84.4
Due after one year through five years	496.2
Due after five years through ten years	218.0
Due after ten years	145.4
Mortgage and asset-backed securities
Trading Securities by Maturity Cost or Amortized Cost
Mortgage and asset-backed securities	136.1
Trading Securities by Maturity, Fair Value
Mortgage and asset-backed securities	$ 133.5

Investment Securities (Cost or Amortized Cost, Gross Unrealized Investment Gains and Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment securities
Sales and maturities of investment securities	$ 2,164.4	$ 2,791.3	$ 3,692.4
Common Stock
Investment securities
Cost or amortized cost	904.7	739.7
Gross unrealized gains	51.0	129.4
Gross unrealized losses	(30.1)	(3.0)
Net foreign currency gains	0.0	0.0
Fixed maturity investments, at fair value	925.6	866.1
Other long-term investments
Investment securities
Cost or amortized cost	330.3	246.6
Gross unrealized gains	52.2	6.8
Gross unrealized losses	(54.9)	(39.7)
Net foreign currency gains	(2.0)	(4.9)
Fixed maturity investments, at fair value	$ 325.6	$ 208.8

Investment Securities (Investments Held on Deposit or as Collateral) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Investments held in trusts	$ 254.3	$ 204.6
Assets held by insurance regulators	$ 6.1	$ 6.0
Percentage of investments recorded at fair value	87.00%	94.00%

Investment Securities (Debt Securities Issued By Corporation) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment securities
Fair value investments	$ 2,328.7	$ 3,208.3	$ 2,547.3
Debt securities issued by corporations
Investment securities
Fair value investments	510.5	880.9
AAA \| Debt securities issued by corporations
Investment securities
Fair value investments	8.9	1.6
AA \| Debt securities issued by corporations
Investment securities
Fair value investments	88.7	42.6
A \| Debt securities issued by corporations
Investment securities
Fair value investments	270.5	192.5
BBB \| Debt securities issued by corporations
Investment securities
Fair value investments	142.4	465.2
BB \| Debt securities issued by corporations
Investment securities
Fair value investments	0.0	161.7
B \| Debt securities issued by corporations
Investment securities
Fair value investments	$ 0.0	$ 17.3

Investment Securities (Mortgage-backed Asset Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment securities
Fair value investments	$ 2,328.7	$ 3,208.3	$ 2,547.3
FNMA
Investment securities
Fair value investments	53.6	84.5
FHLMC
Investment securities
Fair value investments	38.1	62.0
GNMA
Investment securities
Fair value investments	23.7	46.3
Total Agency
Investment securities
Fair value investments	115.4	192.8
Commercial
Investment securities
Fair value investments	0.0	70.5
Total non-agency
Investment securities
Fair value investments	0.0	70.5
Total mortgage-backed securities
Investment securities
Fair value investments	115.4	263.3
Vehicle receivables
Investment securities
Fair value investments	9.2	142.4
Credit card receivables
Investment securities
Fair value investments	8.9	206.0
Other
Investment securities
Fair value investments	0.0	83.0
Total other asset-backed securities
Investment securities
Fair value investments	18.1	431.4
Total mortgage and asset- backed securities
Investment securities
Fair value investments	133.5	694.7
Level 2 Inputs
Investment securities
Fair value investments	1,160.5	2,105.4	$ 2,093.8
Level 2 Inputs \| FNMA
Investment securities
Fair value investments	53.6	84.5
Level 2 Inputs \| FHLMC
Investment securities
Fair value investments	38.1	62.0
Level 2 Inputs \| GNMA
Investment securities
Fair value investments	23.7	46.3
Level 2 Inputs \| Total Agency
Investment securities
Fair value investments	115.4	192.8
Level 2 Inputs \| Commercial
Investment securities
Fair value investments	0.0	70.5
Level 2 Inputs \| Total non-agency
Investment securities
Fair value investments	0.0	70.5
Level 2 Inputs \| Total mortgage-backed securities
Investment securities
Fair value investments	115.4	263.3
Level 2 Inputs \| Vehicle receivables
Investment securities
Fair value investments	9.2	142.4
Level 2 Inputs \| Credit card receivables
Investment securities
Fair value investments	8.9	206.0
Level 2 Inputs \| Other
Investment securities
Fair value investments	0.0	83.0
Level 2 Inputs \| Total other asset-backed securities
Investment securities
Fair value investments	18.1	431.4
Level 2 Inputs \| Total mortgage and asset- backed securities
Investment securities
Fair value investments	133.5	694.7
Level 3 Inputs \| FNMA
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| FHLMC
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| GNMA
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| Total Agency
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| Commercial
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| Total non-agency
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| Total mortgage-backed securities
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| Vehicle receivables
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| Credit card receivables
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| Other
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| Total other asset-backed securities
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| Total mortgage and asset- backed securities
Investment securities
Fair value investments	$ 0.0	$ 0.0

Investment Securities (Other Long-Term Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
DavidShield & PassportCard
Investment securities
Other Investments	$ 75.0	$ 21.0
Kudu
Investment securities
Other Investments	30.7	0.0
Other unconsolidated private equity investments, at fair value
Investment securities
Other Investments	60.0	62.2
Total unconsolidated entities
Investment securities
Other Investments	165.7	83.2
Hedge Funds and Private Equity Funds
Investment securities
Other Investments	146.1	125.3
Forward Contracts
Investment securities
Other Investments	0.0	(3.7)
Other long-term investments
Investment securities
Other Investments	13.8	4.0
White Mountains
Investment securities
Other Investments	$ 325.6	$ 208.8

Investment Securities (Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency) (Details) $ in Millions	Dec. 31, 2018 USD ($) fund	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Private equity funds
Investment securities
Number of investments held \| fund	12
Trading security, fair value		$ 91.8
Hedge funds
Investment securities
Number of investments held \| fund	1
Total private equity funds and hedge funds included in other long-term investments
Investment securities
Other long-term investments largest single investment	$ 54.3		$ 54.9
Trading security, fair value	146.1		125.3
Fair value unfunded commitments	76.7		58.1
Private equity funds \| Private equity funds
Investment securities
Trading security, fair value		91.8	70.4
Fair value unfunded commitments	76.7		58.1
Private equity funds \| Manufacturing/Industrial
Investment securities
Trading security, fair value		42.9	43.3
Fair value unfunded commitments	10.5		10.4
Private equity funds \| Aerospace/Defense/Government
Investment securities
Trading security, fair value		27.6	15.8
Fair value unfunded commitments	34.9		12.9
Private equity funds \| Direct lending
Investment securities
Trading security, fair value		13.0	7.1
Fair value unfunded commitments	17.7		23.1
Private equity funds \| Financial services
Investment securities
Trading security, fair value		8.3	4.2
Fair value unfunded commitments	13.6		11.7
Hedge funds \| Hedge funds
Investment securities
Trading security, fair value	54.3	54.3	54.9
Fair value unfunded commitments	0.0		0.0
Hedge funds \| Hedge Fund, Long/ Short Equity Banks and Financial
Investment securities
Trading security, fair value		$ 54.3	54.9
Fair value unfunded commitments	$ 0.0		$ 0.0

Investment Securities (Investments In Private Equity Funds Subject to Lock-Up Periods) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Private equity funds
Investment securities
Trading security, fair value		$ 91.8
1 – 3 years \| Private equity funds
Investment securities
Trading security, fair value		1.8
3 – 5 years \| Private equity funds
Investment securities
Trading security, fair value		5.5
5 – 10 years \| Private equity funds
Investment securities
Trading security, fair value		63.2
10 years \| Private equity funds
Investment securities
Trading security, fair value		21.3
Hedge funds \| Hedge funds
Investment securities
Trading security, fair value	$ 54.3	$ 54.3	$ 54.9
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Notice Period	45 years

Investment Securities (Rollforward of Fair Value Measurements by Level) (Details)	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) Investment	Dec. 31, 2016 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	$ 3,208,300,000	$ 2,547,300,000
Total realized and unrealized gains (losses)	99,800,000	157,400,000
Amortization/Accretion	(2,500,000)	(9,100,000)
Purchases	1,394,300,000	3,332,500,000
Sales	(2,171,600,000)	(2,814,600,000)
Transfers in	0	18,700,000
Transfers out	0	(18,700,000)
Fair value investments	2,328,700,000	3,208,300,000	$ 2,547,300,000
Net realized and unrealized investment gains (losses), pre-tax	(108,300,000)	133,300,000	(27,400,000)
Net realized and unrealized investment gains	(108,300,000)	133,300,000	(27,400,000)
Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	3,384,400,000
Fair value investments	2,542,900,000	3,384,400,000
Fixed maturity investments \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,129,700,000
Fair value investments	1,077,500,000	2,129,700,000
Other long-term investments \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	77,200,000
Fair value investments	138,700,000	77,200,000
Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized and unrealized investment gains (losses), pre-tax	800,000.0	300,000
Short-term investments \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value disclosure, investments	214,200,000	176,100,000	175,000,000
Foreign Currency on Cash and Open Trades
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized and unrealized investment gains (losses), pre-tax	4,200,000.0
Foreign Exchange Forward
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized and unrealized investment gains (losses), pre-tax	3,500,000.0	23,800,000
Level 1 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	890,400,000	279,500,000
Total realized and unrealized gains (losses)	(64,900,000)	82,700,000
Amortization/Accretion	200,000	0
Purchases	514,700,000	1,209,300,000
Sales	(497,800,000)	(681,100,000)
Transfers in	0	0
Transfers out	0	0
Fair value investments	842,600,000	890,400,000	279,500,000
Level 1 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	1,041,400,000
Fair value investments	1,047,000,000	1,041,400,000
Level 1 Inputs \| Fixed maturity investments \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	296,500,000
Fair value investments	153,200,000	296,500,000
Level 1 Inputs \| Other long-term investments \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	3,208,300,000
Fair value investments	0	3,208,300,000
Level 1 Inputs \| Short-term investments \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	151,000,000
Fair value investments	204,400,000	151,000,000
Level 2 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,105,400,000	2,093,800,000
Total realized and unrealized gains (losses)	(64,400,000)	69,600,000
Amortization/Accretion	(2,700,000)	(9,100,000)
Purchases	783,800,000	2,007,900,000
Sales	(1,661,600,000)	(2,070,300,000)
Transfers in	0	18,700,000
Transfers out	0	0
Fair value investments	1,160,500,000	2,105,400,000	2,093,800,000
Level 2 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,130,500,000
Fair value investments	1,170,300,000	2,130,500,000
Level 2 Inputs \| Fixed maturity investments \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	1,833,200,000
Fair value investments	924,300,000	1,833,200,000
Level 2 Inputs \| Other long-term investments \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 2 Inputs \| Short-term investments \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	25,100,000
Fair value investments	9,800,000	25,100,000
Level 3 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	77,200,000
Fair value investments	138,700,000	77,200,000
Level 3 Inputs \| Fixed maturity investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0	0
Total realized and unrealized gains (losses)		0
Amortization/Accretion		0
Purchases		31,200,000
Sales		(12,500,000)
Transfers in		0
Transfers out		(18,700,000)
Fair value investments		$ 0	0
Number of investments \| Investment		3
Level 3 Inputs \| Fixed maturity investments \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	$ 0
Level 3 Inputs \| Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	77,200,000	91,400,000
Total realized and unrealized gains (losses)	16,200,000	(15,300,000)
Amortization/Accretion	0	0
Purchases	45,300,000	3,100,000
Sales	0	(2,000,000)
Transfers in	0	0
Transfers out	0	0
Fair value investments	138,700,000	77,200,000	91,400,000
Level 3 Inputs \| Other long-term investments \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	77,200,000
Fair value investments	138,700,000	77,200,000
Level 3 Inputs \| Hedge Funds and Private Equity Funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments		82,600,000
Total realized and unrealized gains (losses)		20,400,000
Amortization/Accretion		0
Purchases		81,000,000
Sales		(48,700,000)
Transfers in		0
Transfers out		0
Fair value investments			82,600,000
Level 3 Inputs \| Short-term investments \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 3 Inputs \| Foreign Exchange Forward \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value disclosure, investments		(3,700,000)	(1,200,000)
Fair Value Measured at Net Asset Value Per Share \| Other long-term investments \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	135,300,000
Fair value investments		135,300,000
Fair Value Measured at Net Asset Value Per Share \| Hedge Funds and Private Equity Funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	135,300,000
Total realized and unrealized gains (losses)	13,300,000
Amortization/Accretion	0
Purchases	50,500,000
Sales	(12,200,000)
Transfers in	0
Transfers out	0
Fair value investments	$ 186,900,000	135,300,000
Star & Shield Insurance Exchange
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		(5,200,000)
Star & Shield Insurance Exchange \| Level 1 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		0
Star & Shield Insurance Exchange \| Level 1 Inputs \| Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Short-term investments, at amortized cost (which approximates fair value)			100,000
Star & Shield Insurance Exchange \| Level 2 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		(5,200,000)
Star & Shield Insurance Exchange \| Level 3 Inputs \| Fixed maturity investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		0
Star & Shield Insurance Exchange \| Level 3 Inputs \| Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		0
Star & Shield Insurance Exchange \| Level 3 Inputs \| Hedge Funds and Private Equity Funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		$ 0
Star & Shield Insurance Exchange
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fixed maturity investments held-for-sale			$ 6,600,000

Investment Securities (Significant Unobservable Inputs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
DavidShield & PassportCard \| Standard Poors NR Rating \| Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 75.0
Unobservable Input	0.18
DavidShield & PassportCard \| Standard Poors NR Rating \| Exit Revenue Multiple
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Input	1
PassportCard \| Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value		$ 21.0
Unobservable Input		0.25
PassportCard \| Standard Poors NR Rating \| Exit Revenue Multiple
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Input		1
Compare.com \| Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value		$ 22.1
Unobservable Input		0.35
Compare.com \| Standard Poors NR Rating \| Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 16.9
Unobservable Input	0.22
Compare.com \| Standard Poors NR Rating \| Exit Revenue Multiple
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Input	2.75	1.75
YOUSURE Tarifvergleich GmbH “durchblicker” \| Standard Poors NR Rating \| Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 15.5	$ 11.3
Unobservable Input	0.23	0.21
YOUSURE Tarifvergleich GmbH “durchblicker” \| Standard Poors NR Rating \| Exit Revenue Multiple
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Input	2.25	1.75
Captricity \| Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value		$ 14.5
Unobservable Input		.30
Captricity \| Standard Poors NR Rating \| Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 14.5
Unobservable Input	0.23
Captricity \| Standard Poors NR Rating \| Exit Revenue Multiple
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Input	3.75	3.50
Galvanic \| Multiple of EBITDA
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value		$ 0.6
Unobservable Input		6
Galvanic \| Standard Poors NR Rating \| Multiple of EBITDA
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 3.1
Unobservable Input	6
Private debt instrument \| Standard Poors NR Rating \| Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 10.0
Unobservable Input	0.0962
OneTitle \| Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value		$ 3.6
Unobservable Input		2.521

Goodwill and Other Intangible Assets (Summary of Goodwill and Intangible Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Line Items]
Goodwill, fair value	$ 379.9	$ 25.9
Goodwill	379.9	25.9	$ 25.9
Other intangible assets, accumulated amortization	53.6	34.8
Other intangible assets, net	155.4
Other intangible assets, gross (including goodwill)	591.1	96.9
Goodwill and other intangible assets	537.5	62.1
Goodwill, foreign currency translation gain		0.0
NSM Holding Co LLC
Goodwill [Line Items]
Goodwill, fair value	354.3	0.0
Goodwill	354.3	0.0
Other intangible assets, fair value	140.2	0.0
Other intangible assets, accumulated amortization	8.3	0.0
Other intangible assets, net	131.9	0.0
MediaAlpha
Goodwill [Line Items]
Goodwill, fair value	18.3	18.3
Goodwill	18.3	18.3
Other intangible assets, fair value	69.9	69.9
Other intangible assets, accumulated amortization	44.8	34.5
Other intangible assets, net	25.1	35.4
Buzzmove
Goodwill [Line Items]
Goodwill, fair value	7.3	7.6
Goodwill	7.3	7.6
Other intangible assets, fair value	1.1	1.1
Other intangible assets, accumulated amortization	0.5	0.3
Other intangible assets, net	$ 0.6	0.8
Customer Relationships \| NSM Holding Co LLC
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life	9 years
Other intangible assets, fair value	$ 85.3	0.0
Other intangible assets, accumulated amortization	6.0	0.0
Other intangible assets, net	$ 79.3	0.0
Customer Relationships \| MediaAlpha
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life	9 years
Other intangible assets, fair value	$ 26.8	26.8
Other intangible assets, accumulated amortization	4.9	2.4
Other intangible assets, net	$ 21.9	24.4
Trade Names \| NSM Holding Co LLC
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life	20 years
Other intangible assets, fair value	$ 51.2	0.0
Other intangible assets, accumulated amortization	1.8	0.0
Other intangible assets, net	$ 49.4	0.0
Information Technology \| NSM Holding Co LLC
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life	5 years
Other intangible assets, fair value	$ 3.7	0.0
Other intangible assets, accumulated amortization	0.5	0.0
Other intangible assets, net	$ 3.2	0.0
Information Technology \| MediaAlpha
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life	5 years
Other intangible assets, fair value	$ 33.3	33.3
Other intangible assets, accumulated amortization	30.9	24.3
Other intangible assets, net	$ 2.4	9.0
Information Technology \| Buzzmove
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life	5 years
Other intangible assets, fair value	$ 0.5	0.5
Other intangible assets, accumulated amortization	0.3	0.2
Other intangible assets, net	$ 0.2	0.3
Other \| MediaAlpha
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life	3 years
Other intangible assets, fair value	$ 9.8	9.8
Other intangible assets, accumulated amortization	9.0	7.8
Other intangible assets, net	0.8	2.0
Other Intangible Assets
Goodwill [Line Items]
Other intangible assets, fair value	211.2	71.0
Other intangible assets, accumulated amortization	53.6	34.8
Other intangible assets, net	157.6	36.2
Other Intangible Assets \| NSM Holding Co LLC
Goodwill [Line Items]
Goodwill, foreign currency translation gain	$ (0.7)
Trademarks \| Buzzmove
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life	7 years
Other intangible assets, fair value	$ 0.6	0.6
Other intangible assets, accumulated amortization	0.2	0.1
Other intangible assets, net	0.4	0.5
Non-controlling interests
Goodwill [Line Items]
Goodwill and other intangible assets	(40.6)	(21.1)
Parent
Goodwill [Line Items]
Goodwill and other intangible assets	496.9	41.0
Goodwill
Goodwill [Line Items]
Goodwill	379.9	$ 25.9
Goodwill, foreign currency translation gain	(2.5)
Goodwill \| NSM Holding Co LLC
Goodwill [Line Items]
Goodwill, foreign currency translation gain	(2.2)
Goodwill \| Buzzmove
Goodwill [Line Items]
Goodwill, foreign currency translation gain	$ (0.3)

Goodwill and Other Intangible Assets (Goodwill and Intangible Assets Rollforward) (Details) - USD ($) $ in Millions		12 Months Ended
	Oct. 05, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Beginning balance		$ 25.9	$ 25.9
Acquisitions of businesses			0.0
Foreign currency translation			0.0
Ending balance		379.9	25.9	$ 25.9
Finite-lived Intangible Assets [Roll Forward]
Amortization		(18.8)	(10.7)	(10.5)
Purchase of consolidated subsidiaries, net of cash acquired		295.2	27.6	13.4
Healthplans.com
Finite-lived Intangible Assets [Roll Forward]
Purchase of consolidated subsidiaries, net of cash acquired	$ 28.0
Other Intangible Assets
Finite-lived Intangible Assets [Roll Forward]
Beginning balance		36.2	19.3
Acquisitions of businesses		140.9	0.0
Acquisition of asset groups			27.6
Foreign currency translation		(0.7)	0.0
Amortization		(18.8)	(10.7)
Ending balance		157.6	36.2	$ 19.3
Goodwill
Goodwill [Roll Forward]
Beginning balance		25.9
Acquisitions of businesses		356.5
Foreign currency translation		(2.5)
Ending balance		$ 379.9	$ 25.9

Goodwill and Other Intangible Assets (Amortization Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization of other intangible assets	$ 18.8	$ 10.7	$ 10.5
Finite-lived intangible assets, next twelve months	18.1
Finite-lived intangible assets, year two	16.0
Finite-lived intangible assets, year three	16.0
Finite-lived intangible assets, year four	15.8
Finite-lived intangible assets, year five and thereafter	89.5
Total	$ 155.4

Debt (Debt Outstanding) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument
Debt	$ 192.7	$ 23.8
Other Debt
Debt Instrument
Debt	1.9	0.0
White Mountains \| WTM Bank Facility
Debt Instrument
Debt	0.0	0.0
NSM Holding Co LLC
Debt Instrument
Debt	178.5	0.0
MediaAlpha
Debt Instrument
Debt	14.2	23.8
MediaAlpha \| MediaAlpha Bank Facility
Debt Instrument
Debt instrument at face value	14.3	23.9
Unamortized Debt Issuance Expense	(0.1)	(0.1)
Debt	$ 14.2	$ 23.8
Effective yield (as a percent)	7.10%	5.60%
Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Debt instrument at face value	$ 180.4	$ 0.0
Unamortized Debt Issuance Expense	$ (3.8)	0.0
Debt		$ 0.0
Effective yield (as a percent)	7.40%
Term Loan \| MediaAlpha \| MediaAlpha Bank Facility
Debt Instrument
Debt instrument at face value	$ 14.3

Debt (Schedule of Contractual Repayments) (Details) $ in Millions	Dec. 31, 2018 USD ($)
Debt Disclosure [Abstract]
Due in one year or less	$ 5.2
Due in two to three years	10.4
Due in four to five years	9.4
Due after five years	171.9
Total	$ 196.9

Debt (Narrative) (Details) $ in Millions		8 Months Ended	12 Months Ended
Debt (Narrative) (Details) $ in Millions	May 11, 2018 USD ($) loans	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jun. 15, 2018 USD ($)	Oct. 05, 2017 USD ($)	May 12, 2017 USD ($)	Aug. 14, 2013 USD ($)
Debt Instrument
Interest expense on debt			$ 9.5	$ 2.3	$ 3.0
Repayments of debt, principal			15.4	365.0	404.6
Debt		$ 192.7	192.7	23.8
Draw down of debt and revolving line of credit			84.1	376.0	352.5
Interest paid			8.8	1.4	2.1
WTM Bank Facility
Debt Instrument
Total commitment under revolving credit facility									$ 425.0
Interest expense on debt			0.3	0.6	1.2
NSM Holding Co LLC
Debt Instrument
Debt		178.5	178.5	0.0
NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Total commitment under revolving credit facility	$ 100.0
Number of delayed draw loans \| loans	2
MediaAlpha
Debt Instrument
Debt		14.2	14.2	23.8
MediaAlpha \| MediaAlpha Bank Facility
Debt Instrument
Total commitment under revolving credit facility							$ 28.4	$ 20.0
Interest expense on debt			1.2	1.0	0.9
Debt instrument at face value		14.3	14.3	23.9
Debt		$ 14.2	$ 14.2	$ 23.8
Effective yield (as a percent)		7.10%	7.10%	5.60%
Draw down of debt and revolving line of credit			$ 3.0	$ 20.0
MediaAlpha \| Previous MediaAlpha Bank Facility
Debt Instrument
Total commitment under revolving credit facility		$ 20.0	20.0
Repayments of debt, principal			$ 12.9
Term Loan \| MediaAlpha Bank Facility
Debt Instrument
Basis spread on variable rate			1.50%
Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Repayments of debt, principal		0.8
Debt instrument at face value		$ 180.4	$ 180.4	0.0
Debt				0.0
Effective yield (as a percent)		7.40%	7.40%
Term Loan \| NSM Holding Co LLC \| Ageas Bank Facility
Debt Instrument
Amount outstanding		$ 2.2	$ 2.2
Fair value of amount outstanding		(0.3)	(0.3)
Amortization of purchase accounting adjustment			(0.3)
Term Loan \| MediaAlpha \| MediaAlpha Bank Facility
Debt Instrument
Total commitment under revolving credit facility		18.4	18.4
Repayments of debt, principal			3.6	2.1
Debt instrument at face value		14.3	14.3
Revolving Credit Facility \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Total commitment under revolving credit facility	$ 10.0
Current borrowing capacity	2.0
Repayments of lines of credit		2.0
Revolving Credit Facility \| MediaAlpha \| MediaAlpha Bank Facility
Debt Instrument
Amount outstanding		10.0	10.0
Repayments of debt, principal			$ 9.0	6.0
Basis spread on variable rate			0.25%
Term Loan and Revolving Credit Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Debt instrument at face value		$ 180.4	$ 180.4
London Interbank Offered Rate (LIBOR) \| Term Loan \| Minimum \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Basis spread on variable rate		4.25%	4.25%
London Interbank Offered Rate (LIBOR) \| Term Loan \| Maximum \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Basis spread on variable rate		4.75%	4.75%
Prime Rate \| Term Loan \| Minimum \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Basis spread on variable rate		3.25%	3.25%
Prime Rate \| Term Loan \| Maximum \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Basis spread on variable rate		3.75%	3.75%
Interest Rate Swap \| NSM Holding Co LLC
Debt Instrument
Fixed interest rate		2.97%	2.97%
Variable interest rate		2.52%	2.52%
Interest Rate Swap \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Total commitment under revolving credit facility						$ 151.0
Fixed interest rate		2.97%	2.97%
Variable interest rate		2.52%	2.52%
Debt		$ 150.2	$ 150.2
Debt, weighted average interest rate		7.42%	7.42%
Fresh Insurance Services Group Limited \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Amount outstanding	51.0
KBK Insurance Group, Inc. & KBK Premium Services, Inc. \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Amount outstanding	$ 30.1
NSM Holding Co LLC
Debt Instrument
Interest expense on debt			$ 8.0	$ 0.0	$ 0.0
Designated as Hedging Instrument \| Interest Rate Swap \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Debt		$ 150.2	$ 150.2
Debt, weighted average interest rate		7.47%	7.47%
Not Designated as Hedging Instrument \| Interest Rate Swap \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Debt		$ 30.0	$ 30.0
Effective yield (as a percent)		6.85%	6.85%

Income Taxes (Income Tax (Benefit) Expense (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current income tax (expense) benefit:
U.S. federal									$ (0.1)	$ (0.3)	$ 21.4
State									(1.4)	(1.3)	(0.7)
Non-U.S.									(2.9)	(2.0)	(0.3)
Total current tax expense									(4.4)	(3.6)	20.4
Deferred income tax benefit:
U.S. federal									8.3	11.4	12.5
Non-U.S.									0.1	0.0	0.0
Total deferred income tax benefit									8.4	11.4	12.5
Total income tax benefit	$ 3.6	$ 3.6	$ (2.5)	$ (0.7)	$ 2.5	$ 4.0	$ 1.0	$ 0.3	$ 4.0	$ 7.8	$ 32.9

Income Taxes (Effective Rate Reconciliation) (Details) - USD ($) $ in Millions			3 Months Ended								12 Months Ended
	Dec. 22, 2017	Dec. 21, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Contingency [Line Items]
U.S. statutory rate (as a percent)	21.00%	35.00%									21.00%	35.00%
Tax benefit (expense) at the U.S. statutory rate											$ 37.4	$ (2.7)	$ 51.6
Differences in taxes resulting from:
Change in valuation allowance											(31.0)	42.6	6.9
State taxes											4.0	0.6	(1.2)
Non-U.S. earnings, net of foreign taxes											(2.9)	21.5	(19.2)
Withholding tax											(2.7)	(2.0)	(0.2)
Member’s surplus contributions (“MSC”)											(2.6)	(3.0)	(2.3)
Tax rate changes											1.7	(44.3)	(3.9)
Tax reserve adjustments											(0.8)	(0.3)	0.0
Tax exempt interest and dividends											0.6	0.5	0.1
Officer compensation											0.0	(4.1)	0.0
Other, net											0.3	(1.0)	1.1
Income tax (expense) benefit			$ 3.6	$ 3.6	$ (2.5)	$ (0.7)	$ 2.5	$ 4.0	$ 1.0	$ 0.3	4.0	7.8	32.9
Pre-tax loss			$ (144.1)	$ 44.3	$ (12.4)	$ (66.0)	$ 5.2	$ 8.4	$ (2.2)	$ (3.6)	(178.2)	7.8	(147.3)
Tax Payments and Receipts
Net income tax payments to (receipts from) national governments, total											3.5	2.0	0.3
Non-US
Differences in taxes resulting from:
Pre-tax loss											$ (30.1)	$ 71.3	$ (66.3)

Income Taxes (Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Millions			12 Months Ended
	Dec. 22, 2017	Dec. 21, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets related to:
U.S. federal and state net operating and capital loss carryforwards			$ 95.5	$ 73.0
Non-U.S. net operating loss carryforwards			36.6	33.9
Incentive compensation			14.1	20.4
Investment basis difference			11.1	4.9
Net unrealized investment losses			9.5	0.0
Tax credit carryforwards			4.2	1.3
Deferred acquisition costs			3.5	2.0
Other items			5.8	1.6
Total gross deferred tax assets			180.3	137.1
Less: valuation allowances			139.9	109.6
Total net deferred tax assets			40.4	27.5
Deferred income tax liabilities related to:
Member surplus contributions (“MSC”)			32.8	24.1
Net unrealized investment gains			0.0	1.0
Purchase accounting			4.2	0.2
Other items			1.2	0.9
Total deferred tax liabilities			38.2	26.2
Net deferred tax asset			$ 2.2	$ 1.3
U.S. statutory rate (as a percent)	21.00%	35.00%	21.00%	35.00%
Percentage of utilization of net operating losses				80.00%
Change in valuation allowance			$ 31.0	$ (42.6)	$ (6.9)
United States
Deferred income tax liabilities related to:
Provisional income tax expense			$ 43.1

Income Taxes (Valuation Allowance) (Details) - USD ($) $ in Millions			3 Months Ended								12 Months Ended
	Dec. 22, 2017	Dec. 21, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Valuation Allowance [Line Items]
Valuation allowance			$ 139.9				$ 109.6				$ 139.9	$ 109.6
Change in valuation allowance											$ 31.0	$ (42.6)	$ (6.9)
U.S. statutory rate (as a percent)	21.00%	35.00%									21.00%	35.00%
Income tax (expense) benefit			3.6	$ 3.6	$ (2.5)	$ (0.7)	2.5	$ 4.0	$ 1.0	$ 0.3	$ 4.0	$ 7.8	$ 32.9
U.S. losses and other federal deferred tax benefits
Valuation Allowance [Line Items]
Valuation allowance			102.6				74.8				102.6	74.8
Net operating losses in Luxembourg subsidiaries
Valuation Allowance [Line Items]
Valuation allowance			21.1								21.1
Net Operating Losses in Israel Subsidiaries
Valuation Allowance [Line Items]
Valuation allowance			14.5				13.5				14.5	13.5
Net Operating Losses in Netherlands Subsidiaries
Valuation Allowance [Line Items]
Valuation allowance			1.7								1.7
Luxembourg \| U.S. losses and other federal deferred tax benefits
Valuation Allowance [Line Items]
Change in valuation allowance											1.2	6.4
United States
Valuation Allowance [Line Items]
Provisional income tax expense											43.1
Israel \| Net Operating Losses in Israel Subsidiaries
Valuation Allowance [Line Items]
Change in valuation allowance											2.1	3.0
United Kingdom \| Net Operating Losses in United Kingdom Subsidiaries
Valuation Allowance [Line Items]
Change in valuation allowance											0.7	0.7
Subsidiaries \| Luxembourg \| Net operating losses in Luxembourg subsidiaries
Valuation Allowance [Line Items]
Valuation allowance							20.3					20.3
Subsidiaries \| United Kingdom \| Net operating losses in Luxembourg subsidiaries
Valuation Allowance [Line Items]
Valuation allowance							$ 1.0					1.0
Guilford Holdings, Inc. \| United States
Valuation Allowance [Line Items]
Change in valuation allowance											22.7	(21.5)
U.S. statutory rate (as a percent)	21.00%	35.00%
Provisional income tax expense												20.4
BAM
Valuation Allowance [Line Items]
Change in valuation allowance											1.5	(18.4)
Provisional income tax expense												22.7
Income tax (expense) benefit											(8.7)	$ (10.1)
White Mountains Catskill Holdings, Inc.
Valuation Allowance [Line Items]
Valuation allowance			$ 2.8								$ 2.8

Income Taxes (Net Operating and Capital Loss Carryforwards) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Net deferred tax assets
Total net deferred tax assets	$ 40.4	$ 27.5
Alternative minimum tax credit carryforwards, not subject to expiration	0.7
United States
Net deferred tax assets
Operating loss carryforwards	3.7
Stock Compensation Plan [Member] \| United States
Net deferred tax assets
Operating loss carryforwards	6.8
Operating and Capital Loss Carryforward
Net deferred tax assets
Operating loss carryforwards	577.5
Gross deferred tax asset	126.3
Valuation allowance	(123.1)
Total net deferred tax assets	3.2
Operating and Capital Loss Carryforward \| United States
Net deferred tax assets
Operating loss carryforwards	424.7
Gross deferred tax asset	89.7
Valuation allowance	(87.3)
Total net deferred tax assets	2.4
Operating and Capital Loss Carryforward \| Luxembourg
Net deferred tax assets
Operating loss carryforwards	76.8
Gross deferred tax asset	20.0
Valuation allowance	(20.0)
Total net deferred tax assets	0.0
Operating and Capital Loss Carryforward \| United Kingdom
Net deferred tax assets
Operating loss carryforwards	14.9
Gross deferred tax asset	2.6
Valuation allowance	(1.8)
Total net deferred tax assets	0.8
Operating and Capital Loss Carryforward \| Israel
Net deferred tax assets
Operating loss carryforwards	61.1
Gross deferred tax asset	14.0
Valuation allowance	(14.0)
Total net deferred tax assets	0.0
2019-2023 \| Operating and Capital Loss Carryforward
Net deferred tax assets
Operating loss carryforwards	0.4
2019-2023 \| Operating and Capital Loss Carryforward \| United States
Net deferred tax assets
Operating loss carryforwards	0.4
2019-2023 \| Operating and Capital Loss Carryforward \| Luxembourg
Net deferred tax assets
Operating loss carryforwards	0.0
2019-2023 \| Operating and Capital Loss Carryforward \| United Kingdom
Net deferred tax assets
Operating loss carryforwards	0.0
2019-2023 \| Operating and Capital Loss Carryforward \| Israel
Net deferred tax assets
Operating loss carryforwards	0.0
2024-2028 \| Operating and Capital Loss Carryforward
Net deferred tax assets
Operating loss carryforwards	0.0
2024-2028 \| Operating and Capital Loss Carryforward \| United States
Net deferred tax assets
Operating loss carryforwards	0.0
2024-2028 \| Operating and Capital Loss Carryforward \| Luxembourg
Net deferred tax assets
Operating loss carryforwards	0.0
2024-2028 \| Operating and Capital Loss Carryforward \| United Kingdom
Net deferred tax assets
Operating loss carryforwards	0.0
2024-2028 \| Operating and Capital Loss Carryforward \| Israel
Net deferred tax assets
Operating loss carryforwards	0.0
2029-2038 \| Operating and Capital Loss Carryforward
Net deferred tax assets
Operating loss carryforwards	389.8
2029-2038 \| Operating and Capital Loss Carryforward \| United States
Net deferred tax assets
Operating loss carryforwards	342.8
2029-2038 \| Operating and Capital Loss Carryforward \| Luxembourg
Net deferred tax assets
Operating loss carryforwards	47.0
2029-2038 \| Operating and Capital Loss Carryforward \| United Kingdom
Net deferred tax assets
Operating loss carryforwards	0.0
2029-2038 \| Operating and Capital Loss Carryforward \| Israel
Net deferred tax assets
Operating loss carryforwards	0.0
No expiration date \| Operating and Capital Loss Carryforward
Net deferred tax assets
Operating loss carryforwards	187.3
No expiration date \| Operating and Capital Loss Carryforward \| United States
Net deferred tax assets
Operating loss carryforwards	81.5
No expiration date \| Operating and Capital Loss Carryforward \| Luxembourg
Net deferred tax assets
Operating loss carryforwards	29.8
No expiration date \| Operating and Capital Loss Carryforward \| United Kingdom
Net deferred tax assets
Operating loss carryforwards	14.9
No expiration date \| Operating and Capital Loss Carryforward \| Israel
Net deferred tax assets
Operating loss carryforwards	$ 61.1

Income Taxes (ASC 740-10) (Details) - USD ($) $ in Millions	12 Months Ended
Income Taxes (ASC 740-10) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Contingency [Line Items]
Percentage of likelihood for realization of benefit upon ultimate settlement			50.00%
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns
Balance at beginning	$ 0.3	$ 0.0
Changes in prior year tax positions	0.8	0.1
Tax positions taken during the current year		0.2
Balance at year end	1.1	0.3	$ 0.0
Permanent Differences
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns
Balance at beginning	0.3	0.0
Changes in prior year tax positions	0.8	0.1
Tax positions taken during the current year		0.2
Balance at year end	1.1	0.3	0.0
Temporary Differences
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns
Balance at beginning	0.0	0.0
Changes in prior year tax positions	0.0	0.0
Tax positions taken during the current year		0.0
Balance at year end	0.0	0.0	0.0
Interest and Penalties Net of Tax Benefit
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns
Balance at beginning	0.0	0.0
Changes in prior year tax positions	0.0	0.0
Tax positions taken during the current year		0.0
Balance at year end	$ 0.0	$ 0.0	$ 0.0

Income Taxes (Tax Examinations) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2016
Income Tax Examination [Line Items]
Gross deferred income tax assets	$ 180.3	$ 137.1
Tax Year 2012
Income Tax Examination [Line Items]
Gross deferred income tax assets			$ 0.6
Valuation allowance			$ 0.6

Derivatives (Details) $ in Millions	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Notional amount	$ 206.3

Derivatives (Pre-Tax Operating Results of WM Life Re) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pre-tax operating results of WM Life Re
Total revenues	$ 6.0	$ 198.7	$ 122.3	$ 42.1	$ 114.0	$ 87.5	$ 83.5	$ 88.8	$ 369.1	$ 373.8	$ 157.7
General and administrative expenses									(235.7)	(208.0)	(175.5)
Pre-tax income (loss)	$ (144.1)	$ 44.3	$ (12.4)	$ (66.0)	$ 5.2	$ 8.4	$ (2.2)	$ (3.6)	$ (178.2)	$ 7.8	(147.3)
White Mountains Life Re
Pre-tax operating results of WM Life Re
Fees, included in other revenue											1.2
Change in fair value of variable annuity liability, included in other revenue											(0.3)
Change in fair value of derivatives, included in other revenue											(2.0)
Foreign exchange, included in other revenue											1.3
Total revenues											0.2
Liability for Future Policy Benefits, Other Increase (Decrease)											(0.3)
General and administrative expenses											(2.6)
Pre-tax income (loss)											$ (2.7)

Derivatives (Realized and Unrealized Gains (Losses)) (Details) - White Mountains Life Re $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	$ (2.0)
Fixed income/interest rate
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	1.8
Foreign exchange
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	(4.8)
Equity
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	$ 1.0

Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Changes in derivative instruments
Purchases	$ 1,394.3	$ 3,332.5
Level 2
Changes in derivative instruments
Purchases	783.8	2,007.9
Level 1
Changes in derivative instruments
Purchases	$ 514.7	1,209.3
White Mountains Life Re
Changes in derivative instruments
Realized and unrealized gains (losses)			$ (2.0)
Variable Annuity Liabilities \| White Mountains Life Re \| Level 3
Changes in variable annuity (liabilities)
Beginning balance		0.0	0.3
Purchases			0.0
Realized and unrealized (losses) gains			(0.3)
Transfers in			0.0
Sales/settlements			0.0
Ending balance			0.0
Derivative Instruments \| White Mountains Life Re
Changes in derivative instruments
Beginning of period		0.0	20.1
Purchases			0.0
Realized and unrealized gains (losses)			2.0
Transfers in			0.0
Sales/settlements			(18.1)
Ending balance			0.0
Derivative Instruments \| White Mountains Life Re \| Level 3
Changes in derivative instruments
Beginning of period		0.0	2.7
Purchases			0.0
Realized and unrealized gains (losses)			(2.9)
Transfers in			0.0
Sales/settlements			(5.6)
Ending balance			0.0
Derivative Instruments \| White Mountains Life Re \| Level 2
Changes in derivative instruments
Beginning of period		0.0	16.5
Purchases			0.0
Realized and unrealized gains (losses)			0.7
Transfers in			0.0
Sales/settlements			(15.8)
Ending balance			0.0
Derivative Instruments \| White Mountains Life Re \| Level 1
Changes in derivative instruments
Beginning of period		$ 0.0	0.9
Purchases			0.0
Realized and unrealized gains (losses)			4.2
Transfers in			0.0
Sales/settlements			3.3
Ending balance			$ 0.0

Derivatives (Forward Contracts) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional amount		$ 206.3
Forward Contracts
Derivative [Line Items]
Realized and unrealized gains (losses)	$ 3.5	23.8	$ 1.2
Other unconsolidated investments
Derivative [Line Items]
Net amount recognized in other long-term investments		(3.7)	$ (1.2)
A \| Barclays \| Forward Contracts
Derivative [Line Items]
Notional amount		206.3
Carrying Value		(3.7)
GBP
Derivative [Line Items]
Notional amount		$ 152.0

Derivatives - Interest Rate Swap (Details) - USD ($)	8 Months Ended	12 Months Ended
Derivatives - Interest Rate Swap (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2018	Jun. 15, 2018	May 11, 2018	Dec. 31, 2017
Derivative [Line Items]
Debt	$ 192,700,000	$ 192,700,000			$ 23,800,000
NSM Holding Co LLC
Derivative [Line Items]
Debt	$ 178,500,000	$ 178,500,000			0
NSM Bank Facility \| NSM Holding Co LLC
Derivative [Line Items]
Total commitment under revolving credit facility				$ 100,000,000
NSM Bank Facility \| NSM Holding Co LLC \| Term Loan
Derivative [Line Items]
Debt					$ 0
NSM Bank Facility \| NSM Holding Co LLC \| Term Loan \| London Interbank Offered Rate (LIBOR) \| Minimum
Derivative [Line Items]
Basis spread on variable rate	4.25%	4.25%
NSM Bank Facility \| NSM Holding Co LLC \| Term Loan \| London Interbank Offered Rate (LIBOR) \| Maximum
Derivative [Line Items]
Basis spread on variable rate	4.75%	4.75%
NSM Bank Facility \| NSM Holding Co LLC \| Term Loan \| Prime Rate \| Minimum
Derivative [Line Items]
Basis spread on variable rate	3.25%	3.25%
NSM Bank Facility \| NSM Holding Co LLC \| Term Loan \| Prime Rate \| Maximum
Derivative [Line Items]
Basis spread on variable rate	3.75%	3.75%
Interest Rate Swap \| NSM Holding Co LLC
Derivative [Line Items]
Fixed interest rate	2.97%	2.97%
Variable interest rate	2.52%	2.52%
Net interest expense	$ 700,000.0
Derivative, fair value, net	2,700,000.0	$ 2,700,000.0
Change in the fair value of the swap	$ (2,700,000.0)
Interest Rate Swap \| NSM Bank Facility \| NSM Holding Co LLC \| Term Loan
Derivative [Line Items]
Total commitment under revolving credit facility			$ 151,000,000
Fixed interest rate	2.97%	2.97%
Variable interest rate	2.52%	2.52%
Debt	$ 150,200,000	$ 150,200,000
Debt, weighted average interest rate	7.42%	7.42%
Designated as Hedging Instrument \| Interest Rate Swap \| NSM Bank Facility \| NSM Holding Co LLC \| Term Loan
Derivative [Line Items]
Debt	$ 150,200,000	$ 150,200,000
Debt, weighted average interest rate	7.47%	7.47%

Municipal Bond Guarantee Insurance (Details) $ in Millions	1 Months Ended	12 Months Ended
Municipal Bond Guarantee Insurance (Details) $ in Millions	Apr. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) Contract	Dec. 31, 2017 USD ($) yr Contract	Dec. 31, 2016 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2019
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Supplement Trust Target		$ 603.0
Percentage of par value of policy reinsured		15.00%	15.00%
Percentage of risk premium ceded			60.00%
Repayments of debt, principal		$ 15.4	$ 365.0	$ 404.6
Interest paid		8.8	$ 1.4	$ 2.1
Percentage of discounted future cash flows			10.00%
Unearned premium revenue		176.0	$ 136.8
White Mountains
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Capital contributions					$ 594.5
HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes					503.0
Assets Held-in-trust		$ 100.0
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Year of U.S. Treasury Rate \| yr			1
Contracts outstanding \| Contract		7,525	6,371
Remaining weighted average contract period (in years)		10 years 8 months	10 years 11 months
Municipal bond outstanding principal		$ 52,201.6	$ 42,090.6
Municipal bond outstanding interest		26,560.3	21,057.1
Municipal bond debt service outstanding		78,761.9	$ 63,147.7
Unearned premium revenue		176.0
January 1, 2019 - March 31, 2019		4.1
April 1, 2019 - June 30, 2019		4.0
July 1, 2019 - September 30, 2019		4.0
October 1, 2019 - December 31, 2019		3.8
2019		15.9
2020		15.0
2021		14.0
2022		13.2
2023		12.3
2024 and thereafter		$ 105.6
BAM Management
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Capital contributions					$ 14.5
Preferred stocks \| HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ownership interest (as a percent)		96.90%	96.90%
Common Stock \| HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ownership interest (as a percent)		88.40%	88.40%
HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Percentage of par value of policy reinsured				15.00%
Collateral held in supplement trust		$ 757.4	$ 715.1
Interest receivable		143.7	126.0
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes		$ 481.3	$ 499.0
Surplus Note \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Interest rate (as a percent)		4.60%	3.78%
Basis spread on variable rate		300.00%
Surplus Note \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Repayments of debt, principal		$ 17.7	$ 4.0
Interest paid		5.3	$ 1.0
Combination One \| Surplus Note \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of years of change in interest rate \| yr			5
Fixed interest rate \| Surplus Note \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Interest rate (as a percent)			8.00%
Forecast \| Surplus Note \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Interest rate (as a percent)						5.70%
Series A BAM Surplus Note \| HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes			$ 203.0
Series B BAM Surplus Note \| HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes		$ 300.0
BAM \| Fidus Re Ltd.
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Losses above retention, percentage of liability	90.00%
Fidus Re Ltd. \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Insurance linked securities	$ 100.0
Collateralized financial guarantee, term	12 years
Collateral Financial Guarantee, Callable Term		5 years
Liabilities for guarantees on long-duration contracts, guaranteed benefit liability, gross	$ 165.0
Percentage of Quota Share Reinsurance Agreement		100.00%
Financial guarantee insurance contracts, reimbusement	100.0
Direct written premiums		$ 2,200.0
Minimum \| Fidus Re Ltd. \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Liabilities for guarantees on long-duration contracts, guaranteed benefit liability, gross	$ 165.0
Maximum \| Fidus Re Ltd. \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Liabilities for guarantees on long-duration contracts, guaranteed benefit liability, gross		$ 276.0

Municipal Bond Guarantee Insurance Schedule of Net Written Premiums (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earned premiums:
Net earned premiums	$ 13.9	$ 10.4	$ 13.4
HG/BAM
Written premiums:
Direct	44.8	63.2	38.6
Assumed	8.1	0.0	0.0
Net written premiums	52.9	63.2	38.6
Earned premiums:
Net earned premiums	13.6	9.4	5.9
Assumed	0.3	0.0	0.0
Net earned premiums	$ 13.9	$ 9.4	$ 5.9

Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$ (137.5)	$ 40.8	$ 3.5	$ (48.0)	$ 22.5	$ 562.1	$ 13.6	$ 29.0	$ (141.2)	$ 627.2	$ 401.8
Less: total (loss) income from discontinued operations, net of tax									(17.2)	577.5	523.4
Net income (loss) attributable to White Mountains’ common shareholders									(124.0)	49.7	(121.6)
Allocation of earnings (losses) to participating restricted common shares									1.4	(0.7)	1.5
Basic and diluted earnings per share numerators									$ (122.6)	$ 49.0	$ (120.1)
Basic earnings per share denominators (in thousands):
Weighted Average Number of Shares Issued, Basic									3,382,500	4,293,800	5,014,900
Average unvested restricted common shares									(40,100)	(54,300)	(64,800.00)
Basic (losses) earnings per share denominator									3,342,400	4,239,500	4,950,100
Diluted earnings per share denominator (in thousands):
Average outstanding dilutive options to acquire common shares									3,382,500	4,293,800	5,018,100
Average unvested restricted common shares									(40,100)	(54,300)	(64,800.00)
Diluted earnings (loss) per share denominator									3,342,400.00	4,239,500	4,953,300.00
Diluted earnings per share (in dollars) - continuing operations:
Dividends declared and paid									$ 1.00	$ 1.00	$ 1.00
Undistributed earnings (loss)									(37.67)	10.56	(25.26)
Basic and diluted (losses) earnings per share									$ (36.67)	$ 11.56	$ (24.26)

Earnings Per Share (Footnotes) (Details)	12 Months Ended
Earnings Per Share (Footnotes) (Details)	Dec. 31, 2016 shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Incremental common shares attributable to share-based payment arrangements	3,217
Discontinued Operations
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Incremental common shares attributable to share-based payment arrangements	40,000

Earnings Per Share (Schedule of Undistributed Net Earnings) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net (loss) income attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ (122,600,000)	$ 49,000,000	$ (120,100,000)
Dividends declared, net of restricted common share amounts	(3,700,000)	(4,500,000)	(5,400,000)
Total undistributed net (losses) earnings, net of restricted common share amounts	$ (126,300,000)	$ 44,500,000	$ (125,500,000)

Employee Share-Based Incentive Compensation Plans (WTM Performance Shares) (Details) - Performance Shares $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) multiplier shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares
Range Of Performance Cycle From 2013 to 2015
Unamortized grant date fair value
Vested (in shares)	5,000
White Mountains
Share-based compensation arrangement by share-based payment award
Multiplier for determining payout, low end of range \| multiplier	0
Multiplier for determining payout, high end of range \| multiplier	2
Performance cycle period	3 years
W T M Incentive Plan \| White Mountains
Target Performance Share activity
Beginning of period	50,515	80,353	93,654
Shares paid or expired	(23,186)	(30,838)	(36,294)
New grants	14,105	17,710	22,615
Forfeitures	(818)	(16,710)	378
End of period	40,616	50,515	80,353
Unamortized grant date fair value
Beginning of period \| $	$ 45.8	$ 42.4	$ 57.7
Shares paid or expired \| $	(28.4)	(21.9)	41.0
Forfeitures \| $	0.1	(9.3)	0.5
Expense recognized \| $	14.2	34.6	25.2
End of period \| $	31.7	$ 45.8	$ 42.4
Unamortized grant date fair value \| $	$ 13.8
W T M Incentive Plan \| Sirius Group
Target Performance Share activity
Beginning of period	2,195	7,315
End of period	0	2,195	7,315
W T M Incentive and Phantom Plan \| Range Of Performance Cycle From 2015 to 2017
Unamortized grant date fair value
Performance goal percentage for minimum payout	145.00%
Payout for maximum growth target percentage	147.00%
W T M Incentive and Phantom Plan \| Range Of Performance Cycle From 2014 to 2016
Unamortized grant date fair value
Performance goal percentage for minimum payout		34.00%
Payout for maximum growth target percentage		76.00%
W T M Incentive and Phantom Plan \| Range Of Performance Cycle From 2013 to 2015
Unamortized grant date fair value
Performance goal percentage for minimum payout			140.00%
Payout for maximum growth target percentage			142.00%

Employee Share-Based Incentive Compensation Plans (WTM Performance Shares Granted Under the WTM Incentive Plan) (Details) - W T M Incentive Plan - Performance Shares - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Range Of Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	6.00%
Payout for maximum growth target percentage	200.00%
Range Of Performance Cycle From 2017-2019
Share-based compensation arrangement by share-based payment award
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	5.00%
Payout for maximum growth target percentage	200.00%
Range Of Performance Cycle From 2016-2018
Share-based compensation arrangement by share-based payment award
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	4.00%
Payout for maximum growth target percentage	200.00%
White Mountains
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	41,235
Incentive compensation payable before forfeitures	$ 32.2
Assumed forfeitures (shares)	(619)
Assumed forfeitures in period amount	(0.5)
Shares outstanding (in shares)	40,616	50,515	80,353	93,654
Accrued incentive compensation	$ 31.7	$ 45.8	$ 42.4	$ 57.7
White Mountains \| Range Of Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	13,450
Incentive compensation payable before forfeitures	$ 3.8
White Mountains \| Range Of Performance Cycle From 2017-2019
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	14,070
Incentive compensation payable before forfeitures	$ 11.9
White Mountains \| Range Of Performance Cycle From 2016-2018
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	13,715
Incentive compensation payable before forfeitures	$ 16.5
Minimum \| Range Of Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	2.00%
Minimum \| Range Of Performance Cycle From 2017-2019
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	1.00%
Minimum \| Range Of Performance Cycle From 2016-2018
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	0.00%
Maximum \| Range Of Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	10.00%
Maximum \| Range Of Performance Cycle From 2017-2019
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	9.00%
Maximum \| Range Of Performance Cycle From 2016-2018
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	8.00%

Employee Share-Based Incentive Compensation Plans (WTM Restricted Shares) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
W T M Incentive and Phantom Plan \| Restricted Share Cliff Vesting in January 2021 [Member]
Target Performance Share activity
Issued (in shares)	13,450
W T M Incentive and Phantom Plan \| Restricted Share Cliff Vesting in January 2020
Target Performance Share activity
Issued (in shares)	290	17,485
W T M Incentive and Phantom Plan \| Restricted Share Cliff Vesting in January 2019
Target Performance Share activity
Issued (in shares)	365	250	24,615
W T M Incentive and Phantom Plan \| Restricted Share Cliff Vesting in January 2018
Target Performance Share activity
Issued (in shares)		250	750
White Mountains \| W T M Incentive Plan \| Restricted Stock
Target Performance Share activity
Beginning of period	53,755	70,620	70,675
Issued (in shares)	14,105	17,985	25,365
Vested (in shares)	(25,381)	(28,846)	(24,620)
Assumed forfeitures and cancellations (in shares)	(969)	(6,004)	(800)
End of period	41,510	53,755	70,620
Unamortized grant date fair value
Beginning balance	$ 14.3	$ 19.7	$ 15.7
Issued	11.4	16.3	20.2
Assumed forfeitures and cancellations	(0.2)	(3.5)	(0.3)
Expense recognized	(13.0)	(18.2)	(15.9)
Ending balance	$ 12.5	$ 14.3	$ 19.7
Sirius Group \| W T M Incentive Plan \| Restricted Stock
Target Performance Share activity
Beginning of period	2,195	5,235
End of period	0	2,195	5,235

Employee Share-Based Incentive Compensation Plans (WTM Non-Qualified Options) (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended	12 Months Ended
	Jan. 20, 2017	Mar. 31, 2017	Dec. 31, 2016
Share-based compensation arrangement by share-based payment award
Shares issued, price per share			$ 742
Employee Stock Option \| Combination One
Share-based compensation arrangement by share-based payment award
Non-option equity instruments, exercised			5,000
Exercises in period, intrinsic value			$ 0.4
Shares issued, price per share			$ 742
Employee Stock Option \| Combination Two
Share-based compensation arrangement by share-based payment award
Non-option equity instruments, exercised			80,000
Exercises in period, intrinsic value			$ 8.4
Conversion of stock, shares converted			9,930
Employee Stock Option \| Chief Executive Officer
Share-based compensation arrangement by share-based payment award
Non-option equity instruments, exercised	125,000	40,000
Exercises in period, intrinsic value		$ 4.4
Common Stock
Share-based compensation arrangement by share-based payment award
Conversion of stock, shares converted		5,142

Employee Share-Based Incentive Compensation Plans (MediaAlpha Class B Unit Awards) (Details) - Capital Unit, Class B - MediaAlpha $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)
Share-based compensation arrangement by share-based payment award
Expense recognized	$ 11.7
Compensation not recognized	$ 3.3
Minimum
Share-based compensation arrangement by share-based payment award
Award service period	36 months
Maximum
Share-based compensation arrangement by share-based payment award
Award service period	48 months

Common Shareholders’ Equity and Non-controlling Interests (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period	$ 519.4	$ 723.9	$ 887.2
Value of each common share repurchased and retired during the period (in dollars per share)	$ 876.69	$ 869	$ 802
Common Shares Issued [Abstract]
Stock issued during period, shares, new issues	16,377	25,086	47,030
Shares issued to directors (in shares)	2,272	1,959	1,735
Dividends, Common Stock [Abstract]
Cash dividends declared and paid	$ 3.8	$ 4.6	$ 5.4
Dividends delared and paid (in dollars per share)	$ 1.00
Non-controlling interests	$ (124.9)	$ (131.7)
Common Stock
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period	592,458	832,725	1,106,145
Restricted Stock
Common Shares Issued [Abstract]
Stock issued during period, shares, new issues	14,105	17,985	25,365
Performance Shares
Common Shares Issued [Abstract]
Stock issued during period, shares, new issues			5,000
General Board Authorization
Common Shares Repurchased and Retired [Abstract]
Additional authorized repurchase of common shares	635,705
Common shares repurchased and retired during the period	582,493	821,732	1,098,123
Value of common shares repurchased and retired during the period	$ 511.0	$ 713.1	$ 881.4
Value of each common share repurchased and retired during the period (in dollars per share)	$ 877	$ 870	$ 802.63
Chief Executive Officer \| Restricted Stock
Common Shares Issued [Abstract]
Stock issued during period, shares, new issues		5,142	14,930
Repurchased for Employee Benefit Plans
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period	9,965	10,993	8,022
HG Global
Dividends, Common Stock [Abstract]
Noncontrolling interest, ownership percentage by noncontrolling owners	3.10%	3.10%
Non-controlling interests	$ 14.5	$ 15.9
NSM Insurance HoldCo, LLC
Dividends, Common Stock [Abstract]
Noncontrolling interest, ownership percentage by noncontrolling owners	4.50%	0.00%
Non-controlling interests	$ 13.6	$ 0.0
MediaAlpha
Dividends, Common Stock [Abstract]
Noncontrolling interest, ownership percentage by noncontrolling owners	39.00%	35.70%
Non-controlling interests	$ 16.2	$ 13.1
Buzzmove
Dividends, Common Stock [Abstract]
Noncontrolling interest, ownership percentage by noncontrolling owners	22.90%	22.90%
Non-controlling interests	$ 1.1	$ 2.5
Other
Dividends, Common Stock [Abstract]
Noncontrolling interest, ownership percentage by noncontrolling owners	13.40%
Non-controlling interests	$ 0.3
BAM
Dividends, Common Stock [Abstract]
Noncontrolling interest, ownership percentage by noncontrolling owners	100.00%	100.00%
Non-controlling interests	$ (170.6)	$ (163.2)
Total Non-Controlling Excluding Reciprocals
Dividends, Common Stock [Abstract]
Non-controlling interests	$ 45.7	$ 31.5

Statutory Capital and Surplus (Details) $ in Millions	12 Months Ended
Statutory Capital and Surplus (Details) $ in Millions	Dec. 31, 2018 USD ($) yr	Dec. 31, 2017 USD ($) yr	Dec. 31, 2016 USD ($)	Dec. 31, 2019
Statutory Accounting Practices [Line Items]
Repayments of debt, principal	$ 15.4	$ 365.0	$ 404.6
Interest paid	8.8	1.4	2.1
Other Operations
Statutory Accounting Practices [Line Items]
Unrestricted cash	536.0
Other long-term investments	67.3
HG Re \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	698.9
Cash outside of collateral trusts	2.2
Collateral held in supplement trust	757.4
Cash and investments outside of collateral trusts	3.0
BAM \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	413.7
Statutory net income (loss)	(34.6)	$ (25.4)	$ (32.7)
Surplus required	66.0
HG Global \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Preferred stock, value	$ 619.0
Ownership interest (as a percent)	96.90%
Preferred stock dividend rate	6.00%
NSM Insurance HoldCo, LLC \| NSM Insurance HoldCo, LLC
Statutory Accounting Practices [Line Items]
Cash	$ 16.2
MediaAlpha
Statutory Accounting Practices [Line Items]
Ownership interest (as a percent)	61.00%	64.40%
Cash	$ 5.7	$ 9.1
Distributions made	15.9
Unrestricted cash	5.7
Surplus Note \| BAM
Statutory Accounting Practices [Line Items]
Repayments of debt, principal	17.7	4.0
Interest paid	$ 5.3	$ 1.0
Surplus Note \| BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)	4.60%	3.78%
Basis spread on variable rate	300.00%
Surplus Note \| BAM \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)	8.00%
Preferred stocks \| HG Global \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Dividends payable	$ 288.1
Preferred stocks \| White Mountains \| HG Global \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Dividends payable	278.5
Common Equities \| Other Operations
Statutory Accounting Practices [Line Items]
Distributions made	3.8
Common equity securities	$ 925.6
Combination One \| Surplus Note \| BAM
Statutory Accounting Practices [Line Items]
Number of years of change in interest rate \| yr		5
Combination One \| Surplus Note \| BAM \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Number of years of change in interest rate	5
Combination Two \| Surplus Note \| BAM \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Number of years of change in interest rate \| yr	3
US Treasury Rate \| Surplus Note \| BAM \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)	4.60%
Fixed interest rate \| Surplus Note \| BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)		8.00%
Fixed interest rate \| Surplus Note \| BAM \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)	8.00%
Forecast \| Surplus Note \| BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)				5.70%
Forecast \| US Treasury Rate \| Surplus Note \| BAM \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)				5.70%
Parent \| MediaAlpha
Statutory Accounting Practices [Line Items]
Distributions made	$ 9.8

Segment Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
Segment Information (Details) - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment reporting information
Earned insurance premiums									$ 13.9	$ 10.4	$ 13.4
Net investment income									59.0	56.0	32.1
Gain (Loss) on Investments									$ (108.3)	$ 133.3	(27.4)
Percentage of par value of policy reinsured	15.00%				15.00%				15.00%	15.00%
Commission revenues									$ 394.3	$ 167.0	118.3
Other revenue									10.2	7.1	21.3
Total revenues	$ 6.0	$ 198.7	$ 122.3	$ 42.1	$ 114.0	$ 87.5	$ 83.5	$ 88.8	369.1	373.8	157.7
Loss and loss adjustment expenses										1.1	8.0
Insurance acquisition expenses									5.3	4.1	5.6
Other underwriting expenses									0.4	0.4	0.4
Cost of sales									248.7	139.4	102.0
General and administrative expenses									235.7	208.0	175.5
Broker commission expense									28.9
Amortization of other intangible assets									18.8	10.7	10.5
Interest Expense									9.5	2.3	3.0
Total expenses	150.1	154.4	134.7	108.1	108.8	79.1	85.7	92.4	547.3	366.0	305.0
Pre-tax loss	$ (144.1)	$ 44.3	$ (12.4)	$ (66.0)	$ 5.2	$ 8.4	$ (2.2)	$ (3.6)	(178.2)	7.8	(147.3)
NSM Holding Co LLC
Segment reporting information
Earned insurance premiums									0.0
Net investment income									0.0
Gain (Loss) on Investments									0.0
Commission revenues									94.7	0.0	0.0
Other revenue									6.9	0.0	0.0
Total revenues									101.6	0.0	0.0
Insurance acquisition expenses									0.0
Other underwriting expenses									0.0
Cost of sales									0.0
General and administrative expenses									61.6	0.0	0.0
Broker commission expense									28.9	0.0	0.0
Amortization of other intangible assets									8.3	0.0	0.0
Interest Expense									8.0	0.0	0.0
Total expenses									106.8	0.0	0.0
Pre-tax loss									$ (5.2)
Percentage of commission revenue									33.00%
HG Global-BAM
Segment reporting information
Earned insurance premiums									$ 13.9	9.4	5.9
Net investment income									16.7	12.3	9.0
Gain (Loss) on Investments									(7.5)	0.6	0.7
Commission revenues									0.0	0.0	0.0
Other revenue									1.2	1.0	1.1
Total revenues									24.3	23.3	16.7
Loss and loss adjustment expenses										0.0	0.0
Insurance acquisition expenses									5.3	4.0	3.4
Other underwriting expenses									0.4	0.4	0.4
Cost of sales									0.0	0.0	0.0
General and administrative expenses									48.0	42.9	39.6
Broker commission expense									0.0
Amortization of other intangible assets									0.0	0.0	0.0
Interest Expense									0.0	0.0	0.0
Total expenses									53.7	47.3	43.4
Pre-tax loss									(29.4)	(24.0)	(26.7)
MediaAlpha
Segment reporting information
Earned insurance premiums									0.0	0.0	0.0
Net investment income									0.0	0.0	0.0
Gain (Loss) on Investments									0.0	0.0	0.0
Commission revenues									295.5	163.2	116.5
Other revenue									1.6	0.0	0.0
Total revenues									297.1	163.2	116.5
Loss and loss adjustment expenses										0.0	0.0
Insurance acquisition expenses									0.0	0.0	0.0
Other underwriting expenses									0.0	0.0	0.0
Cost of sales									245.0	135.9	97.8
General and administrative expenses									31.7	16.2	11.8
Broker commission expense									0.0
Amortization of other intangible assets									10.3	10.5	10.1
Interest Expense									1.2	1.0	0.9
Total expenses									288.2	163.6	120.6
Pre-tax loss									$ 8.9	$ (0.4)	$ (4.1)
Percentage of advertising revenue									29.00%	27.00%	24.00%
Other
Segment reporting information
Earned insurance premiums									$ 0.0	$ 1.0	$ 7.5
Net investment income									42.3	43.7	23.1
Gain (Loss) on Investments									(100.8)	132.7	(28.1)
Commission revenues									4.1	3.8	1.8
Other revenue									0.5	6.1	20.2
Total revenues									(53.9)	187.3	24.5
Loss and loss adjustment expenses									0.0	1.1	8.0
Insurance acquisition expenses									0.0	0.1	2.2
Other underwriting expenses									0.0	0.0	0.0
Cost of sales									3.7	3.5	4.2
General and administrative expenses									94.4	148.9	124.1
Broker commission expense									0.0
Amortization of other intangible assets									0.2	0.2	0.4
Interest Expense									0.3	1.3	2.1
Total expenses									98.6	155.1	141.0
Pre-tax loss									$ (152.5)	$ 32.2	$ (116.5)

Segment Information (Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2012
Segment reporting information
Total investments	$ 2,542.9	$ 3,380.7
Assets	3,362.6	3,659.2
Liabilities	644.4	298.4
White Mountains’ common shareholders’ equity	2,843.1	3,492.5
Non-controlling interests	(124.9)	(131.7)
NSM Holding Co LLC
Segment reporting information
Total investments	1.7	0.0
Assets	627.0	0.0
Liabilities	314.8	0.0
White Mountains’ common shareholders’ equity	298.3	0.0
Non-controlling interests	13.9	0.0
MediaAlpha
Segment reporting information
Total investments	0.0	0.0
Assets	88.4	96.5
Liabilities	46.9	59.8
White Mountains’ common shareholders’ equity	25.3	23.6
Non-controlling interests	16.2	13.1
Discontinued Operations, Held-for-sale
Segment reporting information
Total investments	0.0	0.0
Assets	3.3	3.3
Liabilities	0.0	0.0
White Mountains’ common shareholders’ equity	3.3	3.3
Non-controlling interests	0.0	0.0
HG Global-BAM
Segment reporting information
Total investments	768.3	693.4
Assets	816.2	747.4
Liabilities	212.5	167.0
White Mountains’ common shareholders’ equity	759.8	727.7
Non-controlling interests	(156.1)	(147.3)
Other Operations
Segment reporting information
Total investments	1,772.9	2,687.3
Assets	1,827.7	2,812.0
Liabilities	70.2	71.6
White Mountains’ common shareholders’ equity	1,756.4	2,737.9
Non-controlling interests	1.1	2.5
BAM
Segment reporting information
Surplus notes	481.3	499.0
Accrued interest on surplus notes	143.7	126.0
HG Global
Segment reporting information
Non-controlling interests	14.5	15.9
Surplus notes			$ 503.0
Preferred Dividends Payable \| HG Global
Segment reporting information
Liabilities	$ 278.5	$ 227.9

Investments in Unconsolidated Affiliates (Carrying Values of Investments) (Details) - White Mountains - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Other Investments	$ 325.6	$ 208.8
Limited liability companies and private equity securities
Schedule of Equity Method Investments [Line Items]
Other Investments	138.1	58.0
Investments accounted for under the equity method
Schedule of Equity Method Investments [Line Items]
Other Investments	1.3	4.6
Investments In Unconsolidated Entities
Schedule of Equity Method Investments [Line Items]
Other Investments	139.4	62.6
Other unconsolidated investments
Schedule of Equity Method Investments [Line Items]
Other Investments	$ 186.2	$ 146.2

Investments in Unconsolidated Affiliates (Details)	Dec. 31, 2018	Jan. 24, 2018	Dec. 31, 2017
DavidShield & PassportCard
Investments in and Advances to Affiliates [Line Items]
Ownership interest (as a percent)	50.00%
Kudu
Investments in and Advances to Affiliates [Line Items]
Ownership interest (as a percent)	49.50%		0.00%
YOUSURE Tarifvergleich GmbH “durchblicker”
Investments in and Advances to Affiliates [Line Items]
Ownership interest (as a percent)	45.00%		45.00%
Compare.com
Investments in and Advances to Affiliates [Line Items]
Ownership interest (as a percent)	18.40%		22.10%
Tuckerman Capital Fund III, L.P.
Investments in and Advances to Affiliates [Line Items]
Ownership interest (as a percent)	18.50%		21.30%
DavidShield
Investments in and Advances to Affiliates [Line Items]
Ownership interest (as a percent)	50.00%	50.00%
PassportCard
Investments in and Advances to Affiliates [Line Items]
Ownership interest (as a percent)	25.00%	50.00%	50.00%
White Mountains \| DavidShield
Investments in and Advances to Affiliates [Line Items]
Ownership interest (as a percent)	25.00%

Investments in Unconsolidated Affiliates (Balance Sheet and Income Statement Tables) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investments in unconsolidated affiliates
Assets	$ 3,362.6				$ 3,659.2				$ 3,362.6	$ 3,659.2
Liabilities	644.4				298.4				644.4	298.4
Revenues	6.0	$ 198.7	$ 122.3	$ 42.1	114.0	$ 87.5	$ 83.5	$ 88.8	369.1	373.8	$ 157.7
White Mountains
Investments in unconsolidated affiliates
Assets	218.8				75.4				218.8	75.4
Liabilities	$ 46.7				$ 24.2				46.7	24.2
Revenues									134.1	60.0	32.9
Expenses									$ (110.1)	$ (66.8)	$ (76.4)

Variable Interest Entities (Details) $ in Millions	12 Months Ended
Variable Interest Entities (Details) $ in Millions	Dec. 31, 2018 USD ($) director	Feb. 05, 2018 USD ($)	Dec. 31, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured	15.00%		15.00%
HG Global
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured				15.00%
BAM
Variable Interest Entity [Line Items]
Right to elect directors on board number \| director	2
First Loss Reinsurance Treaty \| HG Global
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured	15.00%
First Loss Reinsurance Treaty \| BAM
Variable Interest Entity [Line Items]
Percentage of premiums	60.00%
Other long-term investments \| Kudu
Variable Interest Entity [Line Items]
Unfunded capital commitment		$ 125.0
VIE, carrying amount	$ 30.7

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	$ 192.7	$ 23.8
NSM Holding Co LLC
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	178.5	0.0
NSM Holding Co LLC \| Fair value \| NSM Bank Facility
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt, fair value	176.1	0.0
MediaAlpha
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	14.2	23.8
MediaAlpha \| MediaAlpha Bank Facility
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	14.2	23.8
MediaAlpha \| Fair value \| MediaAlpha Bank Facility
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt, fair value	14.6	23.9
MediaAlpha \| Carrying Value \| MediaAlpha Bank Facility
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	14.2	23.8
Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt		0.0
Term Loan \| NSM Holding Co LLC \| Carrying Value \| NSM Bank Facility
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	$ 176.6	$ 0.0

Transactions with Related Persons (Details) - $ / shares				12 Months Ended
	Jul. 13, 2017	Sep. 15, 2016	Apr. 19, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Value of each common share repurchased and retired during the period (in dollars per share)				$ 876.69	$ 869	$ 802
Franklin Mutual Advisers
Related Party Transaction [Line Items]
Value of each common share repurchased and retired during the period (in dollars per share)	$ 850.00	$ 820.00	$ 807.00
Common Stock
Related Party Transaction [Line Items]
Common shares repurchased and retired during the period				592,458	832,725	1,106,145
Common Stock \| Franklin Mutual Advisers
Related Party Transaction [Line Items]
Common shares repurchased and retired during the period	235,000	305,000	325,000

Commitments and Contingencies (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Rent expense									$ 5.5	$ 3.5	$ 3.4
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Capital lease payments due next year	$ 6.6								6.6
Capital lease payments due in two years	4.9								4.9
Capital lease payments due in three years	4.2								4.2
Capital Lease payments due in four years and thereafter	12.0								12.0
Commitments to fund other-long term investments									170.2
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	0.0	$ (17.3)	$ 0.0	$ 0.1	$ 4.3	$ 539.1	$ 2.8	$ 31.3	0.0	$ 20.5	$ 108.3
Assumed lliability for unpaid claims and claims adjustment expense	$ 82.5								82.5
Sirius Group \| Swedish Tax Authority \| Sirius Group
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest									$ (17.3)

Held for Sale and Discontinued Operations (Details) - USD ($) $ in Millions					3 Months Ended									9 Months Ended	12 Months Ended
	Sep. 28, 2017	Oct. 05, 2016	Jul. 21, 2016	Apr. 18, 2016	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 28, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Discontinued Operations
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold															$ 0.0	$ 1,131.0	$ 2,646.2
Gain (loss) on sale of other discontinued operations, net of tax															(17.2)	557.0	415.1
Net income from discontinued operations, net of tax					$ 0.0	$ (17.3)	$ 0.0	$ 0.1	$ 4.3	$ 539.1	$ 2.8	$ 31.3			0.0	20.5	108.3
Tax benefit (expense)					3.6	$ 3.6	$ (2.5)	$ (0.7)	2.5	$ 4.0	$ 1.0	$ 0.3			4.0	7.8	32.9
Payments to acquire other investments															95.9	84.1	38.5
OneBeacon
Discontinued Operations
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	$ 1,300.0
Gain (loss) on sale of other discontinued operations, net of tax	$ 554.6
Net income from discontinued operations, net of tax														$ 20.5			108.6
OneBeacon \| Discontinued Operations
Discontinued Operations
Gain (loss) on sale of other discontinued operations, net of tax															0.0	554.5	0.0
Net income from discontinued operations, net of tax																20.5	108.6
Tax effect of discontinued operation																(5.7)	(12.5)
Recognition of foreign currency translation from sale of Sirius Group, net of tax																2.9	0.0
Tranzact
Discontinued Operations
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold		$ 1.2	$ 221.3
Gain (loss) on sale of other discontinued operations, net of tax																3.2	51.9
Tax effect of discontinued operation																	30.2
Tranzact \| Discontinued Operations
Discontinued Operations
Gain (loss) on sale of other discontinued operations, net of tax															0.0	3.2	51.9
Net income from discontinued operations, net of tax																0.0	6.1
Tax benefit (expense)																	21.4
Tax effect of extraordinary item, loss																	30.2
Tax effect of discontinued operation																0.0	(6.4)
Recognition of foreign currency translation from sale of Sirius Group, net of tax																0.0	0.0
Sirius Group
Discontinued Operations
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold				$ 2,600.0
Gain (loss) on sale of other discontinued operations, net of tax													$ (4.0)				363.2
Payments to acquire other investments				$ 161.8
Recognition of foreign currency translation from sale of Sirius Group, net of tax																	113.3
Sirius Group \| Discontinued Operations
Discontinued Operations
Gain (loss) on sale of other discontinued operations, net of tax															(17.2)	(0.7)	363.2
Net income from discontinued operations, net of tax																0.0	(6.4)
Tax effect of discontinued operation																0.0	(3.1)
Recognition of foreign currency translation from sale of Sirius Group, net of tax																0.0	113.3
Sale of Sirius Group
Discontinued Operations
Recognition of foreign currency translation from sale of Sirius Group, net of tax																	113.3
Sale of Sirius Group \| Discontinued Operations
Discontinued Operations
Net income from discontinued operations, net of tax																	(4.3)
Net gain (loss) from sale of discontinued operations \| Tranzact \| Discontinued Operations
Discontinued Operations
Tax effect of discontinued operation																	$ 8.8
Connecticut \| Sirius Group, Tranzact, and Star and Shield \| Discontinued Operations, Held-for-sale
Discontinued Operations
Assets held for sale					$ 3.3				$ 3.3						3.3	3.3
Other expenses																$ 3.7
Sirius Group \| Swedish Tax Authority \| Sirius Group
Discontinued Operations
Net income from discontinued operations, net of tax															$ (17.3)

Held for Sale and Discontinued Operations (Loss from Discontinued Operations) (Details) - USD ($) $ in Millions		3 Months Ended									9 Months Ended	12 Months Ended
	Sep. 28, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 28, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues
Earned insurance premiums												$ 13.9	$ 10.4	$ 13.4
Net investment income												59.0	56.0	32.1
Other revenue												10.2	7.1	21.3
Net realized and unrealized investment gains												(108.3)	133.3	(27.4)
Revenues (loss) (including realized and unrealized gains and losses)		$ 6.0	$ 198.7	$ 122.3	$ 42.1	$ 114.0	$ 87.5	$ 83.5	$ 88.8			369.1	373.8	157.7
Expenses
Loss and loss adjustment expenses													1.1	8.0
Other underwriting expenses												0.4	0.4	0.4
General and administrative expenses												235.7	208.0	175.5
Interest expense on debt												9.5	2.3	3.0
Total expenses		150.1	154.4	134.7	108.1	108.8	79.1	85.7	92.4			547.3	366.0	305.0
Net income from discontinued operations, net of tax		$ 0.0	$ (17.3)	$ 0.0	$ 0.1	$ 4.3	$ 539.1	$ 2.8	$ 31.3			0.0	20.5	108.3
Gain (loss) on sale of other discontinued operations, net of tax												(17.2)	557.0	415.1
Total income (loss) from discontinued operations												(17.2)	577.5	523.4
Comprehensive income from discontinued operations												(146.0)	630.7	547.4
Discontinued Operations
Expenses
Total income (loss) from discontinued operations												(17.2)	577.5	523.4
OneBeacon
Expenses
Net income from discontinued operations, net of tax											$ 20.5			108.6
Gain (loss) on sale of other discontinued operations, net of tax	$ 554.6
OneBeacon \| Discontinued Operations
Revenues
Earned insurance premiums													807.6	1,100.6
Net investment income													39.7	50.6
Other revenue													7.7	5.5
Net realized and unrealized investment gains													38.8	37.7
Revenues (loss) (including realized and unrealized gains and losses)													893.8	1,194.4
Expenses
Loss and loss adjustment expenses													546.0	656.0
Insurance and reinsurance acquisition expenses													145.6	206.0
Other underwriting expenses													156.2	209.0
General and administrative expenses													21.2	14.2
Interest expense on debt													10.0	13.1
Total expenses													879.0	1,098.3
Net income from discontinued operations													14.8	96.1
Income tax benefit													5.7	12.5
Net income from discontinued operations, net of tax													20.5	108.6
Gain (loss) on sale of other discontinued operations, net of tax												0.0	554.5	0.0
Total income (loss) from discontinued operations													575.0	108.6
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax													0.3	1.0
Recognition of foreign currency translation from sale of Sirius Group, net of tax													2.9	0.0
Comprehensive income from discontinued operations													578.2	109.6
Sale of Sirius Group
Expenses
Recognition of foreign currency translation from sale of Sirius Group, net of tax														113.3
Sale of Sirius Group \| Discontinued Operations
Expenses
Net income from discontinued operations, net of tax														(4.3)
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax														32.0
Sirius Group
Revenues
Net realized and unrealized investment gains														3.7
Expenses
Gain (loss) on sale of other discontinued operations, net of tax										$ (4.0)				363.2
Recognition of foreign currency translation from sale of Sirius Group, net of tax														113.3
Sirius Group \| Discontinued Operations
Revenues
Earned insurance premiums													0.0	240.1
Net investment income													0.0	14.4
Other revenue													0.0	0.6
Net realized and unrealized investment gains													0.0	(1.5)
Revenues (loss) (including realized and unrealized gains and losses)													0.0	253.6
Expenses
Loss and loss adjustment expenses													0.0	154.9
Insurance and reinsurance acquisition expenses													0.0	59.0
Other underwriting expenses													0.0	30.9
General and administrative expenses													0.0	10.4
Interest expense on debt														7.9
Total expenses													0.0	263.1
Net income from discontinued operations													0.0	(9.5)
Income tax benefit													0.0	3.1
Net income from discontinued operations, net of tax													0.0	(6.4)
Gain (loss) on sale of other discontinued operations, net of tax												(17.2)	(0.7)	363.2
Total income (loss) from discontinued operations													(0.7)	356.8
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax													0.0	32.0
Recognition of foreign currency translation from sale of Sirius Group, net of tax													0.0	113.3
Comprehensive income from discontinued operations													(0.7)	502.1
Tranzact
Expenses
Income tax benefit														(30.2)
Gain (loss) on sale of other discontinued operations, net of tax													3.2	51.9
Tranzact \| Discontinued Operations
Revenues
Earned insurance premiums													0.0	0.0
Net investment income													0.0	0.0
Other revenue													0.0	119.6
Net realized and unrealized investment gains													0.0	0.0
Revenues (loss) (including realized and unrealized gains and losses)													0.0	119.6
Expenses
Loss and loss adjustment expenses													0.0	0.0
Insurance and reinsurance acquisition expenses													0.0	0.0
Other underwriting expenses													0.0	0.0
General and administrative expenses													0.0	116.7
Interest expense on debt														3.2
Total expenses													0.0	119.9
Net income from discontinued operations													0.0	(0.3)
Income tax benefit													0.0	6.4
Net income from discontinued operations, net of tax													0.0	6.1
Gain (loss) on sale of other discontinued operations, net of tax												$ 0.0	3.2	51.9
Total income (loss) from discontinued operations													3.2	58.0
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax													0.0	0.0
Recognition of foreign currency translation from sale of Sirius Group, net of tax													0.0	0.0
Comprehensive income from discontinued operations													3.2	58.0
Total \| Discontinued Operations
Revenues
Earned insurance premiums													807.6	1,340.7
Net investment income													39.7	65.0
Other revenue													7.7	125.7
Net realized and unrealized investment gains													38.8	36.2
Revenues (loss) (including realized and unrealized gains and losses)													893.8	1,567.6
Expenses
Loss and loss adjustment expenses													546.0	810.9
Insurance and reinsurance acquisition expenses													145.6	265.0
Other underwriting expenses													156.2	239.9
General and administrative expenses													21.2	141.3
Interest expense on debt													10.0	24.2
Total expenses													879.0	1,481.3
Net income from discontinued operations													14.8	86.3
Income tax benefit													5.7	22.0
Net income from discontinued operations, net of tax													20.5	108.3
Gain (loss) on sale of other discontinued operations, net of tax													557.0	415.1
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax													0.3	33.0
Recognition of foreign currency translation from sale of Sirius Group, net of tax													2.9	113.3
Comprehensive income from discontinued operations													$ 580.7	$ 669.7

Held for Sale and Discontinued Operations (Cash Flows Table) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net cash provided from discontinued operations (Note 19)	$ 0.0	$ 157.0	$ 23.6
Net cash provided from investing activities — discontinued operations (Note 19)	0.0	3.0	241.4
Net cash used for financing activities — discontinued operations (Note 19)	0.0	(61.9)	(93.8)
Net change in cash during the period		98.1	171.2
Cash balance at beginning of year	97.1	80.2	77.8
Net change in cash during the period - continuing operations	13.2	16.0	2.1
Cash balance at end of year	110.3	97.1	80.2
Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash balance at beginning of year	$ 0.0	70.5	245.4
Net change in cash during the period - continuing operations		(0.9)	(0.3)
Cash sold as part of sale of consolidated subsidiaries		(167.7)	(345.8)
Cash balance at end of year		$ 0.0	$ 70.5

Held for Sale and Discontinued Operations (Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$ (137.5)	$ 40.8	$ 3.5	$ (48.0)	$ 22.5	$ 562.1	$ 13.6	$ 29.0	$ (141.2)	$ 627.2	$ 401.8
Net income (loss) attributable to White Mountains’ common shareholders									(124.0)	49.7	(121.6)
Total income (loss) from discontinued operations									(17.2)	577.5	523.4
Allocation of earnings (losses) to participating restricted common shares									1.4	(0.7)	1.5
Basic and diluted (losses) earnings per share numerators									$ (122.6)	$ 49.0	$ (120.1)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									3,382,500	4,293,800	5,014,900
Average unvested restricted common shares									(40,100)	(54,300)	(64,800.00)
Basic earnings (losses) per share denominator									3,342,400	4,239,500	4,950,100
Diluted earnings per share denominator (in thousands):
Average outstanding dilutive options to acquire common shares									3,382,500	4,293,800	5,018,100
Average unvested restricted common shares									(40,100)	(54,300)	(64,800.00)
Diluted earnings (loss) per share denominator									3,342,400.00	4,239,500	4,953,300.00
Basic (losses) earnings per share (in usd per share)	$ (43.24)	$ 12.83	$ 1.02	$ (12.82)	$ 6.00	$ 130.81	$ 2.97	$ 6.34	$ (41.76)	$ 146.06	$ 80.11
Diluted (losses) earnings per share (in usd per share)	$ (43.24)	$ 12.83	$ 1.02	$ (12.82)	$ 6.00	$ 130.81	$ 2.97	$ 6.34	$ (41.76)	$ 146.06	$ 80.06
Incremental common shares attributable to share-based payment arrangements											3,217
Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders									$ (141.2)	$ 627.2	$ 401.8
Net income (loss) attributable to White Mountains’ common shareholders									(124.0)	49.7	(121.6)
Total income (loss) from discontinued operations									(17.2)	577.5	523.4
Allocation of earnings (losses) to participating restricted common shares									0.2	(7.3)	(6.8)
Basic and diluted (losses) earnings per share numerators									$ (17.0)	$ 570.2	$ 516.6
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									3,382,500	4,293,800	5,014,900
Average unvested restricted common shares									(40,100)	(54,300)	(64,800)
Basic earnings (losses) per share denominator									3,342,400	4,239,500	4,950,100
Diluted earnings per share denominator (in thousands):
Average outstanding dilutive options to acquire common shares									3,382,500	4,293,800	5,018,100
Average unvested restricted common shares									(40,100)	(54,300)	(64,800)
Diluted earnings (loss) per share denominator									3,342,400	4,239,500	4,953,300
Basic (losses) earnings per share (in usd per share)									$ (5.09)	$ 134.50	$ 104.37
Diluted (losses) earnings per share (in usd per share)									$ (5.09)	$ 134.50	$ 104.32
Incremental common shares attributable to share-based payment arrangements											40,000

Subsequent Event (Details) - USD ($) $ in Millions	Feb. 26, 2019	Feb. 14, 2019	Feb. 13, 2019	Dec. 31, 2018	Dec. 31, 2017
MediaAlpha
Subsequent Event [Line Items]
Ownership interest (as a percent)				61.00%	64.40%
MediaAlpha \| Subsequent Event
Subsequent Event [Line Items]
Assets, fair value	$ 350
Ownership interest (as a percent)	42.00%
Equity in earnings of unconsolidated affiliates, net of tax	$ 88
Kudu \| Subsequent Event
Subsequent Event [Line Items]
Ownership interest (as a percent)		99.00%	49.50%
Equity method investment, aggregate cost		$ 50
Unfunded capital commitment		$ 167

- Definition

This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.

+ References + Details

Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 6.0	$ 198.7	$ 122.3	$ 42.1	$ 114.0	$ 87.5	$ 83.5	$ 88.8	$ 369.1	$ 373.8	$ 157.7
Expenses	150.1	154.4	134.7	108.1	108.8	79.1	85.7	92.4	547.3	366.0	305.0
Pre-tax income (loss)	(144.1)	44.3	(12.4)	(66.0)	5.2	8.4	(2.2)	(3.6)	(178.2)	7.8	(147.3)
Tax benefit (expense)	3.6	3.6	(2.5)	(0.7)	2.5	4.0	1.0	0.3	4.0	7.8	32.9
Net income (loss) from continuing operations	(140.5)	47.9	(14.9)	(66.7)	7.7	12.4	(1.2)	(3.3)	(174.2)	15.6	(114.4)
Net income from discontinued operations, net of tax	0.0	(17.3)	0.0	0.1	4.3	539.1	2.8	31.3	0.0	20.5	108.3
Non-controlling interest in consolidated subsidiaries	3.0	10.2	18.4	18.6	10.5	10.6	12.0	1.0	50.2	34.1	(7.2)
Net income attributable to White Mountains’s common shareholders	$ (137.5)	$ 40.8	$ 3.5	$ (48.0)	$ 22.5	$ 562.1	$ 13.6	$ 29.0	$ (141.2)	$ 627.2	$ 401.8
Basic earnings per share
Continuing operations	$ (43.24)	$ 18.27	$ 1.02	$ (12.85)	$ 4.85	$ 5.36	$ 2.36	$ (0.52)	$ (36.67)	$ 11.56	$ (24.26)
Discontinued operations	0.00	(5.44)	0.00	0.03	1.15	125.45	0.61	6.86	(5.09)	134.50	104.37
Total consolidated operations	(43.24)	12.83	1.02	(12.82)	6.00	130.81	2.97	6.34	(41.76)	146.06	80.11
Diluted earnings per share
Continuing operations	(43.24)	18.27	1.02	(12.85)	4.85	5.36	2.36	(0.52)	(36.67)	11.56	(24.26)
Discontinued operations	0.00	(5.44)	0.00	0.03	1.15	125.45	0.61	6.86	(5.09)	134.50	104.32
Total consolidated operations	$ (43.24)	$ 12.83	$ 1.02	$ (12.82)	$ 6.00	$ 130.81	$ 2.97	$ 6.34	$ (41.76)	$ 146.06	$ 80.06

SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Millions	Dec. 31, 2018 USD ($)
Summary of investments other than investments in related parties
Cost	$ 2,537.3
Carrying Value	2,542.9
Fair Value	2,542.9
Fixed maturity investments
Summary of investments other than investments in related parties
Cost	1,088.1
Carrying Value	1,077.5
Fair Value	1,077.5
U.S. Government and agency obligations
Summary of investments other than investments in related parties
Cost	154.0
Carrying Value	153.2
Fair Value	153.2
Debt securities issued by corporations
Summary of investments other than investments in related parties
Cost	519.0
Carrying Value	510.5
Fair Value	510.5
Mortgage and asset-backed securities
Summary of investments other than investments in related parties
Cost	136.1
Carrying Value	133.5
Fair Value	133.5
Municipal obligations
Summary of investments other than investments in related parties
Cost	279.0
Carrying Value	280.3
Fair Value	280.3
Short-term investments
Summary of investments other than investments in related parties
Cost	214.2
Carrying Value	214.2
Fair Value	214.2
Exchange traded funds
Summary of investments other than investments in related parties
Cost	681.8
Carrying Value	675.3
Fair Value	675.3
Common equity securities
Summary of investments other than investments in related parties
Cost	904.7
Carrying Value	925.6
Fair Value	925.6
Banks, trust and insurance companies
Summary of investments other than investments in related parties
Cost	11.8
Carrying Value	13.5
Fair Value	13.5
Industrial, miscellaneous and other
Summary of investments other than investments in related parties
Cost	211.1
Carrying Value	236.8
Fair Value	236.8
Other long-term investments
Summary of investments other than investments in related parties
Cost	330.3
Carrying Value	325.6
Fair Value	$ 325.6

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Total assets	$ 3,362.6	$ 3,659.2
Liabilities
Debt	192.7	23.8
Payable to subsidiary	4.0	11.6	$ 0.2
Total liabilities	644.4	298.4
White Mountains’s common shareholders’ equity	2,718.2	3,360.8
Non-controlling interests	(124.9)	(131.7)
Total liabilities and equity	3,362.6	3,659.2
Fair value investments	2,328.7	3,208.3	$ 2,547.3
White Mountains Insurance Group Ltd.
Assets
Cash	0.7	14.9
Fixed maturity investments, at fair value	0.0	869.6
Common equity securities, at fair value	335.6	641.8
Short-term investments, at amortized cost	28.0	57.2
Other long-term investments	0.0	(3.7)
Other assets	1.8	30.9
Investments in consolidated subsidiaries	2,533.2	1,914.8
Total assets	2,899.3	3,525.5
Liabilities
Payable to subsidiary	15.8	11.8
Other liabilities	25.9	21.2
Total liabilities	41.7	33.0
White Mountains’s common shareholders’ equity	2,843.1	3,492.5
Non-controlling interests	14.5	0.0
Total liabilities and equity	2,899.3	3,525.5
Sirius Group
Liabilities
Other liabilities	$ 17.3
Barclays \| Forward Contracts \| A
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Derivative, fair value, net		$ (3.7)

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - Statement of Operations and Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Payments for (Proceeds from) Short-term Investments									$ 39.0	$ 1.7	$ 27.2
Revenues (loss) (including realized and unrealized gains and losses)	$ 6.0	$ 198.7	$ 122.3	$ 42.1	$ 114.0	$ 87.5	$ 83.5	$ 88.8	369.1	373.8	157.7
Expenses	150.1	154.4	134.7	108.1	108.8	79.1	85.7	92.4	547.3	366.0	305.0
Pre-tax loss	(144.1)	44.3	(12.4)	(66.0)	5.2	8.4	(2.2)	(3.6)	(178.2)	7.8	(147.3)
Income tax (expense) benefit	3.6	3.6	(2.5)	(0.7)	2.5	4.0	1.0	0.3	4.0	7.8	32.9
Net income (loss) from continuing operations	(140.5)	47.9	(14.9)	(66.7)	7.7	12.4	(1.2)	(3.3)	(174.2)	15.6	(114.4)
Net income from discontinued operations, net of tax	0.0	(17.3)	0.0	0.1	4.3	539.1	2.8	31.3	0.0	20.5	108.3
Net loss attributable to non-controlling interests	3.0	10.2	18.4	18.6	10.5	10.6	12.0	1.0	50.2	34.1	(7.2)
Net income attributable to White Mountains’s common shareholders	(137.5)	40.8	3.5	(48.0)	22.5	562.1	13.6	29.0	(141.2)	627.2	401.8
Other comprehensive (loss) income, net of tax									(4.8)	0.3	(0.7)
Comprehensive (loss) income attributable to White Mountains’s common shareholders									(145.7)	630.5	547.1
Computation of net income (loss) available to common shareholders:
Net income attributable to White Mountains’s common shareholders	$ (137.5)	$ 40.8	$ 3.5	$ (48.0)	$ 22.5	$ 562.1	$ 13.6	$ 29.0	(141.2)	627.2	401.8
Comprehensive income (loss)									(146.0)	630.7	547.4
Total income (loss) from discontinued operations									(17.2)	577.5	523.4
Payments for (Proceeds from) Other Investing Activities									4.2	14.7	(4.8)
Net Cash Provided by (Used in) Investing Activities, Continuing Operations									459.2	715.5	1,035.4
Cash Provided by (Used in) Investing Activities, Discontinued Operations									0.0	3.0	241.4
Net Cash Provided by (Used in) Investing Activities									459.2	718.5	1,276.8
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Payments for (Proceeds from) Short-term Investments									(134.0)	24.7	(10.9)
Revenues (loss) (including realized and unrealized gains and losses)									(47.7)	27.3	(1.0)
Expenses									45.9	99.7	68.2
Pre-tax loss									(93.6)	(72.4)	(69.2)
Income tax (expense) benefit									(2.5)	(1.4)	(0.5)
Net income (loss) from continuing operations									(96.1)	(73.8)	(69.7)
Net income from discontinued operations, net of tax									(17.2)	0.0	0.0
Equity in earnings (losses) from consolidated and unconsolidated affiliates									(27.4)	701.0	471.5
Net loss attributable to non-controlling interests									(0.5)	0.0	0.0
Net income attributable to White Mountains’s common shareholders									(141.2)	627.2	401.8
Other comprehensive (loss) income, net of tax									(4.5)	3.3	145.3
Comprehensive (loss) income attributable to White Mountains’s common shareholders									(145.7)	630.5	547.1
Computation of net income (loss) available to common shareholders:
Net income attributable to White Mountains’s common shareholders									(141.2)	627.2	401.8
Payments to Acquire Investments									321.2	474.7	0.0
Proceeds from Sale and Maturity of Debt and Equity Securities, FV-NI, Held-for-investment									967.6	367.1	0.0
Issuance Of Debt To From Subsidiaries									(55.2)	382.0	992.0
Repayment of debt from subsidiaries									(31.0)	0.0	5.0
Payments to acquire interest in subsidiary									258.2		0.0
Net Cash Provided by (Used in) Investing Activities									498.0	(450.3)	997.9
Guilford Holdings, Inc. \| White Mountains Insurance Group Ltd.
Computation of net income (loss) available to common shareholders:
Payments to acquire interest in subsidiary										700.0
Discontinued Operations
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income attributable to White Mountains’s common shareholders									(141.2)	627.2	401.8
Computation of net income (loss) available to common shareholders:
Net income attributable to White Mountains’s common shareholders									(141.2)	627.2	401.8
Total income (loss) from discontinued operations									(17.2)	577.5	523.4
Discontinued Operations \| Sirius Group
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (loss) (including realized and unrealized gains and losses)										0.0	253.6
Expenses										0.0	263.1
Net income from discontinued operations, net of tax										0.0	(6.4)
Computation of net income (loss) available to common shareholders:
Comprehensive income (loss)										(0.7)	502.1
Total income (loss) from discontinued operations										(0.7)	356.8
Discontinued Operations \| OneBeacon, Sirius Group, and Tranzact
Computation of net income (loss) available to common shareholders:
Total income (loss) from discontinued operations										$ 577.5	$ 523.4
Swedish Tax Authority \| Sirius Group \| Sirius Group
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income from discontinued operations, net of tax									$ (17.3)

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income attributable to White Mountains’s common shareholders	$ (137.5)	$ 40.8	$ 3.5	$ (48.0)	$ 22.5	$ 562.1	$ 13.6	$ 29.0	$ (141.2)	$ 627.2	$ 401.8
Charges (credits) to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment losses (gains)									108.3	(133.3)	27.4
Net income from discontinued operations, net of tax	0.0	$ (17.3)	$ 0.0	0.1	4.3	$ 539.1	$ 2.8	31.3	0.0	20.5	108.3
Net change in other assets and liabilities, net									(27.1)	(20.8)	(146.1)
Net cash (used for) provided from operations									(31.1)	94.6	(155.1)
Cash flows from investing activities:
Net decrease in short-term investments									(39.0)	(1.7)	(27.2)
Net cash provided from investing activities									459.2	718.5	1,276.8
Cash flows from financing activities:
Draw down of debt and revolving line of credit									84.1	376.0	352.5
Repayment of debt and revolving line of credit									(15.4)	(365.0)	(404.6)
Proceeds from issuances of common shares									0.0	0.0	3.7
Repurchases and retirement of common shares									(511.9)	(714.6)	(881.3)
Dividends paid on common shares									(3.8)	(4.6)	(5.4)
Net cash used for financing activities									(414.3)	(699.0)	(948.4)
Net change in cash during the period - continuing operations									13.2	16.0	2.1
Cash balance at beginning of year				97.1				80.2	97.1	80.2	77.8
Cash balance at end of year	110.3				97.1				110.3	97.1	80.2
Interest paid									(8.8)	(1.4)	(2.1)
White Mountains’s common shareholders’ equity	2,718.2				3,360.8				2,718.2	3,360.8
Amortization of restricted share and option awards									13.0	14.8	18.5
Payable to subsidiary	4.0				11.6				4.0	11.6	0.2
Lone Tree Holdings Ltd.
Cash flows from financing activities:
White Mountains’s common shareholders’ equity					2,810.4					2,810.4
Other liabilities					14.1					14.1
Investments in consolidated subsidiaries					964.4					964.4
Short-term investments, at amortized cost					13.0					13.0
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income attributable to White Mountains’s common shareholders									(141.2)	627.2	401.8
Charges (credits) to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment losses (gains)									57.8	(18.5)	1.1
Undistributed earnings from subsidiaries									27.4	(701.0)	(471.5)
Net income from discontinued operations, net of tax									(17.2)	0.0	0.0
Other non-cash reconciling items, primarily amortization of restricted share and option awards (3)									34.6	31.1	17.9
Accumulated earnings distributed from subsidiary in cash									0.0	1,256.7	0.0
Net change in other assets and liabilities, net									16.7	(4.9)	(5.6)
Net cash (used for) provided from operations									12.5	1,190.6	(56.3)
Cash flows from investing activities:
Net decrease in short-term investments									134.0	(24.7)	10.9
Payments to Acquire Investments									(321.2)	(474.7)	0.0
Proceeds from Sale and Maturity of Debt and Equity Securities, FV-NI, Held-for-investment									967.6	367.1	0.0
Repayment of debt (to) from subsidiaries									31.0	0.0	(5.0)
Payments to acquire interest in subsidiary									258.2		0.0
Net cash provided from investing activities									498.0	(450.3)	997.9
Cash flows from financing activities:
Draw down of debt and revolving line of credit									0.0	350.0	350.0
Repayment of debt and revolving line of credit									0.0	(350.0)	(400.0)
Proceeds from issuances of common shares									0.0	0.0	3.7
Repurchases and retirement of common shares									(510.9)	(714.6)	(881.3)
Dividends paid on common shares									(3.8)	(4.6)	(5.4)
Payments of restricted shares withholding taxes									(10.0)	(9.3)	(5.8)
Net cash used for financing activities									(524.7)	(728.5)	(938.8)
Net change in cash during the period - continuing operations									(14.2)	11.8	2.8
Cash balance at beginning of year				$ 14.9				$ 3.1	14.9	3.1	0.3
Cash balance at end of year	0.7				14.9				0.7	14.9	3.1
Interest paid									0.0	(0.6)	(1.2)
White Mountains’s common shareholders’ equity	2,843.1				3,492.5				2,843.1	3,492.5
Other liabilities	25.9				21.2				25.9	21.2
Investments in consolidated subsidiaries	2,533.2				1,914.8				2,533.2	1,914.8
Short-term investments, at amortized cost	28.0				57.2				28.0	57.2
Payable to subsidiary	15.8				11.8				15.8	11.8
Transfer from investments										1,238.9
Sirius Group
Cash flows from financing activities:
Other liabilities	$ 17.3								17.3
Lone Tree Holdings Ltd. \| White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Net change in due to affiliates										1,256.7
Proceeds from divestiture of businesses										1,037.6
Bridge Holdings, Ltd. \| White Mountains Insurance Group Ltd.
Cash flows from investing activities:
Payments to acquire interest in subsidiary									$ (255.3)
Cash flows from financing activities:
Noncash Investing and Financing Activities Related Text									22.7
White Mountains Investment, Ltd. \| White Mountains Insurance Group Ltd.
Cash flows from investing activities:
Payments to acquire interest in subsidiary									$ (2.9)
Guilford Holdings, Inc. \| White Mountains Insurance Group Ltd.
Cash flows from investing activities:
Payments to acquire interest in subsidiary										700.0
Intercompany balances \| Lone Tree Holdings Ltd.
Cash flows from financing activities:
Other liabilities					$ 1,863.1					$ 1,863.1
Non-cash reconciling item \| White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Noncash Investing and Financing Activities Related Text										94.2
Non-cash reconciling item \| Guilford Holdings, Inc.
Cash flows from financing activities:
Noncash Investing and Financing Activities Related Text									349.5
Noncash Total \| White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Transfer to investments									$ (603.9)
Transfer from investments									1,065.4
Fixed maturity investments \| White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Transfer to investments									(170.5)
Transfer from investments									373.4		80.0
Fixed maturity investments \| Guilford Holdings, Inc.
Cash flows from financing activities:
Transfer from investments									170.4
Common equity securities \| White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Transfer to investments									(148.8)
Transfer from investments									490.1
Other long-term investments \| White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Transfer to investments									(22.7)
Transfer from investments									22.7
Short-term investments \| White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Transfer to investments									284.6
Transfer from investments									179.2
Short-term investments \| Guilford Holdings, Inc.
Cash flows from financing activities:
Transfer from investments									$ 179.2
Bridge Holdings, Ltd. \| White Mountains Investment, Ltd.
Cash flows from financing activities:
Noncash Investing and Financing Activities Related Text									1.0
Bridge Holdings, Ltd. \| Guilford Holdings, Inc.
Cash flows from financing activities:
Noncash Investing and Financing Activities Related Text									350.0
Sirius Group
Charges (credits) to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment losses (gains)											$ (3.7)
Swedish Tax Authority \| Sirius Group \| Sirius Group
Charges (credits) to reconcile net income to net cash provided from (used for) operations:
Net income from discontinued operations, net of tax									$ (17.3)

SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SUPPLEMENTARY INSURANCE INFORMATION
Net investment income	$ 59.0	$ 56.0	$ 32.1
HG Global-BAM
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	19.0	14.8	10.6
Future policy benefits, losses, claims and loss expenses	0.0	0.0	0.0
Unearned premiums	176.0	136.8	82.9
Other policy claims and benefits payable	0.0	0.0	0.0
Premiums earned	13.9	9.4	5.9
Net investment income	16.7	12.3	9.0
Benefits, claims, losses, and settlement expenses	0.0	0.0	0.0
Amortization of deferred policy acquisition costs	5.3	4.0	3.4
Other operating expenses	0.4	0.4	0.4
Premiums written	52.9	63.2	38.6
Net investment income	16.7	12.3	9.0
Other Operations
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	0.0	0.0	0.0
Future policy benefits, losses, claims and loss expenses	0.0	0.0	0.0
Unearned premiums	0.0	0.0	0.0
Other policy claims and benefits payable	0.0	0.0	0.0
Premiums earned	0.0	1.0	7.5
Net investment income	0.0	0.0	0.2
Benefits, claims, losses, and settlement expenses	0.0	1.1	8.0
Amortization of deferred policy acquisition costs	0.0	0.1	2.2
Other operating expenses	0.0	0.0	0.1
Premiums written	0.0	0.9	6.5
Non-insurance [Member] \| Operating Segments
SUPPLEMENTARY INSURANCE INFORMATION
Net investment income	$ 0.0	$ 43.7	$ 22.9

SCHEDULE IV REINSURANCE (Details) - USD ($) $ in Millions	12 Months Ended
SCHEDULE IV REINSURANCE (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Premiums earned
Net earned premiums	$ 13.9	$ 10.4	$ 13.4
HG Global/BAM
Premiums earned
Gross amount	13.6	9.4	5.9
Ceded to other companies	0.0	0.0	0.0
Assumed from other companies	0.3	0.0	0.0
Net earned premiums	$ 13.9	$ 9.4	$ 5.9
Percentage of amount assumed to net	2.20%	0.00%	0.00%
Other Segments
Premiums earned
Gross amount	$ 0.0	$ 1.0	$ 15.2
Ceded to other companies	0.0	0.0	(7.7)
Assumed from other companies	0.0	0.0	0.0
Net earned premiums	$ 0.0	$ 1.0	$ 7.5
Percentage of amount assumed to net	0.00%	0.00%	0.00%

Investment Securities (Fair Value Measurement by Level) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) Investment	Dec. 31, 2017 USD ($) Investment	Dec. 31, 2016 USD ($)
Investment securities
Percentage of investments recorded at fair value	87.00%	94.00%
Fair value investments	$ 2,328.7	$ 3,208.3	$ 2,547.3
U.S. Government and agency obligations
Investment securities
Cost or amortized cost	154.0	297.8
Debt securities issued by corporations
Investment securities
Cost or amortized cost	519.0	867.6
Fair value investments	510.5	880.9
Mortgage and asset-backed securities
Investment securities
Cost or amortized cost	136.1	697.2
Fair value investments	133.5	694.7
Municipal obligations
Investment securities
Cost or amortized cost	$ 279.0	252.0
Foreign government, agency and provincial obligations
Investment securities
Cost or amortized cost		$ 2.6
Common Stock
Investment securities
Number of investments \| Investment	2	2
Cost or amortized cost	$ 904.7	$ 739.7
Level 1
Investment securities
Fair value investments	842.6	890.4	279.5
Level 2
Investment securities
Fair value investments	1,160.5	2,105.4	2,093.8
Level 2 \| Mortgage and asset-backed securities
Investment securities
Fair value investments	133.5	694.7
Level 3 Inputs \| Mortgage and asset-backed securities
Investment securities
Fair value investments	0.0	$ 0.0
Level 3 Inputs \| Fixed maturity investments
Investment securities
Number of investments \| Investment		3
Fair value investments		$ 0.0	0.0
Level 3 Inputs \| Other long-term investments
Investment securities
Fair value investments	138.7	77.2	91.4
Level 3 Inputs \| Hedge Funds and Private Equity Funds
Investment securities
Fair value investments			$ 82.6
Fair Value Measured at Net Asset Value Per Share \| Hedge Funds and Private Equity Funds
Investment securities
Fair value investments	186.9	135.3
Fair value measured on a recurring basis
Investment securities
Fair value investments	2,542.9	3,384.4
Fair value measured on a recurring basis \| U.S. Government and agency obligations
Investment securities
Fair value investments	153.2	296.5
Fair value measured on a recurring basis \| Debt securities issued by corporations
Investment securities
Fair value investments	510.5	880.9
Fair value measured on a recurring basis \| Debt securities issued by corporations \| Consumer
Investment securities
Fair value investments	68.5	185.1
Fair value measured on a recurring basis \| Debt securities issued by corporations \| Communications
Investment securities
Fair value investments	31.8	127.8
Fair value measured on a recurring basis \| Debt securities issued by corporations \| Financials
Investment securities
Fair value investments	143.4	114.8
Fair value measured on a recurring basis \| Debt securities issued by corporations \| Utilities
Investment securities
Fair value investments	19.8	108.9
Fair value measured on a recurring basis \| Debt securities issued by corporations \| Materials
Investment securities
Fair value investments	26.3	95.5
Fair value measured on a recurring basis \| Debt securities issued by corporations \| Healthcare
Investment securities
Fair value investments	55.0	94.3
Fair value measured on a recurring basis \| Debt securities issued by corporations \| Technology
Investment securities
Fair value investments	60.5	80.5
Fair value measured on a recurring basis \| Debt securities issued by corporations \| Energy
Investment securities
Fair value investments	57.6	48.1
Fair value measured on a recurring basis \| Debt securities issued by corporations \| Industrial
Investment securities
Fair value investments	47.6	25.9
Fair value measured on a recurring basis \| Mortgage and asset-backed securities
Investment securities
Fair value investments	133.5	694.7
Fair value measured on a recurring basis \| Municipal obligations
Investment securities
Fair value investments	280.3	254.9
Fair value measured on a recurring basis \| Foreign government, agency and provincial obligations
Investment securities
Fair value investments		2.7
Fair value measured on a recurring basis \| Fixed maturity investments
Investment securities
Fair value investments	1,077.5	2,129.7
Fair value measured on a recurring basis \| Common equity securities \| Consumer
Investment securities
Fair value investments	6.2	10.7
Fair value measured on a recurring basis \| Common equity securities \| Communications
Investment securities
Fair value investments	12.7	10.9
Fair value measured on a recurring basis \| Common equity securities \| Financials
Investment securities
Fair value investments	13.5	16.3
Fair value measured on a recurring basis \| Common equity securities \| Materials
Investment securities
Fair value investments	3.1
Fair value measured on a recurring basis \| Common equity securities \| Healthcare
Investment securities
Fair value investments	14.0	17.1
Fair value measured on a recurring basis \| Common equity securities \| Technology
Investment securities
Fair value investments	7.4	15.1
Fair value measured on a recurring basis \| Common equity securities \| Energy
Investment securities
Fair value investments	4.1	3.8
Fair value measured on a recurring basis \| Common equity securities \| Industrial
Investment securities
Fair value investments	11.4	11.9
Fair value measured on a recurring basis \| Common equity securities \| Other
Investment securities
Fair value investments	177.9	210.6
Fair value measured on a recurring basis \| Common equity securities \| Exchange traded funds
Investment securities
Fair value investments	675.3	569.7
Fair value measured on a recurring basis \| Common equity securities \| Common equity securities
Investment securities
Fair value investments	925.6	866.1
Fair value measured on a recurring basis \| Other long-term investments
Investment securities
Fair value investments	138.7	77.2
Fair value measured on a recurring basis \| Level 1
Investment securities
Fair value investments	1,047.0	1,041.4
Fair value measured on a recurring basis \| Level 1 \| U.S. Government and agency obligations
Investment securities
Fair value investments	153.2	296.5
Fair value measured on a recurring basis \| Level 1 \| Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 1 \| Debt securities issued by corporations \| Consumer
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 1 \| Debt securities issued by corporations \| Communications
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 1 \| Debt securities issued by corporations \| Financials
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 1 \| Debt securities issued by corporations \| Utilities
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 1 \| Debt securities issued by corporations \| Materials
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 1 \| Debt securities issued by corporations \| Healthcare
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 1 \| Debt securities issued by corporations \| Technology
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 1 \| Debt securities issued by corporations \| Energy
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 1 \| Debt securities issued by corporations \| Industrial
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 1 \| Mortgage and asset-backed securities
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 1 \| Municipal obligations
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 1 \| Foreign government, agency and provincial obligations
Investment securities
Fair value investments		0.0
Fair value measured on a recurring basis \| Level 1 \| Fixed maturity investments
Investment securities
Fair value investments	153.2	296.5
Fair value measured on a recurring basis \| Level 1 \| Short-term investments
Investment securities
Fair value investments	204.4	151.0
Fair value measured on a recurring basis \| Level 1 \| Common equity securities \| Consumer
Investment securities
Fair value investments	6.2	10.7
Fair value measured on a recurring basis \| Level 1 \| Common equity securities \| Communications
Investment securities
Fair value investments	12.7	10.9
Fair value measured on a recurring basis \| Level 1 \| Common equity securities \| Financials
Investment securities
Fair value investments	13.5	16.3
Fair value measured on a recurring basis \| Level 1 \| Common equity securities \| Materials
Investment securities
Fair value investments	3.1
Fair value measured on a recurring basis \| Level 1 \| Common equity securities \| Healthcare
Investment securities
Fair value investments	14.0	17.1
Fair value measured on a recurring basis \| Level 1 \| Common equity securities \| Technology
Investment securities
Fair value investments	7.4	15.1
Fair value measured on a recurring basis \| Level 1 \| Common equity securities \| Energy
Investment securities
Fair value investments	4.1	3.8
Fair value measured on a recurring basis \| Level 1 \| Common equity securities \| Industrial
Investment securities
Fair value investments	11.4	11.9
Fair value measured on a recurring basis \| Level 1 \| Common equity securities \| Other
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 1 \| Common equity securities \| Exchange traded funds
Investment securities
Fair value investments	617.0	508.1
Fair value measured on a recurring basis \| Level 1 \| Common equity securities \| Common equity securities
Investment securities
Fair value investments	689.4	593.9
Fair value measured on a recurring basis \| Level 1 \| Other long-term investments
Investment securities
Fair value investments	0.0	3,208.3
Fair value measured on a recurring basis \| Level 2
Investment securities
Fair value investments	1,170.3	2,130.5
Fair value measured on a recurring basis \| Level 2 \| U.S. Government and agency obligations
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 2 \| Debt securities issued by corporations
Investment securities
Fair value investments	510.5	880.9
Fair value measured on a recurring basis \| Level 2 \| Debt securities issued by corporations \| Consumer
Investment securities
Fair value investments	68.5	185.1
Fair value measured on a recurring basis \| Level 2 \| Debt securities issued by corporations \| Communications
Investment securities
Fair value investments	31.8	127.8
Fair value measured on a recurring basis \| Level 2 \| Debt securities issued by corporations \| Financials
Investment securities
Fair value investments	143.4	114.8
Fair value measured on a recurring basis \| Level 2 \| Debt securities issued by corporations \| Utilities
Investment securities
Fair value investments	19.8	108.9
Fair value measured on a recurring basis \| Level 2 \| Debt securities issued by corporations \| Materials
Investment securities
Fair value investments	26.3	95.5
Fair value measured on a recurring basis \| Level 2 \| Debt securities issued by corporations \| Healthcare
Investment securities
Fair value investments	55.0	94.3
Fair value measured on a recurring basis \| Level 2 \| Debt securities issued by corporations \| Technology
Investment securities
Fair value investments	60.5	80.5
Fair value measured on a recurring basis \| Level 2 \| Debt securities issued by corporations \| Energy
Investment securities
Fair value investments	57.6	48.1
Fair value measured on a recurring basis \| Level 2 \| Debt securities issued by corporations \| Industrial
Investment securities
Fair value investments	47.6	25.9
Fair value measured on a recurring basis \| Level 2 \| Mortgage and asset-backed securities
Investment securities
Fair value investments	133.5	694.7
Fair value measured on a recurring basis \| Level 2 \| Municipal obligations
Investment securities
Fair value investments	280.3	254.9
Fair value measured on a recurring basis \| Level 2 \| Foreign government, agency and provincial obligations
Investment securities
Fair value investments		2.7
Fair value measured on a recurring basis \| Level 2 \| Fixed maturity investments
Investment securities
Fair value investments	924.3	1,833.2
Fair value measured on a recurring basis \| Level 2 \| Short-term investments
Investment securities
Fair value investments	9.8	25.1
Fair value measured on a recurring basis \| Level 2 \| Common equity securities \| Consumer
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 2 \| Common equity securities \| Communications
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 2 \| Common equity securities \| Financials
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 2 \| Common equity securities \| Materials
Investment securities
Fair value investments	0.0
Fair value measured on a recurring basis \| Level 2 \| Common equity securities \| Healthcare
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 2 \| Common equity securities \| Technology
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 2 \| Common equity securities \| Energy
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 2 \| Common equity securities \| Industrial
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 2 \| Common equity securities \| Other
Investment securities
Fair value investments	177.9	210.6
Fair value measured on a recurring basis \| Level 2 \| Common equity securities \| Exchange traded funds
Investment securities
Fair value investments	58.3	61.6
Fair value measured on a recurring basis \| Level 2 \| Common equity securities \| Common equity securities
Investment securities
Fair value investments	236.2	272.2
Fair value measured on a recurring basis \| Level 2 \| Other long-term investments
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs
Investment securities
Fair value investments	138.7	77.2
Fair value measured on a recurring basis \| Level 3 Inputs \| U.S. Government and agency obligations
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Debt securities issued by corporations \| Consumer
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Debt securities issued by corporations \| Communications
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Debt securities issued by corporations \| Financials
Investment securities
Fair value investments		0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Debt securities issued by corporations \| Utilities
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Debt securities issued by corporations \| Materials
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Debt securities issued by corporations \| Healthcare
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Debt securities issued by corporations \| Technology
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Debt securities issued by corporations \| Energy
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Debt securities issued by corporations \| Industrial
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Mortgage and asset-backed securities
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Municipal obligations
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Foreign government, agency and provincial obligations
Investment securities
Fair value investments		0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Fixed maturity investments
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Short-term investments
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Common equity securities \| Consumer
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Common equity securities \| Communications
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Common equity securities \| Financials
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Common equity securities \| Materials
Investment securities
Fair value investments	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Common equity securities \| Healthcare
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Common equity securities \| Technology
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Common equity securities \| Energy
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Common equity securities \| Industrial
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Common equity securities \| Other
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Common equity securities \| Exchange traded funds
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Common equity securities \| Common equity securities
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis \| Level 3 Inputs \| Other long-term investments
Investment securities
Fair value investments	$ 138.7	77.2
Fair value measured on a recurring basis \| Fair Value Measured at Net Asset Value Per Share \| Other long-term investments
Investment securities
Fair value investments		135.3
Foreign Exchange Forward \| Fair value measured on a recurring basis \| Level 3 Inputs \| Other long-term investments
Investment securities
Derivative, fair value, net		$ (3.7)