White Mountains Insurance (WTM) 10-Q 5 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - $ / shares	9 Months Ended
	Sep. 30, 2018	Nov. 02, 2018	Dec. 31, 2017
Document and Entity Information [Abstract]
Entity Registrant Name	WHITE MOUNTAINS INSURANCE GROUP LTD
Entity Central Index Key	776,867
Document Type	10-Q
Document Period End Date	Sep. 30, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		3,180,540
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Entity Emerging Growth Company	false
Entity Small Business	false
Common shares, par value per share (in dollars per share)	$ 1		$ 1

CONSOLIDATED BALANCE SHEETS (St

CONSOLIDATED BALANCE SHEETS (Statement) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Assets
Goodwill and other intangible assets	$ 485.2	$ 62.1
Total assets	3,422.8	3,659.2
Liabilities
Debt	164.3	23.8
Total liabilities	583.2	298.4
White Mountains’s common shareholders’ equity
White Mountains’ common shares	3.2	3.8
Paid-in surplus	576.3	666.8
Retained earnings	2,407.3	2,823.2
Accumulated other comprehensive loss, after tax:
Net unrealized foreign currency translation losses	(2.8)	(1.3)
Total White Mountains’s common shareholders’ equity	2,984	3,492.5
Non-controlling interests
Non-controlling interests	(144.4)	(131.7)
Total equity	2,839.6	3,360.8
Total liabilities and equity	3,422.8	3,659.2
HG Global-BAM
Assets
Fixed maturity investments, at fair value	677.7	623.6
Short-term investments, at fair value	35.9	69.8
Total investments	713.6	693.4
Cash	17.9	25.6
Insurance premiums receivable	6.3	4.5
Deferred acquisition costs	17.6	14.8
Accrued investment income	4.8	3.4
Accounts receivable on unsettled investment sales	0.2	0.1
Other assets	5.1	5.6
Total assets	765.5	747.4
Liabilities
Unearned insurance premiums	156.2	136.8
Accounts payable on unsettled investment purchases	6.5	0.6
Other liabilities	30	29.6
Total liabilities	192.7	167
NSM Holding Co LLC
Assets
Short-term investments, at fair value	0.9	0
Cash	60.6	0
Insurance premiums receivable	29.1	0
Goodwill and other intangible assets	431	0
Other assets	18.8	0
Total assets	540.4	0
Liabilities
Debt	149.3	0
Premiums payable	56.6	0
Contingent consideration earnout liabilities	20.1	0
Other liabilities	27.7	0
Total liabilities	253.7	0
Non-controlling interests
Non-controlling interests	14.1	0
MediaAlpha
Assets
Cash	13	9.1
Goodwill and other intangible assets	45.9	53.7
Accounts receivable from publishers and advertisers	35.9	32.4
Other assets	2.2	1.3
Total assets	97	96.5
Liabilities
Debt	15	23.8
Amounts due to publishers and advertisers	37.7	31.6
Other liabilities	3.9	4.4
Total liabilities	56.6	59.8
Non-controlling interests
Non-controlling interests	15.3	13.1
Other Segment
Assets
Fixed maturity investments, at fair value	354.2	1,506.1
Short-term investments, at fair value	276.9	106.3
Common equity securities, at fair value	1,043.5	866.1
Other long-term investments	289.7	208.8
Total investments	1,964.3	2,687.3
Cash	20.7	62.4
Accrued investment income	5.7	13.9
Accounts receivable on unsettled investment sales	3.3	20.9
Goodwill and other intangible assets	8.3	8.4
Other assets	14.3	19.1
Assets held for sale	3.3	3.3
Total assets	2,019.9	2,815.3
Liabilities
Accrued incentive compensation	40.3	60.6
Accounts payable on unsettled investment purchases	14.2	0
Other liabilities	25.7	11
Total liabilities	$ 80.2	$ 71.6

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Cash	$ 41.2	$ 0
Common shares, par value per share (in dollars per share)	$ 1	$ 1
Common shares, authorized shares	50,000,000	50,000,000
Common shares, issued shares	3,180,540	3,750,171
Common shares, outstanding shares	3,180,540	3,750,171

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues:
Earned insurance premiums	$ 3.3	$ 2.4	$ 9.7	$ 6.6
Net investment income	13.4	12.2	44.9	39.7
Net realized and unrealized investment (losses) gains	66.1	32.5	22.9	102.5
Commission revenues	112.2		278.6	103.9
Other revenue	3.7	1.6	7	7.1
Total revenues	198.7	87.5	363.1	259.8
Expenses:
Insurance acquisition expenses	1.2	0.9	3.9	3
Other underwriting expenses	0.1	0.1	0.3	0.3
Cost of sales	62.9		182	88.7
General and administrative and other expenses	68.5	41.7	174.6	156.1
Broker commission expense	10.9		17.5
Amortization of other intangible assets	7.4	2.4	12.9	7.3
Interest expense	3.4	0.9	6	1.8
Total expenses	154.4	79.1	397.2	257.2
Pre-tax income (loss) from continuing operations	44.3	8.4	(34.1)	2.6
Income tax benefit	3.6	4	0.4	5.3
Net income (loss) from continuing operations	47.9	12.4	(33.7)	7.9
Net (loss) gain from sale of discontinued operations, net of tax		554.3	(17.2)	552.7
Net (loss) income from discontinued operations, net of tax	0	(15.2)	0	20.5
Net income (loss)	30.6	551.5	(50.9)	581.1
Net loss attributable to non-controlling interests	10.2	10.6	47.2	23.6
Net income (loss) attributable to White Mountains’s common shareholders	40.8	562.1	(3.7)	604.7
HG Global-BAM | HG Global-BAM
Revenues:
Earned insurance premiums	3.3	2.4	9.7	6.6
Net investment income	4.8	3.3	12.5	8.9
Net realized and unrealized investment (losses) gains	(4.1)	0.8	(14.4)	3.2
Commission revenues	0		0	0
Other revenue	0.2	0.2	0.8	0.8
Total revenues	4.2	6.7	8.6	19.5
Expenses:
Insurance acquisition expenses	1.2	0.9	3.9	3
Other underwriting expenses	0.1	0.1	0.3	0.3
Cost of sales	0			0
General and administrative and other expenses	11.3	10.6	36.5	31.5
Broker commission expense	0		0
Amortization of other intangible assets	0	0	0
Interest expense	0	0		0
Total expenses	12.6	11.6	40.7	34.8
Pre-tax income (loss) from continuing operations	(8.4)	(4.9)	(32.1)	(15.3)
NSM Holding Co LLC | NSM Holding Co LLC
Revenues:
Earned insurance premiums	0		0
Net investment income	0		0
Net realized and unrealized investment (losses) gains	0		0
Commission revenues	36.6		59.2	0
Other revenue	3.1	0	4	0
Total revenues	39.7	0	63.2	0
Expenses:
Insurance acquisition expenses	0		0
Other underwriting expenses	0		0
Cost of sales	0		0
General and administrative and other expenses	25.8	0	37.9	0
Broker commission expense	10.9	0	17.5	0
Amortization of other intangible assets	5	0	5	0
Interest expense	3.2	0	4.8	0
Total expenses	44.9	0	65.2	0
Pre-tax income (loss) from continuing operations	(5.2)		(2)
MediaAlpha | MediaAlpha
Revenues:
Earned insurance premiums	0	0		0
Net investment income	0	0		0
Net realized and unrealized investment (losses) gains	0	0		0
Commission revenues	74.5	37.9	216.4	101.2
Other revenue	0		1.6	0
Total revenues	74.5	37.9	218	101.2
Expenses:
Insurance acquisition expenses	0	0		0
Other underwriting expenses	0	0		0
Cost of sales	61.8	32.2	179.1	86
General and administrative and other expenses	5.4	3.8	21.2	10.7
Broker commission expense	0		0
Amortization of other intangible assets	2.4	2.3	7.8	7.2
Interest expense	0.2	0.1	0.9	0.6
Total expenses	69.8	38.4	209	104.5
Pre-tax income (loss) from continuing operations	4.7	(0.5)	9	(3.3)
Other | Other Segment
Revenues:
Earned insurance premiums	0	0	0	0
Net investment income	8.6	8.9	32.4	30.8
Net realized and unrealized investment (losses) gains	70.2	31.7	37.3	99.3
Commission revenues	1.1	0.9	3	2.7
Other revenue	0.4	1.4	0.6	6.3
Total revenues	80.3	42.9	73.3	139.1
Expenses:
Insurance acquisition expenses	0	0	0	0
Other underwriting expenses	0	0	0	0
Cost of sales	1.1	0.9	2.9	2.7
General and administrative and other expenses	26	27.3	79	113.9
Broker commission expense	0		0
Amortization of other intangible assets	0	0.1	0.1	0.1
Interest expense	0	0.8	0.3	1.2
Total expenses	27.1	29.1	82.3	117.9
Pre-tax income (loss) from continuing operations	53.2	13.8	(9)	21.2
Advertising and Commission Revenue
Revenues:
Commission revenues		38.8
Advertising and Commission Revenue | HG Global-BAM | HG Global-BAM
Revenues:
Commission revenues		0
Service
Expenses:
Cost of sales		33.1
Service | HG Global-BAM | HG Global-BAM
Expenses:
Cost of sales		0
Discontinued Operations
Expenses:
Net income (loss) attributable to White Mountains’s common shareholders	40.8	562.1	(3.7)	604.7
Discontinued Operations | Sirius Group
Revenues:
Earned insurance premiums		0		0
Net investment income		0		0
Net realized and unrealized investment (losses) gains		0		0
Total revenues		0		0
Expenses:
Insurance acquisition expenses		0		0
Other underwriting expenses		0		0
General and administrative and other expenses		0		0
Interest expense		0		0
Total expenses		0		0
Net (loss) gain from sale of discontinued operations, net of tax	(17.3)	(0.2)	0.1	(0.8)
Net (loss) income from discontinued operations, net of tax		$ 0		$ 0
Swedish Tax Authority | Discontinued Operations | Sirius Group
Expenses:
Net (loss) gain from sale of discontinued operations, net of tax	$ (17.3)		$ (17.2)

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ 40.8	$ 562.1	$ (3.7)	$ 604.7
Other comprehensive income, net of tax:
Other comprehensive (loss) income, net of tax	(0.8)	0	(1.7)	0.2
Comprehensive income from discontinued operations, net of tax	0	3	0	3.2
Comprehensive income	40	565.1	(5.4)	608.1
Comprehensive income (loss) attributable to non-controlling interests	0.2	0	0.2	(0.1)
Comprehensive income (loss) attributable to White Mountains’s common shareholders	$ 40.2	$ 565.1	$ (5.2)	$ 608
Basic earnings (loss) per share
Continuing operations (in usd per share)	$ 18.27	$ 5.36	$ 3.90	$ 7.03
Discontinued operations (in usd per share)	(5.44)	125.45	(4.98)	128.03
Total consolidated operations (in usd per share)	12.83	130.81	(1.08)	135.06
Diluted earnings (loss) per share
Continuing operations (in usd per share)	18.27	5.36	3.90	7.03
Discontinued operations (in usd per share)	(5.44)	125.45	(4.98)	128.03
Total consolidated operations (in usd per share)	12.83	130.81	(1.08)	135.06
Dividends declared per White Mountains’s common share	$ 0	$ 0	$ 1	$ 1

CONSOLIDATED STATEMENTS OF CHAN

CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (Unaudited) - USD ($) $ in Millions	Total	Common shares and paid-in surplus	Retained earnings	AOCI, after tax	Total	Non-controlling interest	Total Equity	Foreign Currency Gain (Loss)Common shares and paid-in surplus	Foreign Currency Gain (Loss)Retained earnings	Foreign Currency Gain (Loss)AOCI, after tax	Foreign Currency Gain (Loss)Total	Foreign Currency Gain (Loss)Non-controlling interest	Foreign Currency Gain (Loss)Total Equity	Pension liability and other accumulated comprehensive itemsAOCI, after tax	Pension liability and other accumulated comprehensive itemsTotal	Pension liability and other accumulated comprehensive itemsTotal Equity	OneBeaconCommon shares and paid-in surplus	OneBeaconRetained earnings	OneBeaconAOCI, after tax	OneBeaconTotal	OneBeaconNon-controlling interest	OneBeaconTotal Equity	Sirius GroupRecognition of foreign currency translation and other accumulated comprehensive items [Member]Common shares and paid-in surplus	Sirius GroupRecognition of foreign currency translation and other accumulated comprehensive items [Member]Retained earnings	Sirius GroupRecognition of foreign currency translation and other accumulated comprehensive items [Member]Non-controlling interest	Sale of Sirius GroupCommon shares and paid-in surplus	Sale of Sirius GroupRetained earnings	Sale of Sirius GroupAOCI, after tax	Sale of Sirius GroupTotal	Sale of Sirius GroupNon-controlling interest	Sale of Sirius GroupTotal Equity	Star & Shield Insurance ExchangeCommon shares and paid-in surplus	Star & Shield Insurance ExchangeRetained earnings	Star & Shield Insurance ExchangeAOCI, after tax	Star & Shield Insurance ExchangeTotal	Star & Shield Insurance ExchangeNon-controlling interest	Star & Shield Insurance ExchangeTotal Equity
Beginning Balances at Dec. 31, 2016		$ 810.7	$ 2,776.6	$ (4.6)	$ 3,582.7	$ 133.3	$ 3,716
Increase (Decrease) in Shareholders' Equity
Net (loss) income	$ 581.1	0	604.7	0	604.7	(23.6)	581.1
Net change in foreign currency translation and other								$ 0	$ 0	$ 0.4	$ 0.4	$ 0.1	$ 0.5
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of Sirius Group														$ 2.9	$ 2.9	$ 2.9							$ 0	$ 0	$ 0
Total comprehensive income (loss)	608.1	0	604.7	3.3	608	(23.5)	584.5
Dividends declared on common shares		0	(4.6)	0	(4.6)	0	(4.6)
Dividends to non-controlling interests		0	0	0	0	(17.7)	(17.7)
Repurchases and retirements of common shares		(147.9)	(575.9)	0	(723.8)	(5.2)	(729)
Issuances of common shares		1.7	0	0	1.7	0	1.7
Dilution from equity-based units of subsidiary																	$ (4.1)	$ 0	$ 0	$ (4.1)	$ 4.7	$ 0.6
Capital contributions from BAM members, net of tax	(25.7)	0	0	0	0	18.7	18.7
Amortization of restricted share awards		10.6	0	0	10.6	0.8	11.4
Deconsolidation of non-controlling interests associated with the sale of Star & Shield																										$ 0	$ 0	$ 0	$ 0	$ (238.3)	$ (238.3)	$ 0	$ 0	$ 0	$ 0	$ (4.4)	$ (4.4)
Net contributions from non-controlling interests		(1.7)	0	0	(1.7)	0	(1.7)
Ending Balances at Sep. 30, 2017		669.3	2,800.8	(1.3)	3,468.8	(131.6)	3,337.2
Beginning Balances at Dec. 31, 2017	3,360.8	670.6	2,823.2	(1.3)	3,492.5	(131.7)	3,360.8
Increase (Decrease) in Shareholders' Equity
Net (loss) income	(50.9)	0	(3.7)	0	(3.7)	(47.2)	(50.9)
Net change in foreign currency translation and other								$ 0	$ 0	$ (1.5)	$ (1.5)	$ 0	$ (1.5)
Total comprehensive income (loss)	(5.4)	0	(3.7)	(1.5)	(5.2)	(47.2)	(52.4)
Dividends declared on common shares		0	(3.8)	0	(3.8)	0	(3.8)
Dividends to non-controlling interests		0	0	0	0	(6)	(6)
Repurchases and retirements of common shares		(104.6)	(408.4)	0	(513)	0	(513)
Issuances of common shares		2	0	0	2	0	2
Recognition of equity-based units of subsidiary		4.5	0	0	4.5	2.3	6.8
Dilution from equity-based units of subsidiary		(1.1)	0	0	(1.1)	1.1	0
Capital contributions from BAM members, net of tax	(26.9)	0	0	0	0	22.9	22.9
Amortization of restricted share awards		9.7	0	0	9.7	0	9.7
Noncontrolling interest from acquisition		0	0	0	0	14.2	14.2
Acquisition from non-controlling interest		(1.6)	0	0	(1.6)	0	(1.6)
Ending Balances at Sep. 30, 2018	$ 2,839.6	$ 579.5	$ 2,407.3	$ (2.8)	$ 2,984	$ (144.4)	$ 2,839.6

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Net Cash Provided by (Used in) Operating Activities, Continuing Operations [Abstract]
Net (loss) income	$ 30.6	$ 551.5	$ (50.9)	$ 581.1
Charges (credits) to reconcile net income to net cash used for operations:
Net realized and unrealized investment gains	(66.1)	(32.5)	(22.9)	(102.5)
Deferred income benefit			(4.7)	(6.9)
Net income from discontinued operations	0	15.2	0	(20.5)
Net loss (gain) from sale of discontinued operations, net of tax		(554.3)	17.2	(552.7)
Amortization of restricted share and option awards			9.7	10.1
Amortization and depreciation			18.4	15.8
Other operating items:
Net change in unearned insurance premiums			19.4	36.2
Net change in deferred acquisition costs			(2.8)	(3.5)
Net change in restricted cash			(12.1)	0
Net change in other assets and liabilities, net			(1.8)	(21.5)
Net cash used for operations - continuing operations			(30.5)	(64.4)
Net cash (used for) provided from operations			(30.5)	92.6
Cash flows from investing activities:
Net change in short-term investments			(138.5)	(612.2)
Sales of fixed maturity and convertible investments			1,768.5	1,631.2
Maturities, calls and paydowns of fixed maturity and convertible investments			124.4	178.6
Sales of common equity securities			129.8	407.8
Distributions and redemptions of other long-term investments and settlements of forward contracts			(5.5)	1.3
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold			0	1,131
Net settlement of investment cash flows and contributions with discontinued operations			0	167.7
Purchases of other long-term investments			(65.9)	(67.7)
Purchases of common equity securities			(253)	(818.6)
Purchases of fixed maturity and convertible investments			(833.1)	(1,189.8)
Purchases of consolidated subsidiaries, net of cash acquired of $71.9			(252.1)	0
Other investing activities, net			33.8	(134.6)
Net cash provided from investing activities - continuing operations			508.4	694.7
Net cash provided from investing activities			508.4	697.7
Cash flows from financing activities:
Draw down of debt and revolving line of credit			50.9	361
Repayment of debt and revolving line of credit			(11.2)	(364.6)
Cash dividends paid to the Company’s common shareholders			(3.8)	(4.6)
Common shares repurchased			(504.7)	(714.6)
Distribution to non-controlling interest shareholders			(4.6)	(0.9)
Contributions from discontinued operations			0	45.2
Payments on contingent consideration earnout liability			(2.6)	0
Capital contributions from BAM members			26.9	25.7
Acquisition of additional subsidiary shares from non-controlling interest			(1.7)	(0.7)
Fidus Re premium payment			(3)	0
Restricted share statutory withholding tax payments			(8.4)	(9.3)
Net cash used for financing activities - continuing operations			(462.2)	(662.8)
Net cash used for financing activities			(462.2)	(724.7)
Effect of exchange rate changes on cash			(0.6)	0
Net change in cash during the period - continuing operations			15.7	(32.5)
Cash balances at beginning of period			97.1	80.2	$ 80.2
Add: cash held for sale, excluding discontinued operations, at the beginning of period	0	0	0	0	0	$ 0.9
Less: cash held for sale, excluding discontinued operations, at the end of period	0	0	0	0	0
Cash balances at end of period	112.2	48.6	112.2	48.6	97.1
Supplemental cash flows information:
Interest paid			(5.3)	(0.5)
Net income tax refunds			(0.5)	0
Discontinued Operations
Other operating items:
Net cash (used for) provided from operations - discontinued operations			0	157
Cash flows from investing activities:
Net cash provided from investing activities - discontinued operations			0	3
Cash flows from financing activities:
Net cash used for financing activities - discontinued operations			0	(61.9)
Cash balances at beginning of period			0	70.5	70.5
Cash balances at end of period	$ 0	$ 71.3	0	71.3	$ 0
Supplemental cash flows information:
Net income tax refunds			$ 0	$ 0

CONSOLIDATED STATEMENTS OF CA_2

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)
Cash balance	$ 112.2
Cash acquired from acquisition	71.9
Discontinued Operations
Cash balance	$ 0

Basis of Presentation and Signi

Basis of Presentation and Significant Accouting Policies	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation and Significant Accouting Policies	Basis of Presentation and Significant Accounting Policies Basis of Presentation The Company is an exempted Bermuda limited liability company whose principal businesses are conducted through its insurance subsidiaries and other affiliates. The Company’s headquarters is located at 26 Reid Street, Hamilton, Bermuda HM 11, its principal executive office is located at 80 South Main Street, Hanover, New Hampshire 03755-2053 and its registered office is located at Clarendon House, 2 Church Street, Hamilton, Bermuda HM 11. The accompanying consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) and include the accounts of White Mountains Insurance Group, Ltd. (the “Company” or the “Registrant”), its subsidiaries (collectively with the Company, “White Mountains”) and other entities required to be consolidated under GAAP. Consolidation Principles Under GAAP, the Company is required to consolidate any entity in which it holds a controlling financial interest. A controlling financial interest is usually in the form of an investment representing the majority of the subsidiary’s voting interests. However, a controlling financial interest may also arise from a financial interest in a variable interest entity (“VIE”) through arrangements that do not involve ownership of voting interests. The Company consolidates a VIE if it determines that it is the primary beneficiary. The primary beneficiary is defined as the entity who holds a variable interest that gives it both the power to direct the VIE’s activities that most significantly impact its economic performance and the obligation to absorb losses of, or the right to receive returns from, the VIE that could potentially be significant to the VIE. Intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. These interim financial statements include all adjustments considered necessary by management to fairly state the financial position, results of operations and cash flows of White Mountains. These interim financial statements may not be indicative of financial results for the full year and should be read in conjunction with the Company’s 2017 Annual Report on Form 10-K. Business Combinations White Mountains accounts for purchases of businesses using the acquisition method, which requires the measurement of assets acquired, including goodwill and other intangible assets and liabilities assumed, including contingent liabilities, at their estimated fair values as of the acquisition date. The acquisition date fair values represent management's best estimates and are based upon established valuation techniques, reasonable assumptions and, where appropriate, valuations performed by independent third parties. In circumstances where additional information is required in order to determine the acquisition date fair value of balance sheet amounts, provisional amounts may be recorded as of the acquisition date and may be subject to subsequent adjustment throughout the measurement period, which is one year from the acquisition date. Measurement period adjustments are recognized in the period in which they are determined. The results of operations and cash flows of businesses acquired are included in the consolidated financial statements from the date of acquisition. 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 12 — “Segment Information” . The HG Global/BAM segment consists of HG Global Ltd. and its wholly-owned subsidiaries (“HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”) (collectively, “HG Global/BAM”). BAM is the first and only mutual municipal bond insurance company in the United States. By insuring the timely payment of principal and interest, BAM provides market access to, and lowers interest expense for, issuers of municipal bonds used to finance essential public purposes such as schools, utilities and transportation facilities. BAM is owned by and operated for the benefit of its members, the municipalities that purchase BAM’s insurance for their debt issuances. HG Global was established to fund the startup of BAM and, through its wholly-owned subsidiary, HG Re Ltd. (“HG Re”), to provide up to 15% -of-par, first loss reinsurance protection for policies underwritten by BAM. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM (the “BAM Surplus Notes”). As of September 30, 2018, $499.0 million of the surplus notes remain outstanding. As of September 30, 2018 and December 31, 2017, White Mountains owned 96.9% of HG Global’s preferred equity and 88.4% of its common equity. White Mountains does not have an ownership interest in BAM. However, White Mountains is required to consolidate BAM’s results in its financial statements because BAM is a VIE for which White Mountains is the primary beneficiary. BAM’s results are attributed to non-controlling interests. The NSM segment consists of NSM Insurance HoldCo, LLC and its wholly-owned subsidiaries (collectively, "NSM"). NSM is a full service managing general agency and program administrator for specialty property and casualty insurance. NSM manages all aspects of the placement process on behalf of its carrier partners, including product development, marketing, underwriting and policy issuance. NSM specializes in niche sectors including collector cars, social services and behavioral health, specialty real estate, sports and fitness centers, and pet insurance. White Mountains acquired a 95% ownership share in NSM on May 11, 2018. The NSM segment also includes White Mountains Catskill Holdings, Inc. 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 develops technology that enables the programmatic buying and selling of vertical-specific, performance-based media between advertisers (buyers of advertising inventory) and publishers (sellers of advertising inventory) through cost-per-click, cost-per-call and cost-per-lead pricing models. MediaAlpha's media buying platform enables advertisers to create and automate data-driven bidding strategies designed to improve the efficiency and enhance overall performance of their marketing campaigns that target high-intent consumers at the time and place they are ready to purchase. MediaAlpha’s publisher platform is used by publishers to sell their vertical-specific, performance-based media to advertisers through transparent, programmatic, auction-based marketplaces. MediaAlpha works with 600 advertisers and 330 publishers across a number of insurance (auto, motorcycle, home, renter, health and life) and non-insurance (travel, education, personal finance and home services) verticals. White Mountains’s Other Operations segment consists of the Company, its wholly-owned subsidiary, White Mountains Capital, Inc. (“WM Capital”), its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), and its other intermediate holding companies, as well as certain consolidated and unconsolidated private capital and other investments. The consolidated private capital investments include Wobi Insurance Agency Ltd. (“Wobi”) and Removal Stars Ltd. (“Buzzmove”). Discontinued Operations and Assets and Liabilities Held for Sale On September 28, 2017, Intact Financial Corporation completed its acquisition of OneBeacon Insurance Group, Ltd. (“OneBeacon”) in an all-cash transaction for $18.10 per share (the “OneBeacon Transaction”). On July 21, 2016, White Mountains completed its sale of Tranzact Holdings, LLC (“Tranzact”) to an affiliate of Clayton, Dubilier & Rice, LLC. On April 18, 2016, White Mountains completed its sale of Sirius International Insurance Group, Ltd. (“Sirius Group”) to CM International Pte. Ltd. and CM Bermuda Limited (collectively “CMI”), the Singapore-based investment arm of China Minsheng Investment Corp., Ltd. White Mountains has presented the results of OneBeacon, Tranzact and Sirius Group as discontinued operations in the statement of operations and comprehensive income and their assets and liabilities as held for sale in the balance sheet for all periods prior to the completion of each transaction. White Mountains has classified its Guilford, Connecticut property, which consists of an office building and adjacent land, as held for sale as of September 30, 2018 and December 31, 2017. See Note 16 — “Held for Sale and Discontinued Operations” . Derivatives White Mountains holds from time to time derivative financial instruments for risk management purposes. White Mountains recognizes all derivatives as either assets or liabilities on the balance sheet measured at fair value. During the quarter ended June 30, 2018, White Mountains entered into an interest rate swap to hedge its exposure to the interest rate risk associated with the interest payments on NSM's variable rate debt. In order to qualify for hedge accounting, a derivative instrument must be both highly effective in offsetting the exposure to the hedged risk and designated as a hedge at inception. The swap meets both of these requirements and is being accounted for as a hedge. Changes in the fair value of the swap are recognized in other comprehensive income until changes in the interest rate on the debt cause an increase in the hedged cash flows. When the hedged cash flows increase, then the effective portion of the gains or losses arising from the change in the fair value of the swap are reclassified into earnings from other comprehensive income. The amounts reclassified are included in the same financial statement caption as the related interest expense, effectively fixing the debt interest at the fixed rate in the swap for the hedged portion of the interest cash flows. Any portion of the change in the fair value of a derivative instrument designated as a cash flow hedge that represents hedge ineffectiveness is recognized in current period pre-tax income. Hedge ineffectiveness is measured as the amount by which changes in the fair value of a derivative designated as a hedge exceed the change in the fair value of the related hedged cash flows. White Mountains formally evaluates and documents the relationship between derivatives used as hedges and the related hedged cash flows, including its risk-management objective and strategy for undertaking a hedging transaction. White Mountains formally assesses the effectiveness of the hedging transaction at both inception and on an ongoing basis. White Mountains also held certain foreign currency forward contracts which were held for risk management purposes, but not designated as hedges. These contracts were measured at fair value with the changes therein recognized through current period pre-tax income. See Note 7 — “Derivatives” . Reinsurance Contracts Accounted for as Deposits Reinsurance contracts that do not meet the risk transfer requirements necessary to be accounted for as reinsurance are accounted for using the deposit method under GAAP. BAM entered into a reinsurance contract agreement with Fidus Reinsurance Ltd. (“Fidus Re”) in 2018, which is accounted for using the deposit method. See Note 8 — “Municipal Bond Guarantee Insurance” . The nonrefundable consideration paid by BAM to Fidus Re is charged to financing expense within general and administrative expenses. Restricted Cash Cash includes amounts on hand and demand deposits with banks and other financial institutions. Cash balances which are not immediately available for general corporate purposes, including fiduciary accounts, are classified as restricted. Restricted amounts included within cash are disclosed parenthetically on the balance sheet. Amounts presented in the statement of cash flows are shown net of balances acquired and sold in the purchase or sale of White Mountains’s consolidated subsidiaries. Changes in restricted cash balances are presented as a separate caption within the operations, investing and financing activities sections of the statement of cash flows. Significant Accounting Policies Refer to the Company’s 2017 Annual Report on Form 10-K for a complete discussion regarding White Mountains’s significant accounting policies. Recently Adopted Changes in Accounting Principles Revenue Recognition On January 1, 2018, White Mountains adopted ASU 2014-09, Revenue from Contracts with Customers (ASC 606), which modifies the guidance for revenue recognition. Under ASU 2014-09, revenue is recognized at an amount that reflects the consideration to which an entity expects to be entitled once it fulfills its performance obligations under the terms of its contract with the customer. The scope of the new guidance includes agent commissions and other non-insurance revenues. Adoption of ASU 2014-09 did not have any impact on White Mountains's financial statements. Share-Based Compensation On January 1, 2018, White Mountains adopted ASU 2017-09, Stock Compensation: Scope of Modification Accounting (ASC 718), which narrows the scope of transactions subject to modification accounting to changes in terms of an award that result in a change in the award’s fair value, vesting conditions or classification. Adoption of ASU 2017-09 did not have any impact on White Mountains’s financial statements. On January 1, 2017, White Mountains adopted ASU 2016-09, Improvements to Employee Share-Based Payment Accounting (ASC 718) which simplifies certain aspects of the accounting for share-based compensation. The new guidance provides an accounting policy election to account for forfeitures by either applying an assumption, as required under existing guidance, or by recognizing forfeitures when they actually occur. At adoption, White Mountains did not change its accounting policy for forfeitures, which is to apply an assumed forfeiture rate. The new guidance has also changed the threshold for partial cash settlement to settle statutory withholding requirements for equity classified awards, increasing the threshold up to the maximum statutory tax rate. As a result of adoption, White Mountains reported $8.4 million and $9.2 million of statutory withholding tax payments made in connection with the settlement of restricted shares as financing cash flows for the nine months ended September 30, 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, regardless of whether it is in a taxes payable position. Business Combinations On January 1, 2018, White Mountains adopted ASU 2017-01, Business Combinations: Clarifying the Definition of a Business (ASC 805), which clarifies the definition of a business and affects the determination of whether acquisitions or disposals are accounted for as assets or as a business. Under the new guidance, when substantially all of the fair value of the assets is concentrated in a single identifiable asset or group of similar assets, it is not a business. 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 FASB issued ASU 2017-08, Premium Amortization on Purchased Callable Debt Securities (ASC 310-20), which changes the amortization period for certain purchased callable debt securities. Under the new guidance, for investments in callable debt securities held at a premium, the premium will be amortized over the period to the earliest call date. The new guidance does not change the amortization period for callable debt securities held at a discount. ASU 2017-08 is not expected to have any impact on White Mountains's financial statements at adoption but may affect the amortization recognized in future periods. Credit Losses In June 2016, the FASB issued ASU 2016-13, Measurement of Credit Losses on Financial Instruments (ASC 326), which establishes new guidance for the recognition of credit losses for financial assets measured at amortized cost. The new ASU requires reporting entities to estimate the credit losses expected over the life of a credit exposure using historical information, current information and reasonable and supportable forecasts that affect the collectability of the financial asset. This differs from current GAAP, which delays recognition until it is probable a loss has been incurred. The new guidance is expected to accelerate recognition of credit losses. The types of assets within the scope of the new guidance include premium receivables, reinsurance recoverables and loans. ASU 2016-13 is effective for annual periods beginning after January 1, 2020, including interim periods. White Mountains measures its portfolio of investment securities at fair value with changes therein recognized through current period earnings and, accordingly, does not expect adoption to have any effect on its financial statements. Leases In February 2016, the FASB issued ASU 2016-02, Leases (ASC 842). The new guidance requires lessees to recognize lease assets and liabilities on the balance sheet for both operating and financing leases, with the exception of leases with an original term of 12 months or less. Under existing guidance recognition of lease assets and liabilities is not required for operating leases. The lease assets and liabilities to be recognized are both measured initially based on the present value of the lease payments. Under the new guidance, a sale-leaseback transaction must meet the recognition criteria under ASC 606, Revenues, in order to be accounted for as sale. The new guidance is effective for White Mountains for years beginning after December 15, 2018, including interim periods therein. White Mountains is evaluating the expected impact of this guidance and available adoption methods.

Significant Transactions

Significant Transactions	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Significant Transactions	Significant Transactions Acquisitions NSM On May 11, 2018, White Mountains closed its acquisition of 95% of NSM for cash consideration of $274.2 million . White Mountains recognized a purchase price adjustment of $2.1 million during the three months ended September 30, 2018 that was paid in October 2018. 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 the second quarter of 2018. On May 18, 2018, NSM acquired 100% of Fresh Insurance Services Group Limited (“Fresh Insurance”), an insurance broker that specializes in non-standard personal lines products, motor trade, and van insurance in the United Kingdom, for cash consideration of $ 49.6 million. NSM recognized a purchase price adjustment of $0.7 million during the three months ended September 30, 2018. 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 . 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 both the three months ended September 30, 2018 and the period from May 11, 2018 through September 30, 2018, NSM recognized pre-tax expense of $2.6 million for the change in the fair value of its contingent consideration earnout liabilities for both 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 September 30, 2018, NSM recorded contingent consideration earnout liabilities of $20.1 million . DavidShield On January 24, 2018, White Mountains acquired 50% of DavidShield Life Insurance Agency (2000) Ltd. (“DavidShield”), its joint venture partner in PassportCard Limited (“PassportCard”). DavidShield is a managing general agency that is the leading provider of expatriate medical insurance in Israel and uses the same card-based delivery system as PassportCard. As part of the transaction, White Mountains restructured its equity stake in PassportCard so that White Mountains and its partner in DavidShield would each own 50% of both businesses. To facilitate the transaction, White Mountains provided financing to its partner in the form of a non-interest bearing loan that is secured by the partner’s equity in PassportCard and DavidShield. The gross purchase price for the 50% of DavidShield was $41.8 million , or $28.3 million net of the financing provided for the restructuring. Kudu On February 5, 2018, White Mountains entered into an agreement to fund up to $127.5 million in Kudu Investment Management, LLC (“Kudu”), a capital provider to asset management and wealth management firms, including $125.0 million for new transactions. Kudu specializes in providing capital solutions to asset managers and registered investment advisers, including generational ownership transfers, management buyouts, acquisition and growth finance, as well as liquidity for legacy partners. As of September 30, 2018 , White Mountains has funded $12.3 million in Kudu. Kudu closed their first transaction in the third quarter of 2018 and announced two more in October, deploying $63.0 million of capital in total, $31.5 million of which is from White Mountains. White Mountains has determined that Kudu is a VIE, however White Mountains is not the primary beneficiary. White Mountains has elected to take the fair value option for its investment in Kudu. MediaAlpha On October 5, 2017, MediaAlpha acquired certain assets associated with the Health, Life and Medicare insurance business of Healthplans.com for an aggregate purchase price of $28.0 million . The majority of assets acquired, which are included in other intangible assets, consist of customer relationships, a non-compete agreement from the seller and domain names. See Note 4 — “Goodwill and Other Intangibles Assets” . On October 5, 2017, White Mountains acquired 131,579 newly-issued Class A common units of MediaAlpha for $12.5 million . As of September 30, 2018 and December 31, 2017 White Mountains’s ownership share in MediaAlpha was 62.2% 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 gain of $554.6 million , net of transaction costs. As a result of the OneBeacon Transaction, OneBeacon’s results have been reported as discontinued operations within White Mountains’s GAAP financial statements. See Note 16 — “Held for Sale and Discontinued Operations” . Star & Shield On March 7, 2017, White Mountains completed the sale of Star & Shield Services LLC, Star & Shield Risk Management LLC, and Star & Shield Claims Services LLC (collectively “Star & Shield”) and its investment in Star & Shield Insurance Exchange (“SSIE”) surplus notes to K2 Insurance Services, LLC. White Mountains did not recognize any gain or loss on the sale.

Investment Securities

Investment Securities	9 Months Ended
	Sep. 30, 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 September 30, 2018 and December 31, 2017 . Other long-term investments consist primarily of hedge funds, private equity funds and unconsolidated private capital investments. Net Investment Income White Mountains’s net investment income is comprised primarily of interest income associated with White Mountains’s fixed maturity investments and short-term investments and dividend income from its common equity securities and other long-term investments. The following table presents pre-tax net investment income for the three and nine months ended September 30, 2018 and 2017 . Three Months Ended Nine Months Ended September 30, September 30, Millions 2018 2017 2018 2017 Investment income: Fixed maturity investments $ 7.5 $ 9.8 $ 27.3 $ 32.9 Short-term investments 1.8 .3 6.5 .6 Common equity securities 3.4 2.7 11.2 7.7 Other long-term investments 1.2 .1 1.7 .5 Total investment income 13.9 12.9 46.7 41.7 Third-party investment expenses (.5 ) (.7 ) (1.8 ) (2.0 ) Net investment income, pre-tax $ 13.4 $ 12.2 $ 44.9 $ 39.7 Net Realized and Unrealized Investment Gains (Losses) The following table presents net realized and unrealized investment gains (losses) for the three and nine months ended September 30, 2018 and 2017 : Three Months Ended Nine Months Ended September 30, September 30, Millions 2018 2017 2018 2017 Net realized investment (losses) gains, pre-tax $ (10.2 ) $ 6.8 $ (13.9 ) $ 20.8 Net unrealized investment gains, pre-tax 76.3 25.7 36.8 81.7 Net realized and unrealized investment gains, pre-tax 66.1 32.5 22.9 102.5 Income tax expense attributable to net realized and unrealized investment gains (9.7 ) (3.9 ) (5.9 ) (9.5 ) Net realized and unrealized investment gains, after-tax $ 56.4 $ 28.6 $ 17.0 $ 93.0 Net Realized Investment Gains (Losses) The following tables present net realized investment gains (losses) for the three and nine months ended September 30, 2018 and 2017 Three Months Ended Three Months Ended September 30, 2018 September 30, 2017 Millions Net Net Total net realized Net Net Total net realized Fixed maturity investments $ (5.1 ) $ — $ (5.1 ) $ .6 $ 1.2 $ 1.8 Short-term investments — — — (.1 ) — (.1 ) Common equity securities (1.5 ) — (1.5 ) 4.9 5.4 10.3 Other long-term investments (3.6 ) — (3.6 ) 2.0 (7.2 ) (5.2 ) Net realized investment (losses) gains, pre-tax (10.2 ) — (10.2 ) 7.4 (.6 ) 6.8 Income tax benefit (expense) attributable to net realized investment (losses) gains 2.3 — 2.3 (.6 ) — (.6 ) Net realized investment (losses) gains, after-tax $ (7.9 ) $ — $ (7.9 ) $ 6.8 $ (.6 ) $ 6.2 Nine Months Ended Nine Months Ended September 30, 2018 September 30, 2017 Millions Net Net Total net realized Net Net Total net realized Fixed maturity investments $ (27.6 ) $ 18.2 $ (9.4 ) $ (.4 ) $ 2.7 $ 2.3 Short-term investments (.8 ) — (.8 ) (.1 ) — (.1 ) Common equity securities 10.5 — 10.5 18.5 6.0 24.5 Other long-term investments (7.0 ) (7.2 ) (14.2 ) 3.0 (8.9 ) (5.9 ) Net realized investment (losses) gains, pre-tax (24.9 ) 11.0 (13.9 ) 21.0 (.2 ) 20.8 Income tax benefit (expense) attributable to net realized investment (losses) gains 11.2 — 11.2 (3.6 ) — (3.6 ) Net realized investment (losses) gains, after-tax $ (13.7 ) $ 11.0 $ (2.7 ) $ 17.4 $ (.2 ) $ 17.2 Net Unrealized Investment Gains (Losses) The following tables present net unrealized investment gains (losses) and changes in the carrying value of investments measured at fair value for the three and nine months ended September 30, 2018 and 2017 : Three Months Ended Three Months Ended September 30, 2018 September 30, 2017 Millions Net unrealized gains (losses) Net Total net unrealized gains (losses) reflected in earnings Net unrealized gains (losses) Net Total net unrealized gains (losses) reflected in earnings Fixed maturity investments $ 1.5 $ — $ 1.5 $ 2.0 $ 5.1 $ 7.1 Short-term investments (.1 ) — (.1 ) — — — Common equity securities 54.9 — 54.9 26.6 (3.1 ) 23.5 Other long-term investments 20.1 (.1 ) 20.0 (2.5 ) (2.4 ) (4.9 ) Net unrealized investment gains (losses), pre-tax 76.4 (.1 ) 76.3 26.1 (.4 ) 25.7 Income tax expense attributable to net unrealized investment gains (losses) (12.0 ) — (12.0 ) (3.3 ) — (3.3 ) Net unrealized investment gains (losses), after-tax $ 64.4 $ (.1 ) $ 64.3 $ 22.8 $ (.4 ) $ 22.4 Nine Months Ended Nine Months Ended September 30, 2018 September 30, 2017 Millions Net unrealized (losses) gains Net gains Total net unrealized (losses) gains reflected in earnings Net unrealized gains Net Total net unrealized gains (losses) reflected in earnings Fixed maturity investments $ (15.7 ) $ (14.8 ) $ (30.5 ) $ 19.4 $ 12.6 $ 32.0 Short-term investments (.1 ) — (.1 ) — — — Common equity securities 43.7 — 43.7 53.5 — 53.5 Other long-term investments 20.5 3.2 23.7 9.2 (13.0 ) (3.8 ) Net unrealized investment gains (losses), pre-tax 48.4 (11.6 ) 36.8 82.1 (.4 ) 81.7 Income tax expense attributable to net unrealized investment gains (losses) (17.1 ) — (17.1 ) (5.9 ) — (5.9 ) Net unrealized investment gains (losses), after-tax $ 31.3 $ (11.6 ) $ 19.7 $ 76.2 $ (.4 ) $ 75.8 The following table presents total gains (losses) included in earnings attributable to net unrealized investment gains (losses) for Level 3 investments for the three and nine months ended September 30, 2018 and 2017 : Three Months Ended Nine Months Ended September 30, September 30, Millions 2018 2017 2018 2017 Other long-term investments $ 4.0 $ (.7 ) $ 2.1 $ (2.2 ) Total net unrealized investment gains (losses) , pre-tax - Level 3 investments $ 4.0 $ (.7 ) $ 2.1 $ (2.2 ) 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 September 30, 2018 and December 31, 2017 . September 30, 2018 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign currency gains Carrying value U.S. Government and agency obligations $ 127.0 $ — $ (1.6 ) $ — $ 125.4 Debt securities issued by corporations 509.4 .2 (9.6 ) — 500.0 Mortgage and asset-backed securities 132.4 — (3.9 ) — 128.5 Municipal obligations 281.1 .4 (3.5 ) — 278.0 Total fixed maturity investments $ 1,049.9 $ .6 $ (18.6 ) $ — $ 1,031.9 December 31, 2017 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign currency gains Carrying value U.S. Government and agency obligations $ 297.8 $ — $ (1.3 ) $ — $ 296.5 Debt securities issued by corporations 867.6 2.9 (4.3 ) 14.7 880.9 Mortgage and asset-backed securities 697.2 1.6 (4.1 ) — 694.7 Municipal obligations 252.0 3.7 (.8 ) — 254.9 Foreign government, agency and provincial obligations 2.6 — — .1 2.7 Total fixed maturity investments $ 2,117.2 $ 8.2 $ (10.5 ) $ 14.8 $ 2,129.7 The following tables present the cost or amortized cost, gross unrealized investment gains (losses), net foreign currency losses, and carrying values of White Mountains’s common equity securities and other long-term investments as of September 30, 2018 and December 31, 2017 : September 30, 2018 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign Carrying value Common equity securities $ 873.4 $ 171.1 $ (1.0 ) $ — $ 1,043.5 Other long-term investments $ 303.8 $ 34.8 $ (47.2 ) $ (1.7 ) $ 289.7 December 31, 2017 Millions Cost or Gross Gross Net foreign Carrying value Common equity securities $ 739.7 $ 129.4 $ (3.0 ) $ — $ 866.1 Other long-term investments $ 246.6 $ 6.8 $ (39.7 ) $ (4.9 ) $ 208.8 Other Long-Term Investments The following table presents the carrying values of White Mountains’s other long-term investments as of September 30, 2018 and December 31, 2017 : Carrying Value at Millions September 30, 2018 December 31, 2017 Hedge funds and private equity funds, at fair value $ 151.6 $ 125.3 Private equity securities, at fair value (1)(2) 126.4 83.2 Foreign currency forward contracts — (3.7 ) Other 11.7 4.0 Total other long-term investments $ 289.7 $ 208.8 (1) See Fair Value Measurements by Level table. (2) Includes White Mountains's non-controlling interests in common equity securities, limited liability companies and private convertible preferred securities. Hedge Funds and Private Equity Funds White Mountains invests in hedge funds and private equity funds, which are included in other long-term investments. The fair value of these investments is generally estimated using the NAV of the funds. As of September 30, 2018 , White Mountains held investments in one hedge fund and thirteen private equity funds. The largest investment in a single fund was $59.6 million as of September 30, 2018 and $54.9 million as of December 31, 2017 . The following table presents investments in hedge funds and private equity funds by investment objective and sector as of September 30, 2018 and December 31, 2017 : September 30, 2018 December 31, 2017 Millions Fair Value Unfunded Commitments Fair Value Unfunded Commitments Hedge funds Long/short banks and financials $ 59.6 $ — $ 54.9 $ — Total hedge funds 59.6 — 54.9 — Private equity funds Manufacturing/Industrial 49.6 10.5 43.3 10.4 Aerospace/Defense/Government 25.5 34.9 15.8 12.9 Direct lending 10.6 19.9 7.1 23.1 Financial services 6.3 15.7 4.2 11.7 Real estate — 50.0 — — Total private equity funds 92.0 131.0 70.4 58.1 Total hedge funds and private equity funds included in other long-term investments $ 151.6 $ 131.0 $ 125.3 $ 58.1 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 September 30, 2018 , White Mountains held one active hedge fund with a fair value of $59.6 million . The hedge fund is subject to a semi-annual restriction on redemptions and an advance notice period requirement of 45 days. 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 September 30, 2018 : Millions 1 – 3 years 3 – 5 years 5 – 10 years >10 years Total Private equity funds — expected lock-up period remaining $14.2 $5.0 $55.9 $16.9 $92.0 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 September 30, 2018 and December 31, 2017, White Mountains is not aware of any indemnification claims relating to its investments in private equity funds. Any future indemnification obligations would be disclosed separately as opposed to being included in the Unfunded Commitments balance. Fair Value Measurements as of September 30, 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 September 30, 2018 and December 31, 2017, White Mountains used quoted market prices or other observable inputs to determine fair value for approximately 89% and 94% of the investment portfolio. Fair Value Measurements by Level The following tables present White Mountains’s fair value measurements for investments as of September 30, 2018 and December 31, 2017 by level. The major security types were based on the legal form of the securities. White Mountains has disaggregated its fixed maturity investments based on the issuing entity type, which impacts credit quality, with debt securities issued by U.S. Government entities carrying minimal credit risk, while the credit and other risks associated with other issuers, such as corporations, foreign governments, municipalities or entities issuing mortgage and asset-backed securities vary depending on the nature of the issuing entity type. White Mountains further disaggregates debt securities issued by corporations and common equity securities by industry sector because investors often reference commonly used benchmarks and their subsectors to monitor risk and performance. Accordingly, White Mountains has further disaggregated these asset classes into subclasses based on the similar sectors and industry classifications it uses to evaluate investment risk and performance against commonly used benchmarks, such as the Bloomberg Barclays U.S. Intermediate Aggregate and S&P 500 indices. September 30, 2018 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 125.4 $ 125.4 $ — $ — Debt securities issued by corporations: Financials 145.9 — 145.9 — Consumer 84.2 — 84.2 — Technology 60.4 — 60.4 — Energy 54.6 — 54.6 — Healthcare 53.9 — 53.9 — Industrial 39.5 — 39.5 — Communications 22.4 — 22.4 — Materials 22.2 — 22.2 — Utilities 16.9 — 16.9 — Total debt securities issued by corporations 500.0 — 500.0 — Mortgage and asset-backed securities 128.5 — 128.5 — Municipal obligations 278.0 — 278.0 — Total fixed maturity investments 1,031.9 125.4 906.5 — Short-term investments (1) 313.7 300.7 13.0 — Common equity securities: Exchange traded funds (2) 742.9 675.3 67.6 — Technology 16.8 16.8 — — Financials 16.7 16.7 — — Healthcare 16.4 16.4 — — Industrial 15.6 15.6 — — Communications 14.8 14.8 — — Consumer 9.8 9.8 — — Energy 6.6 6.6 — — Other (3) 203.9 — 203.9 — Total common equity securities 1,043.5 772.0 271.5 — Other long-term investments (4) 138.1 — — 138.1 Total investments $ 2,527.2 $ 1,198.1 $ 1,191.0 $ 138.1 (1) Short-term investments are measured at amortized cost, which approximates fair value. (2) ETFs traded on foreign exchanges are priced using the fund's published NAV to account for the difference in market close times and are therefore designated a level 2 measurement. (3) Consists of two investments in unit trusts that primarily invest in international equities. (4) Excludes carrying value of $151.6 associated with hedge funds and private equity funds for which fair value is measured at NAV using the practical expedient. December 31, 2017 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 296.5 $ 296.5 $ — $ — Debt securities issued by corporations: Consumer 185.1 — 185.1 — Communications 127.8 — 127.8 — Financials 114.8 — 114.8 — Utilities 108.9 — 108.9 — Materials 95.5 — 95.5 — 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) 87.2 — — 87.2 Total investments $ 3,259.1 $ 1,041.4 $ 2,130.5 $ 87.2 (1) Short-term investments are measured at amortized cost, which approximates fair value. (2) ETFs traded on foreign exchanges are priced using the fund’s published NAV to account for the difference in market close times and are therefore designated a level 2 measurement. (3) Consists of two investments in unit trusts that primarily invests in international equities. (4) Excludes carrying value of $(3.7) related to foreign currency forward contracts and $125.3 associated with hedge funds and private equity funds for which fair value is measured at NAV using the practical expedient. Debt Securities Issued by Corporations The following table presents the ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of September 30, 2018 and December 31, 2017 : Fair Value at Millions September 30, 2018 December 31, 2017 AAA $ 8.1 $ 1.6 AA 83.6 42.6 A 273.3 192.5 BBB 135.0 465.2 BB — 161.7 B — 17.3 Debt securities issued by corporations (1) $ 500.0 $ 880.9 (1) Credit ratings are assigned based on the following hierarchy: (1) Standard & Poor’s Financial Services LLC (“S&P”) and (2) Moody's Investor Service, Inc. (“Moody’s”). Mortgage and Asset-backed Securities The following table presents the fair value of White Mountains’s mortgage and asset-backed securities as of September 30, 2018 and December 31, 2017 : September 30, 2018 December 31, 2017 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: GNMA $ 24.5 $ 24.5 $ — $ 46.3 $ 46.3 $ — FNMA 53.6 53.6 — 84.5 84.5 — FHLMC 39.0 39.0 — 62.0 62.0 — Total agency (1) 117.1 117.1 — 192.8 192.8 — Non-agency: Commercial — — — 70.5 70.5 — Total non-agency — — — 70.5 70.5 — Total mortgage-backed securities 117.1 117.1 — 263.3 263.3 — Other asset-backed securities: Credit card receivables 8.9 8.9 — 206.0 206.0 — Vehicle receivables 2.5 2.5 — 142.4 142.4 — Other — — — 83.0 83.0 — Total other asset-backed securities 11.4 11.4 — 431.4 431.4 — Total mortgage and asset-backed securities $ 128.5 $ 128.5 $ — $ 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 September 30, 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 September 30, 2018 , White Mountains did not hold any mortgage-backed securities classified as non-prime. 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 September 30, 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 nine months ended September 30, 2018 and 2017 : Level 3 Investments Millions Level 1 investments Level 2 investments Fixed maturity investments Other long-term investments Hedge Funds and Private Equity Funds measured at NAV (3) Total Balance at December 31, 2017 $ 890.4 $ 2,105.4 $ — $ 87.2 $ 125.3 $ 3,208.3 (1)(2) Net realized and unrealized gains (losses) 51.9 (33.6 ) — (4.2 ) 17.2 31.3 (4) Amortization/Accretion .1 (2.3 ) — — — (2.2 ) Purchases 412.6 673.5 — 55.1 10.7 1,151.9 Sales (457.6 ) (1,565.0 ) — — (1.6 ) (2,024.2 ) Transfers in — — — — — — Transfers out — — — — — — Balance at September 30, 2018 $ 897.4 $ 1,178.0 $ — $ 138.1 $ 151.6 $ 2,365.1 (2) (1) Excludes carrying value of $(3.7) as of December 31, 2017 associated with foreign currency forward contracts. (2) Excludes carrying value of $ 176.1 and $ 313.7 at December 31, 2017 and September 30, 2018 classified as short-term investments. (3) Investments for which fair value is measured at NAV using the practical expedient are no longer classified within the fair value hierarchy. See Note 1 — “Basis of Presentation and Significant Accounting Policies” . (4) Excludes realized and unrealized losses associated with foreign currency forward contracts, foreign currency on cash and open trades and short-term investments of $3.5 , $4.2 and $0.7 for the nine months ended September 30, 2018. Level 3 Investments Millions Level 1 investments Level 2 investments Fixed maturity investments Other long-term investments Hedge Funds and Private Equity Funds measured at NAV (3) Total Balance at December 31, 2016 $ 279.5 $ 2,093.8 $ — $ 91.4 $ 82.6 $ 2,547.3 (1)(2)(4) Net realized and unrealized gains (losses) 52.4 59.8 — (2.2 ) 15.6 125.6 (5) Amortization/Accretion — (6.6 ) — — — (6.6 ) Purchases 940.7 1,038.5 31.2 2.9 64.9 2,078.2 Sales (539.0 ) (1,668.2 ) (12.5 ) (2.0 ) (8.2 ) (2,229.9 ) Deconsolidation of SSIE — (5.2 ) — — — (5.2 ) Transfers in — 13.1 — — — 13.1 Transfers out — — (13.1 ) — — (13.1 ) Balance at September 30, 2017 $ 733.6 $ 1,525.2 $ 5.6 $ 90.1 $ 154.9 $ 2,509.4 (1)(2) (1) Excludes carrying value of $ 175.0 and $ 786.5 at December 31, 2016 and September 30, 2017 classified as short-term investments, of which $0.1 is classified as held for sale at December 31, 2016 . (2) Excludes carrying value of $(1.2) and $(15.4) as of December 31, 2016 and September 30, 2017 associated with foreign currency forward contracts. (3) Investments for which fair value is measured at NAV using the practical expedient are no longer classified within the fair value hierarchy. See Note 1 — “Basis of Presentation and Significant Accounting Policies” . (4) Includes carrying value of $6.6 of fixed maturity investments at December 31, 2016 that is classified as assets held for sale related to SSIE. (5) Excludes realized and unrealized losses associated with foreign currency forward contracts of $23.1 for the nine months ended September 30, 2017. Fair Value Measurements — Transfers Between Levels - Nine-months ended September 30, 2018 and 2017 Transfers between levels are recorded using the fair value measurement as of the end of the quarterly period in which the event or change in circumstance giving rise to the transfer occurred. During the first nine months of 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 the first nine months of 2017, two fixed maturity investment classified as a Level 3 measurement in the prior period was 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 September 30, 2017. These measurements comprise “Transfers out” of Level 3 and “Transfers in” to Level 2 of $ 13.1 million for the period ended September 30, 2017. Significant Unobservable Inputs The following tables present significant unobservable inputs used in estimating the fair value of other long-term investments, other than hedge funds and private equity funds, classified within Level 3 as of September 30, 2018 and December 31, 2017 . The fair value of investments in hedge funds and private equity funds are generally estimated using the NAV of the funds. $ in millions, except share price September 30, 2018 Description Valuation Technique(s) Fair Value (1) Unobservable Input Private equity security Share price of most recent transaction $28.3 Share price - $56.60 Private equity security Discounted cash flow $21.0 Discount rate - 25.0% Private equity security Discounted cash flow $22.1 Discount rate - 35.0% Private convertible preferred security Discounted cash flow $14.5 Discount rate - 30.0% Private equity security Recent transaction $15.8 Share price - €338.85 Private preferred security Multiple of EBITDA $3.5 EBITDA multiple - 6.00 Private debt instrument Discounted cash flow $10.5 Discount rate - 9.62% (1) Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs. $ in millions, except share price December 31, 2017 Description Valuation Technique(s) Fair Value (1) Unobservable Input Private equity security Discounted cash flow $21.0 Discount rate - 25.0% Private equity security Discounted cash flow $22.1 Discount rate - 35.0% Private convertible preferred security Discounted cash flow $14.5 Discount rate - 30.0% Private equity security Discounted cash flow/ Option pricing method $11.3 Discount rate - 21.0% Time until expiration - 4 years Volatility/Standard deviation - 50.0% Risk free rate - 1.77% Private preferred security Multiple of EBITDA $0.6 EBITDA multiple - 6.00 Private equity security 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 A

Goodwill and Other Intangible Assets	9 Months Ended
	Sep. 30, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Other Intangible Assets	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. On May 11, 2018, White Mountains completed its acquisition of NSM, recognizing $383.0 million of goodwill and other intangible assets, reflecting acquisition date fair values. On May 18, 2018, NSM completed its acquisition of Fresh Insurance, recognizing $54.6 million of goodwill and other intangible assets, reflecting acquisition date fair values. The following tables presents the change in goodwill and other intangible assets: Three Months Ended September 30, 2018 2017 Millions Goodwill Other intangible assets Unallocated acquisition date fair value of intangible assets Total Goodwill and other intangible assets Goodwill Other intangible assets Total Goodwill and other intangible assets Beginning balance $ 25.9 $ 30.7 436.2 $ 492.8 $ 25.9 $ 14.4 $ 40.3 Attribution of acquisition date fair value estimates between goodwill and other intangible assets 295.3 140.9 (436.2 ) — — — — Foreign currency translation (1.2 ) (.4 ) — (1.6 ) — — — Adjustments to reflect acquisition date fair value 1.4 — — 1.4 — — — Amortization (1) — (7.4 ) — (7.4 ) — (2.5 ) (2.5 ) Ending balance $ 321.4 $ 163.8 — $ 485.2 $ 25.9 $ 11.9 $ 37.8 (1) Includes the NSM intangible assets amortization of $1.8 related to the period from May 11, 2018 through June 30, 2018 and $3.2 for the three months ended September 30, 2018. Nine Months Ended September 30, 2018 2017 Millions Goodwill Other intangible assets Total Goodwill and other intangible assets Goodwill Other intangible assets Total Goodwill and other intangible assets Beginning balance $ 25.9 $ 36.2 $ 62.1 $ 25.9 $ 19.3 $ 45.2 Attribution of acquisition date fair value estimates between goodwill and other intangible assets 295.3 140.9 436.2 — — — Foreign currency translation (1.2 ) (.4 ) (1.6 ) — — — — Adjustments to reflect acquisition date fair value 1.4 — 1.4 — — — — Amortization — (12.9 ) (12.9 ) — (7.4 ) (7.4 ) Ending balance $ 321.4 $ 163.8 $ 485.2 $ 25.9 $ 11.9 $ 37.8 The following table presents the acquisition date values, accumulated amortization and net carrying values for other intangible assets and goodwill, by company as of September 30, 2018 and December 31, 2017 : $ in millions September 30, 2018 December 31, 2017 Acquisition date fair value Accumulated amortization Net carrying value Acquisition date fair value Accumulated amortization Net carrying value Goodwill: NSM (1) $ 295.5 $ — $ 295.5 $ — $ — $ — MediaAlpha 18.3 — 18.3 18.3 — 18.3 Buzzmove 7.6 — 7.6 7.6 — 7.6 Total goodwill 321.4 — 321.4 25.9 — 25.9 Other intangible assets: NSM (1) Customer relationships 85.5 3.6 81.9 — — — Trade names 51.2 1.1 50.1 — — — Information technology 3.8 .3 3.5 — — — Subtotal 140.5 5.0 135.5 — — — MediaAlpha Customer relationships 26.8 4.3 22.5 26.8 2.4 24.4 Information technology 33.3 29.2 4.1 33.3 24.3 9.0 Other 9.8 8.8 1.0 9.8 7.8 2.0 Subtotal 69.9 42.3 27.6 69.9 34.5 35.4 Buzzmove Trade names .6 .1 .5 .6 .1 .5 Information technology .5 .3 .2 .5 .2 .3 Subtotal 1.1 .4 .7 1.1 .3 .8 Total other intangible assets 211.5 47.7 163.8 71.0 34.8 36.2 Total goodwill and other intangible assets $ 532.9 $ 47.7 485.2 $ 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 $ 444.6 $ 41.0 (1) Includes the effect of foreign currency translation from the date of acquisition of $1.2 for goodwill and $0.4 for other intangible assets.

Debt

Debt	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Debt	Debt The following table presents White Mountains’s debt outstanding as of September 30, 2018 and December 31, 2017 : Millions September 30, Effective Rate (1) December 31, Effective Rate (1) WTM Bank Facility $ — N/A $ — N/A NSM Bank Facility 150.6 7.5% — Unamortized issuance cost (3.3 ) — NSM Bank Facility, carrying value 147.3 — Other NSM debt 2.0 — MediaAlpha Bank Facility 15.1 6.8% 23.9 5.6% Unamortized issuance cost (.1 ) (.1 ) MediaAlpha Bank Facility, carrying value 15.0 23.8 Total debt $ 164.3 $ 23.8 (1) Effective rate considers the effect of the debt issuance costs. WTM Bank Facility On August 14, 2013, the Company entered into a revolving credit facility with a syndicate of lenders administered by Wells Fargo Bank, N.A., which had a total commitment of $425.0 million and a maturity date of August 14, 2018 (the “WTM Bank Facility”). White Mountains terminated the WTM Bank Facility on May 8, 2018. 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 debt and to fund the acquisition of Fresh Insurance. The NSM Bank Facility is comprised of a term loan of $100.0 million , a delayed-draw term loan of $51.0 million to fund the Fresh Insurance 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 three months ended September 30, 2018 and the period from May 11, 2018 through September 30, 2018, NSM repaid $0.4 million on the term loans. During the period from May 11, 2018 through September 30, 2018, NSM repaid $2.0 million on the revolving credit loan. As of September 30, 2018 , $150.6 million of term loans were outstanding and no revolving credit loans were outstanding under the NSM Bank Facility. 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 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 based on then-current LIBOR. The variable rate received by NSM under the swap agreement was 2.07% . As of September 30, 2018 , NSM’s blended interest rate on the outstanding term loan principal amount of $150.6 million was 6.69% , and 7.47% after consideration of the interest rate swap. See Note 7 — “Derivatives — NSM Interest Rate Swap” . 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 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 $15.1 million as of September 30, 2018 , and a revolving credit loan facility for $10.0 million , which was undrawn as of September 30, 2018 . The MediaAlpha Bank Facility carries a variable interest rate that is based on the Prime Rate, as published by the Wall Street Journal, plus a spread of 1.50% on the term loan facility and 0.25% on the revolving credit loan facility as of September 30, 2018 . During the three and nine months ended September 30, 2018 , MediaAlpha repaid $0.8 million and $2.8 million on the term loan and $4.0 million and $6.0 million on the revolving credit loan under the MediaAlpha Bank Facility. The MediaAlpha Bank Facility is secured by intellectual property and the common stock of MediaAlpha’s subsidiaries, and contains various affirmative, negative and financial covenants that White Mountains considers to be customary for such borrowings, including a fixed charge coverage ratio and an asset coverage ratio covenant. Other 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 September 30, 2018 , the secured term loan facility has an outstanding balance of $2.3 million . The acquisition date fair value of the debt was $2.0 million . Compliance At September 30, 2018 , White Mountains was in compliance with the covenants under all of its debt instruments.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company and its Bermuda domiciled subsidiaries are not subject to Bermuda income tax under current Bermuda law. If there is a change in the law to impose tax, the Company and its Bermuda domiciled subsidiaries would be exempt from such tax until March 31, 2035, pursuant to the Bermuda Exempted Undertakings Tax Protection Act of 1966. The Company has subsidiaries and branches that operate in various other jurisdictions around the world that are subject to tax in the jurisdictions in which they operate. The jurisdictions in which the Company’s subsidiaries and branches are subject to tax are Barbados, Gibraltar, Ireland, Israel, Luxembourg, the United Kingdom and the United States. White Mountains’s income tax benefit related to pre-tax income from continuing operations for the three months ended September 30, 2018 and pre-tax loss from continuing operations for the nine months ended September 30, 2018 represented an effective tax rate of (8.1)% and 1.2% . The effective tax rate was different from the current U.S. statutory rate of 21% , primarily due to a full valuation allowance on most of the net deferred tax assets at U.S. operations, withholding taxes and a tax benefit recorded at BAM. For BAM, member surplus contributions (“MSC”) and the related taxes thereon are recorded directly to non-controlling interest equity, while the valuation allowance on such taxes is recorded through the income statement. For the three and nine months ended September 30, 2018 , BAM recorded a tax benefit of $1.8 million and $4.0 million associated with the valuation allowance on taxes related to MSC that is included in the effective tax rate. White Mountains’s income tax benefit related to pre-tax income from continuing operations for the three and nine months ended September 30, 2017 represented an effective tax rate of (47.6)% and (203.9)% . The effective tax rate was different from the 2017 U.S. statutory rate of 35% , primarily due to a full valuation allowance on all net deferred tax assets at U.S. operations, pre-tax income from continuing operations being near break-even and a tax benefit recorded at BAM. For BAM, MSC and the related taxes thereon are recorded directly to non-controlling interest equity, while the valuation allowance on such taxes is recorded through the income statement. For the three and nine months ended September 30, 2017, BAM recorded a tax benefit of $2.1 million and $6.9 million associated with the valuation allowance on taxes related to MSC that is included in the effective tax rate. In arriving at the effective tax rate for the three and nine months ended September 30, 2018 and 2017, White Mountains forecasted all income and expense items including the change in unrealized investment gains (losses) and realized investment gains (losses) for the years ending December 31, 2018 and 2017. White Mountains records a valuation allowance against deferred tax assets if it becomes more likely than not that all or a portion of a deferred tax asset will not be realized. Changes in valuation allowances from period to period are included in income tax expense in the period of change. In determining whether or not a valuation allowance, or change therein, is warranted, White Mountains considers factors such as prior earnings history, expected future earnings, carryback and carryforward periods and strategies that if executed would result in the realization of a deferred tax asset. With few exceptions, White Mountains is no longer subject to U.S. federal, state, or non-U.S. income tax examinations by tax authorities for years before 2013.

Derivatives

Derivatives	9 Months Ended
	Sep. 30, 2018
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivatives	Derivatives
Interest Rate Swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivatives	NSM Interest Rate Swap On May 11, 2018, NSM entered into the NSM Bank Facility. 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 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 then-current LIBOR. The variable rate received by NSM under the swap agreement was 2.07% at September 30, 2018. Over the term of the swap, the notional amount decreases in accordance with the principal repayments NSM expects to make on its term loans. As of September 30, 2018, NSM’s blended interest rate on the outstanding term loan principal amount of $150.6 million was 6.69% , and 7.47% after consideration of the interest rate swap. NSM’s obligations under the swap are secured by the same collateral securing the NSM Bank Facility on a pari passu basis. NSM does not currently hold any collateral deposits from or provide any collateral deposits to the swap counterparty. NSM evaluated the effectiveness of the swap to hedge its interest rate risk associated with its variable rate debt and concluded at the swap inception date that the swap was highly effective in hedging that risk. NSM will evaluate the effectiveness of the hedging relationship on an ongoing basis. For the three months ended September 30, 2018 and the period from May 11, 2018 through September 30, 2018, NSM recognized net interest expense of $0.3 million and $0.4 million for the periodic net settlement payments on the swap. As of September 30, 2018, the estimated fair value of the swap and the accrual of the periodic net settlement payments recorded in other liabilities was $0.1 million . There was no ineffectiveness in the hedge for the three months ended September 30, 2018 and the period from May 11, 2018 through September 30, 2018. Accordingly, the full amount of the change in the fair value of the swap of $1.0 million and $(0.1) million for the three months ended September 30, 2018 and the period from May 11, 2018 through September 30, 2018 are included within accumulated other comprehensive income (loss).
Forward Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivatives	White Mountains’s investment portfolio includes investments denominated in Japanese Yen, Euros, GBP and other foreign currencies. White Mountains previously entered into foreign currency forward contracts to manage its foreign currency exposure related to these investments. In 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. White Mountains no longer has any open foreign currency forward contracts. As of December 31, 2017, White Mountains held $206.3 million total gross notional value of a foreign currency forward contract with a carrying value of $(3.7) million . The derivative (losses) gains recognized in net realized and unrealized investment gains (losses) for the nine months ended September 30, 2018 were $(3.5) million . The derivative (losses) gains recognized in net realized and unrealized investment gains (losses) for the three and nine months ended September 30, 2017 were $(10.0) million and $(23.1) million .

Municipal Bond Guarantee

Municipal Bond Guarantee	9 Months Ended
	Sep. 30, 2018
Guarantees [Abstract]
Municipal Bond Guarantee Insurance	Municipal Bond Guarantee Insurance In 2012, HG Global was capitalized with $594.5 million from White Mountains and $14.5 million from non-controlling interests to fund BAM, a newly formed mutual municipal bond insurer. As of September 30, 2018 , White Mountains owned 96.9% of HG Global’s preferred equity and 88.4% of its common equity. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of BAM Surplus Notes. At inception, BAM and HG Re also entered into a first loss reinsurance treaty (“FLRT”). HG Re provides first loss reinsurance protection up to 15% of par outstanding on each municipal bond insured by BAM. In return, BAM cedes 60% of the risk premium charged for insuring the municipal bond, net of a ceding commission. During 2017, HG Global and BAM made certain changes to the ceding commission arrangements under the FLRT. These changes serve to accelerate growth in BAM's statutory capital but do not impact the net risk premium ceded from BAM to HG Re. HG Re’s obligations to BAM are collateralized in trusts, and there is an aggregate loss limit that is equal to the total assets in the collateral trusts at any point in time. The interest rate on the BAM Surplus Notes for the five years ending December 31, 2018 is a variable rate equal to the one -year U.S. treasury rate plus 300 basis points, set annually, which is 4.60% for 2018 and was 3.78% for 2017. Prior to the end of 2018, BAM has the option to extend the variable rate period for an additional three years. At the end of the variable rate period, the interest rate will be fixed at the higher of the then current variable rate or 8.0% . No payment of interest or principal on the BAM Surplus Notes may be made without the approval of the New York State Department of Financial Services (“NYDFS”). BAM has stated its intention to seek regulatory approval to pay interest and principal on its surplus notes only to the extent that its remaining qualified statutory capital and other capital resources continue to support its outstanding obligations, business plan and its AA stable rating from S&P. BAM repaid $4.0 million of the BAM Surplus Notes and $1.0 million of the related accrued interest in 2017. There were no repayments for the three and nine months ended September 30, 2018 . In order to further support BAM’s long-term capital position and business prospects, in 2017 HG Global agreed to contribute the $203.0 million of Series A BAM Surplus Notes (“Series A Notes”) into the supplemental collateral trust (the “Supplemental Trust”) at HG Re. The Supplemental Trust already held the $300.0 million of Series B BAM Surplus Notes (“Series B Notes”). Assets held in the Supplemental Trust serve to collateralize HG Re’s obligations to BAM under the FLRT. HG Global and BAM also changed the payment terms of the Series B Notes, so that payments will reduce principal and accrued interest on a pro rata basis, consistent with the payment terms on the Series A Notes. The terms of the Series B Notes had previously stipulated that payments would first reduce interest owed, then reduce principal owed once all accrued interest had been paid. The Supplemental Trust target balance is equal to approximately $603.0 million . As the BAM Surplus Notes are repaid over time, the BAM Surplus Notes will be replaced in the Supplemental Trust by cash and fixed income securities. The collateral trust balances must be at target levels before capital can be distributed out of the Supplemental Trust. In connection with the contribution, the Series A Notes were merged with the Series B Notes. As of September 30, 2018 and December 31, 2017, the collateral trusts held assets of $741.7 million and $715.1 million , which both included $499.0 million of BAM Surplus Notes. As of September 30, 2018 and December 31, 2017, HG Global has accrued $143.2 million and $126.0 million of interest receivable on the BAM Surplus Notes. The following table presents a schedule of BAM’s insured obligations: September 30, 2018 December 31, 2017 Contracts outstanding 7,184 6,371 Remaining weighted average contract period outstanding (in years) 10.8 10.9 Contractual debt service outstanding (in millions): Principal $ 47,259.1 $ 42,090.6 Interest 23,707.4 21,057.1 Total debt service outstanding $ 70,966.5 $ 63,147.7 Gross unearned insurance premiums $ 156.2 $ 136.8 The following table presents a schedule of BAM’s future premium revenues as of September 30, 2018 : Millions September 30, 2018 October 1, 2018 - December 31, 2018 $ 3.3 January 1, 2019 - March 31, 2019 3.3 April 1, 2019 - June 30, 2019 3.3 July 1, 2019 - September 30, 2019 3.3 October 1, 2019 - December 31, 2019 3.2 Total 2019 13.1 2020 12.6 2021 12.1 2022 11.5 2023 and thereafter 103.6 Total gross unearned insurance premiums $ 156.2 The following table presents a schedule of net written premiums included in White Mountains’s HG Global/BAM segment for the three months years ended September 30, 2018 and 2017 : Three Months Ended September 30, Nine Months Ended September 30, Millions 2018 2017 2018 2017 Gross written premiums $ 3.9 $ 10.9 $ 28.9 $ 42.0 Assumed (ceded) written premiums — — — — Net written premiums $ 3.9 $ 10.9 $ 28.9 $ 42.0 In April 2018, BAM entered into a collateralized financial guarantee excess of loss reinsurance agreement with Fidus Re, a special purpose insurer. The agreement has a term of twelve years, but is callable by BAM after five years, and covers all polices written prior to December 31, 2017, excluding non-investment grade bonds and surety policies. 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. Losses above BAM's $165.0 million retention will be split pro rata between BAM and Fidus Re, with BAM retaining 10% and Fidus Re assuming the remaining 90% of the losses under the agreement. The losses covered under the agreement may not exceed the principal and interest amount of the underlying bond guarantee during the risk period. The aggregate loss limit under the agreement is $276.1 million . Because the agreement does not meet the risk transfer requirements necessary to be accounted for as reinsurance, the agreement is accounted for using deposit accounting and any related financing expenses are recorded in general and administrative expenses.

Earnings Per Share

Earnings Per Share	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Earnings Per Share	Earnings Per Share White Mountains calculates earnings per share using the two-class method, which allocates earnings between common shares and unvested restricted common shares. Both classes of shares participate equally in dividends and earnings on a per share basis. Basic earnings per share amounts are based on the weighted average number of common shares outstanding adjusted for unvested restricted common shares. The following table presents the Company’s computation of earnings per share from continuing operations for the three months ended September 30, 2018 and 2017 . See Note 16 — “Held for Sale and Discontinued Operations” . Three Months Ended Nine Months Ended September 30, September 30, 2018 2017 2018 2017 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 40.8 $ 562.1 $ (3.7 ) $ 604.7 Less: total income from discontinued operations, net of tax (17.3 ) 539.1 (17.2 ) 573.2 Net income from continuing operations attributable to White Mountains’s common shareholders $ 58.1 $ 23.0 $ 13.5 $ 31.5 Allocation of earnings to participating restricted common shares (1) (.8 ) (.3 ) (.2 ) (.4 ) Basic and diluted earnings per share numerators $ 57.3 $ 22.7 $ 13.3 $ 31.1 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 3,180.4 4,297.2 3,450.8 4,477.0 Average unvested restricted common shares (2) (41.5 ) (53.7 ) (39.7 ) (54.5 ) Basic earnings per share denominator 3,138.9 4,243.5 3,411.1 4,422.5 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 3,180.4 4,297.2 3,450.8 4,477.0 Average unvested restricted common shares (2) (41.5 ) (53.7 ) (39.7 ) (54.5 ) Diluted earnings per share denominator 3,138.9 4,243.5 3,411.1 4,422.5 Basic and diluted earnings per share (in dollars) - continuing operations: Distributed earnings - dividends declared and paid $ — $ — $ 1.00 $ 1.00 Undistributed earnings (losses) 18.27 5.36 2.90 6.03 Basic and diluted earnings per share $ 18.27 $ 5.36 $ 3.90 $ 7.03 (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (2) Restricted shares outstanding vest either in equal annual installments or upon a stated date. See Note 10 — “Employee Share-Based Incentive Compensation Plans” The following table presents the undistributed net earnings (losses) from continuing operations for the three months ended September 30, 2018 and 2017 . See Note 16 — “Held for Sale and Discontinued Operations” . Three Months Ended Nine Months Ended September 30, September 30, Millions 2018 2017 2018 2017 Undistributed net income (losses) - continuing operations: Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 57.3 $ 22.7 $ 13.3 $ 31.1 Dividends declared net of restricted common share amounts (1) — — (3.7 ) (4.5 ) Total undistributed net losses, net of restricted common share amounts $ 57.3 $ 22.7 $ 9.6 $ 26.6 (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.

Employee Share-Based Incentive

Employee Share-Based Incentive Compensation Plans	9 Months Ended
	Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Employee Share-Based Incentive Compensation Plans	Employee Share-Based Incentive Compensation Plans White Mountains’s Long-Term Incentive Plan (the “WTM Incentive Plan”) provides for grants of various types of share-based and non share-based incentive awards to key employees of White Mountains. As of September 30, 2018 , White Mountains’s share-based compensation incentive awards consist of performance shares and restricted shares. Performance Shares Performance shares are conditional grants of a specified maximum number of common shares or an equivalent amount of cash. Awards generally vest at the end of a three -year period, are subject to the attainment of pre-specified performance goals, and are valued based on the market value of common shares at the time awards are approved for payment. The following table presents the performance share activity for the three and nine months ended September 30, 2018 and 2017 for performance shares granted under the WTM Incentive Plan: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Millions, except share amounts Target Performance Accrued Expense Target Performance Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Beginning of period 40,616 $ 27.2 50,575 $ 31.2 50,515 $ 45.8 80,353 $ 42.4 Shares paid (1) — — — — (23,186 ) (28.4 ) (30,838 ) (21.6 ) New grants — — — — 14,105 — 17,510 — Forfeitures and cancellations (2) — (.1 ) (256 ) (.4 ) (818 ) .1 (16,706 ) (9.4 ) Expense recognized — 7.8 — 7.0 — 17.4 — 26.4 End of period (3) 40,616 $ 34.9 50,319 $ 37.8 40,616 $ 34.9 50,319 $ 37.8 (1) WTM performance share payments in 2018 for the 2015-2017 performance cycle, which were paid in March 2018, ranged from 145% to 147% of target. WTM performance share payments in 2017 for the 2014-2016 performance cycle, which were paid in March 2017, ranged from 34% to 76% of target. (2) Amounts include changes in assumed forfeitures, as required under GAAP. (3) Outstanding performance share awards as of September 30, 2017 excludes 2,195 performance share awards granted to employees of Sirius Group. For performance shares earned in the 2015-2017 and 2014-2016 performance cycles, all performance shares earned were settled in cash. If all the outstanding WTM performance shares had vested on September 30, 2018 , the total additional compensation cost to be recognized would have been $20.4 million , based on accrual factors (common share price and payout assumptions) at September 30, 2018 . The following table presents performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan at September 30, 2018 for each performance cycle: Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Performance cycle: 2016 – 2018 13,715 $ 18.9 2017 – 2019 14,070 13.3 2018 – 2020 13,450 3.2 Sub-total 41,235 35.4 Assumed forfeitures (619 ) (.5 ) September 30, 2018 40,616 $ 34.9 Restricted Shares The following table presents the unrecognized compensation cost associated with the outstanding restricted share awards for the three and nine months ended September 30, 2018 and 2017 : Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Millions, except share amounts Restricted Unamortized Restricted Unamortized Restricted Unamortized Restricted Unamortized Non-vested, Beginning of period 41,510 $ 19.0 53,815 $ 23.5 53,755 $ 14.3 70,620 $ 19.7 Issued — — — — 14,105 11.4 17,785 16.7 Vested — — (260 ) — (25,381 ) — (28,846 ) — Forfeited — — — — (969 ) (.2 ) (6,004 ) (3.5 ) Expense recognized — (3.2 ) — (3.8 ) — (9.7 ) — (13.2 ) End of period (1) 41,510 $ 15.8 53,555 $ 19.7 41,510 $ 15.8 53,555 $ 19.7 (1) Restricted share awards outstanding as of September 30, 2017 includes 2,195 restricted shares issued to employees of Sirius Group, which was accounted for as discontinued operations. During the first quarter of 2018, White Mountains issued 13,450 restricted shares that vest on January 1, 2021, 290 restricted shares that vest on January 1, 2020 and 365 restricted shares that vest on January 1, 2019. During the second quarter of 2017, White Mountains issued 550 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 the first quarter of 2017, White Mountains issued 16,735 restricted shares that vest on January 1, 2020. The unamortized issue date fair value at September 30, 2018 is expected to be recognized ratably over the remaining vesting periods. Non-Qualified Options As of January 20, 2017, the 125,000 Non-Qualified options issued to the Company’s former Chairman and CEO were exercised. During the first quarter of 2017, 40,000 Non-Qualified Options, with an intrinsic value of $4.4 million , were exercised in exchange for 5,142 common shares with an equal total market value. The intrinsic value represents the difference between the market price of the Company’s common shares at the date of exercise and the fixed strike price of $742 per common share. The Non-Qualified Options were fully amortized as of 2011. MediaAlpha Class B Unit Awards MediaAlpha has issued Class B unit awards to certain employees. The units entitle the award recipient to participate in distributions from MediaAlpha, subject to a cumulative distribution threshold, which is a performance condition, and a service period. The grant date fair value of the awards is determined when it is deemed probable that the distribution threshold will be met. The service period ranges from 36 months to 48 months. For the three and nine months ended September 30, 2018 , MediaAlpha recognized $0.2 million and $6.9 million of compensation expense for the vested portion of the awards for which achievement of the performance award is now probable, and $0.1 million of unearned compensation expense for unvested awards, which will be recognized over the remaining service periods of the awards.

Non-controlling Interest

Non-controlling Interest	9 Months Ended
	Sep. 30, 2018
Noncontrolling Interest [Abstract]
Non-controlling Interests	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 September 30, 2018 and December 31, 2017 : September 30, 2018 December 31, 2017 $ in millions Non-controlling Percentage Non-controlling Equity Non-controlling Percentage Non-controlling Equity Other, excluding mutuals HG Global 3.1 % $ 14.8 3.1 % $ 15.9 NSM 5.0 14.1 — — MediaAlpha 37.8 15.3 35.7 13.1 Buzzmove 22.9 1.5 22.9 2.5 Total other, excluding mutuals 45.7 31.5 Mutuals BAM 100.0 (190.1 ) 100.0 (163.2 ) Total non-controlling interests $ (144.4 ) $ (131.7 )

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2018
Segment Reporting [Abstract]
Segment Informartion	Segment Information White Mountains has determined that its reportable segments are HG Global/BAM, NSM, MediaAlpha and Other Operations. White Mountains has made its segment determination based on consideration of the following criteria: (i) the nature of the business activities of each of the Company’s subsidiaries and affiliates; (ii) the manner in which the Company’s subsidiaries and affiliates are organized; (iii) the existence of primary managers responsible for specific subsidiaries and affiliates; and (iv) the organization of information provided to the chief operating decision makers and the Board of Directors. Significant intercompany transactions among White Mountains’s segments have been eliminated herein. The following table presents the financial information for White Mountains’s segments: Millions HG Global/BAM NSM MediaAlpha Other Operations Total Three Months Ended September 30, 2018 Earned insurance premiums $ 3.3 $ — $ — $ — $ 3.3 Net investment income 4.8 — — 8.6 13.4 Net realized and unrealized investment (losses) gains (4.1 ) — — 70.2 66.1 Advertising and commission revenues (1) — 36.6 74.5 1.1 112.2 Other revenue .2 3.1 — .4 3.7 Total revenues 4.2 39.7 74.5 80.3 198.7 Insurance acquisition expenses 1.2 — — — 1.2 Other underwriting expenses .1 — — — .1 Cost of sales — — 61.8 1.1 62.9 General and administrative expenses 11.3 25.8 5.4 26.0 68.5 Broker commission expense — 10.9 — — 10.9 Amortization of other intangible assets — 5.0 2.4 — 7.4 Interest expense — 3.2 .2 — 3.4 Total expenses 12.6 44.9 69.8 27.1 154.4 Pre-tax (loss) income $ (8.4 ) $ (5.2 ) $ 4.7 $ 53.2 $ 44.3 (1) Approximately 30% of MediaAlpha’s advertising revenue for the three months ended September 30, 2018 was associated with one customer. Millions HG Global/BAM MediaAlpha Other Operations Total Three Months Ended September 30, 2017 Earned insurance premiums $ 2.4 $ — $ — $ 2.4 Net investment income 3.3 — 8.9 12.2 Net realized and unrealized investment gains .8 — 31.7 32.5 Advertising and commission revenues (1) — 37.9 .9 38.8 Other revenue .2 — 1.4 1.6 Total revenues 6.7 37.9 42.9 87.5 Insurance acquisition expenses .9 — — .9 Other underwriting expenses .1 — — .1 Cost of sales — 32.2 .9 33.1 General and administrative expenses 10.6 3.8 27.3 41.7 Amortization of other intangible assets — 2.3 .1 2.4 Interest expense — .1 .8 .9 Total expenses 11.6 38.4 29.1 79.1 Pre-tax (loss) income $ (4.9 ) $ (.5 ) $ 13.8 $ 8.4 (1) Approximately 32% of MediaAlpha’s advertising revenue for the three months ended September 30, 2017 was associated with one customer. Millions HG Global/BAM NSM (2) MediaAlpha Other Operations Total Nine Months Ended September 30, 2018 Earned insurance premiums $ 9.7 $ — $ — — $ 9.7 Net investment income 12.5 — — 32.4 44.9 Net realized and unrealized investment (losses) gains (14.4 ) — — 37.3 22.9 Advertising and commission revenues (1) — 59.2 216.4 3.0 278.6 Other revenue .8 4.0 1.6 .6 7.0 Total revenues 8.6 63.2 218.0 73.3 363.1 Insurance acquisition expenses 3.9 — — — 3.9 Other underwriting expenses .3 — — — .3 Cost of sales — — 179.1 2.9 182.0 General and administrative expenses 36.5 37.9 21.2 79.0 174.6 Broker commission expense — 17.5 — — 17.5 Amortization of other intangible assets — 5.0 7.8 .1 12.9 Interest expense — 4.8 .9 .3 6.0 Total expenses 40.7 65.2 209.0 82.3 397.2 Pre-tax (loss) income $ (32.1 ) $ (2.0 ) $ 9.0 $ (9.0 ) $ (34.1 ) (1) Approximately 31% of MediaAlpha’s advertising revenue for the nine months ended September 30, 2018 was associated with one customer. (2) NSM’s results are from May 11, 2018, the date of acquisition, to the end of the third quarter. Millions HG Global/BAM MediaAlpha Other Operations Total Nine Months Ended September 30, 2017 Earned insurance premiums $ 6.6 $ — $ — $ 6.6 Net investment income 8.9 — 30.8 39.7 Net realized and unrealized investment gains 3.2 — 99.3 102.5 Advertising and commission revenues (1) — 101.2 2.7 103.9 Other revenue .8 — 6.3 7.1 Total revenues 19.5 101.2 139.1 259.8 Insurance acquisition expenses 3.0 — — 3.0 Other underwriting expenses .3 — — .3 Cost of sales — 86.0 2.7 88.7 General and administrative expenses 31.5 10.7 113.9 156.1 Amortization of other intangible assets — 7.2 .1 7.3 Interest expense — .6 1.2 1.8 Total expenses 34.8 104.5 117.9 257.2 Pre-tax (loss) income $ (15.3 ) $ (3.3 ) $ 21.2 $ 2.6 (1) Approximately 29% of MediaAlpha’s advertising revenue for the nine months ended September 30, 2017 was associated with one customer.

Investments in Unconsolidated E

Investments in Unconsolidated Entities	9 Months Ended
	Sep. 30, 2018
Equity Method Investments and Joint Ventures [Abstract]
Investments in Unconsolidated Entities	White Mountains’s investments in unconsolidated entities are included within other long-term investments and consist of investments in common equity securities or similar instruments, which give White Mountains the ability to exert significant influence over the investee’s operating and financial policies (“equity method eligible unconsolidated entities”). Such investments may be accounted for under either the equity method or, alternatively, White Mountains may elect to account for them under the fair value option. The following table presents the carrying values of investments in equity method eligible unconsolidated entities recorded within other long-term investments: Millions September 30, 2018 December 31, 2017 Equity method eligible private equity securities, at fair value $ 97.3 $ 58.0 Investments, accounted for under the equity method 11.6 4.6 Total investments in equity method eligible unconsolidated entities 108.9 62.6 Other unconsolidated investments (1) 180.8 146.2 Total other long-term investments $ 289.7 $ 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 September 30, 2018 and December 31, 2017: Ownership Interest Investee September 30, 2018 December 31, 2017 Instrument Held PassportCard (1) 50.0% 50.0% Common shares DavidShield (1) 50.0% — Common shares Kudu 49.5% — Units durchblicker 45.0% 45.0% Common shares Compare.com 22.1% 22.1% Common shares Tuckerman Capital Fund III, L.P. 21.8% 21.3% Limited partnership interest (1) At September 30, 2018, White Mountains's ownership interest in PassportCard comprised a 25% direct ownership interest and a 25% indirect interest through DavidShield. At December 31, 2017, White Mountains's ownership interest was a 50% direct ownership interest. See Note 2 — “Significant Transactions”. For the three and nine months ending September 30, 2018, White Mountains recorded $4.2 million and $8.1 million associated with Tuckerman Capital Fund III L.P. (“Tuckerman III, L.P.”) within unrealized investment gains (losses) and net investment income. Financial data submitted by Tuckerman III L.P. is on a one-quarter lag. For the three and nine trailing months ended June 30, 2018, Tuckerman III L.P.’s total net income was $ 31.9 million and $ 43.7 million . For the three and nine trailing months ended June 30, 2017, Tuckerman III L.P.’s total net income was $3.2 million and $7.1 million . As of June 30, 2018 and December 31, 2017, Tuckerman III, L.P.’s total assets were $61.9 million and $21.8 million .

Fair Value of Financial Instrum

Fair Value of Financial Instruments	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	Fair Value of Financial Instruments White Mountains accounts for all 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 table presents the fair value and carrying value of this financial instrument as of September 30, 2018 and December 31, 2017 : September 30, 2018 December 31, 2017 Millions Fair Value Carrying Value Fair Value Carrying Value NSM Bank Facility $ 147.3 $ 147.3 $ — $ — MediaAlpha Bank Facility $ 15.3 $ 15.0 $ 23.9 $ 23.8 The fair value estimates for the MediaAlpha Bank Facility and the NSM Bank Facility have been determined based on a discounted cash flows approach and are considered to be Level 3 measurements.

Contingencies

Contingencies	9 Months Ended
	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Contingencies	Contingencies Legal Contingencies White Mountains is subject to litigation and arbitration in the normal course of business. White Mountains considers the requirements of ASC 450 when evaluating its exposure to litigation and arbitration. ASC 450 requires that accruals be established for litigation and arbitration if it is probable that a loss has been incurred and it can be reasonably estimated. ASC 450 also requires that litigation and arbitration be disclosed if it is probable that a loss has been incurred or if there is a reasonable possibility that a loss may have been incurred. White Mountains does not have any current litigation that may have a material adverse effect on White Mountains’s financial condition, results of operations or cash flows. The following description presents significant legal contingencies, ongoing non-claims related litigation or arbitration as of September 30, 2018 : Sirius Group Tax Contingency In late 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 2016. In connection with the sale, White Mountains indemnified Sirius Group against the loss of these interest deductions. As a result, in the third quarter of 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 intends to appeal 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 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 greater than $82.5 million and, accordingly, no accrual of a liability is necessary at September 30, 2018 .

Held for Sale and Discontinued

Held for Sale and Discontinued Operations	9 Months Ended
	Sep. 30, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Held for Sale and Discontinued Operations	Held for Sale and Discontinued Operations OneBeacon On September 28, 2017, Intact Financial Corporation completed its acquisition of OneBeacon in an all-cash transaction for $18.10 per share. White Mountains received total proceeds of $1.3 billion and recorded a gain of $554.6 million , net of transaction costs. During the three months ended September 30, 2017, net loss from discontinued operations related to OneBeacon was $(15.2) million . For 2017 through the closing date of the transaction, net income from discontinued operations related to OneBeacon was $20.5 million . Tranzact On July 21, 2016, White Mountains completed its sale of Tranzact to Clayton, Dubilier & Rice, LLC. During the nine months ended September 30, 2017, White Mountains recorded a $1.0 million reduction to the gain from sale of Tranzact in discontinued operations as a result of 2016 state tax payments. Sirius Group On April 18, 2016, White Mountains completed the sale of Sirius Group to CMI. During the three and nine months ended September 30, 2018, White Mountains recorded a $17.3 million loss and $17.2 million loss from the sale of Sirius Group which includes the tax contingency loss of $17.3 million recorded during the third quarter of 2018. See Note 15 — “Contingencies”. During the nine months ended September 30, 2018, White Mountains recorded a $0.1 million gain from the sale of Sirius Group as a result of a change to the valuation of the accrued incentive compensation payable to Sirius Group employees. During the three and nine months ended September 30, 2017, White Mountains recorded a $0.2 million and $0.8 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. Other As of December 31, 2017, White Mountains has classified its Guilford, Connecticut property, which consists of an office building and adjacent land, as held for sale. The property has been measured at its estimated fair value net of costs of disposal, of $3.3 million as of September 30, 2018 and December 31, 2017.
Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Held for Sale and Discontinued Operations	Net Income (Loss) from Discontinued Operations For the three and nine months ended September 30, 2018 , White Mountains recognized net loss from sale of discontinued operations of $17.3 million and $17.2 million , which was primarily due to a tax contingency related to the sale of Sirius Group. See Note 15 — “Contingencies”. The following table presents the results of operations, including related income taxes, associated with the business classified as discontinued operations for the three and nine months ended September 30, 2017 . 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 inured to White Mountains. Given the fixed price nature of the OneBeacon Transaction, OneBeacon’s results were economically transferred to the buyer at signing. Three Months Ended Nine Months Ended September 30, 2017 September 30, 2017 Millions OneBeacon Sirius Group Total OneBeacon Sirius Group Tranzact Total Revenues Earned insurance premiums $ 268.4 $ — $ 268.4 $ 807.6 $ — $ — $ 807.6 Net investment income 13.0 — 13.0 39.7 — — 39.7 Net realized and unrealized gains 11.5 — 11.5 38.8 — — 38.8 Other revenue 2.2 — 2.2 7.7 — — 7.7 Total revenues 295.1 — 295.1 893.8 — — 893.8 Expenses Loss and loss adjustment expenses 206.8 — 206.8 546.0 — — 546.0 Insurance and reinsurance acquisition expenses 51.9 — 51.9 145.6 — — 145.6 Other underwriting expenses 44.9 — 44.9 156.2 — — 156.2 General and administrative expenses 7.4 — 7.4 21.2 — — 21.2 Interest expense 3.4 — 3.4 10.0 — — 10.0 Total expenses 314.4 — 314.4 879.0 — — 879.0 Pre-tax (loss) income (19.3 ) — (19.3 ) 14.8 — — 14.8 Income tax benefit 4.1 — 4.1 5.7 — — 5.7 Net (loss) income from discontinued operations (15.2 ) — (15.2 ) 20.5 — — 20.5 Net gain (loss) from sale of discontinued operations 554.5 (.2 ) 554.3 554.5 (.8 ) (1.0 ) 552.7 Total income (loss) from discontinued operations 539.3 (.2 ) 539.1 575.0 (.8 ) (1.0 ) 573.2 Change in foreign currency translation and other from discontinued operations .1 — .1 .3 — — .3 Recognition of benefit plan assets and obligations from the sale of OneBeacon, net of tax 2.9 — 2.9 2.9 — — 2.9 Comprehensive income (loss) from discontinued operations $ 542.3 $ (.2 ) $ 542.1 $ 578.2 $ (.8 ) $ (1.0 ) $ 576.4 Net Change in Cash from Discontinued Operations The following table presents the net change in cash, including income tax payments to national governments and interest paid associated with the business classified as discontinued operations: Nine Months Ended September 30, Millions 2018 2017 Net cash provided from (used for) operations $ — $ 157.0 Net cash (used for) provided from investing activities — 3.0 Net cash used for financing activities — (61.9 ) Net change in cash during the period — 98.1 Cash balances at beginning of period — 70.5 Net change in cash held for sale, excluding discontinued operations — (.9 ) Cash sold as part of sale of consolidated subsidiaries — 167.7 Cash balances at end of period $ — $ — Supplemental cash flows information: Interest paid $ — $ — Net income tax payment to national governments $ — $ — Earnings Per Share from Discontinued Operations White Mountains calculates earnings per share using the two-class method, which allocates earnings between common and unvested restricted common shares. Both classes of shares participate equally in earnings on a per share basis. Basic earnings per share amounts are based on the weighted average number of common shares outstanding adjusted for unvested restricted common shares. Diluted earnings per share amounts are also impacted by the net effect of potentially dilutive common shares outstanding. The following table presents the Company’s computation of earnings per share for discontinued operations for the three and nine months ended September 30, 2018 and 2017 : Three Months Ended Nine Months Ended September 30, September 30, 2018 2017 2018 2017 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 40.8 $ 562.1 $ (3.7 ) $ 604.7 Less total loss from continuing operations, net of tax 58.1 23.0 13.5 31.5 Net income from discontinued operations attributable to White Mountains’s common shareholders $ (17.3 ) $ 539.1 $ (17.2 ) $ 573.2 Allocation of earnings to participating restricted common shares (1) .2 (6.7 ) .2 (7.0 ) Basic and diluted earnings per share numerators $ (17.1 ) $ 532.4 $ (17.0 ) $ 566.2 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 3,180.4 4,297.2 3,450.8 4,477.0 Average unvested restricted common shares (3) (41.5 ) (53.7 ) (39.7 ) (54.5 ) Basic earnings per share denominator 3,138.9 4,243.5 3,411.1 4,422.5 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 3,180.4 4,297.2 3,450.8 4,477.0 Average unvested restricted common shares (3) (41.5 ) (53.7 ) (39.7 ) (54.5 ) Diluted earnings per share denominator 3,138.9 4,243.5 3,411.1 4,422.5 Basic and diluted earnings per share (in dollars) - discontinued operations: $ (5.44 ) $ 125.45 $ (4.98 ) $ 128.03 (1) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities. (2) Net earnings attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the three and nine months ended September 30, 2018 and 2017 . (3) Restricted common shares outstanding vest either in equal annual installments or upon a stated date. See Note 10 — “Employee Share-Based Incentive Compensation Plans” .

Basis of Presentation and Sig_2

Basis of Presentation and Significant Accouting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The Company is an exempted Bermuda limited liability company whose principal businesses are conducted through its insurance subsidiaries and other affiliates. The Company’s headquarters is located at 26 Reid Street, Hamilton, Bermuda HM 11, its principal executive office is located at 80 South Main Street, Hanover, New Hampshire 03755-2053 and its registered office is located at Clarendon House, 2 Church Street, Hamilton, Bermuda HM 11. The accompanying consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) and include the accounts of White Mountains Insurance Group, Ltd. (the “Company” or the “Registrant”), its subsidiaries (collectively with the Company, “White Mountains”) and other entities required to be consolidated under GAAP. Consolidation Principles Under GAAP, the Company is required to consolidate any entity in which it holds a controlling financial interest. A controlling financial interest is usually in the form of an investment representing the majority of the subsidiary’s voting interests. However, a controlling financial interest may also arise from a financial interest in a variable interest entity (“VIE”) through arrangements that do not involve ownership of voting interests. The Company consolidates a VIE if it determines that it is the primary beneficiary. The primary beneficiary is defined as the entity who holds a variable interest that gives it both the power to direct the VIE’s activities that most significantly impact its economic performance and the obligation to absorb losses of, or the right to receive returns from, the VIE that could potentially be significant to the VIE. Intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. These interim financial statements include all adjustments considered necessary by management to fairly state the financial position, results of operations and cash flows of White Mountains. These interim financial statements may not be indicative of financial results for the full year and should be read in conjunction with the Company’s 2017 Annual Report on Form 10-K.
Business Combinations	Business Combinations White Mountains accounts for purchases of businesses using the acquisition method, which requires the measurement of assets acquired, including goodwill and other intangible assets and liabilities assumed, including contingent liabilities, at their estimated fair values as of the acquisition date. The acquisition date fair values represent management's best estimates and are based upon established valuation techniques, reasonable assumptions and, where appropriate, valuations performed by independent third parties. In circumstances where additional information is required in order to determine the acquisition date fair value of balance sheet amounts, provisional amounts may be recorded as of the acquisition date and may be subject to subsequent adjustment throughout the measurement period, which is one year from the acquisition date. Measurement period adjustments are recognized in the period in which they are determined. The results of operations and cash flows of businesses acquired are included in the consolidated financial statements from the date of acquisition.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Reportable Segments	Reportable Segments White Mountains has determined its reportable segments based on the nature of the underlying businesses, the manner in which the Company’s subsidiaries and affiliates are organized and managed and the organization of the financial information provided to the chief operating decision maker to assess performance and make decisions regarding allocation of resources. White Mountains’s reportable segments are HG Global/BAM, NSM, MediaAlpha and Other Operations. See Note 12 — “Segment Information” . The HG Global/BAM segment consists of HG Global Ltd. and its wholly-owned subsidiaries (“HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”) (collectively, “HG Global/BAM”). BAM is the first and only mutual municipal bond insurance company in the United States. By insuring the timely payment of principal and interest, BAM provides market access to, and lowers interest expense for, issuers of municipal bonds used to finance essential public purposes such as schools, utilities and transportation facilities. BAM is owned by and operated for the benefit of its members, the municipalities that purchase BAM’s insurance for their debt issuances. HG Global was established to fund the startup of BAM and, through its wholly-owned subsidiary, HG Re Ltd. (“HG Re”), to provide up to 15% -of-par, first loss reinsurance protection for policies underwritten by BAM. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM (the “BAM Surplus Notes”). As of September 30, 2018, $499.0 million of the surplus notes remain outstanding. As of September 30, 2018 and December 31, 2017, White Mountains owned 96.9% of HG Global’s preferred equity and 88.4% of its common equity. White Mountains does not have an ownership interest in BAM. However, White Mountains is required to consolidate BAM’s results in its financial statements because BAM is a VIE for which White Mountains is the primary beneficiary. BAM’s results are attributed to non-controlling interests. The NSM segment consists of NSM Insurance HoldCo, LLC and its wholly-owned subsidiaries (collectively, "NSM"). NSM is a full service managing general agency and program administrator for specialty property and casualty insurance. NSM manages all aspects of the placement process on behalf of its carrier partners, including product development, marketing, underwriting and policy issuance. NSM specializes in niche sectors including collector cars, social services and behavioral health, specialty real estate, sports and fitness centers, and pet insurance. White Mountains acquired a 95% ownership share in NSM on May 11, 2018. The NSM segment also includes White Mountains Catskill Holdings, Inc. 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 develops technology that enables the programmatic buying and selling of vertical-specific, performance-based media between advertisers (buyers of advertising inventory) and publishers (sellers of advertising inventory) through cost-per-click, cost-per-call and cost-per-lead pricing models. MediaAlpha's media buying platform enables advertisers to create and automate data-driven bidding strategies designed to improve the efficiency and enhance overall performance of their marketing campaigns that target high-intent consumers at the time and place they are ready to purchase. MediaAlpha’s publisher platform is used by publishers to sell their vertical-specific, performance-based media to advertisers through transparent, programmatic, auction-based marketplaces. MediaAlpha works with 600 advertisers and 330 publishers across a number of insurance (auto, motorcycle, home, renter, health and life) and non-insurance (travel, education, personal finance and home services) verticals. White Mountains’s Other Operations segment consists of the Company, its wholly-owned subsidiary, White Mountains Capital, Inc. (“WM Capital”), its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), and its other intermediate holding companies, as well as certain consolidated and unconsolidated private capital and other investments. The consolidated private capital investments include Wobi Insurance Agency Ltd. (“Wobi”) and Removal Stars Ltd. (“Buzzmove”).
Discontinued Operations and Assets and Liabilities Held for Sale	Discontinued Operations and Assets and Liabilities Held for Sale On September 28, 2017, Intact Financial Corporation completed its acquisition of OneBeacon Insurance Group, Ltd. (“OneBeacon”) in an all-cash transaction for $18.10 per share (the “OneBeacon Transaction”). On July 21, 2016, White Mountains completed its sale of Tranzact Holdings, LLC (“Tranzact”) to an affiliate of Clayton, Dubilier & Rice, LLC. On April 18, 2016, White Mountains completed its sale of Sirius International Insurance Group, Ltd. (“Sirius Group”) to CM International Pte. Ltd. and CM Bermuda Limited (collectively “CMI”), the Singapore-based investment arm of China Minsheng Investment Corp., Ltd. White Mountains has presented the results of OneBeacon, Tranzact and Sirius Group as discontinued operations in the statement of operations and comprehensive income and their assets and liabilities as held for sale in the balance sheet for all periods prior to the completion of each transaction. White Mountains has classified its Guilford, Connecticut property, which consists of an office building and adjacent land, as held for sale as of September 30, 2018 and December 31, 2017.
Reinsurance Contracts Accounted For As Deposits	Reinsurance Contracts Accounted for as Deposits Reinsurance contracts that do not meet the risk transfer requirements necessary to be accounted for as reinsurance are accounted for using the deposit method under GAAP. BAM entered into a reinsurance contract agreement with Fidus Reinsurance Ltd. (“Fidus Re”) in 2018, which is accounted for using the deposit method. See Note 8 — “Municipal Bond Guarantee Insurance” . The nonrefundable consideration paid by BAM to Fidus Re is charged to financing expense within general and administrative expenses.
Restricted Cash	Restricted Cash Cash includes amounts on hand and demand deposits with banks and other financial institutions. Cash balances which are not immediately available for general corporate purposes, including fiduciary accounts, are classified as restricted. Restricted amounts included within cash are disclosed parenthetically on the balance sheet. Amounts presented in the statement of cash flows are shown net of balances acquired and sold in the purchase or sale of White Mountains’s consolidated subsidiaries. Changes in restricted cash balances are presented as a separate caption within the operations, investing and financing activities sections of the statement of cash flows.
Recently Adopted Changes in Accounting Principles and Recently Issued Accounting Pronouncements	Recently Adopted Changes in Accounting Principles Revenue Recognition On January 1, 2018, White Mountains adopted ASU 2014-09, Revenue from Contracts with Customers (ASC 606), which modifies the guidance for revenue recognition. Under ASU 2014-09, revenue is recognized at an amount that reflects the consideration to which an entity expects to be entitled once it fulfills its performance obligations under the terms of its contract with the customer. The scope of the new guidance includes agent commissions and other non-insurance revenues. Adoption of ASU 2014-09 did not have any impact on White Mountains's financial statements. Share-Based Compensation On January 1, 2018, White Mountains adopted ASU 2017-09, Stock Compensation: Scope of Modification Accounting (ASC 718), which narrows the scope of transactions subject to modification accounting to changes in terms of an award that result in a change in the award’s fair value, vesting conditions or classification. Adoption of ASU 2017-09 did not have any impact on White Mountains’s financial statements. On January 1, 2017, White Mountains adopted ASU 2016-09, Improvements to Employee Share-Based Payment Accounting (ASC 718) which simplifies certain aspects of the accounting for share-based compensation. The new guidance provides an accounting policy election to account for forfeitures by either applying an assumption, as required under existing guidance, or by recognizing forfeitures when they actually occur. At adoption, White Mountains did not change its accounting policy for forfeitures, which is to apply an assumed forfeiture rate. The new guidance has also changed the threshold for partial cash settlement to settle statutory withholding requirements for equity classified awards, increasing the threshold up to the maximum statutory tax rate. As a result of adoption, White Mountains reported $8.4 million and $9.2 million of statutory withholding tax payments made in connection with the settlement of restricted shares as financing cash flows for the nine months ended September 30, 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, regardless of whether it is in a taxes payable position. Business Combinations On January 1, 2018, White Mountains adopted ASU 2017-01, Business Combinations: Clarifying the Definition of a Business (ASC 805), which clarifies the definition of a business and affects the determination of whether acquisitions or disposals are accounted for as assets or as a business. Under the new guidance, when substantially all of the fair value of the assets is concentrated in a single identifiable asset or group of similar assets, it is not a business. 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 FASB issued ASU 2017-08, Premium Amortization on Purchased Callable Debt Securities (ASC 310-20), which changes the amortization period for certain purchased callable debt securities. Under the new guidance, for investments in callable debt securities held at a premium, the premium will be amortized over the period to the earliest call date. The new guidance does not change the amortization period for callable debt securities held at a discount. ASU 2017-08 is not expected to have any impact on White Mountains's financial statements at adoption but may affect the amortization recognized in future periods. Credit Losses In June 2016, the FASB issued ASU 2016-13, Measurement of Credit Losses on Financial Instruments (ASC 326), which establishes new guidance for the recognition of credit losses for financial assets measured at amortized cost. The new ASU requires reporting entities to estimate the credit losses expected over the life of a credit exposure using historical information, current information and reasonable and supportable forecasts that affect the collectability of the financial asset. This differs from current GAAP, which delays recognition until it is probable a loss has been incurred. The new guidance is expected to accelerate recognition of credit losses. The types of assets within the scope of the new guidance include premium receivables, reinsurance recoverables and loans. ASU 2016-13 is effective for annual periods beginning after January 1, 2020, including interim periods. White Mountains measures its portfolio of investment securities at fair value with changes therein recognized through current period earnings and, accordingly, does not expect adoption to have any effect on its financial statements. Leases In February 2016, the FASB issued ASU 2016-02, Leases (ASC 842). The new guidance requires lessees to recognize lease assets and liabilities on the balance sheet for both operating and financing leases, with the exception of leases with an original term of 12 months or less. Under existing guidance recognition of lease assets and liabilities is not required for operating leases. The lease assets and liabilities to be recognized are both measured initially based on the present value of the lease payments. Under the new guidance, a sale-leaseback transaction must meet the recognition criteria under ASC 606, Revenues, in order to be accounted for as sale. The new guidance is effective for White Mountains for years beginning after December 15, 2018, including interim periods therein. White Mountains is evaluating the expected impact of this guidance and available adoption methods.

Investment Securities (Tables)

Investment Securities (Tables)	9 Months Ended
	Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Pre-tax net investment income	The following table presents pre-tax net investment income for the three and nine months ended September 30, 2018 and 2017 . Three Months Ended Nine Months Ended September 30, September 30, Millions 2018 2017 2018 2017 Investment income: Fixed maturity investments $ 7.5 $ 9.8 $ 27.3 $ 32.9 Short-term investments 1.8 .3 6.5 .6 Common equity securities 3.4 2.7 11.2 7.7 Other long-term investments 1.2 .1 1.7 .5 Total investment income 13.9 12.9 46.7 41.7 Third-party investment expenses (.5 ) (.7 ) (1.8 ) (2.0 ) Net investment income, pre-tax $ 13.4 $ 12.2 $ 44.9 $ 39.7
Schedule of Realized and Unrealized Gain (Loss) on Investments	The following table presents net realized and unrealized investment gains (losses) for the three and nine months ended September 30, 2018 and 2017 : Three Months Ended Nine Months Ended September 30, September 30, Millions 2018 2017 2018 2017 Net realized investment (losses) gains, pre-tax $ (10.2 ) $ 6.8 $ (13.9 ) $ 20.8 Net unrealized investment gains, pre-tax 76.3 25.7 36.8 81.7 Net realized and unrealized investment gains, pre-tax 66.1 32.5 22.9 102.5 Income tax expense attributable to net realized and unrealized investment gains (9.7 ) (3.9 ) (5.9 ) (9.5 ) Net realized and unrealized investment gains, after-tax $ 56.4 $ 28.6 $ 17.0 $ 93.0
Schedule of realized gain (loss) on investments	The following tables present net realized investment gains (losses) for the three and nine months ended September 30, 2018 and 2017 Three Months Ended Three Months Ended September 30, 2018 September 30, 2017 Millions Net Net Total net realized Net Net Total net realized Fixed maturity investments $ (5.1 ) $ — $ (5.1 ) $ .6 $ 1.2 $ 1.8 Short-term investments — — — (.1 ) — (.1 ) Common equity securities (1.5 ) — (1.5 ) 4.9 5.4 10.3 Other long-term investments (3.6 ) — (3.6 ) 2.0 (7.2 ) (5.2 ) Net realized investment (losses) gains, pre-tax (10.2 ) — (10.2 ) 7.4 (.6 ) 6.8 Income tax benefit (expense) attributable to net realized investment (losses) gains 2.3 — 2.3 (.6 ) — (.6 ) Net realized investment (losses) gains, after-tax $ (7.9 ) $ — $ (7.9 ) $ 6.8 $ (.6 ) $ 6.2 Nine Months Ended Nine Months Ended September 30, 2018 September 30, 2017 Millions Net Net Total net realized Net Net Total net realized Fixed maturity investments $ (27.6 ) $ 18.2 $ (9.4 ) $ (.4 ) $ 2.7 $ 2.3 Short-term investments (.8 ) — (.8 ) (.1 ) — (.1 ) Common equity securities 10.5 — 10.5 18.5 6.0 24.5 Other long-term investments (7.0 ) (7.2 ) (14.2 ) 3.0 (8.9 ) (5.9 ) Net realized investment (losses) gains, pre-tax (24.9 ) 11.0 (13.9 ) 21.0 (.2 ) 20.8 Income tax benefit (expense) attributable to net realized investment (losses) gains 11.2 — 11.2 (3.6 ) — (3.6 ) Net realized investment (losses) gains, after-tax $ (13.7 ) $ 11.0 $ (2.7 ) $ 17.4 $ (.2 ) $ 17.2
Schedule of unrealized gain (loss) on investments	The following tables present net unrealized investment gains (losses) and changes in the carrying value of investments measured at fair value for the three and nine months ended September 30, 2018 and 2017 : Three Months Ended Three Months Ended September 30, 2018 September 30, 2017 Millions Net unrealized gains (losses) Net Total net unrealized gains (losses) reflected in earnings Net unrealized gains (losses) Net Total net unrealized gains (losses) reflected in earnings Fixed maturity investments $ 1.5 $ — $ 1.5 $ 2.0 $ 5.1 $ 7.1 Short-term investments (.1 ) — (.1 ) — — — Common equity securities 54.9 — 54.9 26.6 (3.1 ) 23.5 Other long-term investments 20.1 (.1 ) 20.0 (2.5 ) (2.4 ) (4.9 ) Net unrealized investment gains (losses), pre-tax 76.4 (.1 ) 76.3 26.1 (.4 ) 25.7 Income tax expense attributable to net unrealized investment gains (losses) (12.0 ) — (12.0 ) (3.3 ) — (3.3 ) Net unrealized investment gains (losses), after-tax $ 64.4 $ (.1 ) $ 64.3 $ 22.8 $ (.4 ) $ 22.4 Nine Months Ended Nine Months Ended September 30, 2018 September 30, 2017 Millions Net unrealized (losses) gains Net gains Total net unrealized (losses) gains reflected in earnings Net unrealized gains Net Total net unrealized gains (losses) reflected in earnings Fixed maturity investments $ (15.7 ) $ (14.8 ) $ (30.5 ) $ 19.4 $ 12.6 $ 32.0 Short-term investments (.1 ) — (.1 ) — — — Common equity securities 43.7 — 43.7 53.5 — 53.5 Other long-term investments 20.5 3.2 23.7 9.2 (13.0 ) (3.8 ) Net unrealized investment gains (losses), pre-tax 48.4 (11.6 ) 36.8 82.1 (.4 ) 81.7 Income tax expense attributable to net unrealized investment gains (losses) (17.1 ) — (17.1 ) (5.9 ) — (5.9 ) Net unrealized investment gains (losses), after-tax $ 31.3 $ (11.6 ) $ 19.7 $ 76.2 $ (.4 ) $ 75.8
Net unrealized investment gains (losses) for Level 3 investments	The following table presents total gains (losses) included in earnings attributable to net unrealized investment gains (losses) for Level 3 investments for the three and nine months ended September 30, 2018 and 2017 : Three Months Ended Nine Months Ended September 30, September 30, Millions 2018 2017 2018 2017 Other long-term investments $ 4.0 $ (.7 ) $ 2.1 $ (2.2 ) Total net unrealized investment gains (losses) , pre-tax - Level 3 investments $ 4.0 $ (.7 ) $ 2.1 $ (2.2 )
Investment holdings, equity securities, convertible fixed maturities and other long-term investments	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 September 30, 2018 and December 31, 2017 . September 30, 2018 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign currency gains Carrying value U.S. Government and agency obligations $ 127.0 $ — $ (1.6 ) $ — $ 125.4 Debt securities issued by corporations 509.4 .2 (9.6 ) — 500.0 Mortgage and asset-backed securities 132.4 — (3.9 ) — 128.5 Municipal obligations 281.1 .4 (3.5 ) — 278.0 Total fixed maturity investments $ 1,049.9 $ .6 $ (18.6 ) $ — $ 1,031.9 December 31, 2017 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign currency gains Carrying value U.S. Government and agency obligations $ 297.8 $ — $ (1.3 ) $ — $ 296.5 Debt securities issued by corporations 867.6 2.9 (4.3 ) 14.7 880.9 Mortgage and asset-backed securities 697.2 1.6 (4.1 ) — 694.7 Municipal obligations 252.0 3.7 (.8 ) — 254.9 Foreign government, agency and provincial obligations 2.6 — — .1 2.7 Total fixed maturity investments $ 2,117.2 $ 8.2 $ (10.5 ) $ 14.8 $ 2,129.7 The following tables present the cost or amortized cost, gross unrealized investment gains (losses), net foreign currency losses, and carrying values of White Mountains’s common equity securities and other long-term investments as of September 30, 2018 and December 31, 2017 : September 30, 2018 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign Carrying value Common equity securities $ 873.4 $ 171.1 $ (1.0 ) $ — $ 1,043.5 Other long-term investments $ 303.8 $ 34.8 $ (47.2 ) $ (1.7 ) $ 289.7 December 31, 2017 Millions Cost or Gross Gross Net foreign Carrying value Common equity securities $ 739.7 $ 129.4 $ (3.0 ) $ — $ 866.1 Other long-term investments $ 246.6 $ 6.8 $ (39.7 ) $ (4.9 ) $ 208.8 Other Long-Term Investments The following table presents the carrying values of White Mountains’s other long-term investments as of September 30, 2018 and December 31, 2017 : Carrying Value at Millions September 30, 2018 December 31, 2017 Hedge funds and private equity funds, at fair value $ 151.6 $ 125.3 Private equity securities, at fair value (1)(2) 126.4 83.2 Foreign currency forward contracts — (3.7 ) Other 11.7 4.0 Total other long-term investments $ 289.7 $ 208.8 (1) See Fair Value Measurements by Level table. (2) Includes White Mountains's non-controlling interests in common equity securities, limited liability companies and private convertible preferred securities.
Other long-term investments	The following table presents investments in hedge funds and private equity funds by investment objective and sector as of September 30, 2018 and December 31, 2017 : September 30, 2018 December 31, 2017 Millions Fair Value Unfunded Commitments Fair Value Unfunded Commitments Hedge funds Long/short banks and financials $ 59.6 $ — $ 54.9 $ — Total hedge funds 59.6 — 54.9 — Private equity funds Manufacturing/Industrial 49.6 10.5 43.3 10.4 Aerospace/Defense/Government 25.5 34.9 15.8 12.9 Direct lending 10.6 19.9 7.1 23.1 Financial services 6.3 15.7 4.2 11.7 Real estate — 50.0 — — Total private equity funds 92.0 131.0 70.4 58.1 Total hedge funds and private equity funds included in other long-term investments $ 151.6 $ 131.0 $ 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 September 30, 2018 : Millions 1 – 3 years 3 – 5 years 5 – 10 years >10 years Total Private equity funds — expected lock-up period remaining $14.2 $5.0 $55.9 $16.9 $92.0
Fair value measurements by level, investment securities	The following tables present White Mountains’s fair value measurements for investments as of September 30, 2018 and December 31, 2017 by level. The major security types were based on the legal form of the securities. White Mountains has disaggregated its fixed maturity investments based on the issuing entity type, which impacts credit quality, with debt securities issued by U.S. Government entities carrying minimal credit risk, while the credit and other risks associated with other issuers, such as corporations, foreign governments, municipalities or entities issuing mortgage and asset-backed securities vary depending on the nature of the issuing entity type. White Mountains further disaggregates debt securities issued by corporations and common equity securities by industry sector because investors often reference commonly used benchmarks and their subsectors to monitor risk and performance. Accordingly, White Mountains has further disaggregated these asset classes into subclasses based on the similar sectors and industry classifications it uses to evaluate investment risk and performance against commonly used benchmarks, such as the Bloomberg Barclays U.S. Intermediate Aggregate and S&P 500 indices. September 30, 2018 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 125.4 $ 125.4 $ — $ — Debt securities issued by corporations: Financials 145.9 — 145.9 — Consumer 84.2 — 84.2 — Technology 60.4 — 60.4 — Energy 54.6 — 54.6 — Healthcare 53.9 — 53.9 — Industrial 39.5 — 39.5 — Communications 22.4 — 22.4 — Materials 22.2 — 22.2 — Utilities 16.9 — 16.9 — Total debt securities issued by corporations 500.0 — 500.0 — Mortgage and asset-backed securities 128.5 — 128.5 — Municipal obligations 278.0 — 278.0 — Total fixed maturity investments 1,031.9 125.4 906.5 — Short-term investments (1) 313.7 300.7 13.0 — Common equity securities: Exchange traded funds (2) 742.9 675.3 67.6 — Technology 16.8 16.8 — — Financials 16.7 16.7 — — Healthcare 16.4 16.4 — — Industrial 15.6 15.6 — — Communications 14.8 14.8 — — Consumer 9.8 9.8 — — Energy 6.6 6.6 — — Other (3) 203.9 — 203.9 — Total common equity securities 1,043.5 772.0 271.5 — Other long-term investments (4) 138.1 — — 138.1 Total investments $ 2,527.2 $ 1,198.1 $ 1,191.0 $ 138.1 (1) Short-term investments are measured at amortized cost, which approximates fair value. (2) ETFs traded on foreign exchanges are priced using the fund's published NAV to account for the difference in market close times and are therefore designated a level 2 measurement. (3) Consists of two investments in unit trusts that primarily invest in international equities. (4) Excludes carrying value of $151.6 associated with hedge funds and private equity funds for which fair value is measured at NAV using the practical expedient. December 31, 2017 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 296.5 $ 296.5 $ — $ — Debt securities issued by corporations: Consumer 185.1 — 185.1 — Communications 127.8 — 127.8 — Financials 114.8 — 114.8 — Utilities 108.9 — 108.9 — Materials 95.5 — 95.5 — 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) 87.2 — — 87.2 Total investments $ 3,259.1 $ 1,041.4 $ 2,130.5 $ 87.2 (1) Short-term investments are measured at amortized cost, which approximates fair value. (2) ETFs traded on foreign exchanges are priced using the fund’s published NAV to account for the difference in market close times and are therefore designated a level 2 measurement. (3) Consists of two investments in unit trusts that primarily invests in international equities. (4) Excludes carrying value of $(3.7) related to foreign currency forward contracts and $125.3 associated with hedge funds and private equity funds for which fair value is measured at NAV using the practical expedient.
Debt securities issued by corporations, credit ratings	The following table presents the ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of September 30, 2018 and December 31, 2017 : Fair Value at Millions September 30, 2018 December 31, 2017 AAA $ 8.1 $ 1.6 AA 83.6 42.6 A 273.3 192.5 BBB 135.0 465.2 BB — 161.7 B — 17.3 Debt securities issued by corporations (1) $ 500.0 $ 880.9 (1) Credit ratings are assigned based on the following hierarchy: (1) Standard & Poor’s Financial Services LLC (“S&P”) and (2) Moody's Investor Service, Inc. (“Moody’s”).
Mortgage-backed, asset-backed securities	The following table presents the fair value of White Mountains’s mortgage and asset-backed securities as of September 30, 2018 and December 31, 2017 : September 30, 2018 December 31, 2017 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: GNMA $ 24.5 $ 24.5 $ — $ 46.3 $ 46.3 $ — FNMA 53.6 53.6 — 84.5 84.5 — FHLMC 39.0 39.0 — 62.0 62.0 — Total agency (1) 117.1 117.1 — 192.8 192.8 — Non-agency: Commercial — — — 70.5 70.5 — Total non-agency — — — 70.5 70.5 — Total mortgage-backed securities 117.1 117.1 — 263.3 263.3 — Other asset-backed securities: Credit card receivables 8.9 8.9 — 206.0 206.0 — Vehicle receivables 2.5 2.5 — 142.4 142.4 — Other — — — 83.0 83.0 — Total other asset-backed securities 11.4 11.4 — 431.4 431.4 — Total mortgage and asset-backed securities $ 128.5 $ 128.5 $ — $ 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).
Non-agency commercial mortgage securities, type of interest rate and tranche levels
Rollforward of fair value investments by level	The following tables present the changes in White Mountains’s fair value measurements by level for the nine months ended September 30, 2018 and 2017 : Level 3 Investments Millions Level 1 investments Level 2 investments Fixed maturity investments Other long-term investments Hedge Funds and Private Equity Funds measured at NAV (3) Total Balance at December 31, 2017 $ 890.4 $ 2,105.4 $ — $ 87.2 $ 125.3 $ 3,208.3 (1)(2) Net realized and unrealized gains (losses) 51.9 (33.6 ) — (4.2 ) 17.2 31.3 (4) Amortization/Accretion .1 (2.3 ) — — — (2.2 ) Purchases 412.6 673.5 — 55.1 10.7 1,151.9 Sales (457.6 ) (1,565.0 ) — — (1.6 ) (2,024.2 ) Transfers in — — — — — — Transfers out — — — — — — Balance at September 30, 2018 $ 897.4 $ 1,178.0 $ — $ 138.1 $ 151.6 $ 2,365.1 (2) (1) Excludes carrying value of $(3.7) as of December 31, 2017 associated with foreign currency forward contracts. (2) Excludes carrying value of $ 176.1 and $ 313.7 at December 31, 2017 and September 30, 2018 classified as short-term investments. (3) Investments for which fair value is measured at NAV using the practical expedient are no longer classified within the fair value hierarchy. See Note 1 — “Basis of Presentation and Significant Accounting Policies” . (4) Excludes realized and unrealized losses associated with foreign currency forward contracts, foreign currency on cash and open trades and short-term investments of $3.5 , $4.2 and $0.7 for the nine months ended September 30, 2018. Level 3 Investments Millions Level 1 investments Level 2 investments Fixed maturity investments Other long-term investments Hedge Funds and Private Equity Funds measured at NAV (3) Total Balance at December 31, 2016 $ 279.5 $ 2,093.8 $ — $ 91.4 $ 82.6 $ 2,547.3 (1)(2)(4) Net realized and unrealized gains (losses) 52.4 59.8 — (2.2 ) 15.6 125.6 (5) Amortization/Accretion — (6.6 ) — — — (6.6 ) Purchases 940.7 1,038.5 31.2 2.9 64.9 2,078.2 Sales (539.0 ) (1,668.2 ) (12.5 ) (2.0 ) (8.2 ) (2,229.9 ) Deconsolidation of SSIE — (5.2 ) — — — (5.2 ) Transfers in — 13.1 — — — 13.1 Transfers out — — (13.1 ) — — (13.1 ) Balance at September 30, 2017 $ 733.6 $ 1,525.2 $ 5.6 $ 90.1 $ 154.9 $ 2,509.4 (1)(2) (1) Excludes carrying value of $ 175.0 and $ 786.5 at December 31, 2016 and September 30, 2017 classified as short-term investments, of which $0.1 is classified as held for sale at December 31, 2016 . (2) Excludes carrying value of $(1.2) and $(15.4) as of December 31, 2016 and September 30, 2017 associated with foreign currency forward contracts. (3) Investments for which fair value is measured at NAV using the practical expedient are no longer classified within the fair value hierarchy. See Note 1 — “Basis of Presentation and Significant Accounting Policies” . (4) Includes carrying value of $6.6 of fixed maturity investments at December 31, 2016 that is classified as assets held for sale related to SSIE. (5) Excludes realized and unrealized losses associated with foreign currency forward contracts of $23.1 for the nine months ended September 30, 2017.
Schedule of significant unobservable inputs used in estimating the fair value of investment securities	The following tables present significant unobservable inputs used in estimating the fair value of other long-term investments, other than hedge funds and private equity funds, classified within Level 3 as of September 30, 2018 and December 31, 2017 . The fair value of investments in hedge funds and private equity funds are generally estimated using the NAV of the funds. $ in millions, except share price September 30, 2018 Description Valuation Technique(s) Fair Value (1) Unobservable Input Private equity security Share price of most recent transaction $28.3 Share price - $56.60 Private equity security Discounted cash flow $21.0 Discount rate - 25.0% Private equity security Discounted cash flow $22.1 Discount rate - 35.0% Private convertible preferred security Discounted cash flow $14.5 Discount rate - 30.0% Private equity security Recent transaction $15.8 Share price - €338.85 Private preferred security Multiple of EBITDA $3.5 EBITDA multiple - 6.00 Private debt instrument Discounted cash flow $10.5 Discount rate - 9.62% (1) Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs. $ in millions, except share price December 31, 2017 Description Valuation Technique(s) Fair Value (1) Unobservable Input Private equity security Discounted cash flow $21.0 Discount rate - 25.0% Private equity security Discounted cash flow $22.1 Discount rate - 35.0% Private convertible preferred security Discounted cash flow $14.5 Discount rate - 30.0% Private equity security Discounted cash flow/ Option pricing method $11.3 Discount rate - 21.0% Time until expiration - 4 years Volatility/Standard deviation - 50.0% Risk free rate - 1.77% Private preferred security Multiple of EBITDA $0.6 EBITDA multiple - 6.00 Private equity security 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_2

Goodwill and Other Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Changes in Goodwill and Intangible Assets	The following tables presents the change in goodwill and other intangible assets: Three Months Ended September 30, 2018 2017 Millions Goodwill Other intangible assets Unallocated acquisition date fair value of intangible assets Total Goodwill and other intangible assets Goodwill Other intangible assets Total Goodwill and other intangible assets Beginning balance $ 25.9 $ 30.7 436.2 $ 492.8 $ 25.9 $ 14.4 $ 40.3 Attribution of acquisition date fair value estimates between goodwill and other intangible assets 295.3 140.9 (436.2 ) — — — — Foreign currency translation (1.2 ) (.4 ) — (1.6 ) — — — Adjustments to reflect acquisition date fair value 1.4 — — 1.4 — — — Amortization (1) — (7.4 ) — (7.4 ) — (2.5 ) (2.5 ) Ending balance $ 321.4 $ 163.8 — $ 485.2 $ 25.9 $ 11.9 $ 37.8 (1) Includes the NSM intangible assets amortization of $1.8 related to the period from May 11, 2018 through June 30, 2018 and $3.2 for the three months ended September 30, 2018. Nine Months Ended September 30, 2018 2017 Millions Goodwill Other intangible assets Total Goodwill and other intangible assets Goodwill Other intangible assets Total Goodwill and other intangible assets Beginning balance $ 25.9 $ 36.2 $ 62.1 $ 25.9 $ 19.3 $ 45.2 Attribution of acquisition date fair value estimates between goodwill and other intangible assets 295.3 140.9 436.2 — — — Foreign currency translation (1.2 ) (.4 ) (1.6 ) — — — — Adjustments to reflect acquisition date fair value 1.4 — 1.4 — — — — Amortization — (12.9 ) (12.9 ) — (7.4 ) (7.4 ) Ending balance $ 321.4 $ 163.8 $ 485.2 $ 25.9 $ 11.9 $ 37.8
Schedule of Intangible Assets and Goodwill	The following table presents the acquisition date values, accumulated amortization and net carrying values for other intangible assets and goodwill, by company as of September 30, 2018 and December 31, 2017 : $ in millions September 30, 2018 December 31, 2017 Acquisition date fair value Accumulated amortization Net carrying value Acquisition date fair value Accumulated amortization Net carrying value Goodwill: NSM (1) $ 295.5 $ — $ 295.5 $ — $ — $ — MediaAlpha 18.3 — 18.3 18.3 — 18.3 Buzzmove 7.6 — 7.6 7.6 — 7.6 Total goodwill 321.4 — 321.4 25.9 — 25.9 Other intangible assets: NSM (1) Customer relationships 85.5 3.6 81.9 — — — Trade names 51.2 1.1 50.1 — — — Information technology 3.8 .3 3.5 — — — Subtotal 140.5 5.0 135.5 — — — MediaAlpha Customer relationships 26.8 4.3 22.5 26.8 2.4 24.4 Information technology 33.3 29.2 4.1 33.3 24.3 9.0 Other 9.8 8.8 1.0 9.8 7.8 2.0 Subtotal 69.9 42.3 27.6 69.9 34.5 35.4 Buzzmove Trade names .6 .1 .5 .6 .1 .5 Information technology .5 .3 .2 .5 .2 .3 Subtotal 1.1 .4 .7 1.1 .3 .8 Total other intangible assets 211.5 47.7 163.8 71.0 34.8 36.2 Total goodwill and other intangible assets $ 532.9 $ 47.7 485.2 $ 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 $ 444.6 $ 41.0 (1) Includes the effect of foreign currency translation from the date of acquisition of $1.2 for goodwill and $0.4 for other intangible assets.

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Schedule of debt outstanding	The following table presents White Mountains’s debt outstanding as of September 30, 2018 and December 31, 2017 : Millions September 30, Effective Rate (1) December 31, Effective Rate (1) WTM Bank Facility $ — N/A $ — N/A NSM Bank Facility 150.6 7.5% — Unamortized issuance cost (3.3 ) — NSM Bank Facility, carrying value 147.3 — Other NSM debt 2.0 — MediaAlpha Bank Facility 15.1 6.8% 23.9 5.6% Unamortized issuance cost (.1 ) (.1 ) MediaAlpha Bank Facility, carrying value 15.0 23.8 Total debt $ 164.3 $ 23.8 (1) Effective rate considers the effect of the debt issuance costs.

Municipal Bond Guarantee (Table

Municipal Bond Guarantee (Tables)	9 Months Ended
	Sep. 30, 2018
Guarantees [Abstract]
Schedule of Municipal Bond Guarantee Insured Obligations	The following table presents a schedule of BAM’s insured obligations: September 30, 2018 December 31, 2017 Contracts outstanding 7,184 6,371 Remaining weighted average contract period outstanding (in years) 10.8 10.9 Contractual debt service outstanding (in millions): Principal $ 47,259.1 $ 42,090.6 Interest 23,707.4 21,057.1 Total debt service outstanding $ 70,966.5 $ 63,147.7 Gross unearned insurance premiums $ 156.2 $ 136.8
Financial Guarantee Insurance Contracts, Premium Received over Contract Period	The following table presents a schedule of BAM’s future premium revenues as of September 30, 2018 : Millions September 30, 2018 October 1, 2018 - December 31, 2018 $ 3.3 January 1, 2019 - March 31, 2019 3.3 April 1, 2019 - June 30, 2019 3.3 July 1, 2019 - September 30, 2019 3.3 October 1, 2019 - December 31, 2019 3.2 Total 2019 13.1 2020 12.6 2021 12.1 2022 11.5 2023 and thereafter 103.6 Total gross unearned insurance premiums $ 156.2
Schedule of Net Written Premiums	The following table presents a schedule of net written premiums included in White Mountains’s HG Global/BAM segment for the three months years ended September 30, 2018 and 2017 : Three Months Ended September 30, Nine Months Ended September 30, Millions 2018 2017 2018 2017 Gross written premiums $ 3.9 $ 10.9 $ 28.9 $ 42.0 Assumed (ceded) written premiums — — — — Net written premiums $ 3.9 $ 10.9 $ 28.9 $ 42.0

Earnings Per Share (Tables)

Earnings Per Share (Tables)	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Computation of earnings per share	The following table presents the Company’s computation of earnings per share from continuing operations for the three months ended September 30, 2018 and 2017 . See Note 16 — “Held for Sale and Discontinued Operations” . Three Months Ended Nine Months Ended September 30, September 30, 2018 2017 2018 2017 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 40.8 $ 562.1 $ (3.7 ) $ 604.7 Less: total income from discontinued operations, net of tax (17.3 ) 539.1 (17.2 ) 573.2 Net income from continuing operations attributable to White Mountains’s common shareholders $ 58.1 $ 23.0 $ 13.5 $ 31.5 Allocation of earnings to participating restricted common shares (1) (.8 ) (.3 ) (.2 ) (.4 ) Basic and diluted earnings per share numerators $ 57.3 $ 22.7 $ 13.3 $ 31.1 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 3,180.4 4,297.2 3,450.8 4,477.0 Average unvested restricted common shares (2) (41.5 ) (53.7 ) (39.7 ) (54.5 ) Basic earnings per share denominator 3,138.9 4,243.5 3,411.1 4,422.5 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 3,180.4 4,297.2 3,450.8 4,477.0 Average unvested restricted common shares (2) (41.5 ) (53.7 ) (39.7 ) (54.5 ) Diluted earnings per share denominator 3,138.9 4,243.5 3,411.1 4,422.5 Basic and diluted earnings per share (in dollars) - continuing operations: Distributed earnings - dividends declared and paid $ — $ — $ 1.00 $ 1.00 Undistributed earnings (losses) 18.27 5.36 2.90 6.03 Basic and diluted earnings per share $ 18.27 $ 5.36 $ 3.90 $ 7.03 (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (2) Restricted shares outstanding vest either in equal annual installments or upon a stated date. See Note 10 — “Employee Share-Based Incentive Compensation Plans”
Disposal Groups, Including Discontinued Operations	The following table presents the undistributed net earnings (losses) from continuing operations for the three months ended September 30, 2018 and 2017 . See Note 16 — “Held for Sale and Discontinued Operations” . Three Months Ended Nine Months Ended September 30, September 30, Millions 2018 2017 2018 2017 Undistributed net income (losses) - continuing operations: Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 57.3 $ 22.7 $ 13.3 $ 31.1 Dividends declared net of restricted common share amounts (1) — — (3.7 ) (4.5 ) Total undistributed net losses, net of restricted common share amounts $ 57.3 $ 22.7 $ 9.6 $ 26.6 (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. Net Income (Loss) from Discontinued Operations For the three and nine months ended September 30, 2018 , White Mountains recognized net loss from sale of discontinued operations of $17.3 million and $17.2 million , which was primarily due to a tax contingency related to the sale of Sirius Group. See Note 15 — “Contingencies”. The following table presents the results of operations, including related income taxes, associated with the business classified as discontinued operations for the three and nine months ended September 30, 2017 . 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 inured to White Mountains. Given the fixed price nature of the OneBeacon Transaction, OneBeacon’s results were economically transferred to the buyer at signing. Three Months Ended Nine Months Ended September 30, 2017 September 30, 2017 Millions OneBeacon Sirius Group Total OneBeacon Sirius Group Tranzact Total Revenues Earned insurance premiums $ 268.4 $ — $ 268.4 $ 807.6 $ — $ — $ 807.6 Net investment income 13.0 — 13.0 39.7 — — 39.7 Net realized and unrealized gains 11.5 — 11.5 38.8 — — 38.8 Other revenue 2.2 — 2.2 7.7 — — 7.7 Total revenues 295.1 — 295.1 893.8 — — 893.8 Expenses Loss and loss adjustment expenses 206.8 — 206.8 546.0 — — 546.0 Insurance and reinsurance acquisition expenses 51.9 — 51.9 145.6 — — 145.6 Other underwriting expenses 44.9 — 44.9 156.2 — — 156.2 General and administrative expenses 7.4 — 7.4 21.2 — — 21.2 Interest expense 3.4 — 3.4 10.0 — — 10.0 Total expenses 314.4 — 314.4 879.0 — — 879.0 Pre-tax (loss) income (19.3 ) — (19.3 ) 14.8 — — 14.8 Income tax benefit 4.1 — 4.1 5.7 — — 5.7 Net (loss) income from discontinued operations (15.2 ) — (15.2 ) 20.5 — — 20.5 Net gain (loss) from sale of discontinued operations 554.5 (.2 ) 554.3 554.5 (.8 ) (1.0 ) 552.7 Total income (loss) from discontinued operations 539.3 (.2 ) 539.1 575.0 (.8 ) (1.0 ) 573.2 Change in foreign currency translation and other from discontinued operations .1 — .1 .3 — — .3 Recognition of benefit plan assets and obligations from the sale of OneBeacon, net of tax 2.9 — 2.9 2.9 — — 2.9 Comprehensive income (loss) from discontinued operations $ 542.3 $ (.2 ) $ 542.1 $ 578.2 $ (.8 ) $ (1.0 ) $ 576.4 Net Change in Cash from Discontinued Operations The following table presents the net change in cash, including income tax payments to national governments and interest paid associated with the business classified as discontinued operations: Nine Months Ended September 30, Millions 2018 2017 Net cash provided from (used for) operations $ — $ 157.0 Net cash (used for) provided from investing activities — 3.0 Net cash used for financing activities — (61.9 ) Net change in cash during the period — 98.1 Cash balances at beginning of period — 70.5 Net change in cash held for sale, excluding discontinued operations — (.9 ) Cash sold as part of sale of consolidated subsidiaries — 167.7 Cash balances at end of period $ — $ — Supplemental cash flows information: Interest paid $ — $ — Net income tax payment to national governments $ — $ — Earnings Per Share from Discontinued Operations White Mountains calculates earnings per share using the two-class method, which allocates earnings between common and unvested restricted common shares. Both classes of shares participate equally in earnings on a per share basis. Basic earnings per share amounts are based on the weighted average number of common shares outstanding adjusted for unvested restricted common shares. Diluted earnings per share amounts are also impacted by the net effect of potentially dilutive common shares outstanding. The following table presents the Company’s computation of earnings per share for discontinued operations for the three and nine months ended September 30, 2018 and 2017 : Three Months Ended Nine Months Ended September 30, September 30, 2018 2017 2018 2017 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 40.8 $ 562.1 $ (3.7 ) $ 604.7 Less total loss from continuing operations, net of tax 58.1 23.0 13.5 31.5 Net income from discontinued operations attributable to White Mountains’s common shareholders $ (17.3 ) $ 539.1 $ (17.2 ) $ 573.2 Allocation of earnings to participating restricted common shares (1) .2 (6.7 ) .2 (7.0 ) Basic and diluted earnings per share numerators $ (17.1 ) $ 532.4 $ (17.0 ) $ 566.2 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 3,180.4 4,297.2 3,450.8 4,477.0 Average unvested restricted common shares (3) (41.5 ) (53.7 ) (39.7 ) (54.5 ) Basic earnings per share denominator 3,138.9 4,243.5 3,411.1 4,422.5 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 3,180.4 4,297.2 3,450.8 4,477.0 Average unvested restricted common shares (3) (41.5 ) (53.7 ) (39.7 ) (54.5 ) Diluted earnings per share denominator 3,138.9 4,243.5 3,411.1 4,422.5 Basic and diluted earnings per share (in dollars) - discontinued operations: $ (5.44 ) $ 125.45 $ (4.98 ) $ 128.03 (1) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities. (2) Net earnings attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the three and nine months ended September 30, 2018 and 2017 . (3) Restricted common shares outstanding vest either in equal annual installments or upon a stated date. See Note 10 — “Employee Share-Based Incentive Compensation Plans” .

Employee Share-Based Incentiv_2

Employee Share-Based Incentive Compensation Plans (Tables) - White Mountains	9 Months Ended
	Sep. 30, 2018
Share-based compensation arrangement by share-based payment award
Summary of performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan	The following table presents the performance share activity for the three and nine months ended September 30, 2018 and 2017 for performance shares granted under the WTM Incentive Plan: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Millions, except share amounts Target Performance Accrued Expense Target Performance Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Beginning of period 40,616 $ 27.2 50,575 $ 31.2 50,515 $ 45.8 80,353 $ 42.4 Shares paid (1) — — — — (23,186 ) (28.4 ) (30,838 ) (21.6 ) New grants — — — — 14,105 — 17,510 — Forfeitures and cancellations (2) — (.1 ) (256 ) (.4 ) (818 ) .1 (16,706 ) (9.4 ) Expense recognized — 7.8 — 7.0 — 17.4 — 26.4 End of period (3) 40,616 $ 34.9 50,319 $ 37.8 40,616 $ 34.9 50,319 $ 37.8 (1) WTM performance share payments in 2018 for the 2015-2017 performance cycle, which were paid in March 2018, ranged from 145% to 147% of target. WTM performance share payments in 2017 for the 2014-2016 performance cycle, which were paid in March 2017, ranged from 34% to 76% of target. (2) Amounts include changes in assumed forfeitures, as required under GAAP. (3) Outstanding performance share awards as of September 30, 2017 excludes 2,195 performance share awards granted to employees of Sirius Group.
Share-based Compensation, Performance Shares Award Outstanding Activity	The following table presents performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan at September 30, 2018 for each performance cycle: Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Performance cycle: 2016 – 2018 13,715 $ 18.9 2017 – 2019 14,070 13.3 2018 – 2020 13,450 3.2 Sub-total 41,235 35.4 Assumed forfeitures (619 ) (.5 ) September 30, 2018 40,616 $ 34.9
Summary of restricted shares activity	The following table presents the unrecognized compensation cost associated with the outstanding restricted share awards for the three and nine months ended September 30, 2018 and 2017 : Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Millions, except share amounts Restricted Unamortized Restricted Unamortized Restricted Unamortized Restricted Unamortized Non-vested, Beginning of period 41,510 $ 19.0 53,815 $ 23.5 53,755 $ 14.3 70,620 $ 19.7 Issued — — — — 14,105 11.4 17,785 16.7 Vested — — (260 ) — (25,381 ) — (28,846 ) — Forfeited — — — — (969 ) (.2 ) (6,004 ) (3.5 ) Expense recognized — (3.2 ) — (3.8 ) — (9.7 ) — (13.2 ) End of period (1) 41,510 $ 15.8 53,555 $ 19.7 41,510 $ 15.8 53,555 $ 19.7 (1) Restricted share awards outstanding as of September 30, 2017 includes 2,195 restricted shares issued to employees of Sirius Group, which was accounted for as discontinued operations.

Non-controlling Interest (Table

Non-controlling Interest (Tables)	9 Months Ended
	Sep. 30, 2018
Noncontrolling Interest [Abstract]
Non-controlling Interests	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 September 30, 2018 and December 31, 2017 : September 30, 2018 December 31, 2017 $ in millions Non-controlling Percentage Non-controlling Equity Non-controlling Percentage Non-controlling Equity Other, excluding mutuals HG Global 3.1 % $ 14.8 3.1 % $ 15.9 NSM 5.0 14.1 — — MediaAlpha 37.8 15.3 35.7 13.1 Buzzmove 22.9 1.5 22.9 2.5 Total other, excluding mutuals 45.7 31.5 Mutuals BAM 100.0 (190.1 ) 100.0 (163.2 ) Total non-controlling interests $ (144.4 ) $ (131.7 )

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2018
Segment Reporting [Abstract]
Financial information for White Mountains' segments	The following table presents the financial information for White Mountains’s segments: Millions HG Global/BAM NSM MediaAlpha Other Operations Total Three Months Ended September 30, 2018 Earned insurance premiums $ 3.3 $ — $ — $ — $ 3.3 Net investment income 4.8 — — 8.6 13.4 Net realized and unrealized investment (losses) gains (4.1 ) — — 70.2 66.1 Advertising and commission revenues (1) — 36.6 74.5 1.1 112.2 Other revenue .2 3.1 — .4 3.7 Total revenues 4.2 39.7 74.5 80.3 198.7 Insurance acquisition expenses 1.2 — — — 1.2 Other underwriting expenses .1 — — — .1 Cost of sales — — 61.8 1.1 62.9 General and administrative expenses 11.3 25.8 5.4 26.0 68.5 Broker commission expense — 10.9 — — 10.9 Amortization of other intangible assets — 5.0 2.4 — 7.4 Interest expense — 3.2 .2 — 3.4 Total expenses 12.6 44.9 69.8 27.1 154.4 Pre-tax (loss) income $ (8.4 ) $ (5.2 ) $ 4.7 $ 53.2 $ 44.3 (1) Approximately 30% of MediaAlpha’s advertising revenue for the three months ended September 30, 2018 was associated with one customer. Millions HG Global/BAM MediaAlpha Other Operations Total Three Months Ended September 30, 2017 Earned insurance premiums $ 2.4 $ — $ — $ 2.4 Net investment income 3.3 — 8.9 12.2 Net realized and unrealized investment gains .8 — 31.7 32.5 Advertising and commission revenues (1) — 37.9 .9 38.8 Other revenue .2 — 1.4 1.6 Total revenues 6.7 37.9 42.9 87.5 Insurance acquisition expenses .9 — — .9 Other underwriting expenses .1 — — .1 Cost of sales — 32.2 .9 33.1 General and administrative expenses 10.6 3.8 27.3 41.7 Amortization of other intangible assets — 2.3 .1 2.4 Interest expense — .1 .8 .9 Total expenses 11.6 38.4 29.1 79.1 Pre-tax (loss) income $ (4.9 ) $ (.5 ) $ 13.8 $ 8.4 (1) Approximately 32% of MediaAlpha’s advertising revenue for the three months ended September 30, 2017 was associated with one customer. Millions HG Global/BAM NSM (2) MediaAlpha Other Operations Total Nine Months Ended September 30, 2018 Earned insurance premiums $ 9.7 $ — $ — — $ 9.7 Net investment income 12.5 — — 32.4 44.9 Net realized and unrealized investment (losses) gains (14.4 ) — — 37.3 22.9 Advertising and commission revenues (1) — 59.2 216.4 3.0 278.6 Other revenue .8 4.0 1.6 .6 7.0 Total revenues 8.6 63.2 218.0 73.3 363.1 Insurance acquisition expenses 3.9 — — — 3.9 Other underwriting expenses .3 — — — .3 Cost of sales — — 179.1 2.9 182.0 General and administrative expenses 36.5 37.9 21.2 79.0 174.6 Broker commission expense — 17.5 — — 17.5 Amortization of other intangible assets — 5.0 7.8 .1 12.9 Interest expense — 4.8 .9 .3 6.0 Total expenses 40.7 65.2 209.0 82.3 397.2 Pre-tax (loss) income $ (32.1 ) $ (2.0 ) $ 9.0 $ (9.0 ) $ (34.1 ) (1) Approximately 31% of MediaAlpha’s advertising revenue for the nine months ended September 30, 2018 was associated with one customer. (2) NSM’s results are from May 11, 2018, the date of acquisition, to the end of the third quarter. Millions HG Global/BAM MediaAlpha Other Operations Total Nine Months Ended September 30, 2017 Earned insurance premiums $ 6.6 $ — $ — $ 6.6 Net investment income 8.9 — 30.8 39.7 Net realized and unrealized investment gains 3.2 — 99.3 102.5 Advertising and commission revenues (1) — 101.2 2.7 103.9 Other revenue .8 — 6.3 7.1 Total revenues 19.5 101.2 139.1 259.8 Insurance acquisition expenses 3.0 — — 3.0 Other underwriting expenses .3 — — .3 Cost of sales — 86.0 2.7 88.7 General and administrative expenses 31.5 10.7 113.9 156.1 Amortization of other intangible assets — 7.2 .1 7.3 Interest expense — .6 1.2 1.8 Total expenses 34.8 104.5 117.9 257.2 Pre-tax (loss) income $ (15.3 ) $ (3.3 ) $ 21.2 $ 2.6 (1) Approximately 29% of MediaAlpha’s advertising revenue for the nine months ended September 30, 2017 was associated with one customer.

Investments in Unconsolidated_2

Investments in Unconsolidated Entities (Tables)	9 Months Ended
	Sep. 30, 2018
Schedule of Equity Method Investments [Line Items]
Investments in and Advances to Affiliates	The following table presents the carrying values of investments in equity method eligible unconsolidated entities recorded within other long-term investments: Millions September 30, 2018 December 31, 2017 Equity method eligible private equity securities, at fair value $ 97.3 $ 58.0 Investments, accounted for under the equity method 11.6 4.6 Total investments in equity method eligible unconsolidated entities 108.9 62.6 Other unconsolidated investments (1) 180.8 146.2 Total other long-term investments $ 289.7 $ 208.8 (1) Consists of other long-term investments that are not equity method eligible.
Affiliated Entity
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	The following table presents White Mountains’s investments in equity method eligible unconsolidated entities as of September 30, 2018 and December 31, 2017: Ownership Interest Investee September 30, 2018 December 31, 2017 Instrument Held PassportCard (1) 50.0% 50.0% Common shares DavidShield (1) 50.0% — Common shares Kudu 49.5% — Units durchblicker 45.0% 45.0% Common shares Compare.com 22.1% 22.1% Common shares Tuckerman Capital Fund III, L.P. 21.8% 21.3% Limited partnership interest

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Summary of the fair value and carrying value of financial instruments	The following table presents the fair value and carrying value of this financial instrument as of September 30, 2018 and December 31, 2017 : September 30, 2018 December 31, 2017 Millions Fair Value Carrying Value Fair Value Carrying Value NSM Bank Facility $ 147.3 $ 147.3 $ — $ — MediaAlpha Bank Facility $ 15.3 $ 15.0 $ 23.9 $ 23.8

Held for Sale and Discontinue_2

Held for Sale and Discontinued Operations (Tables)	9 Months Ended
	Sep. 30, 2018
Discontinued Operations and Disposal Groups [Abstract]
Disposal Groups, Including Discontinued Operations	The following table presents the undistributed net earnings (losses) from continuing operations for the three months ended September 30, 2018 and 2017 . See Note 16 — “Held for Sale and Discontinued Operations” . Three Months Ended Nine Months Ended September 30, September 30, Millions 2018 2017 2018 2017 Undistributed net income (losses) - continuing operations: Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 57.3 $ 22.7 $ 13.3 $ 31.1 Dividends declared net of restricted common share amounts (1) — — (3.7 ) (4.5 ) Total undistributed net losses, net of restricted common share amounts $ 57.3 $ 22.7 $ 9.6 $ 26.6 (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. Net Income (Loss) from Discontinued Operations For the three and nine months ended September 30, 2018 , White Mountains recognized net loss from sale of discontinued operations of $17.3 million and $17.2 million , which was primarily due to a tax contingency related to the sale of Sirius Group. See Note 15 — “Contingencies”. The following table presents the results of operations, including related income taxes, associated with the business classified as discontinued operations for the three and nine months ended September 30, 2017 . 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 inured to White Mountains. Given the fixed price nature of the OneBeacon Transaction, OneBeacon’s results were economically transferred to the buyer at signing. Three Months Ended Nine Months Ended September 30, 2017 September 30, 2017 Millions OneBeacon Sirius Group Total OneBeacon Sirius Group Tranzact Total Revenues Earned insurance premiums $ 268.4 $ — $ 268.4 $ 807.6 $ — $ — $ 807.6 Net investment income 13.0 — 13.0 39.7 — — 39.7 Net realized and unrealized gains 11.5 — 11.5 38.8 — — 38.8 Other revenue 2.2 — 2.2 7.7 — — 7.7 Total revenues 295.1 — 295.1 893.8 — — 893.8 Expenses Loss and loss adjustment expenses 206.8 — 206.8 546.0 — — 546.0 Insurance and reinsurance acquisition expenses 51.9 — 51.9 145.6 — — 145.6 Other underwriting expenses 44.9 — 44.9 156.2 — — 156.2 General and administrative expenses 7.4 — 7.4 21.2 — — 21.2 Interest expense 3.4 — 3.4 10.0 — — 10.0 Total expenses 314.4 — 314.4 879.0 — — 879.0 Pre-tax (loss) income (19.3 ) — (19.3 ) 14.8 — — 14.8 Income tax benefit 4.1 — 4.1 5.7 — — 5.7 Net (loss) income from discontinued operations (15.2 ) — (15.2 ) 20.5 — — 20.5 Net gain (loss) from sale of discontinued operations 554.5 (.2 ) 554.3 554.5 (.8 ) (1.0 ) 552.7 Total income (loss) from discontinued operations 539.3 (.2 ) 539.1 575.0 (.8 ) (1.0 ) 573.2 Change in foreign currency translation and other from discontinued operations .1 — .1 .3 — — .3 Recognition of benefit plan assets and obligations from the sale of OneBeacon, net of tax 2.9 — 2.9 2.9 — — 2.9 Comprehensive income (loss) from discontinued operations $ 542.3 $ (.2 ) $ 542.1 $ 578.2 $ (.8 ) $ (1.0 ) $ 576.4 Net Change in Cash from Discontinued Operations The following table presents the net change in cash, including income tax payments to national governments and interest paid associated with the business classified as discontinued operations: Nine Months Ended September 30, Millions 2018 2017 Net cash provided from (used for) operations $ — $ 157.0 Net cash (used for) provided from investing activities — 3.0 Net cash used for financing activities — (61.9 ) Net change in cash during the period — 98.1 Cash balances at beginning of period — 70.5 Net change in cash held for sale, excluding discontinued operations — (.9 ) Cash sold as part of sale of consolidated subsidiaries — 167.7 Cash balances at end of period $ — $ — Supplemental cash flows information: Interest paid $ — $ — Net income tax payment to national governments $ — $ — Earnings Per Share from Discontinued Operations White Mountains calculates earnings per share using the two-class method, which allocates earnings between common and unvested restricted common shares. Both classes of shares participate equally in earnings on a per share basis. Basic earnings per share amounts are based on the weighted average number of common shares outstanding adjusted for unvested restricted common shares. Diluted earnings per share amounts are also impacted by the net effect of potentially dilutive common shares outstanding. The following table presents the Company’s computation of earnings per share for discontinued operations for the three and nine months ended September 30, 2018 and 2017 : Three Months Ended Nine Months Ended September 30, September 30, 2018 2017 2018 2017 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 40.8 $ 562.1 $ (3.7 ) $ 604.7 Less total loss from continuing operations, net of tax 58.1 23.0 13.5 31.5 Net income from discontinued operations attributable to White Mountains’s common shareholders $ (17.3 ) $ 539.1 $ (17.2 ) $ 573.2 Allocation of earnings to participating restricted common shares (1) .2 (6.7 ) .2 (7.0 ) Basic and diluted earnings per share numerators $ (17.1 ) $ 532.4 $ (17.0 ) $ 566.2 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 3,180.4 4,297.2 3,450.8 4,477.0 Average unvested restricted common shares (3) (41.5 ) (53.7 ) (39.7 ) (54.5 ) Basic earnings per share denominator 3,138.9 4,243.5 3,411.1 4,422.5 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 3,180.4 4,297.2 3,450.8 4,477.0 Average unvested restricted common shares (3) (41.5 ) (53.7 ) (39.7 ) (54.5 ) Diluted earnings per share denominator 3,138.9 4,243.5 3,411.1 4,422.5 Basic and diluted earnings per share (in dollars) - discontinued operations: $ (5.44 ) $ 125.45 $ (4.98 ) $ 128.03 (1) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities. (2) Net earnings attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the three and nine months ended September 30, 2018 and 2017 . (3) Restricted common shares outstanding vest either in equal annual installments or upon a stated date. See Note 10 — “Employee Share-Based Incentive Compensation Plans” .

Basis of Presentation and Sig_3

Basis of Presentation and Significant Accouting Policies (Basis of Presentation) (Details) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)advertiserpublisher	Sep. 30, 2017USD ($)	May 11, 2018	Dec. 31, 2017	Sep. 28, 2017$ / shares	Jul. 31, 2012USD ($)
HG Global
Basis of Presentation
Percentage of par value of policy reinsured	15.00%
Surplus notes	$ 499					$ 503
MediaAlpha
Basis of Presentation
Ownership interest (as a percent)	62.20%			64.40%
Number of advertisers | advertiser	600
Number of publishers | publisher	330
Preferred Stock | HG Global
Basis of Presentation
Ownership interest (as a percent)	96.90%			96.90%
Common Stock | HG Global
Basis of Presentation
Ownership interest (as a percent)	88.40%			88.40%
Restricted Stock
Basis of Presentation
Payments related to tax withholding for share-based compensation	$ 8.4	$ 9.2
OneBeacon
Basis of Presentation
Business acquisition, share price | $ / shares					$ 18.10
NSM Insurance HoldCo, LLC
Basis of Presentation
Ownership interest (as a percent)			95.00%

Significant Transactions (Detai

Significant Transactions (Details)	May 11, 2018USD ($)	Jan. 24, 2018USD ($)	Oct. 05, 2017USD ($)shares	Sep. 28, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)earnout_period	Sep. 30, 2017USD ($)	May 18, 2018USD ($)	Feb. 05, 2018USD ($)	Dec. 31, 2017USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchases of consolidated subsidiaries, net of cash acquired							$ 252,100,000	$ 0
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold							0	1,131,000,000
Net (loss) gain from sale of discontinued operations, net of tax						$ 554,300,000	(17,200,000)	$ 552,700,000
NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Contingent consideration					$ 2,600,000		2,600,000
Contingent consideration earnout liabilities					$ 20,100,000		$ 20,100,000				$ 0
Healthplans.com
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchases of consolidated subsidiaries, net of cash acquired			$ 28,000,000
MediaAlpha
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)					62.20%		62.20%				64.40%
OneBeacon
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold				$ 1,300,000,000
Net (loss) gain from sale of discontinued operations, net of tax				$ 554,600,000
NSM Insurance HoldCo, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)	95.00%
Purchases of consolidated subsidiaries, net of cash acquired	$ 274,200,000
Purchase price adjustment					$ 2,100,000
Assets acquired	495,200,000
Goodwill and intangible assets acquired	383,000,000
Liabilities acquired	204,600,000
Noncontrolling interest acquired	14,400,000
Transaction costs					6,300,000
Contingent consideration	$ 10,200,000
Number of earnout periods | earnout_period							2
Fresh Insurance Services Group Limited
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [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
Equity method investment, aggregate cost									49,600,000
Contingent consideration									$ 7,500,000
DavidShield
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)		50.00%			50.00%		50.00%				0.00%
DavidShield & PassportCard
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity method investment, aggregate cost		$ 41,800,000
Payments to acquire business, net		$ 28,300,000
PassportCard
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)		50.00%			50.00%		50.00%				50.00%
PassportCard | White Mountains
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)					25.00%		25.00%
Kudu Investment Management, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)					49.50%		49.50%				0.00%
Equity method investment, aggregate cost					$ 12,300,000		$ 12,300,000			$ 127,500,000
Additional funding					63,000,000		63,000,000			$ 125,000,000
Kudu Investment Management, LLC | White Mountains
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Additional funding					$ 31,500,000		$ 31,500,000
Capital Unit, Class A | MediaAlpha
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Capital units, purchased | shares			131,579
Capital units, value			$ 12,500,000

Investment Securities (Net Inve

Investment Securities (Net Investment Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investment income
Total investment income	$ 13.9	$ 12.9	$ 46.7	$ 41.7
Third-party investment expenses	(0.5)	(0.7)	(1.8)	(2)
Net investment income, pre-tax	13.4	12.2	44.9	39.7
Fixed maturity investments
Investment income
Total investment income	7.5	9.8	27.3	32.9
Short-term investments
Investment income
Total investment income	1.8	0.3	6.5	0.6
Common Stock
Investment income
Total investment income	3.4	2.7	11.2	7.7
Other long-term investments
Investment income
Total investment income	$ 1.2	$ 0.1	$ 1.7	$ 0.5

Investment Securities (Net Real

Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Net realized investment (losses) gains, pre-tax	$ (10.2)	$ 6.8	$ (13.9)	$ 20.8
Net unrealized investment gains, pre-tax	76.3	25.7	36.8	81.7
Net realized and unrealized investment gains, pre-tax	66.1	32.5	22.9	102.5
Income tax expense attributable to net realized and unrealized investment gains	(9.7)	(3.9)	(5.9)	(9.5)
Net realized and unrealized investment gains, after-tax	$ 56.4	$ 28.6	$ 17	$ 93

Investment Securities (Net Re_2

Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt and Equity Securities, FV-NI [Line Items]
Trading Securities, Realized Gain (Loss)	$ (10.2)	$ 7.4	$ (24.9)	$ 21
Trading securities, Tax on Realized Holding Gain (Loss) on Investments	2.3	(0.6)	11.2	(3.6)
Trading securities realized holding gain (loss) on investments after tax	(7.9)	6.8	(13.7)	17.4
Foreign Currency Transaction Gain (Loss), Realized	0	(0.6)	11	(0.2)
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax	0	0	0	0
Trading securities, realized foreign currency gain (loss) net of tax	0	(0.6)	11	(0.2)
Net realized investment (losses) gains, pre-tax	(10.2)	6.8	(13.9)	20.8
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	2.3	(0.6)	11.2	(3.6)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	(7.9)	6.2	(2.7)	17.2
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Trading Securities, Realized Gain (Loss)	(5.1)	0.6	(27.6)	(0.4)
Foreign Currency Transaction Gain (Loss), Realized	0	1.2	18.2	2.7
Net realized investment (losses) gains, pre-tax	(5.1)	1.8	(9.4)	2.3
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Trading Securities, Realized Gain (Loss)	0	(0.1)	(0.8)	(0.1)
Foreign Currency Transaction Gain (Loss), Realized	0	0	0	0
Net realized investment (losses) gains, pre-tax	0	(0.1)	(0.8)	(0.1)
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Trading Securities, Realized Gain (Loss)	(1.5)	4.9	10.5	18.5
Foreign Currency Transaction Gain (Loss), Realized	0	5.4	0	6
Net realized investment (losses) gains, pre-tax	(1.5)	10.3	10.5	24.5
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Trading Securities, Realized Gain (Loss)	(3.6)	2	(7)	3
Foreign Currency Transaction Gain (Loss), Realized	0	(7.2)	(7.2)	(8.9)
Net realized investment (losses) gains, pre-tax	$ (3.6)	$ (5.2)	$ (14.2)	$ (5.9)

Investment Securities (Net Unre

Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ 76.4	$ 26.1	$ 48.4	$ 82.1
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	(12)	(3.3)	(17.1)	(5.9)
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	64.4	22.8	31.3	76.2
Foreign Currency Transaction Gain (Loss), Unrealized	(0.1)	(0.4)	(11.6)	(0.4)
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	0	0	0	0
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	(0.1)	(0.4)	(11.6)	(0.4)
Net unrealized investment gains, pre-tax	76.3	25.7	36.8	81.7
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	(12)	(3.3)	(17.1)	(5.9)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	64.3	22.4	19.7	75.8
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	1.5	2	(15.7)	19.4
Foreign Currency Transaction Gain (Loss), Unrealized	0	5.1	(14.8)	12.6
Net unrealized investment gains, pre-tax	1.5	7.1	(30.5)	32
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	(0.1)	0	(0.1)	0
Foreign Currency Transaction Gain (Loss), Unrealized	0	0	0	0
Net unrealized investment gains, pre-tax	(0.1)	0	(0.1)	0
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	54.9	26.6	43.7	53.5
Foreign Currency Transaction Gain (Loss), Unrealized	0	(3.1)	0	0
Net unrealized investment gains, pre-tax	54.9	23.5	43.7	53.5
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	20.1	(2.5)	20.5	9.2
Foreign Currency Transaction Gain (Loss), Unrealized	(0.1)	(2.4)	3.2	(13)
Net unrealized investment gains, pre-tax	$ 20	$ (4.9)	$ 23.7	$ (3.8)

Investment Securities (Investme

Investment Securities (Investment gains (losses) for Level 3) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ 76.4	$ 26.1	$ 48.4	$ 82.1
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	20.1	(2.5)	20.5	9.2
Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	4	(0.7)	2.1	(2.2)
Level 3 Inputs | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ 4	$ (0.7)	$ 2.1	$ (2.2)

Investment Securities (Invest_2

Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	$ 127	$ 297.8
Trading Securities, Unrealized Holding Gain	0	0
Trading Securities, Unrealized Holding Loss	(1.6)	(1.3)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	125.4	296.5
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	509.4	867.6
Trading Securities, Unrealized Holding Gain	0.2	2.9
Trading Securities, Unrealized Holding Loss	(9.6)	(4.3)
Net foreign currency gains (losses)	0	14.7
Fixed maturity investments, at fair value	500	880.9
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	281.1	252
Trading Securities, Unrealized Holding Gain	0.4	3.7
Trading Securities, Unrealized Holding Loss	(3.5)	(0.8)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	278	254.9
Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	132.4	697.2
Trading Securities, Unrealized Holding Gain	0	1.6
Trading Securities, Unrealized Holding Loss	(3.9)	(4.1)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	128.5	694.7
Foreign government, agency and provincial obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost		2.6
Trading Securities, Unrealized Holding Gain		0
Trading Securities, Unrealized Holding Loss		0
Net foreign currency gains (losses)		0.1
Fixed maturity investments, at fair value		2.7
Fixed Income Investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	1,049.9	2,117.2
Trading Securities, Unrealized Holding Gain	0.6	8.2
Trading Securities, Unrealized Holding Loss	(18.6)	(10.5)
Net foreign currency gains (losses)	0	14.8
Total fixed maturity investments including assets held for sale	$ 1,031.9	$ 2,129.7

Investment Securities (Invest_3

Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Common Stock
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	$ 873.4	$ 739.7
Trading Securities, Unrealized Holding Gain	171.1	129.4
Trading Securities, Unrealized Holding Loss	(1)	(3)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	1,043.5	866.1
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	303.8	246.6
Trading Securities, Unrealized Holding Gain	34.8	6.8
Trading Securities, Unrealized Holding Loss	(47.2)	(39.7)
Net foreign currency gains (losses)	(1.7)	(4.9)
Fixed maturity investments, at fair value	$ 289.7	$ 208.8

Investment Securities (Other lo

Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Hedge and private equity funds included in other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	$ 151.6	$ 125.3
Equity method eligible private equity securities, at fair value
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	126.4	83.2
Forward Contracts
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	0	(3.7)
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	11.7	4
White Mountains
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	289.7	208.8
White Mountains | Equity method eligible private equity securities, at fair value
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	$ 97.3	$ 58

Investment Securities (Hedge Fu

Investment Securities (Hedge Funds and Private Equity Funds) (Details) $ in Millions	Sep. 30, 2018USD ($)fund	Dec. 31, 2017USD ($)
Hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Number of investments	1
Private equity security
Debt and Equity Securities, FV-NI [Line Items]
Number of investments	13
Hedge and private equity funds included in other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Largest single fund investment | $	$ 59.6	$ 54.9

Investment Securities (Fair Val

Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Hedge and private equity funds included in other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	$ 151.6	$ 125.3
Unfunded Commitments	131	58.1
Hedge funds | Hedge Fund, Long/ Short Equity Banks and Financial
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	59.6	54.9
Unfunded Commitments	0	0
Hedge funds | Hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	59.6	54.9
Unfunded Commitments	0	0
Private equity security | Industrial
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	49.6	43.3
Unfunded Commitments	10.5	10.4
Private equity security | Aerospace Defense Government
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	25.5	15.8
Unfunded Commitments	34.9	12.9
Private equity security | Direct Lending
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	10.6	7.1
Unfunded Commitments	19.9	23.1
Private equity security | Financial Services Sector
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	6.3	4.2
Unfunded Commitments	15.7	11.7
Private equity security | Private equity security
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	92	70.4
Unfunded Commitments	131	58.1
Private equity security | Real estate
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Unfunded Commitments	$ 50	$ 0

Investment Securities (Invest_4

Investment Securities (Investments In Private Equity Funds Subject to Lock-Up Periods) (Details) - Private equity security $ in Millions	Sep. 30, 2018USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	$ 92
1-3 years
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	14.2
3-5 years
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	5
5-10 years
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	55.9
Greater than 10 years
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	$ 16.9

Investment Securities (Restrict

Investment Securities (Restrictions on Redemption Frequency and Advance Notice Requirements) (Details) - Hedge funds $ in Millions	Sep. 30, 2018USD ($)fund
Debt and Equity Securities, FV-NI [Line Items]
Number of investments | fund	1
September 2,018
Debt and Equity Securities, FV-NI [Line Items]
Number of hedge funds	1
Other long-term investments | $	$ 59.6

Investment Securities (Fair V_2

Investment Securities (Fair Value Measurements) (Details) (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Percentage of investments recorded at fair value	89.00%	94.00%

Investment Securities (Fair V_3

Investment Securities (Fair Value Measurement by Level) (Details) $ in Millions	Sep. 30, 2018USD ($)Investment	Dec. 31, 2017USD ($)Investment	Sep. 30, 2017USD ($)	Dec. 31, 2016USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 2,365.1	$ 3,208.3	$ 2,509.4	$ 2,547.3
Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	897.4	890.4	733.6	279.5
Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 1,178	$ 2,105.4	1,525.2	2,093.8
Number of hedge funds	2
Common Stock
Debt and Equity Securities, FV-NI [Line Items]
Number of hedge funds | Investment	2	2
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 500	$ 880.9
Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	128.5	694.7
Mortgage-backed and asset-backed securities | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	128.5	694.7
Mortgage-backed and asset-backed securities | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fixed maturity investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0	5.6	0
Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	138.1	87.2	90.1	91.4
Hedge and private equity funds included in other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	151.6	125.3
Hedge and private equity funds included in other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	151.6	125.3	154.9	82.6
Fair value measured on a recurring basis
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	2,527.2	3,259.1
Fair value measured on a recurring basis | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,198.1	1,041.4
Fair value measured on a recurring basis | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,191	2,130.5
Fair value measured on a recurring basis | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	138.1	87.2
Fair value measured on a recurring basis | U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	125.4	296.5
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	125.4	296.5
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	500	880.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	500	880.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	84.2	185.1
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	84.2	185.1
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	16.9	108.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	16.9	108.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	53.9	94.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	53.9	94.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	22.4	127.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	22.4	127.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	22.2	95.5
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	22.2	95.5
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	145.9	114.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	145.9	114.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments		0
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	60.4	80.5
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	60.4	80.5
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	39.5	25.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	39.5	25.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	54.6	48.1
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	54.6	48.1
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	128.5	694.7
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	128.5	694.7
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	278	254.9
Fair value measured on a recurring basis | Municipal obligations | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Municipal obligations | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	278	254.9
Fair value measured on a recurring basis | Municipal obligations | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments		2.7
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments		0
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments		2.7
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments		0
Fair value measured on a recurring basis | Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,031.9	2,129.7
Fair value measured on a recurring basis | Fixed maturity investments | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	125.4	296.5
Fair value measured on a recurring basis | Fixed maturity investments | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	906.5	1,833.2
Fair value measured on a recurring basis | Fixed maturity investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments		176.1
Short-term investments, at fair value	313.7
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	300.7	151
Short-term investments, at fair value		176.1	786.5	175
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	13	25.1
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Consumer
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	9.8	10.7
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	9.8	10.7
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Health Care
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	16.4	17.1
Fair value measured on a recurring basis | Common equity securities | Health Care | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	16.4	17.1
Fair value measured on a recurring basis | Common equity securities | Health Care | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Health Care | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Communications
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	14.8	10.9
Fair value measured on a recurring basis | Common equity securities | Communications | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	14.8	10.9
Fair value measured on a recurring basis | Common equity securities | Communications | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Communications | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Financials
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	16.7	16.3
Fair value measured on a recurring basis | Common equity securities | Financials | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	16.7	16.3
Fair value measured on a recurring basis | Common equity securities | Financials | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Financials | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Technology
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	16.8	15.1
Fair value measured on a recurring basis | Common equity securities | Technology | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	16.8	15.1
Fair value measured on a recurring basis | Common equity securities | Technology | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Technology | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	15.6	11.9
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	15.6	11.9
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Energy
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	6.6	3.8
Fair value measured on a recurring basis | Common equity securities | Energy | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	6.6	3.8
Fair value measured on a recurring basis | Common equity securities | Energy | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Energy | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	742.9	569.7
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	675.3	508.1
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	67.6	61.6
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Other
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	203.9	210.6
Fair value measured on a recurring basis | Common equity securities | Other | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Other | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	203.9	210.6
Fair value measured on a recurring basis | Common equity securities | Other | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,043.5	866.1
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	772	593.9
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	271.5	272.2
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	138.1	87.2
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 138.1	87.2
Forward Contracts | Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Derivative, fair value, net		$ (3.7)	$ (15.4)	$ (1.2)

Investment Securities (Debt sec

Investment Securities (Debt securities issued by corporation) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 2,365.1	$ 3,208.3	$ 2,509.4	$ 2,547.3
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	500	880.9
Fair value investments	500	880.9
AAA | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	8.1	1.6
AA | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	83.6	42.6
A | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	273.3	192.5
BBB | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	135	465.2
BB | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	161.7
B | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 0	$ 17.3

Investment Securities (Mortgage

Investment Securities (Mortgage-backed, Asset-backed Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Fair value investments	$ 2,365.1	$ 3,208.3	$ 2,509.4	$ 2,547.3

Investment Securities (Mortga_2

Investment Securities (Mortgage-backed Asset Securities Table) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 2,365.1	$ 3,208.3	$ 2,509.4	$ 2,547.3
Government National Mortgage Association Certificates and Obligations (GNMA)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	24.5	46.3
Federal National Mortgage Association Certificates and Obligations (FNMA)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	53.6	84.5
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	39	62
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	117.1	192.8
Commercial Mortgage Backed Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	70.5
Private debt instrument
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	70.5
Collateralized Mortgage Backed Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	117.1	263.3
Collateralized Credit Card Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	8.9	206
Collateralized Auto Loans
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	2.5	142.4
Collateralized Securities, Other
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	83
Asset-backed Securities, Securitized Loans and Receivables
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	11.4	431.4
Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	128.5	694.7
Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,178	2,105.4	$ 1,525.2	$ 2,093.8
Level 2 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	24.5	46.3
Level 2 Inputs | Federal National Mortgage Association Certificates and Obligations (FNMA)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	53.6	84.5
Level 2 Inputs | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	39	62
Level 2 Inputs | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	117.1	192.8
Level 2 Inputs | Commercial Mortgage Backed Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	70.5
Level 2 Inputs | Private debt instrument
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	70.5
Level 2 Inputs | Collateralized Mortgage Backed Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	117.1	263.3
Level 2 Inputs | Collateralized Credit Card Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	8.9	206
Level 2 Inputs | Collateralized Auto Loans
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	2.5	142.4
Level 2 Inputs | Collateralized Securities, Other
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	83
Level 2 Inputs | Asset-backed Securities, Securitized Loans and Receivables
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	11.4	431.4
Level 2 Inputs | Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	128.5	694.7
Level 3 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Level 3 Inputs | Federal National Mortgage Association Certificates and Obligations (FNMA)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Level 3 Inputs | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Level 3 Inputs | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Level 3 Inputs | Commercial Mortgage Backed Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Level 3 Inputs | Private debt instrument
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Level 3 Inputs | Collateralized Mortgage Backed Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Level 3 Inputs | Collateralized Credit Card Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Level 3 Inputs | Collateralized Auto Loans
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Level 3 Inputs | Collateralized Securities, Other
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Level 3 Inputs | Asset-backed Securities, Securitized Loans and Receivables
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Level 3 Inputs | Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 0	$ 0

Investment Securities (Rollforw

Investment Securities (Rollforward of Fair Value Measurements by Level) (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			$ 3,208,300,000	$ 2,547,300,000
Net realized and unrealized gains (losses)			31,300,000	125,600,000
Amortization/Accretion			(2,200,000)	(6,600,000)
Purchases			1,151,900,000	2,078,200,000
Sales			(2,024,200,000)	(2,229,900,000)
Transfers in			0	13,100,000
Transfers out			0	(13,100,000)
Ending balance	$ 2,365,100,000	$ 2,509,400,000	2,365,100,000	2,509,400,000
Net realized and unrealized investment (losses) gains	66,100,000	32,500,000	22,900,000	102,500,000
Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			890,400,000	279,500,000
Net realized and unrealized gains (losses)			51,900,000	52,400,000
Amortization/Accretion			100,000	0
Purchases			412,600,000	940,700,000
Sales			(457,600,000)	(539,000,000)
Transfers in			0	0
Transfers out			0	0
Ending balance	897,400,000	733,600,000	897,400,000	733,600,000
Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			2,105,400,000	2,093,800,000
Net realized and unrealized gains (losses)			(33,600,000)	59,800,000
Amortization/Accretion			(2,300,000)	(6,600,000)
Purchases			673,500,000	1,038,500,000
Sales			(1,565,000,000)	(1,668,200,000)
Transfers in			0	13,100,000
Transfers out			0	0
Ending balance	$ 1,178,000,000	1,525,200,000	$ 1,178,000,000	1,525,200,000
Number of Investments	2		2
Fixed maturity investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			$ 0	0
Net realized and unrealized gains (losses)			0	0
Amortization/Accretion			0	0
Purchases			0	31,200,000
Sales			0	(12,500,000)
Transfers in			0	0
Transfers out			0	(13,100,000)
Ending balance	$ 0	5,600,000	0	5,600,000
Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			87,200,000	91,400,000
Net realized and unrealized gains (losses)			(4,200,000)	(2,200,000)
Amortization/Accretion			0	0
Purchases			55,100,000	2,900,000
Sales			0	(2,000,000)
Transfers in			0	0
Transfers out			0	0
Ending balance	138,100,000	90,100,000	138,100,000	90,100,000
Hedge and private equity funds included in other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			125,300,000	82,600,000
Net realized and unrealized gains (losses)			17,200,000	15,600,000
Amortization/Accretion			0	0
Purchases			10,700,000	64,900,000
Sales			(1,600,000)	(8,200,000)
Transfers in			0	0
Transfers out			0	0
Ending balance	151,600,000	154,900,000	151,600,000	154,900,000
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains			(700,000)
Foreign Exchange Forward
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains			(3,500,000)	(23,100,000)
Foreign Currency on Cash and Open Trades
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains			(4,200,000)
Fair value measured on a recurring basis
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			3,259,100,000
Ending balance	2,527,200,000		2,527,200,000
Fair value measured on a recurring basis | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			1,041,400,000
Ending balance	1,198,100,000		1,198,100,000
Fair value measured on a recurring basis | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			2,130,500,000
Ending balance	1,191,000,000		1,191,000,000
Fair value measured on a recurring basis | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			87,200,000
Ending balance	138,100,000		138,100,000
Fair value measured on a recurring basis | Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			2,129,700,000
Ending balance	1,031,900,000		1,031,900,000
Fair value measured on a recurring basis | Fixed maturity investments | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			296,500,000
Ending balance	125,400,000		125,400,000
Fair value measured on a recurring basis | Fixed maturity investments | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			1,833,200,000
Ending balance	906,500,000		906,500,000
Fair value measured on a recurring basis | Fixed maturity investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			0
Ending balance	0		0
Fair value measured on a recurring basis | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			87,200,000
Ending balance	138,100,000		138,100,000
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			0
Ending balance	0		0
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			0
Ending balance	0		0
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			87,200,000
Ending balance	138,100,000		138,100,000
Fair value measured on a recurring basis | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			176,100,000
Short-term investments, at fair value	313,700,000		313,700,000
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			151,000,000
Ending balance	300,700,000		300,700,000
Short-term investments, at fair value		786,500,000		786,500,000	$ 176,100,000	$ 175,000,000
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			25,100,000
Ending balance	13,000,000		13,000,000
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			0
Ending balance	$ 0		$ 0
Forward Contracts | Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Derivative, fair value, net		$ (15,400,000)		(15,400,000)	$ (3,700,000)	(1,200,000)
Star & Shield Insurance Exchange
Debt and Equity Securities, FV-NI [Line Items]
Deconsolidation of SSIE				5,200,000
Star & Shield Insurance Exchange | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Deconsolidation of SSIE				0
Star & Shield Insurance Exchange | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Deconsolidation of SSIE				5,200,000
Star & Shield Insurance Exchange | Fixed maturity investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Deconsolidation of SSIE				0
Star & Shield Insurance Exchange | Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Deconsolidation of SSIE				0
Star & Shield Insurance Exchange | Hedge and private equity funds included in other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Deconsolidation of SSIE				$ 0
Star & Shield Insurance Exchange
Debt and Equity Securities, FV-NI [Line Items]
Trading securities debt reclassified to assets held-for-sale						6,600,000
Star & Shield Insurance Exchange | Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value						$ 100,000

Investment Securities (Signific

Investment Securities (Significant Unobservable Inputs) (Details) - NR - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Private equity security
Quantitative information for Level 3 Fair Value Measurements Assets
Time until expiration of security	4 years	4 years
Volatility rate	50.00%	50.00%
Risk free rate	1.77%	1.77%
Private equity security | Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 15.8
Valuation technique	338.85
Private equity security | Option pricing method
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure		$ 11.3
Valuation technique		0.21
Private debt instrument | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 10.5
Valuation technique	0.0962
Convertible Preferred Stock | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 14.5	$ 14.5
Valuation technique	.30	.30
Convertible Preferred Stock | Multiple of EBITDA
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 3.5	$ 0.6
Valuation technique	6	6
Investment B | Private equity security | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 21	$ 21
Valuation technique	0.25	0.25
Investment E | Private equity security | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 22.1	$ 22.1
Valuation technique	0.35	0.35
Investment C | Private equity security | Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 28.3
Valuation technique	56.60
Investment A | Private equity security | Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure		$ 3.6
Valuation technique		2.521

Goodwill and Other Intangible_3

Goodwill and Other Intangible Assets - Narrative (Details) - USD ($) $ in Millions	May 18, 2018	May 11, 2018
NSM Insurance HoldCo, LLC
Goodwill [Line Items]
Goodwill and intangible assets acquired		$ 383
Fresh Insurance Services Group Limited
Goodwill [Line Items]
Goodwill and intangible assets acquired	$ 54.6

Goodwill and Other Intangible_4

Goodwill and Other Intangible Assets - Goodwill Rollforward (Details) - USD ($) $ in Millions	2 Months Ended	3 Months Ended		5 Months Ended	9 Months Ended
	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017
Goodwill [Roll Forward]
Goodwill, beginning balance					$ 25.9
Unallocated acquisition date fair value of intangible assets, beginning balance		$ 436.2
Intangible assets, gross (including goodwill), beginning balance					96.9
Attribution of acquisition date fair value estimates between goodwill and other intangible assets		(436.2)
Amortization		7.4	$ 2.4		12.9	$ 7.3
Goodwill, ending balance		321.4		$ 321.4	321.4
Intangible assets, net (including goodwill), ending balance		485.2		485.2	485.2
Unallocated acquisition date fair value of intangible assets, ending balance	$ 436.2	0		0	0
Intangible assets, gross (including goodwill), ending balanc		532.9		532.9	532.9
Other Intangible Assets
Goodwill [Roll Forward]
Intangible assets, net (excluding goodwill), beginning balance		30.7	14.4		36.2	19.3
Attribution of acquisition date fair value estimates between goodwill and other intangible assets		140.9			140.9
Foreign currency translation		(0.4)			(0.4)
Amortization		(7.4)	(2.5)		(12.9)	(7.4)
Intangible assets, net (excluding goodwill), ending balance	30.7	163.8	11.9	163.8	163.8	11.9
Intangible Asset and Goodwill
Goodwill [Roll Forward]
Intangible assets, gross (including goodwill), beginning balance		492.8	40.3		62.1	45.2
Attribution of acquisition date fair value estimates between goodwill and other intangible assets					436.2
Foreign currency translation		(1.6)			(1.6)
Adjustments to reflect acquisition date fair value		1.4			1.4
Amortization		(7.4)	(2.5)		(12.9)	(7.4)
Intangible assets, gross (including goodwill), ending balanc	492.8	485.2	37.8	485.2	485.2	37.8
Goodwill
Goodwill [Roll Forward]
Goodwill, beginning balance		25.9	25.9		25.9	25.9
Attribution of acquisition date fair value estimates between goodwill and other intangible assets		295.3			295.3
Foreign currency translation		(1.2)			(1.2)
Adjustments to reflect acquisition date fair value		1.4			1.4
Goodwill, ending balance	25.9	321.4	$ 25.9	321.4	321.4	$ 25.9
NSM Holding Co LLC
Goodwill [Roll Forward]
Intangible assets, net (including goodwill), ending balance		431		431	$ 431
NSM Holding Co LLC | Other Intangible Assets
Goodwill [Roll Forward]
Foreign currency translation				0.4
Amortization	$ 1.8	$ 3.2
NSM Holding Co LLC | Goodwill
Goodwill [Roll Forward]
Foreign currency translation				$ 1.2

Goodwill and Other Intangible_5

Goodwill and Other Intangible Assets - Schedule of Goodwill and Other Intangible Assets (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Goodwill [Line Items]
Goodwill, acquisition date fair value	$ 321.4	$ 25.9
Goodwill	321.4	25.9
Other intangible assets, accumulated amortization	47.7	34.8
Goodwill and intangible assets, acquisition date fair value	532.9	96.9
Goodwill and intangible assets, net carrying value	485.2	62.1
NSM Holding Co LLC
Goodwill [Line Items]
Goodwill, acquisition date fair value	295.5	0
Goodwill	295.5	0
Other intangible assets, acquisition date fair value	140.5	0
Other intangible assets, accumulated amortization	5	0
Other intangible assets, net carrying value	135.5	0
MediaAlpha
Goodwill [Line Items]
Goodwill, acquisition date fair value	18.3	18.3
Goodwill	18.3	18.3
Other intangible assets, acquisition date fair value	69.9	69.9
Other intangible assets, accumulated amortization	42.3	34.5
Other intangible assets, net carrying value	27.6	35.4
Buzzmove
Goodwill [Line Items]
Goodwill, acquisition date fair value	7.6	7.6
Goodwill	7.6	7.6
Other intangible assets, acquisition date fair value	1.1	1.1
Other intangible assets, accumulated amortization	0.4	0.3
Other intangible assets, net carrying value	0.7	0.8
Customer relationships | NSM Holding Co LLC
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	85.5	0
Other intangible assets, accumulated amortization	3.6	0
Other intangible assets, net carrying value	81.9	0
Customer relationships | MediaAlpha
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	26.8	26.8
Other intangible assets, accumulated amortization	4.3	2.4
Other intangible assets, net carrying value	22.5	24.4
Trade names | NSM Holding Co LLC
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	51.2	0
Other intangible assets, accumulated amortization	1.1	0
Other intangible assets, net carrying value	50.1	0
Trade names | Buzzmove
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	0.6	0.6
Other intangible assets, accumulated amortization	0.1	0.1
Other intangible assets, net carrying value	0.5	0.5
Information technology | NSM Holding Co LLC
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	3.8	0
Other intangible assets, accumulated amortization	0.3	0
Other intangible assets, net carrying value	3.5	0
Information technology | MediaAlpha
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	33.3	33.3
Other intangible assets, accumulated amortization	29.2	24.3
Other intangible assets, net carrying value	4.1	9
Information technology | Buzzmove
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	0.5	0.5
Other intangible assets, accumulated amortization	0.3	0.2
Other intangible assets, net carrying value	0.2	0.3
Other | MediaAlpha
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	9.8	9.8
Other intangible assets, accumulated amortization	8.8	7.8
Other intangible assets, net carrying value	1	2
Other Intangible Assets
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	211.5	71
Other intangible assets, accumulated amortization	47.7	34.8
Other intangible assets, net carrying value	163.8	36.2
Non-controlling interest
Goodwill [Line Items]
Goodwill and intangible assets, net carrying value	(40.6)	(21.1)
Total
Goodwill [Line Items]
Goodwill and intangible assets, net carrying value	$ 444.6	$ 41

Goodwill and Other Intangible_6

Goodwill and Other Intangible Assets - Schedule of Goodwill and Other Intangible Assets Footnotes (Details) - USD ($) $ in Millions	3 Months Ended	5 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2018
Other Intangible Assets
Acquired Finite-Lived Intangible Assets [Line Items]
Foreign currency translation	$ (0.4)		$ (0.4)
Goodwill
Acquired Finite-Lived Intangible Assets [Line Items]
Foreign currency translation	$ (1.2)		$ (1.2)
NSM Holding Co LLC | Other Intangible Assets
Acquired Finite-Lived Intangible Assets [Line Items]
Foreign currency translation		$ 0.4
NSM Holding Co LLC | Goodwill
Acquired Finite-Lived Intangible Assets [Line Items]
Foreign currency translation		$ 1.2

Debt (Details)

Debt (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	May 11, 2018	Dec. 31, 2017	Oct. 05, 2017	May 12, 2017	Dec. 12, 2016	Aug. 14, 2013
Debt Instrument
Total debt	$ 164.3	$ 164.3			$ 23.8
Repayments of debt		11.2	$ 364.6
WTM Bank Facility
Debt Instrument
Total commitment under revolving credit facility									$ 425
MediaAlpha
Debt Instrument
Total debt	15	15			23.8
MediaAlpha | MediaAlpha Bank Facility
Debt Instrument
Total debt	15	15			23.8
Face amount	$ 15.1	$ 15.1			$ 23.9
Effective percentage	6.82%	6.82%			5.60%
Unamortized issuance cost	$ (0.1)	$ (0.1)			$ (0.1)
Total commitment under revolving credit facility						$ 28.4	$ 20
NSM Holding Co LLC
Debt Instrument
Total debt	149.3	149.3			0
NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Total commitment under revolving credit facility				$ 100
Revolving Credit Facility | MediaAlpha | MediaAlpha Bank Facility
Debt Instrument
Long-term line of credit	10	$ 10
Revolving Credit Facility | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Total commitment under revolving credit facility				10
Borrowings under facility				2
Term Loan | MediaAlpha Bank Facility
Debt Instrument
Basis spread on variable rate		1.50%
Term Loan | MediaAlpha | MediaAlpha Bank Facility
Debt Instrument
Face amount	15.1	$ 15.1
Total commitment under revolving credit facility	18.4	18.4
Repayments of debt	0.8	2.8
Term Loan | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Total debt	147.3	147.3			0
Face amount	$ 150.6	$ 150.6			0
Effective percentage	7.50%	7.50%
Unamortized issuance cost	$ (3.3)	$ (3.3)			0
Long-term line of credit				$ 51
Repayments of debt	$ 0.4
Debt, weighted average interest rate, all-in	6.69%	6.69%
Term Loan | NSM Holding Co LLC | Other
Debt Instrument
Total debt	$ 2	$ 2			0
Term Loan | NSM Holding Co LLC | Ageas Bank Facility
Debt Instrument
Long-term line of credit	2.3	$ 2.3
Acquisition date fair value of debt								$ 2
Line of Credit | MediaAlpha Bank Facility
Debt Instrument
Basis spread on variable rate		0.25%
Line of Credit | MediaAlpha | MediaAlpha Bank Facility
Debt Instrument
Repayment of lines of credit	4	$ 6
Line of Credit | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Repayment of lines of credit	$ 2
Interest Rate Swap | NSM Holding Co LLC
Debt Instrument
Derivative, fixed interest rate	2.9745%	2.9745%
Derivative, variable interest rate	2.07%	2.07%
Interest Rate Swap | Term Loan | NSM Bank Facility
Debt Instrument
Derivative, variable interest rate	2.07325%	2.07325%
Interest Rate Swap | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Derivative, fixed interest rate	2.9745%	2.9745%
Debt, weighted average interest rate, all-in	7.47%	7.47%
Carrying Value | MediaAlpha | MediaAlpha Bank Facility
Debt Instrument
Total debt	$ 15	$ 15			$ 23.8
Carrying Value | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Total debt	$ 147.3	$ 147.3
London Interbank Offered Rate (LIBOR) | Minimum | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Basis spread on variable rate	4.25%
London Interbank Offered Rate (LIBOR) | Maximum | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Basis spread on variable rate	4.75%
Prime Rate | Minimum | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Basis spread on variable rate	3.25%
Prime Rate | Maximum | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Basis spread on variable rate	3.75%

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Tax Contingency [Line Items]
Effective income tax rate reconciliation, percent	(8.10%)	(47.60%)	1.20%	(203.90%)
U.S. statutory rate (as a percent)			21.00%	35.00%
Income tax expense (benefit)	$ (3.6)	$ (4)	$ (0.4)	$ (5.3)
Continuing Operations | BAM
Income Tax Contingency [Line Items]
Income tax expense (benefit)	$ (1.8)	$ (2.1)	$ (4)	$ (6.9)

Derivatives - NSM Interest Rate

Derivatives - NSM Interest Rate Swap (Details) - USD ($)	3 Months Ended	5 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2018	May 11, 2018	Dec. 31, 2017
NSM Holding Co LLC | NSM Bank Facility
Derivative [Line Items]
Total commitment under revolving credit facility				$ 100,000,000
Term Loan | NSM Holding Co LLC | NSM Bank Facility
Derivative [Line Items]
Face amount	$ 150,600,000	$ 150,600,000	$ 150,600,000		$ 0
Debt, weighted average interest rate, all-in	6.69%	6.69%	6.69%
Interest Rate Swap | NSM Holding Co LLC
Derivative [Line Items]
Derivative, fixed interest rate	2.9745%	2.9745%	2.9745%
Derivative, variable interest rate	2.07%	2.07%	2.07%
Interest expense, net	$ 300,000	$ 400,000
Derivative, fair value, net	100,000	$ 100,000	$ 100,000
Change in fair value of derivative	$ 1,000,000		$ (100,000)
Interest Rate Swap | Term Loan | NSM Bank Facility
Derivative [Line Items]
Derivative, variable interest rate	2.07325%	2.07325%	2.07325%
Interest Rate Swap | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Derivative [Line Items]
Derivative, fixed interest rate	2.9745%	2.9745%	2.9745%
Debt, weighted average interest rate, all-in	7.47%	7.47%	7.47%
Margin over London Interbank Offered Rate (LIBOR) | Minimum | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Derivative [Line Items]
Basis spread on variable rate	4.25%
Margin over London Interbank Offered Rate (LIBOR) | Maximum | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Derivative [Line Items]
Basis spread on variable rate	4.75%
Margin over Prime Rate | Minimum | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Derivative [Line Items]
Basis spread on variable rate	3.25%
Margin over Prime Rate | Maximum | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Derivative [Line Items]
Basis spread on variable rate	3.75%

Derivatives Forward Contracts (

Derivatives Forward Contracts (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Fair value investments	$ 2,509.4	$ 2,365.1	$ 2,509.4	$ 3,208.3	$ 2,547.3
Forward Contracts
Derivative [Line Items]
Notional amount				206.3
Fair value investments				$ (3.7)
Realized and unrealized gains (losses)	$ (10)	$ (3.5)	$ (23.1)

Municipal Bond Guarantee (Detai

Municipal Bond Guarantee (Details) $ in Millions	1 Months Ended	9 Months Ended		12 Months Ended
	Apr. 30, 2018USD ($)	Sep. 30, 2018USD ($)yrContract	Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)Contract	Dec. 31, 2012USD ($)	Jul. 31, 2012USD ($)
Guarantor Obligations [Line Items]
Repayments of debt		$ 11.2	$ 364.6
Interest paid		5.3	$ 0.5
Total gross unearned insurance premiums		156.2		$ 136.8
HG Global
Guarantor Obligations [Line Items]
Partners capital contribution					$ 594.5
Surplus notes		$ 499				$ 503
Assets held-in-trust				$ 603
HG Global | Preferred Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)		96.90%		96.90%
HG Global | Common Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)		88.40%		88.40%
BAM
Guarantor Obligations [Line Items]
Percentage of Risk Premium Ceded		60.00%
Year of U.S. Treasury rate | yr		1
Contracts outstanding | Contract		7,184		6,371
Remaining weighted average contract period outstanding (in years)		10 years 10 months		10 years 11 months
Municipal bond outstanding principal		$ 47,259.1		$ 42,090.6
Municipal bond outstanding interest		23,707.4		21,057.1
Municipal bond debt service outstanding		70,966.5		63,147.7
October 1, 2018 - December 31, 2018		3.3
2,019		13.1
2,020		12.6
2,021		12.1
2,022		11.5
2023 and thereafter		103.6
Total gross unearned insurance premiums		156.2
HG Global
Guarantor Obligations [Line Items]
Collateral held in supplement trust		741.7		715.1
Interest receivable		$ 143.2		126
HG Re
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured		15.00%
BAM
Guarantor Obligations [Line Items]
Surplus notes		$ 499		$ 499
BAM Management
Guarantor Obligations [Line Items]
Partners capital contribution					$ 14.5
Surplus Note | BAM
Guarantor Obligations [Line Items]
Derivative, fixed interest rate (as a percent)		4.60%		3.78%
Surplus Note | BAM
Guarantor Obligations [Line Items]
Repayments of debt				$ 4
Interest paid				1
Combination One | Surplus Note | BAM
Guarantor Obligations [Line Items]
Number of years of change in interest rate | yr		5
Combination Two | Surplus Note | BAM
Guarantor Obligations [Line Items]
Number of years of change in interest rate | yr		3
US Treasury Rate | Surplus Note | BAM
Guarantor Obligations [Line Items]
Basis spread on variable rate		3.00%
Fixed interest rate | Surplus Note | BAM
Guarantor Obligations [Line Items]
Derivative, fixed interest rate (as a percent)		8.00%
First Quarter, year two | BAM
Guarantor Obligations [Line Items]
2,019		$ 3.3
Second Quarter, year two | BAM
Guarantor Obligations [Line Items]
2,019		3.3
Third Quarter, year two | BAM
Guarantor Obligations [Line Items]
2,019		3.3
Fourth Quarter, year two | BAM
Guarantor Obligations [Line Items]
2,019		3.2
Series A BAM Surplus Note | HG Global
Guarantor Obligations [Line Items]
Surplus notes				203
Series B BAM Surplus Note | HG Global
Guarantor Obligations [Line Items]
Surplus notes				$ 300
BAM | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Losses above retention, percentage of liability	90.00%
Fidus Re Ltd. | BAM
Guarantor Obligations [Line Items]
Collateralized financial guarantee, term	12 years
Aggregate losses on ceded business	$ 165
Losses above retention, percentage of liability	10.00%
Minimum | Fidus Re Ltd. | BAM
Guarantor Obligations [Line Items]
Aggregate losses on ceded business	$ 165
Maximum | Fidus Re Ltd. | BAM
Guarantor Obligations [Line Items]
Aggregate losses on ceded business		$ 276

Municipal Bond Guarantee - Sche

Municipal Bond Guarantee - Schedule of Net Written Premiums (Details) - HG/BAM - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Gross written premiums	$ 3.9	$ 10.9	$ 28.9	$ 42
Assumed (ceded) written premiums	0	0	0
Net written premiums	$ 3.9	$ 10.9	$ 28.9	$ 42

Earnings Per Share (Details)

Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 40.8	$ 562.1	$ (3.7)	$ 604.7
Less: total income (loss) from discontinued operations	(17.3)	539.1	(17.2)	573.2
Net income from continuing operations attributable to White Mountains’s common shareholders	58.1	23	13.5	31.5
Allocation of earnings to participating restricted common shares	(0.8)	(0.3)	(0.2)	(0.4)
Basic and diluted earnings per share numerators	$ 57.3	$ 22.7	$ 13.3	$ 31.1
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,180,400	4,297,200	3,450,800	4,477,000
Average unvested restricted common shares	(41,500)	(53,700)	(39,700)	(54,500)
Basic earnings per share denominator	3,138,900	4,243,500	3,411,100	4,422,500
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,180,400	4,297,200	3,450,800	4,477,000
Average unvested restricted common shares	(41,500)	(53,700)	(39,700)	(54,500)
Diluted earnings per share denominator	3,138,900	4,243,500	3,411,100	4,422,500
Basic earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid	$ 0	$ 0	$ 1	$ 1
Undistributed earnings (losses)	18.27	5.36	2.90	6.03
Basic and diluted earnings per share	$ 18.27	$ 5.36	$ 3.90	$ 7.03

Earnings Per Share - Undistribu

Earnings Per Share - Undistributed Net Earnings (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 57.3	$ 22.7	$ 13.3	$ 31.1
Dividends declared net of restricted common share amounts	0	0	(3.7)	(4.5)
Total undistributed net losses, net of restricted common share amounts	$ 57.3	$ 22.7	$ 9.6	$ 26.6

Employee Share-Based Incentiv_3

Employee Share-Based Incentive Compensation Plans (Performance Shares) (Details) - Performance Share - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Range Of Performance Cycle From 2014-2016
Accrued Expense
Percentage of target payout - minimum actual					34.00%
Percentage of target payout - maximum actual					76.00%
Performance Cycle 2015 to 2017
Accrued Expense
Percentage of target payout - minimum actual			145.00%
Percentage of target payout - maximum actual			147.00%
White Mountains
Share-based compensation arrangement by share-based payment award
Vesting Period			3 years
Target Performance Share Activity
Beginning of period (in shares)	40,616	50,575	50,515	80,353	80,353
Shares paid or expired (in shares)	0	0	(23,186)	(30,838)
New grants (in shares)	0	0	14,105	17,510
Assumed forfeitures and cancellations	0	(256)	(818)	(16,706)
Ending of period (in shares)	40,616	50,319	40,616	50,319	50,515
Accrued Expense
Accrued expense at the beginning of period	$ 27.2	$ 31.2	$ 45.8	$ 42.4	$ 42.4
Payments and deferrals	0	0	(28.4)	(21.6)
Assumed forfeitures and cancellations	(0.1)	(0.4)	0.1	(9.4)
Expense recognized	7.8	7	17.4	26.4
Accrued expense at the end of the period	34.9	$ 37.8	34.9	$ 37.8	$ 45.8
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 20.4		$ 20.4
Sirius Group
Target Performance Share Activity
Ending of period (in shares)		2,195		2,195

Employee Share-Based Incentiv_4

Employee Share-Based Incentive Compensation Plans (Performance Shares Granted Under the WTM Incentive Plan) (Details) - White Mountains - Performance Share - W T M Incentive Plan - USD ($) $ in Millions	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	41,235
Accrued Expense	$ 35.4
Target Performance Shares Outstanding, Assumed forfeitures	(619)
Accrued Expense, Assumed forfeitures	(0.5)
Total Target Performance Shares Outstanding	40,616	40,616	50,515	50,319	50,575	80,353
Total Accrued Expense	$ 34.9	$ 27.2	$ 45.8	$ 37.8	$ 31.2	$ 42.4
Performance Cycle 2016 to 2018
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	13,715
Accrued Expense	$ 18.9
Performance Cycle 2017 to 2019
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	14,070
Accrued Expense	$ 13.3
Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	13,450
Accrued Expense	$ 3.2

Employee Share-Based Incentiv_5

Employee Share-Based Incentive Compensation Plans (Restricted Shares) (Details) - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
White Mountains | Restricted Share Cliff Vesting in January 2021
Target Restricted Share Activity
Issued (in shares)	13,450
White Mountains | Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	41,510	53,815	53,755	70,620
Issued (in shares)	0	0	14,105	17,785
Vested (in shares)	0	(260)	(25,381)	(28,846)
Assumed forfeitures and cancellations	0	0	(969)	(6,004)
Ending of period (in shares)	41,510	53,555	41,510	53,555
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 19	$ 23.5	$ 14.3	$ 19.7
Issued	0	0	11.4	16.7
Assumed forfeitures and cancellations	0	0	(0.2)	(3.5)
Expense recognized	(3.2)	(3.8)	(9.7)	(13.2)
Unamortized grant date fair value, ending balance	$ 15.8	$ 19.7	$ 15.8	$ 19.7
White Mountains | Restricted Share Cliff Vesting in January 2019
Target Restricted Share Activity
Issued (in shares)	365	250
White Mountains | Restricted Share Cliff Vesting in January 2018
Target Restricted Share Activity
Issued (in shares)		250
White Mountains | Restricted Share Cliff Vesting in January 2020
Target Restricted Share Activity
Issued (in shares)	290	550		16,735
Sirius Group | Restricted Stock
Target Restricted Share Activity
Ending of period (in shares)		2,195		2,195

Employee Share-Based Incentiv_6

Employee Share-Based Incentive Compensation Plans (Non-Qualified Options & MediaAlpha Class B Share Awards) (Details) - USD ($) $ / shares in Units, $ in Millions	Jan. 20, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2016
Share-based compensation arrangement by share-based payment award
Shares issued, price per share					$ 742
Common Stock
Share-based compensation arrangement by share-based payment award
Conversion of stock, shares converted			5,142
Chief Executive Officer | Employee Stock Option
Share-based compensation arrangement by share-based payment award
Non-option equity instruments, exercised	125,000		40,000
Options, exercises in period, intrinsic value			$ 4.4
MediaAlpha | Class B
Share-based compensation arrangement by share-based payment award
Expense recognized		$ 0.2		$ 6.9
Compensation cost not yet recognized		$ 0.1		$ 0.1
Minimum | MediaAlpha | Class B
Share-based compensation arrangement by share-based payment award
Service period				36 months
Maximum | MediaAlpha | Class B
Share-based compensation arrangement by share-based payment award
Service period				48 months

Non-controlling Interest (Detai

Non-controlling Interest (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ (144.4)	$ (131.7)
Total Non-Controlling Excluding Reciprocals
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ 45.7	$ 31.5
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	3.10%	3.10%
Total non-controlling interests	$ 14.8	$ 15.9
NSM Holding Co LLC
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	5.00%	0.00%
Total non-controlling interests	$ 14.1	$ 0
MediaAlpha
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	37.80%	35.70%
Total non-controlling interests	$ 15.3	$ 13.1
Buzzmove
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	22.90%	22.90%
Total non-controlling interests	$ 1.5	$ 2.5
Build America Mutual
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	$ (190.1)	$ (163.2)

Segment Information (Details)

Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment reporting information
Earned insurance premiums	$ 3.3	$ 2.4	$ 9.7	$ 6.6
Net investment income	13.4	12.2	44.9	39.7
Net realized and unrealized investment (losses) gains	66.1	32.5	22.9	102.5
Commission revenues	112.2		278.6	103.9
Other revenue	3.7	1.6	7	7.1
Total revenues	198.7	87.5	363.1	259.8
Insurance acquisition expenses	1.2	0.9	3.9	3
Other underwriting expenses	0.1	0.1	0.3	0.3
Cost of sales	62.9		182	88.7
General and administrative and other expenses	68.5	41.7	174.6	156.1
Broker commission expense	10.9		17.5
Amortization	7.4	2.4	12.9	7.3
Interest expense	3.4	0.9	6	1.8
Total expenses	154.4	79.1	397.2	257.2
Pre-tax income (loss)	$ 44.3	$ 8.4	$ (34.1)	$ 2.6
MediaAlpha
Segment reporting information
Percentage of advertising revenue	30.00%	32.00%	31.00%	29.00%
HG Global-BAM | HG Global-BAM
Segment reporting information
Earned insurance premiums	$ 3.3	$ 2.4	$ 9.7	$ 6.6
Net investment income	4.8	3.3	12.5	8.9
Net realized and unrealized investment (losses) gains	(4.1)	0.8	(14.4)	3.2
Commission revenues	0		0	0
Other revenue	0.2	0.2	0.8	0.8
Total revenues	4.2	6.7	8.6	19.5
Insurance acquisition expenses	1.2	0.9	3.9	3
Other underwriting expenses	0.1	0.1	0.3	0.3
Cost of sales	0			0
General and administrative and other expenses	11.3	10.6	36.5	31.5
Broker commission expense	0		0
Amortization	0	0	0
Interest expense	0	0		0
Total expenses	12.6	11.6	40.7	34.8
Pre-tax income (loss)	(8.4)	(4.9)	(32.1)	(15.3)
NSM Holding Co LLC | NSM Holding Co LLC
Segment reporting information
Earned insurance premiums	0		0
Net investment income	0		0
Net realized and unrealized investment (losses) gains	0		0
Commission revenues	36.6		59.2	0
Other revenue	3.1	0	4	0
Total revenues	39.7	0	63.2	0
Insurance acquisition expenses	0		0
Other underwriting expenses	0		0
Cost of sales	0		0
General and administrative and other expenses	25.8	0	37.9	0
Broker commission expense	10.9	0	17.5	0
Amortization	5	0	5	0
Interest expense	3.2	0	4.8	0
Total expenses	44.9	0	65.2	0
Pre-tax income (loss)	(5.2)		(2)
MediaAlpha | MediaAlpha
Segment reporting information
Earned insurance premiums	0	0		0
Net investment income	0	0		0
Net realized and unrealized investment (losses) gains	0	0		0
Commission revenues	74.5	37.9	216.4	101.2
Other revenue	0		1.6	0
Total revenues	74.5	37.9	218	101.2
Insurance acquisition expenses	0	0		0
Other underwriting expenses	0	0		0
Cost of sales	61.8	32.2	179.1	86
General and administrative and other expenses	5.4	3.8	21.2	10.7
Broker commission expense	0		0
Amortization	2.4	2.3	7.8	7.2
Interest expense	0.2	0.1	0.9	0.6
Total expenses	69.8	38.4	209	104.5
Pre-tax income (loss)	4.7	(0.5)	9	(3.3)
Other | Other Segment
Segment reporting information
Earned insurance premiums	0	0	0	0
Net investment income	8.6	8.9	32.4	30.8
Net realized and unrealized investment (losses) gains	70.2	31.7	37.3	99.3
Commission revenues	1.1	0.9	3	2.7
Other revenue	0.4	1.4	0.6	6.3
Total revenues	80.3	42.9	73.3	139.1
Insurance acquisition expenses	0	0	0	0
Other underwriting expenses	0	0	0	0
Cost of sales	1.1	0.9	2.9	2.7
General and administrative and other expenses	26	27.3	79	113.9
Broker commission expense	0		0
Amortization	0	0.1	0.1	0.1
Interest expense	0	0.8	0.3	1.2
Total expenses	27.1	29.1	82.3	117.9
Pre-tax income (loss)	$ 53.2	13.8	$ (9)	$ 21.2
Advertising and Commission Revenue
Segment reporting information
Commission revenues		38.8
Advertising and Commission Revenue | HG Global-BAM | HG Global-BAM
Segment reporting information
Commission revenues		0
Service
Segment reporting information
Cost of sales		33.1
Service | HG Global-BAM | HG Global-BAM
Segment reporting information
Cost of sales		$ 0

Investments in Unconsolidated_3

Investments in Unconsolidated Entities (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
White Mountains
Investments in unconsolidated affiliates
Total other long-term investments	$ 289.7	$ 208.8
Equity method eligible private equity securities, at fair value
Investments in unconsolidated affiliates
Total other long-term investments	126.4	83.2
Equity method eligible private equity securities, at fair value | White Mountains
Investments in unconsolidated affiliates
Total other long-term investments	97.3	58
Investments, accounted for under the equity method | White Mountains
Investments in unconsolidated affiliates
Total other long-term investments	11.6	4.6
Total investments in equity method eligible unconsolidated entities | White Mountains
Investments in unconsolidated affiliates
Total other long-term investments	108.9	62.6
Other unconsolidated investments | White Mountains
Investments in unconsolidated affiliates
Total other long-term investments	$ 180.8	$ 146.2

Investments in Unconsolidated_4

Investments in Unconsolidated Entities - White Mountain's Investments (Details)	Sep. 30, 2018	Jan. 24, 2018	Dec. 31, 2017
PassportCard
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%	50.00%	50.00%
DavidShield
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%	50.00%	0.00%
Kudu
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	49.50%		0.00%
durchblicker
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	45.00%		45.00%
Compare.com
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	22.10%		22.10%
Tuckerman Capital Fund III, L.P.
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	21.80%		21.30%
White Mountains | PassportCard
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	25.00%
DavidShield | PassportCard
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	25.00%

Investments in Unconsolidated_5

Investments in Unconsolidated Entities - Narrative (Details) - Tuckerman Capital Fund III, L.P. - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Unrealized investment gains	$ 4.2			$ 8.1
Total net income		$ 31.9	$ 3.2		$ 43.7	$ 7.1
Total assets		$ 61.9			$ 61.9		$ 21.8

Fair Value of Financial Instr_3

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	$ 164.3	$ 23.8
NSM Holding Co LLC
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	149.3	0
NSM Holding Co LLC | Carrying Value | NSM Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	147.3
NSM Holding Co LLC | Estimate of Fair Value Measurement | NSM Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt instrument, fair value disclosure	147.3	0
MediaAlpha
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	15	23.8
MediaAlpha | MediaAlpha Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Face amount	15.1	23.9
Debt	15	23.8
MediaAlpha | Carrying Value | MediaAlpha Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	15	23.8
MediaAlpha | Estimate of Fair Value Measurement | MediaAlpha Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt instrument, fair value disclosure	$ 15.3	$ 23.9

Contingencies (Details)

Contingencies (Details) $ in Millions	Sep. 30, 2018USD ($)
Loss Contingencies [Line Items]
Contingent liability	$ 82.5
Swedish Tax Authority | Sirius Group
Loss Contingencies [Line Items]
Loss contingency, estimate of possible loss	$ 17.3

Held for Sale and Discontinue_3

Held for Sale and Discontinued Operations - (Details) - USD ($) $ / shares in Units, $ in Millions	Sep. 28, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 28, 2017	Dec. 31, 2017
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold				$ 0	$ 1,131
Gain (loss) from sale of discontinued operations, net of tax			$ 554.3	(17.2)	552.7
Net (loss) income from discontinued operations		$ 0	(15.2)	0	20.5
OneBeacon
Discontinued Operations [Line Items}
Sale of stock, price per share	$ 18.10					$ 18.10
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	$ 1,300
Gain (loss) from sale of discontinued operations, net of tax	$ 554.6
Net (loss) income from discontinued operations			(15.2)			$ 20.5
Discontinued Operations | OneBeacon
Discontinued Operations [Line Items}
Net (loss) income from discontinued operations			(15.2)		20.5
Discontinued Operations | Tranzact
Discontinued Operations [Line Items}
Gain (loss) from sale of discontinued operations, net of tax					(1)
Discontinued Operations | Sirius Group
Discontinued Operations [Line Items}
Gain (loss) from sale of discontinued operations, net of tax		(17.3)	(0.2)	0.1	(0.8)
Net (loss) income from discontinued operations			$ 0		$ 0
Connecticut | Discontinued Operations, Held-for-sale
Discontinued Operations [Line Items}
Assets held for sale		3.3		3.3			$ 3.3
Swedish Tax Authority | Discontinued Operations | Sirius Group
Discontinued Operations [Line Items}
Gain (loss) from sale of discontinued operations, net of tax		(17.3)		(17.2)
Swedish Tax Authority | Sirius Group
Discontinued Operations [Line Items}
Loss contingency, estimate of possible loss		$ 17.3		$ 17.3

Held for Sale and Discontinue_4

Held for Sale and Discontinued Operations - Financial Statement Disclosures (Details) - USD ($) $ in Millions	Sep. 28, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 28, 2017
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums		$ 3.3	$ 2.4	$ 9.7	$ 6.6
Net investment income		13.4	12.2	44.9	39.7
Net realized and unrealized investment (losses) gains		66.1	32.5	22.9	102.5
Total revenues		198.7	87.5	363.1	259.8
Insurance acquisition expenses		1.2	0.9	3.9	3
Other underwriting expenses		0.1	0.1	0.3	0.3
General and administrative and other expenses		68.5	41.7	174.6	156.1
Interest expense		3.4	0.9	6	1.8
Total expenses		154.4	79.1	397.2	257.2
Net (loss) income from discontinued operations, net of tax		0	(15.2)	0	20.5
Net (loss) gain from sale of discontinued operations, net of tax			554.3	(17.2)	552.7
Total income (loss) from discontinued operations		(17.3)	539.1	(17.2)	573.2
Total comprehensive income (loss)		40	565.1	(5.4)	608.1
OneBeacon
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net (loss) income from discontinued operations, net of tax			(15.2)			$ 20.5
Net (loss) gain from sale of discontinued operations, net of tax	$ 554.6
Other Discontinued Operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total comprehensive income (loss)					(1)
Discontinued Operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total income (loss) from discontinued operations		(17.3)	539.1	(17.2)	573.2
Discontinued Operations | Net gain (loss) from sale of discontinued operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net (loss) gain from sale of discontinued operations, net of tax			554.5		554.5
Discontinued Operations | OneBeacon
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums			268.4		807.6
Net investment income			13		39.7
Net realized and unrealized investment (losses) gains			11.5		38.8
Other revenue			2.2		7.7
Total revenues			295.1		893.8
Loss and loss adjustment expenses			206.8		546
Insurance acquisition expenses			51.9		145.6
Other underwriting expenses			44.9		156.2
General and administrative and other expenses			7.4		21.2
Interest expense			3.4		10
Total expenses			314.4		879
Pre-tax (loss) income			(19.3)		14.8
Income tax benefit			4.1		5.7
Net (loss) income from discontinued operations, net of tax			(15.2)		20.5
Total income (loss) from discontinued operations			539.3		575
Net change in foreign currency translation and other			0.1		0.3
Recognition of benefit plan assets and obligations from the sale of OneBeacon, net of tax			2.9		2.9
Total comprehensive income (loss)			542.3		578.2
Discontinued Operations | Sirius Group
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums			0		0
Net investment income			0		0
Net realized and unrealized investment (losses) gains			0		0
Other revenue			0		0
Total revenues			0		0
Loss and loss adjustment expenses			0		0
Insurance acquisition expenses			0		0
Other underwriting expenses			0		0
General and administrative and other expenses			0		0
Interest expense			0		0
Total expenses			0		0
Pre-tax (loss) income			0		0
Income tax benefit			0		0
Net (loss) income from discontinued operations, net of tax			0		0
Net (loss) gain from sale of discontinued operations, net of tax		$ (17.3)	(0.2)	$ 0.1	(0.8)
Total income (loss) from discontinued operations			(0.2)		(0.8)
Total comprehensive income (loss)			(0.2)		(0.8)
Discontinued Operations | Sirius Group | Net gain (loss) from sale of discontinued operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net (loss) gain from sale of discontinued operations, net of tax			(0.2)		(0.8)
Discontinued Operations | Other Discontinued Operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums					0
Net investment income					0
Net realized and unrealized investment (losses) gains					0
Other revenue					0
Total revenues					0
Loss and loss adjustment expenses					0
Insurance acquisition expenses					0
Other underwriting expenses					0
General and administrative and other expenses					0
Interest expense					0
Total expenses					0
Pre-tax (loss) income					0
Income tax benefit					0
Net (loss) income from discontinued operations, net of tax					0
Total income (loss) from discontinued operations					(1)
Discontinued Operations | Other Discontinued Operations | Net gain (loss) from sale of discontinued operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net (loss) gain from sale of discontinued operations, net of tax					(1)
Discontinued Operations | Total
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums			268.4		807.6
Net investment income			13		39.7
Net realized and unrealized investment (losses) gains			11.5		38.8
Other revenue			2.2		7.7
Total revenues			295.1		893.8
Loss and loss adjustment expenses			206.8		546
Insurance acquisition expenses			51.9		145.6
Other underwriting expenses			44.9		156.2
General and administrative and other expenses			7.4		21.2
Interest expense			3.4		10
Total expenses			314.4		879
Pre-tax (loss) income			(19.3)		14.8
Income tax benefit			4.1		5.7
Net (loss) income from discontinued operations, net of tax			(15.2)		20.5
Total income (loss) from discontinued operations			539.1		573.2
Net change in foreign currency translation and other			0.1		0.3
Recognition of benefit plan assets and obligations from the sale of OneBeacon, net of tax			2.9		2.9
Total comprehensive income (loss)			542.1		576.4
Discontinued Operations | Total | Net gain (loss) from sale of discontinued operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net (loss) gain from sale of discontinued operations, net of tax			$ 554.3		$ 552.7

Held for Sale and Discontinue_5

Held for Sale and Discontinued Operations - Net Change Cash From Discontinued Operations (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Cash balances at beginning of period	$ 97.1	$ 80.2
Cash balances at end of period	112.2	48.6
Net income tax payment to national governments	0.5	0
Discontinued Operations
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash (used for) provided from operations - discontinued operations	0	157
Net cash provided from investing activities - discontinued operations	0	3
Net cash used for financing activities - discontinued operations	0	(61.9)
Net change in cash during the period	0	98.1
Cash balances at beginning of period	0	70.5
Cash balances at end of period	0	71.3
Interest paid	0	0
Net income tax payment to national governments	0	0
Discontinued Operations, Held-for-sale
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Cash balances at beginning of period	0	70.5
Cash balances at end of period	0	0
Disposal Group, Held-for-sale, Not Discontinued Operations
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net change in cash held for sale, excluding discontinued operations	0	(0.9)
Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Cash sold as part of sale of consolidated subsidiaries	$ 0	$ 167.7

Held for Sale and Discontinue_6

Held for Sale and Discontinued Operations - EPS (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 40.8	$ 562.1	$ (3.7)	$ 604.7
Less total loss from continuing operations, net of tax	58.1	23	13.5	31.5
Net income from discontinued operations attributable to White Mountains’s common shareholders	(17.3)	539.1	(17.2)	573.2
Basic and diluted earnings per share numerators	$ 57.3	$ 22.7	$ 13.3	$ 31.1
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,180,400	4,297,200	3,450,800	4,477,000
Average unvested restricted common shares	(41,500)	(53,700)	(39,700)	(54,500)
Basic earnings per share denominator	3,138,900	4,243,500	3,411,100	4,422,500
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,180,400	4,297,200	3,450,800	4,477,000
Average unvested restricted common shares	(41,500)	(53,700)	(39,700)	(54,500)
Diluted earnings per share denominator	3,138,900	4,243,500	3,411,100	4,422,500
Basic and diluted earnings per share	$ 18.27	$ 5.36	$ 3.90	$ 7.03
Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 40.8	$ 562.1	$ (3.7)	$ 604.7
Less total loss from continuing operations, net of tax	58.1	23	13.5	31.5
Net income from discontinued operations attributable to White Mountains’s common shareholders	(17.3)	539.1	(17.2)	573.2
Allocation of income for participating unvested restricted common shares	0.2	(6.7)	0.2	(7)
Basic and diluted earnings per share numerators	$ (17.1)	$ 532.4	$ (17)	$ 566.2
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,180,400	4,297,200	3,450,800	4,477,000
Average unvested restricted common shares	(41,500)	(53,700)	(39,700)	(54,500)
Basic earnings per share denominator	3,138,900	4,243,500	3,411,100	4,422,500
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,180,400	4,297,200	3,450,800	4,477,000
Average unvested restricted common shares	(41,500)	(53,700)	(39,700)	(54,500)
Diluted earnings per share denominator	3,138,900	4,243,500	3,411,100	4,422,500
Basic and diluted earnings per share	$ (5.44)	$ 125.45	$ (4.98)	$ 128.03