White Mountains Insurance (WTM) 10-Q 6 May 19 2019 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - $ / shares	3 Months Ended
	Mar. 31, 2019	May 02, 2019	Dec. 31, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	WHITE MOUNTAINS INSURANCE GROUP LTD
Entity Central Index Key	0000776867
Document Type	10-Q
Document Period End Date	Mar. 31, 2019
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		3,181,199
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Entity Emerging Growth Company	false
Entity Small Business	false
Common shares, par value per share (in dollars per share)	$ 1		$ 1

CONSOLIDATED BALANCE SHEETS CON

CONSOLIDATED BALANCE SHEETS CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Assets
Goodwill and other intangible assets	$ 510.2	$ 537.5
Total assets	3,629.7	3,362.6
Liabilities
Debt	195.4	192.7
Total liabilities	651.4	644.4
White Mountains’s common shareholders’ equity
White Mountains’ common shares	3.2	3.2
Paid-in surplus	582.6	580.8
Retained earnings	2,542.3	2,264.9
Accumulated other comprehensive loss, after tax:
Net unrealized foreign currency translation losses	(6.1)	(5.8)
Total White Mountains’s common shareholders’ equity	3,122	2,843.1
Non-controlling interests
Non-controlling interests	(143.7)	(124.9)
Total equity	2,978.3	2,718.2
Total liabilities and equity	3,629.7	3,362.6
HG Global-BAM
Assets
Fixed maturity investments, at fair value	737.6	701.4
Short-term investments, at fair value	31.8	66.9
Total investments	769.4	768.3
Cash (restricted $60.8 and $50.0)	10.4	12.5
Insurance premiums receivable	6.5	6.4
Deferred acquisition costs	19.6	19
Accrued investment income	5.4	4.9
Accounts receivable on unsettled investment sales	6.1	0
Other assets	17.1	5.1
Total assets	834.5	816.2
Liabilities
Unearned insurance premiums	179.6	176
Accrued incentive compensation	10.1	20.4
Accounts payable on unsettled investment purchases	1.6	2.2
Other liabilities	28.1	13.9
Total liabilities	219.4	212.5
NSM Holding Co LLC
Assets
Short-term investments, at fair value	0	1.7
Cash (restricted $60.8 and $50.0)	84.5	66.2
Insurance premiums receivable	51.9	44
Goodwill and other intangible assets	488.7	486.2
Other assets	36	28.9
Total assets	661.1	627
Liabilities
Debt	184.5	178.5
Premiums payable	93.4	77.2
Contingent consideration earnout liabilities	27.5	20.2
Other liabilities	46.1	38.9
Total liabilities	351.5	314.8
Non-controlling interests
Non-controlling interests	14.2	13.6
MediaAlpha
Assets
Cash (restricted $60.8 and $50.0)	0	5.7
Goodwill and other intangible assets	0	43.4
Accounts receivable from publishers and advertisers	0	37
Other assets	0	2.3
Total assets	0	88.4
Liabilities
Debt	0	14.2
Amounts due to publishers and advertisers	0	27
Other liabilities	0	5.7
Total liabilities	0	46.9
Non-controlling interests
Non-controlling interests	0	16.2
Other Segment
Assets
Fixed maturity investments, at fair value	394.4	376.1
Short-term investments, at fair value	238	145.6
Common equity securities, at fair value	905	925.6
Other long-term investments	497.6	325.6
Total investments	2,035	1,772.9
Cash (restricted $60.8 and $50.0)	37.3	25.9
Accrued investment income	5.9	5.5
Accounts receivable on unsettled investment sales	2.1	0
Goodwill and other intangible assets	21.5	7.9
Other assets	29	15.5
Assets held for sale	3.3	3.3
Total assets	2,134.1	1,831
Liabilities
Debt	10.9	0
Accrued incentive compensation	26.1	38.9
Accounts payable on unsettled investment purchases	4	5
Other liabilities	39.5	26.3
Total liabilities	$ 80.5	$ 70.2

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Cash	$ 60.8	$ 50
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	3,181,199	3,173,115
Common shares, outstanding shares	3,181,199	3,173,115

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenues:
Net investment income	$ 16	$ 19.7
Net realized and unrealized investment gains (losses)	245.6	(53.7)
Total revenues	433.6	42.1
Expenses:
Amortization of other intangible assets	6.6
Total expenses	151.9	108.1
Pre-tax income (loss) from continuing operations	281.7	(66)
Income tax expense	(10.2)	(0.7)
Net income (loss) from continuing operations	271.5	(66.7)
Net gain from sale of discontinued operations, net of tax	0.7	0.1
Net income (loss)	272.2	(66.6)
Net loss attributable to non-controlling interests	12.2	18.6
Net income (loss) attributable to White Mountains’s common shareholders	284.4	(48)
HG Global-BAM
Revenues:
Earned insurance premiums	4.2	3
Net investment income	5.3	3.7
Net realized and unrealized investment gains (losses)	11.8	(7.9)
Other revenue	0.6	0.2
Expenses:
Insurance acquisition expenses	1.3	1.4
General and administrative expenses	14.4	11.9
NSM Holding Co LLC
Revenues:
Commission revenues	43.3	0
Other revenue	6	0
Total revenues	49.3	0
Expenses:
General and administrative expenses	25.1	0
Broker commission expense	15.7	0
Change in fair value of contingent consideration earnout liabilities	1.3	0
Amortization of other intangible assets	5	0
Interest expense	3.7	0
Total expenses	50.8	0
MediaAlpha
Revenues:
Commission revenues	48.8	70.1
Other revenue	0	1.6
Expenses:
Cost of sales	40.6	57.4
General and administrative expenses	12.5	11.2
Amortization of other intangible assets	1.6	2.9
Interest expense	0.2	0.4
Other Segment
Revenues:
Earned insurance premiums	0	0
Net investment income	10.7	16
Net realized and unrealized investment gains (losses)	119.1	(45.8)
Realized and unrealized gains from the MediaAlpha Transaction		0
Commission revenues	1.3	0.9
Other revenue	0.3	0.3
Expenses:
Insurance acquisition expenses	0	0
Cost of sales	1.1	0.7
General and administrative expenses	29.4	22
Amortization of other intangible assets	0	0
Interest expense	0	0.2
HG Global-BAM | HG Global-BAM
Revenues:
Commission revenues	0	0
Total revenues	21.9	(1)
Expenses:
Cost of sales	0	0
Broker commission expense	0
Change in fair value of contingent consideration earnout liabilities	0
Amortization of other intangible assets	0
Interest expense	0	0
Total expenses	15.7	13.3
Pre-tax income (loss) from continuing operations	6.2	(14.3)
NSM Holding Co LLC | NSM Holding Co LLC
Revenues:
Earned insurance premiums	0
Net investment income	0
Net realized and unrealized investment gains (losses)	0
Total revenues	49.3
Expenses:
Insurance acquisition expenses	0
Cost of sales	0
Pre-tax income (loss) from continuing operations	(1.5)
MediaAlpha | MediaAlpha
Revenues:
Earned insurance premiums		0
Net investment income		0
Net realized and unrealized investment gains (losses)		0
Total revenues	48.8	71.7
Expenses:
Broker commission expense	0
Change in fair value of contingent consideration earnout liabilities	0
Total expenses	54.9	71.9
Pre-tax income (loss) from continuing operations	(6.1)	(0.2)
Other Operations | Other Segment
Revenues:
Total revenues	313.6	(28.6)
Expenses:
Broker commission expense	0
Change in fair value of contingent consideration earnout liabilities	0
Total expenses	30.5	22.9
Pre-tax income (loss) from continuing operations	283.1	(51.5)
Discontinued Operations
Expenses:
Net income (loss) attributable to White Mountains’s common shareholders	284.4	(48)
Swedish Tax Authority | Discontinued Operations | Sirius Group
Expenses:
Net gain from sale of discontinued operations, net of tax		$ 0.1
MediaAlpha | HG Global-BAM
Revenues:
Net realized and unrealized investment gains (losses)	0
MediaAlpha | Other Segment
Revenues:
Realized and unrealized gains from the MediaAlpha Transaction	182.2
MediaAlpha | NSM Holding Co LLC | NSM Holding Co LLC
Revenues:
Net realized and unrealized investment gains (losses)	$ 0

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ 284.4	$ (48)
Other comprehensive loss, net of tax	(0.3)	0
Comprehensive income	284.1	(48)
Comprehensive income (loss) attributable to non-controlling interests	0	0
Comprehensive income (loss) attributable to White Mountains’s common shareholders	$ 284.1	$ (48)
Basic earnings (loss) per share
Continuing operations (in usd per share)	$ 89.42	$ (12.85)
Discontinued operations (in usd per share)	0.22	0.03
Total consolidated operations (in usd per share)	89.64	(12.82)
Diluted earnings (loss) per share
Continuing operations (in usd per share)	89.42	(12.85)
Discontinued operations (in usd per share)	0.22	0.03
Total consolidated operations (in usd per share)	89.64	(12.82)
Dividends declared per White Mountains’s common share	$ 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)AOCI, after tax	Foreign Currency Gain (Loss)Total	Foreign Currency Gain (Loss)Non-controlling interest	Foreign Currency Gain (Loss)Total Equity	BAMNon-controlling interest	BAMTotal Equity	MediaAlphaNon-controlling interest	MediaAlphaTotal Equity
Beginning Balances at Dec. 31, 2017		$ 670.6	$ 2,823.2	$ (1.3)	$ 3,492.5	$ (131.7)	$ 3,360.8
Increase (Decrease) in Shareholders' Equity
Net income (loss)	$ (66.6)		(48)		(48)	(18.6)	(66.6)
Net change in foreign currency translation and other								$ 0	$ 0	$ 0
Total comprehensive income (loss)	(48)		(48)		(48)	(18.6)	(66.6)
Dividends declared on common shares			(3.8)		(3.8)		(3.8)
Dividends to non-controlling interests						(0.3)	(0.3)
Repurchases and retirements of common shares		(1.9)	(6.4)		(8.3)	0	(8.3)
Capital contributions from BAM members, net of tax												$ 4.2	$ 4.2
Amortization of restricted share awards		3.4			3.4		3.4
Recognition of equity-based units of subsidiary		(4.1)			(4.1)	(2.3)	(6.4)
Dilution from equity-based units of subsidiary		(0.9)			(0.9)	0.9	0
Ending Balances at Mar. 31, 2018		675.3	2,765	(1.3)	3,439	(143.2)	3,295.8
Beginning Balances at Dec. 31, 2018	2,718.2	584	2,264.9	(5.8)	2,843.1	(124.9)	2,718.2
Increase (Decrease) in Shareholders' Equity
Net income (loss)	272.2		284.4		284.4	(12.2)	272.2
Net change in foreign currency translation and other								$ (0.3)	$ (0.3)	$ 0	$ (0.3)
Total comprehensive income (loss)	284.1		284.4	(0.3)	284.1	(12.2)	271.9
Dividends declared on common shares			(3.2)		(3.2)		(3.2)
Dividends to non-controlling interests						(0.5)	(0.5)
Repurchases and retirements of common shares		(1)	(3.8)		(4.8)		(4.8)
Capital contributions from BAM members, net of tax												$ 6.8	$ 6.8
Net contributions from non-controlling interests						0.7	0.7
Amortization of restricted share awards		2.5			2.5		2.5
Recognition of equity-based units of subsidiary		0.3			0.3		0.3
Effect of deconsolidation of MediaAlpha														$ (13.8)	$ (13.8)
Acquisition of non-controlling interests		0			0	0.2	0.2
Ending Balances at Mar. 31, 2019	$ 2,978.3	$ 585.8	$ 2,542.3	$ (6.1)	$ 3,122	$ (143.7)	$ 2,978.3

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operations:
Net income (loss)	$ 272.2	$ (66.6)
Adjustments to reconcile net income to net cash used for operations:
Net realized and unrealized investment (gains) losses	(245.6)	53.7
Realized gain from the MediaAlpha Transaction	(66.2)	0
Deferred income expense (benefit)	7.5	(0.8)
Net gain from sale of discontinued operations, net of tax	(0.7)	(0.1)
Amortization of restricted share and option awards	2.8	3.2
Amortization and depreciation	8.3	4.8
Other operating items:
Net change in unearned insurance premiums	3.6	3.4
Net change in deferred acquisition costs	(0.6)	(1.1)
Net change in restricted cash	10.8	0
Net change in other assets and liabilities, net	(33.2)	(41.3)
Net cash used for operations - continuing operations	(41.1)	(44.8)
Net cash provided from operations - discontinued operations	0	0.1
Net cash (used for) provided from operations	(41.1)	(44.7)
Cash flows from investing activities:
Net change in short-term investments	(55.3)	(587.5)
Sales of fixed maturity and convertible investments	93	1,266.5
Maturities, calls and paydowns of fixed maturity and convertible investments	25.6	40.6
Sales of common equity securities	136.5	14.4
Distributions and redemptions of other long-term investments and settlements of forward contracts	1.9	(6.9)
Net settlement of investment cash flows and contributions with discontinued operations	0	0.1
Purchases of other long-term investments	(60.6)	(46)
Purchases of common equity securities	(4.6)	(109.4)
Purchases of fixed maturity and convertible investments	(152.3)	(537.1)
Purchases of consolidated subsidiaries, net of cash acquired of $0.4	(9.6)	0
Other investing activities, net	(17.4)	42.8
Net cash provided from investing activities - continuing operations	(42.8)	77.5
Net cash used for investing activities - discontinued operations	0	(0.1)
Net cash provided from investing activities	(42.8)	77.4
Cash flows from financing activities:
Draw down of debt and revolving line of credit	106.5	0
Repayment of debt and revolving line of credit	(14.8)	(2.3)
Cash dividends paid to the Company’s common shareholders	(3.2)	(3.8)
Distribution to non-controlling interest shareholders	(27.6)	0
Repurchase of shares from non-controlling interest shareholders	(21.1)	0
Proceeds from issuance of shares to non-controlling interest shareholders	62.7	0
Capital contributions to non-controlling interest shareholders	0.7	0
Capital contributions from BAM members	7.9	4.9
Fidus Re premium payment	(0.7)	0
Restricted share statutory withholding tax payments	(4.9)	(8.4)
Net cash used for financing activities - continuing operations	105.5	(9.6)
Net cash used for financing activities - discontinued operations	0	0
Net cash used for financing activities	105.5	(9.6)
Effect of exchange rate changes on cash	0.3	0
Net change in cash during the period - continuing operations	21.9	23.1
Cash balances at beginning of period	110.3	97.1
Cash balances at end of period	132.2	120.2
Supplemental cash flows information:
Interest paid	(3.7)	(0.3)
Net income tax refunds	$ 0	$ 0.2

CONSOLIDATED STATEMENTS OF CA_2

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) $ in Millions	3 Months Ended
	Mar. 31, 2019USD ($)
Cash balance	$ 132.2
Cash acquired from acquisition	0.4
Discontinued Operations
Cash balance	$ 60.8

Basis of Presentation and Signi

Basis of Presentation and Significant Accouting Policies	3 Months Ended
	Mar. 31, 2019
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 2018 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 (through February 26, 2019) and Other Operations. See Note 14 — “Segment Information” . The HG Global/BAM segment consists of HG Global Ltd. and its wholly-owned subsidiaries (“HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”) (collectively, “HG Global/BAM”). BAM is the first and only mutual municipal bond insurance company in the United States. By insuring the timely payment of principal and interest, BAM provides market access to, and lowers interest expense for, issuers of municipal bonds used to finance essential public purposes such as schools, utilities and transportation facilities. BAM is owned by and operated for the benefit of its members, the municipalities that purchase BAM’s insurance for their debt issuances. HG Global was established to fund the startup of BAM and, through its wholly-owned subsidiary, HG Re Ltd. (“HG Re”), to provide up to 15% -of-par, first loss reinsurance protection for policies underwritten by BAM. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM (the “BAM Surplus Notes”). As of March 31, 2019 , $481.3 million of the surplus notes remain outstanding. As of March 31, 2019 and December 31, 2018, White Mountains owned 96.9% of HG Global’s preferred equity and 88.4% of its common equity. White Mountains does not have an ownership interest in BAM. However, White Mountains is required to consolidate BAM’s results in its financial statements because BAM is a VIE for which White Mountains is the primary beneficiary. BAM’s results are attributed to non-controlling interests. The NSM segment consists of NSM Insurance HoldCo, LLC and its wholly-owned subsidiaries (collectively, “NSM”). NSM is a full-service managing general underwriting agency (“MGU”) and program administrator for specialty property and casualty insurance. The company places insurance in niche sectors such as specialty transportation, social services and real estate. On behalf of its insurance carrier partners, NSM manages all aspects of the placement process, including product development, marketing, underwriting, policy issuance and claims. NSM earns commissions based on the volume and profitability of the insurance that it places. NSM does not take insurance risk. As of March 31, 2019 and December 31, 2018, White Mountains owned 95.3% and 95.5% of the basic units outstanding of NSM ( 84.8% and 85.0% on a fully diluted, fully converted basis). The NSM segment also includes White Mountains Catskill Holdings, Inc., the immediate holding company of NSM. See Note 2 — “Significant Transactions ”. The MediaAlpha segment consisted of QL Holdings LLC and its wholly-owned subsidiary QuoteLab, LLC (collectively “MediaAlpha”). MediaAlpha is a marketing technology company that develops technology that enables the programmatic buying and selling of vertical-specific, performance-based media between advertisers (buyers of advertising inventory) and publishers (sellers of advertising inventory) through cost-per-click, cost-per-call and cost-per-lead pricing models. MediaAlpha’s media buying platform enables advertisers to create and automate data-driven bidding strategies designed to improve the efficiency and enhance overall performance of their marketing campaigns that target high-intent consumers at the time and place they are ready to purchase. MediaAlpha’s publisher platform is used by publishers to sell their vertical-specific, performance-based media to advertisers through transparent, programmatic, auction-based marketplaces. White Mountains deconsolidated MediaAlpha on February 26, 2019. White Mountains’s consolidated statement of comprehensive income and its segment disclosures include MediaAlpha’s results of operations for the period from January 1, 2019 through February 26, 2019. See Note 2 — “Significant Transactions” . The Other Operations segment consists of the Company and its wholly-owned subsidiary, White Mountains Capital, Inc. (“WM Capital”), its intermediate holding companies, its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), investment assets managed by WM Advisors, its interests in PassportCard Limited (“PassportCard”) and DavidShield Life Insurance Agency (2000) Ltd. (“DavidShield”) (collectively, “PassportCard/ DavidShield”) and Kudu Investment Management, LLC (“Kudu”), certain other consolidated and unconsolidated entities and certain other strategic investments. Discontinued Operations and Assets Held for Sale 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. Adjustments to the gain on sale subsequent to the transaction have also been classified within discontinued operations. White Mountains has classified its Guilford, Connecticut property, which consists of an office building and adjacent land, as held for sale as of March 31, 2019 and December 31, 2018. See Note 17 — “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 NSM interest rate 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. White Mountains formally evaluates 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 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”) during the quarter ended June 30, 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 2018 Annual Report on Form 10-K for a complete discussion regarding White Mountains’s significant accounting policies. Recently Adopted Changes in Accounting Principles Leases On January 1, 2019, White Mountains adopted ASU 2016-02, Leases (ASC 842), which 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. White Mountains elected the optional transition method that permits prospective adoption with recognition of a cumulative effect adjustment to the opening balance of retained earnings. As a result, White Mountains has presented comparative periods prior to adoption in accordance with previous lease accounting guidance. White Mountains also elected all available practical expedients permitted under ASC 842, which allowed White Mountains to carryforward its historical lease classification and not reassess leases for the definition of a lease under the new guidance. As of January 1, 2019 and March 31, 2019, White Mountains recognized $23.2 million and $21.5 million of lease right-of-use assets and $23.2 million and $ 21.8 million of lease liabilities. Adoption of ASU 2016-02 did not result in an adjustment to opening retained earnings. Premium Amortization on Callable Debt Securities On January 1, 2019, White Mountains adopted 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 is 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. Adoption of ASU 2017-08 did not have any impact on White Mountains’s financial statements. 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. 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 Goodwill In January 2017, the FASB issued ASU 2017-04, Simplifying the Test for Goodwill Impairment (ASC 350), which changes the guidance on goodwill impairment testing. Under the new guidance, the qualitative assessment of the recoverability of goodwill remains the same. However, the second step required under the existing guidance has been eliminated. Goodwill is considered impaired if the carrying value exceeds the estimated fair value. The new guidance is effective for fiscal years beginning after December 15, 2019. Credit Losses In June 2016, the FASB issued ASU 2016-13, Measurement of Credit Losses on Financial Instruments

Significant Transactions

Significant Transactions	3 Months Ended
	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Significant Transactions	Significant Transactions Acquisitions NSM On May 11, 2018, White Mountains acquired 95.0% of the basic units outstanding of NSM. White Mountains funded the acquisition through a combination of cash on hand and new borrowings by NSM. White Mountains paid $274.2 million of cash consideration for its equity interest in NSM, and NSM borrowed $100.0 million in new debt as part of the transaction. During the three months ended September 30, 2018, White Mountains recorded a purchase price adjustment of an additional $2.1 million . White Mountains recognized total assets acquired related to NSM of $495.2 million , including $383.0 million of goodwill and other intangible assets, total liabilities assumed of $204.6 million , including contingent consideration earnout liabilities related to NSM’s previous acquisitions of its U.K.-based operations, of $10.2 million , and non-controlling interest of $14.4 million reflecting acquisition date fair values. In connection with the acquisition, White Mountains incurred transaction costs of $6.3 million in the Other Operations segment, which were expensed in the three months ended June 30, 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 upfront cash consideration of $ 49.6 million. NSM borrowed $51.0 million in new debt to fund the transaction. During the three months ended March 31, 2019, NSM paid a purchase price adjustment of $0.7 million . The purchase price is subject to additional adjustments based upon growth in EBITDA during two earnout periods, ending in February 2020 and February 2022. NSM recognized total assets acquired related to Fresh Insurance of $72.6 million , including $54.6 million of goodwill and other intangible assets, and total liabilities assumed of $22.3 million , reflecting acquisition date fair values. In connection with the acquisition, NSM recorded a contingent consideration earnout liability of $7.5 million . On December 3, 2018, NSM acquired all the net assets of KBK Insurance Group (“KBK”), a specialized MGU focused on the towing and transportation space, for upfront cash consideration of $60.0 million . In order to partially fund the KBK transaction, White Mountains contributed $29.0 million . NSM borrowed $30.1 million in new debt as part of the transaction. White Mountains recognized $59.4 million of goodwill and other intangible assets, reflecting acquisition date fair values, for which the relative fair values of goodwill and other intangible assets had not yet been determined as of December 31, 2018. During the three months ended March 31, 2019 , NSM recorded a purchase price adjustment of $5.9 million relating to the fair value of the contingent consideration earnout liability in connection with the acquisition, which was not previously determined. As of March 31, 2019, White Mountains has determined that the relative values of goodwill and other intangible assets recorded in connection with the KBK transaction are $32.6 million and $32.7 million , reflecting acquisition date fair value. The purchase price is subject to additional adjustments based upon growth in EBITDA during three earnout periods ending in December 2019, December 2020 and December 2021. The contingent consideration earnout liabilities related to these acquisitions are subject to adjustment based upon EBITDA, EBITDA projections, and present value factors for acquired entities. For the three months ended March 31, 2019 , NSM recognized pre-tax expense of $1.3 million for the increase 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 March 31, 2019 and December 31, 2018, NSM recorded contingent consideration earnout liabilities of $27.5 million and $20.2 million . DavidShield On January 24, 2018, White Mountains acquired 50% of the basic shares outstanding of DavidShield, its joint venture partner in PassportCard. DavidShield is a managing general agency that is the leading provider of expatriate medical insurance in Israel and uses the same card-based delivery system as PassportCard. As part of the transaction, White Mountains 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 $125.0 million in Kudu, a provider of capital solutions to asset management and wealth management firms. As of March 31, 2019 and December 31, 2018, White Mountains owned 49.5% of the basic units outstanding of Kudu ( 42.7% on a fully diluted, fully converted basis). Kudu’s solutions are tailored to special situations, including generational ownership transfers, management buyouts, acquisition and growth finance and legacy partner liquidity. Kudu’s solutions typically take the form of revenue shares that generate cash yields and have additional upside equity participation. Kudu provides a range of advisory services to help its asset manager partners grow their businesses. Kudu closed its first three transactions in 2018 deploying $63.0 million of capital in total, of which $31.5 million was from White Mountains. For the three months ended March 31, 2019, Kudu closed two additional transactions, deploying an additional $82.2 million , of which $41.1 million was from White Mountains. As of March 31, 2019, White Mountains has funded $72.6 million of its $125.0 million commitment, plus an additional $3.8 million for working capital. On February 14, 2019, White Mountains entered into a definitive agreement to purchase all the ownership interests in Kudu held by certain funds managed by Oaktree Capital Management, L.P. (“Oaktree”). On April 4, 2019, White Mountains completed its acquisition of Oaktree’s interests in Kudu for $81.4 million . As a result of the purchase, White Mountains’s ownership interest was increased to 99.1% of the basic units outstanding of Kudu ( 85.4% on a fully diluted, fully converted basis). In connection with the transaction, White Mountains assumed all of Oaktree’s unfunded capital commitments to Kudu, increasing White Mountains’s total unfunded Kudu capital commitment to $104.7 million . See Note 18 — “Subsequent Events” . For periods prior to White Mountains’s acquisition of Oaktree’s interest in Kudu, White Mountains determined that Kudu was a VIE, but that White Mountains was not the primary beneficiary. In those periods, White Mountains elected to use the fair value option. For periods subsequent to the acquisition, White Mountains will consolidate Kudu beginning in the second quarter of 2019. Dispositions MediaAlpha On February 26, 2019, MediaAlpha completed the sale of a significant minority stake to Insignia Capital Group in connection with a recapitalization and cash distribution to existing equityholders (the “MediaAlpha Transaction”). White Mountains received net cash proceeds of $88.0 million from the MediaAlpha Transaction. White Mountains expects to receive additional proceeds of $1.3 million during the second quarter of 2019, when funds are released from escrow. White Mountains recognized a realized gain of $ 67.5 million and reduced its ownership interest to 48.3% of the basic units outstanding of MediaAlpha ( 42.0% on a fully diluted, fully converted basis) as a result of the MediaAlpha Transaction. White Mountains’s remaining ownership interest in MediaAlpha no longer meets the criteria for a controlling ownership interest and, accordingly, White Mountains deconsolidated MediaAlpha on February 26, 2019. White Mountains’s consolidated statement of operations and comprehensive income and its segment disclosures include MediaAlpha’s results of operations for the period from January 1, 2019 through February 26, 2019. Upon deconsolidation, White Mountains’s investment in MediaAlpha met the criteria to be accounted for under the equity method or under the fair value option. White Mountains elected the fair value option and the investment in MediaAlpha was measured at its estimated fair value of $114.7 million as of March 31, 2019 , with the change in fair value of $114.7 million recognized as an unrealized investment gain. White Mountains recognized a total of $ 182.2

Investment Securities

Investment Securities	3 Months Ended
	Mar. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Investment Securities	Investment Securities White Mountains’s portfolio of investment securities held for general investment purposes consists of fixed maturity investments, short-term investments, common equity securities 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 March 31, 2019 and December 31, 2018 . Other long-term investments consist primarily of unconsolidated entities, private equity funds and hedge funds. Net Investment Income White Mountains’s net investment income is comprised primarily of interest income associated with White Mountains’s fixed maturity investments and short-term investments and dividend income from its common equity securities and other long-term investments. The following table presents pre-tax net investment income for the three months ended March 31, 2019 and 2018 : Three Months Ended March 31, Millions 2019 2018 Fixed maturity investments $ 8.1 $ 10.9 Short-term investments 1.3 2.6 Common equity securities 5.4 5.0 Other long-term investments 1.6 1.9 Total investment income 16.4 20.4 Third-party investment expenses (.4 ) (.7 ) Net investment income, pre-tax $ 16.0 $ 19.7 Net Realized and Unrealized Investment Gains (Losses) The following table presents net realized and unrealized investment gains (losses) for the three months ended March 31, 2019 and 2018 : Three Months Ended March 31, Millions 2019 2018 Net realized investment gains (losses), pre-tax $ 25.7 $ (5.1 ) Net unrealized investment gains (losses), pre-tax (1) 219.9 (48.6 ) Net realized and unrealized investment gains (losses), pre-tax 245.6 (53.7 ) Income tax (expense) benefit attributable to net realized and unrealized investment gains (losses) (52.2 ) 5.7 Net realized and unrealized investment gains (losses), after-tax $ 193.4 $ (48.0 ) (1) Includes $114.7 in unrealized investment gains associated with the MediaAlpha Transaction. See Note 2 — “Significant Transactions.” For the three months ended March 31, 2019 and 2018, all of White Mountains’s net realized and unrealized investment gains (losses) were recorded in the consolidated statements of operations. There were no investment gains (losses) recorded in other comprehensive income. Net Realized Investment Gains (Losses) The following table presents net realized investment gains (losses) for the three months ended March 31, 2019 and 2018 : Three Months Ended Three Months Ended March 31, 2019 March 31, 2018 Millions Net Net Total Net Realized Net Net Total Net Realized Fixed maturity investments $ (.9 ) $ — $ (.9 ) $ (13.7 ) $ 18.2 $ 4.5 Short-term investments — — — (.1 ) — (.1 ) Common equity securities 25.8 — 25.8 1.2 — 1.2 Other long-term investments .8 — .8 (3.5 ) (7.2 ) (10.7 ) Net realized investment gains (losses), pre-tax 25.7 — 25.7 (16.1 ) 11.0 (5.1 ) Income tax expense attributable to net realized investment gains (losses) (1.7 ) — (1.7 ) (.6 ) — (.6 ) Net realized investment gains (losses), after-tax $ 24.0 $ — $ 24.0 $ (16.7 ) $ 11.0 $ (5.7 ) Net Unrealized Investment Gains (Losses) The following table presents net unrealized investment gains (losses) and changes in the carrying value of investments measured at fair value for the three months ended March 31, 2019 and 2018 : Three Months Ended Three Months Ended March 31, 2019 March 31, 2018 Millions Net Unrealized Gains Net Total Net Unrealized Gains Reflected in Earnings Net Unrealized Losses Net Gains Total Net Unrealized (Losses) Gains Reflected in Earnings Fixed maturity investments $ 22.0 $ — $ 22.0 $ (18.6 ) $ (14.8 ) $ (33.4 ) Short-term investments — — — (.7 ) — (.7 ) Common equity securities 85.5 — 85.5 (16.7 ) — (16.7 ) Other long-term investments (1) 112.7 (.3 ) 112.4 (1.9 ) 4.1 2.2 Net unrealized investment gains (losses), pre-tax 220.2 (.3 ) 219.9 (37.9 ) (10.7 ) (48.6 ) Income tax expense attributable to net unrealized investment gains (losses) (50.5 ) — (50.5 ) 6.3 — 6.3 Net unrealized investment gains (losses), after-tax $ 169.7 $ (.3 ) $ 169.4 $ (31.6 ) $ (10.7 ) $ (42.3 ) (1) Includes $114.7 in unrealized investment gains associated with the MediaAlpha Transaction. See Note 2 — “Significant Transactions.” The following table presents total gains (losses) included in earnings attributable to net unrealized investment gains (losses) for Level 3 investments for the three months ended March 31, 2019 and 2018 : Three Months Ended March 31, Millions 2019 2018 Other long-term investments $ 114.0 $ (5.1 ) Total net unrealized investment gains (losses), pre-tax - Level 3 investments $ 114.0 $ (5.1 ) Investment Holdings The following tables present the cost or amortized cost, gross unrealized investment gains (losses) and carrying values of White Mountains’s fixed maturity investments as of March 31, 2019 and December 31, 2018 : March 31, 2019 Millions Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Carrying Value U.S. Government and agency obligations $ 153.1 $ .4 $ (.5 ) $ 153.0 Debt securities issued by corporations 538.4 7.3 (.9 ) 544.8 Municipal obligations 279.7 6.1 (.4 ) 285.4 Mortgage and asset-backed securities 149.4 .6 (1.2 ) 148.8 Total fixed maturity investments $ 1,120.6 $ 14.4 $ (3.0 ) $ 1,132.0 December 31, 2018 Millions Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Carrying Value U.S. Government and agency obligations $ 154.0 $ .1 $ (.9 ) $ 153.2 Debt securities issued by corporations 519.0 1.0 (9.5 ) 510.5 Municipal obligations 279.0 2.4 (1.1 ) 280.3 Mortgage and asset-backed securities 136.1 .1 (2.7 ) 133.5 Total fixed maturity investments $ 1,088.1 $ 3.6 $ (14.2 ) $ 1,077.5 The following tables present the cost or amortized cost, gross unrealized investment gains (losses), net foreign currency losses, and carrying values of White Mountains’s common equity securities and other long-term investments as of March 31, 2019 and December 31, 2018 : March 31, 2019 Millions Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Net Foreign Currency Losses Carrying Value Common equity securities $ 798.6 $ 107.3 $ (.9 ) $ — $ 905.0 Other long-term investments $ 389.8 $ 164.3 $ (54.3 ) $ (2.2 ) $ 497.6 December 31, 2018 Millions Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Net Foreign Carrying Value Common equity securities $ 904.7 $ 51.0 $ (30.1 ) $ — $ 925.6 Other long-term investments $ 330.3 $ 52.2 $ (54.9 ) $ (2.0 ) $ 325.6 Other Long-Term Investments The following table presents the carrying values of White Mountains’s other long-term investments as of March 31, 2019 and December 31, 2018 : Carrying Value at Millions March 31, 2019 December 31, 2018 MediaAlpha (1) $ 114.7 $ — PassportCard/DavidShield (1) 75.0 75.0 Kudu 71.7 30.7 Other unconsolidated entities (1)(2) 54.4 60.0 Total unconsolidated entities (1)(2) 315.8 165.7 Private equity funds and hedge funds 150.7 146.1 Other 31.1 13.8 Total other long-term investments $ 497.6 $ 325.6 (1) See Fair Value Measurements by Level table. (2) Includes White Mountains’s non-controlling interests in certain private common equity securities, limited liability companies and convertible preferred securities. Private Equity Funds and Hedge Funds White Mountains invests in private equity funds and hedge funds, which are included in other long-term investments. The fair value of these investments is generally estimated using the NAV of the funds. As of March 31, 2019 , White Mountains held investments in twelve private equity funds and one hedge fund. The largest investment in a single fund was $53.1 million as of March 31, 2019 and $54.3 million as of December 31, 2018 . The following table presents investments in private equity funds and hedge funds by investment objective and sector as of March 31, 2019 and December 31, 2018 : March 31, 2019 December 31, 2018 Millions Fair Value Unfunded Commitments Fair Value Unfunded Commitments Private equity funds Manufacturing/Industrial $ 42.1 $ 10.5 $ 42.9 $ 10.5 Aerospace/Defense/Government 32.8 29.8 27.6 34.9 Direct lending 13.6 16.8 13.0 17.7 Financial services 9.1 12.8 8.3 13.6 Total private equity funds 97.6 69.9 91.8 76.7 Hedge funds Long/short banks and financials 53.1 — 54.3 — Total hedge funds 53.1 — 54.3 — Total private equity funds and hedge funds included in other long-term investments $ 150.7 $ 69.9 $ 146.1 $ 76.7 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 March 31, 2019 : Millions 1 – 3 years 3 – 5 years 5 – 10 years >10 years Total Private equity funds — expected lock-up period remaining $7.0 $— $67.9 $22.7 $97.6 Investors in private equity funds are generally subject to indemnification obligations outside of the capital commitment period and prior to the winding up of the fund. As of March 31, 2019 and December 31, 2018, White Mountains is not aware of any indemnification claims relating to its investments in private equity funds. Redemption of investments in most hedge funds is subject to restrictions, including lock-up periods where no redemptions or withdrawals are allowed, restrictions on redemption frequency and advance notice periods for redemptions. Amounts requested for redemptions remain subject to market fluctuations until the redemption effective date, which generally falls at the end of the defined redemption period. As of March 31, 2019 , White Mountains held one active hedge fund with a fair value of $ 53.1 million . The hedge fund is subject to a semi-annual restriction on redemptions and an advance notice period requirement of 45 days. Fair Value Measurements as of March 31, 2019 Fair value measurements are categorized into a hierarchy that distinguishes between inputs based on market data from independent sources (“observable inputs”) and a reporting entity’s internal assumptions based upon the best information available when external market data is limited or unavailable (“unobservable inputs”). Quoted prices in active markets for identical assets or liabilities have the highest priority (“Level 1”), followed by observable inputs other than quoted prices, including prices for similar but not identical assets or liabilities (“Level 2”) and unobservable inputs, including the reporting entity’s estimates of the assumptions that market participants would use, having the lowest priority (“Level 3”). As of March 31, 2019 and December 31, 2018, White Mountains used quoted market prices or other observable inputs to determine fair value for approximately 82% and 87% of the investment portfolio. Fair Value Measurements by Level The following tables present White Mountains’s fair value measurements for investments as of March 31, 2019 and December 31, 2018 by level. The major security types were based on the legal form of the securities. White Mountains has disaggregated its fixed maturity investments based on the issuing entity type, which impacts credit quality, with debt securities issued by U.S. government entities carrying minimal credit risk, while the credit and other risks associated with other issuers, such as corporations, foreign governments, municipalities or entities issuing mortgage and asset-backed securities vary depending on the nature of the issuing entity type. White Mountains further disaggregates debt securities issued by corporations and common equity securities by industry sector because investors often reference commonly used benchmarks and their subsectors to monitor risk and performance. Accordingly, White Mountains has further disaggregated these asset classes into subclasses based on the similar sectors and industry classifications it uses to evaluate investment risk and performance against commonly used benchmarks, such as the Bloomberg Barclays U.S. Intermediate Aggregate and S&P 500 indices. March 31, 2019 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 153.0 $ 153.0 $ — $ — Debt securities issued by corporations: Financials 139.7 — 139.7 — Healthcare 74.7 — 74.7 — Energy 66.0 — 66.0 — Consumer 61.0 — 61.0 — Industrial 57.0 — 57.0 — Technology 55.0 — 55.0 — Communications 40.4 — 40.4 — Utilities 28.5 — 28.5 — Materials 22.5 — 22.5 — Total debt securities issued by corporations 544.8 — 544.8 — Mortgage and asset-backed securities 148.8 — 148.8 — Municipal obligations 285.4 — 285.4 — Total fixed maturity investments 1,132.0 153.0 979.0 — Short-term investments (1) 269.8 249.0 20.8 — Common equity securities: Exchange traded funds (2) 712.3 646.3 66.0 — Other (3) 192.7 — 192.7 — Total common equity securities 905.0 646.3 258.7 — Other long-term investments 275.1 — — 275.1 Other long-term investments — NAV (4) 222.5 — — — Total investments $ 2,804.4 $ 1,048.3 $ 1,258.5 $ 275.1 (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) Consists of unconsolidated entities, private equity funds and one hedge fund for which fair value is measured at NAV using the practical expedient. Investments for which fair value is measured at NAV are not classified within the fair value hierarchy. December 31, 2018 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 153.2 $ 153.2 $ — $ — Debt securities issued by corporations: Financials 143.4 — 143.4 — Consumer 68.5 — 68.5 — Technology 60.5 — 60.5 — Energy 57.6 — 57.6 — Healthcare 55.0 — 55.0 — Industrial 47.6 — 47.6 — Communications 31.8 — 31.8 — Materials 26.3 — 26.3 — Utilities 19.8 — 19.8 — Total debt securities issued by corporations 510.5 — 510.5 — Mortgage and asset-backed securities 133.5 — 133.5 — Municipal obligations 280.3 — 280.3 — Total fixed maturity investments 1,077.5 153.2 924.3 — Short-term investments (1) 214.2 204.4 9.8 — Common equity securities: Exchange traded funds (2) 675.3 617.0 58.3 — Healthcare 14.0 14.0 — — Financials 13.5 13.5 — — Communications 12.7 12.7 — — Industrial 11.4 11.4 — — Technology 7.4 7.4 — — Consumer 6.2 6.2 — — Energy 4.1 4.1 — — Materials 3.1 3.1 — — Other (3) 177.9 — 177.9 — Total common equity securities 925.6 689.4 236.2 — Other long-term investments 138.7 — — 138.7 Other long-term investments — NAV (4) 186.9 — — — Total investments $ 2,542.9 $ 1,047.0 $ 1,170.3 $ 138.7 (1) Short-term investments are measured at amortized cost, which approximates fair value. (2) ETFs traded on foreign exchanges are priced using the fund’s published NAV to account for the difference in market close times and are therefore designated a level 2 measurement. (3) Consists of two investments in unit trusts that primarily invest in international equities. (4) Consists of unconsolidated entities, private equity funds and one hedge fund for which fair value is measured at NAV using the practical expedient. Investments for which fair value is measured at NAV are not classified within the fair value hierarchy. Debt Securities Issued by Corporations The following table presents the ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of March 31, 2019 and December 31, 2018 : Fair Value at Millions March 31, 2019 December 31, 2018 AAA $ 9.6 $ 8.9 AA 83.8 88.7 A 300.3 270.5 BBB 151.1 142.4 Debt securities issued by corporations (1) $ 544.8 $ 510.5 (1) Credit ratings are based upon issuer credit ratings provided by Standard & Poor’s Financial Services LLC (“Standard & Poor’s”), or if unrated by Standard & Poor’s, long-term obligation ratings provided by Moody’s Investor Service, Inc. Mortgage and Asset-backed Securities The following table presents the fair value of White Mountains’s mortgage and asset-backed securities as of March 31, 2019 and December 31, 2018 : March 31, 2019 December 31, 2018 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: FNMA $ 59.8 $ 59.8 $ — $ 53.6 $ 53.6 $ — FHLMC 43.0 43.0 — 38.1 38.1 $ — GNMA 25.6 25.6 — 23.7 23.7 $ — Total agency (1) 128.4 128.4 — 115.4 115.4 — Total mortgage-backed securities 128.4 128.4 — 115.4 115.4 — Other asset-backed securities: Vehicle receivables 11.5 11.5 — 9.2 9.2 — Credit card receivables 8.9 8.9 — 8.9 8.9 — Total other asset-backed securities 20.4 20.4 — 18.1 18.1 — Total mortgage and asset-backed securities $ 148.8 $ 148.8 $ — $ 133.5 $ 133.5 $ — (1) Represents publicly traded mortgage-backed securities which carry the full faith and credit guaranty of the U.S. Government (i.e., GNMA) or are guaranteed by a government sponsored entity (i.e., FNMA, FHLMC). Rollforward of Fair Value Measurements by Level White Mountains uses quoted market prices where available as the inputs to estimate fair value for its investments in active markets. Such measurements are considered to be either Level 1 or Level 2 measurements, depending on whether the quoted market price inputs are for identical securities (Level 1) or similar securities (Level 2). Level 3 measurements for fixed maturity investments, common equity securities and other long-term investments as of March 31, 2019 and 2018 consist of securities for which the estimated fair value has not been determined based upon quoted market price inputs for identical or similar securities. The following tables present the changes in White Mountains’s fair value measurements by level for the three months ended March 31, 2019 and 2018 : Level 3 Investments Unconsolidated Entities, Private Equity Funds and Hedge Funds Measured at NAV (3) Millions Level 1 Investments Level 2 Investments Other Long-term Investments (2) Total Balance at December 31, 2018 $ 842.6 $ 1,160.5 $ 138.7 $ 186.9 $ 2,328.7 (1) Net realized and unrealized gains (losses) 91.7 40.7 114.0 (.8 ) 245.6 Amortization/Accretion .1 (.4 ) — — (.3 ) Purchases 14.6 142.3 11.5 49.2 217.6 Sales (149.7 ) (105.4 ) — (1.9 ) (257.0 ) Transfers in — — 10.9 — 10.9 Transfers out — — — (10.9 ) (10.9 ) Balance at March 31, 2019 $ 799.3 $ 1,237.7 $ 275.1 $ 222.5 $ 2,534.6 (1) (1) Excludes carrying value of $ 269.8 million and $ 214.2 as of March 31, 2019 and December 31, 2018 , respectively classified as short-term investments. (2) Includes $114.7 in unrealized investment gains associated with the MediaAlpha Transaction. See Note 2 — “Significant Transactions”. (3) Investments for which fair value is measured at NAV using the practical expedient are no longer classified within the fair value hierarchy. See Note 1 — “Basis of Presentation and Significant Accounting Policies” . Level 3 Investments Unconsolidated Entities, Private Equity Funds and Hedge Funds Measured at NAV (3) Millions Level 1 Investments Level 2 Investments Other Long-term Investments Total Balance at December 31, 2017 $ 890.4 $ 2,105.4 $ 77.2 $ 135.3 $ 3,208.3 (1)(2) Net realized and unrealized (losses) gains (14.3 ) (26.0 ) (7.8 ) 2.7 (45.4 ) (4) Amortization/Accretion — (1.2 ) — — (1.2 ) Purchases 238.9 407.7 44.0 2.0 692.6 Sales (240.7 ) (1,080.8 ) — (.3 ) (1,321.8 ) Transfers in — — — — — Transfers out — — — — — Balance at March 31, 2018 $ 874.3 $ 1,405.1 $ 113.4 $ 139.7 $ 2,532.5 (1) (1) Excludes carrying value of $ 763.0 and $ 176.1 as of March 31, 2018 and December 31, 2017 , respectively classified as short-term investments. (2) Excludes carrying value of $(3.7) as of December 31, 2017 associated with foreign currency forward contracts. (3) Investments for which fair value is measured at NAV using the practical expedient are no longer classified within the fair value hierarchy. See Note 1 — “Basis of Presentation and Significant Accounting Policies” . (4) Excludes realized and unrealized losses associated with foreign currency forward contracts, foreign currency on cash and open trades and short-term investments of $3.5 , $4.2 and $0.6 for the three months ended March 31, 2018. Fair Value Measurements — Transfers Between Levels - Three-months ended March 31, 2019 and 2018 Transfers between levels are recorded using the fair value measurement as of the end of the quarterly period in which the event or change in circumstance giving rise to the transfer occurred. During the first three months of 2019 and 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. Significant Unobservable Inputs The following tables present significant unobservable inputs used in estimating the fair value of other long-term investments, other than private equity funds and hedge funds, classified within Level 3 as of March 31, 2019 and December 31, 2018 . The fair value of investments in certain unconsolidated entities, private equity funds and hedge funds are generally estimated using the NAV of the funds. $ in Millions, Except Share Price March 31, 2019 Description Valuation Technique(s) Fair Value (1) Unobservable Input MediaAlpha Share price of recent transaction (2) $114.7 Unit price - $208.98 PassportCard/DavidShield Discounted cash flow $75.0 Discount rate - 18.0% Revenue exit multiple - 1.00 Compare.com Discounted cash flow $16.9 Discount rate - 22.0% Revenue exit multiple - 2.75 YOUSURE Tarifvergleich GmbH (“durchblicker”) Discounted cash flow $15.2 Discount rate - 23.0% Revenue exit multiple - 2.25 Captricity, Inc. Discounted cash flow $14.5 Discount rate - 23.0% Revenue exit multiple - 3.75 Galvanic Applied Sciences Multiple of EBITDA $2.7 EBITDA multiple - 6.00 Private debt instrument Discounted cash flow $10.0 Discount rate - 9.23% Private debt instrument Discounted cash flow $10.9 Discount rate - 7.00% (1) Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs. (2) The MediaAlpha Transaction was completed on February 26, 2019. See Note 2 - “Significant Transactions”. $ in Millions, Except Share Price December 31, 2018 Description Valuation Technique(s) Fair Value (1) Unobservable Input PassportCard/DavidShield Discounted cash flow $75.0 Discount rate - 18.0% Revenue exit multiple - 1.00 Compare.com Discounted cash flow $16.9 Discount rate - 22.0% Revenue exit multiple - 2.75 durchblicker Discounted cash flow $15.5 Discount rate - 23.0% Revenue exit multiple - 2.25 Captricity, Inc. Discounted cash flow $14.5 Discount rate - 23.0% Revenue exit multiple - 3.75 Galvanic Applied Sciences Multiple of EBITDA $3.1 EBITDA multiple - 6.00 Private debt instrument Discounted cash flow $10.0 Discount rate - 9.23% (1)

Goodwill and Other Intangible A

Goodwill and Other Intangible Assets	3 Months Ended
	Mar. 31, 2019
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. On February 26, 2019, MediaAlpha completed the sale of a significant minority stake to Insignia Capital Group in connection with a recapitalization and cash distribution to existing equityholders. MediaAlpha also repurchased a portion of the holdings of existing equityholders, including a repurchase of a portion of White Mountains’s holdings. As a result of the repurchase, White Mountains’s ownership interest was reduced to 48.3% of the basic units outstanding of MediaAlpha ( 42.0% on a fully diluted, fully converted basis). White Mountains’s remaining ownership interest in MediaAlpha no longer meets the criteria for a controlling ownership interest and accordingly, White Mountains deconsolidated MediaAlpha on February 26, 2019. The following tables presents the change in goodwill and other intangible assets: Three Months Ended March 31, 2019 2018 Millions Goodwill Other Intangible Assets Total Goodwill and Other Intangible Assets Goodwill Other Intangible Assets Total Goodwill and Other Intangible Assets Beginning balance $ 379.9 $ 157.6 $ 537.5 $ 25.9 $ 36.2 $ 62.1 Deconsolidation of MediaAlpha (18.3 ) (23.5 ) (41.8 ) — — — Attribution of acquisition date fair value estimates between goodwill and other intangible assets (26.8 ) 26.8 — — — — Acquisitions of businesses 13.4 — 13.4 Foreign currency translation 1.2 .3 1.5 — — — — Adjustments to reflect acquisition date fair value .3 5.9 6.2 — — — — Amortization — (6.6 ) (6.6 ) — (2.9 ) (2.9 ) Ending balance $ 349.7 $ 160.5 $ 510.2 $ 25.9 $ 33.3 $ 59.2 The following table presents the acquisition date fair values, accumulated amortization and net carrying values for other intangible assets and goodwill, by company as of March 31, 2019 and December 31, 2018 : $ in Millions March 31, 2019 December 31, 2018 Acquisition Date Fair Value Accumulated Amortization Net Carrying Value Acquisition Date Fair Value Accumulated Amortization Net Carrying Value Goodwill: NSM (1) $ 328.8 $ — $ 328.8 $ 354.3 $ — $ 354.3 MediaAlpha — — — 18.3 — 18.3 Other Operations (2) 20.9 — 20.9 7.3 — 7.3 Total goodwill 349.7 — 349.7 379.9 — 379.9 Other intangible assets: NSM (1) Customer relationships 110.9 9.3 101.6 85.3 6.0 79.3 Trade names 57.1 2.7 54.4 51.2 1.8 49.4 Information technology 3.8 .7 3.1 3.7 .5 3.2 Other 1.4 .6 .8 — — — Subtotal 173.2 13.3 159.9 140.2 8.3 131.9 MediaAlpha Customer relationships — — — 26.8 4.9 21.9 Information technology — — — 33.3 30.9 2.4 Other — — — 9.8 9.0 0.8 Subtotal — — — 69.9 44.8 25.1 Other Operations Trade names .6 .2 .4 .6 .2 .4 Information technology .5 .3 .2 .5 .3 .2 Subtotal 1.1 .5 .6 1.1 .5 .6 Total other intangible assets 174.3 13.8 160.5 211.2 53.6 157.6 Total goodwill and other intangible assets $ 524.0 $ 13.8 510.2 $ 591.1 $ 53.6 537.5 Goodwill and other intangible assets attributed to non-controlling interests (24.1 ) (40.6 ) Goodwill and other intangible assets included in White Mountains’s common shareholders’ equity $ 486.1 $ 496.9 (1) As of March 31, 2019 and December 31, 2018, NSM’s goodwill included $1.0 and $ (2.2) and intangible assets included $0.3 and $ (0.7) of the effect of foreign currency translation. (2) As of March 31, 2019 and December 31, 2018, Other Operations’ goodwill included $0.2 and $ (0.3)

Debt

Debt	3 Months Ended
	Mar. 31, 2019
Debt Disclosure [Abstract]
Debt	Debt The following table presents White Mountains’s debt outstanding as of March 31, 2019 and December 31, 2018 : Millions March 31, Effective Rate (1) December 31, Effective Rate (1) NSM Bank Facility $ 186.4 7.8% $ 180.4 7.4% Unamortized issuance cost (3.6 ) (3.8 ) NSM Bank Facility, carrying value 182.8 176.6 Other NSM debt 1.7 1.9 MediaAlpha Bank Facility — 14.3 7.1% Unamortized issuance cost — (.1 ) MediaAlpha Bank Facility, carrying value — 14.2 Other Operations debt 11.3 — Unamortized issuance cost (.4 ) — Other Operations, carrying value 10.9 — Total debt $ 195.4 $ 192.7 (1) Effective rate considers the effect of the debt issuance costs. NSM Bank Facility On May 11, 2018, NSM entered into a secured credit facility (the “NSM Bank Facility”) with Ares Capital Corporation in order to refinance NSM’s existing debt and to fund the acquisitions of subsidiaries. The NSM Bank Facility is comprised of term loans totaling $181.1 million 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 March 31, 2019 , NSM repaid $0.5 million on the term loans and borrowed $6.5 million on the revolving credit loan. As of March 31, 2019 , $179.9 million of term loans and $6.5 million of 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 $151.0 million of its variable rate term loans. Under the terms of the swap agreement, NSM pays a fixed rate of 2.97% and receives a variable rate, which is reset monthly, based on then-current LIBOR. As of March 31, 2019 , the variable rate received by NSM under the swap agreement was 2.49% . As of March 31, 2019 , the interest rate, including the effect of the swap, for the outstanding term loans of $149.9 million that are hedged by the swap was 7.47% . The effective interest on the outstanding term loans of $30.0 million that are unhedged was 7.30% . The effective interest rate on the total outstanding term loans under the NSM Bank Facility of $186.4 million was 7.44% , excluding the effect of debt issuance costs. 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. Compliance At March 31, 2019

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2019
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. In the event there is a change in the current law and taxes are imposed, the Company and its Bermuda domiciled subsidiaries would be exempt from such tax until March 31, 2035, pursuant to the Bermuda Exempted Undertakings Tax Protection Act of 1966. The Company has subsidiaries and branches that operate in various other jurisdictions around the world that are subject to tax in the jurisdictions in which they operate. The jurisdictions in which the Company’s subsidiaries and branches are subject to tax are Barbados, Ireland, Israel, Luxembourg, the United Kingdom and the United States. White Mountains’s income tax expense related to pre-tax income from continuing operations for the three months ended March 31, 2019 represented an effective tax rate of 3.6% . The effective tax rate was different from the current U.S. statutory rate of 21.0% , primarily due to a release of a full valuation allowance on the net deferred tax assets of the U.S. consolidated group Guilford Holdings, Inc. and subsidiaries (“Guilford”). Guilford includes MediaAlpha, Kudu, various service companies and certain other entities and investments that are included in the Other Operations segment. The effective tax rate was also different due to withholding taxes and a tax benefit recorded at BAM. For BAM, member surplus contributions (“MSC”) and the related taxes thereon are recorded directly to non-controlling interest equity, while the valuation allowance on such taxes is recorded through the income statement. For the three months ended March 31, 2019, BAM recorded a tax benefit of $1.1 million associated with the valuation allowance on taxes related to MSC that is included in the effective tax rate. White Mountains’s income tax expense related to pre-tax loss from continuing operations for the three months ended March 31, 2018 represented an effective tax rate of (1.1)% . The effective tax rate was different from the U.S. statutory rate of 21.0% , primarily due to a full valuation allowance on all net deferred tax assets at U.S. operations, consisting of Guilford and BAM, withholding taxes and a tax benefit recorded at BAM. For BAM, MSC and the related taxes thereon are recorded directly to non-controlling interest equity, while the valuation allowance on such taxes is recorded through the income statement. For the three months ended March 31, 2018, BAM recorded a tax benefit of $0.6 million associated with the valuation allowance on taxes related to MSC that is included in the effective tax rate. In arriving at the effective tax rate for the three months ended March 31, 2019 and 2018, 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, 2019 and 2018. 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. During the three months ended March 31, 2018

Derivatives

Derivatives	3 Months Ended
	Mar. 31, 2019
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 $151.0 million of its variable rate term loans. Under the terms of the swap agreement, NSM pays a fixed rate of 2.97% and receives a variable rate, which is reset monthly, based on the then-current LIBOR. As of March 31, 2019, the variable rate received by NSM under the swap agreement was 2.49% . 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 March 31, 2019, the interest rate, including the effect of the swap, for the outstanding term loans of $149.9 million that are hedged by the swap was 7.47% , excluding the effect of debt issuance costs. 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 March 31, 2019, NSM recognized net interest expense of $0.2 million for the periodic net settlement payments on the swap. As of March 31, 2019 and December 31, 2018, the estimated fair value of the swap and the accrual of the periodic net settlement payments recorded in other liabilities was $4.6 million and $2.7 million . There was no ineffectiveness in the hedge for the period ended March 31, 2019. The $(1.9) million
Forward Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivatives	White Mountains’s investment portfolio includes investments denominated in GBP, Euros, Japanese Yen and other foreign currencies. White Mountains previously entered into foreign currency forward contracts to manage its foreign currency exposure related to certain of these investments. In conjunction with the liquidation of the GBP investment grade corporate bond mandate in the three months ended March 31, 2018 , White Mountains closed the associated foreign currency forward contract. As of March 31, 2019 and December 31, 2018, White Mountains no longer has any open foreign currency forward contracts. The derivative losses recognized in net realized and unrealized investment gains (losses) for the three months ended March 31, 2018 were $(3.5) million

Municipal Bond Guarantee

Municipal Bond Guarantee	3 Months Ended
	Mar. 31, 2019
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 the initial capitalization of BAM, a newly formed mutual municipal bond insurer. As of March 31, 2019 , 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. For capital appreciation bonds, par is adjusted to the estimated equivalent par value for current interest paying bonds. In return, BAM cedes 60% of the risk premium charged for insuring the municipal bond, net of a ceding commission. HG Re’s obligations under the FLRT are limited to the assets in two collateral trusts: a Regulation 114 Trust and a supplemental collateral trust (the “Supplemental Trust” and, together with the Regulation 114 Trust, the “Collateral Trusts”). Losses required to be reimbursed under the FLRT are subject to an aggregate limit equal to the assets held in the Collateral Trusts at any point in time. Effective January 1, 2014, HG Global and BAM agreed to change the interest rate on the BAM Surplus Notes for the five years ending December 31, 2018 from a fixed rate of 8.0% to a variable rate equal to the one -year U.S. treasury rate plus 300 basis points, set annually. In 2018, BAM exercised its option to extend the variable rate period for an additional three years. At the end of the variable rate period, the interest rate will be fixed at the higher of the then current variable rate or 8.0% . The variable rate was set at 5.70% for 2019 and 4.60% for 2018. No payment of interest or principal on the BAM Surplus Notes may be made without the approval of the New York State Department of Financial Services (“NYDFS”). BAM has stated its intention to seek regulatory approval to pay interest and principal on its surplus notes to the extent that its remaining qualified statutory capital and other capital resources continue to support its outstanding obligations, its business plan and its “AA/stable” rating from Standard & Poor’s. BAM repaid $17.7 million of the BAM Surplus Notes and $5.3 million of accrued interest during the year ended December 31, 2018. 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 Trust at HG Re. The Supplemental Trust already held the $300.0 million of Series B BAM Surplus Notes (“Series B Notes”). At the same time HG Global and BAM also changed the payment terms of the Series B Notes, so that payments will reduce principal and accrued interest on a pro rata basis, consistent with the payment terms on the Series A Notes. The terms of the Series B Notes had previously stipulated that payments would first reduce interest owed, then reduce principal owed once all accrued interest had been paid. The NYDFS approved the change during 2017. In connection with the contribution and change in payment terms of the Series B Notes, the Series A Notes were merged into the Series B Notes. The BAM Surplus Notes are currently held in an HG Re sponsored vehicle within the Supplemental Trust. The Regulation 114 Trust target balance is equal to gross ceded unearned premiums and unpaid ceded loss and LAE expenses, if any. The Supplemental Trust target balance is equal to $603.0 million . As the BAM Surplus Notes are repaid over time, the BAM Surplus Notes will be replaced in the Supplemental Trust by cash and fixed income securities. The Collateral Trust balances must be at target levels before excess funds can be distributed out of the Supplemental Trust. Under GAAP, if the terms of a debt instrument are amended, unless there is a greater than 10% change in the expected discounted future cash flows of such instrument, a change in the instrument’s carrying value is not permitted. White Mountains determined that the impact of the changes made during 2017 to the terms of the BAM Surplus Notes on the expected discounted future cash flows was not greater than 10%. As of March 31, 2019 and December 31, 2018 , the collateral trusts held assets of $771.5 million and $757.4 million , which both included $481.3 million of BAM Surplus Notes. As of March 31, 2019 and December 31, 2018 , HG Global has accrued $150.5 million and $143.7 million of interest receivable on the BAM Surplus Notes. The following table presents a schedule of BAM’s insured obligations as of March 31, 2019 and December 31, 2018: March 31, 2019 December 31, 2018 Contracts outstanding 7,786 7,525 Remaining weighted average contract period outstanding (in years) 10.7 10.7 Contractual debt service outstanding (in millions): Principal $ 53,715.2 $ 52,201.6 Interest 27,348.9 26,560.3 Total debt service outstanding $ 81,064.1 $ 78,761.9 Gross unearned insurance premiums $ 179.6 $ 176.0 The following table presents a schedule of BAM’s future premium revenues as of March 31, 2019 : Millions March 31, 2019 April 1, 2019 - December 31, 2019 $ 12.2 January 1, 2020 - March 31, 2020 3.9 April 1, 2020 - June 30, 2020 3.9 July 1, 2020 - September 30, 2020 3.9 October 1, 2020 - December 31, 2020 3.8 Total 2020 15.5 2021 14.5 2022 13.6 2023 12.8 2024 12.0 2025 and thereafter 99.0 Total gross unearned insurance premiums $ 179.6 The following table presents a schedule of net written premiums included in White Mountains’s HG Global/BAM segment for the three months ended March 31, 2019 and 2018 : Three Months Ended March 31, Millions 2019 2018 Written premiums: Direct $ 7.9 $ 6.4 Assumed — — Gross written premiums $ 7.9 $ 6.4 Earned premiums: Direct $ 3.7 $ 3.0 Assumed 0.5 — Gross earned premiums $ 4.2 $ 3.0 In April 2018, BAM entered into a collateralized financial guarantee excess of loss reinsurance agreement with Fidus Re, Ltd. (“Fidus Re”), a Bermuda based special purpose insurer created solely to provide reinsurance protection to BAM. Fidus Re was capitalized by the issuance of $100.0 million of insurance linked securities. The proceeds from issuance were placed in a collateral trust supporting Fidus Re’s obligations to BAM. The insurance linked securities were issued by Fidus Re with an initial term of twelve years and are callable five years after the date of issuance. Under the agreement, BAM retains the first $165.0 million of aggregate losses, before giving effect to HG’s reinsurance coverage, on the ceded business. Fidus Re reinsures 90% of aggregate losses exceeding $165.0 million on a portion of BAM’s financial guarantee portfolio up to a total reimbursement of $100 million . The aggregate loss limit under the agreement is $276.1 million . The agreement is accounted for using deposit accounting, and any related financing expenses are recorded in general and administrative expenses, because the agreement does not meet the risk transfer requirements necessary to be accounted for as reinsurance. In November 2018, BAM entered into a 100% quota share facultative reinsurance agreement under which it assumed a portfolio of municipal bond guarantee contracts with a par value of $2.2 billion . None of the contracts assumed were non-performing and no loss reserves have been established for any of the contracts, either as of the transaction date or as of March 31, 2019. The agreement, which covers future claims exposure only, meets the risk transfer criteria under ASC 944-20, Insurance Activities

Investments in Unconsolidated E

Investments in Unconsolidated Entities	3 Months Ended
	Mar. 31, 2019
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 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 as of March 31 2019 and December 31, 2018: Millions March 31, 2019 December 31, 2018 Equity method eligible unconsolidated entities, at fair value $ 293.6 $ 138.1 Investments accounted for under the equity method 1.0 1.3 Total investments in equity method eligible unconsolidated entities 294.6 139.4 Other unconsolidated investments (1) 203.0 186.2 Total other long-term investments $ 497.6 $ 325.6 (1) Consists of other long-term investments that are not equity method eligible. The following table presents White Mountains’s investments in equity method eligible unconsolidated entities as of March 31, 2019 and 2018: Basic Ownership Interest Investee March 31, 2019 December 31, 2018 Instrument Held MediaAlpha (1) 48.3% n/a Units PassportCard/DavidShield (2) 50.0% 50.0% Common shares Kudu 49.5% 49.5% Units durchblicker 45.0% 45.0% Common shares Tuckerman Capital Fund III, L.P. 16.0% 18.5% Limited partnership interest Compare.com 18.4% 18.4% Common shares (1) As of December 31, 2018, MediaAlpha was a majority-owned consolidated subsidiary of White Mountains. (2) As of March 31, 2019 and December 31, 2018, White Mountains’s ownership interest in DavidShield comprised a 50.0% direct interest and White Mountains’s ownership interest in PassportCard comprised a 25.0% direct ownership interest and a 25.0% indirect interest through DavidShield. See Note 2 — “Significant Transactions”. As a result of the MediaAlpha Transaction, White Mountains’s reduced its ownership interest in MediaAlpha to 48.3% of the basic units outstanding ( 42.0% on a fully diluted, fully converted basis). White Mountains’s remaining ownership interest in MediaAlpha no longer meets the criteria for a controlling ownership interest and, accordingly, White Mountains deconsolidated MediaAlpha on February 26, 2019. Upon deconsolidation, White Mountains’s investment in MediaAlpha met the criteria to be accounted for under the equity method or under the fair value option. White Mountains elected the fair value option and the investment in MediaAlpha was measured at its estimated fair value of $114.7 million as of March 31, 2019 , with the change in fair value of $114.7 million reflected within unrealized investment gains. White Mountains’s consolidated statement of comprehensive income and its segment disclosures include MediaAlpha’s results of operations for the period from January 1, 2019 through February 26, 2019. See Note 2 — “Significant Transactions”. For the period from February 26, 2019 to March 31, 2019, MediaAlpha’s total revenues, total expenses, and pre-tax income were $33.5 million , $32.5 million , and $1.1 million . As of March 31, 2019, MediaAlpha’s total assets and total liabilities were $102.3 million and $148.2 million

Earnings Per Share

Earnings Per Share	3 Months Ended
	Mar. 31, 2019
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 March 31, 2019 and 2018 . See Note 17 — “Held for Sale and Discontinued Operations” . Three Months Ended March 31, 2019 2018 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 284.4 $ (48.0 ) Less: total income from discontinued operations, net of tax .7 .1 Net income (loss) from continuing operations attributable to White Mountains’s common shareholders $ 283.7 $ (48.1 ) Allocation of earnings to participating restricted common shares (1) (2.9 ) .4 Basic and diluted earnings (losses) per share numerators $ 280.8 $ (47.7 ) Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 3,172.2 3,746.1 Average unvested restricted common shares (2) (32.5 ) (35.9 ) Basic earnings (losses) per share denominator 3,139.7 3,710.2 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 3,172.2 3,746.1 Average unvested restricted common shares (2) (32.5 ) (35.9 ) Diluted earnings (losses) per share denominator 3,139.7 3,710.2 Basic and diluted earnings per share (in dollars) - continuing operations: Distributed earnings - dividends declared and paid $ 1.00 $ 1.00 Undistributed earnings (losses) 88.42 (13.85 ) Basic and diluted earnings (losses) per share $ 89.42 $ (12.85 ) (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (2) Restricted shares outstanding vest either in equal annual installments or upon a stated date. See Note 10 — “Employee Share-Based Incentive Compensation Plans”. The following table presents the undistributed net earnings (losses) from continuing operations for the three months ended March 31, 2019 and 2018 . See Note 17 — “Held for Sale and Discontinued Operations” . Three Months Ended March 31, Millions 2019 2018 Undistributed net earnings (losses) - continuing operations: Net income (losses) attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 280.8 $ (47.7 ) Dividends declared, net of restricted common share amounts (1) (3.2 ) (3.7 ) Total undistributed net earnings (losses), net of restricted common share amounts $ 277.6 $ (51.4 ) (1)

Employee Share-Based Incentive

Employee Share-Based Incentive Compensation Plans	3 Months Ended
	Mar. 31, 2019
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 March 31, 2019 , White Mountains’s share-based compensation incentive awards consist of performance shares and restricted shares. Performance Shares Performance shares are conditional grants of a specified maximum number of common shares or an equivalent amount of cash. Awards generally vest at the end of a three -year period, are subject to the attainment of pre-specified performance goals, and are valued based on the market value of common shares at the time awards are approved for payment. The following table presents the performance share activity for the three months ended March 31, 2019 and 2018 for performance shares granted under the WTM Incentive Plan: Three Months Ended March 31, 2019 2018 Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Target Performance Accrued Expense Beginning of period 40,616 $ 31.7 50,515 $ 45.8 Shares paid (1) (13,715 ) (18.1 ) (23,186 ) (28.4 ) New grants 13,700 — 14,105 — Forfeitures and cancellations (2) 1 .2 (818 ) .3 Expense recognized — 9.3 — 3.3 End of period 40,602 $ 23.1 40,616 $ 21.0 (1) WTM performance share payments in 2019 for the 2016-2018 performance cycle, which were paid in cash in March 2019, ranged from 139% to 166% of target. WTM performance share payments in 2018 for the 2015-2017 performance cycle, which were paid in cash in March 2018, ranged from 145% to 147% of target. (2) Amounts include changes in assumed forfeitures, as required under GAAP. For performance shares earned in the 2016-2018 and 2015-2017 performance cycles, all performance shares earned were settled in cash. If all the outstanding WTM performance shares had vested on March 31, 2019 , the total additional compensation cost to be recognized would have been $24.4 million , based on accrual factors (common share price and payout assumptions) as of March 31, 2019 . The following table presents performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan as of March 31, 2019 for each performance cycle: Three Months Ended March 31, 2019 Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Performance cycle: 2017 – 2019 14,070 $ 16.9 2018 – 2020 13,450 5.5 2019 – 2021 13,700 1.0 Sub-total 41,220 23.4 Assumed forfeitures (618 ) (.3 ) March 31, 2019 40,602 $ 23.1 Restricted Shares The following table presents the unrecognized compensation cost associated with the outstanding restricted share awards for the three months ended March 31, 2019 and 2018 : Three Months Ended March 31, 2019 2018 Millions, except share amounts Restricted Unamortized Issue Date Restricted Unamortized Issue Date Non-vested, Beginning of period 41,510 $ 12.5 53,755 $ 14.3 Issued 13,700 12.8 14,105 11.4 Vested (13,715 ) — (25,381 ) — Forfeited — — (969 ) (.2 ) Expense recognized — (2.5 ) — (3.3 ) End of period 41,495 $ 22.8 41,510 $ 22.2 During the three months ended March 31, 2019 , White Mountains issued 13,700 restricted shares that vest on January 1, 2022. During the three months ended March 31, 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 vested on January 1, 2019. The unamortized issue date fair value as of March 31, 2019

Leases

Leases	3 Months Ended
	Mar. 31, 2019
Leases [Abstract]
Leases	Leases White Mountains has entered into lease agreements, primarily for office space. These leases are classified as operating leases, with lease expense recognized on a straight-line basis over the term of the lease. Lease incentives, such as free rent or landlord reimbursements for leasehold improvements, are recognized at lease inception and amortized on a straight-line basis over the term of the lease. Lease expense and the amortization of leasehold improvements are recognized within general and administrative expenses. Lease payments related to options to extend or renew the lease term are excluded from the calculation of lease liabilities unless White Mountains is reasonably certain of exercising those options. On January 1, 2019, White Mountains adopted ASU 2016-02, Leases (ASC 842). See Note 1 — “Basis of Presentation and Significant Accounting Policies” — Basis of Presentation and Significant Accounting Policies . Prior to adoption of ASU 2016-02, White Mountains recognized lease expense for operating leases on a straight-line basis, but did not recognize lease assets or liabilities on its consolidated balance sheet. Upon adoption on January 1, 2019 , White Mountains recognized lease right-of-use (“ROU”) assets of $23.2 million and lease liabilities of $23.2 million . As of March 31, 2019, the ROU asset was $21.5 million and lease liabilities were $ 21.8 million . The following table summarizes net lease expense recognized in White Mountains’s consolidated statement of operations for the three months ended March 31, 2019: Millions Lease Cost Three Months Ended March 31, 2019 Lease cost $ 1.7 Less: sublease income .1 Net lease cost $ 1.6 The following table presents the contractual maturities of the lease liabilities associated with White Mountains’s operating lease agreements as of March 31, 2019: Millions As of March 31, 2019 Remainder of 2019 $ 4.2 2020 4.6 2021 3.9 2022 3.7 2023 3.1 Thereafter 5.1 Total undiscounted lease payments 24.6 Less: present value adjustment 2.8 Operating lease liability $ 21.8 The following table presents lease related assets and liabilities by reportable segment as of March 31, 2019: As of March 31, 2019 Millions HG/BAM NSM Other Operations Total Weighted Average Incremental Borrowing Rate (1) ROU lease asset $ 11.8 $ 4.7 $ 5.0 $ 21.5 4.7% Lease liability $ 11.8 $ 4.7 $ 5.3 $ 21.8 (1)

Non-controlling Interests

Non-controlling Interests	3 Months Ended
	Mar. 31, 2019
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 March 31, 2019 and December 31, 2018 : March 31, 2019 December 31, 2018 $ in Millions Non-controlling Percentage Non-controlling Equity Non-controlling Percentage Non-controlling Equity Non-controlling interests, excluding BAM HG Global 3.1 % $ 14.8 3.1 % $ 14.5 NSM 4.7 14.2 4.5 13.6 Other NSM 13.4 .2 13.4 .3 MediaAlpha — — 39.0 16.2 Other Operations various .9 various 1.1 Total, excluding BAM 30.1 45.7 BAM 100.0 (173.8 ) 100.0 (170.6 ) Total non-controlling interests $ (143.7 ) $ (124.9 )

Segment Information

Segment Information	3 Months Ended
	Mar. 31, 2019
Segment Reporting [Abstract]
Segment Informartion	Segment Information White Mountains has determined that its reportable segments are HG Global/BAM, NSM, MediaAlpha (through February 26, 2019) 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. White Mountains deconsolidated MediaAlpha on February 26, 2019. White Mountains’s segment disclosures include MediaAlpha’s results of operations for the period from January 1, 2019 through February 26, 2019. See Note 2 — “Significant Transactions” . 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 (1) Other Operations Total Three Months Ended March 31, 2019 Earned insurance premiums $ 4.2 $ — $ — $ — $ 4.2 Net investment income 5.3 — — 10.7 16.0 Net realized and unrealized investment gains 11.8 — — 119.1 130.9 Realized and unrealized gains from the MediaAlpha Transaction — — — 182.2 182.2 Advertising and commission revenues (1) — 43.3 48.8 1.3 93.4 Other revenue .6 6.0 — .3 6.9 Total revenues 21.9 49.3 48.8 313.6 433.6 Insurance acquisition expenses 1.3 — — — 1.3 Cost of sales — — 40.6 1.1 41.7 General and administrative expenses 14.4 25.1 12.5 29.4 81.4 Broker commission expense — 15.7 — — 15.7 Change in fair value of contingent consideration earnout liabilities — 1.3 — — 1.3 Amortization of other intangible assets — 5.0 1.6 — 6.6 Interest expense — 3.7 .2 — 3.9 Total expenses 15.7 50.8 54.9 30.5 151.9 Pre-tax income (loss) $ 6.2 $ (1.5 ) $ (6.1 ) $ 283.1 $ 281.7 (1) MediaAlpha’s results are from January 1, 2019 to February 26, 2019, the date of the MediaAlpha Transaction. Millions HG Global/BAM MediaAlpha Other Operations Total Three Months Ended March 31, 2018 Earned insurance premiums $ 3.0 $ — $ — $ 3.0 Net investment income 3.7 — 16.0 19.7 Net realized and unrealized investment losses (7.9 ) — (45.8 ) (53.7 ) Advertising and commission revenues (1) — 70.1 .9 71.0 Other revenue .2 1.6 .3 2.1 Total revenues (1.0 ) 71.7 (28.6 ) 42.1 Insurance acquisition expenses 1.4 — — 1.4 Cost of sales — 57.4 .7 58.1 General and administrative expenses 11.9 11.2 22.0 45.1 Amortization of other intangible assets — 2.9 — 2.9 Interest expense — .4 .2 .6 Total expenses 13.3 71.9 22.9 108.1 Pre-tax loss $ (14.3 ) $ (.2 ) $ (51.5 ) $ (66.0 ) (1) Approximately 31% of MediaAlpha’s advertising revenue for the three months ended March 31, 2018

Fair Value of Financial Instrum

Fair Value of Financial Instruments	3 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	Fair Value of Financial Instruments White Mountains records its financial instruments at fair value with the exception of the NSM Bank Facility and the MediaAlpha Bank Facility, which are recorded as debt at face value less unamortized original issue discount. The following table presents the fair value and carrying value of this financial instrument as of March 31, 2019 and December 31, 2018 : March 31, 2019 December 31, 2018 Millions Fair Value Carrying Value Fair Value Carrying Value NSM Bank Facility $ 182.9 $ 182.8 $ 176.1 $ 176.6 MediaAlpha Bank Facility $ — $ — $ 14.6 $ 14.2 Other Operations debt $ 10.9 (1) $ 10.9 $ — $ — (1) White Mountains measured the fair value of the Other Operations debt at the carrying value as a result of the debt being acquired on March 12, 2019. The fair value estimates for the NSM Bank Facility and the MediaAlpha Bank Facility have been determined based on a discounted cash flows approach and are considered to be Level 3 measurements. As a result of the MediaAlpha Transaction, White Mountains’s remaining ownership interest in MediaAlpha no longer meets the criteria for a controlling ownership interest and accordingly, White Mountains deconsolidated MediaAlpha on February 26, 2019. See

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Contingencies NSM Contingent Liability In connection with White Mountains’s acquisition of NSM, White Mountains and NSM entered into an agreement with American International Group, Inc. (“AIG”) to help 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 the renewal rights are not sold by December 31, 2019, AIG has the right to require NSM to purchase the renewal rights for $82.5 million . The Company has guaranteed NSM’s obligations under the agreement with AIG. The manner in which these obligations are ultimately discharged depends on a number of factors, including the market value of the renewal rights, the number of potential buyers and the current and prospective environment for U.S. collector car insurance. White Mountains believes that the estimated fair value of the renewal rights is equal to or greater than $82.5 million and, accordingly, no accrual of a liability is necessary as of March 31, 2019 . Sirius Group Tax Contingency A subsidiary of Sirius Group, which was sold by White Mountains in 2016, has been denied interest deductions by the Swedish Tax Authority (“STA”) for tax years 2013-2016. In October 2018, the Swedish Administrative Court ruled against Sirius Group on its appeal of the Swedish Tax Agency’s denial of certain interest deductions relating to periods prior to the sale of Sirius Group to CMI. In connection with the sale, White Mountains indemnified Sirius Group against the loss of certain tax attributes, including those related to these interest deductions. As a result, as of March 31, 2019 and December 31, 2018, White Mountains recorded a liability of $16.6 million and $17.3 million , reflecting the value of these interest deductions. For the three months ended March 31, 2019, the change in the liability, of $0.7 million , is related to foreign currency translation and is included within net gain on sale of discontinued operations. Sirius Group has appealed the decision to the Swedish Administrative Court of Appeal. Legal Contingencies White Mountains is subject to litigation and arbitration in the normal course of business. White Mountains recognizes an accrual for litigation and arbitration if it is probable that a loss has been incurred and the amount can be reasonably estimated. White Mountains discloses litigation and arbitration 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 as of March 31, 2019

Held for Sale and Discontinued

Held for Sale and Discontinued Operations	3 Months Ended
	Mar. 31, 2019
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 Sirius Group On April 18, 2016, White Mountains completed the sale of Sirius Group to CMI. During the three months ended March 31, 2018, White Mountains recorded a $0.1 million gain from sale of Sirius Group as a result of a change to the valuation of the accrued incentive compensation payable to Sirius Group employees. In October 2018, the Swedish Administrative Court ruled against Sirius Group on its appeal of the Swedish Tax Agency’s denial of certain interest deductions relating to periods prior to the sale of Sirius Group to CMI. In connection with the sale, White Mountains indemnified Sirius Group against the loss of certain tax attributes, including those related to these interest deductions. As a result, as of March 31, 2019 and December 31, 2018, White Mountains recorded a liability of $16.6 million and $17.3 million , reflecting the value of these interest deductions. For the three months ended March 31, 2019, the change in the liability, of $ 0.7 million , is related to foreign currency translation and is included within net gain on sale of discontinued operations. See Note 16 — “Commitments and Contingencies” . Other As of December 31, 2017, White Mountains has classified its Guilford, Connecticut property, which consists of an office building and adjacent land, as held for sale. The property has been measured at its estimated fair value net of costs of disposal, of $3.3 million as of March 31, 2019
Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Held for Sale and Discontinued Operations	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: Three Months Ended Millions 2018 Net cash provided from operations $ .1 Net cash used for investing activities (.1 ) Net cash used for financing activities — Net change in cash during the period — Cash balances at beginning of period — 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 months ended March 31, 2019 and 2018 : Three Months Ended March 31, 2019 2018 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 284.4 $ (48.0 ) Less: total income (loss) from continuing operations, net of tax 283.7 (48.1 ) Net income from discontinued operations attributable to White Mountains’s common shareholders $ .7 $ .1 Allocation of earnings to participating restricted common shares (1) — — Basic and diluted earnings per share numerators (2) $ .7 $ .1 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 3,172.2 3,746.1 Average unvested restricted common shares (3) (32.5 ) (35.9 ) Basic earnings per share denominator 3,139.7 3,710.2 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 3,172.2 3,746.1 Average unvested restricted common shares (3) (32.5 ) (35.9 ) Diluted earnings per share denominator 3,139.7 3,710.2 Basic and diluted earnings per share (in dollars) - discontinued operations: $ .22 $ .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 months ended March 31, 2019 and 2018 . (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”

Subsequent Events

Subsequent Events	3 Months Ended
	Mar. 31, 2019
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events Embrace On April 1, 2019, NSM acquired 100% of Embrace Pet Insurance, a nationwide provider of pet health insurance for dogs and cats. NSM paid $73.0 million of cash consideration, net of cash acquired, for its equity interest in Embrace Pet Insurance. Kudu On April 4, 2019, White Mountains completed its acquisition of Oaktree’s interests in Kudu for $81.4 million . As a result of the purchase, White Mountains’s ownership interest was increased to 99.1% of the basic units outstanding of Kudu ( 85.4% on a fully diluted, fully converted basis). In connection with the transaction, White Mountains assumed all of Oaktree’s unfunded capital commitments to Kudu, increasing White Mountains’s total unfunded Kudu capital commitment to $104.7 million

Basis of Presentation and Sig_2

Basis of Presentation and Significant Accouting Policies (Policies)	3 Months Ended
	Mar. 31, 2019
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.
Business Combinations	Business CombinationsWhite 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 EstimatesThe 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 (through February 26, 2019) and Other Operations. See Note 14 — “Segment Information” . The HG Global/BAM segment consists of HG Global Ltd. and its wholly-owned subsidiaries (“HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”) (collectively, “HG Global/BAM”). BAM is the first and only mutual municipal bond insurance company in the United States. By insuring the timely payment of principal and interest, BAM provides market access to, and lowers interest expense for, issuers of municipal bonds used to finance essential public purposes such as schools, utilities and transportation facilities. BAM is owned by and operated for the benefit of its members, the municipalities that purchase BAM’s insurance for their debt issuances. HG Global was established to fund the startup of BAM and, through its wholly-owned subsidiary, HG Re Ltd. (“HG Re”), to provide up to 15% -of-par, first loss reinsurance protection for policies underwritten by BAM. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM (the “BAM Surplus Notes”). As of March 31, 2019 , $481.3 million of the surplus notes remain outstanding. As of March 31, 2019 and December 31, 2018, White Mountains owned 96.9% of HG Global’s preferred equity and 88.4% of its common equity. White Mountains does not have an ownership interest in BAM. However, White Mountains is required to consolidate BAM’s results in its financial statements because BAM is a VIE for which White Mountains is the primary beneficiary. BAM’s results are attributed to non-controlling interests. The NSM segment consists of NSM Insurance HoldCo, LLC and its wholly-owned subsidiaries (collectively, “NSM”). NSM is a full-service managing general underwriting agency (“MGU”) and program administrator for specialty property and casualty insurance. The company places insurance in niche sectors such as specialty transportation, social services and real estate. On behalf of its insurance carrier partners, NSM manages all aspects of the placement process, including product development, marketing, underwriting, policy issuance and claims. NSM earns commissions based on the volume and profitability of the insurance that it places. NSM does not take insurance risk. As of March 31, 2019 and December 31, 2018, White Mountains owned 95.3% and 95.5% of the basic units outstanding of NSM ( 84.8% and 85.0% on a fully diluted, fully converted basis). The NSM segment also includes White Mountains Catskill Holdings, Inc., the immediate holding company of NSM. See Note 2 — “Significant Transactions ”. The MediaAlpha segment consisted of QL Holdings LLC and its wholly-owned subsidiary QuoteLab, LLC (collectively “MediaAlpha”). MediaAlpha is a marketing technology company that develops technology that enables the programmatic buying and selling of vertical-specific, performance-based media between advertisers (buyers of advertising inventory) and publishers (sellers of advertising inventory) through cost-per-click, cost-per-call and cost-per-lead pricing models. MediaAlpha’s media buying platform enables advertisers to create and automate data-driven bidding strategies designed to improve the efficiency and enhance overall performance of their marketing campaigns that target high-intent consumers at the time and place they are ready to purchase. MediaAlpha’s publisher platform is used by publishers to sell their vertical-specific, performance-based media to advertisers through transparent, programmatic, auction-based marketplaces. White Mountains deconsolidated MediaAlpha on February 26, 2019. White Mountains’s consolidated statement of comprehensive income and its segment disclosures include MediaAlpha’s results of operations for the period from January 1, 2019 through February 26, 2019. See Note 2 — “Significant Transactions” . The Other Operations segment consists of the Company and its wholly-owned subsidiary, White Mountains Capital, Inc. (“WM Capital”), its intermediate holding companies, its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), investment assets managed by WM Advisors, its interests in PassportCard Limited (“PassportCard”) and DavidShield Life Insurance Agency (2000) Ltd. (“DavidShield”) (collectively, “PassportCard/ DavidShield”) and Kudu Investment Management, LLC (“Kudu”), certain other consolidated and unconsolidated entities and certain other strategic investments.
Discontinued Operations and Assets and Liabilities Held for Sale	Discontinued Operations and Assets Held for Sale 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. Adjustments to the gain on sale subsequent to the transaction have also been classified within discontinued operations. White Mountains has classified its Guilford, Connecticut property, which consists of an office building and adjacent land, as held for sale as of March 31, 2019
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”) during the quarter ended June 30, 2018, which is accounted for using the deposit method. See Note 8 — “Municipal Bond Guarantee Insurance”
Restricted Cash	Restricted CashCash 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 Leases On January 1, 2019, White Mountains adopted ASU 2016-02, Leases (ASC 842), which 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. White Mountains elected the optional transition method that permits prospective adoption with recognition of a cumulative effect adjustment to the opening balance of retained earnings. As a result, White Mountains has presented comparative periods prior to adoption in accordance with previous lease accounting guidance. White Mountains also elected all available practical expedients permitted under ASC 842, which allowed White Mountains to carryforward its historical lease classification and not reassess leases for the definition of a lease under the new guidance. As of January 1, 2019 and March 31, 2019, White Mountains recognized $23.2 million and $21.5 million of lease right-of-use assets and $23.2 million and $ 21.8 million of lease liabilities. Adoption of ASU 2016-02 did not result in an adjustment to opening retained earnings. Premium Amortization on Callable Debt Securities On January 1, 2019, White Mountains adopted 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 is 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. Adoption of ASU 2017-08 did not have any impact on White Mountains’s financial statements. 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. 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 Goodwill In January 2017, the FASB issued ASU 2017-04, Simplifying the Test for Goodwill Impairment (ASC 350), which changes the guidance on goodwill impairment testing. Under the new guidance, the qualitative assessment of the recoverability of goodwill remains the same. However, the second step required under the existing guidance has been eliminated. Goodwill is considered impaired if the carrying value exceeds the estimated fair value. The new guidance is effective for fiscal years beginning after December 15, 2019. Credit Losses In June 2016, the FASB issued ASU 2016-13, Measurement of Credit Losses on Financial Instruments

Investment Securities (Tables)

Investment Securities (Tables)	3 Months Ended
	Mar. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Pre-tax net investment income	The following table presents pre-tax net investment income for the three months ended March 31, 2019 and 2018 : Three Months Ended March 31, Millions 2019 2018 Fixed maturity investments $ 8.1 $ 10.9 Short-term investments 1.3 2.6 Common equity securities 5.4 5.0 Other long-term investments 1.6 1.9 Total investment income 16.4 20.4 Third-party investment expenses (.4 ) (.7 ) Net investment income, pre-tax $ 16.0 $ 19.7
Schedule of Realized and Unrealized Gain (Loss) on Investments	The following table presents net realized and unrealized investment gains (losses) for the three months ended March 31, 2019 and 2018 : Three Months Ended March 31, Millions 2019 2018 Net realized investment gains (losses), pre-tax $ 25.7 $ (5.1 ) Net unrealized investment gains (losses), pre-tax (1) 219.9 (48.6 ) Net realized and unrealized investment gains (losses), pre-tax 245.6 (53.7 ) Income tax (expense) benefit attributable to net realized and unrealized investment gains (losses) (52.2 ) 5.7 Net realized and unrealized investment gains (losses), after-tax $ 193.4 $ (48.0 ) (1) Includes $114.7 in unrealized investment gains associated with the MediaAlpha Transaction. See Note 2 — “Significant Transactions.” For the three months ended March 31, 2019 and 2018, all of White Mountains’s net realized and unrealized investment gains (losses) were recorded in the consolidated statements of operations. There were no investment gains (losses) recorded in other comprehensive income.
Schedule of realized gain (loss) on investments	The following table presents net realized investment gains (losses) for the three months ended March 31, 2019 and 2018 : Three Months Ended Three Months Ended March 31, 2019 March 31, 2018 Millions Net Net Total Net Realized Net Net Total Net Realized Fixed maturity investments $ (.9 ) $ — $ (.9 ) $ (13.7 ) $ 18.2 $ 4.5 Short-term investments — — — (.1 ) — (.1 ) Common equity securities 25.8 — 25.8 1.2 — 1.2 Other long-term investments .8 — .8 (3.5 ) (7.2 ) (10.7 ) Net realized investment gains (losses), pre-tax 25.7 — 25.7 (16.1 ) 11.0 (5.1 ) Income tax expense attributable to net realized investment gains (losses) (1.7 ) — (1.7 ) (.6 ) — (.6 ) Net realized investment gains (losses), after-tax $ 24.0 $ — $ 24.0 $ (16.7 ) $ 11.0 $ (5.7 )
Schedule of unrealized gain (loss) on investments	The following table presents net unrealized investment gains (losses) and changes in the carrying value of investments measured at fair value for the three months ended March 31, 2019 and 2018 : Three Months Ended Three Months Ended March 31, 2019 March 31, 2018 Millions Net Unrealized Gains Net Total Net Unrealized Gains Reflected in Earnings Net Unrealized Losses Net Gains Total Net Unrealized (Losses) Gains Reflected in Earnings Fixed maturity investments $ 22.0 $ — $ 22.0 $ (18.6 ) $ (14.8 ) $ (33.4 ) Short-term investments — — — (.7 ) — (.7 ) Common equity securities 85.5 — 85.5 (16.7 ) — (16.7 ) Other long-term investments (1) 112.7 (.3 ) 112.4 (1.9 ) 4.1 2.2 Net unrealized investment gains (losses), pre-tax 220.2 (.3 ) 219.9 (37.9 ) (10.7 ) (48.6 ) Income tax expense attributable to net unrealized investment gains (losses) (50.5 ) — (50.5 ) 6.3 — 6.3 Net unrealized investment gains (losses), after-tax $ 169.7 $ (.3 ) $ 169.4 $ (31.6 ) $ (10.7 ) $ (42.3 )
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 months ended March 31, 2019 and 2018 : Three Months Ended March 31, Millions 2019 2018 Other long-term investments $ 114.0 $ (5.1 ) Total net unrealized investment gains (losses), pre-tax - Level 3 investments $ 114.0 $ (5.1 )
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) and carrying values of White Mountains’s fixed maturity investments as of March 31, 2019 and December 31, 2018 : March 31, 2019 Millions Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Carrying Value U.S. Government and agency obligations $ 153.1 $ .4 $ (.5 ) $ 153.0 Debt securities issued by corporations 538.4 7.3 (.9 ) 544.8 Municipal obligations 279.7 6.1 (.4 ) 285.4 Mortgage and asset-backed securities 149.4 .6 (1.2 ) 148.8 Total fixed maturity investments $ 1,120.6 $ 14.4 $ (3.0 ) $ 1,132.0 December 31, 2018 Millions Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Carrying Value U.S. Government and agency obligations $ 154.0 $ .1 $ (.9 ) $ 153.2 Debt securities issued by corporations 519.0 1.0 (9.5 ) 510.5 Municipal obligations 279.0 2.4 (1.1 ) 280.3 Mortgage and asset-backed securities 136.1 .1 (2.7 ) 133.5 Total fixed maturity investments $ 1,088.1 $ 3.6 $ (14.2 ) $ 1,077.5 The following tables present the cost or amortized cost, gross unrealized investment gains (losses), net foreign currency losses, and carrying values of White Mountains’s common equity securities and other long-term investments as of March 31, 2019 and December 31, 2018 : March 31, 2019 Millions Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Net Foreign Currency Losses Carrying Value Common equity securities $ 798.6 $ 107.3 $ (.9 ) $ — $ 905.0 Other long-term investments $ 389.8 $ 164.3 $ (54.3 ) $ (2.2 ) $ 497.6 December 31, 2018 Millions Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Net Foreign Carrying Value Common equity securities $ 904.7 $ 51.0 $ (30.1 ) $ — $ 925.6 Other long-term investments $ 330.3 $ 52.2 $ (54.9 ) $ (2.0 ) $ 325.6 Other Long-Term Investments The following table presents the carrying values of White Mountains’s other long-term investments as of March 31, 2019 and December 31, 2018 : Carrying Value at Millions March 31, 2019 December 31, 2018 MediaAlpha (1) $ 114.7 $ — PassportCard/DavidShield (1) 75.0 75.0 Kudu 71.7 30.7 Other unconsolidated entities (1)(2) 54.4 60.0 Total unconsolidated entities (1)(2) 315.8 165.7 Private equity funds and hedge funds 150.7 146.1 Other 31.1 13.8 Total other long-term investments $ 497.6 $ 325.6 (1) See Fair Value Measurements by Level table. (2)
Other long-term investments	The following table presents investments in private equity funds and hedge funds by investment objective and sector as of March 31, 2019 and December 31, 2018 : March 31, 2019 December 31, 2018 Millions Fair Value Unfunded Commitments Fair Value Unfunded Commitments Private equity funds Manufacturing/Industrial $ 42.1 $ 10.5 $ 42.9 $ 10.5 Aerospace/Defense/Government 32.8 29.8 27.6 34.9 Direct lending 13.6 16.8 13.0 17.7 Financial services 9.1 12.8 8.3 13.6 Total private equity funds 97.6 69.9 91.8 76.7 Hedge funds Long/short banks and financials 53.1 — 54.3 — Total hedge funds 53.1 — 54.3 — Total private equity funds and hedge funds included in other long-term investments $ 150.7 $ 69.9 $ 146.1 $ 76.7
Fair Value of private equity funds subject to lock-up periods	The following table presents investments in private equity funds that were subject to lock-up periods as of March 31, 2019 : Millions 1 – 3 years 3 – 5 years 5 – 10 years >10 years Total Private equity funds — expected lock-up period remaining $7.0 $— $67.9 $22.7 $97.6
Fair value measurements by level, investment securities	The following tables present White Mountains’s fair value measurements for investments as of March 31, 2019 and December 31, 2018 by level. The major security types were based on the legal form of the securities. White Mountains has disaggregated its fixed maturity investments based on the issuing entity type, which impacts credit quality, with debt securities issued by U.S. government entities carrying minimal credit risk, while the credit and other risks associated with other issuers, such as corporations, foreign governments, municipalities or entities issuing mortgage and asset-backed securities vary depending on the nature of the issuing entity type. White Mountains further disaggregates debt securities issued by corporations and common equity securities by industry sector because investors often reference commonly used benchmarks and their subsectors to monitor risk and performance. Accordingly, White Mountains has further disaggregated these asset classes into subclasses based on the similar sectors and industry classifications it uses to evaluate investment risk and performance against commonly used benchmarks, such as the Bloomberg Barclays U.S. Intermediate Aggregate and S&P 500 indices. March 31, 2019 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 153.0 $ 153.0 $ — $ — Debt securities issued by corporations: Financials 139.7 — 139.7 — Healthcare 74.7 — 74.7 — Energy 66.0 — 66.0 — Consumer 61.0 — 61.0 — Industrial 57.0 — 57.0 — Technology 55.0 — 55.0 — Communications 40.4 — 40.4 — Utilities 28.5 — 28.5 — Materials 22.5 — 22.5 — Total debt securities issued by corporations 544.8 — 544.8 — Mortgage and asset-backed securities 148.8 — 148.8 — Municipal obligations 285.4 — 285.4 — Total fixed maturity investments 1,132.0 153.0 979.0 — Short-term investments (1) 269.8 249.0 20.8 — Common equity securities: Exchange traded funds (2) 712.3 646.3 66.0 — Other (3) 192.7 — 192.7 — Total common equity securities 905.0 646.3 258.7 — Other long-term investments 275.1 — — 275.1 Other long-term investments — NAV (4) 222.5 — — — Total investments $ 2,804.4 $ 1,048.3 $ 1,258.5 $ 275.1 (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) Consists of unconsolidated entities, private equity funds and one hedge fund for which fair value is measured at NAV using the practical expedient. Investments for which fair value is measured at NAV are not classified within the fair value hierarchy. December 31, 2018 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 153.2 $ 153.2 $ — $ — Debt securities issued by corporations: Financials 143.4 — 143.4 — Consumer 68.5 — 68.5 — Technology 60.5 — 60.5 — Energy 57.6 — 57.6 — Healthcare 55.0 — 55.0 — Industrial 47.6 — 47.6 — Communications 31.8 — 31.8 — Materials 26.3 — 26.3 — Utilities 19.8 — 19.8 — Total debt securities issued by corporations 510.5 — 510.5 — Mortgage and asset-backed securities 133.5 — 133.5 — Municipal obligations 280.3 — 280.3 — Total fixed maturity investments 1,077.5 153.2 924.3 — Short-term investments (1) 214.2 204.4 9.8 — Common equity securities: Exchange traded funds (2) 675.3 617.0 58.3 — Healthcare 14.0 14.0 — — Financials 13.5 13.5 — — Communications 12.7 12.7 — — Industrial 11.4 11.4 — — Technology 7.4 7.4 — — Consumer 6.2 6.2 — — Energy 4.1 4.1 — — Materials 3.1 3.1 — — Other (3) 177.9 — 177.9 — Total common equity securities 925.6 689.4 236.2 — Other long-term investments 138.7 — — 138.7 Other long-term investments — NAV (4) 186.9 — — — Total investments $ 2,542.9 $ 1,047.0 $ 1,170.3 $ 138.7 (1) Short-term investments are measured at amortized cost, which approximates fair value. (2) ETFs traded on foreign exchanges are priced using the fund’s published NAV to account for the difference in market close times and are therefore designated a level 2 measurement. (3) Consists of two investments in unit trusts that primarily invest in international equities. (4)
Debt securities issued by corporations, credit ratings	The following table presents the ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of March 31, 2019 and December 31, 2018 : Fair Value at Millions March 31, 2019 December 31, 2018 AAA $ 9.6 $ 8.9 AA 83.8 88.7 A 300.3 270.5 BBB 151.1 142.4 Debt securities issued by corporations (1) $ 544.8 $ 510.5 (1) Credit ratings are based upon issuer credit ratings provided by Standard & Poor’s Financial Services LLC (“Standard & Poor’s”), or if unrated by Standard & Poor’s, long-term obligation ratings provided by Moody’s Investor Service, Inc.
Mortgage-backed, asset-backed securities	The following table presents the fair value of White Mountains’s mortgage and asset-backed securities as of March 31, 2019 and December 31, 2018 : March 31, 2019 December 31, 2018 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: FNMA $ 59.8 $ 59.8 $ — $ 53.6 $ 53.6 $ — FHLMC 43.0 43.0 — 38.1 38.1 $ — GNMA 25.6 25.6 — 23.7 23.7 $ — Total agency (1) 128.4 128.4 — 115.4 115.4 — Total mortgage-backed securities 128.4 128.4 — 115.4 115.4 — Other asset-backed securities: Vehicle receivables 11.5 11.5 — 9.2 9.2 — Credit card receivables 8.9 8.9 — 8.9 8.9 — Total other asset-backed securities 20.4 20.4 — 18.1 18.1 — Total mortgage and asset-backed securities $ 148.8 $ 148.8 $ — $ 133.5 $ 133.5 $ — (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 three months ended March 31, 2019 and 2018 : Level 3 Investments Unconsolidated Entities, Private Equity Funds and Hedge Funds Measured at NAV (3) Millions Level 1 Investments Level 2 Investments Other Long-term Investments (2) Total Balance at December 31, 2018 $ 842.6 $ 1,160.5 $ 138.7 $ 186.9 $ 2,328.7 (1) Net realized and unrealized gains (losses) 91.7 40.7 114.0 (.8 ) 245.6 Amortization/Accretion .1 (.4 ) — — (.3 ) Purchases 14.6 142.3 11.5 49.2 217.6 Sales (149.7 ) (105.4 ) — (1.9 ) (257.0 ) Transfers in — — 10.9 — 10.9 Transfers out — — — (10.9 ) (10.9 ) Balance at March 31, 2019 $ 799.3 $ 1,237.7 $ 275.1 $ 222.5 $ 2,534.6 (1) (1) Excludes carrying value of $ 269.8 million and $ 214.2 as of March 31, 2019 and December 31, 2018 , respectively classified as short-term investments. (2) Includes $114.7 in unrealized investment gains associated with the MediaAlpha Transaction. See Note 2 — “Significant Transactions”. (3) Investments for which fair value is measured at NAV using the practical expedient are no longer classified within the fair value hierarchy. See Note 1 — “Basis of Presentation and Significant Accounting Policies” . Level 3 Investments Unconsolidated Entities, Private Equity Funds and Hedge Funds Measured at NAV (3) Millions Level 1 Investments Level 2 Investments Other Long-term Investments Total Balance at December 31, 2017 $ 890.4 $ 2,105.4 $ 77.2 $ 135.3 $ 3,208.3 (1)(2) Net realized and unrealized (losses) gains (14.3 ) (26.0 ) (7.8 ) 2.7 (45.4 ) (4) Amortization/Accretion — (1.2 ) — — (1.2 ) Purchases 238.9 407.7 44.0 2.0 692.6 Sales (240.7 ) (1,080.8 ) — (.3 ) (1,321.8 ) Transfers in — — — — — Transfers out — — — — — Balance at March 31, 2018 $ 874.3 $ 1,405.1 $ 113.4 $ 139.7 $ 2,532.5 (1) (1) Excludes carrying value of $ 763.0 and $ 176.1 as of March 31, 2018 and December 31, 2017 , respectively classified as short-term investments. (2) Excludes carrying value of $(3.7) as of December 31, 2017 associated with foreign currency forward contracts. (3) Investments for which fair value is measured at NAV using the practical expedient are no longer classified within the fair value hierarchy. See Note 1 — “Basis of Presentation and Significant Accounting Policies” .
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 private equity funds and hedge funds, classified within Level 3 as of March 31, 2019 and December 31, 2018 . The fair value of investments in certain unconsolidated entities, private equity funds and hedge funds are generally estimated using the NAV of the funds. $ in Millions, Except Share Price March 31, 2019 Description Valuation Technique(s) Fair Value (1) Unobservable Input MediaAlpha Share price of recent transaction (2) $114.7 Unit price - $208.98 PassportCard/DavidShield Discounted cash flow $75.0 Discount rate - 18.0% Revenue exit multiple - 1.00 Compare.com Discounted cash flow $16.9 Discount rate - 22.0% Revenue exit multiple - 2.75 YOUSURE Tarifvergleich GmbH (“durchblicker”) Discounted cash flow $15.2 Discount rate - 23.0% Revenue exit multiple - 2.25 Captricity, Inc. Discounted cash flow $14.5 Discount rate - 23.0% Revenue exit multiple - 3.75 Galvanic Applied Sciences Multiple of EBITDA $2.7 EBITDA multiple - 6.00 Private debt instrument Discounted cash flow $10.0 Discount rate - 9.23% Private debt instrument Discounted cash flow $10.9 Discount rate - 7.00% (1) Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs. (2) The MediaAlpha Transaction was completed on February 26, 2019. See Note 2 - “Significant Transactions”. $ in Millions, Except Share Price December 31, 2018 Description Valuation Technique(s) Fair Value (1) Unobservable Input PassportCard/DavidShield Discounted cash flow $75.0 Discount rate - 18.0% Revenue exit multiple - 1.00 Compare.com Discounted cash flow $16.9 Discount rate - 22.0% Revenue exit multiple - 2.75 durchblicker Discounted cash flow $15.5 Discount rate - 23.0% Revenue exit multiple - 2.25 Captricity, Inc. Discounted cash flow $14.5 Discount rate - 23.0% Revenue exit multiple - 3.75 Galvanic Applied Sciences Multiple of EBITDA $3.1 EBITDA multiple - 6.00 Private debt instrument Discounted cash flow $10.0 Discount rate - 9.23% (1)

Goodwill and Other Intangible_2

Goodwill and Other Intangible Assets (Tables)	3 Months Ended
	Mar. 31, 2019
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 March 31, 2019 2018 Millions Goodwill Other Intangible Assets Total Goodwill and Other Intangible Assets Goodwill Other Intangible Assets Total Goodwill and Other Intangible Assets Beginning balance $ 379.9 $ 157.6 $ 537.5 $ 25.9 $ 36.2 $ 62.1 Deconsolidation of MediaAlpha (18.3 ) (23.5 ) (41.8 ) — — — Attribution of acquisition date fair value estimates between goodwill and other intangible assets (26.8 ) 26.8 — — — — Acquisitions of businesses 13.4 — 13.4 Foreign currency translation 1.2 .3 1.5 — — — — Adjustments to reflect acquisition date fair value .3 5.9 6.2 — — — — Amortization — (6.6 ) (6.6 ) — (2.9 ) (2.9 ) Ending balance $ 349.7 $ 160.5 $ 510.2 $ 25.9 $ 33.3 $ 59.2
Schedule of Intangible Assets and Goodwill	The following table presents the acquisition date fair values, accumulated amortization and net carrying values for other intangible assets and goodwill, by company as of March 31, 2019 and December 31, 2018 : $ in Millions March 31, 2019 December 31, 2018 Acquisition Date Fair Value Accumulated Amortization Net Carrying Value Acquisition Date Fair Value Accumulated Amortization Net Carrying Value Goodwill: NSM (1) $ 328.8 $ — $ 328.8 $ 354.3 $ — $ 354.3 MediaAlpha — — — 18.3 — 18.3 Other Operations (2) 20.9 — 20.9 7.3 — 7.3 Total goodwill 349.7 — 349.7 379.9 — 379.9 Other intangible assets: NSM (1) Customer relationships 110.9 9.3 101.6 85.3 6.0 79.3 Trade names 57.1 2.7 54.4 51.2 1.8 49.4 Information technology 3.8 .7 3.1 3.7 .5 3.2 Other 1.4 .6 .8 — — — Subtotal 173.2 13.3 159.9 140.2 8.3 131.9 MediaAlpha Customer relationships — — — 26.8 4.9 21.9 Information technology — — — 33.3 30.9 2.4 Other — — — 9.8 9.0 0.8 Subtotal — — — 69.9 44.8 25.1 Other Operations Trade names .6 .2 .4 .6 .2 .4 Information technology .5 .3 .2 .5 .3 .2 Subtotal 1.1 .5 .6 1.1 .5 .6 Total other intangible assets 174.3 13.8 160.5 211.2 53.6 157.6 Total goodwill and other intangible assets $ 524.0 $ 13.8 510.2 $ 591.1 $ 53.6 537.5 Goodwill and other intangible assets attributed to non-controlling interests (24.1 ) (40.6 ) Goodwill and other intangible assets included in White Mountains’s common shareholders’ equity $ 486.1 $ 496.9 (1) As of March 31, 2019 and December 31, 2018, NSM’s goodwill included $1.0 and $ (2.2) and intangible assets included $0.3 and $ (0.7) of the effect of foreign currency translation. (2) As of March 31, 2019 and December 31, 2018, Other Operations’ goodwill included $0.2 and $ (0.3)

Debt (Tables)

Debt (Tables)	3 Months Ended
	Mar. 31, 2019
Debt Disclosure [Abstract]
Schedule of debt outstanding	The following table presents White Mountains’s debt outstanding as of March 31, 2019 and December 31, 2018 : Millions March 31, Effective Rate (1) December 31, Effective Rate (1) NSM Bank Facility $ 186.4 7.8% $ 180.4 7.4% Unamortized issuance cost (3.6 ) (3.8 ) NSM Bank Facility, carrying value 182.8 176.6 Other NSM debt 1.7 1.9 MediaAlpha Bank Facility — 14.3 7.1% Unamortized issuance cost — (.1 ) MediaAlpha Bank Facility, carrying value — 14.2 Other Operations debt 11.3 — Unamortized issuance cost (.4 ) — Other Operations, carrying value 10.9 — Total debt $ 195.4 $ 192.7 (1)

Municipal Bond Guarantee (Table

Municipal Bond Guarantee (Tables)	3 Months Ended
	Mar. 31, 2019
Guarantees [Abstract]
Schedule of Municipal Bond Guarantee Insured Obligations	The following table presents a schedule of BAM’s insured obligations as of March 31, 2019 and December 31, 2018: March 31, 2019 December 31, 2018 Contracts outstanding 7,786 7,525 Remaining weighted average contract period outstanding (in years) 10.7 10.7 Contractual debt service outstanding (in millions): Principal $ 53,715.2 $ 52,201.6 Interest 27,348.9 26,560.3 Total debt service outstanding $ 81,064.1 $ 78,761.9 Gross unearned insurance premiums $ 179.6 $ 176.0
Financial Guarantee Insurance Contracts, Premium Received over Contract Period	The following table presents a schedule of BAM’s future premium revenues as of March 31, 2019 : Millions March 31, 2019 April 1, 2019 - December 31, 2019 $ 12.2 January 1, 2020 - March 31, 2020 3.9 April 1, 2020 - June 30, 2020 3.9 July 1, 2020 - September 30, 2020 3.9 October 1, 2020 - December 31, 2020 3.8 Total 2020 15.5 2021 14.5 2022 13.6 2023 12.8 2024 12.0 2025 and thereafter 99.0 Total gross unearned insurance premiums $ 179.6
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 ended March 31, 2019 and 2018 : Three Months Ended March 31, Millions 2019 2018 Written premiums: Direct $ 7.9 $ 6.4 Assumed — — Gross written premiums $ 7.9 $ 6.4 Earned premiums: Direct $ 3.7 $ 3.0 Assumed 0.5 — Gross earned premiums $ 4.2 $ 3.0

Investments in Unconsolidated_2

Investments in Unconsolidated Entities (Tables)	3 Months Ended
	Mar. 31, 2019
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 as of March 31 2019 and December 31, 2018: Millions March 31, 2019 December 31, 2018 Equity method eligible unconsolidated entities, at fair value $ 293.6 $ 138.1 Investments accounted for under the equity method 1.0 1.3 Total investments in equity method eligible unconsolidated entities 294.6 139.4 Other unconsolidated investments (1) 203.0 186.2 Total other long-term investments $ 497.6 $ 325.6 (1) Consists of other long-term investments that are not equity method eligible.
Affiliated Entity
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	The following table presents White Mountains’s investments in equity method eligible unconsolidated entities as of March 31, 2019 and 2018: Basic Ownership Interest Investee March 31, 2019 December 31, 2018 Instrument Held MediaAlpha (1) 48.3% n/a Units PassportCard/DavidShield (2) 50.0% 50.0% Common shares Kudu 49.5% 49.5% Units durchblicker 45.0% 45.0% Common shares Tuckerman Capital Fund III, L.P. 16.0% 18.5% Limited partnership interest Compare.com 18.4% 18.4% Common shares (1) As of December 31, 2018, MediaAlpha was a majority-owned consolidated subsidiary of White Mountains. (2) As of March 31, 2019 and December 31, 2018, White Mountains’s ownership interest in DavidShield comprised a 50.0% direct interest and White Mountains’s ownership interest in PassportCard comprised a 25.0% direct ownership interest and a 25.0% indirect interest through DavidShield. See Note 2 — “Significant Transactions”. As a result of the MediaAlpha Transaction, White Mountains’s reduced its ownership interest in MediaAlpha to 48.3% of the basic units outstanding ( 42.0% on a fully diluted, fully converted basis). White Mountains’s remaining ownership interest in MediaAlpha no longer meets the criteria for a controlling ownership interest and, accordingly, White Mountains deconsolidated MediaAlpha on February 26, 2019. Upon deconsolidation, White Mountains’s investment in MediaAlpha met the criteria to be accounted for under the equity method or under the fair value option. White Mountains elected the fair value option and the investment in MediaAlpha was measured at its estimated fair value of $114.7 million as of March 31, 2019 , with the change in fair value of $114.7 million reflected within unrealized investment gains. White Mountains’s consolidated statement of comprehensive income and its segment disclosures include MediaAlpha’s results of operations for the period from January 1, 2019 through February 26, 2019. See Note 2 — “Significant Transactions”. For the period from February 26, 2019 to March 31, 2019, MediaAlpha’s total revenues, total expenses, and pre-tax income were $33.5 million , $32.5 million , and $1.1 million . As of March 31, 2019, MediaAlpha’s total assets and total liabilities were $102.3 million and $148.2 million

Earnings Per Share (Tables)

Earnings Per Share (Tables)	3 Months Ended
	Mar. 31, 2019
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 March 31, 2019 and 2018 . See Note 17 — “Held for Sale and Discontinued Operations” . Three Months Ended March 31, 2019 2018 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 284.4 $ (48.0 ) Less: total income from discontinued operations, net of tax .7 .1 Net income (loss) from continuing operations attributable to White Mountains’s common shareholders $ 283.7 $ (48.1 ) Allocation of earnings to participating restricted common shares (1) (2.9 ) .4 Basic and diluted earnings (losses) per share numerators $ 280.8 $ (47.7 ) Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 3,172.2 3,746.1 Average unvested restricted common shares (2) (32.5 ) (35.9 ) Basic earnings (losses) per share denominator 3,139.7 3,710.2 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 3,172.2 3,746.1 Average unvested restricted common shares (2) (32.5 ) (35.9 ) Diluted earnings (losses) per share denominator 3,139.7 3,710.2 Basic and diluted earnings per share (in dollars) - continuing operations: Distributed earnings - dividends declared and paid $ 1.00 $ 1.00 Undistributed earnings (losses) 88.42 (13.85 ) Basic and diluted earnings (losses) per share $ 89.42 $ (12.85 ) (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 March 31, 2019 and 2018 . See Note 17 — “Held for Sale and Discontinued Operations” . Three Months Ended March 31, Millions 2019 2018 Undistributed net earnings (losses) - continuing operations: Net income (losses) attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 280.8 $ (47.7 ) Dividends declared, net of restricted common share amounts (1) (3.2 ) (3.7 ) Total undistributed net earnings (losses), net of restricted common share amounts $ 277.6 $ (51.4 ) (1) 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: Three Months Ended Millions 2018 Net cash provided from operations $ .1 Net cash used for investing activities (.1 ) Net cash used for financing activities — Net change in cash during the period — Cash balances at beginning of period — 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 months ended March 31, 2019 and 2018 : Three Months Ended March 31, 2019 2018 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 284.4 $ (48.0 ) Less: total income (loss) from continuing operations, net of tax 283.7 (48.1 ) Net income from discontinued operations attributable to White Mountains’s common shareholders $ .7 $ .1 Allocation of earnings to participating restricted common shares (1) — — Basic and diluted earnings per share numerators (2) $ .7 $ .1 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 3,172.2 3,746.1 Average unvested restricted common shares (3) (32.5 ) (35.9 ) Basic earnings per share denominator 3,139.7 3,710.2 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 3,172.2 3,746.1 Average unvested restricted common shares (3) (32.5 ) (35.9 ) Diluted earnings per share denominator 3,139.7 3,710.2 Basic and diluted earnings per share (in dollars) - discontinued operations: $ .22 $ .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 months ended March 31, 2019 and 2018 . (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	3 Months Ended
	Mar. 31, 2019
Share-based compensation arrangement by share-based payment award
Summary of performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan	The following table presents the performance share activity for the three months ended March 31, 2019 and 2018 for performance shares granted under the WTM Incentive Plan: Three Months Ended March 31, 2019 2018 Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Target Performance Accrued Expense Beginning of period 40,616 $ 31.7 50,515 $ 45.8 Shares paid (1) (13,715 ) (18.1 ) (23,186 ) (28.4 ) New grants 13,700 — 14,105 — Forfeitures and cancellations (2) 1 .2 (818 ) .3 Expense recognized — 9.3 — 3.3 End of period 40,602 $ 23.1 40,616 $ 21.0 (1) WTM performance share payments in 2019 for the 2016-2018 performance cycle, which were paid in cash in March 2019, ranged from 139% to 166% of target. WTM performance share payments in 2018 for the 2015-2017 performance cycle, which were paid in cash in March 2018, ranged from 145% to 147% of target. (2) Amounts include changes in assumed forfeitures, as required under GAAP.
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 as of March 31, 2019 for each performance cycle: Three Months Ended March 31, 2019 Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Performance cycle: 2017 – 2019 14,070 $ 16.9 2018 – 2020 13,450 5.5 2019 – 2021 13,700 1.0 Sub-total 41,220 23.4 Assumed forfeitures (618 ) (.3 ) March 31, 2019 40,602 $ 23.1
Summary of restricted shares activity	The following table presents the unrecognized compensation cost associated with the outstanding restricted share awards for the three months ended March 31, 2019 and 2018 : Three Months Ended March 31, 2019 2018 Millions, except share amounts Restricted Unamortized Issue Date Restricted Unamortized Issue Date Non-vested, Beginning of period 41,510 $ 12.5 53,755 $ 14.3 Issued 13,700 12.8 14,105 11.4 Vested (13,715 ) — (25,381 ) — Forfeited — — (969 ) (.2 ) Expense recognized — (2.5 ) — (3.3 ) End of period 41,495 $ 22.8 41,510 $ 22.2

Leases (Tables)

Leases (Tables)	3 Months Ended
	Mar. 31, 2019
Leases [Abstract]
Lease, Cost	The following table summarizes net lease expense recognized in White Mountains’s consolidated statement of operations for the three months ended March 31, 2019: Millions Lease Cost Three Months Ended March 31, 2019 Lease cost $ 1.7 Less: sublease income .1 Net lease cost $ 1.6
Lessee, Operating Lease, Liability, Maturity	The following table presents the contractual maturities of the lease liabilities associated with White Mountains’s operating lease agreements as of March 31, 2019: Millions As of March 31, 2019 Remainder of 2019 $ 4.2 2020 4.6 2021 3.9 2022 3.7 2023 3.1 Thereafter 5.1 Total undiscounted lease payments 24.6 Less: present value adjustment 2.8 Operating lease liability $ 21.8
Schedule of Right-of-Use Assets and Lease Liabilities	The following table presents lease related assets and liabilities by reportable segment as of March 31, 2019: As of March 31, 2019 Millions HG/BAM NSM Other Operations Total Weighted Average Incremental Borrowing Rate (1) ROU lease asset $ 11.8 $ 4.7 $ 5.0 $ 21.5 4.7% Lease liability $ 11.8 $ 4.7 $ 5.3 $ 21.8 (1) The present value of the remaining lease payments were determined by discounting the lease payments using the incremental borrowing rate.

Non-controlling Interests (Tabl

Non-controlling Interests (Tables)	3 Months Ended
	Mar. 31, 2019
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 March 31, 2019 and December 31, 2018 : March 31, 2019 December 31, 2018 $ in Millions Non-controlling Percentage Non-controlling Equity Non-controlling Percentage Non-controlling Equity Non-controlling interests, excluding BAM HG Global 3.1 % $ 14.8 3.1 % $ 14.5 NSM 4.7 14.2 4.5 13.6 Other NSM 13.4 .2 13.4 .3 MediaAlpha — — 39.0 16.2 Other Operations various .9 various 1.1 Total, excluding BAM 30.1 45.7 BAM 100.0 (173.8 ) 100.0 (170.6 ) Total non-controlling interests $ (143.7 ) $ (124.9 )

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2019
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 (1) Other Operations Total Three Months Ended March 31, 2019 Earned insurance premiums $ 4.2 $ — $ — $ — $ 4.2 Net investment income 5.3 — — 10.7 16.0 Net realized and unrealized investment gains 11.8 — — 119.1 130.9 Realized and unrealized gains from the MediaAlpha Transaction — — — 182.2 182.2 Advertising and commission revenues (1) — 43.3 48.8 1.3 93.4 Other revenue .6 6.0 — .3 6.9 Total revenues 21.9 49.3 48.8 313.6 433.6 Insurance acquisition expenses 1.3 — — — 1.3 Cost of sales — — 40.6 1.1 41.7 General and administrative expenses 14.4 25.1 12.5 29.4 81.4 Broker commission expense — 15.7 — — 15.7 Change in fair value of contingent consideration earnout liabilities — 1.3 — — 1.3 Amortization of other intangible assets — 5.0 1.6 — 6.6 Interest expense — 3.7 .2 — 3.9 Total expenses 15.7 50.8 54.9 30.5 151.9 Pre-tax income (loss) $ 6.2 $ (1.5 ) $ (6.1 ) $ 283.1 $ 281.7 (1) MediaAlpha’s results are from January 1, 2019 to February 26, 2019, the date of the MediaAlpha Transaction. Millions HG Global/BAM MediaAlpha Other Operations Total Three Months Ended March 31, 2018 Earned insurance premiums $ 3.0 $ — $ — $ 3.0 Net investment income 3.7 — 16.0 19.7 Net realized and unrealized investment losses (7.9 ) — (45.8 ) (53.7 ) Advertising and commission revenues (1) — 70.1 .9 71.0 Other revenue .2 1.6 .3 2.1 Total revenues (1.0 ) 71.7 (28.6 ) 42.1 Insurance acquisition expenses 1.4 — — 1.4 Cost of sales — 57.4 .7 58.1 General and administrative expenses 11.9 11.2 22.0 45.1 Amortization of other intangible assets — 2.9 — 2.9 Interest expense — .4 .2 .6 Total expenses 13.3 71.9 22.9 108.1 Pre-tax loss $ (14.3 ) $ (.2 ) $ (51.5 ) $ (66.0 ) (1) Approximately 31% of MediaAlpha’s advertising revenue for the three months ended March 31, 2018

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Summary of the fair value and carrying value of financial instruments	The following table presents the fair value and carrying value of this financial instrument as of March 31, 2019 and December 31, 2018 : March 31, 2019 December 31, 2018 Millions Fair Value Carrying Value Fair Value Carrying Value NSM Bank Facility $ 182.9 $ 182.8 $ 176.1 $ 176.6 MediaAlpha Bank Facility $ — $ — $ 14.6 $ 14.2 Other Operations debt $ 10.9 (1) $ 10.9 $ — $ — (1)

Held for Sale and Discontinue_2

Held for Sale and Discontinued Operations (Tables)	3 Months Ended
	Mar. 31, 2019
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 March 31, 2019 and 2018 . See Note 17 — “Held for Sale and Discontinued Operations” . Three Months Ended March 31, Millions 2019 2018 Undistributed net earnings (losses) - continuing operations: Net income (losses) attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 280.8 $ (47.7 ) Dividends declared, net of restricted common share amounts (1) (3.2 ) (3.7 ) Total undistributed net earnings (losses), net of restricted common share amounts $ 277.6 $ (51.4 ) (1) 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: Three Months Ended Millions 2018 Net cash provided from operations $ .1 Net cash used for investing activities (.1 ) Net cash used for financing activities — Net change in cash during the period — Cash balances at beginning of period — 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 months ended March 31, 2019 and 2018 : Three Months Ended March 31, 2019 2018 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 284.4 $ (48.0 ) Less: total income (loss) from continuing operations, net of tax 283.7 (48.1 ) Net income from discontinued operations attributable to White Mountains’s common shareholders $ .7 $ .1 Allocation of earnings to participating restricted common shares (1) — — Basic and diluted earnings per share numerators (2) $ .7 $ .1 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 3,172.2 3,746.1 Average unvested restricted common shares (3) (32.5 ) (35.9 ) Basic earnings per share denominator 3,139.7 3,710.2 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 3,172.2 3,746.1 Average unvested restricted common shares (3) (32.5 ) (35.9 ) Diluted earnings per share denominator 3,139.7 3,710.2 Basic and diluted earnings per share (in dollars) - discontinued operations: $ .22 $ .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 months ended March 31, 2019 and 2018 . (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) - USD ($) $ in Millions	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	May 11, 2018	Jul. 31, 2012
Basis of Presentation
ROU lease asset	$ 21.5
Operating lease liability	$ 21.8
HG Global
Basis of Presentation
Percentage of par value of policy reinsured	15.00%
Surplus notes	$ 481.3				$ 503
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%
NSM Insurance HoldCo, LLC
Basis of Presentation
Ownership interest (as a percent)	95.30%		95.50%	95.00%
Fully diluted ownership interest (as a percent)	84.80%		85.00%
Accounting Standards Update 2016-02
Basis of Presentation
ROU lease asset	$ 21.5	$ 23.2
Operating lease liability	$ 21.8	$ 23.2

Significant Transactions (Detai

Significant Transactions (Details)	Feb. 26, 2019USD ($)	Dec. 03, 2018USD ($)	May 11, 2018USD ($)	Jan. 24, 2018USD ($)	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)earnout_period	Mar. 31, 2018USD ($)	Apr. 04, 2019USD ($)	Dec. 31, 2018USD ($)	May 18, 2018USD ($)	Feb. 05, 2018USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchases of consolidated subsidiaries, net of cash acquired						$ 9,600,000	$ 0
Net gain from sale of discontinued operations, net of tax						66,200,000	$ 0
NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Contingent consideration						1,300,000
Contingent consideration earnout liabilities						27,500,000			$ 20,200,000
MediaAlpha
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)	48.30%
Proceeds from Dividends Received	$ 88,000,000
Fully diluted ownership interest (as a percent)	42.00%
Net gain from sale of discontinued operations, net of tax						67,500,000
Assets, fair value						114,700,000
Unrealized investment gain						114,700,000
Total realized and unrealized gain on sale of minority stake						$ 182,200,000
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%			95.30%			95.50%
Purchases of consolidated subsidiaries, net of cash acquired			$ 274,200,000
Purchase price adjustment						$ 2,100,000
Number of earnout periods | earnout_period						2
Assets acquired			495,200,000
Goodwill and intangible assets acquired			383,000,000
Liabilities acquired			204,600,000
Contingent consideration			10,200,000
Noncontrolling interest acquired			14,400,000
Transaction costs						$ 6,300,000
Fully diluted ownership interest (as a percent)						84.80%			85.00%
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
Contingent consideration										7,500,000
Equity method investment, aggregate cost										49,600,000
Fresh Insurance Services Group Limited | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchase price adjustment						5,900,000
KBK Insurance Group, Inc. & KBK Premium Services, Inc.
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchases of consolidated subsidiaries, net of cash acquired		$ 60,000,000
Goodwill and intangible assets acquired		59,400,000				32,600,000
Intangible assets acquired						$ 32,700,000
Stock issued during period, value, acquisitions		29,000,000
Proceeds from borrowings		$ 30,100,000
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%
DavidShield | 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%
DavidShield & 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%
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%		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%
Equity method investment, aggregate cost											$ 125,000,000
Commitment for working capital						$ 3,800,000
Additional funding						82,200,000			$ 63,000,000
Fully diluted ownership interest (as a percent)									42.70%
Kudu Investment Management, LLC | White Mountains
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity method investment, aggregate cost						72,600,000
Additional funding						41,100,000			$ 31,500,000
Scenario, Forecast | MediaAlpha
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax					$ 1,300,000
Subsequent Event | Kudu Investment Management, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)								99.10%
Equity method investment, aggregate cost								$ 81,400,000
Unfunded capital commitment								$ 104,700,000
Fully diluted ownership interest (as a percent)								85.40%
NSM Bank Facility | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long-term line of credit			100,000,000
NSM Bank Facility | Term Loan | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long-term line of credit			$ 181,100,000			$ 186,400,000			$ 180,400,000	$ 51,000,000

Investment Securities (Net Inve

Investment Securities (Net Investment Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Investment income
Total investment income	$ 16.4	$ 20.4
Third-party investment expenses	(0.4)	(0.7)
Net investment income, pre-tax	16	19.7
Fixed maturity investments
Investment income
Total investment income	8.1	10.9
Short-term investments
Investment income
Total investment income	1.3	2.6
Common Stock
Investment income
Total investment income	5.4	5
Other long-term investments
Investment income
Total investment income	$ 1.6	$ 1.9

Investment Securities (Net Real

Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Net realized investment gains (losses), pre-tax	$ 25.7	$ (5.1)
Net unrealized investment gains (losses), pre-tax (1)	219.9	(48.6)
Net realized and unrealized investment gains (losses), pre-tax	245.6	(53.7)
Income tax (expense) benefit attributable to net realized and unrealized investment gains (losses)	(52.2)	5.7
Net realized and unrealized investment gains (losses), after-tax	193.4	$ (48)
MediaAlpha
Unrealized investment gain	$ 114.7

Investment Securities (Net Re_2

Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Trading Securities, Realized (Losses) Gains	$ 25.7	$ (16.1)
Trading securities, Tax on Realized Holding (Losses) Gains on Investments	(1.7)	(0.6)
Trading securities realized holding (losses) gain on investments after tax	24	(16.7)
Foreign Currency Transaction Gains (Losses), Realized	0	11
Trading Securities, Tax on Realized Foreign Currency Transaction Gains (Losses) Tax	0	0
Trading securities, realized foreign currency gain (loss) net of tax	0	11
Net realized investment gains (losses), pre-tax	25.7	(5.1)
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(1.7)	(0.6)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	24	(5.7)
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Trading Securities, Realized (Losses) Gains	(0.9)	(13.7)
Foreign Currency Transaction Gains (Losses), Realized	0	18.2
Net realized investment gains (losses), pre-tax	(0.9)	4.5
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Trading Securities, Realized (Losses) Gains	0	(0.1)
Foreign Currency Transaction Gains (Losses), Realized	0	0
Net realized investment gains (losses), pre-tax	0	(0.1)
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Trading Securities, Realized (Losses) Gains	25.8	1.2
Foreign Currency Transaction Gains (Losses), Realized	0	0
Net realized investment gains (losses), pre-tax	25.8	1.2
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Trading Securities, Realized (Losses) Gains	0.8	(3.5)
Foreign Currency Transaction Gains (Losses), Realized	0	(7.2)
Net realized investment gains (losses), pre-tax	$ 0.8	$ (10.7)

Investment Securities (Net Unre

Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ 220.2	$ (37.9)
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	(50.5)	6.3
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	169.7	(31.6)
Foreign Currency Transaction Gain (Loss), Unrealized	(0.3)	(10.7)
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	0	0
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	(0.3)	(10.7)
Net unrealized investment gains (losses), pre-tax (1)	219.9	(48.6)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	(50.5)	6.3
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	169.4	(42.3)
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	22	(18.6)
Foreign Currency Transaction Gain (Loss), Unrealized	0	(14.8)
Net unrealized investment gains (losses), pre-tax (1)	22	(33.4)
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	0	(0.7)
Foreign Currency Transaction Gain (Loss), Unrealized	0	0
Net unrealized investment gains (losses), pre-tax (1)	0	(0.7)
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	85.5	(16.7)
Foreign Currency Transaction Gain (Loss), Unrealized	0	0
Net unrealized investment gains (losses), pre-tax (1)	85.5	(16.7)
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	112.7	(1.9)
Foreign Currency Transaction Gain (Loss), Unrealized	(0.3)	4.1
Net unrealized investment gains (losses), pre-tax (1)	112.4	$ 2.2
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Unrealized investment gain	$ 114.7

Investment Securities (Investme

Investment Securities (Investment gains (losses) for Level 3) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ 220.2	$ (37.9)
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	112.7	(1.9)
Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	114	(5.1)
Level 3 Inputs | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ 114	$ (5.1)

Investment Securities (Invest_2

Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	$ 153.1	$ 154
Trading Securities, Unrealized Holding Gain	0.4	0.1
Trading Securities, Unrealized Holding Loss	(0.5)	(0.9)
Fixed maturity investments, at fair value	153	153.2
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	538.4	519
Trading Securities, Unrealized Holding Gain	7.3	1
Trading Securities, Unrealized Holding Loss	(0.9)	(9.5)
Fixed maturity investments, at fair value	544.8	510.5
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	279.7	279
Trading Securities, Unrealized Holding Gain	6.1	2.4
Trading Securities, Unrealized Holding Loss	(0.4)	(1.1)
Fixed maturity investments, at fair value	285.4	280.3
Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	149.4	136.1
Trading Securities, Unrealized Holding Gain	0.6	0.1
Trading Securities, Unrealized Holding Loss	(1.2)	(2.7)
Fixed maturity investments, at fair value	148.8	133.5
Fixed Income Investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	1,120.6	1,088.1
Trading Securities, Unrealized Holding Gain	14.4	3.6
Trading Securities, Unrealized Holding Loss	(3)	(14.2)
Total fixed maturity investments including assets held for sale	$ 1,132	$ 1,077.5

Investment Securities (Invest_3

Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Common Stock
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	$ 798.6	$ 904.7
Trading Securities, Unrealized Holding Gain	107.3	51
Trading Securities, Unrealized Holding Loss	(0.9)	(30.1)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	905	925.6
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	389.8	330.3
Trading Securities, Unrealized Holding Gain	164.3	52.2
Trading Securities, Unrealized Holding Loss	(54.3)	(54.9)
Net foreign currency gains (losses)	(2.2)	(2)
Fixed maturity investments, at fair value	$ 497.6	$ 325.6

Investment Securities (Other lo

Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	$ 114.7	$ 0
DavidShield & PassportCard
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	75	75
Kudu Investment Management, LLC
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	71.7	30.7
Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	54.4	60
Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	315.8	165.7
Hedge and private equity funds included in other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	150.7	146.1
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	31.1	13.8
White Mountains
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	$ 497.6	$ 325.6

Investment Securities (Hedge Fu

Investment Securities (Hedge Funds and Private Equity Funds) (Details) $ in Millions	Mar. 31, 2019USD ($)fund	Dec. 31, 2018USD ($)
Private equity security
Debt and Equity Securities, FV-NI [Line Items]
Number of investments	12
Hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Number of investments	1
Hedge and private equity funds included in other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Largest single fund investment | $	$ 53.1	$ 54.3

Investment Securities (Fair Val

Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Private equity security
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	$ 97.6
Hedge and private equity funds included in other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	150.7	$ 146.1
Unfunded Commitments	69.9	76.7
Hedge funds | Hedge Fund, Long/ Short Equity Banks and Financial
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	53.1	54.3
Unfunded Commitments	0	0
Hedge funds | Hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	53.1	54.3
Unfunded Commitments	0	0
Private equity security | Manufacturing/Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	42.1	42.9
Unfunded Commitments	10.5	10.5
Private equity security | Aerospace/Defense/Government
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	32.8	27.6
Unfunded Commitments	29.8	34.9
Private equity security | Direct lending
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	13.6	13
Unfunded Commitments	16.8	17.7
Private equity security | Financial services
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	9.1	8.3
Unfunded Commitments	12.8	13.6
Private equity security | Private equity security
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	97.6	91.8
Unfunded Commitments	$ 69.9	$ 76.7

Investment Securities (Invest_4

Investment Securities (Investments In Private Equity Funds Subject to Lock-Up Periods) (Details) - Private equity security $ in Millions	Mar. 31, 2019USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	$ 97.6
1-3 years
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	7
3-5 years
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	0
5-10 years
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	67.9
Greater than 10 years
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	$ 22.7

Investment Securities (Restrict

Investment Securities (Restrictions on Redemption Frequency and Advance Notice Requirements) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019USD ($)fund	Dec. 31, 2018USD ($)
Hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Number of investments | fund	1
Hedge funds | Hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	$ 53.1	$ 54.3
Investment redemption, notice period	45 days
Hedge funds | Hedge Fund, Long/ Short Equity Banks and Financial
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	$ 53.1	$ 54.3

Investment Securities (Fair V_2

Investment Securities (Fair Value Measurements) (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Percentage of investments recorded at fair value	82.00%	87.00%

Investment Securities (Fair V_3

Investment Securities (Fair Value Measurement by Level) (Details) $ in Millions	Mar. 31, 2019USD ($)Investment	Dec. 31, 2018USD ($)Investment	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 2,534.6	$ 2,328.7	$ 2,532.5	$ 3,208.3
Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	799.3	842.6	874.3	890.4
Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 1,237.7	$ 1,160.5	1,405.1	2,105.4
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	$ 544.8	$ 510.5
Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	148.8	133.5
Mortgage-backed and asset-backed securities | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	148.8	133.5
Mortgage-backed and asset-backed securities | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	275.1	138.7	113.4	77.2
Hedge and private equity funds included in other long-term investments | NAV
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	222.5	186.9	139.7	135.3
Fair value measured on a recurring basis
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	2,804.4	2,542.9
Fair value measured on a recurring basis | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,048.3	1,047
Fair value measured on a recurring basis | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,258.5	1,170.3
Fair value measured on a recurring basis | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	275.1	138.7
Fair value measured on a recurring basis | U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	153	153.2
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	153	153.2
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	544.8	510.5
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	544.8	510.5
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 | Financials
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	139.7	143.4
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	139.7	143.4
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 | Health Care
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	74.7	55
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	74.7	55
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 | Energy
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	66	57.6
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	66	57.6
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 | Debt securities issued by corporations | Consumer
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	61	68.5
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	61	68.5
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 | Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	57	47.6
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	57	47.6
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 | Technology
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	55	60.5
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	55	60.5
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 | Communications
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	40.4	31.8
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	40.4	31.8
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 | Utilities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	28.5	19.8
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	28.5	19.8
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 | Materials
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	22.5	26.3
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	22.5	26.3
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 | Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	148.8	133.5
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	148.8	133.5
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	285.4	280.3
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	285.4	280.3
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 | Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,132	1,077.5
Fair value measured on a recurring basis | Fixed maturity investments | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	153	153.2
Fair value measured on a recurring basis | Fixed maturity investments | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	979	924.3
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]
Short-term investments, at fair value	269.8	214.2
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	249	204.4	$ 763	$ 176.1
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	20.8	9.8
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	0	0
Fair value measured on a recurring basis | Common equity securities | Financials
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments		13.5
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		13.5
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
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
Fair value measured on a recurring basis | Common equity securities | Health Care
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments		14
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		14
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
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
Fair value measured on a recurring basis | Common equity securities | Energy
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments		4.1
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		4.1
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
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
Fair value measured on a recurring basis | Common equity securities | Consumer
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments		6.2
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		6.2
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
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
Fair value measured on a recurring basis | Common equity securities | Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments		11.4
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		11.4
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
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
Fair value measured on a recurring basis | Common equity securities | Technology
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments		7.4
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		7.4
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
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
Fair value measured on a recurring basis | Common equity securities | Communications
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments		12.7
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		12.7
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
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
Fair value measured on a recurring basis | Common equity securities | Materials
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments		3.1
Fair value measured on a recurring basis | Common equity securities | Materials | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments		3.1
Fair value measured on a recurring basis | Common equity securities | Materials | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments		0
Fair value measured on a recurring basis | Common equity securities | Materials | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments		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	712.3	675.3
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	646.3	617
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	66	58.3
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	192.7	177.9
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	192.7	177.9
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	905	925.6
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	646.3	689.4
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	258.7	236.2
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	275.1	138.7
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
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
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	$ 275.1	$ 138.7

Investment Securities (Debt sec

Investment Securities (Debt securities issued by corporation) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 2,534.6	$ 2,328.7	$ 2,532.5	$ 3,208.3
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	544.8	510.5
Fair value investments	544.8	510.5
AAA | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	9.6	8.9
AA | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	83.8	88.7
A | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	300.3	270.5
BBB | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 151.1	$ 142.4

Investment Securities (Mortgage

Investment Securities (Mortgage-backed Asset Securities Table) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 2,534.6	$ 2,328.7	$ 2,532.5	$ 3,208.3
Federal National Mortgage Association Certificates and Obligations (FNMA)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	59.8	53.6
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	43	38.1
Government National Mortgage Association Certificates and Obligations (GNMA)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	25.6	23.7
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	128.4	115.4
Collateralized Mortgage Backed Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	128.4	115.4
Collateralized Auto Loans
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	11.5	9.2
Collateralized Credit Card Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	8.9	8.9
Asset-backed Securities, Securitized Loans and Receivables
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	20.4	18.1
Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	148.8	133.5
Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,237.7	1,160.5	$ 1,405.1	$ 2,105.4
Level 2 Inputs | Federal National Mortgage Association Certificates and Obligations (FNMA)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	59.8	53.6
Level 2 Inputs | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	43	38.1
Level 2 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	25.6	23.7
Level 2 Inputs | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	128.4	115.4
Level 2 Inputs | Collateralized Mortgage Backed Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	128.4	115.4
Level 2 Inputs | Collateralized Auto Loans
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	11.5	9.2
Level 2 Inputs | Collateralized Credit Card Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	8.9	8.9
Level 2 Inputs | Asset-backed Securities, Securitized Loans and Receivables
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	20.4	18.1
Level 2 Inputs | Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	148.8	133.5
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 | Government National Mortgage Association Certificates and Obligations (GNMA)
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 | Collateralized Mortgage Backed 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 Credit Card Securities
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) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	$ 2,328.7	$ 3,208.3
Net realized and unrealized gains (losses)	245.6	(45.4)
Amortization/Accretion	(0.3)	(1.2)
Purchases	217.6	692.6
Sales	(257)	(1,321.8)
Transfers in	10.9	0
Transfers out	(10.9)	0
Ending balance	2,534.6	2,532.5
Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	842.6	890.4
Net realized and unrealized gains (losses)	91.7	(14.3)
Amortization/Accretion	0.1	0
Purchases	14.6	238.9
Sales	(149.7)	(240.7)
Transfers in	0	0
Transfers out	0	0
Ending balance	799.3	874.3
Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	1,160.5	2,105.4
Net realized and unrealized gains (losses)	40.7	(26)
Amortization/Accretion	(0.4)	(1.2)
Purchases	142.3	407.7
Sales	(105.4)	(1,080.8)
Transfers in	0	0
Transfers out	0	0
Ending balance	1,237.7	1,405.1
Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	138.7	77.2
Net realized and unrealized gains (losses)	114	(7.8)
Amortization/Accretion	0	0
Purchases	11.5	44
Sales	0
Transfers in	10.9	0
Transfers out	0	0
Ending balance	275.1	113.4
Hedge and private equity funds included in other long-term investments | NAV
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	186.9	135.3
Net realized and unrealized gains (losses)	(0.8)	2.7
Amortization/Accretion	0	0
Purchases	49.2	2
Sales	(1.9)	(0.3)
Transfers in	0	0
Transfers out	(10.9)	0
Ending balance	222.5	139.7
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Realized and unrealized losses		(0.6)
Foreign Exchange Forward
Debt and Equity Securities, FV-NI [Line Items]
Realized and unrealized losses		(3.5)
Foreign Currency on Cash and Open Trades
Debt and Equity Securities, FV-NI [Line Items]
Realized and unrealized losses		(4.2)
Fair value measured on a recurring basis
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	2,542.9
Ending balance	2,804.4
Fair value measured on a recurring basis | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	1,047
Ending balance	1,048.3
Fair value measured on a recurring basis | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	1,170.3
Ending balance	1,258.5
Fair value measured on a recurring basis | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	138.7
Ending balance	275.1
Fair value measured on a recurring basis | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	138.7
Ending balance	275.1
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Ending balance	0
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Ending balance	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	138.7
Ending balance	275.1
Fair value measured on a recurring basis | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	269.8		$ 214.2
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	249	$ 763	204.4	$ 176.1
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	20.8		9.8
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	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				$ (3.7)
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Unrealized investment gain	$ 114.7

Investment Securities (Signific

Investment Securities (Significant Unobservable Inputs) (Details) - NR $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019USD ($)$ / shares	Dec. 31, 2018USD ($)
MediaAlpha
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 114.7
DavidShield & PassportCard | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	75	$ 75
Compare.com | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	16.9	16.9
durchblicker | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	15.2	15.5
Captricity | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	14.5	14.5
Galvanic | Multiple of EBITDA
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	2.7	3.1
Private Debt Instrument | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	10	$ 10
Private Debt Instrument | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 10.9
Unit price | MediaAlpha
Quantitative information for Level 3 Fair Value Measurements Assets
Share Price | $ / shares	$ 208.98
Discount rate | DavidShield & PassportCard
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	18.00%	18.00%
Discount rate | Compare.com
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	22.00%	22.00%
Discount rate | durchblicker
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	23.00%	23.00%
Discount rate | Captricity
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	23.00%	23.00%
Discount rate | Private Debt Instrument
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	9.23%	9.23%
Discount rate | Private Debt Instrument
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	7.00%
Revenue exit multiple | DavidShield & PassportCard
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input	1	1
Revenue exit multiple | Compare.com
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input	2.75	2.75
Revenue exit multiple | durchblicker
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input	2.25	2.25
Revenue exit multiple | Captricity
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input	3.75	3.75
EBITDA multiple | Galvanic
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input	6	6

Goodwill and Other Intangible_3

Goodwill and Other Intangible Assets - Narrative (Details) - USD ($) $ in Millions	Mar. 31, 2019	Feb. 26, 2019	Dec. 31, 2018	May 18, 2018	May 11, 2018
NSM Insurance HoldCo, LLC
Goodwill [Line Items]
Goodwill and intangible assets acquired					$ 383
Ownership interest (as a percent)	95.30%		95.50%		95.00%
Fully diluted ownership interest (as a percent)	84.80%		85.00%
Fresh Insurance Services Group Limited
Goodwill [Line Items]
Goodwill and intangible assets acquired				$ 54.6
Ownership interest (as a percent)				100.00%
MediaAlpha
Goodwill [Line Items]
Ownership interest (as a percent)		48.30%
Fully diluted ownership interest (as a percent)		42.00%

Goodwill and Other Intangible_4

Goodwill and Other Intangible Assets - Goodwill Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 379.9	$ 25.9	$ 25.9
Intangible assets, gross (including goodwill), beginning balance	591.1
Amortization	(6.6)
Goodwill, ending balance	349.7		379.9
Intangible assets, gross (including goodwill), ending balance	524		591.1
Other Intangible Assets
Goodwill [Roll Forward]
Intangible assets, net (excluding goodwill), beginning balance	157.6	36.2	36.2
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	26.8	0
Acquisitions of businesses	0
Foreign currency translation	0.3
Adjustments to reflect acquisition date fair value	5.9
Amortization	(6.6)	(2.9)
Intangible assets, net (excluding goodwill), ending balance	160.5	33.3	157.6
Intangible Asset and Goodwill
Goodwill [Roll Forward]
Intangible assets, gross (including goodwill), beginning balance	537.5	62.1	62.1
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	0	0
Acquisitions of businesses	13.4
Foreign currency translation	1.5
Adjustments to reflect acquisition date fair value	6.2
Amortization	6.6	2.9
Intangible assets, gross (including goodwill), ending balance	510.2	59.2	537.5
Goodwill
Goodwill [Roll Forward]
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	(26.8)	0
Acquisitions of businesses	13.4
Foreign currency translation	1.2
Adjustments to reflect acquisition date fair value	0.3
Goodwill, ending balance	349.7	25.9
MediaAlpha | Other Intangible Assets
Goodwill [Roll Forward]
Deconsolidation of MediaAlpha	(23.5)	$ 0
MediaAlpha | Intangible Asset and Goodwill
Goodwill [Roll Forward]
Deconsolidation of MediaAlpha	(41.8)
MediaAlpha | Goodwill
Goodwill [Roll Forward]
Deconsolidation of MediaAlpha	(18.3)
NSM Holding Co LLC | Other Intangible Assets
Goodwill [Roll Forward]
Foreign currency translation	0.3		(0.7)
NSM Holding Co LLC | Goodwill
Goodwill [Roll Forward]
Foreign currency translation	$ 1		$ (2.2)

Goodwill and Other Intangible_5

Goodwill and Other Intangible Assets - Schedule of Goodwill and Other Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Line Items]
Goodwill, acquisition date fair value	$ 349.7	$ 379.9
Goodwill	349.7	379.9	$ 25.9
Other intangible assets, accumulated amortization	13.8	53.6
Goodwill and intangible assets, acquisition date fair value	524	591.1
Goodwill and intangible assets, net carrying value	510.2	537.5
NSM Holding Co LLC
Goodwill [Line Items]
Goodwill, acquisition date fair value	328.8	354.3
Goodwill	328.8	354.3
Other intangible assets, acquisition date fair value	173.2	140.2
Other intangible assets, accumulated amortization	13.3	8.3
Other intangible assets, net carrying value	159.9	131.9
MediaAlpha
Goodwill [Line Items]
Goodwill, acquisition date fair value	0	18.3
Goodwill	0	18.3
Other intangible assets, acquisition date fair value	0	69.9
Other intangible assets, accumulated amortization	0	44.8
Other intangible assets, net carrying value	0	25.1
Other Operations
Goodwill [Line Items]
Goodwill, acquisition date fair value	20.9	7.3
Goodwill	20.9	7.3
Other intangible assets, acquisition date fair value	1.1	1.1
Other intangible assets, accumulated amortization	0.5	0.5
Other intangible assets, net carrying value	0.6	0.6
Customer relationships | NSM Holding Co LLC
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	110.9	85.3
Other intangible assets, accumulated amortization	9.3	6
Other intangible assets, net carrying value	101.6	79.3
Customer relationships | MediaAlpha
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	0	26.8
Other intangible assets, accumulated amortization	0	4.9
Other intangible assets, net carrying value	0	21.9
Trade names | NSM Holding Co LLC
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	57.1	51.2
Other intangible assets, accumulated amortization	2.7	1.8
Other intangible assets, net carrying value	54.4	49.4
Trade names | Other Operations
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	0.6	0.6
Other intangible assets, accumulated amortization	0.2	0.2
Other intangible assets, net carrying value	0.4	0.4
Information technology | NSM Holding Co LLC
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	3.8	3.7
Other intangible assets, accumulated amortization	0.7	0.5
Other intangible assets, net carrying value	3.1	3.2
Information technology | MediaAlpha
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	0	33.3
Other intangible assets, accumulated amortization	0	30.9
Other intangible assets, net carrying value	0	2.4
Information technology | Other Operations
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	0.5	0.5
Other intangible assets, accumulated amortization	0.3	0.3
Other intangible assets, net carrying value	0.2	0.2
Other Operations | NSM Holding Co LLC
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	1.4
Other intangible assets, accumulated amortization	0.6
Other intangible assets, net carrying value	0.8
Other Operations | MediaAlpha
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	0	9.8
Other intangible assets, accumulated amortization	0	9
Other intangible assets, net carrying value	0	0.8
Other Intangible Assets
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	174.3	211.2
Other intangible assets, accumulated amortization	13.8	53.6
Other intangible assets, net carrying value	160.5	157.6
Non-controlling interest
Goodwill [Line Items]
Goodwill and intangible assets, net carrying value	(24.1)	(40.6)
Total
Goodwill [Line Items]
Goodwill and intangible assets, net carrying value	$ 486.1	$ 496.9

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	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Goodwill
Acquired Finite-Lived Intangible Assets [Line Items]
Foreign currency translation	$ 1.2
NSM Holding Co LLC | Other Intangible Assets
Acquired Finite-Lived Intangible Assets [Line Items]
Foreign currency translation	0.3	$ (0.7)
NSM Holding Co LLC | Goodwill
Acquired Finite-Lived Intangible Assets [Line Items]
Foreign currency translation	1	(2.2)
Buzzmove | Goodwill
Acquired Finite-Lived Intangible Assets [Line Items]
Foreign currency translation	$ 0.2	$ (0.3)

Debt - Narrative (Details)

Debt - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		8 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Jun. 15, 2018	May 18, 2018	May 11, 2018
Debt Instrument
Total debt	$ 195.4		$ 192.7
Repayments of debt	14.8	$ 2.3
Other Operations
Debt Instrument
Face amount	11.3		0
Unamortized issuance cost	(0.4)		0
MediaAlpha
Debt Instrument
Total debt	0		14.2
MediaAlpha | MediaAlpha Bank Facility
Debt Instrument
Total debt	0		14.2
Face amount	$ 0		$ 14.3
Effective percentage			7.10%
Unamortized issuance cost	$ 0		$ (0.1)
NSM Holding Co LLC
Debt Instrument
Total debt	184.5		178.5
NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Total debt	182.8		176.6
Face amount						$ 100
Term Loan | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Total debt	179.9
Face amount	$ 186.4		$ 180.4		$ 51	181.1
Effective percentage	7.80%		7.40%
Unamortized issuance cost	$ (3.6)		$ (3.8)
Repayments of debt	0.5
Term Loan | NSM Holding Co LLC | Other Operations
Debt Instrument
Total debt	1.7		$ 1.9
Revolving Credit Facility | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Total debt	6.5
Total commitment under revolving credit facility						10
Borrowings under facility						$ 2
Repayments of debt	$ 6.5
Interest Rate Swap | NSM Holding Co LLC
Debt Instrument
Derivative, fixed interest rate				2.97%
Derivative, variable interest rate	2.49%
Interest Rate Swap | Term Loan | NSM Bank Facility
Debt Instrument
Derivative, variable interest rate	2.49%
Interest Rate Swap | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Total debt	$ 149.9
Total commitment under revolving credit facility				$ 151
Derivative, fixed interest rate	2.97%
Debt, weighted average interest rate, all-in	7.44%
London Interbank Offered Rate (LIBOR) | Minimum | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Basis spread on variable rate	4.25%		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%		4.75%
Prime Rate | Minimum | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Basis spread on variable rate	3.25%		3.25%
Prime Rate | Maximum | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Basis spread on variable rate	3.75%		3.75%
Designated as Hedging Instrument | Interest Rate Swap | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Total debt	$ 149.9
Debt, weighted average interest rate, all-in	7.47%
Not Designated as Hedging Instrument | Interest Rate Swap | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Total debt	$ 30
Effective percentage	7.30%
Carrying Value | MediaAlpha | MediaAlpha Bank Facility
Debt Instrument
Total debt	$ 0		$ 14.2
Carrying Value | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Total debt	182.8		176.6
Carrying Value | Other Operations
Debt Instrument
Total debt	$ 10.9		$ 0

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Tax Contingency [Line Items]
Effective income tax rate reconciliation, percent	3.60%	(1.10%)
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	21.00%
Income tax expense (benefit)	$ 10.2	$ 0.7
Continuing Operations | BAM
Income Tax Contingency [Line Items]
Income tax expense (benefit)	$ (1.1)	$ (0.6)

Derivatives - NSM Interest Rate

Derivatives - NSM Interest Rate Swap (Details) - USD ($)	3 Months Ended	8 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Jun. 15, 2018	May 18, 2018	May 11, 2018
Derivative [Line Items]
Debt	$ 195,400,000	$ 192,700,000
NSM Holding Co LLC
Derivative [Line Items]
Debt	184,500,000	178,500,000
NSM Holding Co LLC | NSM Bank Facility
Derivative [Line Items]
Debt	182,800,000	176,600,000
Face amount					$ 100,000,000
Term Loan | NSM Holding Co LLC | NSM Bank Facility
Derivative [Line Items]
Debt	179,900,000
Face amount	$ 186,400,000	180,400,000		$ 51,000,000	$ 181,100,000
Interest Rate Swap | NSM Holding Co LLC
Derivative [Line Items]
Derivative, fixed interest rate			2.97%
Derivative, variable interest rate	2.49%
Interest expense, net	$ 200,000
Derivative, fair value, net	4,600,000	$ 2,700,000
Change in fair value of derivative	$ (1,900,000)
Interest Rate Swap | Term Loan | NSM Bank Facility
Derivative [Line Items]
Derivative, variable interest rate	2.49%
Interest Rate Swap | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Derivative [Line Items]
Derivative, fixed interest rate	2.97%
Debt	$ 149,900,000
Total commitment under revolving credit facility			$ 151,000,000
Debt, weighted average interest rate, all-in	7.44%
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%	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%	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%	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%	3.75%
Designated as Hedging Instrument | Interest Rate Swap | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Derivative [Line Items]
Debt	$ 149,900,000
Debt, weighted average interest rate, all-in	7.47%

Derivatives Forward Contracts (

Derivatives Forward Contracts (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018USD ($)
Forward Contracts
Derivative [Line Items]
Realized and unrealized gains (losses)	$ (3.5)

Municipal Bond Guarantee - Narr

Municipal Bond Guarantee - Narrative (Details) $ in Millions	1 Months Ended	3 Months Ended		12 Months Ended
	Apr. 30, 2018USD ($)	Mar. 31, 2019USD ($)yrContract	Mar. 31, 2018USD ($)	Dec. 31, 2018USD ($)Contract	Dec. 31, 2017USD ($)	Dec. 31, 2012USD ($)	Dec. 31, 2019	Jul. 31, 2012USD ($)
Guarantor Obligations [Line Items]
Repayments of debt		$ 14.8	$ 2.3
Interest paid		3.7	$ 0.3
Percentage of discounted future cash flows					10.00%
Total gross unearned insurance premiums		179.6		$ 176
HG Global
Guarantor Obligations [Line Items]
Partners capital contribution						$ 594.5
Surplus notes		$ 481.3						$ 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		7,786		7,525
Remaining weighted average contract period outstanding (in years)		10 years 8 months 12 days		10 years 8 months 12 days
Municipal bond outstanding principal		$ 53,715.2		$ 52,201.6
Municipal bond outstanding interest		27,348.9		26,560.3
Municipal bond debt service outstanding		81,064.1		78,761.9
April 1, 2019 - December 31, 2019		12.2
2020		15.5
2021		14.5
2022		13.6
2023		12.8
2024		12
2025 and thereafter		99
Total gross unearned insurance premiums		179.6
HG Global
Guarantor Obligations [Line Items]
Collateral held in supplement trust		771.5		757.4
Interest receivable		$ 150.5		143.7
HG Re
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured		15.00%
BAM
Guarantor Obligations [Line Items]
Surplus notes		$ 481.3		$ 481.3
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%
Surplus Note | BAM
Guarantor Obligations [Line Items]
Repayments of debt				$ 17.7
Interest paid				$ 5.3
Combination One | Surplus Note | BAM
Guarantor Obligations [Line Items]
Number of years of change in interest rate | yr		5
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%		8.00%
First Quarter, year two | BAM
Guarantor Obligations [Line Items]
2020		$ 3.9
Second Quarter, year two | BAM
Guarantor Obligations [Line Items]
2020		3.9
Third Quarter, year two | BAM
Guarantor Obligations [Line Items]
2020		3.9
Fourth Quarter, year two | BAM
Guarantor Obligations [Line Items]
2020		3.8
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]
Secured Debt	$ 100
Collateralized financial guarantee, term	12 years
Collateral financial guarantee, callable term				5 years
Aggregate losses on ceded business	$ 165
Premium receivable, Premium expected to be collected	100
Percentage of Quota Share Reinsurance Agreement				100.00%
Direct written premiums				$ 2,200
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		$ 276.1
Scenario, Forecast | Surplus Note | BAM
Guarantor Obligations [Line Items]
Derivative, fixed interest rate (as a percent)							5.70%

Municipal Bond Guarantee - Sche

Municipal Bond Guarantee - Schedule of Net Written Premiums (Details) - HG/BAM - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Direct written premiums	$ 7.9	$ 6.4
Assumed written premiums	0	0
Gross written premiums	7.9	6.4
Direct premiums earned	3.7	3
Assumed premiums earned	0.5	0
Gross earned premiums	$ 4.2	$ 3

Investments in Unconsolidated_3

Investments in Unconsolidated Entities (Details) - White Mountains - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Investments in unconsolidated affiliates
Other Investments	$ 497.6	$ 325.6
Equity method eligible private equity securities, at fair value
Investments in unconsolidated affiliates
Other Investments	293.6	138.1
Investments, accounted for under the equity method
Investments in unconsolidated affiliates
Other Investments	1	1.3
Total investments in equity method eligible unconsolidated entities
Investments in unconsolidated affiliates
Other Investments	294.6	139.4
Other unconsolidated investments
Investments in unconsolidated affiliates
Other Investments	$ 203	$ 186.2

Investments in Unconsolidated_4

Investments in Unconsolidated Entities - White Mountain's Investments (Details)	Mar. 31, 2019	Dec. 31, 2018	Jan. 24, 2018
MediaAlpha
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	48.30%
DavidShield & PassportCard
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%	50.00%
PassportCard
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	25.00%	25.00%	50.00%
DavidShield
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%	50.00%	50.00%
Kudu
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	49.50%	49.50%
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)	18.40%	18.40%
Tuckerman Capital Fund III, L.P.
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	16.00%	18.50%
White Mountains | DavidShield
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	25.00%	25.00%

Investments in Unconsolidated_5

Investments in Unconsolidated Entities - Narrative (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2019	Mar. 31, 2018	Feb. 26, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Total revenues		$ 433.6	$ 42.1
Total assets	$ 3,629.7	3,629.7			$ 3,362.6
Total liabilities	651.4	651.4			644.4
MediaAlpha
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)				48.30%
Fully diluted ownership interest (as a percent)				42.00%
Assets, fair value	114.7	114.7
Unrealized investment gain		114.7
Total assets	0	0			88.4
Total liabilities	0	0			$ 46.9
MediaAlpha | MediaAlpha
Schedule of Equity Method Investments [Line Items]
Total revenues	33.5	48.8	$ 71.7
Expenses	32.5
Pre-tax income	1.1
Total assets	102.3	102.3
Total liabilities	$ 148.2	$ 148.2

Earnings Per Share (Details)

Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 284.4	$ (48)
Less: total income (loss) from discontinued operations	0.7	0.1
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	283.7	(48.1)
Allocation of earnings to participating restricted common shares	(2.9)	0.4
Basic and diluted earnings (losses) per share numerators	$ 280.8	$ (47.7)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,172,200	3,746,100
Average unvested restricted common shares	(32,500)	(35,900)
Basic earnings (losses) per share denominator	3,139,700	3,710,200
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,172,200	3,746,100
Average unvested restricted common shares	(32,500)	(35,900)
Diluted earnings (losses) per share denominator	3,139,700	3,710,200
Basic earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid	$ 1	$ 1
Undistributed earnings (losses)	88.42	(13.85)
Basic and diluted earnings (losses) per share	$ 89.42	$ (12.85)

Earnings Per Share - Undistribu

Earnings Per Share - Undistributed Net Earnings (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Earnings Per Share [Abstract]
Net income (losses) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 280.8	$ (47.7)
Dividends declared net of restricted common share amounts	(3.2)	(3.7)
Total undistributed net earnings (losses), net of restricted common share amounts	$ 277.6	$ (51.4)

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
	Mar. 31, 2019	Mar. 31, 2018
Performance Cycle 2016 to 2018
Accrued Expense
Percentage of target payout - minimum actual	139.00%
Percentage of target payout - maximum actual	166.00%
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	50,515
Shares paid or expired (in shares)	(13,715)	(23,186)
New grants (in shares)	13,700	14,105
Assumed forfeitures and cancellations	1	(818)
Ending of period (in shares)	40,602	40,616
Accrued Expense
Accrued expense at the beginning of period	$ 31.7	$ 45.8
Payments and deferrals	(18.1)	(28.4)
Assumed forfeitures and cancellations	0.2	0.3
Expense recognized	9.3	3.3
Accrued expense at the end of the period	23.1	$ 21
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 24.4

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	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	41,220
Accrued Expense	$ 23.4
Target Performance Shares Outstanding, Assumed forfeitures	(618)
Accrued Expense, Assumed forfeitures	(0.3)
Total Target Performance Shares Outstanding	40,602	40,616	40,616	50,515
Total Accrued Expense	$ 23.1	$ 31.7	$ 21	$ 45.8
Performance Cycle 2017 to 2019
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	14,070
Accrued Expense	$ 16.9
Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	13,450
Accrued Expense	$ 5.5
Performance Cycle 2019 to 2021
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	13,700
Accrued Expense	$ 1

Employee Share-Based Incentiv_5

Employee Share-Based Incentive Compensation Plans (Restricted Shares) (Details) - White Mountains - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	41,510	53,755
Issued (in shares)	13,700	14,105
Vested (in shares)	(13,715)	(25,381)
Assumed forfeitures and cancellations	0	(969)
Ending of period (in shares)	41,495	41,510
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 12.5	$ 14.3
Issued	12.8	11.4
Assumed forfeitures and cancellations	0	(0.2)
Expense recognized	(2.5)	(3.3)
Unamortized grant date fair value, ending balance	$ 22.8	$ 22.2
Restricted Share Cliff Vesting in January 2022
Target Restricted Share Activity
Issued (in shares)	13,700
Restricted Share Cliff Vesting in January 2021
Target Restricted Share Activity
Issued (in shares)		13,450
Restricted Share Cliff Vesting in January 2020
Target Restricted Share Activity
Issued (in shares)		290
Restricted Share Cliff Vesting in January 2019
Target Restricted Share Activity
Issued (in shares)		365

Leases - Narrative (Details)

Leases - Narrative (Details) - USD ($) $ in Millions	Mar. 31, 2019	Jan. 01, 2019
Lessee, Lease, Description [Line Items]
ROU lease asset	$ 21.5
Operating lease liability	21.8
Accounting Standards Update 2016-02
Lessee, Lease, Description [Line Items]
ROU lease asset	21.5	$ 23.2
Operating lease liability	$ 21.8	$ 23.2

Leases - Schedule of Lease Cost

Leases - Schedule of Lease Cost (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019USD ($)
Leases [Abstract]
Lease cost	$ 1.7
Less: sublease income	0.1
Net lease cost	$ 1.6

Leases - Schedule of Future Min

Leases - Schedule of Future Minimum Payments on Operating Leases (Details) $ in Millions	Mar. 31, 2019USD ($)
Leases [Abstract]
Remainder of 2019	$ 4.2
2020	4.6
2021	3.9
2022	3.7
2023	3.1
Thereafter	5.1
Total undiscounted lease payments	24.6
Less: present value adjustment	2.8
Operating lease liability	$ 21.8

Leases - Schedule of Right-of-U

Leases - Schedule of Right-of-Use Assets and Lease Liabilities (Details) $ in Millions	Mar. 31, 2019USD ($)
Lessee, Lease, Description [Line Items]
ROU lease asset	$ 21.5
Lease liability	$ 21.8
Operating lease, weighted average incremental borrowing rate, percent	4.70%
HG/BAM
Lessee, Lease, Description [Line Items]
ROU lease asset	$ 11.8
Lease liability	11.8
NSM Holding Co LLC
Lessee, Lease, Description [Line Items]
ROU lease asset	4.7
Lease liability	4.7
Other Operations
Lessee, Lease, Description [Line Items]
ROU lease asset	5
Lease liability	$ 5.3

Non-controlling Interests (Deta

Non-controlling Interests (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ (143.7)	$ (124.9)
Total Non-Controlling Excluding Reciprocals
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ 30.1	$ 45.7
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	3.10%	3.10%
Total non-controlling interests	$ 14.8	$ 14.5
NSM Holding Co LLC
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	4.70%	4.50%
Total non-controlling interests	$ 14.2	$ 13.6
Other NSM
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	13.40%	13.40%
Total non-controlling interests	$ 0.2	$ 0.3
MediaAlpha
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	0.00%	39.00%
Total non-controlling interests	$ 0	$ 16.2
Other Operations
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ 0.9	$ 1.1
Build America Mutual
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	$ (173.8)	$ (170.6)

Segment Information (Details)

Segment Information (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2019	Mar. 31, 2018
Segment reporting information
Net investment income		$ 16	$ 19.7
Net realized and unrealized investment gains (losses)		245.6	(53.7)
Total revenues		433.6	42.1
Amortization		6.6
Total expenses		151.9	108.1
Pre-tax income (loss)		281.7	(66)
HG Global-BAM
Segment reporting information
Earned insurance premiums		4.2	3
Net investment income		5.3	3.7
Net realized and unrealized investment gains (losses)		11.8	(7.9)
Other revenue		0.6	0.2
Insurance acquisition expenses		1.3	1.4
General and administrative expenses		14.4	11.9
NSM Holding Co LLC
Segment reporting information
Commission revenues		43.3	0
Other revenue		6	0
Total revenues		49.3	0
General and administrative expenses		25.1	0
Broker commission expense		15.7	0
Change in fair value of contingent consideration earnout liabilities		1.3	0
Amortization		5	0
Interest expense		3.7	0
Total expenses		50.8	0
MediaAlpha
Segment reporting information
Commission revenues		48.8	70.1
Other revenue		0	1.6
Cost of sales		40.6	57.4
General and administrative expenses		12.5	11.2
Amortization		1.6	2.9
Interest expense		0.2	$ 0.4
Percentage of advertising revenue			31.00%
Other Segment
Segment reporting information
Earned insurance premiums		0	$ 0
Net investment income		10.7	16
Net realized and unrealized investment gains (losses)		119.1	(45.8)
Commission revenues		1.3	0.9
Other revenue		0.3	0.3
Insurance acquisition expenses		0	0
Cost of sales		1.1	0.7
General and administrative expenses		29.4	22
Amortization		0	0
Interest expense		0	0.2
Total
Segment reporting information
Earned insurance premiums		4.2	3
Net investment income		16	19.7
Net realized and unrealized investment gains (losses)		130.9	(53.7)
Commission revenues		93.4	71
Other revenue		6.9	2.1
Total revenues		433.6	42.1
Insurance acquisition expenses		1.3	1.4
Cost of sales		41.7	58.1
General and administrative expenses		81.4	45.1
Broker commission expense		15.7
Change in fair value of contingent consideration earnout liabilities		1.3
Amortization			2.9
Interest expense		3.9	0.6
Total expenses		151.9	108.1
Pre-tax income (loss)		281.7	(66)
HG Global-BAM | HG Global-BAM
Segment reporting information
Commission revenues		0	0
Total revenues		21.9	(1)
Cost of sales		0	0
Broker commission expense		0
Change in fair value of contingent consideration earnout liabilities		0
Amortization		0
Interest expense		0	0
Total expenses		15.7	13.3
Pre-tax income (loss)		6.2	(14.3)
NSM Holding Co LLC | NSM Holding Co LLC
Segment reporting information
Earned insurance premiums		0
Net investment income		0
Net realized and unrealized investment gains (losses)		0
Total revenues		49.3
Insurance acquisition expenses		0
Cost of sales		0
Pre-tax income (loss)		(1.5)
MediaAlpha | MediaAlpha
Segment reporting information
Earned insurance premiums			0
Net investment income			0
Net realized and unrealized investment gains (losses)			0
Total revenues	$ 33.5	48.8	71.7
Broker commission expense		0
Change in fair value of contingent consideration earnout liabilities		0
Total expenses		54.9	71.9
Pre-tax income (loss)		(6.1)	(0.2)
Other Operations | Other Segment
Segment reporting information
Total revenues		313.6	(28.6)
Broker commission expense		0
Change in fair value of contingent consideration earnout liabilities		0
Total expenses		30.5	22.9
Pre-tax income (loss)		283.1	$ (51.5)
MediaAlpha | HG Global-BAM
Segment reporting information
Net realized and unrealized investment gains (losses)		0
MediaAlpha | Total
Segment reporting information
Net realized and unrealized investment gains (losses)		182.2
MediaAlpha | NSM Holding Co LLC | NSM Holding Co LLC
Segment reporting information
Net realized and unrealized investment gains (losses)		$ 0

Fair Value of Financial Instr_3

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	$ 195.4	$ 192.7
NSM Holding Co LLC
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	184.5	178.5
NSM Holding Co LLC | NSM Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	182.8	176.6
NSM Holding Co LLC | Carrying Value | NSM Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	182.8	176.6
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	182.9	176.1
MediaAlpha
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	0	14.2
MediaAlpha | MediaAlpha Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	0	14.2
MediaAlpha | Carrying Value | MediaAlpha Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	0	14.2
MediaAlpha | Estimate of Fair Value Measurement | MediaAlpha Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt instrument, fair value disclosure	0	14.6
Other Operations | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	10.9	0
Other Operations | Estimate of Fair Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt instrument, fair value disclosure	$ 10.9	$ 0

Commitments and Contingencies (

Commitments and Contingencies (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
American International Group | NSM Insurance HoldCo, LLC
Loss Contingencies [Line Items]
Contingent liability	$ 82.5
Sirius Group | Swedish Tax Authority | Sirius Group
Loss Contingencies [Line Items]
Contingent liability	16.6		$ 17.3
Change in contingent liability	$ (0.7)
Scenario, Forecast | American International Group | NSM Insurance HoldCo, LLC
Loss Contingencies [Line Items]
Contingent liability		$ 82.5

Held for Sale and Discontinue_3

Held for Sale and Discontinued Operations - (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Discontinued Operations [Line Items}
Gain (loss) from sale of discontinued operations, net of tax	$ 0.7	$ 0.1
Connecticut | Discontinued Operations, Held-for-sale
Discontinued Operations [Line Items}
Assets held for sale	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		$ 0.1
Sirius Group | Swedish Tax Authority | Sirius Group
Discontinued Operations [Line Items}
Contingent liability	16.6		$ 17.3
Change in contingent liability	$ (0.7)

Held for Sale and Discontinue_4

Held for Sale and Discontinued Operations - Net Change Cash From Discontinued Operations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from operations - discontinued operations	$ 0	$ 0.1
Net cash provided from investing activities - discontinued operations	0	(0.1)
Net cash used for financing activities - discontinued operations	0	0
Cash balances at beginning of period	110.3	97.1
Cash balances at end of period	132.2	120.2
Net income tax payment to national governments	0	(0.2)
Discontinued Operations
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net change in cash during the period		0
Cash balances at beginning of period	50	0
Cash balances at end of period	$ 60.8	0
Interest paid		0
Net income tax payment to national governments		0
Discontinued Operations, Held-for-sale
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Cash balances at beginning of period		0
Cash balances at end of period		$ 0

Held for Sale and Discontinue_5

Held for Sale and Discontinued Operations - EPS (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 284.4	$ (48)
Less: total income (loss) from continuing operations, net of tax	283.7	(48.1)
Net income from discontinued operations attributable to White Mountains’s common shareholders	0.7	0.1
Basic and diluted earnings (losses) per share numerators	$ 280.8	$ (47.7)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,172,200	3,746,100
Average unvested restricted common shares	(32,500)	(35,900)
Basic earnings (losses) per share denominator	3,139,700	3,710,200
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,172,200	3,746,100
Average unvested restricted common shares	(32,500)	(35,900)
Diluted earnings per share denominator	3,139,700	3,710,200
Basic and diluted earnings (losses) per share	$ 89.42	$ (12.85)
Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 284.4	$ (48)
Less: total income (loss) from continuing operations, net of tax	283.7	(48.1)
Net income from discontinued operations attributable to White Mountains’s common shareholders	0.7	0.1
Allocation of income for participating unvested restricted common shares	0	0
Basic and diluted earnings (losses) per share numerators	$ 0.7	$ 0.1
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,172,200	3,746,100
Average unvested restricted common shares	(32,500)	(35,900)
Basic earnings (losses) per share denominator	3,139,700	3,710,200
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,172,200	3,746,100
Average unvested restricted common shares	(32,500)	(35,900)
Diluted earnings per share denominator	3,139,700	3,710,200
Basic and diluted earnings (losses) per share	$ 0.22	$ 0.03

Subsequent Events (Details)

Subsequent Events (Details) - USD ($) $ in Millions	Apr. 01, 2019	Apr. 04, 2019	Mar. 31, 2019	Dec. 31, 2018	Feb. 05, 2018
Kudu Investment Management, LLC
Subsequent Event [Line Items]
Equity method investment, aggregate cost					$ 125
Ownership interest (as a percent)			49.50%	49.50%
Fully diluted ownership interest (as a percent)				42.70%
Kudu Investment Management, LLC | Subsequent Event
Subsequent Event [Line Items]
Equity method investment, aggregate cost		$ 81.4
Ownership interest (as a percent)		99.10%
Fully diluted ownership interest (as a percent)		85.40%
Unfunded capital commitment		$ 104.7
Embrace Pet Insurance | NSM Holding Co LLC | Subsequent Event
Subsequent Event [Line Items]
Business acquisition, percentage acquired	100.00%
Cash consideration	$ 73