White Mountains Insurance (WTM) 10-Q 6 Nov 19 2019 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2019	Nov. 05, 2019
Cover page.
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2019
Document Transition Report	false
Entity File Number	1-8993
Entity Registrant Name	WHITE MOUNTAINS INSURANCE GROUP, LTD
Entity Incorporation, State or Country Code	D0
Entity Tax Identification Number	94-2708455
Entity Address, Address Line One	80 South Main Street,
Entity Address, Postal Zip Code	03755-2053
Entity Address, City or Town	Hanover,
Entity Address, State or Province	NH
City Area Code	603
Local Phone Number	640-2200
Title of 12(b) Security	Common Shares, par value $1.00
Trading Symbol	WTM
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		3,185,353
Entity Central Index Key	0000776867
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Assets
Goodwill and other intangible assets	$ 660.9	$ 537.5
Total assets	3,836.3	3,362.6
Liabilities
Debt	230.2	192.7
Total liabilities	765.4	644.4
White Mountains’s common shareholders’ equity
White Mountains’s common shares at $1 par value per share—authorized 50,000,000 shares; issued and outstanding 3,185,353 and 3,173,115 shares	3.2	3.2
Paid-in surplus	590	580.8
Retained earnings	2,611.5	2,264.9
Accumulated other comprehensive loss, after tax:
Net unrealized foreign currency translation and interest rate swap losses	(11.3)	(5.8)
Total White Mountains’s common shareholders’ equity	3,193.4	2,843.1
Non-controlling interests
Non-controlling interests	(122.5)	(124.9)
Total equity	3,070.9	2,718.2
Total liabilities and equity	3,836.3	3,362.6
HG Global-BAM
Assets
Fixed maturity investments, at fair value	794.5	701.4
Short-term investments, at fair value	57.1	66.9
Total investments	851.6	768.3
Cash	11.3	12.5
Insurance premiums receivable	6.7	6.4
Deferred acquisition costs	21.4	19
Accrued investment income	5.5	4.9
Other assets	16.4	5.1
Total assets	912.9	816.2
Liabilities
Unearned insurance premiums	203.5	176
Accrued incentive compensation	19	20.4
Accounts payable on unsettled investment purchases	3.6	2.2
Other liabilities	27.4	13.9
Total liabilities	253.5	212.5
NSM Holding Co LLC
Assets
Short-term investments, at fair value	0	1.7
Cash	105.7	66.2
Insurance premiums receivable	53.6	44
Goodwill and other intangible assets	630.6	486.2
Other assets	36.8	28.9
Total assets	826.7	627
Liabilities
Debt	219.4	178.5
Premiums payable	92.6	77.2
Contingent consideration earnout liabilities	28.7	20.2
Other liabilities	60.1	38.9
Total liabilities	400.8	314.8
Non-controlling interests
Non-controlling interests	15.2	13.6
Kudu Investment Management, LLC
Assets
Short-term investments, at fair value	0.1	0
Other long-term investments	219.3	0
Cash	4.9	0
Accrued investment income	4.7	0
Goodwill and other intangible assets	9.6	0
Other assets	3.7	0
Total assets	242.3	0
Liabilities
Other liabilities	5.3	0
Total liabilities	5.3	0
Non-controlling interests
Non-controlling interests	2.3	0
MediaAlpha
Assets
Cash	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	371.5	376.1
Short-term investments, at fair value	215.3	145.6
Common equity securities, at fair value	629.7	925.6
Other long-term investments	554.4	325.6
Total investments	1,770.9	1,772.9
Cash	27.3	25.9
Accrued investment income	5.1	5.5
Accounts receivable on unsettled investment sales	0.3	0
Goodwill and other intangible assets	20.7	7.9
Other assets	27.3	15.5
Assets held for sale	2.8	3.3
Total assets	1,854.4	1,831
Liabilities
Debt	10.8	0
Accrued incentive compensation	44.9	38.9
Accounts payable on unsettled investment purchases	0	5
Other liabilities	50.1	26.3
Total liabilities	$ 105.8	$ 70.2

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Cash	$ 67	$ 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	3,185,353	3,173,115
Common shares, outstanding shares	3,185,353	3,173,115

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenues:
Net investment income	$ 18.4	$ 13.4	$ 55.4	$ 44.9
Net realized and unrealized investment gains (losses)			347.8	22.9
Total revenues	155.5	198.7	718.1	363.1
Expenses:
Total expenses	104.4	154.4	369.5	397.2
Pre-Tax and Net Income [Abstract]
Pre-tax income (loss) from continuing operations	51.1	44.3	348.6	(34.1)
Income (expense) tax benefit	(8.8)	3.6	(18.9)	0.4
Net income (loss) from continuing operations	42.3	47.9	329.7	(33.7)
Net gain from sale of discontinued operations, net of tax	0.9	(17.3)	1.6	(17.2)
Net income (loss)	43.2	30.6	331.3	(50.9)
Net loss attributable to non-controlling interests	5.5	10.2	22.3	47.2
Net income (loss) attributable to White Mountains’s common shareholders	48.7	40.8	353.6	(3.7)
HG Global-BAM
Revenues:
Earned insurance premiums	5.2	3.3	13.7	9.7
Net investment income	5.4	4.8	16.1	12.5
Net realized and unrealized investment gains (losses)	7.1	(4.1)	30.4	(14.4)
Commission revenues	0	0	0	0
Other revenue	0.3	0.2	1.3	0.8
Total revenues	18	4.2	61.5	8.6
Expenses:
Deferred Policy Acquisition Costs, Amortization Expense	1.4	1.2	4.1	3.9
Other underwriting expenses	0.1	0.1	0.3	0.3
General and administrative expenses	12.5	11.3	39.1	36.5
Cost of sales	0	0	0	0
Amortization of other intangible assets	0	0	0	0
Interest expense	0	0	0	0
Total expenses	14	12.6	43.5	40.7
Pre-Tax and Net Income [Abstract]
Pre-tax income (loss) from continuing operations	4	(8.4)	18	(32.1)
NSM Holding Co LLC
Revenues:
Earned insurance premiums	0		0
Net investment income	0		0
Net realized and unrealized investment gains (losses)	0		0
Commission revenues	49.4	35	147.6	56.4
Other revenue	10.5	4.7	27.7	6.8
Total revenues	59.9	39.7	175.3	63.2
Expenses:
Deferred Policy Acquisition Costs, Amortization Expense	0		0
Other underwriting expenses	0		0
General and administrative expenses	33.3	23.5	91.4	35.7
Broker commission expense	15.3	10.6	48.9	17.1
Change in fair value of contingent consideration earnout liabilities	(2)	2.6	5.6	2.6
Cost of sales	0		0
Amortization of other intangible assets	4.4	5	13.5	5
Interest expense	4.4	3.2	12.2	4.8
Total expenses	55.4	44.9	171.6	65.2
Pre-Tax and Net Income [Abstract]
Pre-tax income (loss) from continuing operations	4.5	(5.2)	3.7	(2)
Kudu Investment Management, LLC
Revenues:
Earned insurance premiums	0
Net investment income	4.6	0	8.6	0
Net realized and unrealized investment gains (losses)	2.7	0	3.1	0
Commission revenues	0
Other revenue	0.2		0.2	0
Total revenues	7.5	0	11.9	0
Expenses:
Deferred Policy Acquisition Costs, Amortization Expense	0
Other underwriting expenses	0
General and administrative expenses	4.5	0	6.7	0
Cost of sales	0
Amortization of other intangible assets	0.2	0	0.2	0
Total expenses	4.7	0	6.9	0
Pre-Tax and Net Income [Abstract]
Pre-tax income (loss) from continuing operations	2.8		5
MediaAlpha
Revenues:
Earned insurance premiums		0	0	0
Net investment income		0	0	0
Net realized and unrealized investment gains (losses)		0	0	0
Commission revenues	0	74.5	48.8	216.4
Other revenue	0	0	0	1.6
Total revenues	0	74.5	48.8	218
Expenses:
Deferred Policy Acquisition Costs, Amortization Expense		0	0	0
Other underwriting expenses		0	0	0
General and administrative expenses	0	5.4	12.5	21.2
Cost of sales	0	61.8	40.6	179.1
Amortization of other intangible assets	0	2.4	1.6	7.8
Interest expense	0	0.2	0.2	0.9
Total expenses	0	69.8	54.9	209
Pre-Tax and Net Income [Abstract]
Pre-tax income (loss) from continuing operations		4.7	(6.1)	9
Other Segment
Revenues:
Earned insurance premiums	0	0	0	0
Net investment income	8.4	8.6	30.7	32.4
Net realized and unrealized investment gains (losses)	57.2	70.2	199.6	37.3
Realized gain and unrealized investment gain from the MediaAlpha Transaction			182.2	0
Commission revenues	1.9	1.1	4.5	3
Other revenue	2.6	0.4	3.6	0.6
Total revenues	70.1	80.3	420.6	73.3
Expenses:
Deferred Policy Acquisition Costs, Amortization Expense	0		0	0
Other underwriting expenses	0		0	0
General and administrative expenses	27.7	26	87.2	79
Cost of sales	2.3	1.1	5	2.9
Amortization of other intangible assets	0	0	0.1	0.1
Interest expense	0.3	0	0.3	0.3
Total expenses	30.3	27.1	92.6	82.3
Pre-Tax and Net Income [Abstract]
Pre-tax income (loss) from continuing operations	39.8	53.2	328	(9)
Discontinued Operations
Pre-Tax and Net Income [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ 48.7	$ 40.8	353.6	(3.7)
Swedish Tax Authority | Discontinued Operations | Sirius Group
Pre-Tax and Net Income [Abstract]
Net gain from sale of discontinued operations, net of tax				$ 0.1
MediaAlpha | HG Global-BAM
Revenues:
Realized gain and unrealized investment gain from the MediaAlpha Transaction			0
MediaAlpha | NSM Holding Co LLC
Revenues:
Realized gain and unrealized investment gain from the MediaAlpha Transaction			$ 0

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ 48.7	$ 40.8	$ 353.6	$ (3.7)
Other comprehensive loss, net of tax	(2.4)	(0.8)	(5.8)	(1.7)
Comprehensive income (loss)	46.3	40	347.8	(5.4)
Comprehensive loss attributable to non-controlling interests	0.1	0.2	0.3	0.2
Comprehensive income (loss) attributable to White Mountains’s common shareholders	$ 46.4	$ 40.2	$ 348.1	$ (5.2)
Basic earnings (loss) per share
Continuing operations (in usd per share)	$ 15.01	$ 18.27	$ 110.67	$ 3.90
Discontinued operations (in usd per share)	0.28	(5.44)	0.50	(4.98)
Total consolidated operations (in usd per share)	15.29	12.83	111.17	(1.08)
Diluted earnings (loss) per share
Continuing operations (in usd per share)	15.01	18.27	110.67	3.90
Discontinued operations (in usd per share)	0.28	(5.44)	0.50	(4.98)
Total consolidated operations (in usd per share)	15.29	12.83	111.17	(1.08)
Dividends declared per White Mountains’s common share	$ 0	$ 0	$ 1	$ 1

CONSOLIDATED STATEMENTS OF CHAN

CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (Unaudited) - USD ($) shares in Millions, $ 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)	$ (50.9)		(3.7)		(3.7)	(47.2)	(50.9)
Net change in foreign currency translation and other				(1.5)				$ (1.5)	$ (1.5)	$ 0	$ (1.5)
Total comprehensive income (loss)	(5.4)		(3.7)		(5.2)	(47.2)	(52.4)
Dividends declared on common shares			(3.8)		(3.8)		(3.8)
Dividends to non-controlling interests						(6)	(6)
Repurchases and retirements of common shares		$ (104.6)	(408.4)		(513)		(513)
Issuance of common shares (shares)		2
Issuance of common shares					2		2
Capital contributions from BAM members, net of tax												$ 22.9	$ 22.9
Amortization of restricted share awards		$ 9.7			9.7		9.7
Recognition of equity-based units of subsidiary		4.5			4.5	2.3	6.8
Dilution from equity-based units of subsidiary		(1.1)			(1.1)	1.1	0
Acquisition of subsidiary		(1.6)			(1.6)		(1.6)
Acquisition of non-controlling interests						14.2	14.2
Ending Balances at Sep. 30, 2018		579.5	2,407.3	(2.8)	2,984	(144.4)	2,839.6
Beginning Balances at Jun. 30, 2018		$ 575.8	2,366.5	(2.2)	2,940.1	(142.6)	2,797.5
Increase (Decrease) in Shareholders' Equity
Net income (loss)	30.6		40.8		40.8	(10.2)	30.6
Net change in foreign currency translation and other				(0.6)				(0.6)	(0.6)	0	(0.6)
Total comprehensive income (loss)	40		40.8		$ 40.2	(10.2)	30
Dividends to non-controlling interests						$ (1.9)	$ (1.9)
Issuance of common shares (shares)		0.2			0.2	0	0.2
Capital contributions from BAM members, net of tax												10.3	10.3
Amortization of restricted share awards		$ 3.3			$ 3.3		$ 3.3
Recognition of equity-based units of subsidiary		0.2			0.2	$ 0	0.2
Ending Balances at Sep. 30, 2018		579.5	2,407.3	(2.8)	2,984	(144.4)	2,839.6
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)	331.3		353.6		353.6	(22.3)	331.3
Net change in foreign currency translation and other								(5.5)	(5.5)	(0.3)	(5.8)
Total comprehensive income (loss)	347.8		353.6	(5.5)	348.1	(22.6)	325.5
Dividends declared on common shares			(3.2)		(3.2)		(3.2)
Dividends to non-controlling interests						(1.6)	(1.6)
Repurchases and retirements of common shares		(1.1)	(3.8)		(4.9)		(4.9)
Issuance of common shares		2.2			2.2		2.2
Capital contributions from BAM members, net of tax												36.3	36.3
Net contributions from non-controlling interests						2.3	2.3
Amortization of restricted share awards		7.8			7.8		7.8
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						1.8	1.8
Ending Balances at Sep. 30, 2019	3,070.9	593.2	2,611.5	(11.3)	3,193.4	(122.5)	3,070.9
Beginning Balances at Jun. 30, 2019		590.5	2,562.8	(9)	3,144.3	(136.4)	3,007.9
Increase (Decrease) in Shareholders' Equity
Net income (loss)	43.2		48.7		48.7	(5.5)	43.2
Net change in foreign currency translation and other								$ (2.3)	$ (2.3)	$ (0.1)	$ (2.4)
Total comprehensive income (loss)	46.3		48.7	(2.3)	46.4	(5.6)	40.8
Dividends to non-controlling interests						(0.6)	(0.6)
Capital contributions from BAM members, net of tax												$ 19.3	$ 19.3
Net contributions from non-controlling interests						0.8	0.8
Amortization of restricted share awards		2.7			2.7		2.7
Ending Balances at Sep. 30, 2019	$ 3,070.9	$ 593.2	$ 2,611.5	$ (11.3)	$ 3,193.4	$ (122.5)	$ 3,070.9

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operations:
Net income (loss)	$ 331.3	$ (50.9)
Adjustments to reconcile net income to net cash used for operations:
Net realized and unrealized investment gains	(347.8)	(22.9)
Realized gain from the MediaAlpha Transaction	(67.5)	0
Deferred income expense (benefit)	11.4	(4.7)
Net (gain) loss from sale of discontinued operations, net of tax	(1.6)	17.2
Amortization of restricted share and option awards	8.3	9.7
Amortization and depreciation	19.8	18.4
Other operating items:
Net change in unearned insurance premiums	27.5	19.4
Net change in deferred acquisition costs	(2.4)	(2.8)
Net change in restricted cash	17	(12.1)
Investments in asset management firms - Kudu	(74.4)	0
Net change in other assets and liabilities, net	5.9	(1.8)
Net cash used for operations	(72.5)	(30.5)
Cash flows from investing activities:
Net change in short-term investments	(54.5)	(138.5)
Sales of fixed maturity and convertible investments	292.6	1,768.5
Maturities, calls and paydowns of fixed maturity and convertible investments	92.7	124.4
Sales of common equity securities	442.1	129.8
Distributions and redemptions of other long-term investments and settlements of forward contracts	28.7	(5.5)
Purchases of other long-term investments	(173)	(65.9)
Purchases of common equity securities	(4.8)	(253)
Purchases of fixed maturity and convertible investments	(428)	(833.1)
Purchases of consolidated subsidiaries, net of cash acquired of $12.8 and $71.6	(243.4)	(252.1)
Other investing activities, net	(12.9)	33.8
Net cash (used for) provided from investing activities	(60.5)	508.4
Cash flows from financing activities:
Draw down of debt and revolving line of credit	149.4	50.9
Repayment of debt and revolving line of credit	(22.2)	(11.2)
Cash dividends paid to the Company’s common shareholders	(3.2)	(3.8)
Common shares repurchased	0	(504.7)
Distribution to non-controlling interest shareholders	(27.7)	(4.6)
Payments on contingent consideration earnout liability	(2.6)	(2.6)
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	1.6	0
Capital contributions from BAM members	42.9	26.9
Acquisition of additional subsidiary shares from non-controlling interest	0	(1.7)
Fidus Re premium payment	(2.2)	(3)
Restricted share statutory withholding tax payments	(4.9)	(8.4)
Net cash provided from (used for) financing activities	172.7	(462.2)
Effect of exchange rate changes on cash	(0.8)	(0.6)
Net change in cash during the period - continuing operations, including the effect of exchange rate changes	38.9	15.1
Cash balances at beginning of period	110.3	97.1
Cash balances at end of period	149.2	112.2
Supplemental cash flows information:
Interest paid	(11.7)	(5.3)
Net income tax refunds	$ (0.3)	$ (0.5)

CONSOLIDATED STATEMENTS OF CA_2

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Cash balance	$ 149.2	$ 112.2	$ 110.3	$ 97.1
Cash acquired from acquisition	12.8	17.6
Discontinued Operations
Cash balance	$ 67	$ 41.2	$ 50	$ 0

Basis of Presentation and Signi

Basis of Presentation and Significant Accouting Policies	9 Months Ended
	Sep. 30, 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 Company’s website is located at www.whitemountains.com. The information contained on White Mountains’s website is not incorporated by reference into, and is not a part of, this report. The accompanying consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) and include the accounts of 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. 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 up to one year from the acquisition date. Measurement period adjustments are recognized in the period in which they are determined. The results of operations and cash flows of businesses acquired are included in the consolidated financial statements from the date of acquisition. White Mountains accounts for purchases of other intangible assets that do not meet the definition of a business as asset acquisitions. Asset acquisitions are recognized at the amount of consideration paid, which is deemed to equal fair value. 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, Kudu, 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 September 30, 2019 , $481.3 million of the surplus notes remain outstanding. As of September 30, 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, real estate, social services and pet health insurance. 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 September 30, 2019 and December 31, 2018, White Mountains owned 96.4% and 95.5% of the basic units outstanding of NSM ( 88.4% and 85.0% on a fully diluted, fully converted basis). See Note 2 — “Significant Transactions ”. The Kudu segment consists of Kudu Investment Management, LLC (“Kudu”), a capital provider to global asset management and wealth management firms. Kudu specializes in providing capital solutions to asset managers and registered investment advisers for purposes including generational ownership transfers, management buyouts, acquisition and growth finance and legacy partner liquidity. Kudu also provides strategic assistance to investees from time to time. Kudu’s capital solutions typically are structured as long-term or permanent revenue shares that generate cash yields and have additional upside equity participation. On April 4, 2019, White Mountains completed its acquisition of the ownership interests in Kudu (the “Kudu Transaction”) held by certain funds managed by Oaktree Capital Management, L.P. (“Oaktree”). In addition, White Mountains assumed all of Oaktree’s unfunded capital commitments to Kudu, increasing White Mountains’s total capital commitment to $250 million , of which $28.1 million is unfunded as of September 30, 2019. As a result of the Kudu Transaction, White Mountains’s basic unit ownership of Kudu increased from 49.5% to 99.1% ( 42.7% to 85.4% on a fully diluted, fully converted basis), and White Mountains began consolidating Kudu in its financial statements during the second quarter of 2019. The MediaAlpha segment consisted of QL Holdings LLC and its wholly-owned subsidiary QuoteLab, LLC (collectively “MediaAlpha”). MediaAlpha is a marketing technology company that enables the programmatic buying and selling of vertical-specific, performance-based media between advertisers (buyers of advertising inventory) and publishers (sellers of advertising inventory). MediaAlpha operates in insurance and non-insurance verticals and uses 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 media to advertisers through transparent, programmatic, auction-based marketplaces. 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 deconsolidated MediaAlpha as a result of the MediaAlpha Transaction. White Mountains’s consolidated statement of comprehensive income and its segment disclosures include MediaAlpha’s results of operations through the date of the MediaAlpha Transaction. 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”), Kudu (for periods prior to the Kudu Transaction), MediaAlpha (for periods after the MediaAlpha Transaction), and Elementum Holdings LP (“Elementum”) and 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 September 30, 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 may also hold foreign currency forward contracts for risk management purposes that are not designated as hedges. In these circumstances, these contracts are 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 that 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 Notes to Consolidated Financial Statements in 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, White Mountains recognized $23.2 million for both the lease right-of-use assets and lease liabilities. As of September 30, 2019, White Mountains recognized $23.0 million and $ 23.3 million of lease right-of-use assets and 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 (ASC 326), which establishes new guidance for the recognition of credit losses for financial assets measured at amortized cost. The new ASU requires reporting entities to estimate the credit losses expected over the life of a credit exposure using historical information, current information and reasonable and supportable forecasts that affect the collectability of the financial asset. This differs from current GAAP, which delays recognition until it is probable a loss has been incurred. The new guidance is expected to accelerate recognition of credit losses. The types of assets within the scope of the new guidance include premium receivables, reinsurance recoverables and loans. ASU 2016-13 is effective for annual periods beginning after January 1, 2020, including interim periods. White Mountains measures its portfolio of investment securities at fair value with changes therein recognized through current period earnings and, accordingly, does not expect adoption to have any effect on its financial statements.

Significant Transactions

Significant Transactions	9 Months Ended
	Sep. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Significant Transactions	Significant Transactions Acquisitions Elementum On May 31, 2019, White Mountains acquired a 30.0% limited partnership interest in Elementum for $55.1 million (the “Elementum Transaction”). Elementum manages portfolios across a range of liquidity and risk/return profiles, including catastrophe bonds and collateralized reinsurance investments. As part of the Elementum Transaction, White Mountains also committed to invest $50.0 million in insurance-linked securities funds managed by Elementum. As of September 30, 2019, White Mountains has invested $40.0 million into three Elementun-managed funds. White Mountains has elected to use the fair value option for its investment in Elementum. Both the investment in Elementum and the investments in insurance-linked securities funds managed by Elementum are included within other long-term investments. 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 as part of 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 . White Mountains contributed $29.0 million and NSM borrowed $30.1 million 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 finalized 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. During the three months ended March 31, 2019, White Mountains determined that the relative values of goodwill and other intangible assets recorded in connection with the KBK transaction were $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. On April 1, 2019, NSM acquired 100% of Embrace Pet Insurance (“Embrace”), a nationwide provider of pet health insurance for dogs and cats, for upfront cash consideration of $71.5 million , net of cash acquired. White Mountains contributed $58.2 million to NSM and NSM borrowed $20.4 million as part of the transaction. White Mountains recognized $70.6 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 finalized as of September 30, 2019. On June 28, 2019, NSM acquired the renewal rights on its U.S. collector car business (the “Renewal Rights”) from American International Group, Inc. (“AIG”) for $82.5 million . The acquisition satisfied NSM’s obligation to acquire the Renewal Rights from AIG. See Note 16 — “Commitments and Contingencies” . White Mountains contributed $59.1 million to NSM and NSM borrowed $22.5 million as part of the transaction. White Mountains recognized $82.5 million of other intangible assets, reflecting acquisition date fair values. See Note 4 — “Goodwill and Other Intangible Assets” . The contingent consideration earnout liabilities related to the NSM, Fresh Insurance and KBK acquisitions are subject to adjustments based upon EBITDA, EBITDA projections, and present value factors for acquired entities. For the three and nine months ended September 30, 2019 , NSM recognized pre-tax (income) expense of $(2.0) million and $5.6 million for the change in the fair value of its contingent consideration earnout liabilities. For the three months ended September 30, 2019, the change was driven by a reduction in the earnout liability related to Fresh Insurance and NSM’s other U.K.-based operations. For the nine months ended September 30, 2019, the change was driven by an increase in the earnout liability related to KBK, partially offset by a reduction in the earnout liability related to Fresh Insurance and its other U.K.-based operations. For both the three months ended September 30, 2018 and the period from May 11, 2018 through September 30, 2018, NSM recognized pre-tax expense of $2.6 million for the change in the fair value of its contingent consideration earnout liabilities. The change was driven by an increase on the earnout liability related to Fresh Insurance and NSM’s other U.K.-based operations. Any future adjustments to contingent consideration earnout liabilities under the agreements will also be recognized through pre-tax income. As of September 30, 2019 and December 31, 2018, NSM recorded total contingent consideration earnout liabilities of $28.7 million and $20.2 million . For the nine months ended September 30, 2019, NSM paid $2.6 million related to its U.K.-based operations’ contingent consideration earnout liabilities. 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. 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). On April 4, 2019, White Mountains completed the Kudu Transaction for cash consideration of $81.4 million . In addition, White Mountains assumed all of Oaktree’s unfunded capital commitments to Kudu, increasing White Mountains’s total capital commitment to $250.0 million , of which $28.1 million was unfunded as of September 30, 2019. As a result of the Kudu Transaction, White Mountains’s basic unit ownership of Kudu increased from 49.5% to 99.1% ( 42.7% to 85.4% on a fully diluted, fully converted basis), and White Mountains began consolidating Kudu in its financial statements during the second quarter of 2019. White Mountains’s consolidated financial statements and its segment disclosures include Kudu’s results for the period from April 4, 2019 to September 30, 2019. For periods prior to the Kudu Transaction, White Mountains determined that Kudu was a VIE, but White Mountains was not the primary beneficiary. In those periods, White Mountains elected to use the fair value option. Kudu closed three transactions in 2018 deploying $63.0 million , including transaction costs, of which $31.5 million was funded by White Mountains. As of September 30, 2019, Kudu has deployed a total of $221.9 million , including transaction costs, in eight investment management firms with combined assets under management of over $30 billion . Dispositions MediaAlpha On February 26, 2019, MediaAlpha completed the MediaAlpha Transaction. White Mountains received net cash proceeds of $89.3 million from the MediaAlpha Transaction. 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 initially 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 million of realized gain and unrealized investment gain on the MediaAlpha Transaction.

Investment Securities

Investment Securities	9 Months Ended
	Sep. 30, 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 substantially all classified as trading securities. Trading securities are reported at fair value as of the balance sheet date. Net realized and unrealized investment gains (losses) on trading securities are reported in pre-tax revenues. White Mountains’s fixed maturity investments are generally valued using industry standard pricing methodologies. Key inputs include benchmark yields, benchmark securities, reported trades, issuer spreads, bids, offers, credit ratings and prepayment speeds. Income on mortgage and asset-backed securities is recognized using an effective yield based on anticipated prepayments and the estimated economic life of the securities. When actual prepayments differ significantly from anticipated prepayments, the estimated economic life is recalculated and the remaining unamortized premium or discount is amortized prospectively over the remaining economic life. Realized investment gains (losses) resulting from sales of investment securities are accounted for using the specific identification method. Premiums and discounts on all fixed maturity investments are amortized or accreted to income over the anticipated life of the investment. Short-term investments consist of interest-bearing money market funds, certificates of deposit and other securities, which at the time of purchase, mature or become available for use within one year. Short-term investments are carried at amortized or accreted cost, which approximated fair value as of September 30, 2019 and December 31, 2018 . Other long-term investments consist primarily of unconsolidated entities, Kudu’s investments (non-controlling equity interests in the form of revenue and earnings participations), insurance-linked securities (“ILS”) funds, 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 and nine months ended September 30, 2019 and 2018 : Three Months Ended Nine Months Ended September 30, September 30, Millions 2019 2018 2019 2018 Fixed maturity investments $ 8.0 $ 7.5 $ 24.4 $ 27.3 Short-term investments 1.3 1.8 3.9 6.5 Common equity securities 2.5 3.4 10.8 11.2 Other long-term investments 7.0 1.2 17.4 1.7 Total investment income 18.8 13.9 56.5 46.7 Third-party investment expenses (.4 ) (.5 ) (1.1 ) (1.8 ) Net investment income, pre-tax $ 18.4 $ 13.4 $ 55.4 $ 44.9 Net Realized and Unrealized Investment Gains (Losses) The following table presents net realized and unrealized investment gains (losses) for the three and nine months ended September 30, 2019 and 2018 : Three Months Ended Nine Months Ended September 30, September 30, Millions 2019 2018 2019 2018 Net realized investment gains (losses), pre-tax (1) $ 19.6 $ (10.2 ) $ 90.8 $ (13.9 ) Net unrealized investment gains, pre-tax (2) 47.4 76.3 257.0 36.8 Net realized and unrealized investment gains, pre-tax 67.0 66.1 347.8 22.9 Income tax expense attributable to net realized and unrealized investment gains (13.2 ) (9.7 ) (69.5 ) (5.9 ) Net realized and unrealized investment gains, after-tax $ 53.8 $ 56.4 $ 278.3 $ 17.0 (1) Excludes $67.5 realized gain associated with the MediaAlpha Transaction, which is recorded in a separate line item in the statement of operations titled realized gain and unrealized investment gain from the MediaAlpha Transaction. See Note 2 — “Significant Transactions.” (2) Includes $114.7 unrealized investment gain associated with the MediaAlpha Transaction, which is recorded in a separate line item in the statement of operations titled realized gain and unrealized investment gain from the MediaAlpha Transaction. See Note 2 — “Significant Transactions.” For the three and nine months ended September 30, 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 tables present net realized investment gains (losses) for the three and nine months ended September 30, 2019 and 2018 : Three Months Ended Three Months Ended September 30, 2019 September 30, 2018 Millions Net Net Total Net Realized Net Net Total Net Realized Fixed maturity investments $ 1.4 $ — $ 1.4 $ (5.1 ) $ — $ (5.1 ) Common equity securities 14.0 — 14.0 (1.5 ) — (1.5 ) Other long-term investments 4.2 — 4.2 (3.6 ) — (3.6 ) Net realized investment gains (losses), pre-tax 19.6 — 19.6 (10.2 ) — (10.2 ) Income tax (expense) benefit attributable to net realized investment gains (losses) (.2 ) — (.2 ) 2.3 — 2.3 Net realized investment gains (losses), after-tax $ 19.4 $ — $ 19.4 $ (7.9 ) $ — $ (7.9 ) Nine Months Ended Nine Months Ended September 30, 2019 September 30, 2018 Millions Net Net Total Net Realized Net Net Total Net Realized Fixed maturity investments $ 3.5 $ — $ 3.5 $ (27.6 ) $ 18.2 $ (9.4 ) Short-term investments .1 — .1 (.8 ) — (.8 ) Common equity securities 81.4 — 81.4 10.5 — 10.5 Other long-term investments (1) 5.8 — 5.8 (7.0 ) (7.2 ) (14.2 ) Net realized investment gains (losses), pre-tax 90.8 — 90.8 (24.9 ) 11.0 (13.9 ) Income tax (expense) benefit attributable to net realized investment gains (losses) (9.0 ) — (9.0 ) 11.2 — 11.2 Net realized investment gains (losses), after-tax $ 81.8 $ — $ 81.8 $ (13.7 ) $ 11.0 $ (2.7 ) (1) Excludes $67.5 realized gain associated with the MediaAlpha Transaction, which is recorded in a separate line item in the statement of operations titled realized gain and unrealized investment gain from the MediaAlpha Transaction. See Note 2 — “Significant Transactions.” Net Unrealized Investment Gains (Losses) The following tables present net unrealized investment gains (losses) and changes in the carrying value of investments measured at fair value for the three and nine months ended September 30, 2019 and 2018 : Three Months Ended Three Months Ended September 30, 2019 September 30, 2018 Millions Net Unrealized Gains (Losses) Net Total Net Unrealized Gains (Losses) Reflected in Earnings Net Unrealized Gains (Losses) Net Total Net Unrealized Gains (Losses) Reflected in Earnings Fixed maturity investments $ 7.5 $ — $ 7.5 $ 1.5 $ — $ 1.5 Short-term investments — — — (.1 ) — (.1 ) Common equity securities (11.6 ) — (11.6 ) 54.9 — 54.9 Other long-term investments 52.3 (.8 ) 51.5 20.1 (.1 ) 20.0 Net unrealized investment gains (losses), pre-tax 48.2 (.8 ) 47.4 76.4 (.1 ) 76.3 Income tax expense attributable to net unrealized investment gains (13.0 ) — (13.0 ) (12.0 ) — (12.0 ) Net unrealized investment gains (losses), after-tax $ 35.2 $ (.8 ) $ 34.4 $ 64.4 $ (.1 ) $ 64.3 Nine Months Ended Nine Months Ended September 30, 2019 September 30, 2018 Millions Net Unrealized Gains Net Total Net Unrealized Gains Reflected in Earnings Net Unrealized (Losses) Gains Net Gains Total Net Unrealized (Losses) Gains Reflected in Earnings Fixed maturity investments $ 43.4 $ — $ 43.4 $ (15.7 ) $ (14.8 ) $ (30.5 ) Short-term investments .2 — .2 (.1 ) — (.1 ) Common equity securities 60.1 — 60.1 43.7 — 43.7 Other long-term investments (1) 154.1 (.8 ) 153.3 20.5 3.2 23.7 Net unrealized investment gains (losses), pre-tax 257.8 (.8 ) 257.0 48.4 (11.6 ) 36.8 Income tax expense attributable to net unrealized investment gains (60.5 ) — (60.5 ) (17.1 ) — (17.1 ) Net unrealized investment gains (losses), after-tax $ 197.3 $ (.8 ) $ 196.5 $ 31.3 $ (11.6 ) $ 19.7 (1) Includes $114.7 unrealized investment gain associated with the MediaAlpha Transaction, which is recorded in a separate line item in the statement of operations titled realized gain and unrealized investment gain from 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 and nine months ended September 30, 2019 and 2018 for investments held at September 30, 2019 and 2018: Three Months Ended Nine Months Ended September 30, September 30, Millions 2019 2018 2019 2018 Other long-term investments $ 54.4 $ 4.0 $ 154.4 $ 2.1 Total net unrealized investment gains, pre-tax - Level 3 investments $ 54.4 $ 4.0 $ 154.4 $ 2.1 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 September 30, 2019 and December 31, 2018 : September 30, 2019 Millions Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Carrying Value U.S. Government and agency obligations $ 196.3 $ 1.3 $ — $ 197.6 Debt securities issued by corporations 449.5 13.8 (.1 ) 463.2 Municipal obligations 292.4 15.7 (.1 ) 308.0 Mortgage and asset-backed securities 195.0 2.5 (.3 ) 197.2 Total fixed maturity investments $ 1,133.2 $ 33.3 $ (.5 ) $ 1,166.0 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 September 30, 2019 and December 31, 2018 : September 30, 2019 Millions Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Net Foreign Currency Losses Carrying Value Common equity securities $ 548.8 $ 80.9 $ — $ — $ 629.7 Other long-term investments $ 619.5 $ 222.8 $ (65.9 ) $ (2.7 ) $ 773.7 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 September 30, 2019 and December 31, 2018 : Carrying Value at Millions September 30, 2019 December 31, 2018 Kudu investments (1)(2) $ 219.3 $ — MediaAlpha (1)(3) 150.0 — PassportCard/DavidShield (1) 90.0 75.0 Elementum (1) 55.1 — Other unconsolidated entities (1)(4) 39.5 60.0 Kudu (5) — 30.7 Total unconsolidated entities (1) 553.9 165.7 Private equity funds and hedge funds 145.5 146.1 Insurance-linked securities funds 41.4 — Other 32.9 13.8 Total other long-term investments $ 773.7 $ 325.6 (1) See Fair Value Measurements by Level table. (2) Includes Kudu’s non-controlling equity interests in the form of revenue and earnings participations. As of September 30, 2019 White Mountains consolidates Kudu. See Note 2 — “Significant Transactions”. (3) The MediaAlpha Transaction was completed on February 26, 2019. See Note 2 — “Significant Transactions”. (4) Includes White Mountains’s non-controlling interests in certain private common equity securities, limited liability companies and convertible preferred securities. (5) Includes White Mountains non-controlling interest in Kudu prior to consolidation. See Note 2 — “Significant Transactions”. 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 net asset value (“NAV”) of the funds. As of September 30, 2019 , White Mountains held investments in twelve private equity funds and one hedge fund. The largest investment in a single fund was $54.9 million as of September 30, 2019 and $54.3 million as of December 31, 2018 . The following table presents investments and unfunded commitments in private equity funds and hedge funds by investment objective and sector as of September 30, 2019 and December 31, 2018 : September 30, 2019 December 31, 2018 Millions Fair Value Unfunded Commitments Fair Value Unfunded Commitments Private equity funds Manufacturing/Industrial $ 45.3 $ 4.1 $ 42.9 $ 10.5 Aerospace/Defense/Government 32.9 24.5 27.6 34.9 Financial services 12.4 26.2 8.3 13.6 Direct lending — — 13.0 17.7 Total private equity funds 90.6 54.8 91.8 76.7 Hedge funds Long/short banks and financials 54.9 — 54.3 — Total hedge funds 54.9 — 54.3 — Total private equity funds and hedge funds included in other long-term investments $ 145.5 $ 54.8 $ 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 September 30, 2019 : Millions 1 – 3 years 3 – 5 years 5 – 10 years >10 years Total Private equity funds — expected lock-up period remaining $5.4 $16.5 $56.3 $12.4 $90.6 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 September 30, 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 September 30, 2019 , White Mountains held one active hedge fund with a fair value of $ 54.9 million . The hedge fund is subject to a semi-annual restriction on redemptions and an advance notice period requirement of 45 days. Insurance-Linked Securities Funds White Mountains’s other long-term investments include insurance-linked securities (“ILS”) funds. The fair value of these investments are generally estimated using the NAV of the funds. As of September 30, 2019 , White Mountains held investments in three ILS funds with a fair value of $41.4 million . Investments in ILS funds are generally subject to restrictions, including lock-up periods where no redemption or withdrawal are allowed, restrictions on redemption frequency and advance notice periods for redemptions. From time to time, natural catastrophe, liquidity, market or other events will occur that make the determination of fair value for underlying investments in an ILS fund less certain due to the potential for loss development. In such circumstances, the impacted investments may be subject to additional lock-up provisions. As of September 30, 2019 , White Mountains holds one ILS fund subject to a lock-up period that expires on June 1, 2020. White Mountains’s ILS funds are subject to monthly and annual restrictions on redemptions and advance redemption notice period requirements that range between 30 and 90 days. 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. Fair Value Measurements as of September 30, 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 September 30, 2019 and December 31, 2018, White Mountains used quoted market prices or other observable inputs to determine fair value for approximately 73% 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 September 30, 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. September 30, 2019 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 197.6 $ 197.6 $ — $ — Debt securities issued by corporations: Financials 143.3 — 143.3 — Industrial 60.5 — 60.5 — Healthcare 55.0 — 55.0 — Consumer 49.3 — 49.3 — Energy 45.0 — 45.0 — Technology 44.5 — 44.5 — Communications 27.1 — 27.1 — Utilities 22.3 — 22.3 — Materials 16.2 — 16.2 — Total debt securities issued by corporations 463.2 — 463.2 — Mortgage and asset-backed securities 197.2 — 197.2 — Municipal obligations 308.0 — 308.0 — Total fixed maturity investments 1,166.0 197.6 968.4 — Short-term investments (1) 272.5 261.0 11.5 — Common equity securities: Exchange traded funds (2) 519.7 506.1 13.6 — Other (3) 110.0 — 110.0 — Total common equity securities 629.7 506.1 123.6 — Other long-term investments 586.8 — — 586.8 Other long-term investments — NAV (4) 186.9 — — — Total investments $ 2,841.9 $ 964.7 $ 1,103.5 $ 586.8 (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 private equity funds, one hedge fund and ILS funds for which fair value is measured at NAV using the practical expedient. Investments for which fair value is measured at NAV are not classified within the fair value hierarchy. 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 September 30, 2019 and December 31, 2018 : Fair Value at Millions September 30, 2019 December 31, 2018 AAA $ 9.9 $ 8.9 AA 79.9 88.7 A 286.7 270.5 BBB 86.7 142.4 Debt securities issued by corporations (1) $ 463.2 $ 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 September 30, 2019 and December 31, 2018 : September 30, 2019 December 31, 2018 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: FNMA $ 79.8 $ 79.8 $ — $ 53.6 $ 53.6 $ — FHLMC 53.5 53.5 — 38.1 38.1 $ — GNMA 33.5 33.5 — 23.7 23.7 $ — Total agency (1) 166.8 166.8 — 115.4 115.4 — Total mortgage-backed securities 166.8 166.8 — 115.4 115.4 — Other asset-backed securities: Vehicle receivables 16.5 16.5 — 9.2 9.2 — Credit card receivables 13.9 13.9 — 8.9 8.9 — Total other asset-backed securities 30.4 30.4 — 18.1 18.1 — Total mortgage and asset-backed securities $ 197.2 $ 197.2 $ — $ 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 September 30, 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 September 30, 2019 and 2018 : Level 3 Investments Unconsolidated Entities, Private Equity Funds, Hedge Funds and ILS Funds Measured at NAV (3)(5) Millions Level 1 Investments Level 2 Investments Other Long-term Investments (2)(5)(6) Total Balance at December 31, 2018 $ 842.6 $ 1,160.5 $ 138.7 $ 186.9 $ 2,328.7 (1) Net realized and unrealized gains 121.8 66.6 154.2 4.9 347.5 (2,4) Amortization/Accretion .2 (1.2 ) — — (1.0 ) Purchases 101.4 331.3 211.3 106.4 750.4 (6) Sales (362.3 ) (465.2 ) — (28.7 ) (856.2 ) Transfers in — — 82.6 — 82.6 Transfers out — — — (82.6 ) (82.6 ) Balance at September 30, 2019 $ 703.7 $ 1,092.0 $ 586.8 $ 186.9 $ 2,569.4 (1) (1) Excludes carrying value of $ 272.5 million and $ 214.2 as of September 30, 2019 and December 31, 2018 classified as short-term investments. (2) Includes $114.7 unrealized investment gain associated with the MediaAlpha Transaction, which is recorded in a separate line item in the statement of operations titled realized gain and unrealized investment gain from 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” . (4) Excludes realized and unrealized losses associated with short-term investments of $0.3 for the nine months ended September 30, 2019. (5) Transfers include $71.7 associated with the Kudu Transaction. See Note 2 — “Significant Transactions”. (6) Purchases include $70.1 associated with the Kudu Transaction. See Note 2 — “Significant Transactions”. 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 gains (losses) 51.9 (33.6 ) (5.3 ) 18.3 31.3 (4) Amortization/Accretion .1 (2.3 ) — — (2.2 ) Purchases 412.6 673.5 55.1 10.7 1,151.9 Sales (457.6 ) (1,565.0 ) — (1.6 ) (2,024.2 ) Transfers in — — — — — Transfers out — — — — — Balance at September 30, 2018 $ 897.4 $ 1,178.0 $ 127.0 $ 162.7 $ 2,365.1 (1) (1) Excludes carrying value of $ 313.7 and $ 176.1 as of September 30, 2018 and December 31, 2017 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 $0.7 for the nine months ended September 30, 2018. Fair Value Measurements — Transfers Between Levels - Nine-months ended September 30, 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 nine 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. During the first nine months of 2019 and 2018, there were no fixed maturity investments or other long-term investments classified as Level 2 measurements in the prior period that were transferred to Level 3 measurements. Significant Unobservable Inputs The following tables present significant unobservable inputs used in estimating the fair value of other long-term investments, other than private equity funds, hedge funds and ILS funds, classified within Level 3 as of September 30, 2019 and December 31, 2018 . The fair value of investments in private equity funds, hedge funds and ILS funds are generally estimated using the NAV of the funds. $ in Millions September 30, 2019 Description Valuation Technique(s) Fair Value (1) Unobservable Inputs Discount Rate Terminal revenue exit multiple (x) or terminal revenue growth rate (%) Kudu investments Discounted cash flow $219.3 15% - 20% 6x - 12x MediaAlpha Discounted cash flow $150.0 17% 4% PassportCard/DavidShield Discounted cash flow $90.0 18% 1x Private debt instruments Discounted cash flow $22.7 6% - 9% N/A All other Discounted cash flow $29.3 23% 2x - 4x Elementum Holdings, L.P. Price of recent transaction (2) $55.1 Transaction price: $55.1 Compare.com Estimated net realizable value $2.5 Net asset value: $2.5 Galvanic Applied Sciences Multiple of EBITDA $2.7 EBITDA multiple: 6 (1) Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs. (2) The Elementum Transaction was completed on May 31, 2019. See Note 2 — “Significant Transactions”. $ in Millions December 31, 2018 Description Valuation Technique(s) Fair Value (1) Unobservable Inputs Discount Rate Terminal revenue exit multiple (x) PassportCard/DavidShield Discounted cash flow $75.0 18% 1x Compare.com Discounted cash flow $16.9 22% 3x Private debt instrument Discounted cash flow $10.0 9% N/A All other Discounted cash flow $30.0 23% 2x - 4x Galvanic Applied Sciences Multiple of EBITDA $3.1 EBITDA multiple: 6 (1) Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs.

Goodwill and Other Intangible A

Goodwill and Other Intangible Assets	9 Months Ended
	Sep. 30, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets White Mountains accounts for business combinations using the acquisition method. Under the acquisition method, White Mountains recognizes and measures the assets acquired, liabilities assumed and any noncontrolling interest in the acquired entities at their acquisition date fair values. The estimated acquisition date fair values, generally consisting of intangible assets and liabilities for contingent consideration, may be recorded at provisional amounts in circumstances where the information necessary to complete the acquisition accounting is not available at the reporting date. Any such provisional amounts are finalized as measurement period adjustments within one year of the acquisition date. The following table presents a summary of the acquisition date fair values of goodwill and other intangible assets for acquisitions during the nine months ended September 30, 2019 and 2018: $ in Millions Acquisition of subsidiary/ asset Goodwill and Other intangible asset Acquisition Date NSM $ 383.0 May 11, 2018 Fresh Insurance 54.6 May 18, 2018 KBK 59.4 December 3, 2018 Embrace 70.6 (1) April 1, 2019 Renewal Rights (2) 82.5 June 28, 2019 Total NSM segment $ 650.1 Kudu Transaction $ 9.8 April 4, 2019 Other Operations $ 13.1 (1) March 12, 2019 (1) The relative fair values of goodwill and of other intangible assets recognized in connection with the acquisition of Embrace and the acquisition in Other Operations had not yet been finalized at September 30, 2019. (2) NSM’s acquisition of the Renewal Rights was an asset purchase. The following tables presents the change in goodwill and other intangible assets for the three and nine months ended September 30, 2019 and 2018 : Three Months Ended September 30, 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 $ 428.6 $ 239.0 $ 667.6 $ 25.9 $ 30.7 $ 56.6 Attribution of acquisition date fair value estimates between goodwill and other intangible assets (2.2 ) 2.2 — 295.3 140.9 436.2 Foreign currency translation (1.6 ) (.5 ) (2.1 ) (1.2 ) (.4 ) — (1.6 ) Adjustments to reflect acquisition date fair value — — — 1.4 — — 1.4 Amortization — (4.6 ) (4.6 ) — (7.4 ) (7.4 ) Ending balance $ 424.8 $ 236.1 $ 660.9 $ 321.4 $ 163.8 $ 485.2 Nine Months Ended September 30, 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 (29.0 ) 29.0 — 295.3 140.9 436.2 Acquisitions of businesses 93.8 (1) — 93.8 — — — Acquisition of collector car renewal rights — 82.5 82.5 — — — Foreign currency translation (1.6 ) — (1.6 ) (1.2 ) (.4 ) — (1.6 ) Adjustments to reflect acquisition date fair value — 5.9 5.9 1.4 — — 1.4 Amortization — (15.4 ) (15.4 ) — (12.9 ) (12.9 ) Ending balance $ 424.8 $ 236.1 $ 660.9 $ 321.4 $ 163.8 $ 485.2 (1) The relative fair values of goodwill and other intangible assets recognized in connection with the acquisition of Embrace and the acquisition in Other Operations had not yet been finalized at September 30, 2019. White Mountains evaluates goodwill for potential impairment at least annually, generally in the interim period prior to the one-year anniversary of the acquisition date. An impairment of goodwill or other intangible assets occurs when the carrying value of the asset exceeds its fair value. Between annual evaluations, White Mountains considers changes in circumstances or events subsequent to the most recent evaluation that may indicate that an impairment may exist and, if necessary will perform an interim review for potential impairment. During the second quarter of 2019, White Mountains performed a periodic review of NSM’s goodwill and other intangible assets and concluded that goodwill and other intangible assets were not impaired. There have been no changes in circumstances or events for the quarterly period ended September 30, 2019 that would indicate the existence of an impairment of goodwill or other intangible assets for any acquired entity. 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 September 30, 2019 and December 31, 2018 : $ in Millions Weighted Average Economic life (in years) September 30, 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) N/A $ 397.0 $ — $ 397.0 $ 354.3 $ — $ 354.3 Kudu N/A 7.6 — 7.6 — — — MediaAlpha N/A — — — 18.3 — 18.3 Other Operations (2) N/A 20.2 — 20.2 7.3 — 7.3 Total goodwill 424.8 — 424.8 379.9 — 379.9 Other intangible assets: NSM (1) Customer relationships 9 110.5 15.5 95.0 85.3 6.0 79.3 Trade names 19 57.1 4.2 52.9 51.2 1.8 49.4 Information technology 5 3.6 1.1 2.5 3.7 .5 3.2 Renewal rights 12 82.5 .3 82.2 — — — Other 3 1.7 .7 1.0 — — — Subtotal 255.4 21.8 233.6 140.2 8.3 131.9 Kudu Trade names 7 2.2 .2 2.0 — — — MediaAlpha Customer relationships 9 — — — 26.8 4.9 21.9 Information technology 5 — — — 33.3 30.9 2.4 Other 3 — — — 9.8 9.0 .8 Subtotal — — — 69.9 44.8 25.1 Other Operations Trade names 7 .6 .2 .4 .6 .2 .4 Information technology 5 .4 .3 .1 .5 .3 .2 Subtotal 1.0 .5 .5 1.1 .5 .6 Total other intangible assets 258.6 22.5 236.1 211.2 53.6 157.6 Total goodwill and other intangible assets $ 683.4 $ 22.5 660.9 $ 591.1 $ 53.6 537.5 Goodwill and other intangible assets attributed to non-controlling interests (25.0 ) (40.6 ) Goodwill and other intangible assets included in White Mountains’s common shareholders’ equity $ 635.9 $ 496.9 (1) As of September 30, 2019 , NSM’s goodwill included $(1.5) and $ (2.2) of the effect of foreign currency translation. As of December 31, 2018, NSM’s intangible assets include $ (0.7) of the effect of foreign currency translation. (2) As of September 30, 2019 and December 31, 2018, Other Operations’ goodwill included $(0.1) and $ (0.3) of the effect of foreign currency translation.

Debt

Debt	9 Months Ended
	Sep. 30, 2019
Debt Disclosure [Abstract]
Debt	Debt The following table presents White Mountains’s debt outstanding as of September 30, 2019 and December 31, 2018: Millions September 30, Effective Rate (1) December 31, Effective Rate (1) NSM Bank Facility $ 221.8 7.7% $ 180.4 7.4% Unamortized issuance cost (4.1 ) (3.8 ) NSM Bank Facility, carrying value 217.7 176.6 Other NSM debt 1.7 2.5% 1.9 2.2% MediaAlpha Bank Facility — 14.3 7.1% Unamortized issuance cost — (.1 ) MediaAlpha Bank Facility, carrying value — 14.2 Other Operations debt 11.2 9.6% — Unamortized issuance cost (.4 ) — Other Operations, carrying value 10.8 — Total debt $ 230.2 $ 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 $224.0 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 and nine months ended September 30, 2019 , NSM repaid $0.5 million and $1.4 million on the term loans. During the three months ended September 30, 2019, NSM did not make any borrowings on the term loans or revolving credit loans. During the nine months ended September 30, 2019, NSM repaid $6.5 million on the revolving credit loan. During the nine months ended September 30, 2019 , NSM borrowed $42.9 million on the term loans, which included $20.4 million and $22.5 million for the funding of the acquisitions of Embrace and the Renewal Rights. During the nine months ended September 30, 2019 , NSM borrowed $6.5 million on the revolving credit loan. As of September 30, 2019 , the term loans had an outstanding balance of $221.8 million and the revolving credit loans were undrawn. Interest on the NSM Bank Facility accrues at a floating interest rate equal to the three month LIBOR or the Prime Rate, as published by the Wall Street Journal plus, in each case, an applicable margin. The margin over LIBOR may vary between 4.25% and 4.75% , and the margin over the Prime Rate may vary between 3.25% and 3.75% , in each case, depending on the consolidated total leverage ratio of the borrower. On June 15, 2018, NSM entered into an interest rate swap agreement to hedge its exposure to interest rate risk on $151.0 million of its variable rate term loans. Under the terms of the swap agreement, NSM pays a fixed rate of 2.97% and receives a variable rate, which is reset monthly, based on then-current LIBOR. As of September 30, 2019 , the variable rate received by NSM under the swap agreement was 2.11% . As of September 30, 2019 , the interest rate, including the effect of the swap, for the outstanding term loans of $149.1 million that are hedged by the swap was 7.47% . The effective interest on the outstanding term loans of $72.7 million that are unhedged was 7.12% . The effective interest rate on the total outstanding term loans under the NSM Bank Facility of $221.8 million was 7.17% , 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 September 30, 2019 , White Mountains was in compliance with the covenants under all of its debt instruments.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 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 benefit (expense) related to pre-tax income (loss) from continuing operations for the three and nine months ended September 30, 2019 represented an effective tax rate of 17.2% and 5.4% . The effective tax rate for the three and nine months ended September 30, 2019 was different from the current U.S. statutory rate of 21.0% , due to income generated in jurisdictions with lower tax rates than the United States, state income taxes and a tax benefit recorded at BAM. Also, the effective tax rate for the nine months ended September 30, 2019 was different from the current U.S. statutory rate of 21.0% , due to the 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 Kudu, White Mountains’s investment in MediaAlpha, various service companies and certain other entities and investments that are included in the Other Operations segment. For BAM, member surplus contributions (“MSC”) and the related taxes thereon are recorded directly to non-controlling interest equity, while the valuation allowance on such taxes is recorded through the income statement. For the three and nine months ended September 30, 2019, BAM recorded a tax benefit of $3.6 million and $6.6 million associated with the valuation allowance on taxes related to MSC that is included in the effective tax rate. White Mountains’s income tax expense related to pre-tax loss from continuing operations for the three and nine months ended September 30, 2018 represented an effective tax rate of (8.1)% and (1.2)% . The effective tax rate was different from the U.S. statutory rate of 21.0% , due to a full valuation allowance on net deferred tax assets at U.S operations, consisting of Guilford and BAM, withholding taxes, state income 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 and nine months ended September 30, 2018, BAM recorded a tax benefit of $1.8 million and $4.0 million associated with the valuation allowance on taxes related to MSC that is included in the effective tax rate. In arriving at the effective tax rate for the three and nine months ended September 30, 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.

Derivatives

Derivatives	9 Months Ended
	Sep. 30, 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 September 30, 2019, the variable rate received by NSM under the swap agreement was 2.11% . Over the term of the swap, the notional amount decreases in accordance with the principal repayments NSM expects to make on its term loans. As of September 30, 2019, the interest rate, including the effect of the swap, for the outstanding term loans of $149.1 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 and nine months ended September 30, 2019, White Mountains recognized net interest expense of $0.3 million and $0.6 million for the periodic net settlement payments on the swap. For the three months and nine months ended September 30, 2018, White Mountains recognized net interest expense of $0.3 million and $0.4 million for the periodic net settlement payments on the swap. As of September 30, 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 $7.8 million and $2.7 million . There was no ineffectiveness in the hedge for the three and nine months ended September 30, 2019 or the three and nine months ended September 30, 2018. For the three and nine months ended September 30, 2019, the $(0.8) million and $(5.1) million change in the fair value of the swap is included within accumulated other comprehensive income (loss). For the three months and nine months ended September 30, 2018, the $1.0 million and $(0.1) million change in the fair value of the swap is included within White Mountains’s accumulated other comprehensive income (loss).
Forward Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivatives	Foreign Currency Forward Contracts White Mountains’s investment portfolio includes investments denominated in GBP, Japanese Yen, Euros 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 the first quarter of 2018, White Mountains closed an outstanding foreign currency forward contract in conjunction with the termination of its GBP investment grade corporate bond mandate. As of September 30, 2019 and December 31, 2018, White Mountains no longer had any open foreign currency forward contracts. The derivative losses recognized in net realized and unrealized investment gains (losses) for the nine months ended September 30, 2018 were $(3.5) million

Municipal Bond Guarantee

Municipal Bond Guarantee	9 Months Ended
	Sep. 30, 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 then newly formed mutual municipal bond insurer. As of September 30, 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 September 30, 2019 and December 31, 2018 , the collateral trusts held assets of $787.4 million and $757.4 million , which both included $481.3 million of BAM Surplus Notes. As of September 30, 2019 and December 31, 2018 , HG Global has accrued $164.2 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 September 30, 2019 and December 31, 2018: September 30, 2019 December 31, 2018 Contracts outstanding 8,508 7,525 Remaining weighted average contract period outstanding (in years) 10.7 10.7 Contractual debt service outstanding (in millions): Principal $ 59,190.9 $ 52,201.6 Interest 30,701.7 26,560.3 Total debt service outstanding $ 89,892.6 $ 78,761.9 Gross unearned insurance premiums (in millions) $ 203.5 $ 176.0 The following table presents a schedule of BAM’s future premium revenues as of September 30, 2019 : Millions September 30, 2019 October 1, 2019 - December 31, 2019 $ 4.9 January 1, 2020 - March 31, 2020 4.8 April 1, 2020 - June 30, 2020 4.8 July 1, 2020 - September 30, 2020 4.6 October 1, 2020 - December 31, 2020 4.5 Total 2020 18.7 2021 17.4 2022 16.3 2023 15.3 2024 14.3 2025 and thereafter 116.6 Total gross unearned insurance premiums $ 203.5 The following table presents a schedule of net written premiums included in White Mountains’s HG Global/BAM segment for the three and nine months ended September 30, 2019 and 2018 : Three Months Ended September 30, Nine Months Ended September 30, Millions 2019 2018 2019 2018 Written premiums: Direct $ 10.2 $ 3.9 $ 30.6 $ 28.9 Assumed 10.6 — 10.6 — Gross written premiums $ 20.8 $ 3.9 $ 41.2 $ 28.9 Earned premiums: Direct $ 4.4 $ 3.3 $ 11.9 $ 9.7 Assumed .8 — 1.8 — Gross earned premiums $ 5.2 $ 3.3 $ 13.7 $ 9.7 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 $100.0 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 . In September 2019, BAM entered into facultative quota share reinsurance agreements under which it assumed a portfolio of municipal bond guarantee contracts with a par value of $1.1 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 September 30, 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	9 Months Ended
	Sep. 30, 2019
Equity Method Investments and Joint Ventures [Abstract]
Investments in Unconsolidated Entities	The following table presents the carrying values of White Mountains’s equity method eligible investments recorded within other long-term investments as of September 30, 2019 and December 31, 2018: Millions September 30, 2019 December 31, 2018 Equity method eligible investments, at fair value $ 673.2 $ 269.7 Other (1) 100.5 55.9 Total other long-term investments $ 773.7 $ 325.6 (1) Consists of other long-term investments that are not equity method eligible. The following table presents White Mountains’s significant equity method eligible investments as of September 30, 2019 and December 31, 2018: Basic Ownership Interest Investee September 30, 2019 December 31, 2018 Instrument Held PassportCard/DavidShield 50.0% 50.0% Common shares MediaAlpha (1) 48.3% n/a Units durchblicker 45.0% 45.0% Common shares Elementum Holdings, L.P. 30.0% n/a Limited partnership interest Tuckerman Capital Funds (2) 18.8 - 62.4% 18.5 - 62.3% Limited and general partnership interests Compare.com 18.4% 18.4% Common shares JAM Partners L.P. 11.1% 12.3% Limited partnership interest Enlightenment Capital Funds 10.0 - 36.5% 10.3 - 36.5% Limited and general partnership interests Kudu investments (3) 3.2 - 30.0% n/a Revenue and earnings participations Kudu (3) n/a 49.5% Units (1) As of December 31, 2018, MediaAlpha was a majority-owned consolidated subsidiary of White Mountains. See Note 2 — “Significant Transactions” (2) Includes White Mountains’s direct investment in Galvanic Applied Sciences. (3) As of September 30, 2019 White Mountains consolidates Kudu. See Note 2 — “Significant Transactions”. As a result of the MediaAlpha Transaction, White Mountains 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 as of 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 initially 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. For the three and nine months ended September 30, 2019, White Mountains recognized $35.3 million and $150.0 million in unrealized investment gains associated with its investment in MediaAlpha. 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 three months ended September 30, 2019, MediaAlpha’s total revenues, total expenses, gross profit and pre-tax income were $110.5 million , $102.1 million , $18.4 million and $8.4 million . For the period from February 26, 2019 to September 30, 2019, MediaAlpha’s total revenues, total expenses, gross profit and pre-tax income were $233.2 million , $217.9 million , $38.8 million and $15.3 million . As of September 30, 2019, MediaAlpha’s total assets and total liabilities were $103.4 million and $145.8 million

Earnings Per Share

Earnings Per Share	9 Months Ended
	Sep. 30, 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 and nine months ended September 30, 2019 and 2018 . See Note 17 — “Held for Sale and Discontinued Operations” . Three Months Ended Nine Months Ended September 30, September 30, 2019 2018 2019 2018 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 48.7 $ 40.8 $ 353.6 $ (3.7 ) Less: total income (loss) from discontinued operations, net of tax .9 (17.3 ) 1.6 (17.2 ) Net income from continuing operations attributable to White Mountains’s common shareholders $ 47.8 $ 58.1 $ 352.0 $ 13.5 Allocation of earnings to participating restricted common shares (1) (.7 ) (.8 ) (4.4 ) (.2 ) Basic and diluted earnings per share numerators $ 47.1 $ 57.3 $ 347.6 $ 13.3 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 3,185.4 3,180.4 3,180.4 3,450.8 Average unvested restricted common shares (2) (43.4 ) (41.5 ) (39.6 ) (39.7 ) Basic earnings per share denominator 3,142.0 3,138.9 3,140.8 3,411.1 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 3,185.4 3,180.4 3,180.4 3,450.8 Average unvested restricted common shares (2) (43.4 ) (41.5 ) (39.6 ) (39.7 ) Diluted earnings per share denominator 3,142.0 3,138.9 3,140.8 3,411.1 Basic and diluted earnings per share (in dollars) - continuing operations: Distributed earnings - dividends declared and paid $ — $ — $ 1.00 $ 1.00 Undistributed earnings 15.01 18.27 109.67 2.90 Basic and diluted earnings per share $ 15.01 $ 18.27 $ 110.67 $ 3.90 (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 and nine months ended September 30, 2019 and 2018 . See Note 17 — “Held for Sale and Discontinued Operations” . Three Months Ended Nine Months Ended September 30, September 30, Millions 2019 2018 2019 2018 Undistributed net earnings - continuing operations: Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 47.1 $ 57.3 $ 347.6 $ 13.3 Dividends declared, net of restricted common share amounts (1) — — (3.2 ) (3.7 ) Total undistributed net earnings, net of restricted common share amounts $ 47.1 $ 57.3 $ 344.4 $ 9.6 (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.

Employee Share-Based Incentive

Employee Share-Based Incentive Compensation Plans	9 Months Ended
	Sep. 30, 2019
Share-based Payment Arrangement [Abstract]
Employee Share-Based Incentive Compensation Plans	Employee Share-Based Incentive Compensation Plans White Mountains’s Long-Term Incentive Plan (the “WTM Incentive Plan”) provides for grants of various types of share-based and non-share-based incentive awards to key employees of White Mountains. As of September 30, 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 and nine months ended September 30, 2019 and 2018 for performance shares granted under the WTM Incentive Plan: Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Millions, except share amounts Target Performance Accrued Expense Target Performance Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Accrued Expense Beginning of period 42,473 $ 30.8 40,616 $ 27.2 40,616 $ 31.7 50,515 $ 45.8 Shares paid (1) — — — — (13,715 ) (18.1 ) (23,186 ) (28.4 ) New grants — — — — 15,600 — 14,105 — Forfeitures and cancellations (2) — — — (.1 ) (28 ) — (818 ) .1 Expense recognized — 5.6 — 7.8 — 22.8 — 17.4 End of period 42,473 $ 36.4 40,616 $ 34.9 42,473 $ 36.4 40,616 $ 34.9 (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 September 30, 2019 , the total additional compensation cost to be recognized would have been $21.1 million , based on accrual factors (common share price and payout assumptions) as of September 30, 2019 . The following table presents performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan as of September 30, 2019 for each performance cycle: Nine Months Ended September 30, 2019 Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Performance cycle: 2017 – 2019 14,070 $ 24.4 2018 – 2020 13,450 8.5 2019 – 2021 15,600 4.0 Sub-total 43,120 36.9 Assumed forfeitures (647 ) (.5 ) September 30, 2019 42,473 $ 36.4 Restricted Shares The following table presents the unrecognized compensation cost associated with the outstanding restricted share awards for the three and nine months ended September 30, 2019 and 2018 : Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Millions, except share amounts Restricted Unamortized Restricted Unamortized Restricted Unamortized Issue Date Restricted Unamortized Issue Date Non-vested, Beginning of period 43,395 $ 21.9 41,510 $ 19.0 41,510 $ 12.5 53,755 $ 14.3 Issued — — — — 15,600 14.5 14,105 11.4 Vested — — — — (13,715 ) — (25,381 ) — Forfeited — — — — — — (969 ) (.2 ) Expense recognized — (2.6 ) — (3.2 ) — (7.7 ) — (9.7 ) End of period 43,395 $ 19.3 41,510 $ 15.8 43,395 $ 19.3 41,510 $ 15.8 During the nine months ended September 30, 2019 , White Mountains issued 15,600 restricted shares that vest on January 1, 2022. During the nine months ended September 30, 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 September 30, 2019

Leases

Leases	9 Months Ended
	Sep. 30, 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 September 30, 2019, the ROU asset was $23.0 million and lease liabilities were $ 23.3 million . The following table summarizes net lease expense recognized in White Mountains’s consolidated statement of operations for the three and nine months ended September 30, 2019: Millions Lease Cost Three Months Ended September 30, 2019 Nine Months Ended September 30, 2019 Lease cost $ 1.9 $ 5.4 Less: sublease income .1 .3 Net lease cost $ 1.8 $ 5.1 The following table presents the contractual maturities of the lease liabilities associated with White Mountains’s operating lease agreements as of September 30, 2019 : Millions As of September 30, 2019 Remainder of 2019 $ 1.7 2020 5.7 2021 4.7 2022 4.2 2023 3.6 Thereafter 6.3 Total undiscounted lease payments 26.2 Less: present value adjustment 2.9 Operating lease liability $ 23.3 The following table presents lease related assets and liabilities by reportable segment as of September 30, 2019: As of September 30, 2019 Millions HG/BAM NSM Kudu Other Operations Total Weighted Average Incremental Borrowing Rate (1) ROU lease asset $ 10.8 $ 4.4 $ 2.3 $ 5.5 $ 23.0 4.5% Lease liability $ 10.8 $ 4.4 $ 2.3 $ 5.8 $ 23.3 (1) The present value of the remaining lease payments were determined by discounting the lease payments using the incremental borrowing rate.

Non-controlling Interests

Non-controlling Interests	9 Months Ended
	Sep. 30, 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 September 30, 2019 and December 31, 2018 : September 30, 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 % $ 14.9 3.1 % $ 14.5 NSM 3.6 15.2 4.5 13.6 Other NSM 13.4 .1 13.4 .3 Kudu 1.0 2.3 — — MediaAlpha — — 39.0 16.2 Other Operations various .2 various 1.1 Total, excluding BAM 32.7 45.7 BAM 100.0 (155.2 ) 100.0 (170.6 ) Total non-controlling interests $ (122.5 ) $ (124.9 )

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2019
Segment Reporting [Abstract]
Segment Informartion	Segment Information White Mountains has determined that its reportable segments are HG Global/BAM, NSM, Kudu, 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 as of February 26, 2019, the date of the MediaAlpha Transaction. Kudu became a reportable segment as of April 4, 2019, the date of the Kudu Transaction. 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 Kudu Other Operations Total Three Months Ended September 30, 2019 Earned insurance premiums $ 5.2 $ — $ — $ — $ 5.2 Net investment income 5.4 — 4.6 8.4 18.4 Net realized and unrealized investment gains 7.1 — 2.7 57.2 67.0 Advertising and commission revenues — 49.4 — 1.9 51.3 Other revenue .3 10.5 .2 2.6 13.6 Total revenues 18.0 59.9 7.5 70.1 155.5 Insurance acquisition expenses 1.4 — — — 1.4 Other underwriting expenses .1 — — — .1 Cost of sales — — — 2.3 2.3 General and administrative expenses 12.5 33.3 4.5 27.7 78.0 Broker commission expense — 15.3 — — 15.3 Change in fair value of contingent consideration — (2.0 ) — — (2.0 ) Amortization of other intangible assets — 4.4 .2 — 4.6 Interest expense — 4.4 — .3 4.7 Total expenses 14.0 55.4 4.7 30.3 104.4 Pre-tax income $ 4.0 $ 4.5 $ 2.8 $ 39.8 $ 51.1 Millions HG Global/ BAM NSM Kudu (1) MediaAlpha (2) Other Operations Total Nine Months Ended September 30, 2019 Earned insurance premiums $ 13.7 $ — $ — $ — $ — $ 13.7 Net investment income 16.1 — 8.6 — 30.7 55.4 Net realized and unrealized investment gains 30.4 — 3.1 — 199.6 233.1 Realized gain and unrealized investment gain from the MediaAlpha Transaction — — — — 182.2 182.2 Advertising and commission revenues — 147.6 — 48.8 4.5 200.9 Other revenue 1.3 27.7 .2 — 3.6 32.8 Total revenues 61.5 175.3 11.9 48.8 420.6 718.1 Insurance acquisition expenses 4.1 — — — — 4.1 Other underwriting expenses .3 — — — — .3 Cost of sales — — — 40.6 5.0 45.6 General and administrative expenses 39.1 91.4 6.7 12.5 87.2 236.9 Broker commission expense — 48.9 — — — 48.9 Change in fair value of contingent consideration — 5.6 — — — 5.6 Amortization of other intangible assets — 13.5 .2 1.6 .1 15.4 Interest expense — 12.2 .2 .3 12.7 Total expenses 43.5 171.6 6.9 54.9 92.6 369.5 Pre-tax income (loss) $ 18.0 $ 3.7 $ 5.0 $ (6.1 ) $ 328.0 $ 348.6 (1) Kudu’s results are from April 4, 2019, the date of the Kudu Transaction, to September 30, 2019. (2) MediaAlpha's results are from January 1, 2019 to February 26, 2019, the date of the MediaAlpha Transaction. Millions HG Global/BAM NSM MediaAlpha Other Operations Total Three Months Ended September 30, 2018 Earned insurance premiums $ 3.3 $ — $ — $ — $ 3.3 Net investment income 4.8 — — 8.6 13.4 Net realized and unrealized investment (losses) gains (4.1 ) — — 70.2 66.1 Advertising and commission revenues (1) — 35.0 74.5 1.1 110.6 Other revenue .2 4.7 — .4 5.3 Total revenues 4.2 39.7 74.5 80.3 198.7 Insurance acquisition expenses 1.2 — — — 1.2 Other underwriting expenses .1 — — — .1 Cost of sales — — 61.8 1.1 62.9 General and administrative expenses 11.3 23.5 5.4 26.0 66.2 Broker commission expense — 10.6 — — 10.6 Change in fair value of contingent consideration — 2.6 — — 2.6 Amortization of other intangible assets — 5.0 2.4 — 7.4 Interest expense — 3.2 .2 — 3.4 Total expenses 12.6 44.9 69.8 27.1 154.4 Pre-tax (loss) gains $ (8.4 ) $ (5.2 ) $ 4.7 $ 53.2 $ 44.3 (1) Approximately 30% of MediaAlpha’s advertising revenue for the three months ended September 30, 2018 was associated with one customer. Millions HG Global/BAM NSM (1) MediaAlpha Other Operations Total Nine Months Ended September 30, 2018 Earned insurance premiums $ 9.7 $ — $ — $ — $ 9.7 Net investment income 12.5 — — 32.4 44.9 Net realized and unrealized investment (losses) gains (14.4 ) — — 37.3 22.9 Advertising and commission revenues (2) — 56.4 216.4 3.0 275.8 Other revenue .8 6.8 1.6 .6 9.8 Total revenues 8.6 63.2 218.0 73.3 363.1 Loss and loss adjustment expenses — — — — Insurance acquisition expenses 3.9 — — — 3.9 Other underwriting expenses .3 — — — .3 Cost of sales — — 179.1 2.9 182.0 General and administrative expenses 36.5 35.7 21.2 79.0 172.4 Broker commission expense — 17.1 — — 17.1 Change in fair value of contingent consideration — 2.6 — — 2.6 Amortization of other intangible assets — 5.0 7.8 .1 12.9 Interest expense — 4.8 .9 .3 6.0 Total expenses 40.7 65.2 209.0 82.3 397.2 Pre-tax (loss) gains $ (32.1 ) $ (2.0 ) $ 9.0 $ (9.0 ) $ (34.1 ) (1) NSM’s results are from May 11, 2018, the date of acquisition, to September 30, 2018. (2) Approximately 31% of MediaAlpha’s advertising revenue for the nine months ended September 30, 2018 was associated with one customer.

Fair Value of Financial Instrum

Fair Value of Financial Instruments	9 Months Ended
	Sep. 30, 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 September 30, 2019 and December 31, 2018 : September 30, 2019 December 31, 2018 Millions Fair Value Carrying Value Fair Value Carrying Value NSM Bank Facility $ 219.7 $ 217.7 $ 176.1 $ 176.6 MediaAlpha Bank Facility $ — $ — $ 14.6 $ 14.2 Other Operations debt $ 11.3 $ 10.8 $ — $ — The fair value estimates for the NSM Bank Facility and the MediaAlpha Bank Facility have been determined based on a discounted cash flows approach and are considered to be Level 3 measurements. 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 as of February 26, 2019. See Note 2 — “Significant Transactions”.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 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 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 were not sold by December 31, 2019, AIG had the right to require NSM to purchase the Renewal Rights for $82.5 million . On June 28, 2019, NSM acquired the Renewal Rights from AIG for $82.5 million . The transaction satisfied NSM’s obligation to acquire the Renewal Rights from AIG. 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 of September 30, 2019 and December 31, 2018, White Mountains reported a liability of $15.7 million and $17.3 million , reflecting the value of these interest deductions. For the three and nine months ended September 30, 2019 , the decrease in the liability of $0.9 million and $1.6 million is related to foreign currency translation and 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 September 30, 2019

Held for Sale and Discontinued

Held for Sale and Discontinued Operations	9 Months Ended
	Sep. 30, 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 nine months ended September 30, 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 of September 30, 2019 and December 31, 2018, White Mountains recorded a liability of $15.7 million and $17.3 million , reflecting the value of these interest deductions. For the three and nine months ended September 30, 2019 , the decrease in the liability, of $ 0.9 million and $ 1.6 million , is related to foreign currency translation and 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 $2.8 million and $3.3 million as of September 30, 2019
Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Held for Sale and Discontinued Operations	Earnings Per Share from Discontinued Operations White Mountains calculates earnings per share using the two-class method, which allocates earnings between common and unvested restricted common shares. Both classes of shares participate equally in earnings on a per share basis. Basic earnings per share amounts are based on the weighted average number of common shares outstanding adjusted for unvested restricted common shares. Diluted earnings per share amounts are also impacted by the net effect of potentially dilutive common shares outstanding. The following table presents the Company’s computation of earnings per share for discontinued operations for the three and nine months ended September 30, 2019 and 2018 : Three Months Ended Nine Months Ended September 30, September 30, 2019 2018 2019 2018 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 48.7 $ 40.8 $ 353.6 $ (3.7 ) Less: total income from continuing operations, net of tax 47.8 58.1 352.0 13.5 Net income (loss) from discontinued operations attributable to White Mountains’s common shareholders $ .9 $ (17.3 ) $ 1.6 $ (17.2 ) Allocation of earnings to participating restricted common shares (1) — .2 — .2 Basic and diluted earnings (loss) per share numerators (2) $ .9 $ (17.1 ) $ 1.6 $ (17.0 ) Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 3,185.4 3,180.4 3,180.4 3,450.8 Average unvested restricted common shares (3) (43.4 ) (41.5 ) (39.6 ) (39.7 ) Basic earnings per share denominator 3,142.0 3,138.9 3,140.8 3,411.1 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 3,185.4 3,180.4 3,180.4 3,450.8 Average unvested restricted common shares (3) (43.4 ) (41.5 ) (39.6 ) (39.7 ) Diluted earnings per share denominator 3,142.0 3,138.9 3,140.8 3,411.1 Basic and diluted earnings (loss) per share (in dollars) - discontinued operations: $ .28 $ (5.44 ) $ .50 $ (4.98 ) (1) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities. (2) Net earnings attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the three and nine months ended September 30, 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_2

Basis of Presentation and Significant Accouting Policies (Policies)	9 Months Ended
	Sep. 30, 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 Company’s website is located at www.whitemountains.com. The information contained on White Mountains’s website is not incorporated by reference into, and is not a part of, this report. The accompanying consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) and include the accounts of 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. 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	Business Combinations
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Reportable Segments	Reportable Segments White Mountains has determined its reportable segments based on the nature of the underlying businesses, the manner in which the Company’s subsidiaries and affiliates are organized and managed and the organization of the financial information provided to the chief operating decision maker to assess performance and make decisions regarding allocation of resources. White Mountains’s reportable segments are HG Global/BAM, NSM, Kudu, 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 September 30, 2019 , $481.3 million of the surplus notes remain outstanding. As of September 30, 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, real estate, social services and pet health insurance. 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 September 30, 2019 and December 31, 2018, White Mountains owned 96.4% and 95.5% of the basic units outstanding of NSM ( 88.4% and 85.0% on a fully diluted, fully converted basis). See Note 2 — “Significant Transactions ”. The Kudu segment consists of Kudu Investment Management, LLC (“Kudu”), a capital provider to global asset management and wealth management firms. Kudu specializes in providing capital solutions to asset managers and registered investment advisers for purposes including generational ownership transfers, management buyouts, acquisition and growth finance and legacy partner liquidity. Kudu also provides strategic assistance to investees from time to time. Kudu’s capital solutions typically are structured as long-term or permanent revenue shares that generate cash yields and have additional upside equity participation. On April 4, 2019, White Mountains completed its acquisition of the ownership interests in Kudu (the “Kudu Transaction”) held by certain funds managed by Oaktree Capital Management, L.P. (“Oaktree”). In addition, White Mountains assumed all of Oaktree’s unfunded capital commitments to Kudu, increasing White Mountains’s total capital commitment to $250 million , of which $28.1 million is unfunded as of September 30, 2019. As a result of the Kudu Transaction, White Mountains’s basic unit ownership of Kudu increased from 49.5% to 99.1% ( 42.7% to 85.4% on a fully diluted, fully converted basis), and White Mountains began consolidating Kudu in its financial statements during the second quarter of 2019. The MediaAlpha segment consisted of QL Holdings LLC and its wholly-owned subsidiary QuoteLab, LLC (collectively “MediaAlpha”). MediaAlpha is a marketing technology company that enables the programmatic buying and selling of vertical-specific, performance-based media between advertisers (buyers of advertising inventory) and publishers (sellers of advertising inventory). MediaAlpha operates in insurance and non-insurance verticals and uses 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 media to advertisers through transparent, programmatic, auction-based marketplaces. 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 deconsolidated MediaAlpha as a result of the MediaAlpha Transaction. White Mountains’s consolidated statement of comprehensive income and its segment disclosures include MediaAlpha’s results of operations through the date of the MediaAlpha Transaction. 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”), Kudu (for periods prior to the Kudu Transaction), MediaAlpha (for periods after the MediaAlpha Transaction), and Elementum Holdings LP (“Elementum”) and 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 September 30, 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 Cash
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, White Mountains recognized $23.2 million for both the lease right-of-use assets and lease liabilities. As of September 30, 2019, White Mountains recognized $23.0 million and $ 23.3 million of lease right-of-use assets and 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 (ASC 326), which establishes new guidance for the recognition of credit losses for financial assets measured at amortized cost. The new ASU requires reporting entities to estimate the credit losses expected over the life of a credit exposure using historical information, current information and reasonable and supportable forecasts that affect the collectability of the financial asset. This differs from current GAAP, which delays recognition until it is probable a loss has been incurred. The new guidance is expected to accelerate recognition of credit losses. The types of assets within the scope of the new guidance include premium receivables, reinsurance recoverables and loans. ASU 2016-13 is effective for annual periods beginning after January 1, 2020, including interim periods. White Mountains measures its portfolio of investment securities at fair value with changes therein recognized through current period earnings and, accordingly, does not expect adoption to have any effect on its financial statements.

Investment Securities (Tables)

Investment Securities (Tables)	9 Months Ended
	Sep. 30, 2019
Investments, Debt and Equity Securities [Abstract]
Pre-tax net investment income	The following table presents pre-tax net investment income for the three and nine months ended September 30, 2019 and 2018 : Three Months Ended Nine Months Ended September 30, September 30, Millions 2019 2018 2019 2018 Fixed maturity investments $ 8.0 $ 7.5 $ 24.4 $ 27.3 Short-term investments 1.3 1.8 3.9 6.5 Common equity securities 2.5 3.4 10.8 11.2 Other long-term investments 7.0 1.2 17.4 1.7 Total investment income 18.8 13.9 56.5 46.7 Third-party investment expenses (.4 ) (.5 ) (1.1 ) (1.8 ) Net investment income, pre-tax $ 18.4 $ 13.4 $ 55.4 $ 44.9
Schedule of Realized and Unrealized Gain (Loss) on Investments	The following table presents net realized and unrealized investment gains (losses) for the three and nine months ended September 30, 2019 and 2018 : Three Months Ended Nine Months Ended September 30, September 30, Millions 2019 2018 2019 2018 Net realized investment gains (losses), pre-tax (1) $ 19.6 $ (10.2 ) $ 90.8 $ (13.9 ) Net unrealized investment gains, pre-tax (2) 47.4 76.3 257.0 36.8 Net realized and unrealized investment gains, pre-tax 67.0 66.1 347.8 22.9 Income tax expense attributable to net realized and unrealized investment gains (13.2 ) (9.7 ) (69.5 ) (5.9 ) Net realized and unrealized investment gains, after-tax $ 53.8 $ 56.4 $ 278.3 $ 17.0 (1) Excludes $67.5 realized gain associated with the MediaAlpha Transaction, which is recorded in a separate line item in the statement of operations titled realized gain and unrealized investment gain from the MediaAlpha Transaction. See Note 2 — “Significant Transactions.” (2) Includes $114.7 unrealized investment gain associated with the MediaAlpha Transaction, which is recorded in a separate line item in the statement of operations titled realized gain and unrealized investment gain from the MediaAlpha Transaction. See Note 2 — “Significant Transactions.”
Schedule of realized gain (loss) on investments	The following tables present net realized investment gains (losses) for the three and nine months ended September 30, 2019 and 2018 : Three Months Ended Three Months Ended September 30, 2019 September 30, 2018 Millions Net Net Total Net Realized Net Net Total Net Realized Fixed maturity investments $ 1.4 $ — $ 1.4 $ (5.1 ) $ — $ (5.1 ) Common equity securities 14.0 — 14.0 (1.5 ) — (1.5 ) Other long-term investments 4.2 — 4.2 (3.6 ) — (3.6 ) Net realized investment gains (losses), pre-tax 19.6 — 19.6 (10.2 ) — (10.2 ) Income tax (expense) benefit attributable to net realized investment gains (losses) (.2 ) — (.2 ) 2.3 — 2.3 Net realized investment gains (losses), after-tax $ 19.4 $ — $ 19.4 $ (7.9 ) $ — $ (7.9 ) Nine Months Ended Nine Months Ended September 30, 2019 September 30, 2018 Millions Net Net Total Net Realized Net Net Total Net Realized Fixed maturity investments $ 3.5 $ — $ 3.5 $ (27.6 ) $ 18.2 $ (9.4 ) Short-term investments .1 — .1 (.8 ) — (.8 ) Common equity securities 81.4 — 81.4 10.5 — 10.5 Other long-term investments (1) 5.8 — 5.8 (7.0 ) (7.2 ) (14.2 ) Net realized investment gains (losses), pre-tax 90.8 — 90.8 (24.9 ) 11.0 (13.9 ) Income tax (expense) benefit attributable to net realized investment gains (losses) (9.0 ) — (9.0 ) 11.2 — 11.2 Net realized investment gains (losses), after-tax $ 81.8 $ — $ 81.8 $ (13.7 ) $ 11.0 $ (2.7 )
Schedule of unrealized gain (loss) on investments	The following tables present net unrealized investment gains (losses) and changes in the carrying value of investments measured at fair value for the three and nine months ended September 30, 2019 and 2018 : Three Months Ended Three Months Ended September 30, 2019 September 30, 2018 Millions Net Unrealized Gains (Losses) Net Total Net Unrealized Gains (Losses) Reflected in Earnings Net Unrealized Gains (Losses) Net Total Net Unrealized Gains (Losses) Reflected in Earnings Fixed maturity investments $ 7.5 $ — $ 7.5 $ 1.5 $ — $ 1.5 Short-term investments — — — (.1 ) — (.1 ) Common equity securities (11.6 ) — (11.6 ) 54.9 — 54.9 Other long-term investments 52.3 (.8 ) 51.5 20.1 (.1 ) 20.0 Net unrealized investment gains (losses), pre-tax 48.2 (.8 ) 47.4 76.4 (.1 ) 76.3 Income tax expense attributable to net unrealized investment gains (13.0 ) — (13.0 ) (12.0 ) — (12.0 ) Net unrealized investment gains (losses), after-tax $ 35.2 $ (.8 ) $ 34.4 $ 64.4 $ (.1 ) $ 64.3 Nine Months Ended Nine Months Ended September 30, 2019 September 30, 2018 Millions Net Unrealized Gains Net Total Net Unrealized Gains Reflected in Earnings Net Unrealized (Losses) Gains Net Gains Total Net Unrealized (Losses) Gains Reflected in Earnings Fixed maturity investments $ 43.4 $ — $ 43.4 $ (15.7 ) $ (14.8 ) $ (30.5 ) Short-term investments .2 — .2 (.1 ) — (.1 ) Common equity securities 60.1 — 60.1 43.7 — 43.7 Other long-term investments (1) 154.1 (.8 ) 153.3 20.5 3.2 23.7 Net unrealized investment gains (losses), pre-tax 257.8 (.8 ) 257.0 48.4 (11.6 ) 36.8 Income tax expense attributable to net unrealized investment gains (60.5 ) — (60.5 ) (17.1 ) — (17.1 ) Net unrealized investment gains (losses), after-tax $ 197.3 $ (.8 ) $ 196.5 $ 31.3 $ (11.6 ) $ 19.7 (1) Includes $114.7 unrealized investment gain associated with the MediaAlpha Transaction, which is recorded in a separate line item in the statement of operations titled realized gain and unrealized investment gain from the MediaAlpha Transaction. See Note 2 — “Significant Transactions.”
Net unrealized investment gains (losses) for Level 3 investments	The following table presents total gains (losses) included in earnings attributable to net unrealized investment gains (losses) for Level 3 investments for the three and nine months ended September 30, 2019 and 2018 for investments held at September 30, 2019 and 2018: Three Months Ended Nine Months Ended September 30, September 30, Millions 2019 2018 2019 2018 Other long-term investments $ 54.4 $ 4.0 $ 154.4 $ 2.1 Total net unrealized investment gains, pre-tax - Level 3 investments $ 54.4 $ 4.0 $ 154.4 $ 2.1
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 September 30, 2019 and December 31, 2018 : September 30, 2019 Millions Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Carrying Value U.S. Government and agency obligations $ 196.3 $ 1.3 $ — $ 197.6 Debt securities issued by corporations 449.5 13.8 (.1 ) 463.2 Municipal obligations 292.4 15.7 (.1 ) 308.0 Mortgage and asset-backed securities 195.0 2.5 (.3 ) 197.2 Total fixed maturity investments $ 1,133.2 $ 33.3 $ (.5 ) $ 1,166.0 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 September 30, 2019 and December 31, 2018 : September 30, 2019 Millions Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Net Foreign Currency Losses Carrying Value Common equity securities $ 548.8 $ 80.9 $ — $ — $ 629.7 Other long-term investments $ 619.5 $ 222.8 $ (65.9 ) $ (2.7 ) $ 773.7 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 September 30, 2019 and December 31, 2018 : Carrying Value at Millions September 30, 2019 December 31, 2018 Kudu investments (1)(2) $ 219.3 $ — MediaAlpha (1)(3) 150.0 — PassportCard/DavidShield (1) 90.0 75.0 Elementum (1) 55.1 — Other unconsolidated entities (1)(4) 39.5 60.0 Kudu (5) — 30.7 Total unconsolidated entities (1) 553.9 165.7 Private equity funds and hedge funds 145.5 146.1 Insurance-linked securities funds 41.4 — Other 32.9 13.8 Total other long-term investments $ 773.7 $ 325.6 (1) See Fair Value Measurements by Level table. (2) Includes Kudu’s non-controlling equity interests in the form of revenue and earnings participations. As of September 30, 2019 White Mountains consolidates Kudu. See Note 2 — “Significant Transactions”. (3) The MediaAlpha Transaction was completed on February 26, 2019. See Note 2 — “Significant Transactions”. (4) Includes White Mountains’s non-controlling interests in certain private common equity securities, limited liability companies and convertible preferred securities. (5) Includes White Mountains non-controlling interest in Kudu prior to consolidation. See Note 2 — “Significant Transactions”.
Other long-term investments	The following table presents investments and unfunded commitments in private equity funds and hedge funds by investment objective and sector as of September 30, 2019 and December 31, 2018 : September 30, 2019 December 31, 2018 Millions Fair Value Unfunded Commitments Fair Value Unfunded Commitments Private equity funds Manufacturing/Industrial $ 45.3 $ 4.1 $ 42.9 $ 10.5 Aerospace/Defense/Government 32.9 24.5 27.6 34.9 Financial services 12.4 26.2 8.3 13.6 Direct lending — — 13.0 17.7 Total private equity funds 90.6 54.8 91.8 76.7 Hedge funds Long/short banks and financials 54.9 — 54.3 — Total hedge funds 54.9 — 54.3 — Total private equity funds and hedge funds included in other long-term investments $ 145.5 $ 54.8 $ 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 September 30, 2019 : Millions 1 – 3 years 3 – 5 years 5 – 10 years >10 years Total Private equity funds — expected lock-up period remaining $5.4 $16.5 $56.3 $12.4 $90.6
Fair value measurements by level, investment securities	The following tables present White Mountains’s fair value measurements for investments as of September 30, 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. September 30, 2019 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 197.6 $ 197.6 $ — $ — Debt securities issued by corporations: Financials 143.3 — 143.3 — Industrial 60.5 — 60.5 — Healthcare 55.0 — 55.0 — Consumer 49.3 — 49.3 — Energy 45.0 — 45.0 — Technology 44.5 — 44.5 — Communications 27.1 — 27.1 — Utilities 22.3 — 22.3 — Materials 16.2 — 16.2 — Total debt securities issued by corporations 463.2 — 463.2 — Mortgage and asset-backed securities 197.2 — 197.2 — Municipal obligations 308.0 — 308.0 — Total fixed maturity investments 1,166.0 197.6 968.4 — Short-term investments (1) 272.5 261.0 11.5 — Common equity securities: Exchange traded funds (2) 519.7 506.1 13.6 — Other (3) 110.0 — 110.0 — Total common equity securities 629.7 506.1 123.6 — Other long-term investments 586.8 — — 586.8 Other long-term investments — NAV (4) 186.9 — — — Total investments $ 2,841.9 $ 964.7 $ 1,103.5 $ 586.8 (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 private equity funds, one hedge fund and ILS funds for which fair value is measured at NAV using the practical expedient. Investments for which fair value is measured at NAV are not classified within the fair value hierarchy. 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, credit ratings	The following table presents the ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of September 30, 2019 and December 31, 2018 : Fair Value at Millions September 30, 2019 December 31, 2018 AAA $ 9.9 $ 8.9 AA 79.9 88.7 A 286.7 270.5 BBB 86.7 142.4 Debt securities issued by corporations (1) $ 463.2 $ 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 September 30, 2019 and December 31, 2018 : September 30, 2019 December 31, 2018 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: FNMA $ 79.8 $ 79.8 $ — $ 53.6 $ 53.6 $ — FHLMC 53.5 53.5 — 38.1 38.1 $ — GNMA 33.5 33.5 — 23.7 23.7 $ — Total agency (1) 166.8 166.8 — 115.4 115.4 — Total mortgage-backed securities 166.8 166.8 — 115.4 115.4 — Other asset-backed securities: Vehicle receivables 16.5 16.5 — 9.2 9.2 — Credit card receivables 13.9 13.9 — 8.9 8.9 — Total other asset-backed securities 30.4 30.4 — 18.1 18.1 — Total mortgage and asset-backed securities $ 197.2 $ 197.2 $ — $ 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 September 30, 2019 and 2018 : Level 3 Investments Unconsolidated Entities, Private Equity Funds, Hedge Funds and ILS Funds Measured at NAV (3)(5) Millions Level 1 Investments Level 2 Investments Other Long-term Investments (2)(5)(6) Total Balance at December 31, 2018 $ 842.6 $ 1,160.5 $ 138.7 $ 186.9 $ 2,328.7 (1) Net realized and unrealized gains 121.8 66.6 154.2 4.9 347.5 (2,4) Amortization/Accretion .2 (1.2 ) — — (1.0 ) Purchases 101.4 331.3 211.3 106.4 750.4 (6) Sales (362.3 ) (465.2 ) — (28.7 ) (856.2 ) Transfers in — — 82.6 — 82.6 Transfers out — — — (82.6 ) (82.6 ) Balance at September 30, 2019 $ 703.7 $ 1,092.0 $ 586.8 $ 186.9 $ 2,569.4 (1) (1) Excludes carrying value of $ 272.5 million and $ 214.2 as of September 30, 2019 and December 31, 2018 classified as short-term investments. (2) Includes $114.7 unrealized investment gain associated with the MediaAlpha Transaction, which is recorded in a separate line item in the statement of operations titled realized gain and unrealized investment gain from 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” . (4) Excludes realized and unrealized losses associated with short-term investments of $0.3 for the nine months ended September 30, 2019. (5) Transfers include $71.7 associated with the Kudu Transaction. See Note 2 — “Significant Transactions”. (6) Purchases include $70.1 associated with the Kudu Transaction. See Note 2 — “Significant Transactions”. 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 gains (losses) 51.9 (33.6 ) (5.3 ) 18.3 31.3 (4) Amortization/Accretion .1 (2.3 ) — — (2.2 ) Purchases 412.6 673.5 55.1 10.7 1,151.9 Sales (457.6 ) (1,565.0 ) — (1.6 ) (2,024.2 ) Transfers in — — — — — Transfers out — — — — — Balance at September 30, 2018 $ 897.4 $ 1,178.0 $ 127.0 $ 162.7 $ 2,365.1 (1) (1) Excludes carrying value of $ 313.7 and $ 176.1 as of September 30, 2018 and December 31, 2017 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 $0.7 for the nine months ended September 30, 2018.
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, hedge funds and ILS funds, classified within Level 3 as of September 30, 2019 and December 31, 2018 . The fair value of investments in private equity funds, hedge funds and ILS funds are generally estimated using the NAV of the funds. $ in Millions September 30, 2019 Description Valuation Technique(s) Fair Value (1) Unobservable Inputs Discount Rate Terminal revenue exit multiple (x) or terminal revenue growth rate (%) Kudu investments Discounted cash flow $219.3 15% - 20% 6x - 12x MediaAlpha Discounted cash flow $150.0 17% 4% PassportCard/DavidShield Discounted cash flow $90.0 18% 1x Private debt instruments Discounted cash flow $22.7 6% - 9% N/A All other Discounted cash flow $29.3 23% 2x - 4x Elementum Holdings, L.P. Price of recent transaction (2) $55.1 Transaction price: $55.1 Compare.com Estimated net realizable value $2.5 Net asset value: $2.5 Galvanic Applied Sciences Multiple of EBITDA $2.7 EBITDA multiple: 6 (1) Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs. (2) The Elementum Transaction was completed on May 31, 2019. See Note 2 — “Significant Transactions”. $ in Millions December 31, 2018 Description Valuation Technique(s) Fair Value (1) Unobservable Inputs Discount Rate Terminal revenue exit multiple (x) PassportCard/DavidShield Discounted cash flow $75.0 18% 1x Compare.com Discounted cash flow $16.9 22% 3x Private debt instrument Discounted cash flow $10.0 9% N/A All other Discounted cash flow $30.0 23% 2x - 4x Galvanic Applied Sciences Multiple of EBITDA $3.1 EBITDA multiple: 6 (1) Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs.

Goodwill and Other Intangible_2

Goodwill and Other Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Recognized Identified Assets Acquired and Liabilities Assumed	The following table presents a summary of the acquisition date fair values of goodwill and other intangible assets for acquisitions during the nine months ended September 30, 2019 and 2018: $ in Millions Acquisition of subsidiary/ asset Goodwill and Other intangible asset Acquisition Date NSM $ 383.0 May 11, 2018 Fresh Insurance 54.6 May 18, 2018 KBK 59.4 December 3, 2018 Embrace 70.6 (1) April 1, 2019 Renewal Rights (2) 82.5 June 28, 2019 Total NSM segment $ 650.1 Kudu Transaction $ 9.8 April 4, 2019 Other Operations $ 13.1 (1) March 12, 2019 (1) The relative fair values of goodwill and of other intangible assets recognized in connection with the acquisition of Embrace and the acquisition in Other Operations had not yet been finalized at September 30, 2019.
Changes in Goodwill and Intangible Assets	he following tables presents the change in goodwill and other intangible assets for the three and nine months ended September 30, 2019 and 2018 : Three Months Ended September 30, 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 $ 428.6 $ 239.0 $ 667.6 $ 25.9 $ 30.7 $ 56.6 Attribution of acquisition date fair value estimates between goodwill and other intangible assets (2.2 ) 2.2 — 295.3 140.9 436.2 Foreign currency translation (1.6 ) (.5 ) (2.1 ) (1.2 ) (.4 ) — (1.6 ) Adjustments to reflect acquisition date fair value — — — 1.4 — — 1.4 Amortization — (4.6 ) (4.6 ) — (7.4 ) (7.4 ) Ending balance $ 424.8 $ 236.1 $ 660.9 $ 321.4 $ 163.8 $ 485.2 Nine Months Ended September 30, 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 (29.0 ) 29.0 — 295.3 140.9 436.2 Acquisitions of businesses 93.8 (1) — 93.8 — — — Acquisition of collector car renewal rights — 82.5 82.5 — — — Foreign currency translation (1.6 ) — (1.6 ) (1.2 ) (.4 ) — (1.6 ) Adjustments to reflect acquisition date fair value — 5.9 5.9 1.4 — — 1.4 Amortization — (15.4 ) (15.4 ) — (12.9 ) (12.9 ) Ending balance $ 424.8 $ 236.1 $ 660.9 $ 321.4 $ 163.8 $ 485.2 (1) The relative fair values of goodwill and other intangible assets recognized in connection with the acquisition of Embrace and the acquisition in Other Operations had not yet been finalized at September 30, 2019.
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 September 30, 2019 and December 31, 2018 : $ in Millions Weighted Average Economic life (in years) September 30, 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) N/A $ 397.0 $ — $ 397.0 $ 354.3 $ — $ 354.3 Kudu N/A 7.6 — 7.6 — — — MediaAlpha N/A — — — 18.3 — 18.3 Other Operations (2) N/A 20.2 — 20.2 7.3 — 7.3 Total goodwill 424.8 — 424.8 379.9 — 379.9 Other intangible assets: NSM (1) Customer relationships 9 110.5 15.5 95.0 85.3 6.0 79.3 Trade names 19 57.1 4.2 52.9 51.2 1.8 49.4 Information technology 5 3.6 1.1 2.5 3.7 .5 3.2 Renewal rights 12 82.5 .3 82.2 — — — Other 3 1.7 .7 1.0 — — — Subtotal 255.4 21.8 233.6 140.2 8.3 131.9 Kudu Trade names 7 2.2 .2 2.0 — — — MediaAlpha Customer relationships 9 — — — 26.8 4.9 21.9 Information technology 5 — — — 33.3 30.9 2.4 Other 3 — — — 9.8 9.0 .8 Subtotal — — — 69.9 44.8 25.1 Other Operations Trade names 7 .6 .2 .4 .6 .2 .4 Information technology 5 .4 .3 .1 .5 .3 .2 Subtotal 1.0 .5 .5 1.1 .5 .6 Total other intangible assets 258.6 22.5 236.1 211.2 53.6 157.6 Total goodwill and other intangible assets $ 683.4 $ 22.5 660.9 $ 591.1 $ 53.6 537.5 Goodwill and other intangible assets attributed to non-controlling interests (25.0 ) (40.6 ) Goodwill and other intangible assets included in White Mountains’s common shareholders’ equity $ 635.9 $ 496.9 (1) As of September 30, 2019 , NSM’s goodwill included $(1.5) and $ (2.2) of the effect of foreign currency translation. As of December 31, 2018, NSM’s intangible assets include $ (0.7) of the effect of foreign currency translation. (2) As of September 30, 2019 and December 31, 2018, Other Operations’ goodwill included $(0.1) and $ (0.3) of the effect of foreign currency translation.

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2019
Debt Disclosure [Abstract]
Schedule of debt outstanding	The following table presents White Mountains’s debt outstanding as of September 30, 2019 and December 31, 2018: Millions September 30, Effective Rate (1) December 31, Effective Rate (1) NSM Bank Facility $ 221.8 7.7% $ 180.4 7.4% Unamortized issuance cost (4.1 ) (3.8 ) NSM Bank Facility, carrying value 217.7 176.6 Other NSM debt 1.7 2.5% 1.9 2.2% MediaAlpha Bank Facility — 14.3 7.1% Unamortized issuance cost — (.1 ) MediaAlpha Bank Facility, carrying value — 14.2 Other Operations debt 11.2 9.6% — Unamortized issuance cost (.4 ) — Other Operations, carrying value 10.8 — Total debt $ 230.2 $ 192.7 (1) Effective rate considers the effect of the debt issuance costs.

Municipal Bond Guarantee (Table

Municipal Bond Guarantee (Tables)	9 Months Ended
	Sep. 30, 2019
Guarantees [Abstract]
Schedule of Municipal Bond Guarantee Insured Obligations	The following table presents a schedule of BAM’s insured obligations as of September 30, 2019 and December 31, 2018: September 30, 2019 December 31, 2018 Contracts outstanding 8,508 7,525 Remaining weighted average contract period outstanding (in years) 10.7 10.7 Contractual debt service outstanding (in millions): Principal $ 59,190.9 $ 52,201.6 Interest 30,701.7 26,560.3 Total debt service outstanding $ 89,892.6 $ 78,761.9 Gross unearned insurance premiums (in millions) $ 203.5 $ 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 September 30, 2019 : Millions September 30, 2019 October 1, 2019 - December 31, 2019 $ 4.9 January 1, 2020 - March 31, 2020 4.8 April 1, 2020 - June 30, 2020 4.8 July 1, 2020 - September 30, 2020 4.6 October 1, 2020 - December 31, 2020 4.5 Total 2020 18.7 2021 17.4 2022 16.3 2023 15.3 2024 14.3 2025 and thereafter 116.6 Total gross unearned insurance premiums $ 203.5
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 and nine months ended September 30, 2019 and 2018 : Three Months Ended September 30, Nine Months Ended September 30, Millions 2019 2018 2019 2018 Written premiums: Direct $ 10.2 $ 3.9 $ 30.6 $ 28.9 Assumed 10.6 — 10.6 — Gross written premiums $ 20.8 $ 3.9 $ 41.2 $ 28.9 Earned premiums: Direct $ 4.4 $ 3.3 $ 11.9 $ 9.7 Assumed .8 — 1.8 — Gross earned premiums $ 5.2 $ 3.3 $ 13.7 $ 9.7

Investments in Unconsolidated_2

Investments in Unconsolidated Entities (Tables)	9 Months Ended
	Sep. 30, 2019
Schedule of Equity Method Investments [Line Items]
Investments in and Advances to Affiliates	The following table presents the carrying values of White Mountains’s equity method eligible investments recorded within other long-term investments as of September 30, 2019 and December 31, 2018: Millions September 30, 2019 December 31, 2018 Equity method eligible investments, at fair value $ 673.2 $ 269.7 Other (1) 100.5 55.9 Total other long-term investments $ 773.7 $ 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 significant equity method eligible investments as of September 30, 2019 and December 31, 2018: Basic Ownership Interest Investee September 30, 2019 December 31, 2018 Instrument Held PassportCard/DavidShield 50.0% 50.0% Common shares MediaAlpha (1) 48.3% n/a Units durchblicker 45.0% 45.0% Common shares Elementum Holdings, L.P. 30.0% n/a Limited partnership interest Tuckerman Capital Funds (2) 18.8 - 62.4% 18.5 - 62.3% Limited and general partnership interests Compare.com 18.4% 18.4% Common shares JAM Partners L.P. 11.1% 12.3% Limited partnership interest Enlightenment Capital Funds 10.0 - 36.5% 10.3 - 36.5% Limited and general partnership interests Kudu investments (3) 3.2 - 30.0% n/a Revenue and earnings participations Kudu (3) n/a 49.5% Units (1) As of December 31, 2018, MediaAlpha was a majority-owned consolidated subsidiary of White Mountains. See Note 2 — “Significant Transactions” (2) Includes White Mountains’s direct investment in Galvanic Applied Sciences. (3) As of September 30, 2019 White Mountains consolidates Kudu. See Note 2 — “Significant Transactions”.

Earnings Per Share (Tables)

Earnings Per Share (Tables)	9 Months Ended
	Sep. 30, 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 and nine months ended September 30, 2019 and 2018 . See Note 17 — “Held for Sale and Discontinued Operations” . Three Months Ended Nine Months Ended September 30, September 30, 2019 2018 2019 2018 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 48.7 $ 40.8 $ 353.6 $ (3.7 ) Less: total income (loss) from discontinued operations, net of tax .9 (17.3 ) 1.6 (17.2 ) Net income from continuing operations attributable to White Mountains’s common shareholders $ 47.8 $ 58.1 $ 352.0 $ 13.5 Allocation of earnings to participating restricted common shares (1) (.7 ) (.8 ) (4.4 ) (.2 ) Basic and diluted earnings per share numerators $ 47.1 $ 57.3 $ 347.6 $ 13.3 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 3,185.4 3,180.4 3,180.4 3,450.8 Average unvested restricted common shares (2) (43.4 ) (41.5 ) (39.6 ) (39.7 ) Basic earnings per share denominator 3,142.0 3,138.9 3,140.8 3,411.1 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 3,185.4 3,180.4 3,180.4 3,450.8 Average unvested restricted common shares (2) (43.4 ) (41.5 ) (39.6 ) (39.7 ) Diluted earnings per share denominator 3,142.0 3,138.9 3,140.8 3,411.1 Basic and diluted earnings per share (in dollars) - continuing operations: Distributed earnings - dividends declared and paid $ — $ — $ 1.00 $ 1.00 Undistributed earnings 15.01 18.27 109.67 2.90 Basic and diluted earnings per share $ 15.01 $ 18.27 $ 110.67 $ 3.90 (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 and nine months ended September 30, 2019 and 2018 . See Note 17 — “Held for Sale and Discontinued Operations” . Three Months Ended Nine Months Ended September 30, September 30, Millions 2019 2018 2019 2018 Undistributed net earnings - continuing operations: Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 47.1 $ 57.3 $ 347.6 $ 13.3 Dividends declared, net of restricted common share amounts (1) — — (3.2 ) (3.7 ) Total undistributed net earnings, net of restricted common share amounts $ 47.1 $ 57.3 $ 344.4 $ 9.6 (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. three and nine months ended September 30, 2019 and 2018 : Three Months Ended Nine Months Ended September 30, September 30, 2019 2018 2019 2018 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 48.7 $ 40.8 $ 353.6 $ (3.7 ) Less: total income from continuing operations, net of tax 47.8 58.1 352.0 13.5 Net income (loss) from discontinued operations attributable to White Mountains’s common shareholders $ .9 $ (17.3 ) $ 1.6 $ (17.2 ) Allocation of earnings to participating restricted common shares (1) — .2 — .2 Basic and diluted earnings (loss) per share numerators (2) $ .9 $ (17.1 ) $ 1.6 $ (17.0 ) Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 3,185.4 3,180.4 3,180.4 3,450.8 Average unvested restricted common shares (3) (43.4 ) (41.5 ) (39.6 ) (39.7 ) Basic earnings per share denominator 3,142.0 3,138.9 3,140.8 3,411.1 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 3,185.4 3,180.4 3,180.4 3,450.8 Average unvested restricted common shares (3) (43.4 ) (41.5 ) (39.6 ) (39.7 ) Diluted earnings per share denominator 3,142.0 3,138.9 3,140.8 3,411.1 Basic and diluted earnings (loss) per share (in dollars) - discontinued operations: $ .28 $ (5.44 ) $ .50 $ (4.98 ) (1) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities. (2) Net earnings attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the three and nine months ended September 30, 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	9 Months Ended
	Sep. 30, 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 and nine months ended September 30, 2019 and 2018 for performance shares granted under the WTM Incentive Plan: Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Millions, except share amounts Target Performance Accrued Expense Target Performance Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Accrued Expense Beginning of period 42,473 $ 30.8 40,616 $ 27.2 40,616 $ 31.7 50,515 $ 45.8 Shares paid (1) — — — — (13,715 ) (18.1 ) (23,186 ) (28.4 ) New grants — — — — 15,600 — 14,105 — Forfeitures and cancellations (2) — — — (.1 ) (28 ) — (818 ) .1 Expense recognized — 5.6 — 7.8 — 22.8 — 17.4 End of period 42,473 $ 36.4 40,616 $ 34.9 42,473 $ 36.4 40,616 $ 34.9 (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 September 30, 2019 for each performance cycle: Nine Months Ended September 30, 2019 Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Performance cycle: 2017 – 2019 14,070 $ 24.4 2018 – 2020 13,450 8.5 2019 – 2021 15,600 4.0 Sub-total 43,120 36.9 Assumed forfeitures (647 ) (.5 ) September 30, 2019 42,473 $ 36.4
Summary of restricted shares activity	The following table presents the unrecognized compensation cost associated with the outstanding restricted share awards for the three and nine months ended September 30, 2019 and 2018 : Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Millions, except share amounts Restricted Unamortized Restricted Unamortized Restricted Unamortized Issue Date Restricted Unamortized Issue Date Non-vested, Beginning of period 43,395 $ 21.9 41,510 $ 19.0 41,510 $ 12.5 53,755 $ 14.3 Issued — — — — 15,600 14.5 14,105 11.4 Vested — — — — (13,715 ) — (25,381 ) — Forfeited — — — — — — (969 ) (.2 ) Expense recognized — (2.6 ) — (3.2 ) — (7.7 ) — (9.7 ) End of period 43,395 $ 19.3 41,510 $ 15.8 43,395 $ 19.3 41,510 $ 15.8

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2019
Leases [Abstract]
Lease, Cost	The following table summarizes net lease expense recognized in White Mountains’s consolidated statement of operations for the three and nine months ended September 30, 2019: Millions Lease Cost Three Months Ended September 30, 2019 Nine Months Ended September 30, 2019 Lease cost $ 1.9 $ 5.4 Less: sublease income .1 .3 Net lease cost $ 1.8 $ 5.1
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 September 30, 2019 : Millions As of September 30, 2019 Remainder of 2019 $ 1.7 2020 5.7 2021 4.7 2022 4.2 2023 3.6 Thereafter 6.3 Total undiscounted lease payments 26.2 Less: present value adjustment 2.9 Operating lease liability $ 23.3
Schedule of Right-of-Use Assets and Lease Liabilities	The following table presents lease related assets and liabilities by reportable segment as of September 30, 2019: As of September 30, 2019 Millions HG/BAM NSM Kudu Other Operations Total Weighted Average Incremental Borrowing Rate (1) ROU lease asset $ 10.8 $ 4.4 $ 2.3 $ 5.5 $ 23.0 4.5% Lease liability $ 10.8 $ 4.4 $ 2.3 $ 5.8 $ 23.3 (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)	9 Months Ended
	Sep. 30, 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 September 30, 2019 and December 31, 2018 : September 30, 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 % $ 14.9 3.1 % $ 14.5 NSM 3.6 15.2 4.5 13.6 Other NSM 13.4 .1 13.4 .3 Kudu 1.0 2.3 — — MediaAlpha — — 39.0 16.2 Other Operations various .2 various 1.1 Total, excluding BAM 32.7 45.7 BAM 100.0 (155.2 ) 100.0 (170.6 ) Total non-controlling interests $ (122.5 ) $ (124.9 )

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2019
Segment Reporting [Abstract]
Financial information for White Mountains' segments	in. The following table presents the financial information for White Mountains’s segments: Millions HG Global/ BAM NSM Kudu Other Operations Total Three Months Ended September 30, 2019 Earned insurance premiums $ 5.2 $ — $ — $ — $ 5.2 Net investment income 5.4 — 4.6 8.4 18.4 Net realized and unrealized investment gains 7.1 — 2.7 57.2 67.0 Advertising and commission revenues — 49.4 — 1.9 51.3 Other revenue .3 10.5 .2 2.6 13.6 Total revenues 18.0 59.9 7.5 70.1 155.5 Insurance acquisition expenses 1.4 — — — 1.4 Other underwriting expenses .1 — — — .1 Cost of sales — — — 2.3 2.3 General and administrative expenses 12.5 33.3 4.5 27.7 78.0 Broker commission expense — 15.3 — — 15.3 Change in fair value of contingent consideration — (2.0 ) — — (2.0 ) Amortization of other intangible assets — 4.4 .2 — 4.6 Interest expense — 4.4 — .3 4.7 Total expenses 14.0 55.4 4.7 30.3 104.4 Pre-tax income $ 4.0 $ 4.5 $ 2.8 $ 39.8 $ 51.1 Millions HG Global/ BAM NSM Kudu (1) MediaAlpha (2) Other Operations Total Nine Months Ended September 30, 2019 Earned insurance premiums $ 13.7 $ — $ — $ — $ — $ 13.7 Net investment income 16.1 — 8.6 — 30.7 55.4 Net realized and unrealized investment gains 30.4 — 3.1 — 199.6 233.1 Realized gain and unrealized investment gain from the MediaAlpha Transaction — — — — 182.2 182.2 Advertising and commission revenues — 147.6 — 48.8 4.5 200.9 Other revenue 1.3 27.7 .2 — 3.6 32.8 Total revenues 61.5 175.3 11.9 48.8 420.6 718.1 Insurance acquisition expenses 4.1 — — — — 4.1 Other underwriting expenses .3 — — — — .3 Cost of sales — — — 40.6 5.0 45.6 General and administrative expenses 39.1 91.4 6.7 12.5 87.2 236.9 Broker commission expense — 48.9 — — — 48.9 Change in fair value of contingent consideration — 5.6 — — — 5.6 Amortization of other intangible assets — 13.5 .2 1.6 .1 15.4 Interest expense — 12.2 .2 .3 12.7 Total expenses 43.5 171.6 6.9 54.9 92.6 369.5 Pre-tax income (loss) $ 18.0 $ 3.7 $ 5.0 $ (6.1 ) $ 328.0 $ 348.6 (1) Kudu’s results are from April 4, 2019, the date of the Kudu Transaction, to September 30, 2019. (2) MediaAlpha's results are from January 1, 2019 to February 26, 2019, the date of the MediaAlpha Transaction. Millions HG Global/BAM NSM MediaAlpha Other Operations Total Three Months Ended September 30, 2018 Earned insurance premiums $ 3.3 $ — $ — $ — $ 3.3 Net investment income 4.8 — — 8.6 13.4 Net realized and unrealized investment (losses) gains (4.1 ) — — 70.2 66.1 Advertising and commission revenues (1) — 35.0 74.5 1.1 110.6 Other revenue .2 4.7 — .4 5.3 Total revenues 4.2 39.7 74.5 80.3 198.7 Insurance acquisition expenses 1.2 — — — 1.2 Other underwriting expenses .1 — — — .1 Cost of sales — — 61.8 1.1 62.9 General and administrative expenses 11.3 23.5 5.4 26.0 66.2 Broker commission expense — 10.6 — — 10.6 Change in fair value of contingent consideration — 2.6 — — 2.6 Amortization of other intangible assets — 5.0 2.4 — 7.4 Interest expense — 3.2 .2 — 3.4 Total expenses 12.6 44.9 69.8 27.1 154.4 Pre-tax (loss) gains $ (8.4 ) $ (5.2 ) $ 4.7 $ 53.2 $ 44.3 (1) Approximately 30% of MediaAlpha’s advertising revenue for the three months ended September 30, 2018 was associated with one customer. Millions HG Global/BAM NSM (1) MediaAlpha Other Operations Total Nine Months Ended September 30, 2018 Earned insurance premiums $ 9.7 $ — $ — $ — $ 9.7 Net investment income 12.5 — — 32.4 44.9 Net realized and unrealized investment (losses) gains (14.4 ) — — 37.3 22.9 Advertising and commission revenues (2) — 56.4 216.4 3.0 275.8 Other revenue .8 6.8 1.6 .6 9.8 Total revenues 8.6 63.2 218.0 73.3 363.1 Loss and loss adjustment expenses — — — — Insurance acquisition expenses 3.9 — — — 3.9 Other underwriting expenses .3 — — — .3 Cost of sales — — 179.1 2.9 182.0 General and administrative expenses 36.5 35.7 21.2 79.0 172.4 Broker commission expense — 17.1 — — 17.1 Change in fair value of contingent consideration — 2.6 — — 2.6 Amortization of other intangible assets — 5.0 7.8 .1 12.9 Interest expense — 4.8 .9 .3 6.0 Total expenses 40.7 65.2 209.0 82.3 397.2 Pre-tax (loss) gains $ (32.1 ) $ (2.0 ) $ 9.0 $ (9.0 ) $ (34.1 ) (1) NSM’s results are from May 11, 2018, the date of acquisition, to September 30, 2018. (2) Approximately 31% of MediaAlpha’s advertising revenue for the nine months ended September 30, 2018 was associated with one customer.

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 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 September 30, 2019 and December 31, 2018 : September 30, 2019 December 31, 2018 Millions Fair Value Carrying Value Fair Value Carrying Value NSM Bank Facility $ 219.7 $ 217.7 $ 176.1 $ 176.6 MediaAlpha Bank Facility $ — $ — $ 14.6 $ 14.2 Other Operations debt $ 11.3 $ 10.8 $ — $ —

Held for Sale and Discontinue_2

Held for Sale and Discontinued Operations (Tables)	9 Months Ended
	Sep. 30, 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 and nine months ended September 30, 2019 and 2018 . See Note 17 — “Held for Sale and Discontinued Operations” . Three Months Ended Nine Months Ended September 30, September 30, Millions 2019 2018 2019 2018 Undistributed net earnings - continuing operations: Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 47.1 $ 57.3 $ 347.6 $ 13.3 Dividends declared, net of restricted common share amounts (1) — — (3.2 ) (3.7 ) Total undistributed net earnings, net of restricted common share amounts $ 47.1 $ 57.3 $ 344.4 $ 9.6 (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. three and nine months ended September 30, 2019 and 2018 : Three Months Ended Nine Months Ended September 30, September 30, 2019 2018 2019 2018 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 48.7 $ 40.8 $ 353.6 $ (3.7 ) Less: total income from continuing operations, net of tax 47.8 58.1 352.0 13.5 Net income (loss) from discontinued operations attributable to White Mountains’s common shareholders $ .9 $ (17.3 ) $ 1.6 $ (17.2 ) Allocation of earnings to participating restricted common shares (1) — .2 — .2 Basic and diluted earnings (loss) per share numerators (2) $ .9 $ (17.1 ) $ 1.6 $ (17.0 ) Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 3,185.4 3,180.4 3,180.4 3,450.8 Average unvested restricted common shares (3) (43.4 ) (41.5 ) (39.6 ) (39.7 ) Basic earnings per share denominator 3,142.0 3,138.9 3,140.8 3,411.1 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 3,185.4 3,180.4 3,180.4 3,450.8 Average unvested restricted common shares (3) (43.4 ) (41.5 ) (39.6 ) (39.7 ) Diluted earnings per share denominator 3,142.0 3,138.9 3,140.8 3,411.1 Basic and diluted earnings (loss) per share (in dollars) - discontinued operations: $ .28 $ (5.44 ) $ .50 $ (4.98 ) (1) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities. (2) Net earnings attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the three and nine months ended September 30, 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	Sep. 30, 2019	Apr. 04, 2019	Apr. 03, 2019	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	May 11, 2018	Jul. 31, 2012
Basis of Presentation
Ownership interest (as a percent)	50.00%					50.00%
ROU lease asset	$ 23
Operating lease liability	$ 23.3
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)	96.40%					95.50%	95.00%
Fully diluted ownership interest (as a percent)	88.40%					85.00%
Kudu Investment Management, LLC
Basis of Presentation
Ownership interest (as a percent)		99.10%	49.50%	49.50%		49.50%
Fully diluted ownership interest (as a percent)		85.40%	42.70%	42.70%		42.70%
Accounting Standards Update 2016-02
Basis of Presentation
ROU lease asset	$ 23				$ 23.2
Operating lease liability	$ 23.3				$ 23.2

Significant Transactions (Detai

Significant Transactions (Details)	Jun. 28, 2019USD ($)	Apr. 04, 2019USD ($)	Apr. 01, 2019USD ($)	Feb. 26, 2019USD ($)	Dec. 03, 2018USD ($)	May 11, 2018USD ($)	Jan. 24, 2018USD ($)	Sep. 30, 2019USD ($)	Mar. 31, 2019USD ($)	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2019USD ($)earnout_period	Sep. 30, 2018USD ($)	May 31, 2019USD ($)	Apr. 03, 2019USD ($)	Dec. 31, 2018USD ($)	May 18, 2018USD ($)	Feb. 05, 2018USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)								50.00%					50.00%				50.00%
Equity method investment, aggregate cost																			$ 125,000,000
Purchases of consolidated subsidiaries, net of cash acquired													$ 243,400,000	$ 252,100,000
Payments to acquire business, net		$ 81,400,000
Net gain from sale of discontinued operations, net of tax													67,500,000	$ 0
NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Change in fair value of contingent consideration earnout liabilities								$ (2,000,000)		$ 2,600,000		$ 2,600,000	5,600,000
Payment for contingent consideration liability													2,600,000
Contingent consideration earnout liabilities								28,700,000					28,700,000				$ 20,200,000
White Mountains
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Other Investments								773,700,000					773,700,000				$ 325,600,000
MediaAlpha
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)				48.30%
Fully diluted ownership interest (as a percent)				42.00%
Proceeds from dividends received				$ 89,300,000
Net gain from sale of discontinued operations, net of tax													67,500,000
Assets, fair value									$ 114,700,000
Unrealized investment gain								$ 35,300,000	114,700,000				150,000,000
Total realized and unrealized gain on sale of minority stake													$ 182,200,000
Elementum Holdings, L.P.
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)								30.00%					30.00%		30.00%
Other Investments															$ 55,100,000
Insurance-Linked Securities Funds
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Other Commitment															$ 50,000,000
Elementum Funds
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity method investment, aggregate cost								$ 40,000,000					$ 40,000,000
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%		96.40%					96.40%				95.50%
Purchases of consolidated subsidiaries, net of cash acquired						$ 274,200,000
Purchase price adjustment								$ 2,100,000
Assets acquired						495,200,000
Goodwill and intangible assets acquired						383,000,000
Liabilities acquired						204,600,000
Contingent consideration						10,200,000
Noncontrolling interest acquired						14,400,000
Transaction costs											$ 6,300,000
Number of earnout periods | earnout_period													2
Fully diluted ownership interest (as a percent)								88.40%					88.40%				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%
Equity method investment, aggregate cost																		$ 49,600,000
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
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
Stock issued during period, value, acquisitions					29,000,000
Proceeds from borrowings					$ 30,100,000
Intangible assets acquired									$ 32,700,000
AIG Collector Car Business
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchases of consolidated subsidiaries, net of cash acquired	$ 82,500,000
Intangible assets acquired	82,500,000
AIG Collector Car Business | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from borrowings	22,500,000
AIG Collector Car Business | White Mountains
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Stock issued during period, value, acquisitions	$ 59,100,000
DavidShield
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)							50.00%
PassportCard
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)							50.00%
DavidShield & PassportCard
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity method investment, aggregate cost							$ 41,800,000
Payments to acquire business, net							$ 28,300,000
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%							49.50%							49.50%	49.50%
Fully diluted ownership interest (as a percent)		85.40%							42.70%							42.70%	42.70%
Additional funding																	$ 63,000,000
Assets under management								$ 30,000,000,000					$ 30,000,000,000
Unfunded capital commitment		$ 250,000,000						28,100,000					28,100,000			$ 250,000,000
Amount deployed													221,900,000
Kudu Investment Management, LLC | White Mountains
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Additional funding																	31,500,000
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						$ 224,000,000		$ 221,800,000					221,800,000				$ 180,400,000	$ 51,000,000
Proceeds from borrowings													42,900,000
Embrace Pet Insurance
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Goodwill and intangible assets acquired			$ 70,600,000
Embrace Pet Insurance | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Stock issued during period, value, acquisitions			58,200,000
Proceeds from borrowings			$ 20,400,000
Percentage of voting interests acquired			100.00%
Payments to acquire businesses, gross			$ 71,500,000
Embrace Pet Insurance | NSM Bank Facility | Term Loan | NSM Holding Co LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from borrowings													$ 20,400,000

Investment Securities (Net Inve

Investment Securities (Net Investment Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Investment income
Total investment income	$ 18.8	$ 13.9	$ 56.5	$ 46.7
Third-party investment expenses	(0.4)	(0.5)	(1.1)	(1.8)
Net investment income, pre-tax	18.4	13.4	55.4	44.9
Fixed maturity investments
Investment income
Total investment income	8	7.5	24.4	27.3
Short-term investments
Investment income
Total investment income	1.3	1.8	3.9	6.5
Common Stock
Investment income
Total investment income	2.5	3.4	10.8	11.2
Other long-term investments
Investment income
Total investment income	$ 7	$ 1.2	$ 17.4	$ 1.7

Investment Securities (Net Real

Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Net realized investment gains (losses), pre-tax (1)	$ 19,600,000		$ (10,200,000)	$ 90,800,000	$ (13,900,000)
Net unrealized investment gains (losses), pre-tax	47,400,000		76,300,000	257,000,000	36,800,000
Net realized and unrealized investment gains, pre-tax	67,000,000		66,100,000	347,800,000	22,900,000
Income tax expense attributable to net realized and unrealized investment gains	(13,200,000)		(9,700,000)	(69,500,000)	(5,900,000)
Net realized and unrealized investment gains, after-tax	53,800,000		$ 56,400,000	278,300,000	17,000,000
Net gain from sale of discontinued operations, net of tax				67,500,000	$ 0
MediaAlpha
Net gain from sale of discontinued operations, net of tax				67,500,000
Unrealized investment gain	$ 35,300,000	$ 114,700,000		$ 150,000,000

Investment Securities (Net Re_2

Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Debt and Equity Securities, FV-NI [Line Items]
Trading securities, net realized gains	$ 19.6	$ (10.2)	$ 90.8	$ (24.9)
Trading securities, Tax on Realized Holding (Losses) Gains on Investments	(0.2)	2.3	(9)	11.2
Trading securities realized holding (losses) gain on investments after tax	19.4	(7.9)	81.8	(13.7)
Foreign Currency Transaction Gains (Losses), Realized	0	0	0	11
Trading Securities, Tax on Realized Foreign Currency Transaction Gains (Losses) Tax	0	0	0	0
Trading securities, realized foreign currency gain (loss) net of tax	0	0	0	11
Net realized investment gains (losses), pre-tax (1)	19.6	(10.2)	90.8	(13.9)
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(0.2)	2.3	(9)	11.2
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	19.4	(7.9)	81.8	(2.7)
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Trading securities, net realized gains	1.4	(5.1)	3.5	(27.6)
Foreign Currency Transaction Gains (Losses), Realized	0	0	0	18.2
Net realized investment gains (losses), pre-tax (1)	1.4	(5.1)	3.5	(9.4)
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Trading securities, net realized gains			0.1	(0.8)
Foreign Currency Transaction Gains (Losses), Realized			0	0
Net realized investment gains (losses), pre-tax (1)			0.1	(0.8)
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Trading securities, net realized gains	14	(1.5)	81.4	10.5
Foreign Currency Transaction Gains (Losses), Realized	0	0	0	0
Net realized investment gains (losses), pre-tax (1)	14	(1.5)	81.4	10.5
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Trading securities, net realized gains	4.2	(3.6)	5.8	(7)
Foreign Currency Transaction Gains (Losses), Realized	0	0	0	(7.2)
Net realized investment gains (losses), pre-tax (1)	$ 4.2	$ (3.6)	$ 5.8	$ (14.2)

Investment Securities (Net Unre

Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ 48,200,000		$ 76,400,000	$ 257,800,000	$ 48,400,000
Net Foreign Exchange Gains, pre-tax	(800,000)		(100,000)	(800,000)	(11,600,000)
Net unrealized investment gains (losses), pre-tax	47,400,000		76,300,000	257,000,000	36,800,000
Net Unrealized Gains (Losses), tax	13,000,000		12,000,000	(60,500,000)	17,100,000
Net Foreign Exchange Gains, tax	0		0	0	0
Total Net Unrealized Gains (Losses) Reflected in Earnings, tax	13,000,000		12,000,000	(60,500,000)	17,100,000
Net Unrealized Gains (Losses), after-tax	35,200,000		64,400,000	197,300,000	31,300,000
Net Foreign Exchange Gains, after-tax	(800,000)		(100,000)	(800,000)	(11,600,000)
Total Net Unrealized Gains (Losses) Reflected in Earnings, after-tax	34,400,000		64,300,000	196,500,000	19,700,000
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	7,500,000		1,500,000	43,400,000	(15,700,000)
Net Foreign Exchange Gains, pre-tax	0		0	0	(14,800,000)
Net unrealized investment gains (losses), pre-tax	7,500,000		1,500,000	43,400,000	(30,500,000)
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	0		(100,000)	200,000	(100,000)
Net Foreign Exchange Gains, pre-tax	0		0	0	0
Net unrealized investment gains (losses), pre-tax	0		(100,000)	200,000	(100,000)
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	(11,600,000)		54,900,000	60,100,000	43,700,000
Net Foreign Exchange Gains, pre-tax	0		0	0	0
Net unrealized investment gains (losses), pre-tax	(11,600,000)		54,900,000	60,100,000	43,700,000
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	52,300,000		20,100,000	154,100,000	20,500,000
Net Foreign Exchange Gains, pre-tax	(800,000)		(100,000)	(800,000)	3,200,000
Net unrealized investment gains (losses), pre-tax	51,500,000		$ 20,000,000	153,300,000	$ 23,700,000
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Unrealized investment gain	$ 35,300,000	$ 114,700,000		$ 150,000,000

Investment Securities (Investme

Investment Securities (Investment gains (losses) for Level 3) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ 48.2	$ 76.4	$ 257.8	$ 48.4
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	52.3	20.1	154.1	20.5
Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	54.4	4	154.4	2.1
Level 3 Inputs | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ 54.4	$ 4	$ 154.4	$ 2.1

Investment Securities (Invest_2

Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	$ 196.3	$ 154
Trading Securities, Unrealized Holding Gain	1.3	0.1
Trading Securities, Unrealized Holding Loss	0	(0.9)
Fixed maturity investments, at fair value	197.6	153.2
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	449.5	519
Trading Securities, Unrealized Holding Gain	13.8	1
Trading Securities, Unrealized Holding Loss	(0.1)	(9.5)
Fixed maturity investments, at fair value	463.2	510.5
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	292.4	279
Trading Securities, Unrealized Holding Gain	15.7	2.4
Trading Securities, Unrealized Holding Loss	(0.1)	(1.1)
Fixed maturity investments, at fair value	308	280.3
Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	195	136.1
Trading Securities, Unrealized Holding Gain	2.5	0.1
Trading Securities, Unrealized Holding Loss	(0.3)	(2.7)
Fixed maturity investments, at fair value	197.2	133.5
Fixed Income Investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	1,133.2	1,088.1
Trading Securities, Unrealized Holding Gain	33.3	3.6
Trading Securities, Unrealized Holding Loss	(0.5)	(14.2)
Total fixed maturity investments including assets held for sale	$ 1,166	$ 1,077.5

Investment Securities (Invest_3

Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Common Stock
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	$ 548.8	$ 904.7
Trading Securities, Unrealized Holding Gain	80.9	51
Trading Securities, Unrealized Holding Loss	0	(30.1)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	629.7	925.6
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	619.5	330.3
Trading Securities, Unrealized Holding Gain	222.8	52.2
Trading Securities, Unrealized Holding Loss	(65.9)	(54.9)
Net foreign currency gains (losses)	(2.7)	(2)
Fixed maturity investments, at fair value	$ 773.7	$ 325.6

Investment Securities (Other lo

Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Kudu Investment Management, LLC
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	$ 219.3	$ 0
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	150	0
DavidShield & PassportCard
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	90	75
Elementum Holdings, L.P.
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	55.1	0
Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	39.5	60
Kudu
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	0	30.7
Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	553.9	165.7
Hedge and private equity funds included in other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	145.5	146.1
Insurance-linked securities funds
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	41.4	0
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	32.9	13.8
White Mountains
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	$ 773.7	$ 325.6

Investment Securities (Hedge Fu

Investment Securities (Hedge Funds and Private Equity Funds) (Details) $ in Millions	Sep. 30, 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 | $	$ 54.9	$ 54.3

Investment Securities (Fair Val

Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Private equity security
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	$ 90.6
Hedge and private equity funds included in other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	145.5	$ 146.1
Unfunded Commitments	54.8	76.7
Hedge funds | Hedge Fund, Long/ Short Equity Banks and Financial
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	54.9	54.3
Unfunded Commitments	0	0
Hedge funds | Hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	54.9	54.3
Unfunded Commitments	0	0
Private equity security | Manufacturing/Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	45.3	42.9
Unfunded Commitments	4.1	10.5
Private equity security | Aerospace/Defense/Government
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	32.9	27.6
Unfunded Commitments	24.5	34.9
Private equity security | Direct lending
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	0	13
Unfunded Commitments	0	17.7
Private equity security | Financial services
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	12.4	8.3
Unfunded Commitments	26.2	13.6
Private equity security | Private equity security
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	90.6	91.8
Unfunded Commitments	$ 54.8	$ 76.7

Investment Securities (Invest_4

Investment Securities (Investments In Private Equity Funds Subject to Lock-Up Periods) (Details) - Private equity security $ in Millions	Sep. 30, 2019USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	$ 90.6
1-3 years
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	5.4
3-5 years
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	16.5
5-10 years
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	56.3
Greater than 10 years
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	$ 12.4

Investment Securities (Restrict

Investment Securities (Restrictions on Redemption Frequency and Advance Notice Requirements) (Details) $ in Millions	9 Months Ended
	Sep. 30, 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	$ 54.9	$ 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	$ 54.9	$ 54.3

Investment Securities (Insuranc

Investment Securities (Insurance-Linked Securities Funds) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2019USD ($)	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Dec. 31, 2017USD ($)
Investment [Line Items]
Fair value investments	$ 2,569.4	$ 2,328.7	$ 2,365.1	$ 3,208.3
Insurance-Linked Securities Funds
Investment [Line Items]
Number of Investments	3
Fair value investments	$ 41.4
Minimum
Investment [Line Items]
Redemptions and advanced redemption notice period	30 days
Maximum
Investment [Line Items]
Redemptions and advanced redemption notice period	90 days

Investment Securities (Fair V_2

Investment Securities (Fair Value Measurements) (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Percentage of investments recorded at fair value	73.00%	87.00%

Investment Securities (Fair V_3

Investment Securities (Fair Value Measurement by Level) (Details) $ in Millions	Sep. 30, 2019USD ($)Investment	Dec. 31, 2018USD ($)Investment	Sep. 30, 2018USD ($)	Dec. 31, 2017USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 2,569.4	$ 2,328.7	$ 2,365.1	$ 3,208.3
Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	703.7	842.6	897.4	890.4
Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 1,092	$ 1,160.5	1,178	2,105.4
Common Stock
Debt and Equity Securities, FV-NI [Line Items]
Number of Investments | Investment	2	2
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 463.2	$ 510.5
Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	197.2	133.5
Mortgage-backed and asset-backed securities | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	197.2	133.5
Mortgage-backed and asset-backed securities | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0	0
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	272.5	214.2
Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	586.8	138.7	127	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	186.9	186.9	162.7	135.3
Fair value measured on a recurring basis
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	2,841.9	2,542.9
Fair value measured on a recurring basis | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	964.7	1,047
Fair value measured on a recurring basis | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,103.5	1,170.3
Fair value measured on a recurring basis | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	586.8	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	197.6	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	197.6	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	463.2	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	463.2	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	143.3	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	143.3	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	55	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	55	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 | Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	60.5	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	60.5	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	44.5	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	44.5	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 | Energy
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	45	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	45	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	49.3	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	49.3	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 | Communications
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	27.1	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	27.1	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	22.3	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	22.3	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	16.2	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	16.2	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	197.2	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	197.2	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	308	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	308	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	1,166	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	197.6	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	968.4	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	272.5	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	261	204.4	$ 313.7	$ 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	11.5	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 | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	506.1
Fair value measured on a recurring basis | Common equity securities | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	123.6
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 | 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 | 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 | 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	519.7	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	506.1	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	13.6	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	110	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	110	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	629.7	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		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		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	586.8	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	$ 586.8	$ 138.7

Investment Securities (Debt sec

Investment Securities (Debt securities issued by corporation) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 2,569.4	$ 2,328.7	$ 2,365.1	$ 3,208.3
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	463.2	510.5
Fair value investments	463.2	510.5
AAA | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	9.9	8.9
AA | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	79.9	88.7
A | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	286.7	270.5
BBB | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 86.7	$ 142.4

Investment Securities (Mortgage

Investment Securities (Mortgage-backed Asset Securities Table) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 2,569.4	$ 2,328.7	$ 2,365.1	$ 3,208.3
Federal National Mortgage Association Certificates and Obligations (FNMA)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	79.8	53.6
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	53.5	38.1
Government National Mortgage Association Certificates and Obligations (GNMA)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	33.5	23.7
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	166.8	115.4
Collateralized Mortgage Backed Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	166.8	115.4
Collateralized Auto Loans
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	16.5	9.2
Collateralized Credit Card Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	13.9	8.9
Asset-backed Securities, Securitized Loans and Receivables
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	30.4	18.1
Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	197.2	133.5
Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,092	1,160.5	$ 1,178	$ 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	79.8	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	53.5	38.1
Level 2 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	33.5	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	166.8	115.4
Level 2 Inputs | Collateralized Mortgage Backed Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	166.8	115.4
Level 2 Inputs | Collateralized Auto Loans
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	16.5	9.2
Level 2 Inputs | Collateralized Credit Card Securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	13.9	8.9
Level 2 Inputs | Asset-backed Securities, Securitized Loans and Receivables
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	30.4	18.1
Level 2 Inputs | Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	197.2	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 ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		$ 2,328,700,000	$ 2,328,700,000	$ 3,208,300,000
Net realized and unrealized gains			347,500,000	31,300,000
Amortization/Accretion			(1,000,000)	(2,200,000)
Purchases			750,400,000	1,151,900,000
Sales			(856,200,000)	(2,024,200,000)
Transfers in			82,600,000	0
Transfers out			(82,600,000)	0
Ending balance	$ 2,569,400,000		2,569,400,000	2,365,100,000
Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		842,600,000	842,600,000	890,400,000
Net realized and unrealized gains			121,800,000	51,900,000
Amortization/Accretion			200,000	100,000
Purchases			101,400,000	412,600,000
Sales			(362,300,000)	(457,600,000)
Transfers in			0	0
Transfers out			0	0
Ending balance	703,700,000		703,700,000	897,400,000
Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		1,160,500,000	1,160,500,000	2,105,400,000
Net realized and unrealized gains			66,600,000	(33,600,000)
Amortization/Accretion			(1,200,000)	(2,300,000)
Purchases			331,300,000	673,500,000
Sales			(465,200,000)	(1,565,000,000)
Transfers in			0	0
Transfers out			0	0
Ending balance	1,092,000,000		1,092,000,000	1,178,000,000
Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		138,700,000	138,700,000	77,200,000
Net realized and unrealized gains			154,200,000	(5,300,000)
Amortization/Accretion			0	0
Purchases			211,300,000	55,100,000
Sales			0
Transfers in			82,600,000	0
Transfers out			0	0
Ending balance	586,800,000		586,800,000	127,000,000
Hedge and private equity funds included in other long-term investments | NAV
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		186,900,000	186,900,000	135,300,000
Net realized and unrealized gains			4,900,000	18,300,000
Amortization/Accretion			0	0
Purchases			106,400,000	10,700,000
Sales			(28,700,000)	(1,600,000)
Transfers in			0	0
Transfers out			(82,600,000)	0
Ending balance	186,900,000		186,900,000	162,700,000
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	272,500,000		272,500,000		$ 214,200,000
Realized and unrealized losses			(300,000)	(700,000)
Foreign Exchange Forward
Debt and Equity Securities, FV-NI [Line Items]
Realized and unrealized losses				(3,500,000)
Foreign Currency on Cash and Open Trades
Debt and Equity Securities, FV-NI [Line Items]
Realized and unrealized losses				(4,200,000)
Fair value measured on a recurring basis
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		2,542,900,000	2,542,900,000
Ending balance	2,841,900,000		2,841,900,000
Fair value measured on a recurring basis | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		1,047,000,000	1,047,000,000
Ending balance	964,700,000		964,700,000
Fair value measured on a recurring basis | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		1,170,300,000	1,170,300,000
Ending balance	1,103,500,000		1,103,500,000
Fair value measured on a recurring basis | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		138,700,000	138,700,000
Ending balance	586,800,000		586,800,000
Fair value measured on a recurring basis | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		138,700,000	138,700,000
Ending balance	586,800,000		586,800,000
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		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		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		138,700,000	138,700,000
Ending balance	586,800,000		586,800,000
Fair value measured on a recurring basis | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	272,500,000		272,500,000		214,200,000
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	261,000,000		261,000,000	$ 313,700,000	204,400,000	$ 176,100,000
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	11,500,000		11,500,000		9,800,000
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		$ 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						$ (3,700,000)
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Unrealized investment gain	$ 35,300,000	$ 114,700,000	150,000,000
Kudu
Debt and Equity Securities, FV-NI [Line Items]
Purchases			70,100,000
Transfers in			$ 71,700,000

Investment Securities (Signific

Investment Securities (Significant Unobservable Inputs) (Details) - NR $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019USD ($)	Dec. 31, 2018USD ($)
Discount rate
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate		9.00%
Kudu Investments | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 219.3
Kudu Investments | Minimum | Discount rate
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	15.00%
Kudu Investments | Maximum | Discount rate
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	20.00%
MediaAlpha | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 150
MediaAlpha | Discount rate
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	17.00%
MediaAlpha | Long-term revenue growth
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input	4.00%
DavidShield & PassportCard | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 90	$ 75
DavidShield & PassportCard | Discount rate
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	18.00%	18.00%
DavidShield & PassportCard | Revenue exit multiple
Quantitative information for Level 3 Fair Value Measurements Assets
Equity Securities, FV-NI, Measurement Input	1	1
Private Debt Instrument | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 22.7	$ 10
Private Debt Instrument | Minimum | Discount rate
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	6.00%
Private Debt Instrument | Maximum | Discount rate
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	9.00%
Elementum Holdings, L.P. | Price of Recent Transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 55.1
Other | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 29.3	$ 30
Other | Discount rate
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	23.00%	23.00%
Other | Minimum | Revenue exit multiple
Quantitative information for Level 3 Fair Value Measurements Assets
Equity Securities, FV-NI, Measurement Input	2	2
Other | Maximum | Revenue exit multiple
Quantitative information for Level 3 Fair Value Measurements Assets
Equity Securities, FV-NI, Measurement Input	4	4
Compare.com | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value		$ 16.9
Compare.com | Estimated Net Realizable Value
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 2.5
Compare.com | Discount rate
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate		22.00%
Compare.com | Revenue exit multiple
Quantitative information for Level 3 Fair Value Measurements Assets
Equity Securities, FV-NI, Measurement Input		3
Galvanic Applied Sciences | Multiple of EBITDA
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 2.7	$ 3.1
Equity Securities, FV-NI, Measurement Input	6	6

Goodwill and Other Intangible_3

Goodwill and Other Intangible Assets - Fair Value of Goodwill and Other Intangible Assets (Details) - USD ($) $ in Millions	Sep. 30, 2019	Jun. 28, 2019	Apr. 04, 2019	Apr. 01, 2019	Mar. 12, 2019	Dec. 03, 2018	May 18, 2018	May 11, 2018
NSM
Goodwill [Line Items]
Goodwill and intangible assets acquired								$ 383
Fresh Insurance
Goodwill [Line Items]
Goodwill and intangible assets acquired							$ 54.6
KBK
Goodwill [Line Items]
Goodwill and intangible assets acquired						$ 59.4
Other Operations
Goodwill [Line Items]
Goodwill and intangible assets acquired					$ 13.1
Embrace Pet Insurance
Goodwill [Line Items]
Goodwill and intangible assets acquired				$ 70.6
Kudu
Goodwill [Line Items]
Goodwill and intangible assets acquired			$ 9.8
Renewal Rights
Goodwill [Line Items]
Goodwill and intangible assets acquired		$ 82.5
NSM
Goodwill [Line Items]
Goodwill and intangible assets acquired	$ 650.1

Goodwill and Other Intangible_4

Goodwill and Other Intangible Assets - Goodwill Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 428.6	$ 25.9	$ 379.9	$ 25.9	$ 25.9
Intangible assets, gross (including goodwill), beginning balance			591.1
Foreign currency translation		0		0
Goodwill, ending balance	424.8		424.8		379.9
Intangible assets, gross (including goodwill), ending balance	683.4		683.4		591.1
Other Intangible Assets
Goodwill [Roll Forward]
Intangible assets, net (excluding goodwill), beginning balance	239	30.7	157.6	36.2	36.2
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	2.2	140.9	29	140.9
Acquisitions of businesses			0	0
Foreign currency translation	(0.5)	(0.4)	0	(0.4)
Adjustments to reflect acquisition date fair value	0	0	5.9	0
Amortization	4.6	7.4	(15.4)	(12.9)
Intangible assets, net (excluding goodwill), ending balance	236.1	163.8	236.1	163.8	157.6
Renewal Rights
Goodwill [Roll Forward]
Acquisitions of businesses			82.5
Intangible Asset and Goodwill
Goodwill [Roll Forward]
Intangible assets, gross (including goodwill), beginning balance	667.6	56.6	537.5	62.1	62.1
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	0	436.2	0	436.2
Acquisitions of businesses			93.8	0
Foreign currency translation	(2.1)	(1.6)	(1.6)	(1.6)
Adjustments to reflect acquisition date fair value	0	1.4	5.9	1.4
Amortization	4.6	7.4	15.4	12.9
Intangible assets, gross (including goodwill), ending balance	660.9	485.2	660.9	485.2	537.5
Intangible Asset and Goodwill | Renewal Rights
Goodwill [Roll Forward]
Acquisitions of businesses			82.5
Goodwill
Goodwill [Roll Forward]
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	(2.2)	(295.3)	(29)	(295.3)
Acquisitions of businesses			93.8	0
Foreign currency translation	(1.6)	(1.2)	(1.6)	(1.2)
Adjustments to reflect acquisition date fair value	0	1.4	0	1.4
Goodwill, ending balance	424.8	$ 321.4	424.8	321.4
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)	0
MediaAlpha | Goodwill
Goodwill [Roll Forward]
Deconsolidation of MediaAlpha			$ (18.3)	$ 0
NSM Holding Co LLC | Other Intangible Assets
Goodwill [Roll Forward]
Foreign currency translation					(0.7)
NSM Holding Co LLC | Goodwill
Goodwill [Roll Forward]
Foreign currency translation	$ (1.5)				$ (2.2)

Goodwill and Other Intangible_5

Goodwill and Other Intangible Assets - Schedule of Goodwill and Other Intangible Assets (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Goodwill [Line Items]
Goodwill, acquisition date fair value	$ 424.8		$ 379.9
Goodwill	424.8	$ 428.6	379.9	$ 25.9	$ 25.9
Other intangible assets, accumulated amortization	22.5		53.6
Goodwill and intangible assets, acquisition date fair value	683.4		591.1
Goodwill and intangible assets, net carrying value	660.9		537.5
NSM Holding Co LLC
Goodwill [Line Items]
Goodwill, acquisition date fair value	397		354.3
Goodwill	397		354.3
Other intangible assets, acquisition date fair value	255.4		140.2
Other intangible assets, accumulated amortization	21.8		8.3
Other intangible assets, net carrying value	233.6		131.9
Kudu investments
Goodwill [Line Items]
Goodwill, acquisition date fair value	7.6		0
Goodwill	7.6		0
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	20.2		7.3
Goodwill	20.2		7.3
Other intangible assets, acquisition date fair value	1		1.1
Other intangible assets, accumulated amortization	0.5		0.5
Other intangible assets, net carrying value	0.5		0.6
Customer relationships | NSM Holding Co LLC
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	110.5		85.3
Other intangible assets, accumulated amortization	15.5		6
Other intangible assets, net carrying value	$ 95		79.3
Acquired finite-lived intangible assets, weighted average useful life	9 years
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
Acquired finite-lived intangible assets, weighted average useful life	9 years
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	4.2		1.8
Other intangible assets, net carrying value	$ 52.9		49.4
Acquired finite-lived intangible assets, weighted average useful life	19 years
Trade names | Kudu investments
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	$ 2.2		0
Other intangible assets, accumulated amortization	0.2		0
Other intangible assets, net carrying value	$ 2		0
Acquired finite-lived intangible assets, weighted average useful life	7 years
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
Acquired finite-lived intangible assets, weighted average useful life	7 years
Information technology | NSM Holding Co LLC
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	$ 3.6		3.7
Other intangible assets, accumulated amortization	1.1		0.5
Other intangible assets, net carrying value	$ 2.5		3.2
Acquired finite-lived intangible assets, weighted average useful life	5 years
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
Acquired finite-lived intangible assets, weighted average useful life	5 years
Information technology | Other Operations
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	$ 0.4		0.5
Other intangible assets, accumulated amortization	0.3		0.3
Other intangible assets, net carrying value	$ 0.1		0.2
Acquired finite-lived intangible assets, weighted average useful life	5 years
Other Operations | NSM Holding Co LLC
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	$ 1.7
Other intangible assets, accumulated amortization	0.7
Other intangible assets, net carrying value	$ 1
Acquired finite-lived intangible assets, weighted average useful life	3 years
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
Acquired finite-lived intangible assets, weighted average useful life	3 years
Renewal Rights | NSM Holding Co LLC
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	$ 82.5
Other intangible assets, accumulated amortization	0.3
Other intangible assets, net carrying value	$ 82.2
Acquired finite-lived intangible assets, weighted average useful life	12 years
Other Intangible Assets
Goodwill [Line Items]
Other intangible assets, acquisition date fair value	$ 258.6		211.2
Other intangible assets, accumulated amortization	22.5		53.6
Other intangible assets, net carrying value	236.1		157.6
Non-controlling interest
Goodwill [Line Items]
Goodwill and intangible assets, net carrying value	(25)		(40.6)
Total
Goodwill [Line Items]
Goodwill and intangible assets, net carrying value	$ 635.9		$ 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		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Acquired Finite-Lived Intangible Assets [Line Items]
Foreign currency translation		$ 0		$ 0
Goodwill
Acquired Finite-Lived Intangible Assets [Line Items]
Foreign currency translation	$ (1.6)	$ (1.2)	$ (1.6)	$ (1.2)
NSM Holding Co LLC | Other Intangible Assets
Acquired Finite-Lived Intangible Assets [Line Items]
Foreign currency translation					$ (0.7)
NSM Holding Co LLC | Goodwill
Acquired Finite-Lived Intangible Assets [Line Items]
Foreign currency translation	$ (1.5)				(2.2)
Buzzmove | Goodwill
Acquired Finite-Lived Intangible Assets [Line Items]
Foreign currency translation			$ (0.1)		$ (0.3)

Debt - Narrative (Details)

Debt - Narrative (Details) - USD ($)	Apr. 01, 2019	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Jun. 15, 2018	May 18, 2018	May 11, 2018
Debt Instrument
Total debt		$ 230,200,000	$ 192,700,000	$ 230,200,000
Repayments of debt				22,200,000	$ 11,200,000
Other Operations
Debt Instrument
Face amount		$ 11,200,000	0	$ 11,200,000
Effective percentage		9.60%		9.60%
Unamortized issuance cost		$ (400,000)	0	$ (400,000)
MediaAlpha
Debt Instrument
Total debt		0	14,200,000	0
MediaAlpha | MediaAlpha Bank Facility
Debt Instrument
Total debt		0	14,200,000	0
Face amount		0	$ 14,300,000	0
Effective percentage			7.10%
Unamortized issuance cost		0	$ (100,000)	0
NSM Holding Co LLC
Debt Instrument
Total debt		219,400,000	178,500,000	219,400,000
NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Total debt		217,700,000	176,600,000	217,700,000
Face amount								$ 100,000,000
Term Loan | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Total debt		221,800,000		221,800,000
Face amount		$ 221,800,000	$ 180,400,000	$ 221,800,000			$ 51,000,000	224,000,000
Effective percentage		7.70%	7.40%	7.70%
Unamortized issuance cost		$ (4,100,000)	$ (3,800,000)	$ (4,100,000)
Repayments of debt		500,000		1,400,000
Proceeds from borrowings				42,900,000
Term Loan | NSM Holding Co LLC | Other Operations
Debt Instrument
Total debt		1,700,000	1,900,000	1,700,000
Unamortized issuance cost		(0.025)	$ (0.022)	(0.025)
Revolving Credit Facility | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Total commitment under revolving credit facility								10,000,000
Borrowings under facility								$ 2,000,000
Repayments of debt		$ 6,500,000
Proceeds from borrowings				$ 6,500,000
Interest Rate Swap | NSM Holding Co LLC
Debt Instrument
Derivative, fixed interest rate						2.97%
Derivative, variable interest rate		2.11%		2.11%
Interest Rate Swap | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Total debt		$ 149,100,000		$ 149,100,000
Total commitment under revolving credit facility						$ 151,000,000
Hedged Liability, Fair Value Hedge						$ 151,000,000
Derivative, fixed interest rate		2.97%		2.97%
Debt, weighted average interest rate, all-in		7.17%		7.17%
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		$ 149,100,000		$ 149,100,000
Debt, weighted average interest rate, all-in		7.47%		7.47%
Not Designated as Hedging Instrument | Interest Rate Swap | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Total debt		$ 72,700,000		$ 72,700,000
Effective percentage		7.12%		7.12%
Embrace | NSM Holding Co LLC
Debt Instrument
Proceeds from borrowings	$ 20,400,000
Embrace | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Proceeds from borrowings				$ 20,400,000
Carrying Value | MediaAlpha | MediaAlpha Bank Facility
Debt Instrument
Total debt		$ 0	$ 14,200,000	0
Carrying Value | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Total debt		217,700,000	176,600,000	217,700,000
Carrying Value | Other Operations
Debt Instrument
Total debt		$ 10,800,000	$ 0	10,800,000
Renewal Rights | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Debt Instrument
Proceeds from borrowings				$ 22,500,000

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Tax Contingency [Line Items]
Effective income tax rate reconciliation, percent	17.20%	(8.10%)	5.40%	(1.20%)
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%		21.00%	21.00%
Income tax expense (benefit)	$ 8.8	$ (3.6)	$ 18.9	$ (0.4)
Continuing Operations | BAM
Income Tax Contingency [Line Items]
Income tax expense (benefit)	$ (3.6)	$ (1.8)	$ (6.6)	$ (4)

Derivatives - NSM Interest Rate

Derivatives - NSM Interest Rate Swap (Details) - USD ($) $ in Millions	3 Months Ended		5 Months Ended	8 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Jun. 15, 2018
Derivative [Line Items]
Debt	$ 230.2			$ 192.7	$ 230.2
NSM Holding Co LLC
Derivative [Line Items]
Debt	219.4			178.5	219.4
NSM Holding Co LLC | NSM Bank Facility
Derivative [Line Items]
Debt	217.7			176.6	217.7
Term Loan | NSM Holding Co LLC | NSM Bank Facility
Derivative [Line Items]
Debt	$ 221.8				$ 221.8
Interest Rate Swap | NSM Holding Co LLC
Derivative [Line Items]
Derivative, fixed interest rate							2.97%
Derivative, variable interest rate	2.11%				2.11%
Interest expense, net	$ 0.3		$ 0.3		$ 0.6	$ 0.4
Derivative, fair value, net	7.8			$ 2.7	7.8
Change in fair value of derivative	$ (0.8)	$ 1			$ (5.1)	$ (0.1)
Interest Rate Swap | Term Loan | NSM Holding Co LLC | NSM Bank Facility
Derivative [Line Items]
Total commitment under revolving credit facility							$ 151
Derivative, fixed interest rate	2.97%				2.97%
Debt	$ 149.1				$ 149.1
Debt, weighted average interest rate, all-in	7.17%				7.17%
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	$ 149.1				$ 149.1
Debt, weighted average interest rate, all-in	7.47%				7.47%

Derivatives Forward Contracts (

Derivatives Forward Contracts (Details) $ in Millions	9 Months Ended
	Sep. 30, 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		9 Months Ended		12 Months Ended
	Sep. 30, 2019USD ($)Contract	Apr. 30, 2018USD ($)	Sep. 30, 2019USD ($)yrContract	Sep. 30, 2018USD ($)	Dec. 31, 2018USD ($)Contract	Dec. 31, 2017USD ($)	Dec. 31, 2012USD ($)	Dec. 31, 2019	Jul. 31, 2012USD ($)
Guarantor Obligations [Line Items]
Ownership interest (as a percent)	50.00%		50.00%		50.00%
Repayments of debt			$ 22.2	$ 11.2
Interest paid			11.7	$ 5.3
Percentage of discounted future cash flows						10.00%
HG Global
Guarantor Obligations [Line Items]
Partners capital contribution							$ 594.5
Surplus notes	$ 481.3		$ 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%		96.90%
HG Global | Common Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)	88.40%		88.40%		88.40%
BAM
Guarantor Obligations [Line Items]
Percentage of risk premium ceded	60.00%		60.00%
Year of U.S. Treasury rate | yr			1
Contracts outstanding | Contract	8,508		8,508		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	$ 59,190.9		$ 59,190.9		$ 52,201.6
Municipal bond outstanding interest	30,701.7		30,701.7		26,560.3
Municipal bond debt service outstanding	89,892.6		89,892.6		78,761.9
October 1, 2019 - December 31, 2019	4.9		4.9
2020	18.7		18.7
2021	17.4		17.4
2022	16.3		16.3
2023	15.3		15.3
2024	14.3		14.3
2025 and thereafter	116.6		116.6
Total gross unearned insurance premiums	203.5		203.5		176
HG Global
Guarantor Obligations [Line Items]
Collateral held in supplement trust	787.4		787.4		757.4
Interest receivable	164.2		$ 164.2		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		$ 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%		8.00%
First Quarter, year two | BAM
Guarantor Obligations [Line Items]
2020	$ 4.8		$ 4.8
Second Quarter, year two | BAM
Guarantor Obligations [Line Items]
2020	4.8		4.8
Third Quarter, year two | BAM
Guarantor Obligations [Line Items]
2020	4.6		4.6
Fourth Quarter, year two | BAM
Guarantor Obligations [Line Items]
2020	4.5		$ 4.5
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	$ 1,100				$ 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		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Direct written premiums	$ 10.2	$ 3.9	$ 30.6	$ 28.9
Assumed written premiums	10.6	0	10.6	0
Gross written premiums	20.8	3.9	41.2	28.9
Direct premiums earned	4.4	3.3	11.9	9.7
Assumed premiums earned	0.8	0	1.8	0
Gross earned premiums	$ 5.2	$ 3.3	$ 13.7	$ 9.7

Investments in Unconsolidated_3

Investments in Unconsolidated Entities (Details) - White Mountains - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Investments in unconsolidated affiliates
Other Investments	$ 773.7	$ 325.6
Equity method eligible private equity securities, at fair value
Investments in unconsolidated affiliates
Other Investments	673.2	269.7
Other unconsolidated investments
Investments in unconsolidated affiliates
Other Investments	$ 100.5	$ 55.9

Investments in Unconsolidated_4

Investments in Unconsolidated Entities - White Mountain's Investments (Details)	Sep. 30, 2019	May 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%		50.00%
MediaAlpha
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	48.30%
durchblicker
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	45.00%		45.00%
Elementum Holdings, L.P.
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	30.00%	30.00%
Compare.com
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	18.40%		18.40%
JAM Partners L.P.
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	11.10%		12.30%
Kudu
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)			49.50%
Minimum | Tuckerman Capital Funds
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	18.80%		18.50%
Minimum | Enlightenment Capital Funds
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	10.00%		10.30%
Minimum | Kudu Investments
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	3.20%
Maximum | Tuckerman Capital Funds
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	62.40%		62.30%
Maximum | Enlightenment Capital Funds
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	36.50%		36.50%
Maximum | Kudu Investments
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	30.00%

Investments in Unconsolidated_5

Investments in Unconsolidated Entities - Narrative (Details) - USD ($)	3 Months Ended			7 Months Ended	9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Feb. 26, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%			50.00%	50.00%			50.00%
Total revenues	$ 155,500,000		$ 198,700,000		$ 718,100,000	$ 363,100,000
Total assets	3,836,300,000			$ 3,836,300,000	3,836,300,000			$ 3,362,600,000
Total liabilities	765,400,000			765,400,000	765,400,000			644,400,000
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		$ 114,700,000
Unrealized investment gain	35,300,000	$ 114,700,000			150,000,000
Total assets	0			0	0			88,400,000
Total liabilities	0			0	0			$ 46,900,000
MediaAlpha
Schedule of Equity Method Investments [Line Items]
Total revenues	0		$ 74,500,000		48,800,000	$ 218,000,000
MediaAlpha | MediaAlpha
Schedule of Equity Method Investments [Line Items]
Total revenues	110,500,000			233,200,000
Expenses	102,100,000			217,900,000
Gross profit	18,400,000			38,800,000
Pre-tax income	8,400,000			15,300,000
Total assets	103,400,000			103,400,000	103,400,000
Total liabilities	$ 145,800,000			$ 145,800,000	$ 145,800,000

Earnings Per Share (Details)

Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 48.7	$ 40.8	$ 353.6	$ (3.7)
Less: total income (loss) from discontinued operations	0.9	(17.3)	1.6	(17.2)
Net income from continuing operations attributable to White Mountains’s common shareholders	47.8	58.1	352	13.5
Allocation of earnings to participating restricted common shares	(0.7)	(0.8)	(4.4)	(0.2)
Basic and diluted earnings per share numerators	$ 47.1	$ 57.3	$ 347.6	$ 13.3
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,185,400	3,180,400	3,180,400	3,450,800
Average unvested restricted common shares	(43,400)	(41,500)	(39,600)	(39,700)
Basic earnings per share denominator	3,142,000	3,138,900	3,140,800	3,411,100
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,185,400	3,180,400	3,180,400	3,450,800
Average unvested restricted common shares	(43,400)	(41,500)	(39,600)	(39,700)
Diluted earnings per share denominator	3,142,000	3,138,900	3,140,800	3,411,100
Basic earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid	$ 0	$ 0	$ 1	$ 1
Undistributed earnings	15.01	18.27	109.67	2.90
Basic and diluted earnings per share	$ 15.01	$ 18.27	$ 110.67	$ 3.90

Earnings Per Share - Undistribu

Earnings Per Share - Undistributed Net Earnings (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 47,100,000	$ 57,300,000	$ 347,600,000	$ 13,300,000
Dividends declared net of restricted common share amounts	0	0	(3,200,000)	(3,700,000)
Total undistributed net earnings, net of restricted common share amounts	$ 47,100,000	$ 57,300,000	$ 344,400,000	$ 9,600,000

Employee Share-Based Incentiv_3

Employee Share-Based Incentive Compensation Plans (Performance Shares) (Details) - Performance Share - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 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)	42,473	40,616	40,616	50,515
Shares paid or expired (in shares)	0	0	(13,715)	(23,186)
New grants (in shares)	0	0	15,600	14,105
Assumed forfeitures and cancellations	0	0	(28)	(818)
Ending of period (in shares)	42,473	40,616	42,473	40,616
Accrued Expense
Accrued expense at the beginning of period	$ 30.8	$ 27.2	$ 31.7	$ 45.8
Payments and deferrals	0	0	(18.1)	(28.4)
Assumed forfeitures and cancellations	0	(0.1)	0	0.1
Expense recognized	5.6	7.8	22.8	17.4
Accrued expense at the end of the period	36.4	$ 34.9	36.4	$ 34.9
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 21.1		$ 21.1

Employee Share-Based Incentiv_4

Employee Share-Based Incentive Compensation Plans (Performance Shares Granted Under the WTM Incentive Plan) (Details) - White Mountains - Performance Share - W T M Incentive Plan - USD ($) $ in Millions	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	43,120
Accrued Expense	$ 36.9
Target Performance Shares Outstanding, Assumed forfeitures	(647)
Accrued Expense, Assumed forfeitures	(0.5)
Total Target Performance Shares Outstanding	42,473	42,473	40,616	40,616	40,616	50,515
Total Accrued Expense	$ 36.4	$ 30.8	$ 31.7	$ 34.9	$ 27.2	$ 45.8
Performance Cycle 2017 to 2019
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	14,070
Accrued Expense	$ 24.4
Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	13,450
Accrued Expense	$ 8.5
Performance Cycle 2019 to 2021
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	15,600
Accrued Expense	$ 4

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		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	43,395	41,510	41,510	53,755
Issued (in shares)	0	0	15,600	14,105
Vested (in shares)	0	0	(13,715)	(25,381)
Assumed forfeitures and cancellations	0	0	0	(969)
Ending of period (in shares)	43,395	41,510	43,395	41,510
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 21.9	$ 19	$ 12.5	$ 14.3
Issued	0	0	14.5	11.4
Assumed forfeitures and cancellations	0	0	0	(0.2)
Expense recognized	(2.6)	(3.2)	(7.7)	(9.7)
Unamortized grant date fair value, ending balance	$ 19.3	$ 15.8	$ 19.3	$ 15.8
Restricted Share Cliff Vesting in January 2022
Target Restricted Share Activity
Issued (in shares)			15,600
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	Sep. 30, 2019	Jan. 01, 2019
Lessee, Lease, Description [Line Items]
ROU lease asset	$ 23
Operating lease liability	23.3
Accounting Standards Update 2016-02
Lessee, Lease, Description [Line Items]
ROU lease asset	23	$ 23.2
Operating lease liability	$ 23.3	$ 23.2

Leases - Schedule of Lease Cost

Leases - Schedule of Lease Cost (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Leases [Abstract]
Lease cost	$ 1.9	$ 5.4
Less: sublease income	0.1	0.3
Net lease cost	$ 1.8	$ 5.1

Leases - Schedule of Future Min

Leases - Schedule of Future Minimum Payments on Operating Leases (Details) $ in Millions	Sep. 30, 2019USD ($)
Leases [Abstract]
Remainder of 2019	$ 1.7
2020	5.7
2021	4.7
2022	4.2
2023	3.6
Thereafter	6.3
Total undiscounted lease payments	26.2
Less: present value adjustment	2.9
Operating lease liability	$ 23.3

Leases - Schedule of Right-of-U

Leases - Schedule of Right-of-Use Assets and Lease Liabilities (Details) $ in Millions	Sep. 30, 2019USD ($)
Lessee, Lease, Description [Line Items]
ROU lease asset	$ 23
Lease liability	$ 23.3
Operating lease, weighted average incremental borrowing rate, percent	4.50%
HG/BAM
Lessee, Lease, Description [Line Items]
ROU lease asset	$ 10.8
Lease liability	10.8
NSM Holding Co LLC
Lessee, Lease, Description [Line Items]
ROU lease asset	4.4
Lease liability	4.4
Kudu investments
Lessee, Lease, Description [Line Items]
ROU lease asset	2.3
Lease liability	2.3
Other Operations
Lessee, Lease, Description [Line Items]
ROU lease asset	5.5
Lease liability	$ 5.8

Non-controlling Interests (Deta

Non-controlling Interests (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ (122.5)	$ (124.9)
Total Non-Controlling Excluding Reciprocals
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ 32.7	$ 45.7
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	3.10%	3.10%
Total non-controlling interests	$ 14.9	$ 14.5
NSM Holding Co LLC
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	3.60%	4.50%
Total non-controlling interests	$ 15.2	$ 13.6
Other NSM
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	13.40%	13.40%
Total non-controlling interests	$ 0.1	$ 0.3
Kudu Investment Management, LLC
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	1.00%	0.00%
Total non-controlling interests	$ 2.3	$ 0
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	$ 0.2	$ 1.1
Build America Mutual
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	$ (155.2)	$ (170.6)

Segment Information (Details)

Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		5 Months Ended	7 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018
Segment reporting information
Net investment income	$ 18.4	$ 13.4			$ 55.4	$ 44.9
Net realized and unrealized investment gains (losses)					347.8	22.9
Total revenues	155.5	198.7			718.1	363.1
Loss and loss adjustment expenses						0
Total expenses	104.4	154.4			369.5	397.2
Pre-tax income (loss)	51.1	44.3			348.6	(34.1)
Kudu investments
Segment reporting information
Earned insurance premiums	0
Net investment income	4.6	0			8.6	0
Net realized and unrealized investment gains (losses)	2.7	0			3.1	0
Commission revenues	0
Other revenue	0.2				0.2	0
Total revenues	7.5	0			11.9	0
Insurance acquisition expenses	0
Other underwriting expenses	0
Cost of sales	0
General and administrative expenses	4.5	0			6.7	0
Amortization	0.2	0			0.2	0
Total expenses	4.7	0			6.9	0
Pre-tax income (loss)	2.8				5
HG Global-BAM
Segment reporting information
Earned insurance premiums	5.2	3.3			13.7	9.7
Net investment income	5.4	4.8			16.1	12.5
Net realized and unrealized investment gains (losses)	7.1	(4.1)			30.4	(14.4)
Commission revenues	0	0			0	0
Other revenue	0.3	0.2			1.3	0.8
Total revenues	18	4.2			61.5	8.6
Insurance acquisition expenses	1.4	1.2			4.1	3.9
Other underwriting expenses	0.1	0.1			0.3	0.3
Cost of sales	0	0			0	0
General and administrative expenses	12.5	11.3			39.1	36.5
Amortization	0	0			0	0
Interest expense	0	0			0	0
Total expenses	14	12.6			43.5	40.7
Pre-tax income (loss)	4	(8.4)			18	(32.1)
NSM Holding Co LLC
Segment reporting information
Earned insurance premiums	0				0
Net investment income	0				0
Net realized and unrealized investment gains (losses)	0				0
Commission revenues	49.4	35			147.6	56.4
Other revenue	10.5	4.7			27.7	6.8
Total revenues	59.9	39.7			175.3	63.2
Insurance acquisition expenses	0				0
Other underwriting expenses	0				0
Cost of sales	0				0
General and administrative expenses	33.3	23.5			91.4	35.7
Broker commission expense	15.3	10.6			48.9	17.1
Change in fair value of contingent consideration earnout liabilities	(2)	2.6			5.6	2.6
Amortization	4.4	5			13.5	5
Interest expense	4.4	3.2			12.2	4.8
Total expenses	55.4	44.9			171.6	65.2
Pre-tax income (loss)	4.5	(5.2)			3.7	(2)
MediaAlpha
Segment reporting information
Earned insurance premiums		0			0	0
Net investment income		0			0	0
Net realized and unrealized investment gains (losses)		0			0	0
Commission revenues	0	74.5			48.8	216.4
Other revenue	0	0			0	1.6
Total revenues	0	74.5			48.8	218
Insurance acquisition expenses		0			0	0
Other underwriting expenses		0			0	0
Cost of sales	0	61.8			40.6	179.1
General and administrative expenses	0	5.4			12.5	21.2
Amortization	0	2.4			1.6	7.8
Interest expense	0	0.2			0.2	0.9
Total expenses	0	69.8			54.9	209
Pre-tax income (loss)		$ 4.7			(6.1)	$ 9
Percentage of advertising revenue		30.00%				31.00%
Other Segment
Segment reporting information
Earned insurance premiums	0	$ 0			0	$ 0
Net investment income	8.4	8.6			30.7	32.4
Net realized and unrealized investment gains (losses)	57.2	70.2			199.6	37.3
Realized gain and unrealized investment gain from the MediaAlpha Transaction					182.2	0
Commission revenues	1.9	1.1			4.5	3
Other revenue	2.6	0.4			3.6	0.6
Total revenues	70.1	80.3			420.6	73.3
Loss and loss adjustment expenses						0
Insurance acquisition expenses	0				0	0
Other underwriting expenses	0				0	0
Cost of sales	2.3	1.1			5	2.9
General and administrative expenses	27.7	26			87.2	79
Amortization	0	0			0.1	0.1
Interest expense	0.3	0			0.3	0.3
Total expenses	30.3	27.1			92.6	82.3
Pre-tax income (loss)	39.8	53.2			328	(9)
Total
Segment reporting information
Earned insurance premiums	5.2	3.3			13.7	9.7
Net investment income	18.4	13.4			55.4	44.9
Net realized and unrealized investment gains (losses)	67	66.1			233.1	22.9
Realized gain and unrealized investment gain from the MediaAlpha Transaction					182.2
Commission revenues	51.3	110.6			200.9	275.8
Other revenue	13.6	5.3			32.8	9.8
Total revenues	155.5	198.7			718.1	363.1
Insurance acquisition expenses	1.4	1.2			4.1	3.9
Other underwriting expenses	0.1	0.1			0.3	0.3
Cost of sales	2.3	62.9			45.6	182
General and administrative expenses	78	66.2			236.9	172.4
Broker commission expense	15.3	10.6			48.9	17.1
Change in fair value of contingent consideration earnout liabilities	(2)	2.6			5.6	2.6
Amortization	4.6	7.4			15.4	12.9
Interest expense	4.7	3.4			12.7	6
Total expenses	104.4	154.4			369.5	397.2
Pre-tax income (loss)	51.1	44.3			348.6	(34.1)
Kudu investments | Kudu investments
Segment reporting information
Other revenue		0
HG Global-BAM | HG Global-BAM
Segment reporting information
Loss and loss adjustment expenses						0
NSM Holding Co LLC
Segment reporting information
Change in fair value of contingent consideration earnout liabilities	(2)	$ 2.6	$ 2.6		5.6
MediaAlpha | MediaAlpha
Segment reporting information
Total revenues	$ 110.5			$ 233.2
Loss and loss adjustment expenses						$ 0
MediaAlpha | HG Global-BAM
Segment reporting information
Realized gain and unrealized investment gain from the MediaAlpha Transaction					0
MediaAlpha | NSM Holding Co LLC
Segment reporting information
Realized gain and unrealized investment gain from the MediaAlpha Transaction					$ 0

Fair Value of Financial Instr_3

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	$ 230.2	$ 192.7
NSM Holding Co LLC
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	219.4	178.5
NSM Holding Co LLC | NSM Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	217.7	176.6
NSM Holding Co LLC | Carrying Value | NSM Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	217.7	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	219.7	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	10.8	0
Other Operations | Estimate of Fair Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt instrument, fair value disclosure	$ 11.3	$ 0

Commitments and Contingencies (

Commitments and Contingencies (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2019	Jun. 28, 2019	Dec. 31, 2018
American International Group | NSM Insurance HoldCo, LLC
Loss Contingencies [Line Items]
Intangible assets acquired				$ 82.5
Sirius Group | Swedish Tax Authority | Sirius Group
Loss Contingencies [Line Items]
Contingent liability	$ 15.7	$ 15.7			$ 17.3
Change in contingent liability	$ 0.9	$ (1.6)
Scenario, Forecast | American International Group | NSM Insurance HoldCo, LLC
Loss Contingencies [Line Items]
Purchase Obligation			$ 82.5

Held for Sale and Discontinue_3

Held for Sale and Discontinued Operations - (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Discontinued Operations [Line Items}
Net gain from sale of discontinued operations, net of tax	$ 0.9	$ (17.3)	$ 1.6	$ (17.2)
Connecticut | Discontinued Operations, Held-for-sale
Discontinued Operations [Line Items}
Assets held for sale	2.8		2.8		$ 3.3
Swedish Tax Authority | Discontinued Operations | Sirius Group
Discontinued Operations [Line Items}
Net gain from sale of discontinued operations, net of tax				$ 0.1
Sirius Group | Swedish Tax Authority | Sirius Group
Discontinued Operations [Line Items}
Contingent liability	$ 15.7		$ 15.7		$ 17.3

Held for Sale and Discontinue_4

Held for Sale and Discontinued Operations - EPS (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 48.7	$ 40.8	$ 353.6	$ (3.7)
Less: total income from continuing operations, net of tax	47.8	58.1	352	13.5
Net income (loss) from discontinued operations attributable to White Mountains’s common shareholders	0.9	(17.3)	1.6	(17.2)
Basic and diluted earnings per share numerators	$ 47.1	$ 57.3	$ 347.6	$ 13.3
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,185,400	3,180,400	3,180,400	3,450,800
Average unvested restricted common shares	(43,400)	(41,500)	(39,600)	(39,700)
Basic earnings per share denominator	3,142,000	3,138,900	3,140,800	3,411,100
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,185,400	3,180,400	3,180,400	3,450,800
Average unvested restricted common shares	(43,400)	(41,500)	(39,600)	(39,700)
Diluted earnings per share denominator	3,142,000	3,138,900	3,140,800	3,411,100
Basic and diluted earnings per share	$ 15.01	$ 18.27	$ 110.67	$ 3.90
Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 48.7	$ 40.8	$ 353.6	$ (3.7)
Less: total income from continuing operations, net of tax	47.8	58.1	352	13.5
Net income (loss) from discontinued operations attributable to White Mountains’s common shareholders	0.9	(17.3)	1.6	(17.2)
Allocation of income for participating unvested restricted common shares	0	0.2	0	0.2
Basic and diluted earnings per share numerators	$ 0.9	$ (17.1)	$ 1.6	$ (17)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,185,400	3,180,400	3,180,400	3,450,800
Average unvested restricted common shares	(43,400)	(41,500)	(39,600)	(39,700)
Basic earnings per share denominator	3,142,000	3,138,900	3,140,800	3,411,100
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,185,400	3,180,400	3,180,400	3,450,800
Average unvested restricted common shares	(43,400)	(41,500)	(39,600)	(39,700)
Diluted earnings per share denominator	3,142,000	3,138,900	3,140,800	3,411,100
Basic and diluted earnings per share	$ 0.28	$ (5.44)	$ 0.50	$ (4.98)