White Mountains Insurance (WTM) 10-Q 6 Nov 23 2023 Q3 Quarterly report Financial data

Cover Page

Cover Page - shares	9 Months Ended
	Sep. 30, 2023	Nov. 02, 2023
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2023
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	23 South Main Street, Suite 3B
Entity Address, City or Town	Hanover,
Entity Address, State or Province	NH
Entity Address, Postal Zip Code	03755-2053
City Area Code	603
Local Phone Number	640-2200
Title of 12(b) Security	Common Shares, par value $1.00 per share
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 (in shares)		2,560,452
Entity Central Index Key	0000776867
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q3
Amendment Flag	false

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Assets
Common equity securities, at fair value	$ 637	$ 668.4
Other long-term investments	1,895.8	1,488
Goodwill and other intangible assets	372.1	392.4
Total assets	8,142.8	7,389.3
Liabilities
Debt	564.7	575.2
Total liabilities	3,978.6	3,454.3
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 2,560,452 and 2,572,156 shares	2.6	2.6
Paid-in surplus	544.9	536
Retained earnings	3,404.3	3,211.8
Accumulated other comprehensive income (loss), after-tax:
Net unrealized gains (losses) from foreign currency translation and interest rate swap	(2.7)	(3.5)
Total White Mountains’s common shareholders’ equity	3,949.1	3,746.9
Noncontrolling interests	215.1	188.1
Total equity	4,164.2	3,935
Total liabilities and equity	8,142.8	7,389.3
Other Operations
Assets
Fixed maturity investments, at fair value	270.7	238.2
Common equity securities, at fair value	252.1	333.8
Investment in MediaAlpha, at fair value	188.8	168.6
Short-term investments, at fair value	215.5	373.6
Other long-term investments	698.5	418.5
Total investments	1,625.6	1,532.7
Cash	22.9	33.9
Goodwill and other intangible assets	71.2	91.3
Other assets	89.6	155.6
Total assets	1,809.3	1,813.5
Liabilities
Debt	29.6	36.7
Accrued incentive compensation	64	86.1
Other liabilities	27.3	34.3
Total liabilities	120.9	157.1
HG Global/BAM | Operating Segments
Assets
Fixed maturity investments, at fair value	932.2	909.9
Short-term investments, at fair value	80.1	65.9
Total investments	1,012.3	975.8
Cash	4.9	18.2
Insurance premiums receivable	5.5	6.6
Deferred acquisition costs	38.5	36
Other assets	22.2	21.9
Total assets	1,083.4	1,058.5
Liabilities
Unearned insurance premiums	312.3	298.3
Debt	146.8	146.5
Accrued incentive compensation	19.9	28
Other liabilities	40	29
Total liabilities	519	501.8
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liabilities
Loss and loss adjustment expense reserves	1,571.8	1,296.5
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Assets
Fixed maturity investments, at fair value	775	772.8
Common equity securities, at fair value	384.9	334.6
Short-term investments, at fair value	850	484.6
Other long-term investments	416.6	373.6
Total investments	2,426.5	1,965.6
Cash	129.6	101.5
Reinsurance recoverables	538.4	595.3
Insurance premiums receivable	781.6	544.1
Deferred acquisition costs	178.7	127.2
Goodwill and other intangible assets	292.5	292.5
Other assets	66.9	65.2
Total assets	4,414.2	3,691.4
Liabilities
Loss and loss adjustment expense reserves	1,571.8	1,296.5
Unearned insurance premiums	984.6	623.2
Debt	184.6	183.7
Reinsurance payable	146.3	251.1
Contingent consideration	62.1	45.3
Other liabilities	132	122.3
Total liabilities	3,081.4	2,522.1
Kudu | Operating Segments
Assets
Short-term investments, at fair value	17.3	0
Other long-term investments	780.7	695.9
Total investments	798	695.9
Cash	5.4	101.4
Accrued investment income	15.8	12.4
Goodwill and other intangible assets	8.4	8.6
Other assets	8.3	7.6
Total assets	835.9	825.9
Liabilities
Debt	203.7	208.3
Other liabilities	53.6	65
Total liabilities	$ 257.3	$ 273.3

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Common shares, par value per share (in dollars per share)	$ 1	$ 1
Common shares, authorized shares (in shares)	50,000,000	50,000,000
Common shares, issued shares (in shares)	2,560,452	2,572,156
Common shares, outstanding shares (in shares)	2,560,452	2,572,156
Kudu | Operating Segments
Restricted cash	$ 0	$ 12.2

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Revenues:
Earned insurance premiums	$ 506.8	$ 353.2	$ 1,070.6	$ 783.8
Net investment income	48.1	33.9	130.1	79.6
Net realized and unrealized investment gains (losses)	(58.3)	(48)	151.2	(255.2)
Commission revenues	3.5	3.2	10	8.7
Other revenues	19.5	40.9	68	103.4
Total revenues	519.6	383.2	1,429.9	720.3
Expenses:
Loss and loss adjustment expenses	265.8	213.7	581.1	456.2
Acquisition expenses	92.4	76.5	217.8	184.4
Cost of sales	8	25	33.5	68.8
General and administrative expenses	99.6	85.8	298.2	256.9
Change in fair value of contingent consideration	17	2.7	16.8	4.9
Interest expense	15.7	10.5	44.9	27.5
Total expenses	498.5	414.2	1,192.3	998.7
Pre-tax income (loss) from continuing operations	21.1	(31)	237.6	(278.4)
Income tax (expense) benefit	(7.3)	7.4	(19.4)	26.1
Net income (loss) from continuing operations	13.8	(23.6)	218.2	(252.3)
Net income (loss) from discontinued operations, net of tax - NSM Group	0	6.3	0	16.4
Net gain (loss) from sale of discontinued operations, net of tax - NSM Group	0	886.8	0	886.8
Net income (loss)	13.8	869.5	218.2	650.9
Net (income) loss attributable to noncontrolling interests	9.8	18.7	4.5	101.5
Net income (loss) attributable to White Mountains’s common shareholders	23.6	888.2	222.7	752.4
Other Operations
Revenues:
Earned insurance premiums	0	0	0	0
Net investment income	8	8.5	22	13.6
Commission revenues	3.5	3.2	10	8.7
Other revenues	15.2	33	67.1	89.6
Total revenues	(12.1)	8.8	186	1.4
Expenses:
Loss and loss adjustment expenses	0	0	0	0
Acquisition expenses	0	0	0	0
Cost of sales	8	25	33.5	68.8
General and administrative expenses	42.2	41.3	130.7	122.1
Change in fair value of contingent consideration	0	0	0	0
Interest expense	0.9	0.6	2.9	1.2
Total expenses	51.1	66.9	167.1	192.1
Pre-tax income (loss) from continuing operations	(63.2)	(58.1)	18.9	(190.7)
Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	(11.5)	(29.4)	190.1	(141.9)
Investments, Excluding MediaAlpha | Other Operations
Revenues:
Net realized and unrealized investment gains (losses)	8	(17.3)	125.8	2.8
MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	(46.8)	(18.6)	(38.9)	(113.3)
MediaAlpha | Other Operations
Revenues:
Net realized and unrealized investment gains (losses)	(46.8)	(18.6)	(38.9)	(113.3)
HG Global/BAM | Operating Segments
Revenues:
Earned insurance premiums	7.9	7.1	23.3	26
Net investment income	8.1	5.7	22.9	15.1
Net realized and unrealized investment gains (losses)	(24.1)	(38.8)	(17)	(114)
Other revenues	0.7	1.3	2	3.7
Total revenues	(7.4)	(24.7)	31.2	(69.2)
Expenses:
Acquisition expenses	2.1	1.7	6.3	9.5
General and administrative expenses	17.5	15.8	49.7	49.5
Interest expense	3.8	2	10.8	5.4
Total expenses	23.4	19.5	66.8	64.4
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Expenses:
Loss and loss adjustment expenses	265.8	213.7	581.1	456.2
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Revenues:
Earned insurance premiums	498.9	346.1	1,047.3	757.8
Net investment income	16.9	4.9	41.2	9.7
Net realized and unrealized investment gains (losses)	(6.6)	(14.4)	35.9	(76.5)
Other revenues	3.6	6.6	(1.1)	10.1
Total revenues	512.8	343.2	1,123.3	701.1
Expenses:
Loss and loss adjustment expenses	265.8	213.7	581.1	456.2
Acquisition expenses	90.3	74.8	211.5	174.9
General and administrative expenses	35.4	24.2	105.5	74.9
Change in fair value of contingent consideration	17	2.7	16.8	4.9
Interest expense	5.5	3.7	15.7	10.6
Total expenses	414	319.1	930.6	721.5
Kudu | Operating Segments
Revenues:
Earned insurance premiums	0	0	0	0
Net investment income	15.1	14.8	44	41.2
Net realized and unrealized investment gains (losses)	11.2	41.1	45.4	45.8
Commission revenues	0	0	0	0
Other revenues	0	0	0	0
Total revenues	26.3	55.9	89.4	87
Expenses:
Loss and loss adjustment expenses	0	0	0	0
Acquisition expenses	0	0	0	0
Cost of sales	0	0	0	0
General and administrative expenses	4.5	4.5	12.3	10.4
Change in fair value of contingent consideration	0	0	0	0
Interest expense	5.5	4.2	15.5	10.3
Total expenses	10	8.7	27.8	20.7
Pre-tax income (loss) from continuing operations	16.3	47.2	61.6	66.3
Kudu | Investments, Excluding MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	11.2	41.1	45.4	45.8
Kudu | MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	$ 0	$ 0	$ 0	$ 0

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Statement of Comprehensive Income [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ 23.6	$ 888.2	$ 222.7	$ 752.4
Other comprehensive income, net of tax:
Other comprehensive income (loss), net of tax	(1.7)	(1.4)	0.9	(3)
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	0	0.7	0	(5.2)
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	0	2.9	0	2.9
Comprehensive income (loss)	21.9	890.4	223.6	747.1
Other comprehensive (income) loss attributable to noncontrolling interests	0.7	(0.3)	(0.1)	0.4
Comprehensive income (loss) attributable to White Mountains’s common shareholders	$ 22.6	$ 890.1	$ 223.5	$ 747.5
Basic earnings (loss) per share
Basic and diluted earnings (losses) per share (in usd per share)	$ 9.19	$ (1.66)	$ 86.82	$ (50.73)
Discontinued operations (in usd per share)	0	308.59	0	304.97
Total consolidated operations (in usd per share)	9.19	306.93	86.82	254.24
Diluted earnings (loss) per share
Continuing operations (in usd per share)	9.19	(1.66)	86.82	(50.73)
Discontinued operations (in usd per share)	0	308.59	0	304.97
Total consolidated operations (in usd per share)	9.19	306.93	86.82	254.24
Dividends declared per White Mountains’s common share (in usd per share)	0	0	1	1
Dividends paid per White Mountains’s common share (in usd per share)	$ 0	$ 0	$ 1	$ 1

CONSOLIDATED STATEMENTS OF CHAN

CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Millions	Total	Total Equity	Total	Common shares and paid-in surplus	Retained earnings	AOCI, after tax	Non-controlling interest
Beginning Balances at Dec. 31, 2021		$ 3,704.7	$ 3,548.1	$ 588.9	$ 2,957.5	$ 1.7	$ 156.6
Increase (Decrease) in Shareholders' Equity
Net income (loss)	$ 752.4	650.9	752.4		752.4		(101.5)
Other comprehensive income (loss), net of tax	(3)	(3)	(2.6)			(2.6)	(0.4)
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	(5.2)	(5.2)	(5.2)			(5.2)
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	2.9	2.9	2.9			2.9
Total comprehensive income (loss)	747.1	645.6	747.5		752.4	(4.9)	(101.9)
Dividends declared on common shares		(3)	(3)		(3)
Dividends to noncontrolling interests		(7.5)					(7.5)
Issuances of common shares		3	3	3
Issuances of shares of non-controlling interests		74.6					74.6
Repurchases and retirements of common shares		(610.5)	(610.5)	(89.2)	(521.3)
BAM member surplus contribution, net of tax		62.3					62.3
Amortization of restricted share awards		10.3	10.3	10.3
Recognition of equity-based compensation expense of subsidiaries		8.9	8.1	8.1			0.8
Net contributions and dilution from other noncontrolling interests		(4.6)	4.5	14.2	(9.7)		(9.1)
Disposition of noncontrolling interests		(17.5)					(17.5)
Ending Balances at Sep. 30, 2022		3,866.3	3,708	535.3	3,175.9	(3.2)	158.3
Beginning Balances at Jun. 30, 2022		3,492	3,323.3	603.2	2,725.1	(5)	168.7
Increase (Decrease) in Shareholders' Equity
Net income (loss)	888.2	869.5	888.2		888.2		(18.7)
Other comprehensive income (loss), net of tax	(1.4)	(1.5)	(1.8)			(1.8)	0.3
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	0.7	0.7	0.7			0.7
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	2.9	2.9	2.9			2.9
Total comprehensive income (loss)	890.4	871.6	890		888.2	1.8	(18.4)
Dividends to noncontrolling interests		(0.5)					(0.5)
Repurchases and retirements of common shares		(508.9)	(508.9)	(71.5)	(437.4)
BAM member surplus contribution, net of tax		26					26
Amortization of restricted share awards		3.8	3.8	3.8
Recognition of equity-based compensation expense of subsidiaries		0.6	0.4	0.4			0.2
Net contributions and dilution from other noncontrolling interests		(0.8)	(0.6)	(0.6)			(0.2)
Disposition of noncontrolling interests		(17.5)					(17.5)
Ending Balances at Sep. 30, 2022		3,866.3	3,708	535.3	3,175.9	(3.2)	158.3
Beginning Balances at Dec. 31, 2022	3,935	3,935	3,746.9	538.6	3,211.8	(3.5)	188.1
Increase (Decrease) in Shareholders' Equity
Net income (loss)	222.7	218.2	222.7		222.7		(4.5)
Other comprehensive income (loss), net of tax	0.9	0.9	0.8			0.8	0.1
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	0
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	0
Total comprehensive income (loss)	223.6	219.1	223.5		222.7	0.8	(4.4)
Dividends declared on common shares		(2.6)	(2.6)		(2.6)
Dividends to noncontrolling interests		(8.3)					(8.3)
Issuances of common shares		2.1	2.1	2.1
Repurchases and retirements of common shares		(32.7)	(32.7)	(5.1)	(27.6)
BAM member surplus contribution, net of tax		46.4					46.4
Amortization of restricted share awards		11.2	11.2	11.2
Recognition of equity-based compensation expense of subsidiaries		1.8	1.3	1.3			0.5
Net contributions and dilution from other noncontrolling interests		1.6	(0.6)	(0.6)			2.2
Disposition of noncontrolling interests		(9.4)					(9.4)
Ending Balances at Sep. 30, 2023	4,164.2	4,164.2	3,949.1	547.5	3,404.3	(2.7)	215.1
Beginning Balances at Jun. 30, 2023		4,125.9	3,922.2	543.2	3,380.7	(1.7)	203.7
Increase (Decrease) in Shareholders' Equity
Net income (loss)	23.6	13.8	23.6		23.6		(9.8)
Other comprehensive income (loss), net of tax	(1.7)	(1.7)	(1)			(1)	(0.7)
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	0
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	0
Total comprehensive income (loss)	21.9	12.1	22.6		23.6	(1)	(10.5)
Dividends to noncontrolling interests		(0.5)					(0.5)
BAM member surplus contribution, net of tax		19.9					19.9
Amortization of restricted share awards		4	4	4
Recognition of equity-based compensation expense of subsidiaries		0.6	0.4	0.4			0.2
Net contributions and dilution from other noncontrolling interests		2.2	(0.1)	(0.1)			2.3
Ending Balances at Sep. 30, 2023	$ 4,164.2	$ 4,164.2	$ 3,949.1	$ 547.5	$ 3,404.3	$ (2.7)	$ 215.1

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Cash flows from operations:
Net income (loss)	$ 218.2	$ 650.9
Adjustments to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses from investment in MediaAlpha	(151.2)	255.2
Deferred income tax expense (benefit)	1.9	22.1
Amortization of restricted share awards	11.2	10.3
Amortization (accretion) and depreciation	(10.3)	7
Net (income) loss from discontinued operation, net of tax - NSM Group	0	(16.4)
Net gain (loss) from sale of discontinued operations, net of tax	0	(886.8)
Other operating items:
Net change in reinsurance recoverables	56.9	(95.1)
Net change in insurance premiums receivable	(236.4)	(282.3)
Net change in deferred acquisition costs	(54)	(45)
Net change in loss and loss adjustment expense reserves	275.3	434.7
Net change in unearned insurance premiums	375.4	305.8
Net change in reinsurance payable	(104.8)	(111.4)
Net change in restricted cash	(12.2)	4.4
Contributions to Kudu’s Participation Contracts	(108.3)	(97.9)
Proceeds from Kudu’s Participation Contracts sold	74.6	0
Net other operating activities	43.3	(37.3)
Net cash provided from (used for) operations - continuing operations	379.6	118.2
Net cash provided from (used for) operations - NSM Group discontinued operations (Note 19)	0	38.7
Net cash provided from (used for) operations	379.6	156.9
Cash flows from investing activities:
Net change in short-term investments	(194.9)	(735.4)
Sales of fixed maturity investments	107.9	212.4
Maturities, calls and paydowns of fixed maturity investments	212.6	107.3
Sales of common equity securities	129.9	0
Distributions and redemptions of other long-term investments	26.8	55
Proceeds from the sale of NSM Group and Other Operating Businesses, net of cash sold of $0.8 and $144.4	17.3	1,384.3
Purchases of consolidated subsidiaries, net of cash acquired of $0.0 and $0.3	0	(67.9)
Purchases of fixed maturity investments	(392.9)	(474.1)
Purchases of common equity securities and investment in MediaAlpha	(114.2)	(88)
Purchases of other long-term investments	(237.7)	(136.9)
Net other investing activities	(9.8)	3.7
Net cash provided from (used for) investing activities - continuing operations	(455)	260.4
Net cash provided from (used for) investing activities - NSM Group discontinued operations (Note 19)	0	7.1
Net cash provided from (used for) investing activities	(455)	267.5
Cash flows from financing activities:
Draw down of debt and revolving lines of credit	12.2	211.8
Repayment of debt and revolving lines of credit	(24.3)	(10.5)
Cash dividends paid to common shareholders	(2.6)	(3)
Repurchases and retirements of common shares	(32.7)	(610.5)
Net contributions from (distributions to) other noncontrolling interests	(5.3)	(9.2)
Net (contributions to) distributions from discontinued operations	0	11.6
Issuances of shares to noncontrolling interests	0	74.6
BAM member surplus contributions	46.4	62.3
Fidus Re premium payments	(10.3)	(5.6)
Net other financing activities	(0.2)	(2.1)
Net cash provided from (used for) financing activities - continuing operations	(16.8)	(280.6)
Net cash provided from (used for) financing activities - NSM Group discontinued operations (Note 19)	0	(17.5)
Net cash provided from (used for) financing activities	(16.8)	(298.1)
Net change in cash during the period	(92.2)	98
Cash balances at beginning of period (includes restricted cash balances of $12.2 and $4.5 and excludes discontinued operations cash balances of $0.0 and $111.6)	255	147.7
Cash balances at end of period (includes restricted cash balances of $0.0 and $8.9 and excludes discontinued operations cash balances of $0.0 and $0.0)	162.8	245.7
Supplemental cash flows information:
Interest paid	(18)	(10.9)
Net income tax payments	(29.9)	(3.2)
Investments, Excluding MediaAlpha
Adjustments to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses from investment in MediaAlpha	(190.1)	141.9
MediaAlpha
Adjustments to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses from investment in MediaAlpha	$ 38.9	$ 113.3

CONSOLIDATED STATEMENTS OF CA_2

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Cash sold in subsidiary transaction	$ 0.8	$ 144.4
Cash acquired from acquisition	0	0.3
Restricted cash balance	0	8.9
NSM Insurance Group | Discontinued Operations, Disposed of by Sale
Discontinued operations cash balance	$ 0	$ 0

Basis of Presentation and Signi

Basis of Presentation and Significant Accounting Policies	9 Months Ended
	Sep. 30, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation and Significant Accounting Policies	Basis of Presentation and Significant Accounting Policies Basis of Presentation White Mountains Insurance Group, Ltd. (the “Company” or the “Registrant”) is an exempted Bermuda limited liability company whose principal businesses are conducted through its 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 23 South Main Street, Suite 3B, 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 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 the Company, its subsidiaries (collectively with the Company, “White Mountains”) and other entities required to be consolidated under GAAP. Intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. 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. 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 2022 Annual Report on Form 10-K. Reportable Segments As of September 30, 2023, White Mountains conducted its operations through three reportable segments: (1) HG Global/BAM, (2) Ark/WM Outrigger, and (3) Kudu, with our remaining operating businesses, holding companies and other assets included in 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 Company’s chief operating decision makers and its Board of Directors. See Note 14 — “Segment Information.” The HG Global/BAM segment consists of HG Global Ltd. and its wholly-owned subsidiaries (collectively, “HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”) (collectively with HG Global, “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 purpose projects, such as schools, utilities and transportation facilities. BAM is owned by and operated for the benefit of its members, the municipalities that purchase BAM’s insurance for their debt issuances. HG Global was established to fund the startup of BAM and, through its reinsurance subsidiary, HG Re Ltd. (“HG Re”), to provide up to 15%-of-par, first loss reinsurance protection for policies underwritten by BAM. HG Global, together with its subsidiaries, funded the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM (the “BAM Surplus Notes”). As of September 30, 2023 and December 31, 2022, 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 variable interest entity (“VIE”) for which White Mountains is the primary beneficiary. BAM’s results are all attributed to noncontrolling interests. The Ark/WM Outrigger segment consists of Ark Insurance Holdings Limited and its subsidiaries (collectively, “Ark”) and Outrigger Re Ltd. Segregated Account 2023-1 (“WM Outrigger Re”) (collectively with Ark, “Ark/WM Outrigger”). Ark is a specialty property and casualty insurance and reinsurance company that offers a wide range of niche insurance and reinsurance products, including property, specialty, marine & energy, casualty and accident & health. Ark underwrites select coverages through Lloyd’s Syndicates 4020 and 3902 (the “Syndicates”) and its wholly-owned subsidiary Group Ark Insurance Limited (“GAIL”). White Mountains acquired a controlling ownership interest in Ark on January 1, 2021 (the “Ark Transaction”). As of September 30, 2023 and December 31, 2022, White Mountains owned 72.0% of Ark on a basic shares outstanding basis (63.0% after taking account of management’s equity incentives). The remaining shares are owned by current and former employees. In the future, management rollover shareholders could earn additional shares in Ark if and to the extent that White Mountains achieves certain multiple of invested capital return thresholds. If fully earned, these additional shares would represent 12.5% of the shares outstanding at closing. The liability related to these additional shares is recorded as contingent consideration. For the years of account prior to the Ark Transaction, a significant proportion of the Syndicates’ underwriting capital was provided by third-party insurance and reinsurance groups (“TPC Providers”) using whole account reinsurance contracts with Ark’s corporate member. For the years of account subsequent to the Ark Transaction, Ark is no longer using TPC Providers to provide underwriting capital for the Syndicates. Captions within results of operations and other comprehensive income for the three and nine months ended September 30, 2022 are shown net of amounts relating to the TPC Providers’ share of the Syndicates’ results, including investment results. During the fourth quarter of 2022, Ark sponsored the formation of Outrigger Re Ltd., a Bermuda company registered as a special purpose insurer and segregated accounts company, to provide collateralized reinsurance protection on Ark’s Bermuda global property catastrophe excess of loss portfolio written in calendar year 2023. Outrigger Re Ltd. issued non-voting redeemable preference shares on behalf of four segregated accounts to White Mountains and unrelated third-party investors. White Mountains consolidates the results of its segregated account, WM Outrigger Re, in its financial statements. See Note 2 — “Significant Transactions.” As of September 30, 2023 and December 31, 2022, White Mountains owned 100.0% of WM Outrigger Re’s preferred equity. The Kudu segment consists of Kudu Investment Management, LLC and its subsidiaries (collectively “Kudu”). Kudu provides capital solutions for boutique asset and wealth managers for a variety of 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 generally are structured as minority preferred equity stakes with distribution rights, typically tied to gross revenues and designed to generate immediate cash yields. As of September 30, 2023 and December 31, 2022, White Mountains owned 89.2% and 89.3% of Kudu’s basic units outstanding (76.1% and 76.1% on a fully diluted, fully converted basis). White Mountains’s Other Operations consists of the Company and its wholly-owned subsidiary, White Mountains Capital, LLC (“WM Capital”), its other intermediate holding companies, its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), investment assets managed by WM Advisors, its interests in MediaAlpha, Inc. (“MediaAlpha”), PassportCard Limited (“PassportCard”) and DavidShield Life Insurance Agency (2000) Ltd. (“DavidShield”) (collectively, “PassportCard/ DavidShield”), Elementum Holdings LP (“Elementum”), certain other consolidated and unconsolidated entities (“Other Operating Businesses”) and certain other assets. Discontinued Operations On August 1, 2022, White Mountains Holdings (Luxembourg) S.à r.l. (“WTM Holdings Seller”), an indirect wholly owned subsidiary of White Mountains, completed the sale of White Mountains Catskill Holdings, Inc. and NSM Insurance HoldCo, LLC (“NSM” and, collectively with White Mountains Catskill Holdings, Inc., the “NSM Group”) to Riser Merger Sub, Inc., an affiliate of The Carlyle Group Inc. (the “NSM Transaction”), pursuant to the terms of the securities purchase agreement dated as of May 9, 2022. See Note 2 — “Significant Transactions. ” NSM is a full-service managing general agent (“MGA”) and program administrator with delegated binding authorities for specialty property and casualty insurance. As a result of the NSM Transaction, the assets and liabilities of NSM Group have been presented in the balance sheet as held for sale for periods prior to the closing of the transaction, and the results of operations for NSM Group have been classified as discontinued operations in the statements of operations and comprehensive income through the closing of the transaction. Prior period amounts have been reclassified to conform to the current period’s presentation. See Note 19 — “Held for Sale and Discontinued Operations.” Significant Accounting Policies Refer to the Company’s 2022 Annual Report on Form 10-K for a complete discussion regarding White Mountains’s significant accounting policies.

Significant Transactions

Significant Transactions	9 Months Ended
	Sep. 30, 2023
Significant Transactions [Abstract]
Significant Transactions	Significant Transactions NSM On August 1, 2022, the NSM Transaction closed. White Mountains received $1.4 billion in net cash proceeds at closing and recognized a net transaction gain of $875.7 million, which was comprised of $886.8 million of net gain from sale of discontinued operations and $2.9 million of comprehensive income related to the recognition of foreign currency translation gains (losses) from the sale, partially offset by $14.0 million of compensation and other costs related to the transaction recorded in Other Operations. WM Outrigger Re During the fourth quarter of 2022, Ark sponsored the formation of Outrigger Re Ltd., a Bermuda company registered as a special purpose insurer and segregated accounts company, to provide reinsurance capacity to Ark. On December 20, 2022, Outrigger Re Ltd. issued $250.0 million of non-voting redeemable preference shares on behalf of four segregated accounts to White Mountains and unrelated third-party investors. Upon issuance of the preference shares, Outrigger Re Ltd. entered into collateralized quota share agreements with GAIL to provide reinsurance protection on Ark’s Bermuda global property catastrophe excess of loss portfolio written in calendar year 2023. The proceeds from the issuance of the preference shares were deposited into collateral trust accounts to fund any potential obligations under the reinsurance agreements with GAIL. Outrigger Re Ltd.’s obligations under the reinsurance agreements with GAIL are subject to an aggregate limit equal to the assets in the collateral trusts at any point in time. The terms of the reinsurance agreements are renewable upon the mutual agreement of Ark and the applicable preference shareholder. White Mountains purchased 100% of the preference shares issued by its segregated account, WM Outrigger Re, for $205.0 million. White Mountains consolidates WM Outrigger Re’s results in its financial statements. WM Outrigger Re’s quota share reinsurance agreement with GAIL eliminates in White Mountains’s consolidated financial statements. Kudu On May 26, 2022, Kudu raised $114.5 million of equity capital (the “Kudu Transaction”) from Massachusetts Mutual Life Insurance Company (“Mass Mutual”), White Mountains and Kudu management. Mass Mutual, White Mountains and Kudu management contributed $64.1 million, $50.0 million and $0.4 million at a pre-money valuation of 1.3x book value, or $114.0 million above the December 31, 2021 equity value of Kudu’s go-forward portfolio of revenue and earnings participation contracts (“Participation Contracts”). The go-forward portfolio of Kudu’s Participation Contracts excluded $54.3 million of enterprise value as of December 31, 2021 relating to two portfolio companies that had announced sale transactions prior to the capital raise. As a result of the Kudu Transaction, White Mountains’s basic ownership of Kudu decreased from 99.1% to 89.3%.

Investment Securities

Investment Securities	9 Months Ended
	Sep. 30, 2023
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, its investment in MediaAlpha and other long-term investments, which are 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 fair value, which approximated amortized cost, as of September 30, 2023 and December 31, 2022. Other long-term investments consist primarily of unconsolidated entities, Kudu’s Participation Contracts, private equity funds and hedge funds, a bank loan fund, Lloyd’s trust deposits, insurance-linked securities (“ILS”) funds and private debt instruments. 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, dividend income from common equity securities, distributions from its investment in MediaAlpha and distributions from other long-term investments. The following table presents pre-tax net investment income for the three and nine months ended September 30, 2023 and 2022: Three Months Ended September 30, Nine Months Ended September 30, Millions 2023 2022 2023 2022 Fixed maturity investments $ 16.6 $ 11.1 $ 46.6 $ 28.6 Short-term investments 13.8 6.7 34.8 7.2 Common equity securities 1.0 .1 3.6 .2 Other long-term investments 17.4 17.1 46.8 45.9 Amount attributable to TPC Providers — (.3) — (.8) Total investment income 48.8 34.7 131.8 81.1 Third-party investment expenses (.7) (.8) (1.7) (1.5) Net investment income, pre-tax $ 48.1 $ 33.9 $ 130.1 $ 79.6 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, 2023 and 2022: Three Months Ended September 30, Nine Months Ended September 30, Millions 2023 2022 2023 2022 Realized investment gains (losses) Fixed maturity investments $ (.3) $ (4.9) $ (1.7) $ (10.8) Short-term investments (.2) (.1) (.3) (.4) Common equity securities 1.2 — 13.8 — Other long-term investments .6 3.4 48.9 24.1 Net realized investment gains (losses) 1.3 (1.6) 60.7 12.9 Unrealized investment gains (losses) Fixed maturity investments (27.9) (58.8) (9.7) (192.9) Short-term investments (.6) (2.1) .7 (2.6) Common equity securities (18.5) 4.3 29.8 (6.3) Investment in MediaAlpha (46.8) (18.6) (38.9) (113.3) Other long-term investments 34.2 27.5 108.6 40.8 Net unrealized investment gains (losses) (59.6) (47.7) 90.5 (274.3) Net realized and unrealized investment gains (losses), before amount attributable to TPC providers (1) (58.3) (49.3) 151.2 (261.4) Amount attributable to TPC Providers — 1.3 — 6.2 Net realized and unrealized investment gains (losses) $ (58.3) $ (48.0) $ 151.2 $ (255.2) Fixed maturity and short-term investments Net realized and unrealized investment gains (losses) $ (29.0) $ (65.9) $ (11.0) $ (206.7) Less: net realized and unrealized gains (losses) on investment 2.5 .1 4.7 (3.0) Net unrealized investment gains (losses) recognized during the period on $ (31.5) $ (66.0) $ (15.7) $ (203.7) Common equity securities and investment in MediaAlpha Net realized and unrealized investment gains (losses) on common $ (17.3) $ 4.3 $ 43.6 $ (6.3) Net realized and unrealized investment gains (losses) from (46.8) (18.6) (38.9) (113.3) Total net realized and unrealized investment gains (losses) (64.1) (14.3) 4.7 (119.6) Less: net realized and unrealized gains (losses) on investment (.1) — 3.7 — Net unrealized investment gains (losses) recognized during the period on $ (64.0) $ (14.3) $ 1.0 $ (119.6) (1) For the three months ended September 30, 2023 and 2022, includes $(12.8) and $(22.6) of net realized and unrealized investment gains (losses) related to foreign currency exchange. For the nine months ending September 30, 2023 and 2022, includes $(1.9) and $(44.4) of net realized and unrealized investment gains (losses) related to foreign currency exchange. The following table presents total net unrealized gains (losses) attributable to Level 3 investments for the three and nine months ended September 30, 2023 and 2022 for investments still held at the end of the period: Three Months Ended September 30, Nine Months Ended September 30, Millions 2023 2022 2023 2022 Total net unrealized investment gains on other long-term $ 24.6 $ 40.8 $ 76.0 $ 57.7 Investment Holdings The following tables present the cost or amortized cost, gross unrealized investment gains (losses), net foreign currency gains (losses) and carrying values of White Mountains’s fixed maturity investments as of September 30, 2023 and December 31, 2022: September 30, 2023 Millions Cost or Gross Gross Net Foreign Carrying U.S. Government and agency obligations $ 270.1 $ — $ (9.0) $ — $ 261.1 Debt securities issued by corporations 1,038.8 .1 (74.6) (2.7) 961.6 Municipal obligations 264.6 — (26.4) — 238.2 Mortgage and asset-backed securities 372.6 — (47.2) — 325.4 Collateralized loan obligations 196.5 .3 (3.1) (2.1) 191.6 Total fixed maturity investments $ 2,142.6 $ .4 $ (160.3) $ (4.8) $ 1,977.9 December 31, 2022 Millions Cost or Gross Gross Net Foreign Carrying U.S. Government and agency obligations $ 216.6 $ — $ (10.2) $ — $ 206.4 Debt securities issued by corporations 1,098.3 .6 (78.3) (1.8) 1,018.8 Municipal obligations 281.6 .4 (23.4) — 258.6 Mortgage and asset-backed securities 288.7 — (34.5) — 254.2 Collateralized loan obligations 190.8 .1 (6.0) (2.0) 182.9 Total fixed maturity investments $ 2,076.0 $ 1.1 $ (152.4) $ (3.8) $ 1,920.9 The following table presents the cost or amortized cost and carrying values of White Mountains’s fixed maturity investments by contractual maturity as of September 30, 2023. Actual maturities could differ from contractual maturities because borrowers may have the right to call or prepay certain obligations with or without call or prepayment penalties. September 30, 2023 Millions Cost or Amortized Cost Carrying Value Due in one year or less $ 307.6 $ 301.2 Due after one year through five years 870.9 813.4 Due after five years through ten years 302.7 267.6 Due after ten years 92.3 78.7 Mortgage and asset-backed securities and collateralized loan obligations 569.1 517.0 Total fixed maturity investments $ 2,142.6 $ 1,977.9 The following tables present the cost or amortized cost, gross unrealized investment gains (losses), net foreign currency gains (losses), and carrying values of common equity securities, White Mountains’s investment in MediaAlpha and other long-term investments as of September 30, 2023 and December 31, 2022: September 30, 2023 Millions Cost or Gross Gross Net Foreign Carrying Common equity securities $ 599.5 $ 51.6 $ (4.8) $ (9.3) $ 637.0 Investment in MediaAlpha $ 59.2 $ 139.9 $ (10.3) $ — $ 188.8 Other long-term investments $ 1,639.9 $ 390.9 $ (116.6) $ (18.4) $ 1,895.8 December 31, 2022 Millions Cost or Gross Gross Net Foreign Carrying Common equity securities $ 660.6 $ 26.7 $ (8.4) $ (10.5) $ 668.4 Investment in MediaAlpha $ — $ 168.6 $ — $ — $ 168.6 Other long-term investments $ 1,340.8 $ 271.1 $ (107.1) $ (16.8) $ 1,488.0 Fair Value Measurements 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). See Note 17 — “Fair Value of Financial Instruments.” As of September 30, 2023 and December 31, 2022, White Mountains used quoted market prices or other observable inputs to determine fair value for approximately 68% and 72% 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, 2023 and December 31, 2022 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, 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 by industry sector because investors often reference commonly used benchmarks and their subsectors to monitor risk and performance. Accordingly, White Mountains has further disaggregated this asset class 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. September 30, 2023 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 261.1 $ 261.1 $ — $ — Debt securities issued by corporations: Financials 290.1 — 290.1 — Consumer 188.3 — 188.3 — Healthcare 109.1 — 109.1 — Industrial 107.8 — 107.8 — Technology 107.5 — 107.5 — Utilities 66.6 — 66.6 — Communications 42.4 — 42.4 — Energy 29.1 — 29.1 — Materials 20.7 — 20.7 — Total debt securities issued by corporations 961.6 — 961.6 — Municipal obligations 238.2 — 238.2 — Mortgage and asset-backed securities 325.4 — 325.4 — Collateralized loan obligations 191.6 — 191.6 — Total fixed maturity investments 1,977.9 261.1 1,716.8 — Short-term investments 1,162.9 1,149.7 13.2 — Common equity securities: Exchange-traded funds 252.1 252.1 — — Other (1) 384.9 — 384.9 — Total common equity securities 637.0 252.1 384.9 — Investment in MediaAlpha 188.8 188.8 — — Other long-term investments 1,090.7 — 15.4 1,075.3 Other long-term investments — NAV (2) 805.1 — — — Total other long-term investments 1,895.8 — 15.4 1,075.3 Total investments $ 5,862.4 $ 1,851.7 $ 2,130.3 $ 1,075.3 (1) Consists of investments in listed funds that predominantly invest in international equities. (2) Consists of private equity funds and hedge funds, a bank loan fund, Lloyd’s trust deposits and ILS funds for which fair value is measured using NAV as a practical expedient. Investments for which fair value is measured at NAV are not classified within the fair value hierarchy. December 31, 2022 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 206.4 $ 206.4 $ — $ — Debt securities issued by corporations: Financials 291.2 — 291.2 — Consumer 191.9 — 191.9 — Healthcare 121.3 — 121.3 — Industrial 115.4 — 115.4 — Technology 123.7 — 123.7 — Utilities 73.8 — 73.8 — Communications 47.9 — 47.9 — Energy 33.9 — 33.9 — Materials 19.7 — 19.7 — Total debt securities issued by corporations 1,018.8 — 1,018.8 — Municipal obligations 258.6 — 258.6 — Mortgage and asset-backed securities 254.2 — 254.2 — Collateralized loan obligations 182.9 — 182.9 — Total fixed maturity investments 1,920.9 206.4 1,714.5 — Short-term investments 924.1 924.1 — — Common equity securities: Exchange-traded funds 333.8 333.8 — — Other (1) 334.6 — 334.6 — Total common equity securities 668.4 333.8 334.6 — Investment in MediaAlpha 168.6 168.6 — — Other long-term investments 926.4 — 14.8 911.6 Other long-term investments — NAV (2) 561.6 — — — Total other long-term investments 1,488.0 — 14.8 911.6 Total investments $ 5,170.0 $ 1,632.9 $ 2,063.9 $ 911.6 (1) Consists of investments in listed funds that predominantly invest in international equities. (2) Consists of private equity funds and hedge funds, a bank loan fund, Lloyd’s trust deposits and ILS funds for which fair value is measured using NAV as a practical expedient. Investments for which fair value is measured at NAV are not classified within the fair value hierarchy. Investments Held on Deposit or as Collateral As of September 30, 2023 and December 31, 2022, investments of $531.8 million and $500.5 million were held in trusts required to be maintained in relation to HG Global’s reinsurance agreements with BAM. HG Global is required to maintain an interest reserve account in connection with its senior notes issued in 2022. As of September 30, 2023 and December 31, 2022, the fair value of the interest reserve account, which is included in short-term investments, was $29.9 million and $31.2 million. See Note 7 — “Debt.” BAM is required to maintain deposits with certain insurance regulatory agencies in order to maintain its insurance licenses. The fair value of such deposits, which represent state deposits and are included within the investment portfolio, totaled $4.6 million as of both September 30, 2023 and December 31, 2022. Lloyd’s trust deposits are generally required of Lloyd's syndicates to protect policyholders in non-U.K. markets and are pledged into Lloyd’s trust accounts to provide a portion of the capital needed to support obligations at Lloyd’s. As of September 30, 2023 and December 31, 2022, Ark held Lloyd’s trust deposits with a fair value of $150.3 million and $137.4 million. The underwriting capacity of a member of Lloyd’s must be supported by providing a deposit (“Funds at Lloyd’s”) in the form of cash, securities or letters of credit in an amount determined by Lloyd’s. The amount of such deposit is calculated for each member through the completion of an annual capital adequacy exercise. These requirements allow Lloyd’s to evaluate whether each member has sufficient assets to meet its underwriting liabilities plus a required solvency margin. As of September 30, 2023 and December 31, 2022, the fair value of Ark’s Funds at Lloyd’s investment deposits totaled $335.2 million and $319.2 million. As of September 30, 2023 and December 31, 2022, Ark held additional investments on deposit or as collateral for insurance regulators and reinsurance counterparties of $232.4 million and $257.0 million. As of September 30, 2023 and December 31, 2022, Ark had $147.2 million and $90.3 million of short-term investments pledged as collateral under uncommitted standby letters of credit. See Note 7 — “Debt.” As of September 30, 2023 and December 31, 2022, short-term investments of $233.3 million and $203.7 million were held in a collateral trust account required to be maintained in relation to WM Outrigger Re’s reinsurance agreement with GAIL. Kudu is required to maintain an interest reserve account in connection with its credit facility. As of September 30, 2023, the interest reserve account of $14.1 million was held in short term investments. As of December 31, 2022, the interest reserve account of $12.2 million was held in restricted cash. See Note 7 - “Debt.” Debt Securities Issued by Corporations The following table presents the credit ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of September 30, 2023 and December 31, 2022: Fair Value at Millions September 30, 2023 December 31, 2022 AAA $ 12.6 $ 11.3 AA 84.3 96.0 A 509.6 567.9 BBB 349.1 337.7 Other 6.0 5.9 Debt securities issued by corporations (1) $ 961.6 $ 1,018.8 (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 and Collateralized Loan Obligations The following table presents the fair value of White Mountains’s mortgage and asset-backed securities and collateralized loan obligations as of September 30, 2023 and December 31, 2022: September 30, 2023 December 31, 2022 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: FNMA $ 162.5 $ 162.5 $ — $ 124.5 $ 124.5 $ — FHLMC 114.3 114.3 — 78.8 78.8 — GNMA 29.2 29.2 — 28.3 28.3 — Total agency (1) 306.0 306.0 — 231.6 231.6 — Non-agency: Residential .2 .2 — .3 .3 — Total non-agency .2 .2 — .3 .3 — Total mortgage-backed securities 306.2 306.2 — 231.9 231.9 — Other asset-backed securities: Credit card receivables 3.2 3.2 — 11.9 11.9 — Vehicle receivables 13.5 13.5 — 10.4 10.4 — Other 2.5 2.5 — — — — Total other asset-backed securities 19.2 19.2 — 22.3 22.3 — Total mortgage and asset-backed securities 325.4 325.4 — 254.2 254.2 — Collateralized loan obligations 191.6 191.6 — 182.9 182.9 — Total mortgage and asset-backed securities $ 517.0 $ 517.0 $ — $ 437.1 $ 437.1 $ — (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). As of September 30, 2023 and December 31, 2022, White Mountains’s investment portfolio included $191.6 million and $182.9 million of collateralized loan obligations that are within the senior tranches of their respective fund securitization structures. All of White Mountains’s collateral loan obligations were rated AAA or AA as of September 30, 2023 and December 31, 2022. Investment in MediaAlpha In 2020, MediaAlpha completed an initial public offering (the “MediaAlpha IPO”). Following the MediaAlpha IPO, White Mountains’s investment in MediaAlpha is accounted for at fair value based on the publicly traded share price of MediaAlpha’s common stock and is presented as a separate line item on the balance sheet. During the second quarter of 2023, White Mountains completed a tender offer to purchase 5.9 million additional shares of MediaAlpha at a purchase price of $10.00 per share. As of September 30, 2023, White Mountains owned 22.9 million shares, representing a 35.3% basic ownership interest (33.0% on a fully diluted, fully converted basis). See Note 16 — “Equity Method Eligible Investments.” At White Mountains’s current level of ownership, each $1.00 per share increase or decrease in the share price of MediaAlpha will result in an approximate $9.00 per share increase or decrease in White Mountains’s book value per share. At the September 30, 2023 closing price of $8.26 per share, the fair value of White Mountains’s investment in MediaAlpha was $188.8 million. Other Long-Term Investments The following table presents the carrying values of White Mountains’s other long-term investments by reportable segment as of September 30, 2023 and December 31, 2022: Fair Value at September 30, 2023 Millions Ark/ WM Outrigger Kudu Other Total Kudu’s Participation Contracts $ — $ 775.3 $ — $ 775.3 PassportCard/DavidShield — — 150.0 150.0 Elementum Holdings, L.P. — — 35.0 35.0 Mandatorily redeemable preferred — — 58.5 58.5 Other unconsolidated entities (1) — — 41.8 41.8 Total unconsolidated entities — 775.3 285.3 1,060.6 Private equity funds and hedge funds 55.4 — 243.9 299.3 Bank loan fund 189.3 — — 189.3 ILS funds — — 159.5 159.5 Lloyd’s trust deposits 150.3 — — 150.3 Private debt instruments — 5.4 9.4 14.8 Other 21.6 — .4 22.0 Total other long-term investments $ 416.6 $ 780.7 $ 698.5 $ 1,895.8 (1) Includes White Mountains’s noncontrolling equity interests in certain preferred securities, private common equity securities, limited liability company units and Simple Agreement for Future Equity (“SAFE”) investments. Fair Value at December 31, 2022 Millions Ark/ WM Outrigger Kudu Other Total Kudu’s Participation Contracts $ — $ 695.9 $ — $ 695.9 PassportCard/DavidShield — — 135.0 135.0 Elementum Holdings, L.P. — — 30.0 30.0 Other unconsolidated entities (1) — — 37.2 37.2 Total unconsolidated entities — 695.9 202.2 898.1 Private equity funds and hedge funds 40.4 — 157.4 197.8 Bank loan fund 174.8 — — 174.8 ILS funds — — 49.3 49.3 Lloyd’s trust deposits 137.4 — — 137.4 Private debt instruments — — 9.6 9.6 Other 21.0 — — 21.0 Total other long-term investments $ 373.6 $ 695.9 $ 418.5 $ 1,488.0 (1) Includes White Mountains’s noncontrolling equity interests in certain preferred securities, private common equity securities, limited liability company units and Simple Agreement for Future Equity (“SAFE”) investments. Private Equity Funds and Hedge Funds White Mountains invests in private equity funds and hedge funds, which are included in other long-term investments. The fair value of these investments is generally estimated using the NAV of the funds. As of September 30, 2023, White Mountains held investments in eighteen private equity funds and two hedge funds. The largest investment in a single private equity fund or hedge fund was $53.4 million as of September 30, 2023 and $49.0 million as of December 31, 2022. The following table presents the fair value of investments and unfunded commitments in private equity funds and hedge funds by investment objective and sector as of September 30, 2023 and December 31, 2022: September 30, 2023 December 31, 2022 Millions Fair Value Unfunded Fair Value Unfunded Private equity funds Aerospace/Defense/Government $ 146.4 $ 25.5 $ 59.4 $ 37.5 Financial services 88.6 41.8 77.1 54.3 Real estate 4.2 2.5 4.1 2.5 Total private equity funds 239.2 69.8 140.6 94.3 Hedge funds Long/short equity financials and business services 51.1 — 49.0 — European small/mid cap 9.0 — 8.2 — Total hedge funds 60.1 — 57.2 — Total private equity funds and hedge funds included in other long-term investments $ 299.3 $ 69.8 $ 197.8 $ 94.3 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, 2023: Millions 1 – 3 years 3 – 5 years 5 – 10 years >10 years Total Private equity funds — expected lock-up period remaining $4.4 $72.1 $148.8 $13.9 $239.2 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, 2023 and December 31, 2022, 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. Advance notice requirements for redemptions from White Mountains’s hedge fund investments range from 45 to 90 days. One of White Mountains’s hedge fund investments also limits redemptions to every second anniversary following the date of the initial investment. Bank Loan Fund White Mountains’s other long-term investments include a bank loan fund with a fair value of $189.3 million and $174.8 million as of September 30, 2023 and December 31, 2022. The fair value of this investment is estimated using the NAV of the fund. The bank loan fund’s investment objective is to provide, on an unleveraged basis, high current income consistent with preservation of capital and low duration. The bank loan fund primarily invests in a broad portfolio of U.S. dollar-denominated, non-investment grade, floating-rate senior secured loans and may invest in other financial instruments, such as secured and unsecured corporate debt, credit default swaps, reverse repurchase agreements, synthetic indices and cash and cash equivalents. The investment in the bank loan fund is subject to 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. White Mountains may redeem all or a portion of its bank loan fund investment as of any calendar month-end upon 15 calendar days advanced written notice. Lloyd’s Trust Deposits White Mountains’s other long-term investments include Lloyd’s trust deposits, which consist of non-U.K. deposits and Canadian comingled pooled funds. The Lloyd’s trust deposits invest primarily in short-term government securities, agency securities and corporate bonds held in trusts that are managed by Lloyd's of London. These investments are generally required of Lloyd's syndicates to protect policyholders in non-U.K. markets and are pledged into Lloyd’s trust accounts to provide a portion of the capital needed to support obligations at Lloyd’s. The fair value of the Lloyd’s trust deposits is generally estimated using the NAV of the funds. As of September 30, 2023 and December 31, 2022, White Mountains held Lloyd’s trust deposits with a fair value of $150.3 million and $137.4 million. Insurance-Linked Securities Funds White Mountains’s other long-term investments include ILS fund investments. The fair value of these investments is generally estimated using the NAV of the funds. As of September 30, 2023 and December 31, 2022, White Mountains held investments in ILS funds with a fair value of $159.5 million and $49.3 million. Investments in ILS funds are generally subject to restrictions, including lock-up periods where no redemptions or withdrawals are allowed, non-renewal clauses, 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 ILS funds less certain due to the potential for loss development. In such circumstances, the impacted investments may be subject to additional lock-up provisions. ILS funds are typically subject to monthly and annual restrictions on redemptions and advance redemption notice period requirements that range between 30 and 90 days. Amounts requested for redemption remain subject to market fluctuations until the redemption effective date, which generally falls at the end of the defined redemption period, or until the underlying investment has fully matured or been commuted. Rollforward of Level 3 Investments Level 3 measurements as of September 30, 2023 and 2022 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 table presents the changes in White Mountains’s fair value measurements for Level 3 investments for the nine months ended September 30, 2023 and 2022: Level 3 Investments Millions Other Long-term Other Long-term Balance at December 31, 2022 $ 911.6 Balance at December 31, 2021 $ 890.6 Net realized and unrealized gains 74.0 Net realized and unrealized gains 57.3 Purchases and contributions 200.1 Purchases and contributions 127.9 Sales and distributions (110.4) Sales and distributions (31.7) Transfers in — Transfers in — Transfers out — Transfers out — Balance at September 30, 2023 $ 1,075.3 Balance at September 30, 2022 $ 1,044.1 Fair Value Measurements — Transfers Between Levels - Nine months ended September 30, 2023 and 2022 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 nine months ended September 30, 2023 and 2022, 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 nine months ended September 30, 2023 and 2022, 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 White Mountains’s other long-term investments, classified within Level 3 as of September 30, 2023 and December 31, 2022. The tables below exclude $41.7 million and $41.1 million of Level 3 other long-term investments generally valued based on recent or expected transaction prices. The fair value of investments in private equity funds and hedge funds, bank loan funds, Lloyd’s trust deposits and ILS funds are generally estimated using the NAV of the funds. $ in Millions September 30, 2023 Description Valuation Technique(s) (1) Fair Value (2) Unobservable Inputs Discount Rate (5) Terminal Cash Flow Exit Multiple (x) or Terminal Revenue Growth Rate (%) (5) Kudu’s Participation Contracts (3)(4) Discounted cash flow $775.3 19% - 26% 7x - 22x PassportCard/DavidShield Discounted cash flow $150.0 25% 4% Elementum Holdings, L.P. Discounted cash flow $35.0 25% 4% Mandatorily redeemable preferred Discounted cash flow $58.5 15% N/A Private debt instruments Discounted cash flow $14.8 12% - 13% N/A (1) Key inputs to the discounted cash flow analysis generally include projections of future revenue and earnings, discount rates and terminal exit multiples or growth rates. (2) Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs. (3) Since Kudu’s Participation Contracts are not subject to corporate taxes within Kudu Investment Management, LLC, pre-tax discount rates are applied to pre-tax cash flows in determining fair values. The weighted average discount rate and weighted average terminal cash flow exit multiple applied to Kudu’s Participation Contracts was 21% and 13x. (4) In the first nine months of 2023, Kudu deployed a total of $108.3 into new and existing Participation Contracts. (5) Increases (decreases) to the discount rates in isolation would result in lower (higher) fair value measurements, while increases (decreases) to the terminal cash flow exit multiples or terminal revenue growth rates in isolation would result in higher (lower) fair value measurements. $ in Millions December 31, 2022 Description Valuation Technique(s) (1) Fair Value (2) Unobservable Inputs Discount Rate (6) Terminal Cash Flow Exit Multiple (x) or Terminal Revenue Growth Rate (%) (6) Kudu’s Participation Contracts (3)(4)(5) Discounted cash flow $695.9 18% - 25% 7x - 16x PassportCard/DavidShield Discounted cash flow $135.0 24% 4% Elementum Holdings, L.P. Discounted cash flow $30.0 21% 4% Private debt instruments Discounted cash flow $9.6 11% N/A (1) Key inputs to the discounted cash flow analysis generally include projections of future revenue and earnings, discount rates and terminal exit multiples or growth rates. (2) Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs. (3

Goodwill and Other Intangible A

Goodwill and Other Intangible Assets	9 Months Ended
	Sep. 30, 2023
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. Goodwill represents the excess of the amount paid to acquire businesses over the fair value of identifiable net assets at the date of acquisition. 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 the economic lives, acquisition date fair values, accumulated amortization and net carrying values for other intangible assets and goodwill as of September 30, 2023 and December 31, 2022: $ in Millions Weighted Average Economic September 30, 2023 December 31, 2022 Acquisition Date Fair Value Accumulated Amortization Net Carrying Value Acquisition Date Fair Value Accumulated Amortization Net Carrying Value Goodwill: Ark N/A $ 116.8 $ — $ 116.8 $ 116.8 $ — $ 116.8 Kudu N/A 7.6 — 7.6 7.6 — 7.6 Other Operations N/A 44.4 — 44.4 52.1 — 52.1 Total goodwill 168.8 — 168.8 176.5 — 176.5 Other intangible assets: Ark Underwriting capacity N/A 175.7 — 175.7 175.7 — 175.7 Kudu Trade names 7 2.2 1.4 .8 2.2 1.2 1.0 Other Operations Trade names 13.3 13.3 3.7 9.6 17.9 3.0 14.9 Customer relationships 10.9 24.8 9.4 15.4 29.5 7.5 22.0 Other 12.1 2.8 1.0 1.8 2.8 .5 2.3 Subtotal 40.9 14.1 26.8 50.2 11.0 39.2 Total other intangible assets 218.8 15.5 203.3 228.1 12.2 215.9 Total goodwill and other intangible assets $ 387.6 $ 15.5 $ 372.1 $ 404.6 $ 12.2 $ 392.4 Intangible Assets Valuation Methods The goodwill recognized for the entities shown above is attributed to expected future cash flows. The acquisition date fair values of other intangible assets with finite lives are estimated using income approach techniques, which use future expected cash flows to develop a discounted present value amount. The multi-period-excess-earnings method estimates fair value using the present value of the incremental after-tax cash flows attributable solely to the other intangible asset over its remaining life. This approach was used to estimate the fair value of other intangible assets associated with the underwriting capacity and customer relationships. The relief-from-royalty method was used to estimate fair value for other intangible assets that relate to rights that could be obtained via a license from a third-party owner. Under this method, the fair value is estimated using the present value of license fees avoided by owning rather than leasing the asset. This technique was used to estimate the fair value of trade names and other intangible assets. The with-or-without method estimates the fair value of other intangible assets that provide an incremental benefit. Under this method, the fair value of the other intangible asset is calculated by comparing the value of the entity with and without the other intangible asset. This approach was used to estimate the fair value of favorable lease terms. On at least an annual basis beginning no later than the interim period included in the one-year anniversary of an acquisition, White Mountains evaluates goodwill and other intangible assets for potential impairment. Between annual evaluations, White Mountains considers changes in circumstances or events subsequent to the most recent evaluation that may indicate an impairment may exist and, if necessary will perform an interim review for potential impairment. During the three and nine months ended September 30, 2023 and 2022, White Mountains did not recognize any impairments to goodwill and other intangible assets. Rollforward of Goodwill and Other Intangible Assets The following table presents the change in goodwill and other intangible assets for the three and nine months ended September 30, 2023 and 2022: Three Months Ended September 30, 2023 2022 Millions Goodwill Other Intangible Assets Total Goodwill and Other Intangible Assets Goodwill Other Intangible Assets Total Goodwill and Other Intangible Assets Beginning balance $ 168.8 $ 204.8 $ 373.6 $ 201.8 $ 196.2 $ 398.0 Attribution of acquisition date fair value estimates between goodwill and other intangible assets (1) — — — (22.9) 22.9 — Measurement period adjustments (2) — — — (3.4) — (3.4) Amortization — (1.5) (1.5) — (1.4) (1.4) Ending balance $ 168.8 $ 203.3 $ 372.1 $ 175.5 $ 217.7 $ 393.2 (1) Relates to acquisitions within Other Operations. The relative fair values of goodwill and other intangible assets recognized in connection with acquisitions during 2022 had not yet been finalized as of the end of the period. (2) Measurement period adjustments relate to updated information about acquisition date fair values of assets acquired and liabilities assumed. During the three months ended September 30, 2022, adjustments relate to acquisitions within Other Operations. Nine Months Ended September 30, 2023 2022 Millions Goodwill Other Intangible Assets Total Goodwill and Other Intangible Assets Goodwill Other Intangible Assets Total Goodwill and Other Intangible Assets Beginning balance $ 176.5 $ 215.9 $ 392.4 $ 142.3 $ 198.2 $ 340.5 Acquisition of businesses (1) — — — 59.5 — 59.5 Dispositions (2) (6.7) (6.9) (13.6) — — — Attribution of acquisition date fair value estimates between goodwill and other intangible assets (1) — — — (22.9) 22.9 — Measurement period adjustments (3) (1.0) — (1.0) (3.4) — (3.4) Amortization — (5.7) (5.7) — (3.4) (3.4) Ending balance $ 168.8 $ 203.3 $ 372.1 $ 175.5 $ 217.7 $ 393.2 (1) Relates to acquisitions within Other Operations. The relative fair values of goodwill and other intangible assets recognized in connection with acquisitions during 2022 had not yet been finalized as of the end of the period. (2) Relates to a disposition within Other Operations.

Loss and Loss Adjustment Expens

Loss and Loss Adjustment Expense Reserves	9 Months Ended
	Sep. 30, 2023
Insurance [Abstract]
Loss and Loss Adjustment Expense Reserves	Loss and Loss Adjustment Expense Reserves P&C Insurance and Reinsurance The following table summarizes the loss and loss adjustment expense (“LAE”) reserve activity of the Ark/WM Outrigger segment for the three and nine months ended September 30, 2023 and 2022: Three Months Ended September 30, Nine Months Ended September 30, Millions 2023 2022 2023 2022 Gross beginning balance $ 1,421.0 $ 1,022.1 $ 1,296.5 $ 894.7 Less: beginning reinsurance recoverable on unpaid losses (1) (362.7) (375.5) (505.0) (428.9) Net loss and LAE reserves 1,058.3 646.6 791.5 465.8 Loss and LAE incurred relating to: Current year losses 266.4 222.9 562.1 491.8 Prior year losses (.6) (9.2) 19.0 (35.6) Net incurred losses and LAE 265.8 213.7 581.1 456.2 Loss and LAE paid relating to: Current year losses (8.8) (6.1) (18.4) (20.8) Prior year losses (90.4) (37.0) (281.1) (130.8) Net paid loss and LAE (99.2) (43.1) (299.5) (151.6) Change in TPC Providers’ participation (2) — — 145.4 57.5 Foreign currency translation and other adjustments to loss (5.5) (9.2) .9 (19.9) Net ending balance 1,219.4 808.0 1,219.4 808.0 Plus: ending reinsurance recoverable on unpaid losses (3) 352.4 521.4 352.4 521.4 Gross ending balance $ 1,571.8 $ 1,329.4 $ 1,571.8 $ 1,329.4 (1) The beginning reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $0.0 and $182.4 for the three months ended September 30, 2023 and 2022 and $145.4 and $276.8 for the nine months ended September 30, 2023 and 2022. (2) Amount represents the impact to net loss and LAE reserves due to a change in the TPC Providers’ participation related to the annual reinsurance to close process. (3) The ending reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $0.0 and $168.0 as of September 30, 2023 and 2022. For the three and nine months ended September 30, 2023, the Ark/WM Outrigger segment recognized $0.6 million of net favorable prior year loss reserve development and $19.0 million of net unfavorable prior year loss reserve development. The net unfavorable prior year loss reserve development for the nine months ended September 30, 2023 was driven primarily by Winter Storm Elliot and three large claims in the property and marine & energy lines of business. For the three and nine months ended September 30, 2022, the Ark/WM Outrigger segment recognized $9.2 million and $35.6 million of net favorable prior year loss reserve development, driven primarily by good claims experience across most classes including the property and specialty lines of business, predominantly from business underwritten in London. Impact of Third-Party Capital For the years of account prior to the Ark Transaction, a significant proportion of the Syndicates’ underwriting capital was provided by TPC Providers using whole account reinsurance contracts with Ark’s corporate member. For the years of account subsequent to the Ark Transaction, Ark is no longer using TPC Providers to provide underwriting capital for the Syndicates. A reinsurance to close (“RITC”) agreement is generally put in place after the third year of operations for a year of account such that the outstanding loss and LAE reserves, including future development thereon, are reinsured into the next year of account. As a result, and in combination with the changing participation provided by TPC Providers, Ark’s participation on outstanding loss and LAE reserves reinsured into the next year of account may change, perhaps significantly. During the first quarter of 2023, an RITC agreement was executed such that the outstanding loss and LAE reserves for claims arising out of the 2020 year of account, for which the TPC Providers’ participation in the total net results of the Syndicates was 42.8%, were reinsured into the 2021 year of account, for which the TPC Providers’ participation in the total net results of the Syndicates was 0.0%. During the first quarter of 2022, an RITC agreement was executed such that the outstanding loss and LAE reserves for claims arising out of the 2019 year of account, for which the TPC Providers’ participation in the total net results of the Syndicates was 58.3%, were reinsured into the 2020 year of account, for which the TPC Providers’ participation in the total net results of the Syndicates was 42.8%. Municipal Bond Guarantee Insurance HG Re and BAM do not have any outstanding loss and LAE reserves related to BAM’s municipal bond guarantee insurance business.

Third-Party Reinsurance

Third-Party Reinsurance	9 Months Ended
	Sep. 30, 2023
Reinsurance Disclosures [Abstract]
Third-Party Reinsurance	Third-Party Reinsurance P&C Insurance and Reinsurance In the normal course of business, Ark may seek to limit losses that may arise from catastrophes or other events by reinsuring certain risks with third-party reinsurers. Ark remains liable for risks reinsured in the event that the reinsurer does not honor its obligations under reinsurance contracts. The following table summarizes the effects of reinsurance on written and earned premiums and on losses and LAE for the Ark/WM Outrigger segment for the three and nine months ended September 30, 2023 and 2022: Millions Three Months Ended September 30, Nine Months Ended September 30, 2023 2022 2023 2022 Written premiums: Direct $ 189.4 $ 118.6 $ 727.1 $ 563.8 Assumed 61.8 96.9 939.6 688.7 Gross written premiums 251.2 215.5 1,666.7 1,252.5 Ceded (20.2) (1) (22.9) (360.3) (1) (246.0) Net written premiums $ 231.0 $ 192.6 $ 1,306.4 $ 1,006.5 Earned premiums: Direct $ 273.1 $ 193.5 $ 624.1 $ 492.9 Assumed 370.0 261.4 684.6 473.7 Gross earned premiums 643.1 454.9 1,308.7 966.6 Ceded (144.2) (2) (108.8) (261.4) (2) (208.8) Net earned premiums $ 498.9 $ 346.1 $ 1,047.3 $ 757.8 Losses and LAE: Gross $ 269.3 $ 391.4 $ 627.5 $ 712.9 Ceded (3.5) (3) (177.7) (46.4) (3) (256.7) Net losses and LAE $ 265.8 $ 213.7 $ 581.1 $ 456.2 (1) The ceded written premiums exclude $6.0 and $108.4 ceded by Ark to WM Outrigger Re for the three and nine months ended September 30, 2023, which eliminate in White Mountains’s consolidated financial statements. (2) The ceded earned premiums exclude $60.6 and $75.4 ceded by Ark to WM Outrigger Re for the three and nine months ended September 30, 2023, which eliminate in White Mountains’s consolidated financial statements. (3) The ceded loss and LAE exclude $7.3 and $7.9 ceded by Ark to WM Outrigger Re for the three and nine months ended September 30, 2023, which eliminate in White Mountains’s consolidated financial statements. The following table presents the Ark/WM Outrigger segment’s reinsurance recoverables as of September 30, 2023 and December 31, 2022: Millions September 30, 2023 December 31, 2022 Reinsurance recoverables on unpaid losses $ 352.4 (1) $ 505.0 (3) Reinsurance recoverables on paid losses 26.3 31.1 Ceded unearned premiums 159.7 (2) 59.2 Reinsurance recoverables $ 538.4 $ 595.3 (1) The reinsurance recoverables on unpaid losses exclude $7.9 ceded by Ark to WM Outrigger Re as of September 30, 2023, which eliminate in White Mountains’s consolidated financial statements. (2) The ceded unearned premiums exclude $33.0 ceded by Ark to WM Outrigger Re as of September 30, 2023, which eliminate in White Mountains’s consolidated financial statements. (3) The reinsurance recoverables on unpaid losses include $145.4 attributable to TPC Providers as of December 31, 2022, which are collateralized. As reinsurance contracts do not relieve Ark of its obligation to its policyholders, Ark seeks to reduce the credit risk associated with reinsurance balances by avoiding over-reliance on specific reinsurers through the application of concentration limits and thresholds. Ark is selective with its reinsurers, placing reinsurance with only those reinsurers having a strong financial condition. Ark monitors the financial strength of its reinsurers on an ongoing basis. The following table presents the Ark/WM Outrigger segment’s gross and net reinsurance recoverables by the reinsurer’s A.M. Best Company, Inc (“A.M. Best”) ratings as of September 30, 2023: $ in Millions As of September 30, 2023 A.M. Best Rating (1) Gross Collateral Net % of Total A+ or better $ 203.7 $ — $ 203.7 69.6 % A- to A 78.8 — 78.8 26.9 B++ or lower and not rated 96.2 (2) 85.9 (2) 10.3 3.5 Total $ 378.7 $ 85.9 $ 292.8 100.0 % (1) A.M. Best ratings as detailed above are: “A+ or better” (Superior) “A- to A” (Excellent), “B++” (Good). (2) Excludes $7.9 ceded by Ark to WM Outrigger Re as of September 30, 2023, which eliminates in White Mountains’s consolidated financial statements. Reinsurance Contracts Accounted for as Deposits Ark has an aggregate excess of loss contract with SiriusPoint Ltd. (“SiriusPoint”), which is accounted for using the deposit method and recorded within other assets. Ark earns an annual crediting rate of 3.0%, which is recorded within other revenue. As of September 30, 2023 and December 31, 2022, the carrying value of Ark’s deposit in SiriusPoint, including accrued interest, was $20.3 million and $20.4 million. Municipal Bond Guarantee Insurance See Note 10 — “Municipal Bond Guarantee Insurance” for third-party reinsurance balances and reinsurance contracts accounted for as deposits related to White Mountains’s financial guarantee business.

Debt

Debt	9 Months Ended
	Sep. 30, 2023
Debt Disclosure [Abstract]
Debt	Debt The following table presents White Mountains’s debt outstanding as of September 30, 2023 and December 31, 2022: $ in Millions September 30, 2023 Effective (1) December 31, 2022 Effective (1) HG Global Senior Notes $ 150.0 11.8% $ 150.0 8.9% Unamortized discount and issuance cost (3.2) (3.5) HG Global Senior Notes, carrying value 146.8 146.5 Ark 2007 Subordinated Notes, carrying value 30.0 30.0 Ark 2021 Subordinated Notes Tranche 1 41.9 41.3 Ark 2021 Subordinated Notes Tranche 2 47.0 47.0 Ark 2021 Subordinated Notes Tranche 3 70.0 70.0 Unamortized issuance cost (4.3) (4.6) Ark 2021 Subordinated Notes, carrying value 154.6 153.7 Total Ark Subordinated Notes, carrying value 184.6 10.8% 183.7 7.6% Kudu Credit Facility 210.3 9.9% 215.2 6.1% Unamortized issuance cost (6.6) (6.9) Kudu Credit Facility, carrying value 203.7 208.3 Other Operations debt 30.1 10.3% 37.4 6.6% Unamortized issuance cost (.5) (.7) Other Operations debt, carrying value 29.6 36.7 Total debt $ 564.7 $ 575.2 (1) Effective rate includes the effect of the amortization of debt issuance costs and, where applicable, the original issue discount. HG Global Senior Notes On April 29, 2022, HG Global received the proceeds from the issuance of its $150.0 million face value floating rate secured senior notes (the “HG Global Senior Notes”). The HG Global Senior Notes, which mature in April 2032, accrue interest at a floating rate equal to the three-month Secured Overnight Financing Rate (“SOFR”) plus 6.3% per annum. Subsequent to the five-year anniversary of the funding date, absent the occurrence of an early amortization trigger event, HG Global will be required to make payments of principal on a quarterly basis totaling $15.0 million annually. Upon the occurrence of an early amortization trigger event, HG Global is required to use all available cash flow to repay the notes. Early amortization trigger events include scenarios in which HG Re is effectively in runoff. HG Global has the option to redeem, in whole or in part, the HG Global Senior Notes after the five-year anniversary of the funding date at the outstanding principal amount plus accrued interest. On June 16, 2022, HG entered into an interest rate cap agreement, effective on July 25, 2022, to limit its exposure to the risk of interest rate increases on the HG Global Senior Notes. The notional amount of the interest rate cap is $150.0 million and the termination date is July 25, 2025. See Note 9 — “Derivatives.” The HG Global Senior Notes require HG Global to maintain an interest reserve account of eight times the interest accrued for the most recent quarterly interest period. While the interest rate cap is in force, the interest reserve account is subject to a maximum required balance of $29.2 million. As of September 30, 2023, the fair value of the interest reserve account, which is included in short-term investments, was $29.9 million. The HG Global Senior Notes are secured by the capital stock and other equity interests of HG Global’s subsidiaries, the interest reserve account, and all cash and non-cash proceeds from such collateral. The HG Global Senior Notes contain various affirmative and negative covenants that White Mountains considers to be customary for such borrowings. If the payment of principal and interest under the HG Global Senior Notes becomes subject to tax withholding on behalf of a relevant governmental authority for certain indemnified taxes, the HG Global Senior Notes require the payment of additional amounts such that the amount received by the noteholders is the same as would have been received absent the tax withholding being imposed. The HG Global Senior Notes require the payment of additional interest of 1.0% per annum if the HG Global Senior Notes receive a non-investment grade rating or are no longer rated. As of September 30, 2023, the HG Global Senior Notes had an investment grade rating. As of September 30, 2023, the HG Global Senior Notes had an outstanding balance of $150.0 million. Ark Subordinated Notes In March 2007, GAIL issued $30.0 million face value of floating rate unsecured junior subordinated deferrable interest notes to Alesco Preferred Funding XII Ltd., Alesco Preferred Funding XIII Ltd. and Alesco Preferred Funding XIV Ltd (the “Ark 2007 Subordinated Notes”). The Ark 2007 Subordinated Notes, which mature in June 2037, accrue interest at a floating rate equal to the three-month U.S. London Inter-Bank Offered Rate (“LIBOR”) plus 4.6% per annum. As of September 30, 2023, the Ark 2007 Subordinated Notes had an outstanding balance of $30.0 million. In the third quarter of 2021, GAIL issued $163.3 million face value floating rate unsecured subordinated notes at par in three separate transactions for proceeds of $157.8 million, net of debt issuance costs (collectively, the “Ark 2021 Subordinated Notes”). The Ark 2021 Subordinated Notes were issued in private placement offerings that were exempt from the registration requirements of the Securities Act of 1933. On July 13, 2021, Ark issued €39.1 million ($46.3 million based upon the foreign exchange spot rate as of the date of the transaction) face value floating rate unsecured subordinated notes (“Ark 2021 Subordinated Notes Tranche 1”). The Ark 2021 Subordinated Notes Tranche 1, which mature in July 2041, accrue interest at a floating rate equal to the three-month Euro Interbank Offered Rate (“EURIBOR”) plus 5.75% per annum. On August 11, 2021, Ark issued $47.0 million face value floating rate unsecured subordinated notes (“Ark 2021 Subordinated Notes Tranche 2”). The Ark 2021 Subordinated Notes Tranche 2, which mature in August 2041, accrue interest at a floating rate equal to the three-month U.S. LIBOR plus 5.75% per annum. On September 8, 2021, Ark issued $70.0 million face value floating rate unsecured subordinated notes (“Ark 2021 Subordinated Notes Tranche 3”). The Ark 2021 Subordinated Notes Tranche 3, which mature in September 2041, accrue interest at a floating rate equal to the three-month U.S. LIBOR plus 6.1% per annum. On the ten-year anniversary of the issue dates, the interest rate for the Ark 2021 Subordinated Notes will increase by 1.0% per annum. Ark has the option to redeem, in whole or in part, the Ark 2021 Subordinated Notes ahead of contractual maturity at the outstanding principal amounts plus accrued interest at the ten-year anniversary or any subsequent interest payment date. All payments of principal and interest under the Ark 2021 Subordinated Notes are conditional upon GAIL’s solvency and compliance with the enhanced capital requirements of the Bermuda Monetary Authority (“BMA”). The deferral of payments of principal and interest under these conditions does not constitute a default by Ark and does not give the noteholders any rights to accelerate repayment of the Ark 2021 Subordinated Notes or take any enforcement action under the Ark 2021 Subordinated Notes. If the payments of principal and interest under the Ark 2021 Subordinated Notes become subject to tax withholding on behalf of Bermuda or any political subdivision there, the Ark 2021 Subordinated Notes require the payment of additional amounts such that the amount received by the noteholders is the same as would have been received absent the tax withholding being imposed. The Ark 2021 Subordinated Notes Tranche 3 require the payment of additional interest of 1.0% per annum upon the occurrence of a premium load event until such event is remedied. Premium load events include the failure to meet payment obligations of the Ark 2021 Subordinated Notes Tranche 3 when due, failure of GAIL to maintain an investment grade credit rating, failure to maintain 120% of GAIL’s Bermuda solvency capital requirement, failure of GAIL to maintain a debt to capital ratio below 40%, late filing of GAIL’s or Ark’s financial information, and making a restricted payment or distribution on GAIL’s common stock or other securities that rank junior or pari passu with the Ark 2021 Subordinated Notes Tranche 3 when a different premium load event exists or will be caused by the restricted payment. As of September 30, 2023, there were no premium load events. As of September 30, 2023, the Ark 2021 Subordinated Notes Tranche 1 had an outstanding balance of €39.1 million ($41.9 million based upon the foreign exchange spot rate as of September 30, 2023), the Ark 2021 Subordinated Notes Tranche 2 had an outstanding balance of $47.0 million, and the Ark 2021 Subordinated Notes Tranche 3 had an outstanding balance of $70.0 million. The Ark Subordinated Notes contain various affirmative and negative covenants that White Mountains considers to be customary for such borrowings. Ark Standby Letter of Credit Facilities In December 2021, Ark entered into two uncommitted secured standby letter of credit facility agreements to support the continued growth and expansion of its GAIL insurance and reinsurance operations. The standby letter of credit facility agreements were executed with ING Bank N.V., London Branch (the “ING LOC Facility”) with capacity of $50.0 million on an uncollateralized basis and with Citibank Europe Plc (the “Citibank LOC Facility”) with capacity of $100.0 million on a collateralized basis. In September 2022, Ark entered into an additional uncommitted standby letter of credit facility agreement with Lloyds Bank Corporate Markets PLC (the “Lloyds LOC Facility”) with capacity of $50.0 million on a collateralized basis. As of September 30, 2023, the ING LOC Facility was undrawn. As of September 30, 2023, the Citibank LOC Facility had an outstanding balance of $93.2 million and short-term investments pledged as collateral of $108.6 million. As of September 30, 2023, the Lloyds LOC Facility had an outstanding balance of $25.9 million and short-term investments pledged as collateral of $38.6 million. Ark’s uncommitted secured standby letter of credit facility agreements contain various representations, warranties and covenants that White Mountains considers to be customary for such borrowings. Kudu Credit Facility On March 23, 2021, Kudu entered into a secured revolving credit facility (the “Kudu Credit Facility”) with Mass Mutual to repay its prior credit facility and to fund new investments and related transaction expenses. The maximum borrowing capacity of the Kudu Credit Facility is $300.0 million. The Kudu Credit Facility matures on March 23, 2036. Through June 30, 2023, interest on the Kudu Credit Facility accrued at a floating interest rate equal to the greater of the three-month LIBOR or 0.25% plus, in each case, 4.30%. Effective July 1, 2023, the Kudu Credit Facility was amended such that the interest rate is equal to the three month Secured Overnight Financing Rate (“SOFR”) plus a credit adjustment spread of 0.26% and a stated margin of 4.30%. The Kudu Credit Facility requires Kudu to maintain an interest reserve account. As of September 30, 2023, the interest reserve account of $14.1 million was held in short term investments. As of December 31, 2022, the interest reserve account of $12.2 million was held in restricted cash. The Kudu Credit Facility requires Kudu to maintain a ratio of the outstanding balance to the sum of the fair market value of Kudu’s Participation Contracts and cash held in certain accounts (the “LTV Percentage”) of less than 50% in years 0-3, 40% in years 4-6, 25% in years 7-8, 15% in years 9-10, and 0% thereafter. As of September 30, 2023, Kudu had a 27.9% LTV Percentage. Kudu may borrow undrawn balances within the initial three-year availability period, subject to customary terms and conditions, to the extent the amount borrowed under the Kudu Credit Facility does not exceed the borrowing base, which is equal to 35% of the fair value of Kudu’s qualifying Participation Contracts. When considering the fair value of Kudu’s qualifying Participation Contracts as of September 30, 2023, the available undrawn balance was $74.1 million. The following table presents the change in debt under the Kudu Credit Facility for the three and nine months ended September 30, 2023 and 2022: Three Months Ended September 30, Nine Months Ended September 30, Millions 2023 2022 2023 2022 Kudu Credit Facility Beginning balance $ 210.3 $ 260.4 $ 215.2 $ 225.4 Term loans Borrowings — — 12.0 35.0 Repayments — — (16.9) — Ending balance $ 210.3 $ 260.4 $ 210.3 $ 260.4 The Kudu Credit Facility is secured by all property of the loan parties and contains various affirmative and negative covenants that White Mountains considers to be customary for such borrowings. Other Operations Debt As of September 30, 2023, White Mountains’s Other Operations had debt with an outstanding balance of $30.1 million, which consisted of four secured credit facilities (collectively, “Other Operations debt”). Compliance At September 30, 2023, White Mountains was in compliance, in all material respects, with all of the covenants under its debt instruments.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2023
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company and its Bermuda domiciled subsidiaries are not subject to income tax under current Bermuda law. The Company has subsidiaries and branches that operate in various other jurisdictions around the world and are subject to tax in the jurisdictions in which they operate. As of September 30, 2023, the primary jurisdictions in which the Company’s subsidiaries and branches were subject to tax include Ireland, Israel, Luxembourg, the United Kingdom and the United States. White Mountains’s income tax expense related to pre-tax income from continuing operations for the three months ended September 30, 2023 represented an effective tax rate of 34.6%. The effective tax rate was different from the U.S. statutory rate of 21.0%, driven primarily by an increase in the valuation allowance at BAM, as well as withholding taxes and state income taxes, partially offset by full year forecasted income in jurisdictions with lower tax rates than the United States. White Mountains’s income tax expense related to pre-tax income from continuing operations for the nine months ended September 30, 2023 represented an effective tax rate of 8.2%. The effective tax rate was different from the U.S. statutory rate of 21.0%, driven primarily by full year forecasted income in jurisdictions with lower tax rates than the United States. White Mountains’s income tax benefit related to pre-tax loss from continuing operations for the three and nine months ended September 30, 2022 represented an effective tax rate of 23.9% and 9.4%. The effective tax rate for the nine months ended September 30, 2022 was different from the U.S. statutory rate of 21.0%, driven primarily by full year forecasted income in jurisdictions with lower tax rates than the United States, partially offset by an increase in the valuation allowance on net deferred tax assets in certain U.S. operations within Other Operations, an increase in the valuation allowance at BAM, as well as state income taxes. On April 1, 2023, the U.K. corporate tax rate increased from 19.0% to 25.0%. On August 16, 2022, the U.S. enacted the Inflation Reduction Act (the “IRA”). White Mountains has evaluated the tax provisions of the IRA, the most significant of which relate to the corporate alternative minimum tax and the tax on share repurchases, and does not expect the legislation to have a material impact on the results of its operations. In arriving at the effective tax rate for the three and nine months ended September 30, 2023 and 2022, White Mountains forecasted all income and expense items including the change in net unrealized investment gains (losses) and net realized investment gains (losses) for the years ending December 31, 2023 and 2022. 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 2017.

Derivatives

Derivatives	9 Months Ended
	Sep. 30, 2023
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives	Derivatives HG Global Interest Rate Cap On June 16, 2022, HG entered into an interest rate cap agreement, effective on July 25, 2022, to limit its exposure to the risk of interest rate increases on the HG Global Senior Notes. The notional amount of the interest rate cap is $150.0 million and the termination date is July 25, 2025. HG paid initial premiums of $3.3 million for the interest rate cap. Under the terms of the interest rate cap agreement, if the current three-month SOFR rate at the measurement date exceeds 3.5%, HG will receive payments from the counterparty equal to the difference between the three-month SOFR rate on the determination date and 3.5%, multiplied by the notional amount of the cap based on the number of days in the quarter. As of September 30, 2023, the three-month SOFR rate was 5.4%.

Municipal Bond Guarantee Insura

Municipal Bond Guarantee Insurance	9 Months Ended
	Sep. 30, 2023
Guarantees [Abstract]
Municipal Bond Guarantee Insurance	Municipal Bond Guarantee Insurance HG Global was established to fund the startup of BAM, a mutual municipal bond insurer. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of the BAM Surplus Notes. Reinsurance Treaties FLRT BAM is a party to a first loss reinsurance treaty (“FLRT”) with HG Re under which HG Re provides first loss protection up to 15%-of-par outstanding on each municipal bond insured by BAM. For capital appreciation bonds, par is adjusted to the estimated equivalent par value for current interest paying bonds. In return, BAM cedes approximately 60% of the risk premium charged for insuring the municipal bond, which is net of a ceding commission. The FLRT is a perpetual agreement with terms that can be renegotiated after a specified period of time. During 2021, BAM and HG Re agreed that the terms may be renegotiated at the end of 2024, and each subsequent five-year period thereafter. Fidus Re BAM is party to a collateralized financial guarantee excess of loss reinsurance agreement that serves to increase BAM’s claims paying resources and is provided by Fidus Re Ltd. (“Fidus Re”). In 2018, Fidus Re was initially capitalized by the issuance of $100.0 million of insurance-linked securities (the “Fidus Re 2018 Agreement”), which have an initial term of 12 years and are callable five years after the date of issuance. The proceeds from issuance were placed in a collateral trust supporting Fidus Re’s obligations to BAM. Under the Fidus Re 2018 Agreement, Fidus Re reinsures 90% of aggregate losses exceeding $165.0 million on a portion of BAM’s financial guarantee portfolio (the “2018 Covered Portfolio”) up to a total reimbursement of $100.0 million. The Fidus Re 2018 Agreement does not provide coverage for losses in excess of $276.1 million. The 2018 Covered Portfolio consists of approximately 24% of BAM’s gross par outstanding as of September 30, 2023. In 2021, Fidus Re issued an additional $150.0 million of insurance-linked securities (the “Fidus Re 2021 Agreement”), which have an initial term of 12 years and are callable five years after the date of issuance. The proceeds from issuance were placed in a collateral trust supporting Fidus Re’s obligations to BAM. Under the Fidus Re 2021 Agreement, Fidus Re reinsures 90% of aggregate losses exceeding $135.0 million on a portion of BAM’s financial guarantee portfolio (the “2021 Covered Portfolio”) up to a total reimbursement of $150.0 million. The Fidus Re 2021 Agreement does not provide coverage for losses in excess of $301.7 million. The 2021 Covered Portfolio consists of approximately 29% of BAM’s gross par outstanding as of September 30, 2023. In 2022, Fidus Re issued an additional $150.0 million of insurance linked securities (the “Fidus Re 2022 Agreement”), which have an initial term of 12 years and are callable seven years after the date of issuance. The proceeds from issuance were placed in a collateral trust supporting Fidus Re’s obligations to BAM. Under the Fidus Re 2022 Agreement, Fidus Re reinsures 90% of aggregate losses exceeding $110.0 million on a portion of BAM’s financial guarantee portfolio (the “2022 Covered Portfolio”) up to a total reimbursement of $150.0 million. The Fidus Re 2022 Agreement does not provide coverage for losses in excess of $276.7 million. The 2022 Covered Portfolio consists of approximately 30% of BAM’s gross par outstanding as of September 30, 2023. The Fidus Re agreements are accounted for using deposit accounting and any related financing expenses are recorded in general and administrative expenses as they do not meet the risk transfer requirements necessary to be accounted for as reinsurance. XOLT In January 2020, BAM entered into an excess of loss reinsurance agreement (the “XOLT”) with HG Re. Under the XOLT, HG Re provides last dollar protection for exposures on municipal bonds insured by BAM in excess of the New York State Department of Financial Services (“NYDFS”) single issuer limits. As of September 30, 2023, the XOLT is subject to an aggregate limit equal to the lesser of $125.0 million or the assets held in the supplemental collateral trust (the “Supplemental Trust”) at any point in time. The agreement is accounted for using deposit accounting and any related financing expenses are recorded in general and administrative expenses as the agreement does not meet the risk transfer requirements necessary to be accounted for as reinsurance. Collateral Trusts HG Re’s obligations under the FLRT are subject to an aggregate limit equal to the assets in two collateral trusts, the Supplemental Trust and the Regulation 114 Trust (together, the “Collateral Trusts”), at any point in time. On a monthly basis, BAM deposits cash equal to ceded premiums, net of ceding commissions, due to HG Re under the FLRT directly into the Regulation 114 Trust. The Regulation 114 Trust target balance is equal to HG Re’s unearned premiums and unpaid loss and LAE reserves, if any. If, at the end of any quarter, the Regulation 114 Trust balance is below the target balance, funds will be withdrawn from the Supplemental Trust and deposited into the Regulation 114 Trust in an amount equal to the shortfall. If, at the end of any quarter, the Regulation 114 Trust balance is above 102% of the target balance, funds will be withdrawn from the Regulation 114 Trust and deposited into the Supplemental Trust. The Regulation 114 Trust balance as of September 30, 2023 and December 31, 2022 was $311.2 million and $288.6 million. The Supplemental Trust target balance is $603.0 million, less the amount of cash and securities in the Regulation 114 Trust in excess of its target balance (the “Supplemental Trust Target Balance”). If, at the end of any quarter, the Supplemental Trust balance exceeds the Supplemental Trust Target Balance, such excess may be distributed to HG Re. The distribution will be made first as an assignment of accrued interest on the BAM Surplus Notes and second in cash and/or fixed income securities. 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 Supplemental Trust balance as of September 30, 2023 and December 31, 2022 was $588.5 million and $568.3 million. As of September 30, 2023 and December 31, 2022, the Collateral Trusts held assets of $899.7 million and $856.9 million, which included $526.5 million and $503.3 million of cash and investments, $340.0 million and $340.0 million of BAM Surplus Notes and $33.2 million and $13.6 million of interest receivable on the BAM Surplus Notes. BAM Surplus Notes Through 2024, the interest rate on the BAM Surplus Notes is a variable rate equal to the one-year U.S. Treasury rate plus 300 basis points, set annually. During 2023, the interest rate on the BAM Surplus Notes is 7.7%. Beginning in 2025, the interest rate will be fixed at the higher of the then current variable rate or 8.0%. Under its agreements with HG Global, BAM is required to seek regulatory approval to pay principal and interest on the BAM Surplus Notes only 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. No payment of principal or interest on the BAM Surplus Notes may be made without the approval of the NYDFS. In December 2022, BAM made a $36.0 million cash payment of principal and interest on the BAM Surplus Notes held by HG Global. Of this payment, $24.6 million was a repayment of principal held in the Supplemental Trust, $1.0 million was a payment of accrued interest held in the Supplemental Trust and $10.4 million was a payment of accrued interest held outside the Supplemental Trust. During the three and nine months ended September 30, 2023 and 2022, BAM made no repayments of the BAM Surplus Notes or accrued interest. As of September 30, 2023 and December 31, 2022, the principal balance on the BAM Surplus Notes was $340.0 million for both periods, and total interest receivable on the BAM Surplus Notes was $177.6 million and $157.9 million. Insured Obligations and Premiums The following table presents a schedule of BAM’s insured obligations as of September 30, 2023 and December 31, 2022: September 30, 2023 December 31, 2022 Contracts outstanding 14,084 13,382 Remaining weighted average contract period (in years) 11.0 10.8 Contractual debt service outstanding (in millions): Principal $ 105,918.1 $ 99,996.9 Interest 52,628.8 48,880.6 Total debt service outstanding $ 158,546.9 $ 148,877.5 Gross unearned insurance premiums (in millions) $ 312.3 $ 298.3 The following table presents a schedule of BAM’s future premium revenues as of September 30, 2023: Millions September 30, 2023 October 1, 2023 - December 31, 2023 $ 7.3 January 1, 2024 - March 31, 2024 7.2 April 1, 2024 - June 30, 2024 7.1 July 1, 2024 - September 30, 2024 7.0 October 1, 2024 - December 31, 2024 6.9 Total 2024 28.2 2025 26.4 2026 24.8 2027 23.2 2028 21.4 2029 and thereafter 181.0 Total gross unearned insurance premiums $ 312.3 The following table presents a schedule of written premiums and earned premiums included in the HG Global/BAM segment for the three and nine months ended September 30, 2023 and 2022: Three Months Ended September 30, Nine Months Ended September 30, Millions 2023 2022 2023 2022 Written premiums: Direct $ 16.5 $ 18.4 $ 37.3 $ 44.9 Assumed — 1.3 — 1.3 Gross written premiums (1) $ 16.5 $ 19.7 $ 37.3 $ 46.2 Earned premiums: Direct $ 7.4 $ 6.5 $ 21.4 $ 22.0 Assumed .5 .6 1.9 4.0 Gross earned premiums (1) $ 7.9 $ 7.1 $ 23.3 $ 26.0 (1) There are no ceded premium amounts in the periods presented, and gross earned premiums are equivalent to net written premiums and net earned premiums. In September 2022, BAM entered into a 100% facultative quota share reinsurance agreement under which it assumed a portfolio of municipal bond guarantee contracts with a par value of $42.5 million. During 2023, the contracts associated with this assumed reinsurance transaction were novated.

Earnings Per Share

Earnings Per Share	9 Months Ended
	Sep. 30, 2023
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, 2023 and 2022. See Note 19 — “Held for Sale and Discontinued Operations.” Three Months Ended September 30, Nine Months Ended September 30, 2023 2022 2023 2022 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common $ 23.6 $ 888.2 $ 222.7 $ 752.4 Less: total income (loss) from discontinued operations, net of tax (1) — 893.1 — 903.2 Less: net (income) loss from discontinued operations — (.1) — (.7) Net income (loss) from continuing operations attributable to 23.6 (4.8) 222.7 (150.1) Allocation of (earnings) losses to participating restricted common shares (2) (.3) .1 (3.1) 1.8 Basic and diluted earnings (losses) per share numerators $ 23.3 $ (4.7) $ 219.6 $ (148.3) Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 2,560.5 2,893.8 2,564.2 2,959.3 Average unvested restricted common shares (3) (37.6) (38.3) (35.5) (35.5) Basic earnings (losses) per share denominator 2,522.9 2,855.5 2,528.7 2,923.8 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 2,560.5 2,893.8 2,564.2 2,959.3 Average unvested restricted common shares (3) (37.6) (38.3) (35.5) (35.5) Diluted earnings (losses) per share denominator 2,522.9 2,855.5 2,528.7 2,923.8 Basic and diluted earnings per share (in dollars) - continuing Distributed earnings - dividends declared and paid $ — $ — $ 1.00 $ 1.00 Undistributed earnings (losses) 9.19 (1.66) 85.82 (51.73) Basic and diluted earnings (losses) per share $ 9.19 $ (1.66) $ 86.82 $ (50.73) (1) Includes net income (loss) from discontinued operations, net of tax - NSM Group, net gain (loss) from sale of discontinued operations, net of tax - NSM Group and net (income) loss from discontinued operations attributable to non-controlling interests. See Note 19 — “Held for Sale and Discontinued Operations. ” (2) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (3) Restricted shares outstanding vest upon a stated date. See Note 12 — “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, 2023 and 2022. See Note 19 — “Held for Sale and Discontinued Operations.” Three Months Ended September 30, Nine Months Ended September 30, Millions 2023 2022 2023 2022 Undistributed net earnings - continuing operations: Net income (loss) attributable to White Mountains’s common $ 23.3 $ (4.7) $ 219.6 $ (148.3) Dividends declared, net of restricted common share amounts (1) — — (2.5) (3.0) Total undistributed net earnings (losses), net of restricted $ 23.3 $ (4.7) $ 217.1 $ (151.3) (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, 2023
Share-Based Payment Arrangement [Abstract]
Employee Share-Based Incentive Compensation Plans	Employee Share-Based Incentive Compensation Plans White Mountains’s share-based incentive compensation plans are designed to incentivize key employees to maximize shareholder value over long periods of time. White Mountains believes that this is best pursued by utilizing a pay-for-performance program that closely aligns the financial interests of management with those of its shareholders while rewarding appropriate risk taking. White Mountains accomplishes this by emphasizing variable long-term compensation that is contingent on performance over a number of years rather than fixed entitlements. White Mountains expenses all of its share-based compensation. As a result, White Mountains’s calculation of its owners’ returns includes the expense of all outstanding share-based compensation awards. White Mountains’s Long-Term Incentive Plan (the “WTM Incentive Plan”) provides for grants of various types of share-based and non-share-based incentive awards to key employees and directors of White Mountains. As of September 30, 2023 and 2022, White Mountains’s share-based incentive compensation awards consist of performance shares and restricted shares. Performance Shares Performance shares are designed to reward employees for meeting company-wide performance targets. Performance shares are conditional grants of a specified maximum number of common shares or an equivalent amount of cash. Awards generally vest at the end of a three-year service period, are subject to the attainment of pre-specified performance goals, and are valued based on the market value of common shares at the time awards are paid. Performance shares earned under the WTM Incentive Plan are typically paid in cash but may be paid in common shares. Compensation expense is recognized for the vested portion of the awards over the related service periods. The level of payout ranges from zero to two times the number of shares initially granted, depending on White Mountains’s financial performance. Performance shares become payable at the conclusion of a performance cycle (typically three years) if pre-defined financial targets are met. The performance measures used for determining performance share payouts are growth in White Mountains’s adjusted book value per share and intrinsic value per share. Intrinsic value per share is generally calculated by adjusting adjusted book value per share for differences between the adjusted book value of certain assets and liabilities and White Mountains’s estimate of their underlying intrinsic values. The following table presents the performance share activity for the three and nine months ended September 30, 2023 and 2022 for performance shares granted under the WTM Incentive Plan: Three Months Ended September 30, Nine Months Ended September 30, 2023 2022 2023 2022 $ in Millions Target Performance Accrued Target Performance Accrued Target Performance Accrued Target Performance Accrued Beginning of period 37,031 $ 44.6 39,449 $ 41.2 39,449 $ 67.5 40,828 $ 42.2 Shares paid (1) — — — — (13,350) (40.8) (14,625) (26.4) New grants — — — — 10,895 — 13,225 — Forfeitures and cancellations (2) — — — (.1) 37 .2 21 (.2) Expense recognized — 10.5 — 11.9 — 28.2 — 37.4 End of period 37,031 $ 55.1 39,449 $ 53.0 37,031 $ 55.1 39,449 $ 53.0 (1) WTM performance share payments in 2023 for the 2020-2022 performance cycle, which were paid in March 2023 at 200% of target. WTM performance share payments in 2022 for the 2019-2021 performance cycle, which were paid in March 2022 at 172% of target. (2) Amounts include changes in assumed forfeitures, as required under GAAP. During the nine months ended September 30, 2023, White Mountains granted 10,895 performance shares for the 2023-2025 performance cycle. During the nine months ended September 30, 2022, White Mountains granted 13,225 performance shares for the 2022-2024 performance cycle. For the 2020-2022 performance cycle, all performance shares earned were settled in cash. For the 2019-2021 performance cycle, the Company issued common shares for 750 performance shares earned, and all other performance shares earned were settled in cash. If the outstanding performance shares had vested on September 30, 2023, the total additional compensation cost to be recognized would have been $32.9 million, based on accrual factors as of September 30, 2023 (common share price and payout assumptions). The following table presents performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan as of September 30, 2023 for each performance cycle: September 30, 2023 $ in Millions Target Performance Accrued Performance cycle: 2021 – 2023 13,475 $ 30.4 2022 – 2024 13,225 22.1 2023 – 2025 10,895 3.4 Sub-total 37,595 55.9 Assumed forfeitures (564) (.8) Total 37,031 $ 55.1 Restricted Shares Restricted shares are grants of a specified number of common shares that generally vest at the end of a 34-month service period. The following table presents the unrecognized compensation cost associated with the outstanding restricted share awards under the WTM Incentive Plan for the three and nine months ended September 30, 2023 and 2022: Three Months Ended September 30, Nine Months Ended September 30, 2023 2022 2023 2022 $ in Millions Restricted Unamortized Restricted Unamortized Restricted Unamortized Issue Date Restricted Unamortized Issue Date Non-vested, Beginning of period 37,595 $ 24.3 38,350 $ 23.0 38,350 $ 15.5 37,850 $ 15.9 Issued — — — — 10,895 16.0 13,225 13.8 Vested — — — — (11,650) — (12,725) — Forfeited — — — — — — — — Expense recognized — (4.1) — (3.8) — (11.3) — (10.5) End of period 37,595 $ 20.2 38,350 $ 19.2 37,595 $ 20.2 38,350 $ 19.2

Noncontrolling Interests

Noncontrolling Interests	9 Months Ended
	Sep. 30, 2023
Noncontrolling Interest [Abstract]
Noncontrolling Interests	Noncontrolling Interests Noncontrolling interests consist of the ownership interests of noncontrolling shareholders in consolidated entities and are presented separately on the balance sheet. The following table presents the balance of noncontrolling interests included in White Mountains’s total equity and the related percentage of each consolidated entity’s total equity owned by noncontrolling shareholders as of September 30, 2023 and December 31, 2022: September 30, 2023 December 31, 2022 $ in Millions Noncontrolling Percentage (1) Noncontrolling Equity Noncontrolling Percentage (1) Noncontrolling Equity Noncontrolling interests, excluding BAM HG Global 3.1 % $ (3.3) 3.1 % $ (.6) Ark 28.0 % 279.8 28.0 % 247.9 Kudu 10.8 % 94.5 10.8 % 75.1 Other various 10.0 various 20.4 Total, excluding BAM 381.0 342.8 BAM 100.0 % (165.9) 100.0 % (154.7) Total noncontrolling interests $ 215.1 $ 188.1 (1) The noncontrolling percentage represents the basic ownership interests held by noncontrolling shareholders with the exception of HG Global, for which the noncontrolling percentage represents the preferred share ownership held by noncontrolling shareholders.

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2023
Segment Reporting [Abstract]
Segment Information	Segment Information As of September 30, 2023, White Mountains conducted its operations through three reportable segments: (1) HG Global/BAM, (2) Ark/WM Outrigger, and (3) Kudu, with our remaining operating businesses, holding companies and other assets included in Other Operations. White Mountains has made its segment determination based on consideration of the following criteria: (i) the nature of the business activities of each of the Company’s subsidiaries and affiliates; (ii) the manner in which the Company’s subsidiaries and affiliates are organized; (iii) the existence of primary managers responsible for specific subsidiaries and affiliates; and (iv) the organization of information provided to the chief operating decision makers and the Board of Directors. Significant intercompany transactions among White Mountains’s segments have been eliminated herein. During the fourth quarter of 2022, Ark sponsored the formation of Outrigger Re Ltd. to provide reinsurance protection on Ark’s Bermuda global property catastrophe excess of loss portfolio written in calendar year 2023. White Mountains consolidates its segregated account of Outrigger Re Ltd., WM Outrigger Re, in its financial statements. WM Outrigger Re’s quota share reinsurance agreement with GAIL eliminates in White Mountains’s consolidated financial statements. WM Outrigger Re exclusively provides reinsurance protection to Ark. As a result, WM Outrigger Re was aggregated with Ark within the Ark/WM Outrigger segment starting in 2023. See Note 2 — “Significant Transactions.” As a result of the NSM Transaction, the results of operations for NSM, previously reported as a segment, have been classified as discontinued operations in the statements of operations and comprehensive income through the closing of the transaction. See Note 19 — “Held for Sale and Discontinued Operations.” Prior period amounts have been reclassified to conform to the current period’s presentation. The following tables present the financial information for White Mountains’s segments for the three and nine months ended September 30, 2023 and 2022: HG Global/BAM Ark/WM Outrigger Other Operations Total Millions HG Global BAM (1) Ark WM Outrigger Re Kudu Three Months Ended September 30, 2023 Earned insurance premiums $ 6.6 $ 1.3 $ 438.3 $ 60.6 $ — $ — $ 506.8 Net investment income 4.3 3.8 13.9 3.0 15.1 8.0 48.1 Net investment income (expense) - BAM 6.6 (6.6) — — — — — Net realized and unrealized investment gains (losses) (13.6) (10.5) (6.6) — 11.2 8.0 (11.5) Net realized and unrealized investment gains — — — — — (46.8) (46.8) Commission revenues — — — — — 3.5 3.5 Other revenues — .7 3.6 — — 15.2 19.5 Total revenues 3.9 (11.3) 449.2 63.6 26.3 (12.1) 519.6 Loss and loss adjustment expenses — — 258.5 7.3 — — 265.8 Acquisition expenses 2.0 .1 71.0 19.3 — — 92.4 Cost of sales — — — — — 8.0 8.0 General and administrative expenses .5 17.0 35.3 .1 4.5 42.2 99.6 Change in fair value of contingent consideration — — 17.0 — — — 17.0 Interest expense 3.8 — 5.5 — 5.5 .9 15.7 Total expenses 6.3 17.1 387.3 26.7 10.0 51.1 498.5 Pre-tax income (loss) from continuing operations $ (2.4) $ (28.4) $ 61.9 $ 36.9 $ 16.3 $ (63.2) $ 21.1 (1) BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the BAM Surplus Notes and is not reduced by accruals of interest expense on the BAM Surplus Notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the NYDFS. HG Global/BAM Ark/WM Outrigger Other Operations Millions HG Global BAM (1) Ark Kudu Total Three Months Ended September 30, 2022 Earned insurance premiums $ 5.9 $ 1.2 $ 346.1 $ — $ — $ 353.2 Net investment income 2.8 2.9 4.9 14.8 8.5 33.9 Net investment income (expense) - BAM 2.9 (2.9) — — — — Net realized and unrealized investment gains (losses) (19.6) (19.2) (14.4) 41.1 (17.3) (29.4) Net realized and unrealized investment gains — — — — (18.6) (18.6) Commission revenues — — — — 3.2 3.2 Other revenues .1 1.2 6.6 — 33.0 40.9 Total revenues (7.9) (16.8) 343.2 55.9 8.8 383.2 Losses and loss adjustment expenses — — 213.7 — — 213.7 Acquisition expenses 1.6 .1 74.8 — — 76.5 Cost of sales — — — — 25.0 25.0 General and administrative expenses .6 15.2 24.2 4.5 41.3 85.8 Change in fair value of contingent consideration — — 2.7 — — 2.7 Interest expense 2.0 — 3.7 4.2 .6 10.5 Total expenses 4.2 15.3 319.1 8.7 66.9 414.2 Pre-tax income (loss) from continuing operations $ (12.1) $ (32.1) $ 24.1 $ 47.2 $ (58.1) $ (31.0) (1) BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the BAM Surplus Notes and is not reduced by accruals of interest expense on the BAM Surplus Notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the NYDFS. HG Global/BAM Ark/WM Outrigger Other Operations Millions HG Global BAM (1) Ark WM Outrigger Re Kudu Total Nine Months Ended September 30, 2023 Earned insurance premiums $ 19.4 $ 3.9 $ 971.9 $ 75.4 $ — $ — $ 1,070.6 Net investment income 12.4 10.5 33.5 7.7 44.0 22.0 130.1 Net investment income (expense) - 19.7 (19.7) — — — — — Net realized and unrealized investment (11.4) (5.6) 35.9 — 45.4 125.8 190.1 Net realized and unrealized investment gains — — — — — (38.9) (38.9) Commission revenues — — — — — 10.0 10.0 Other revenues — 2.0 (1.1) — — 67.1 68.0 Total revenues 40.1 (8.9) 1,040.2 83.1 89.4 186.0 1,429.9 Loss and loss adjustment expenses — — 573.2 7.9 — — 581.1 Acquisition expenses 5.6 .7 189.3 22.2 — — 217.8 Cost of sales — — — — — 33.5 33.5 General and administrative expenses 1.9 47.8 105.3 .2 12.3 130.7 298.2 Change in fair value of contingent consideration — — 16.8 — — — 16.8 Interest expense 10.8 — 15.7 — 15.5 2.9 44.9 Total expenses 18.3 48.5 900.3 30.3 27.8 167.1 1,192.3 Pre-tax income (loss) from continuing $ 21.8 $ (57.4) $ 139.9 $ 52.8 $ 61.6 $ 18.9 $ 237.6 (1) BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the BAM Surplus Notes and is not reduced by accruals of interest expense on the BAM Surplus Notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the NYDFS. HG Global/BAM Ark/WM Outrigger Other Operations Millions HG Global BAM (1) Ark Kudu Total Nine Months Ended September 30, 2022 Earned insurance premiums $ 21.5 $ 4.5 $ 757.8 $ — $ — $ 783.8 Net investment income 7.1 8.0 9.7 41.2 13.6 79.6 Net investment income (expense) - 8.8 (8.8) — — — — Net realized and unrealized investment (57.8) (56.2) (76.5) 45.8 2.8 (141.9) Net realized and unrealized investment gains — — — — (113.3) (113.3) Commission revenues — — — — 8.7 8.7 Other revenues .3 3.4 10.1 — 89.6 103.4 Total revenues (20.1) (49.1) 701.1 87.0 1.4 720.3 Losses and loss adjustment expenses — — 456.2 — — 456.2 Acquisition expenses 7.6 1.9 174.9 — — 184.4 Cost of sales — — — — 68.8 68.8 General and administrative expenses 2.1 47.4 74.9 10.4 122.1 256.9 Change in fair value of contingent consideration — — 4.9 — — 4.9 Interest expense 5.4 — 10.6 10.3 1.2 27.5 Total expenses 15.1 49.3 721.5 20.7 192.1 998.7 Pre-tax income (loss) from continuing operations $ (35.2) $ (98.4) $ (20.4) $ 66.3 $ (190.7) $ (278.4)

Variable Interest Entities

Variable Interest Entities	9 Months Ended
	Sep. 30, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Variable Interest Entities	Variable Interest Entities BAM BAM is owned by and operated for the benefit of its members, the municipalities that purchase BAM’s insurance for their debt issuances. However, the equity at risk funded by BAM’s members is not sufficient to fund its operations without the additional financial support provided by the BAM Surplus Notes and, accordingly, White Mountains has determined that BAM is a VIE. Pursuant to the FLRT, BAM’s underwriting guidelines may only be amended with the consent of HG Re. In addition, HG Holdings Ltd, a subsidiary of HG Global, has the right to designate two directors for election to BAM’s Board of Directors. As a result, we have determined that White Mountains is the primary beneficiary and is required to consolidate BAM’s results in its financial statements. Since BAM is owned by its members, its equity and results of operations are included in noncontrolling interests. HG Re’s obligations under the FLRT are subject to an aggregate limit equal to the assets in the Collateral Trusts at any point in time. WM Outrigger Re White Mountains has determined that Outrigger Re Ltd. and WM Outrigger Re are VIEs. White Mountains is not the primary beneficiary of Outrigger Re Ltd. or the other segregated accounts. White Mountains is the primary beneficiary of WM Outrigger Re, as it has both the power to direct the activities that most significantly impact WM Outrigger Re’s economic performance and the obligation to absorb losses, or the right to receive returns, that could potentially be significant to WM Outrigger Re. As a result, White Mountains consolidates WM Outrigger Re’s results in its financial statements. The assets of WM Outrigger Re can only be used to settle the liabilities of WM Outrigger Re, and there is no recourse to the Company for any creditors of WM Outrigger Re. PassportCard/DavidShield As of September 30, 2023, White Mountains’s ownership interest in PassportCard/DavidShield was 53.8%. White Mountains has determined that both PassportCard and DavidShield are VIEs but that White Mountains is not the primary beneficiary and therefore does not consolidate either PassportCard or DavidShield. The governance structures for both PassportCard and DavidShield were designed to give White Mountains and its co-investor equal power to make the decisions that most significantly impact operations. White Mountains does not have the unilateral power to direct the operations of PassportCard or DavidShield and does not hold a controlling financial interest. White Mountains’s ownership interest gives White Mountains the ability to exert significant influence over the significant financial and operating activities of PassportCard/DavidShield. Accordingly, White Mountains’s investment in PassportCard/DavidShield meets the criteria to be accounted for under the equity method. White Mountains has taken the fair value option for its investment in PassportCard/DavidShield. Changes in the fair value of PassportCard/DavidShield are recorded in net realized and unrealized investment gains (losses). As of September 30, 2023, White Mountains’s maximum exposure to loss on its equity investment in PassportCard/DavidShield and the non-interest-bearing loan to its partner is the total carrying value of $159.4 million. Elementum As of September 30, 2023, White Mountains’s ownership interest in Elementum was 26.6%. White Mountains has determined that Elementum is a VIE but that White Mountains is not the primary beneficiary and therefore does not consolidate Elementum. White Mountains’s ownership interest gives White Mountains the ability to exert significant influence over the significant financial and operating activities of Elementum. Accordingly, Elementum meets the criteria to be accounted for under the equity method. White Mountains has taken the fair value option for its investment in Elementum. Changes in the fair value of Elementum are recorded in net realized and unrealized investment gains (losses). As of September 30, 2023, White Mountains’s maximum exposure to loss on its limited partnership interest in Elementum is the carrying value of $35.0 million. Limited Partnerships White Mountains’s investments in limited partnerships are generally considered VIEs because the limited partnership interests do not have substantive kick-out rights or participating rights. White Mountains does not have the unilateral power to direct the operations of these limited partnerships, and therefore White Mountains is not the primary beneficiary and does not consolidate the limited partnerships. White Mountains has taken the fair value option for its investments in limited partnerships, which are generally measured at NAV as a practical expedient. As of September 30, 2023, White Mountains’s maximum exposure to loss on its investments in limited partnerships is the carrying value of $239.2 million.

Equity Method Eligible Investme

Equity Method Eligible Investments	9 Months Ended
	Sep. 30, 2023
Equity Method Investments and Joint Ventures [Abstract]
Equity Method Eligible Investments	Equity Method Eligible Investments White Mountains’s equity method eligible investments include Kudu’s Participation Contracts, White Mountains’s investment in MediaAlpha, PassportCard/DavidShield, Elementum Holdings, L.P. and certain other unconsolidated entities, private equity funds and hedge funds in which White Mountains has the ability to exert significant influence over the investee’s operating and financial policies. The following table presents the ownership interests and carrying values of White Mountains’s equity method eligible investments as of September 30, 2023 and December 31, 2022: September 30, 2023 December 31, 2022 $ in Millions Ownership Interest (1) Carrying Value Ownership Interest (1) Carrying Value Kudu’s Participation Contracts 4.1% - 30.0% $ 775.3 4.1% - 30.0% $ 695.9 Investment in MediaAlpha 35.3% 188.8 27.1% 168.6 PassportCard/DavidShield 53.8% 150.0 53.8% 135.0 Elementum Holdings, L.P. 26.6% 35.0 29.7% 30.0 Other equity method eligible investments, at fair value Under 50.0% 279.0 Under 50.0% 84.4 Other equity method eligible investments, at fair value 50.0% and over 25.2 50.0% and over — (1) Ownership interest generally references basic ownership interest with the exception of Kudu’s Participation Contracts, which are noncontrolling equity interests in the form of revenue and earnings participation contracts. For the three and nine months ended September 30, 2023, White Mountains received dividend and income distributions from equity method eligible investments of $13.2 million and $42.1 million, which were recorded within net investment income in the consolidated statements of operations. For the three and nine months ended September 30, 2022, White Mountains received dividend and income distributions from equity method eligible investments of $16.1 million and $42.9 million.

Fair Value of Financial Instrum

Fair Value of Financial Instruments	9 Months Ended
	Sep. 30, 2023
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 debt obligations, which are recorded as debt at face value less unamortized original issue discount. See Note 7 — “Debt.” The following table presents the fair value and carrying value of these financial instruments as of September 30, 2023 and December 31, 2022: September 30, 2023 December 31, 2022 Millions Fair Value Carrying Value Fair Value Carrying Value HG Global Senior Notes $ 157.6 $ 146.8 $ 155.7 $ 146.5 Ark 2007 Subordinated Notes $ 29.1 $ 30.0 $ 28.4 $ 30.0 Ark 2021 Subordinated Notes $ 164.5 $ 154.6 $ 163.1 $ 153.7 Kudu Credit Facility $ 223.0 $ 203.7 $ 223.9 $ 208.3 Other Operations debt $ 31.1 $ 29.6 $ 38.2 $ 36.7 The fair value estimates for the HG Global Senior Notes, Ark 2007 Subordinated Notes, Ark 2021 Subordinated Notes, Kudu Credit Facility and Other Operations debt have been determined based on a discounted cash flow approach and are considered to be Level 3 measurements. For the fair value level measurements associated with White Mountains’s investment securities see Note 3 — “Investment Securities.” For the fair value level measurements associated with White Mountains’s derivative instruments see Note 9 — “Derivatives.”

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2023
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Legal Contingencies White Mountains, and the insurance industry in general, is routinely subject to claims related litigation and arbitration in the normal course of business, as well as litigation and arbitration that do not arise from, nor are directly related to, claims activity. White Mountains’s estimates of the costs of settling matters routinely encountered in claims activity are reflected in the reserves for unpaid loss and LAE. See Note 5 — “Losses and Loss Adjustment Expense Reserves.”

Held for Sale and Discontinued

Held for Sale and Discontinued Operations	9 Months Ended
	Sep. 30, 2023
Discontinued Operations and Disposal Groups [Abstract]
Held for Sale and Discontinued Operations	Held for Sale and Discontinued Operations NSM On August 1, 2022, White Mountains closed the NSM Transaction. See Note 2 — “Significant Transactions.” As a result of the NSM Transaction, the assets and liabilities of NSM Group have been presented in the balance sheet as held for sale for periods prior to the closing of the transaction, and the results of operations for NSM Group have been classified as discontinued operations in the statements of operations and comprehensive income through the closing of the transaction. Prior period amounts have been reclassified to conform to the current period’s presentation. Net Income (Loss) from Discontinued Operations The following table summarizes the results of operations, including related income taxes associated with the businesses classified as discontinued operations for the three and nine months ended September 30, 2022: Millions Three Months Ended September 30, 2022 (1) Nine Months Ended September 30, 2022 (1) Revenues Commission revenues $ 26.6 $ 176.9 Other revenues 7.0 48.1 Total revenues 33.6 225.0 Expenses General and administrative expenses 15.7 126.8 Broker commission expenses 8.2 52.9 Change in fair value of contingent consideration — .1 Amortization of other intangible assets — 9.1 Interest expense 1.6 12.1 Total expenses 25.5 201.0 Pre-tax income (loss) from discontinued operations 8.1 24.0 Income tax (expense) benefit (1.8) (7.6) Net income (loss) from discontinued operations, net of tax - NSM Group 6.3 16.4 Net gain (loss) from sale of discontinued operations, net of tax - NSM Group 886.8 886.8 Total income (loss) from discontinued operations, net of tax 893.1 903.2 Net (income) loss from discontinued operations attributable to noncontrolling (.2) (.8) Total income (loss) from discontinued operations attributable to White 892.9 902.4 Other comprehensive income (loss) from discontinued operations, net of tax - .7 (5.2) Net gain (loss) from foreign currency translation from sale of discontinued 2.9 2.9 Comprehensive income (loss) from discontinued operations 896.5 900.1 Other comprehensive (income) loss from discontinued operations attributable to (.1) .2 Comprehensive income (loss) from discontinued operations attributable to $ 896.4 $ 900.3 (1) As a result of the NSM Transaction, the results of operations for NSM Group are presented for the periods from July 1, 2022 to August 1, 2022 and January 1, 2022 to August 1, 2022. Net Change in Cash from Discontinued Operations The following table summarizes the net change in cash associated with the businesses classified as discontinued operations for the six months ended September 30, 2022: Millions Nine Months Ended September 30, 2022 Net cash provided from (used for) operations $ 38.7 Net cash provided from (used for) investing activities 7.1 Net cash used from (used for) financing activities (17.5) Effect of exchange rate changes on cash 4.0 Net change in cash during the period 32.3 Cash balances at beginning of period (includes restricted cash of $89.2) 111.6 Cash sold as part of the sale of NSM Group (143.9) Cash balances at end of period $ — Supplemental cash flows information: Interest paid $ (12.0) Net income tax payments $ — 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, 2022: Three Months Ended September 30, 2022 Nine Months Ended September 30, 2022 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 888.2 $ 752.4 Less: net income (loss) from continuing operations (23.6) (252.3) Less: net (income) loss from continuing operations attributable to noncontrolling 18.8 102.2 Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders (1) 893.0 902.5 Allocation of earnings to participating restricted common shares (2) (11.8) (10.8) Basic and diluted earnings per share numerators (3) $ 881.2 $ 891.7 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 2,893.8 2,959.3 Average unvested restricted common shares (4) (38.3) (35.5) Basic earnings per share denominator 2,855.5 2,923.8 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 2,893.8 2,959.3 Average unvested restricted common shares (4) (38.3) (35.5) Diluted earnings per share denominator 2,855.5 2,923.8 Basic and diluted earnings (losses) per share (in dollars) - discontinued operations $ 308.59 $ 304.97 (1) Includes net income (loss) from discontinued operations, net of tax - NSM Group, net gain (loss) from sale of discontinued operations, net of tax - NSM Group and net (income) loss from discontinued operations attributable to non-controlling interests. (2) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (3) 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, 2022. (4) Restricted shares outstanding vest upon a stated date. See Note 12 — “Employee Share-Based Incentive Compensation Plans.”

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2023
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events Agreement to Acquire Bamboo On October 19, 2023, White Mountains entered into an agreement and plan of merger (the “Bamboo Merger Agreement”) with Bamboo IDE8 Insurance Services, LLC (“Bamboo”) and John Chu, as the unitholders’ representative. Bamboo is a capital-light, tech- and data-enabled insurance distribution platform providing insurance offerings to the residential property market in California. Bamboo operates primarily through its full-service MGA business, where the company manages all aspects of the placement process on behalf of its fronting and reinsurance partners, including product development, marketing, underwriting, policy issuance and claims oversight, and earns commissions based on the volume and profitability of the insurance that it places. Immediately following the merger, White Mountains will make a primary investment in Bamboo (together with the merger and the rollover transactions described below, the “Bamboo Transaction”). As a result of the Bamboo Transaction, White Mountains expects to acquire approximately 70% of the issued and outstanding equity interests of Bamboo for approximately $285 million in cash. The precise size of White Mountains’s ownership interest and equity investment will vary based on rollover elections by certain unitholders prior to closing. The Bamboo Transaction is expected to close in the first quarter of 2024. Completion of the Bamboo Transaction is subject to receipt of certain regulatory approvals and other customary closing conditions. The Bamboo Transaction is not subject to a financing condition.

Basis of Presentation and Sig_2

Basis of Presentation and Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation White Mountains Insurance Group, Ltd. (the “Company” or the “Registrant”) is an exempted Bermuda limited liability company whose principal businesses are conducted through its 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 23 South Main Street, Suite 3B, 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 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 the Company, its subsidiaries (collectively with the Company, “White Mountains”) and other entities required to be consolidated under GAAP. Intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. 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 As of September 30, 2023, White Mountains conducted its operations through three reportable segments: (1) HG Global/BAM, (2) Ark/WM Outrigger, and (3) Kudu, with our remaining operating businesses, holding companies and other assets included in 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 Company’s chief operating decision makers and its Board of Directors. See Note 14 — “Segment Information.” The HG Global/BAM segment consists of HG Global Ltd. and its wholly-owned subsidiaries (collectively, “HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”) (collectively with HG Global, “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 purpose projects, such as schools, utilities and transportation facilities. BAM is owned by and operated for the benefit of its members, the municipalities that purchase BAM’s insurance for their debt issuances. HG Global was established to fund the startup of BAM and, through its reinsurance subsidiary, HG Re Ltd. (“HG Re”), to provide up to 15%-of-par, first loss reinsurance protection for policies underwritten by BAM. HG Global, together with its subsidiaries, funded the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM (the “BAM Surplus Notes”). As of September 30, 2023 and December 31, 2022, 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 variable interest entity (“VIE”) for which White Mountains is the primary beneficiary. BAM’s results are all attributed to noncontrolling interests. The Ark/WM Outrigger segment consists of Ark Insurance Holdings Limited and its subsidiaries (collectively, “Ark”) and Outrigger Re Ltd. Segregated Account 2023-1 (“WM Outrigger Re”) (collectively with Ark, “Ark/WM Outrigger”). Ark is a specialty property and casualty insurance and reinsurance company that offers a wide range of niche insurance and reinsurance products, including property, specialty, marine & energy, casualty and accident & health. Ark underwrites select coverages through Lloyd’s Syndicates 4020 and 3902 (the “Syndicates”) and its wholly-owned subsidiary Group Ark Insurance Limited (“GAIL”). White Mountains acquired a controlling ownership interest in Ark on January 1, 2021 (the “Ark Transaction”). As of September 30, 2023 and December 31, 2022, White Mountains owned 72.0% of Ark on a basic shares outstanding basis (63.0% after taking account of management’s equity incentives). The remaining shares are owned by current and former employees. In the future, management rollover shareholders could earn additional shares in Ark if and to the extent that White Mountains achieves certain multiple of invested capital return thresholds. If fully earned, these additional shares would represent 12.5% of the shares outstanding at closing. The liability related to these additional shares is recorded as contingent consideration. For the years of account prior to the Ark Transaction, a significant proportion of the Syndicates’ underwriting capital was provided by third-party insurance and reinsurance groups (“TPC Providers”) using whole account reinsurance contracts with Ark’s corporate member. For the years of account subsequent to the Ark Transaction, Ark is no longer using TPC Providers to provide underwriting capital for the Syndicates. Captions within results of operations and other comprehensive income for the three and nine months ended September 30, 2022 are shown net of amounts relating to the TPC Providers’ share of the Syndicates’ results, including investment results. During the fourth quarter of 2022, Ark sponsored the formation of Outrigger Re Ltd., a Bermuda company registered as a special purpose insurer and segregated accounts company, to provide collateralized reinsurance protection on Ark’s Bermuda global property catastrophe excess of loss portfolio written in calendar year 2023. Outrigger Re Ltd. issued non-voting redeemable preference shares on behalf of four segregated accounts to White Mountains and unrelated third-party investors. White Mountains consolidates the results of its segregated account, WM Outrigger Re, in its financial statements. See Note 2 — “Significant Transactions.” As of September 30, 2023 and December 31, 2022, White Mountains owned 100.0% of WM Outrigger Re’s preferred equity. The Kudu segment consists of Kudu Investment Management, LLC and its subsidiaries (collectively “Kudu”). Kudu provides capital solutions for boutique asset and wealth managers for a variety of 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 generally are structured as minority preferred equity stakes with distribution rights, typically tied to gross revenues and designed to generate immediate cash yields. As of September 30, 2023 and December 31, 2022, White Mountains owned 89.2% and 89.3% of Kudu’s basic units outstanding (76.1% and 76.1% on a fully diluted, fully converted basis). White Mountains’s Other Operations consists of the Company and its wholly-owned subsidiary, White Mountains Capital, LLC (“WM Capital”), its other intermediate holding companies, its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), investment assets managed by WM Advisors, its interests in MediaAlpha, Inc. (“MediaAlpha”), PassportCard Limited (“PassportCard”) and DavidShield Life Insurance Agency (2000) Ltd. (“DavidShield”) (collectively, “PassportCard/ DavidShield”), Elementum Holdings LP (“Elementum”), certain other consolidated and unconsolidated entities (“Other Operating Businesses”) and certain other assets.
Discontinued Operations	Discontinued Operations On August 1, 2022, White Mountains Holdings (Luxembourg) S.à r.l. (“WTM Holdings Seller”), an indirect wholly owned subsidiary of White Mountains, completed the sale of White Mountains Catskill Holdings, Inc. and NSM Insurance HoldCo, LLC (“NSM” and, collectively with White Mountains Catskill Holdings, Inc., the “NSM Group”) to Riser Merger Sub, Inc., an affiliate of The Carlyle Group Inc. (the “NSM Transaction”), pursuant to the terms of the securities purchase agreement dated as of May 9, 2022. See Note 2 — “Significant Transactions. ” NSM is a full-service managing general agent (“MGA”) and program administrator with delegated binding authorities for specialty property and casualty insurance.

Investment Securities (Tables)

Investment Securities (Tables)	9 Months Ended
	Sep. 30, 2023
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, 2023 and 2022: Three Months Ended September 30, Nine Months Ended September 30, Millions 2023 2022 2023 2022 Fixed maturity investments $ 16.6 $ 11.1 $ 46.6 $ 28.6 Short-term investments 13.8 6.7 34.8 7.2 Common equity securities 1.0 .1 3.6 .2 Other long-term investments 17.4 17.1 46.8 45.9 Amount attributable to TPC Providers — (.3) — (.8) Total investment income 48.8 34.7 131.8 81.1 Third-party investment expenses (.7) (.8) (1.7) (1.5) Net investment income, pre-tax $ 48.1 $ 33.9 $ 130.1 $ 79.6
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, 2023 and 2022: Three Months Ended September 30, Nine Months Ended September 30, Millions 2023 2022 2023 2022 Realized investment gains (losses) Fixed maturity investments $ (.3) $ (4.9) $ (1.7) $ (10.8) Short-term investments (.2) (.1) (.3) (.4) Common equity securities 1.2 — 13.8 — Other long-term investments .6 3.4 48.9 24.1 Net realized investment gains (losses) 1.3 (1.6) 60.7 12.9 Unrealized investment gains (losses) Fixed maturity investments (27.9) (58.8) (9.7) (192.9) Short-term investments (.6) (2.1) .7 (2.6) Common equity securities (18.5) 4.3 29.8 (6.3) Investment in MediaAlpha (46.8) (18.6) (38.9) (113.3) Other long-term investments 34.2 27.5 108.6 40.8 Net unrealized investment gains (losses) (59.6) (47.7) 90.5 (274.3) Net realized and unrealized investment gains (losses), before amount attributable to TPC providers (1) (58.3) (49.3) 151.2 (261.4) Amount attributable to TPC Providers — 1.3 — 6.2 Net realized and unrealized investment gains (losses) $ (58.3) $ (48.0) $ 151.2 $ (255.2) Fixed maturity and short-term investments Net realized and unrealized investment gains (losses) $ (29.0) $ (65.9) $ (11.0) $ (206.7) Less: net realized and unrealized gains (losses) on investment 2.5 .1 4.7 (3.0) Net unrealized investment gains (losses) recognized during the period on $ (31.5) $ (66.0) $ (15.7) $ (203.7) Common equity securities and investment in MediaAlpha Net realized and unrealized investment gains (losses) on common $ (17.3) $ 4.3 $ 43.6 $ (6.3) Net realized and unrealized investment gains (losses) from (46.8) (18.6) (38.9) (113.3) Total net realized and unrealized investment gains (losses) (64.1) (14.3) 4.7 (119.6) Less: net realized and unrealized gains (losses) on investment (.1) — 3.7 — Net unrealized investment gains (losses) recognized during the period on $ (64.0) $ (14.3) $ 1.0 $ (119.6) (1) For the three months ended September 30, 2023 and 2022, includes $(12.8) and $(22.6) of net realized and unrealized investment gains (losses) related to foreign currency exchange. For the nine months ending September 30, 2023 and 2022, includes $(1.9) and $(44.4) of net realized and unrealized investment gains (losses) related to foreign currency exchange. The following table presents total net unrealized gains (losses) attributable to Level 3 investments for the three and nine months ended September 30, 2023 and 2022 for investments still held at the end of the period: Three Months Ended September 30, Nine Months Ended September 30, Millions 2023 2022 2023 2022 Total net unrealized investment gains on other long-term $ 24.6 $ 40.8 $ 76.0 $ 57.7
Investment holdings, equity securities, convertible fixed maturities and other long-term investments	The following tables present the cost or amortized cost, gross unrealized investment gains (losses), net foreign currency gains (losses) and carrying values of White Mountains’s fixed maturity investments as of September 30, 2023 and December 31, 2022: September 30, 2023 Millions Cost or Gross Gross Net Foreign Carrying U.S. Government and agency obligations $ 270.1 $ — $ (9.0) $ — $ 261.1 Debt securities issued by corporations 1,038.8 .1 (74.6) (2.7) 961.6 Municipal obligations 264.6 — (26.4) — 238.2 Mortgage and asset-backed securities 372.6 — (47.2) — 325.4 Collateralized loan obligations 196.5 .3 (3.1) (2.1) 191.6 Total fixed maturity investments $ 2,142.6 $ .4 $ (160.3) $ (4.8) $ 1,977.9 December 31, 2022 Millions Cost or Gross Gross Net Foreign Carrying U.S. Government and agency obligations $ 216.6 $ — $ (10.2) $ — $ 206.4 Debt securities issued by corporations 1,098.3 .6 (78.3) (1.8) 1,018.8 Municipal obligations 281.6 .4 (23.4) — 258.6 Mortgage and asset-backed securities 288.7 — (34.5) — 254.2 Collateralized loan obligations 190.8 .1 (6.0) (2.0) 182.9 Total fixed maturity investments $ 2,076.0 $ 1.1 $ (152.4) $ (3.8) $ 1,920.9 The following tables present the cost or amortized cost, gross unrealized investment gains (losses), net foreign currency gains (losses), and carrying values of common equity securities, White Mountains’s investment in MediaAlpha and other long-term investments as of September 30, 2023 and December 31, 2022: September 30, 2023 Millions Cost or Gross Gross Net Foreign Carrying Common equity securities $ 599.5 $ 51.6 $ (4.8) $ (9.3) $ 637.0 Investment in MediaAlpha $ 59.2 $ 139.9 $ (10.3) $ — $ 188.8 Other long-term investments $ 1,639.9 $ 390.9 $ (116.6) $ (18.4) $ 1,895.8 December 31, 2022 Millions Cost or Gross Gross Net Foreign Carrying Common equity securities $ 660.6 $ 26.7 $ (8.4) $ (10.5) $ 668.4 Investment in MediaAlpha $ — $ 168.6 $ — $ — $ 168.6 Other long-term investments $ 1,340.8 $ 271.1 $ (107.1) $ (16.8) $ 1,488.0
Schedule of fixed maturity investment holdings	The following table presents the cost or amortized cost and carrying values of White Mountains’s fixed maturity investments by contractual maturity as of September 30, 2023. Actual maturities could differ from contractual maturities because borrowers may have the right to call or prepay certain obligations with or without call or prepayment penalties. September 30, 2023 Millions Cost or Amortized Cost Carrying Value Due in one year or less $ 307.6 $ 301.2 Due after one year through five years 870.9 813.4 Due after five years through ten years 302.7 267.6 Due after ten years 92.3 78.7 Mortgage and asset-backed securities and collateralized loan obligations 569.1 517.0 Total fixed maturity investments $ 2,142.6 $ 1,977.9
Fair value measurements by level, investment securities	The following tables present White Mountains’s fair value measurements for investments as of September 30, 2023 and December 31, 2022 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, 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 by industry sector because investors often reference commonly used benchmarks and their subsectors to monitor risk and performance. Accordingly, White Mountains has further disaggregated this asset class 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. September 30, 2023 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 261.1 $ 261.1 $ — $ — Debt securities issued by corporations: Financials 290.1 — 290.1 — Consumer 188.3 — 188.3 — Healthcare 109.1 — 109.1 — Industrial 107.8 — 107.8 — Technology 107.5 — 107.5 — Utilities 66.6 — 66.6 — Communications 42.4 — 42.4 — Energy 29.1 — 29.1 — Materials 20.7 — 20.7 — Total debt securities issued by corporations 961.6 — 961.6 — Municipal obligations 238.2 — 238.2 — Mortgage and asset-backed securities 325.4 — 325.4 — Collateralized loan obligations 191.6 — 191.6 — Total fixed maturity investments 1,977.9 261.1 1,716.8 — Short-term investments 1,162.9 1,149.7 13.2 — Common equity securities: Exchange-traded funds 252.1 252.1 — — Other (1) 384.9 — 384.9 — Total common equity securities 637.0 252.1 384.9 — Investment in MediaAlpha 188.8 188.8 — — Other long-term investments 1,090.7 — 15.4 1,075.3 Other long-term investments — NAV (2) 805.1 — — — Total other long-term investments 1,895.8 — 15.4 1,075.3 Total investments $ 5,862.4 $ 1,851.7 $ 2,130.3 $ 1,075.3 (1) Consists of investments in listed funds that predominantly invest in international equities. (2) Consists of private equity funds and hedge funds, a bank loan fund, Lloyd’s trust deposits and ILS funds for which fair value is measured using NAV as a practical expedient. Investments for which fair value is measured at NAV are not classified within the fair value hierarchy. December 31, 2022 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 206.4 $ 206.4 $ — $ — Debt securities issued by corporations: Financials 291.2 — 291.2 — Consumer 191.9 — 191.9 — Healthcare 121.3 — 121.3 — Industrial 115.4 — 115.4 — Technology 123.7 — 123.7 — Utilities 73.8 — 73.8 — Communications 47.9 — 47.9 — Energy 33.9 — 33.9 — Materials 19.7 — 19.7 — Total debt securities issued by corporations 1,018.8 — 1,018.8 — Municipal obligations 258.6 — 258.6 — Mortgage and asset-backed securities 254.2 — 254.2 — Collateralized loan obligations 182.9 — 182.9 — Total fixed maturity investments 1,920.9 206.4 1,714.5 — Short-term investments 924.1 924.1 — — Common equity securities: Exchange-traded funds 333.8 333.8 — — Other (1) 334.6 — 334.6 — Total common equity securities 668.4 333.8 334.6 — Investment in MediaAlpha 168.6 168.6 — — Other long-term investments 926.4 — 14.8 911.6 Other long-term investments — NAV (2) 561.6 — — — Total other long-term investments 1,488.0 — 14.8 911.6 Total investments $ 5,170.0 $ 1,632.9 $ 2,063.9 $ 911.6 (1) Consists of investments in listed funds that predominantly invest in international equities. (2) Consists of private equity funds and hedge funds, a bank loan fund, Lloyd’s trust deposits and ILS funds for which fair value is measured using NAV as a practical expedient. Investments for which fair value is measured at NAV are not classified within the fair value hierarchy. The following table presents the fair value of White Mountains’s mortgage and asset-backed securities and collateralized loan obligations as of September 30, 2023 and December 31, 2022: September 30, 2023 December 31, 2022 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: FNMA $ 162.5 $ 162.5 $ — $ 124.5 $ 124.5 $ — FHLMC 114.3 114.3 — 78.8 78.8 — GNMA 29.2 29.2 — 28.3 28.3 — Total agency (1) 306.0 306.0 — 231.6 231.6 — Non-agency: Residential .2 .2 — .3 .3 — Total non-agency .2 .2 — .3 .3 — Total mortgage-backed securities 306.2 306.2 — 231.9 231.9 — Other asset-backed securities: Credit card receivables 3.2 3.2 — 11.9 11.9 — Vehicle receivables 13.5 13.5 — 10.4 10.4 — Other 2.5 2.5 — — — — Total other asset-backed securities 19.2 19.2 — 22.3 22.3 — Total mortgage and asset-backed securities 325.4 325.4 — 254.2 254.2 — Collateralized loan obligations 191.6 191.6 — 182.9 182.9 — Total mortgage and asset-backed securities $ 517.0 $ 517.0 $ — $ 437.1 $ 437.1 $ — (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).
Debt securities issued by corporations, credit ratings	The following table presents the credit ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of September 30, 2023 and December 31, 2022: Fair Value at Millions September 30, 2023 December 31, 2022 AAA $ 12.6 $ 11.3 AA 84.3 96.0 A 509.6 567.9 BBB 349.1 337.7 Other 6.0 5.9 Debt securities issued by corporations (1) $ 961.6 $ 1,018.8 (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.
Schedule of carrying values for other long-term investments	The following table presents the carrying values of White Mountains’s other long-term investments by reportable segment as of September 30, 2023 and December 31, 2022: Fair Value at September 30, 2023 Millions Ark/ WM Outrigger Kudu Other Total Kudu’s Participation Contracts $ — $ 775.3 $ — $ 775.3 PassportCard/DavidShield — — 150.0 150.0 Elementum Holdings, L.P. — — 35.0 35.0 Mandatorily redeemable preferred — — 58.5 58.5 Other unconsolidated entities (1) — — 41.8 41.8 Total unconsolidated entities — 775.3 285.3 1,060.6 Private equity funds and hedge funds 55.4 — 243.9 299.3 Bank loan fund 189.3 — — 189.3 ILS funds — — 159.5 159.5 Lloyd’s trust deposits 150.3 — — 150.3 Private debt instruments — 5.4 9.4 14.8 Other 21.6 — .4 22.0 Total other long-term investments $ 416.6 $ 780.7 $ 698.5 $ 1,895.8 (1) Includes White Mountains’s noncontrolling equity interests in certain preferred securities, private common equity securities, limited liability company units and Simple Agreement for Future Equity (“SAFE”) investments. Fair Value at December 31, 2022 Millions Ark/ WM Outrigger Kudu Other Total Kudu’s Participation Contracts $ — $ 695.9 $ — $ 695.9 PassportCard/DavidShield — — 135.0 135.0 Elementum Holdings, L.P. — — 30.0 30.0 Other unconsolidated entities (1) — — 37.2 37.2 Total unconsolidated entities — 695.9 202.2 898.1 Private equity funds and hedge funds 40.4 — 157.4 197.8 Bank loan fund 174.8 — — 174.8 ILS funds — — 49.3 49.3 Lloyd’s trust deposits 137.4 — — 137.4 Private debt instruments — — 9.6 9.6 Other 21.0 — — 21.0 Total other long-term investments $ 373.6 $ 695.9 $ 418.5 $ 1,488.0 (1) Includes White Mountains’s noncontrolling equity interests in certain preferred securities, private common equity securities, limited liability company units and Simple Agreement for Future Equity (“SAFE”) investments.
Other long-term investments	The following table presents the fair value of investments and unfunded commitments in private equity funds and hedge funds by investment objective and sector as of September 30, 2023 and December 31, 2022: September 30, 2023 December 31, 2022 Millions Fair Value Unfunded Fair Value Unfunded Private equity funds Aerospace/Defense/Government $ 146.4 $ 25.5 $ 59.4 $ 37.5 Financial services 88.6 41.8 77.1 54.3 Real estate 4.2 2.5 4.1 2.5 Total private equity funds 239.2 69.8 140.6 94.3 Hedge funds Long/short equity financials and business services 51.1 — 49.0 — European small/mid cap 9.0 — 8.2 — Total hedge funds 60.1 — 57.2 — Total private equity funds and hedge funds included in other long-term investments $ 299.3 $ 69.8 $ 197.8 $ 94.3
Fair value of private equity funds subject to lock-up periods	The following table presents investments in private equity funds that were subject to lock-up periods as of September 30, 2023: Millions 1 – 3 years 3 – 5 years 5 – 10 years >10 years Total Private equity funds — expected lock-up period remaining $4.4 $72.1 $148.8 $13.9 $239.2
Rollforward of fair value investments by level	The following table presents the changes in White Mountains’s fair value measurements for Level 3 investments for the nine months ended September 30, 2023 and 2022: Level 3 Investments Millions Other Long-term Other Long-term Balance at December 31, 2022 $ 911.6 Balance at December 31, 2021 $ 890.6 Net realized and unrealized gains 74.0 Net realized and unrealized gains 57.3 Purchases and contributions 200.1 Purchases and contributions 127.9 Sales and distributions (110.4) Sales and distributions (31.7) Transfers in — Transfers in — Transfers out — Transfers out — Balance at September 30, 2023 $ 1,075.3 Balance at September 30, 2022 $ 1,044.1
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 White Mountains’s other long-term investments, classified within Level 3 as of September 30, 2023 and December 31, 2022. The tables below exclude $41.7 million and $41.1 million of Level 3 other long-term investments generally valued based on recent or expected transaction prices. The fair value of investments in private equity funds and hedge funds, bank loan funds, Lloyd’s trust deposits and ILS funds are generally estimated using the NAV of the funds. $ in Millions September 30, 2023 Description Valuation Technique(s) (1) Fair Value (2) Unobservable Inputs Discount Rate (5) Terminal Cash Flow Exit Multiple (x) or Terminal Revenue Growth Rate (%) (5) Kudu’s Participation Contracts (3)(4) Discounted cash flow $775.3 19% - 26% 7x - 22x PassportCard/DavidShield Discounted cash flow $150.0 25% 4% Elementum Holdings, L.P. Discounted cash flow $35.0 25% 4% Mandatorily redeemable preferred Discounted cash flow $58.5 15% N/A Private debt instruments Discounted cash flow $14.8 12% - 13% N/A (1) Key inputs to the discounted cash flow analysis generally include projections of future revenue and earnings, discount rates and terminal exit multiples or growth rates. (2) Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs. (3) Since Kudu’s Participation Contracts are not subject to corporate taxes within Kudu Investment Management, LLC, pre-tax discount rates are applied to pre-tax cash flows in determining fair values. The weighted average discount rate and weighted average terminal cash flow exit multiple applied to Kudu’s Participation Contracts was 21% and 13x. (4) In the first nine months of 2023, Kudu deployed a total of $108.3 into new and existing Participation Contracts. (5) Increases (decreases) to the discount rates in isolation would result in lower (higher) fair value measurements, while increases (decreases) to the terminal cash flow exit multiples or terminal revenue growth rates in isolation would result in higher (lower) fair value measurements. $ in Millions December 31, 2022 Description Valuation Technique(s) (1) Fair Value (2) Unobservable Inputs Discount Rate (6) Terminal Cash Flow Exit Multiple (x) or Terminal Revenue Growth Rate (%) (6) Kudu’s Participation Contracts (3)(4)(5) Discounted cash flow $695.9 18% - 25% 7x - 16x PassportCard/DavidShield Discounted cash flow $135.0 24% 4% Elementum Holdings, L.P. Discounted cash flow $30.0 21% 4% Private debt instruments Discounted cash flow $9.6 11% N/A (1) Key inputs to the discounted cash flow analysis generally include projections of future revenue and earnings, discount rates and terminal exit multiples or growth rates. (2) Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs. (3) Since Kudu’s Participation Contracts are not subject to corporate taxes within Kudu Investment Management, LLC, pre-tax discount rates are applied to pre-tax cash flows in determining fair values. The weighted average discount rate and weighted average terminal cash flow exit multiple applied to Kudu’s Participation Contracts was 21% and 12x. (4) In 2022, Kudu deployed a total of $99.8 into new and existing Participation Contracts. (5) As of December 31, 2022, two of Kudu’s Participation Contracts with a total fair value of $189.0 were valued using a probability weighted expected return method, which takes into account factors such as a discounted cash flow analysis, the expected value to be received in a pending sale transaction and the likelihood that a sales transaction will take place. (6) Increases (decreases) to the discount rates in isolation would result in lower (higher) fair value measurements, while increases (decreases) to the terminal cash flow exit multiples or terminal revenue growth rates in isolation would result in higher (lower) fair value measurements.

Goodwill and Other Intangible_2

Goodwill and Other Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of intangible assets and goodwill	The following table presents the economic lives, acquisition date fair values, accumulated amortization and net carrying values for other intangible assets and goodwill as of September 30, 2023 and December 31, 2022: $ in Millions Weighted Average Economic September 30, 2023 December 31, 2022 Acquisition Date Fair Value Accumulated Amortization Net Carrying Value Acquisition Date Fair Value Accumulated Amortization Net Carrying Value Goodwill: Ark N/A $ 116.8 $ — $ 116.8 $ 116.8 $ — $ 116.8 Kudu N/A 7.6 — 7.6 7.6 — 7.6 Other Operations N/A 44.4 — 44.4 52.1 — 52.1 Total goodwill 168.8 — 168.8 176.5 — 176.5 Other intangible assets: Ark Underwriting capacity N/A 175.7 — 175.7 175.7 — 175.7 Kudu Trade names 7 2.2 1.4 .8 2.2 1.2 1.0 Other Operations Trade names 13.3 13.3 3.7 9.6 17.9 3.0 14.9 Customer relationships 10.9 24.8 9.4 15.4 29.5 7.5 22.0 Other 12.1 2.8 1.0 1.8 2.8 .5 2.3 Subtotal 40.9 14.1 26.8 50.2 11.0 39.2 Total other intangible assets 218.8 15.5 203.3 228.1 12.2 215.9 Total goodwill and other intangible assets $ 387.6 $ 15.5 $ 372.1 $ 404.6 $ 12.2 $ 392.4 The following table presents the change in goodwill and other intangible assets for the three and nine months ended September 30, 2023 and 2022: Three Months Ended September 30, 2023 2022 Millions Goodwill Other Intangible Assets Total Goodwill and Other Intangible Assets Goodwill Other Intangible Assets Total Goodwill and Other Intangible Assets Beginning balance $ 168.8 $ 204.8 $ 373.6 $ 201.8 $ 196.2 $ 398.0 Attribution of acquisition date fair value estimates between goodwill and other intangible assets (1) — — — (22.9) 22.9 — Measurement period adjustments (2) — — — (3.4) — (3.4) Amortization — (1.5) (1.5) — (1.4) (1.4) Ending balance $ 168.8 $ 203.3 $ 372.1 $ 175.5 $ 217.7 $ 393.2 (1) Relates to acquisitions within Other Operations. The relative fair values of goodwill and other intangible assets recognized in connection with acquisitions during 2022 had not yet been finalized as of the end of the period. (2) Measurement period adjustments relate to updated information about acquisition date fair values of assets acquired and liabilities assumed. During the three months ended September 30, 2022, adjustments relate to acquisitions within Other Operations. Nine Months Ended September 30, 2023 2022 Millions Goodwill Other Intangible Assets Total Goodwill and Other Intangible Assets Goodwill Other Intangible Assets Total Goodwill and Other Intangible Assets Beginning balance $ 176.5 $ 215.9 $ 392.4 $ 142.3 $ 198.2 $ 340.5 Acquisition of businesses (1) — — — 59.5 — 59.5 Dispositions (2) (6.7) (6.9) (13.6) — — — Attribution of acquisition date fair value estimates between goodwill and other intangible assets (1) — — — (22.9) 22.9 — Measurement period adjustments (3) (1.0) — (1.0) (3.4) — (3.4) Amortization — (5.7) (5.7) — (3.4) (3.4) Ending balance $ 168.8 $ 203.3 $ 372.1 $ 175.5 $ 217.7 $ 393.2 (1) Relates to acquisitions within Other Operations. The relative fair values of goodwill and other intangible assets recognized in connection with acquisitions during 2022 had not yet been finalized as of the end of the period. (2) Relates to a disposition within Other Operations.

Loss and Loss Adjustment Expe_2

Loss and Loss Adjustment Expense Reserves (Tables)	9 Months Ended
	Sep. 30, 2023
Insurance [Abstract]
Schedule of liability for unpaid claims and claims adjustment expense	The following table summarizes the loss and loss adjustment expense (“LAE”) reserve activity of the Ark/WM Outrigger segment for the three and nine months ended September 30, 2023 and 2022: Three Months Ended September 30, Nine Months Ended September 30, Millions 2023 2022 2023 2022 Gross beginning balance $ 1,421.0 $ 1,022.1 $ 1,296.5 $ 894.7 Less: beginning reinsurance recoverable on unpaid losses (1) (362.7) (375.5) (505.0) (428.9) Net loss and LAE reserves 1,058.3 646.6 791.5 465.8 Loss and LAE incurred relating to: Current year losses 266.4 222.9 562.1 491.8 Prior year losses (.6) (9.2) 19.0 (35.6) Net incurred losses and LAE 265.8 213.7 581.1 456.2 Loss and LAE paid relating to: Current year losses (8.8) (6.1) (18.4) (20.8) Prior year losses (90.4) (37.0) (281.1) (130.8) Net paid loss and LAE (99.2) (43.1) (299.5) (151.6) Change in TPC Providers’ participation (2) — — 145.4 57.5 Foreign currency translation and other adjustments to loss (5.5) (9.2) .9 (19.9) Net ending balance 1,219.4 808.0 1,219.4 808.0 Plus: ending reinsurance recoverable on unpaid losses (3) 352.4 521.4 352.4 521.4 Gross ending balance $ 1,571.8 $ 1,329.4 $ 1,571.8 $ 1,329.4 (1) The beginning reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $0.0 and $182.4 for the three months ended September 30, 2023 and 2022 and $145.4 and $276.8 for the nine months ended September 30, 2023 and 2022. (2) Amount represents the impact to net loss and LAE reserves due to a change in the TPC Providers’ participation related to the annual reinsurance to close process. (3) The ending reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $0.0 and $168.0 as of September 30, 2023 and 2022.

Third-Party Reinsurance (Tables

Third-Party Reinsurance (Tables)	9 Months Ended
	Sep. 30, 2023
Reinsurance Disclosures [Abstract]
Effects of reinsurance	The following table summarizes the effects of reinsurance on written and earned premiums and on losses and LAE for the Ark/WM Outrigger segment for the three and nine months ended September 30, 2023 and 2022: Millions Three Months Ended September 30, Nine Months Ended September 30, 2023 2022 2023 2022 Written premiums: Direct $ 189.4 $ 118.6 $ 727.1 $ 563.8 Assumed 61.8 96.9 939.6 688.7 Gross written premiums 251.2 215.5 1,666.7 1,252.5 Ceded (20.2) (1) (22.9) (360.3) (1) (246.0) Net written premiums $ 231.0 $ 192.6 $ 1,306.4 $ 1,006.5 Earned premiums: Direct $ 273.1 $ 193.5 $ 624.1 $ 492.9 Assumed 370.0 261.4 684.6 473.7 Gross earned premiums 643.1 454.9 1,308.7 966.6 Ceded (144.2) (2) (108.8) (261.4) (2) (208.8) Net earned premiums $ 498.9 $ 346.1 $ 1,047.3 $ 757.8 Losses and LAE: Gross $ 269.3 $ 391.4 $ 627.5 $ 712.9 Ceded (3.5) (3) (177.7) (46.4) (3) (256.7) Net losses and LAE $ 265.8 $ 213.7 $ 581.1 $ 456.2 (1) The ceded written premiums exclude $6.0 and $108.4 ceded by Ark to WM Outrigger Re for the three and nine months ended September 30, 2023, which eliminate in White Mountains’s consolidated financial statements. (2) The ceded earned premiums exclude $60.6 and $75.4 ceded by Ark to WM Outrigger Re for the three and nine months ended September 30, 2023, which eliminate in White Mountains’s consolidated financial statements. (3) The ceded loss and LAE exclude $7.3 and $7.9 ceded by Ark to WM Outrigger Re for the three and nine months ended September 30, 2023, which eliminate in White Mountains’s consolidated financial statements. The following table presents the Ark/WM Outrigger segment’s reinsurance recoverables as of September 30, 2023 and December 31, 2022: Millions September 30, 2023 December 31, 2022 Reinsurance recoverables on unpaid losses $ 352.4 (1) $ 505.0 (3) Reinsurance recoverables on paid losses 26.3 31.1 Ceded unearned premiums 159.7 (2) 59.2 Reinsurance recoverables $ 538.4 $ 595.3 (1) The reinsurance recoverables on unpaid losses exclude $7.9 ceded by Ark to WM Outrigger Re as of September 30, 2023, which eliminate in White Mountains’s consolidated financial statements. (2) The ceded unearned premiums exclude $33.0 ceded by Ark to WM Outrigger Re as of September 30, 2023, which eliminate in White Mountains’s consolidated financial statements. (3) The reinsurance recoverables on unpaid losses include $145.4 attributable to TPC Providers as of December 31, 2022, which are collateralized. The following table presents a schedule of written premiums and earned premiums included in the HG Global/BAM segment for the three and nine months ended September 30, 2023 and 2022: Three Months Ended September 30, Nine Months Ended September 30, Millions 2023 2022 2023 2022 Written premiums: Direct $ 16.5 $ 18.4 $ 37.3 $ 44.9 Assumed — 1.3 — 1.3 Gross written premiums (1) $ 16.5 $ 19.7 $ 37.3 $ 46.2 Earned premiums: Direct $ 7.4 $ 6.5 $ 21.4 $ 22.0 Assumed .5 .6 1.9 4.0 Gross earned premiums (1) $ 7.9 $ 7.1 $ 23.3 $ 26.0 (1) There are no ceded premium amounts in the periods presented, and gross earned premiums are equivalent to net written premiums and net earned premiums.
Gross and net reinsurance recoverables by reinsurers ratings	The following table presents the Ark/WM Outrigger segment’s gross and net reinsurance recoverables by the reinsurer’s A.M. Best Company, Inc (“A.M. Best”) ratings as of September 30, 2023: $ in Millions As of September 30, 2023 A.M. Best Rating (1) Gross Collateral Net % of Total A+ or better $ 203.7 $ — $ 203.7 69.6 % A- to A 78.8 — 78.8 26.9 B++ or lower and not rated 96.2 (2) 85.9 (2) 10.3 3.5 Total $ 378.7 $ 85.9 $ 292.8 100.0 % (1) A.M. Best ratings as detailed above are: “A+ or better” (Superior) “A- to A” (Excellent), “B++” (Good). (2) Excludes $7.9 ceded by Ark to WM Outrigger Re as of September 30, 2023, which eliminates in White Mountains’s consolidated financial statements.

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2023
Debt Disclosure [Abstract]
Schedule of debt outstanding	The following table presents White Mountains’s debt outstanding as of September 30, 2023 and December 31, 2022: $ in Millions September 30, 2023 Effective (1) December 31, 2022 Effective (1) HG Global Senior Notes $ 150.0 11.8% $ 150.0 8.9% Unamortized discount and issuance cost (3.2) (3.5) HG Global Senior Notes, carrying value 146.8 146.5 Ark 2007 Subordinated Notes, carrying value 30.0 30.0 Ark 2021 Subordinated Notes Tranche 1 41.9 41.3 Ark 2021 Subordinated Notes Tranche 2 47.0 47.0 Ark 2021 Subordinated Notes Tranche 3 70.0 70.0 Unamortized issuance cost (4.3) (4.6) Ark 2021 Subordinated Notes, carrying value 154.6 153.7 Total Ark Subordinated Notes, carrying value 184.6 10.8% 183.7 7.6% Kudu Credit Facility 210.3 9.9% 215.2 6.1% Unamortized issuance cost (6.6) (6.9) Kudu Credit Facility, carrying value 203.7 208.3 Other Operations debt 30.1 10.3% 37.4 6.6% Unamortized issuance cost (.5) (.7) Other Operations debt, carrying value 29.6 36.7 Total debt $ 564.7 $ 575.2 (1) Effective rate includes the effect of the amortization of debt issuance costs and, where applicable, the original issue discount. The following table presents the change in debt under the Kudu Credit Facility for the three and nine months ended September 30, 2023 and 2022: Three Months Ended September 30, Nine Months Ended September 30, Millions 2023 2022 2023 2022 Kudu Credit Facility Beginning balance $ 210.3 $ 260.4 $ 215.2 $ 225.4 Term loans Borrowings — — 12.0 35.0 Repayments — — (16.9) — Ending balance $ 210.3 $ 260.4 $ 210.3 $ 260.4

Municipal Bond Guarantee Insu_2

Municipal Bond Guarantee Insurance (Tables)	9 Months Ended
	Sep. 30, 2023
Guarantees [Abstract]
Schedule of municipal bond guarantee insured obligations	The following table presents a schedule of BAM’s insured obligations as of September 30, 2023 and December 31, 2022: September 30, 2023 December 31, 2022 Contracts outstanding 14,084 13,382 Remaining weighted average contract period (in years) 11.0 10.8 Contractual debt service outstanding (in millions): Principal $ 105,918.1 $ 99,996.9 Interest 52,628.8 48,880.6 Total debt service outstanding $ 158,546.9 $ 148,877.5 Gross unearned insurance premiums (in millions) $ 312.3 $ 298.3
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, 2023: Millions September 30, 2023 October 1, 2023 - December 31, 2023 $ 7.3 January 1, 2024 - March 31, 2024 7.2 April 1, 2024 - June 30, 2024 7.1 July 1, 2024 - September 30, 2024 7.0 October 1, 2024 - December 31, 2024 6.9 Total 2024 28.2 2025 26.4 2026 24.8 2027 23.2 2028 21.4 2029 and thereafter 181.0 Total gross unearned insurance premiums $ 312.3
Effects of reinsurance	The following table summarizes the effects of reinsurance on written and earned premiums and on losses and LAE for the Ark/WM Outrigger segment for the three and nine months ended September 30, 2023 and 2022: Millions Three Months Ended September 30, Nine Months Ended September 30, 2023 2022 2023 2022 Written premiums: Direct $ 189.4 $ 118.6 $ 727.1 $ 563.8 Assumed 61.8 96.9 939.6 688.7 Gross written premiums 251.2 215.5 1,666.7 1,252.5 Ceded (20.2) (1) (22.9) (360.3) (1) (246.0) Net written premiums $ 231.0 $ 192.6 $ 1,306.4 $ 1,006.5 Earned premiums: Direct $ 273.1 $ 193.5 $ 624.1 $ 492.9 Assumed 370.0 261.4 684.6 473.7 Gross earned premiums 643.1 454.9 1,308.7 966.6 Ceded (144.2) (2) (108.8) (261.4) (2) (208.8) Net earned premiums $ 498.9 $ 346.1 $ 1,047.3 $ 757.8 Losses and LAE: Gross $ 269.3 $ 391.4 $ 627.5 $ 712.9 Ceded (3.5) (3) (177.7) (46.4) (3) (256.7) Net losses and LAE $ 265.8 $ 213.7 $ 581.1 $ 456.2 (1) The ceded written premiums exclude $6.0 and $108.4 ceded by Ark to WM Outrigger Re for the three and nine months ended September 30, 2023, which eliminate in White Mountains’s consolidated financial statements. (2) The ceded earned premiums exclude $60.6 and $75.4 ceded by Ark to WM Outrigger Re for the three and nine months ended September 30, 2023, which eliminate in White Mountains’s consolidated financial statements. (3) The ceded loss and LAE exclude $7.3 and $7.9 ceded by Ark to WM Outrigger Re for the three and nine months ended September 30, 2023, which eliminate in White Mountains’s consolidated financial statements. The following table presents the Ark/WM Outrigger segment’s reinsurance recoverables as of September 30, 2023 and December 31, 2022: Millions September 30, 2023 December 31, 2022 Reinsurance recoverables on unpaid losses $ 352.4 (1) $ 505.0 (3) Reinsurance recoverables on paid losses 26.3 31.1 Ceded unearned premiums 159.7 (2) 59.2 Reinsurance recoverables $ 538.4 $ 595.3 (1) The reinsurance recoverables on unpaid losses exclude $7.9 ceded by Ark to WM Outrigger Re as of September 30, 2023, which eliminate in White Mountains’s consolidated financial statements. (2) The ceded unearned premiums exclude $33.0 ceded by Ark to WM Outrigger Re as of September 30, 2023, which eliminate in White Mountains’s consolidated financial statements. (3) The reinsurance recoverables on unpaid losses include $145.4 attributable to TPC Providers as of December 31, 2022, which are collateralized. The following table presents a schedule of written premiums and earned premiums included in the HG Global/BAM segment for the three and nine months ended September 30, 2023 and 2022: Three Months Ended September 30, Nine Months Ended September 30, Millions 2023 2022 2023 2022 Written premiums: Direct $ 16.5 $ 18.4 $ 37.3 $ 44.9 Assumed — 1.3 — 1.3 Gross written premiums (1) $ 16.5 $ 19.7 $ 37.3 $ 46.2 Earned premiums: Direct $ 7.4 $ 6.5 $ 21.4 $ 22.0 Assumed .5 .6 1.9 4.0 Gross earned premiums (1) $ 7.9 $ 7.1 $ 23.3 $ 26.0 (1) There are no ceded premium amounts in the periods presented, and gross earned premiums are equivalent to net written premiums and net earned premiums.

Earnings Per Share (Tables)

Earnings Per Share (Tables)	9 Months Ended
	Sep. 30, 2023
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, 2023 and 2022. See Note 19 — “Held for Sale and Discontinued Operations.” Three Months Ended September 30, Nine Months Ended September 30, 2023 2022 2023 2022 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common $ 23.6 $ 888.2 $ 222.7 $ 752.4 Less: total income (loss) from discontinued operations, net of tax (1) — 893.1 — 903.2 Less: net (income) loss from discontinued operations — (.1) — (.7) Net income (loss) from continuing operations attributable to 23.6 (4.8) 222.7 (150.1) Allocation of (earnings) losses to participating restricted common shares (2) (.3) .1 (3.1) 1.8 Basic and diluted earnings (losses) per share numerators $ 23.3 $ (4.7) $ 219.6 $ (148.3) Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 2,560.5 2,893.8 2,564.2 2,959.3 Average unvested restricted common shares (3) (37.6) (38.3) (35.5) (35.5) Basic earnings (losses) per share denominator 2,522.9 2,855.5 2,528.7 2,923.8 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 2,560.5 2,893.8 2,564.2 2,959.3 Average unvested restricted common shares (3) (37.6) (38.3) (35.5) (35.5) Diluted earnings (losses) per share denominator 2,522.9 2,855.5 2,528.7 2,923.8 Basic and diluted earnings per share (in dollars) - continuing Distributed earnings - dividends declared and paid $ — $ — $ 1.00 $ 1.00 Undistributed earnings (losses) 9.19 (1.66) 85.82 (51.73) Basic and diluted earnings (losses) per share $ 9.19 $ (1.66) $ 86.82 $ (50.73) (1) Includes net income (loss) from discontinued operations, net of tax - NSM Group, net gain (loss) from sale of discontinued operations, net of tax - NSM Group and net (income) loss from discontinued operations attributable to non-controlling interests. See Note 19 — “Held for Sale and Discontinued Operations. ” (2) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (3) Restricted shares outstanding vest upon a stated date. See Note 12 — “Employee Share-Based Incentive Compensation Plans.” Three Months Ended September 30, 2022 Nine Months Ended September 30, 2022 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 888.2 $ 752.4 Less: net income (loss) from continuing operations (23.6) (252.3) Less: net (income) loss from continuing operations attributable to noncontrolling 18.8 102.2 Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders (1) 893.0 902.5 Allocation of earnings to participating restricted common shares (2) (11.8) (10.8) Basic and diluted earnings per share numerators (3) $ 881.2 $ 891.7 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 2,893.8 2,959.3 Average unvested restricted common shares (4) (38.3) (35.5) Basic earnings per share denominator 2,855.5 2,923.8 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 2,893.8 2,959.3 Average unvested restricted common shares (4) (38.3) (35.5) Diluted earnings per share denominator 2,855.5 2,923.8 Basic and diluted earnings (losses) per share (in dollars) - discontinued operations $ 308.59 $ 304.97 (1) Includes net income (loss) from discontinued operations, net of tax - NSM Group, net gain (loss) from sale of discontinued operations, net of tax - NSM Group and net (income) loss from discontinued operations attributable to non-controlling interests. (2) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (3) 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, 2022. (4) Restricted shares outstanding vest upon a stated date. See Note 12 — “Employee Share-Based Incentive Compensation Plans.”
Schedule of undistributed earnings	The following table presents the undistributed net earnings (losses) from continuing operations for the three and nine months ended September 30, 2023 and 2022. See Note 19 — “Held for Sale and Discontinued Operations.” Three Months Ended September 30, Nine Months Ended September 30, Millions 2023 2022 2023 2022 Undistributed net earnings - continuing operations: Net income (loss) attributable to White Mountains’s common $ 23.3 $ (4.7) $ 219.6 $ (148.3) Dividends declared, net of restricted common share amounts (1) — — (2.5) (3.0) Total undistributed net earnings (losses), net of restricted $ 23.3 $ (4.7) $ 217.1 $ (151.3) (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.

Employee Share-Based Incentiv_2

Employee Share-Based Incentive Compensation Plans (Tables)	9 Months Ended
	Sep. 30, 2023
Share-Based Payment Arrangement [Abstract]
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, 2023 and 2022 for performance shares granted under the WTM Incentive Plan: Three Months Ended September 30, Nine Months Ended September 30, 2023 2022 2023 2022 $ in Millions Target Performance Accrued Target Performance Accrued Target Performance Accrued Target Performance Accrued Beginning of period 37,031 $ 44.6 39,449 $ 41.2 39,449 $ 67.5 40,828 $ 42.2 Shares paid (1) — — — — (13,350) (40.8) (14,625) (26.4) New grants — — — — 10,895 — 13,225 — Forfeitures and cancellations (2) — — — (.1) 37 .2 21 (.2) Expense recognized — 10.5 — 11.9 — 28.2 — 37.4 End of period 37,031 $ 55.1 39,449 $ 53.0 37,031 $ 55.1 39,449 $ 53.0 (1) WTM performance share payments in 2023 for the 2020-2022 performance cycle, which were paid in March 2023 at 200% of target. WTM performance share payments in 2022 for the 2019-2021 performance cycle, which were paid in March 2022 at 172% 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, 2023 for each performance cycle: September 30, 2023 $ in Millions Target Performance Accrued Performance cycle: 2021 – 2023 13,475 $ 30.4 2022 – 2024 13,225 22.1 2023 – 2025 10,895 3.4 Sub-total 37,595 55.9 Assumed forfeitures (564) (.8) Total 37,031 $ 55.1
Unrecognized compensation cost for restricted share awards	The following table presents the unrecognized compensation cost associated with the outstanding restricted share awards under the WTM Incentive Plan for the three and nine months ended September 30, 2023 and 2022: Three Months Ended September 30, Nine Months Ended September 30, 2023 2022 2023 2022 $ in Millions Restricted Unamortized Restricted Unamortized Restricted Unamortized Issue Date Restricted Unamortized Issue Date Non-vested, Beginning of period 37,595 $ 24.3 38,350 $ 23.0 38,350 $ 15.5 37,850 $ 15.9 Issued — — — — 10,895 16.0 13,225 13.8 Vested — — — — (11,650) — (12,725) — Forfeited — — — — — — — — Expense recognized — (4.1) — (3.8) — (11.3) — (10.5) End of period 37,595 $ 20.2 38,350 $ 19.2 37,595 $ 20.2 38,350 $ 19.2

Noncontrolling Interests (Table

Noncontrolling Interests (Tables)	9 Months Ended
	Sep. 30, 2023
Noncontrolling Interest [Abstract]
Non-controlling interests	The following table presents the balance of noncontrolling interests included in White Mountains’s total equity and the related percentage of each consolidated entity’s total equity owned by noncontrolling shareholders as of September 30, 2023 and December 31, 2022: September 30, 2023 December 31, 2022 $ in Millions Noncontrolling Percentage (1) Noncontrolling Equity Noncontrolling Percentage (1) Noncontrolling Equity Noncontrolling interests, excluding BAM HG Global 3.1 % $ (3.3) 3.1 % $ (.6) Ark 28.0 % 279.8 28.0 % 247.9 Kudu 10.8 % 94.5 10.8 % 75.1 Other various 10.0 various 20.4 Total, excluding BAM 381.0 342.8 BAM 100.0 % (165.9) 100.0 % (154.7) Total noncontrolling interests $ 215.1 $ 188.1 (1) The noncontrolling percentage represents the basic ownership interests held by noncontrolling shareholders with the exception of HG Global, for which the noncontrolling percentage represents the preferred share ownership held by noncontrolling shareholders.

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2023
Segment Reporting [Abstract]
Schedule of segments	The following tables present the financial information for White Mountains’s segments for the three and nine months ended September 30, 2023 and 2022: HG Global/BAM Ark/WM Outrigger Other Operations Total Millions HG Global BAM (1) Ark WM Outrigger Re Kudu Three Months Ended September 30, 2023 Earned insurance premiums $ 6.6 $ 1.3 $ 438.3 $ 60.6 $ — $ — $ 506.8 Net investment income 4.3 3.8 13.9 3.0 15.1 8.0 48.1 Net investment income (expense) - BAM 6.6 (6.6) — — — — — Net realized and unrealized investment gains (losses) (13.6) (10.5) (6.6) — 11.2 8.0 (11.5) Net realized and unrealized investment gains — — — — — (46.8) (46.8) Commission revenues — — — — — 3.5 3.5 Other revenues — .7 3.6 — — 15.2 19.5 Total revenues 3.9 (11.3) 449.2 63.6 26.3 (12.1) 519.6 Loss and loss adjustment expenses — — 258.5 7.3 — — 265.8 Acquisition expenses 2.0 .1 71.0 19.3 — — 92.4 Cost of sales — — — — — 8.0 8.0 General and administrative expenses .5 17.0 35.3 .1 4.5 42.2 99.6 Change in fair value of contingent consideration — — 17.0 — — — 17.0 Interest expense 3.8 — 5.5 — 5.5 .9 15.7 Total expenses 6.3 17.1 387.3 26.7 10.0 51.1 498.5 Pre-tax income (loss) from continuing operations $ (2.4) $ (28.4) $ 61.9 $ 36.9 $ 16.3 $ (63.2) $ 21.1 (1) BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the BAM Surplus Notes and is not reduced by accruals of interest expense on the BAM Surplus Notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the NYDFS. HG Global/BAM Ark/WM Outrigger Other Operations Millions HG Global BAM (1) Ark Kudu Total Three Months Ended September 30, 2022 Earned insurance premiums $ 5.9 $ 1.2 $ 346.1 $ — $ — $ 353.2 Net investment income 2.8 2.9 4.9 14.8 8.5 33.9 Net investment income (expense) - BAM 2.9 (2.9) — — — — Net realized and unrealized investment gains (losses) (19.6) (19.2) (14.4) 41.1 (17.3) (29.4) Net realized and unrealized investment gains — — — — (18.6) (18.6) Commission revenues — — — — 3.2 3.2 Other revenues .1 1.2 6.6 — 33.0 40.9 Total revenues (7.9) (16.8) 343.2 55.9 8.8 383.2 Losses and loss adjustment expenses — — 213.7 — — 213.7 Acquisition expenses 1.6 .1 74.8 — — 76.5 Cost of sales — — — — 25.0 25.0 General and administrative expenses .6 15.2 24.2 4.5 41.3 85.8 Change in fair value of contingent consideration — — 2.7 — — 2.7 Interest expense 2.0 — 3.7 4.2 .6 10.5 Total expenses 4.2 15.3 319.1 8.7 66.9 414.2 Pre-tax income (loss) from continuing operations $ (12.1) $ (32.1) $ 24.1 $ 47.2 $ (58.1) $ (31.0) (1) BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the BAM Surplus Notes and is not reduced by accruals of interest expense on the BAM Surplus Notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the NYDFS. HG Global/BAM Ark/WM Outrigger Other Operations Millions HG Global BAM (1) Ark WM Outrigger Re Kudu Total Nine Months Ended September 30, 2023 Earned insurance premiums $ 19.4 $ 3.9 $ 971.9 $ 75.4 $ — $ — $ 1,070.6 Net investment income 12.4 10.5 33.5 7.7 44.0 22.0 130.1 Net investment income (expense) - 19.7 (19.7) — — — — — Net realized and unrealized investment (11.4) (5.6) 35.9 — 45.4 125.8 190.1 Net realized and unrealized investment gains — — — — — (38.9) (38.9) Commission revenues — — — — — 10.0 10.0 Other revenues — 2.0 (1.1) — — 67.1 68.0 Total revenues 40.1 (8.9) 1,040.2 83.1 89.4 186.0 1,429.9 Loss and loss adjustment expenses — — 573.2 7.9 — — 581.1 Acquisition expenses 5.6 .7 189.3 22.2 — — 217.8 Cost of sales — — — — — 33.5 33.5 General and administrative expenses 1.9 47.8 105.3 .2 12.3 130.7 298.2 Change in fair value of contingent consideration — — 16.8 — — — 16.8 Interest expense 10.8 — 15.7 — 15.5 2.9 44.9 Total expenses 18.3 48.5 900.3 30.3 27.8 167.1 1,192.3 Pre-tax income (loss) from continuing $ 21.8 $ (57.4) $ 139.9 $ 52.8 $ 61.6 $ 18.9 $ 237.6 (1) BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the BAM Surplus Notes and is not reduced by accruals of interest expense on the BAM Surplus Notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the NYDFS. HG Global/BAM Ark/WM Outrigger Other Operations Millions HG Global BAM (1) Ark Kudu Total Nine Months Ended September 30, 2022 Earned insurance premiums $ 21.5 $ 4.5 $ 757.8 $ — $ — $ 783.8 Net investment income 7.1 8.0 9.7 41.2 13.6 79.6 Net investment income (expense) - 8.8 (8.8) — — — — Net realized and unrealized investment (57.8) (56.2) (76.5) 45.8 2.8 (141.9) Net realized and unrealized investment gains — — — — (113.3) (113.3) Commission revenues — — — — 8.7 8.7 Other revenues .3 3.4 10.1 — 89.6 103.4 Total revenues (20.1) (49.1) 701.1 87.0 1.4 720.3 Losses and loss adjustment expenses — — 456.2 — — 456.2 Acquisition expenses 7.6 1.9 174.9 — — 184.4 Cost of sales — — — — 68.8 68.8 General and administrative expenses 2.1 47.4 74.9 10.4 122.1 256.9 Change in fair value of contingent consideration — — 4.9 — — 4.9 Interest expense 5.4 — 10.6 10.3 1.2 27.5 Total expenses 15.1 49.3 721.5 20.7 192.1 998.7 Pre-tax income (loss) from continuing operations $ (35.2) $ (98.4) $ (20.4) $ 66.3 $ (190.7) $ (278.4)

Equity Method Eligible Invest_2

Equity Method Eligible Investments (Tables)	9 Months Ended
	Sep. 30, 2023
Equity Method Investments and Joint Ventures [Abstract]
Equity method investments	The following table presents the ownership interests and carrying values of White Mountains’s equity method eligible investments as of September 30, 2023 and December 31, 2022: September 30, 2023 December 31, 2022 $ in Millions Ownership Interest (1) Carrying Value Ownership Interest (1) Carrying Value Kudu’s Participation Contracts 4.1% - 30.0% $ 775.3 4.1% - 30.0% $ 695.9 Investment in MediaAlpha 35.3% 188.8 27.1% 168.6 PassportCard/DavidShield 53.8% 150.0 53.8% 135.0 Elementum Holdings, L.P. 26.6% 35.0 29.7% 30.0 Other equity method eligible investments, at fair value Under 50.0% 279.0 Under 50.0% 84.4 Other equity method eligible investments, at fair value 50.0% and over 25.2 50.0% and over — (1) Ownership interest generally references basic ownership interest with the exception of Kudu’s Participation Contracts, which are noncontrolling equity interests in the form of revenue and earnings participation contracts.

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2023
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 these financial instruments as of September 30, 2023 and December 31, 2022: September 30, 2023 December 31, 2022 Millions Fair Value Carrying Value Fair Value Carrying Value HG Global Senior Notes $ 157.6 $ 146.8 $ 155.7 $ 146.5 Ark 2007 Subordinated Notes $ 29.1 $ 30.0 $ 28.4 $ 30.0 Ark 2021 Subordinated Notes $ 164.5 $ 154.6 $ 163.1 $ 153.7 Kudu Credit Facility $ 223.0 $ 203.7 $ 223.9 $ 208.3 Other Operations debt $ 31.1 $ 29.6 $ 38.2 $ 36.7

Held for Sale and Discontinue_2

Held for Sale and Discontinued Operations (Tables)	9 Months Ended
	Sep. 30, 2023
Discontinued Operations and Disposal Groups [Abstract]
Schedule of discontinued operations	The following table summarizes the results of operations, including related income taxes associated with the businesses classified as discontinued operations for the three and nine months ended September 30, 2022: Millions Three Months Ended September 30, 2022 (1) Nine Months Ended September 30, 2022 (1) Revenues Commission revenues $ 26.6 $ 176.9 Other revenues 7.0 48.1 Total revenues 33.6 225.0 Expenses General and administrative expenses 15.7 126.8 Broker commission expenses 8.2 52.9 Change in fair value of contingent consideration — .1 Amortization of other intangible assets — 9.1 Interest expense 1.6 12.1 Total expenses 25.5 201.0 Pre-tax income (loss) from discontinued operations 8.1 24.0 Income tax (expense) benefit (1.8) (7.6) Net income (loss) from discontinued operations, net of tax - NSM Group 6.3 16.4 Net gain (loss) from sale of discontinued operations, net of tax - NSM Group 886.8 886.8 Total income (loss) from discontinued operations, net of tax 893.1 903.2 Net (income) loss from discontinued operations attributable to noncontrolling (.2) (.8) Total income (loss) from discontinued operations attributable to White 892.9 902.4 Other comprehensive income (loss) from discontinued operations, net of tax - .7 (5.2) Net gain (loss) from foreign currency translation from sale of discontinued 2.9 2.9 Comprehensive income (loss) from discontinued operations 896.5 900.1 Other comprehensive (income) loss from discontinued operations attributable to (.1) .2 Comprehensive income (loss) from discontinued operations attributable to $ 896.4 $ 900.3 (1) As a result of the NSM Transaction, the results of operations for NSM Group are presented for the periods from July 1, 2022 to August 1, 2022 and January 1, 2022 to August 1, 2022. Net Change in Cash from Discontinued Operations The following table summarizes the net change in cash associated with the businesses classified as discontinued operations for the six months ended September 30, 2022: Millions Nine Months Ended September 30, 2022 Net cash provided from (used for) operations $ 38.7 Net cash provided from (used for) investing activities 7.1 Net cash used from (used for) financing activities (17.5) Effect of exchange rate changes on cash 4.0 Net change in cash during the period 32.3 Cash balances at beginning of period (includes restricted cash of $89.2) 111.6 Cash sold as part of the sale of NSM Group (143.9) Cash balances at end of period $ — Supplemental cash flows information: Interest paid $ (12.0) Net income tax payments $ —
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, 2023 and 2022. See Note 19 — “Held for Sale and Discontinued Operations.” Three Months Ended September 30, Nine Months Ended September 30, 2023 2022 2023 2022 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common $ 23.6 $ 888.2 $ 222.7 $ 752.4 Less: total income (loss) from discontinued operations, net of tax (1) — 893.1 — 903.2 Less: net (income) loss from discontinued operations — (.1) — (.7) Net income (loss) from continuing operations attributable to 23.6 (4.8) 222.7 (150.1) Allocation of (earnings) losses to participating restricted common shares (2) (.3) .1 (3.1) 1.8 Basic and diluted earnings (losses) per share numerators $ 23.3 $ (4.7) $ 219.6 $ (148.3) Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 2,560.5 2,893.8 2,564.2 2,959.3 Average unvested restricted common shares (3) (37.6) (38.3) (35.5) (35.5) Basic earnings (losses) per share denominator 2,522.9 2,855.5 2,528.7 2,923.8 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 2,560.5 2,893.8 2,564.2 2,959.3 Average unvested restricted common shares (3) (37.6) (38.3) (35.5) (35.5) Diluted earnings (losses) per share denominator 2,522.9 2,855.5 2,528.7 2,923.8 Basic and diluted earnings per share (in dollars) - continuing Distributed earnings - dividends declared and paid $ — $ — $ 1.00 $ 1.00 Undistributed earnings (losses) 9.19 (1.66) 85.82 (51.73) Basic and diluted earnings (losses) per share $ 9.19 $ (1.66) $ 86.82 $ (50.73) (1) Includes net income (loss) from discontinued operations, net of tax - NSM Group, net gain (loss) from sale of discontinued operations, net of tax - NSM Group and net (income) loss from discontinued operations attributable to non-controlling interests. See Note 19 — “Held for Sale and Discontinued Operations. ” (2) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (3) Restricted shares outstanding vest upon a stated date. See Note 12 — “Employee Share-Based Incentive Compensation Plans.” Three Months Ended September 30, 2022 Nine Months Ended September 30, 2022 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 888.2 $ 752.4 Less: net income (loss) from continuing operations (23.6) (252.3) Less: net (income) loss from continuing operations attributable to noncontrolling 18.8 102.2 Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders (1) 893.0 902.5 Allocation of earnings to participating restricted common shares (2) (11.8) (10.8) Basic and diluted earnings per share numerators (3) $ 881.2 $ 891.7 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 2,893.8 2,959.3 Average unvested restricted common shares (4) (38.3) (35.5) Basic earnings per share denominator 2,855.5 2,923.8 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 2,893.8 2,959.3 Average unvested restricted common shares (4) (38.3) (35.5) Diluted earnings per share denominator 2,855.5 2,923.8 Basic and diluted earnings (losses) per share (in dollars) - discontinued operations $ 308.59 $ 304.97 (1) Includes net income (loss) from discontinued operations, net of tax - NSM Group, net gain (loss) from sale of discontinued operations, net of tax - NSM Group and net (income) loss from discontinued operations attributable to non-controlling interests. (2) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (3) 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, 2022. (4) Restricted shares outstanding vest upon a stated date. See Note 12 — “Employee Share-Based Incentive Compensation Plans.”

Basis of Presentation and Sig_3

Basis of Presentation and Significant Accounting Policies (Details) $ in Millions	9 Months Ended
	Sep. 30, 2023 USD ($) segment	Dec. 31, 2022	Dec. 20, 2022 account	May 26, 2022	May 25, 2022
Basis of Presentation
Number of reportable segments | segment	3
Percent of shares outstanding	12.50%
Subsidiary - HG Global
Basis of Presentation
Purchase of surplus notes | $	$ 503
Subsidiary - HG Global | Maximum
Basis of Presentation
Percentage of par value of policy reinsured	15%
Subsidiary - HG Global | Preferred Stock
Basis of Presentation
Ownership percentage	96.90%	96.90%
Subsidiary - HG Global | Common Stock
Basis of Presentation
Ownership percentage		88.40%
Subsidiary - Ark Insurance Holdings Limited
Basis of Presentation
Ownership percentage	72%	72%
Ownership percentage, after equity incentives	63%	63%
Subsidiary - WM Outrigger Re | Preferred Stock
Basis of Presentation
Ownership percentage	100%	100%
Kudu
Basis of Presentation
Ownership percentage		89.20%
Ownership percentage of basic units outstanding		89.30%		89.30%	99.10%
Ownership percentage, fully diluted	76.10%
Ownership percentage, fully converted		76.10%
Outrigger Re Ltd | Redeemable Preferred Stock
Basis of Presentation
Number of segregated accounts | account			4

Significant Transactions - NSM

Significant Transactions - NSM (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Aug. 01, 2022	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Other Significant Transactions [Line Items]
Net gain (loss) from sale of discontinued operations, net of tax - NSM Group		$ 0	$ 886.8	$ 0	$ 886.8
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group		$ 0	2.9	$ 0	2.9
Discontinued Operations, Disposed of by Sale | NSM Insurance Group
Other Significant Transactions [Line Items]
Transaction value	$ 1,400
Net transaction gain	875.7
Net gain (loss) from sale of discontinued operations, net of tax - NSM Group	886.8		886.8		886.8
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	2.9		2.9		2.9
Compensation and other costs	$ 14		$ 15.7		$ 126.8

Significant Transactions - Outr

Significant Transactions - Outrigger Re (Details) $ in Millions		3 Months Ended
	Dec. 20, 2022 USD ($) account	Dec. 31, 2022 USD ($)	Sep. 30, 2023
Outrigger Re
Other Significant Transactions [Line Items]
Purchase price		$ 205
Subsidiary - WM Outrigger Re | Preferred Stock
Other Significant Transactions [Line Items]
Basic ownership		100%	100%
Redeemable Preferred Stock | Outrigger Re Ltd
Other Significant Transactions [Line Items]
Issuances of common shares	$ 250
Number of segregated accounts | account	4

Significant Transactions - Kudu

Significant Transactions - Kudu (Details) $ in Millions	May 26, 2022 USD ($)	Dec. 31, 2022	May 25, 2022	Dec. 31, 2021 USD ($) portfolioCompany
Other Significant Transactions [Line Items]
Number of portfolio companies | portfolioCompany				2
Kudu
Other Significant Transactions [Line Items]
Equity capital raised	$ 114.5
Pre-money valuation amount	$ 114
Pre-money valuation ratio	1.3
Kudu | Kudu’s Participation Contracts
Other Significant Transactions [Line Items]
Enterprise value excluded				$ 54.3
Kudu
Other Significant Transactions [Line Items]
Ownership percentage of basic units outstanding	89.30%	89.30%	99.10%
Massachusetts Mutual Life Insurance Company | Kudu
Other Significant Transactions [Line Items]
Equity capital raised	$ 64.1
White Mountains | Kudu
Other Significant Transactions [Line Items]
Equity capital raised	50
Kudu Investment Management, LLC | Kudu
Other Significant Transactions [Line Items]
Equity capital raised	$ 0.4

Investment Securities - Schedul

Investment Securities - Schedule of Pre-Tax Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Net Investment Income [Line Items]
Total investment income	$ 48.8	$ 34.7	$ 131.8	$ 81.1
Third-party investment expenses	(0.7)	(0.8)	(1.7)	(1.5)
Net investment income	48.1	33.9	130.1	79.6
Fixed maturity investments
Net Investment Income [Line Items]
Total investment income	16.6	11.1	46.6	28.6
Short-term investments
Net Investment Income [Line Items]
Total investment income	13.8	6.7	34.8	7.2
Common equity securities
Net Investment Income [Line Items]
Total investment income	1	0.1	3.6	0.2
Other long-term investments
Net Investment Income [Line Items]
Total investment income	17.4	17.1	46.8	45.9
Amount attributable to TPC Providers
Net Investment Income [Line Items]
Total investment income	$ 0	$ 0.3	$ 0	$ 0.8

Investment Securities - Net Rea

Investment Securities - Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Gain (Loss) on Securities [Line Items]
Net realized and unrealized investment gains (losses)	$ (58.3)	$ (48)	$ 151.2	$ (255.2)
Amount attributable to TPC Providers	0	1.3	0	6.2
MediaAlpha
Gain (Loss) on Securities [Line Items]
Net realized and unrealized investment gains (losses)	(46.8)	(18.6)	(38.9)	(113.3)
Net realized and unrealized investment (losses) gains
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	1.3	(1.6)	60.7	12.9
Net unrealized investment gains (losses)	(59.6)	(47.7)	90.5	(274.3)
Net realized and unrealized investment gains (losses)	(58.3)	(49.3)	151.2	(261.4)
Fixed maturity investments
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	(0.3)	(4.9)	(1.7)	(10.8)
Net unrealized investment gains (losses)	(27.9)	(58.8)	(9.7)	(192.9)
Short-term investments
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	(0.2)	(0.1)	(0.3)	(0.4)
Net unrealized investment gains (losses)	(0.6)	(2.1)	0.7	(2.6)
Common equity securities
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	1.2	0	13.8	0
Net unrealized investment gains (losses)	(18.5)	4.3	29.8	(6.3)
Net realized and unrealized investment gains (losses)	(17.3)	4.3	43.6	(6.3)
Equity Method Investments | MediaAlpha
Gain (Loss) on Securities [Line Items]
Net unrealized investment gains (losses)	(46.8)	(18.6)	(38.9)	(113.3)
Net realized and unrealized investment gains (losses)	(46.8)	(18.6)	(38.9)	(113.3)
Other long-term investments
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	0.6	3.4	48.9	24.1
Net unrealized investment gains (losses)	34.2	27.5	108.6	40.8
Fixed maturity and short-term investments
Gain (Loss) on Securities [Line Items]
Net realized and unrealized investment gains (losses)	(29)	(65.9)	(11)	(206.7)
Less: net realized and unrealized gains (losses) on investment securities sold during the period	2.5	0.1	4.7	(3)
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	(31.5)	(66)	(15.7)	(203.7)
Common equity securities and investment in MediaAlpha
Gain (Loss) on Securities [Line Items]
Net realized and unrealized investment gains (losses)	(64.1)	(14.3)	4.7	(119.6)
Less: net realized and unrealized gains (losses) on investment securities sold during the period	(0.1)	0	3.7	0
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	(64)	(14.3)	1	(119.6)
Total Investments After TPC Providers - Investments
Gain (Loss) on Securities [Line Items]
Gain (loss) on investments, exchange rate effect	$ (12.8)	$ (22.6)	$ (1.9)	$ (44.4)

Investment Securities - Investm

Investment Securities - Investment Gains (Losses) for Level 3 (Details) - Other long-term investments - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Gain (Loss) on Securities [Line Items]
Net unrealized investment gains (losses)	$ 34.2	$ 27.5	$ 108.6	$ 40.8
Level 3
Gain (Loss) on Securities [Line Items]
Net unrealized investment gains (losses)	$ 24.6	$ 40.8	$ 76	$ 57.7

Investment Securities - Fixed M

Investment Securities - Fixed Maturity Investments (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	$ 2,142.6	$ 2,076
Gross Unrealized Gains	0.4	1.1
Gross Unrealized Losses	(160.3)	(152.4)
Net Foreign Currency Gains (Losses)	(4.8)	(3.8)
Carrying Value	1,977.9	1,920.9
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	270.1	216.6
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(9)	(10.2)
Net Foreign Currency Gains (Losses)	0	0
Carrying Value	261.1	206.4
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	1,038.8	1,098.3
Gross Unrealized Gains	0.1	0.6
Gross Unrealized Losses	(74.6)	(78.3)
Net Foreign Currency Gains (Losses)	(2.7)	(1.8)
Carrying Value	961.6	1,018.8
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	264.6	281.6
Gross Unrealized Gains	0	0.4
Gross Unrealized Losses	(26.4)	(23.4)
Net Foreign Currency Gains (Losses)	0	0
Carrying Value	238.2	258.6
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	372.6	288.7
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(47.2)	(34.5)
Net Foreign Currency Gains (Losses)	0	0
Carrying Value	325.4	254.2
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	196.5	190.8
Gross Unrealized Gains	0.3	0.1
Gross Unrealized Losses	(3.1)	(6)
Net Foreign Currency Gains (Losses)	(2.1)	(2)
Carrying Value	$ 191.6	$ 182.9

Investment Securities - Cost an

Investment Securities - Cost and Amortized Cost Maturity Schedule (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Fixed Maturities, Excluding Asset-Backed Securities And Collateralized Loan Obligations
Cost or Amortized Cost
Due in one year or less	$ 307.6
Due after one year through five years	870.9
Due after five years through ten years	302.7
Due after ten years	92.3
Carrying Value
Due in one year or less	301.2
Due after one year through five years	813.4
Due after five years through ten years	267.6
Due after ten years	78.7
Mortgage and asset-backed securities and collateralized loan obligations
Cost or Amortized Cost
Mortgage and asset-backed securities and collateralized loan obligations	569.1
Carrying Value
Mortgage and asset-backed securities and collateralized loan obligations	517
Fixed maturity investments
Cost or Amortized Cost
Total fixed maturity investments	2,142.6	$ 2,076
Carrying Value
Total fixed maturity investments	$ 1,977.9	$ 1,920.9

Investment Securities - Common

Investment Securities - Common Equity Securities and Other Long Term Investments (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
Common equity securities
Cost or Amortized Cost	$ 599.5	$ 660.6
Gross Unrealized Gains	51.6	26.7
Gross Unrealized Losses	(4.8)	(8.4)
Net Foreign Currency Gains (Losses)	(9.3)	(10.5)
Carrying Value	637	668.4
Other long-term investments
Cost or Amortized Cost	1,639.9	1,340.8
Gross Unrealized Gains	390.9	271.1
Other Long-Term Investments, Unrealized Loss	(116.6)	(107.1)
Net Foreign Currency Gains (Losses)	(18.4)	(16.8)
Carrying Value	1,895.8	1,488
MediaAlpha
Investment in MediaAlpha
Cost or Amortized Cost	59.2	0
Gross Unrealized Gains	139.9	168.6
Gross Unrealized Losses	(10.3)	0
Net Foreign Currency Gains (Losses)	0	0
Carrying Value	$ 188.8	$ 168.6

Investment Securities - Narrati

Investment Securities - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Jun. 30, 2023 USD ($) $ / shares	Sep. 30, 2023 USD ($) fund $ / shares shares	Dec. 31, 2022 USD ($)
Concentration Risk [Line Items]
Percentage of investment fair value		68%	72%
Investments held in trusts		$ 531.8	$ 500.5
Assets held by insurance regulators		4.6	4.6
Other long-term investments		$ 1,895.8	1,488
MediaAlpha | IPO
Concentration Risk [Line Items]
Increase in book value per share (in dollars per share) | $ / shares		$ 9
Fair value measured on a recurring basis | MediaAlpha
Concentration Risk [Line Items]
Other long-term investments		$ 188.8	168.6
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments		150.3	137.4
Lloyds Bank
Concentration Risk [Line Items]
Fair value investments		335.2	319.2
Outrigger Re
Concentration Risk [Line Items]
Short-term investments		$ 233.3	$ 203.7
MediaAlpha
Concentration Risk [Line Items]
Tender offer purchase price	$ 5.9
Purchase price (in dollars per share) | $ / shares	$ 10	$ 8.26
Investment owned (in shares) | shares		22.9
Ownership interest (as a percent)		35.30%	27.10%
Fully diluted ownership interest (as a percent)		33%
Increase in investment owned (in dollars per share) | $ / shares		$ 1
Largest Investment | Private Equity and Hedge Fund Concentration Risk | Investments
Concentration Risk [Line Items]
Other long-term investments		$ 53.4	$ 49
HG Global Senior Notes | Senior Notes
Concentration Risk [Line Items]
Interest reserve account		29.9	31.2
Kudu Credit Facility | Secured Debt
Concentration Risk [Line Items]
Interest reserve account		14.1
Restricted cash			12.2
Asset Pledged as Collateral
Concentration Risk [Line Items]
Short-term investments		147.2	90.3
Asset Pledged as Collateral | Additional Investments, Insurance Regulators and Reinsurance Counterparties
Concentration Risk [Line Items]
Fair value investments		232.4	257
Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value		191.6	182.9
Collateralized loan obligations | Fair value measured on a recurring basis
Concentration Risk [Line Items]
Fixed maturity investments, at fair value		191.6	182.9
Private equity funds
Concentration Risk [Line Items]
Other long-term investments		$ 239.2
Number of investments | fund		18
Hedge funds
Concentration Risk [Line Items]
Number of investments | fund		2
Hedge funds | Minimum
Concentration Risk [Line Items]
Notice period		45 days
Hedge funds | Maximum
Concentration Risk [Line Items]
Notice period		90 days
Bank loan fund
Concentration Risk [Line Items]
Other long-term investments		$ 189.3	174.8
Foreign regulatory deposits
Concentration Risk [Line Items]
Other long-term investments		150.3	137.4
ILS funds
Concentration Risk [Line Items]
Other long-term investments		$ 159.5	$ 49.3
ILS funds | Minimum
Concentration Risk [Line Items]
Notice period		30 days
ILS funds | Maximum
Concentration Risk [Line Items]
Notice period		90 days

Investment Securities - Fair Va

Investment Securities - Fair Value Measurement by Level (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	$ 637	$ 668.4
Other long-term investments	1,895.8	1,488
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	261.1	206.4
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	961.6	1,018.8
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	238.2	258.6
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	325.4	254.2
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	191.6	182.9
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	1,977.9	1,920.9
Level 2 | Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	325.4	254.2
Level 2 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	191.6	182.9
Level 3 | Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Level 3 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	5,862.4	5,170
Fair value measured on a recurring basis | MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	188.8	168.6
Fair value measured on a recurring basis | U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	261.1	206.4
Fair value measured on a recurring basis | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	961.6	1,018.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	290.1	291.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	188.3	191.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Healthcare
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	109.1	121.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	107.8	115.4
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	107.5	123.7
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	66.6	73.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	42.4	47.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	29.1	33.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	20.7	19.7
Fair value measured on a recurring basis | Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	238.2	258.6
Fair value measured on a recurring basis | Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	325.4	254.2
Fair value measured on a recurring basis | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	191.6	182.9
Fair value measured on a recurring basis | Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	1,977.9	1,920.9
Fair value measured on a recurring basis | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	1,162.9	924.1
Fair value measured on a recurring basis | Exchange-traded funds
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	252.1	333.8
Fair value measured on a recurring basis | Common stock, other
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	384.9	334.6
Fair value measured on a recurring basis | Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	637	668.4
Fair value measured on a recurring basis | Other Long-Term Investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,895.8	1,488
Fair value measured on a recurring basis | Fair Value, Inputs, Level 1, Level 2, and Level 3 | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,090.7	926.4
Fair value measured on a recurring basis | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,851.7	1,632.9
Fair value measured on a recurring basis | Level 1 | MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	188.8	168.6
Fair value measured on a recurring basis | Level 1 | U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	261.1	206.4
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Financials
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Consumer
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Healthcare
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Technology
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Utilities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Communications
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Energy
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Materials
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	261.1	206.4
Fair value measured on a recurring basis | Level 1 | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	1,149.7	924.1
Fair value measured on a recurring basis | Level 1 | Exchange-traded funds
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	252.1	333.8
Fair value measured on a recurring basis | Level 1 | Common stock, other
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	252.1	333.8
Fair value measured on a recurring basis | Level 1 | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Fair value measured on a recurring basis | Level 1 | Other Long-Term Investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Fair value measured on a recurring basis | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	2,130.3	2,063.9
Fair value measured on a recurring basis | Level 2 | MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Fair value measured on a recurring basis | Level 2 | U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	961.6	1,018.8
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Financials
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	290.1	291.2
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Consumer
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	188.3	191.9
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Healthcare
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	109.1	121.3
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	107.8	115.4
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Technology
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	107.5	123.7
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Utilities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	66.6	73.8
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Communications
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	42.4	47.9
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Energy
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	29.1	33.9
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Materials
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	20.7	19.7
Fair value measured on a recurring basis | Level 2 | Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	238.2	258.6
Fair value measured on a recurring basis | Level 2 | Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	325.4	254.2
Fair value measured on a recurring basis | Level 2 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	191.6	182.9
Fair value measured on a recurring basis | Level 2 | Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	1,716.8	1,714.5
Fair value measured on a recurring basis | Level 2 | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	13.2	0
Fair value measured on a recurring basis | Level 2 | Exchange-traded funds
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	0	0
Fair value measured on a recurring basis | Level 2 | Common stock, other
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	384.9	334.6
Fair value measured on a recurring basis | Level 2 | Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	384.9	334.6
Fair value measured on a recurring basis | Level 2 | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	15.4	14.8
Fair value measured on a recurring basis | Level 2 | Other Long-Term Investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	15.4	14.8
Fair value measured on a recurring basis | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,075.3	911.6
Fair value measured on a recurring basis | Level 3 | MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Fair value measured on a recurring basis | Level 3 | U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Financials
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Consumer
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Healthcare
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Technology
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Utilities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Communications
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Energy
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Materials
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Exchange-traded funds
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Common stock, other
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,075.3	911.6
Fair value measured on a recurring basis | Level 3 | Other Long-Term Investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,075.3	911.6
Fair value measured on a recurring basis | NAV | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	$ 805.1	$ 561.6

Investment Securities - Debt Se

Investment Securities - Debt Securities Issued by Corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	$ 961.6	$ 1,018.8
AAA
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	12.6	11.3
AA
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	84.3	96
A
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	509.6	567.9
BBB
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	349.1	337.7
Other
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	$ 6	$ 5.9

Investment Securities - Mortgag

Investment Securities - Mortgage and Asset-Backed Securities and Collateralized Loan Obligations (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Total mortgage and asset-backed securities and collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	$ 517	$ 437.1
Total mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	325.4	254.2
Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	306.2	231.9
Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	19.2	22.3
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	191.6	182.9
Level 2 | Total mortgage and asset-backed securities and collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	517	437.1
Level 2 | Total mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	325.4	254.2
Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	306.2	231.9
Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	19.2	22.3
Level 2 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	191.6	182.9
Level 3 | Total mortgage and asset-backed securities and collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Level 3 | Total mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Level 3 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Level 3 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Total non-agency | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.2	0.3
Total non-agency | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.2	0.3
Total non-agency | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Total agency | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	306	231.6
Total agency | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	306	231.6
Total agency | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
FNMA | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	162.5	124.5
FNMA | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	162.5	124.5
FNMA | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
FHLMC | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	114.3	78.8
FHLMC | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	114.3	78.8
FHLMC | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
GNMA | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	29.2	28.3
GNMA | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	29.2	28.3
GNMA | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Non-agency: Residential | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.2	0.3
Non-agency: Residential | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.2	0.3
Non-agency: Residential | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Credit card receivables | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	3.2	11.9
Credit card receivables | Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	3.2	11.9
Credit card receivables | Level 3 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Vehicle receivables | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	13.5	10.4
Vehicle receivables | Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	13.5	10.4
Vehicle receivables | Level 3 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Other | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	2.5	0
Other | Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	2.5	0
Other | Level 3 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	$ 0	$ 0

Investment Securities - Other L

Investment Securities - Other Long-Term Investments (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	$ 1,895.8	$ 1,488
PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	150	135
Elementum Holdings, L.P.
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	35	30
Mandatorily redeemable preferred securities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	58.5
Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	41.8	37.2
Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	299.3	197.8
Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	189.3	174.8
ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	159.5	49.3
Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	150.3	137.4
Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	14.8	9.6
Other
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	22	21
Kudu’s Participation Contracts
Debt and Equity Securities, FV-NI [Line Items]
Equity method investments	775.3	695.9
Kudu’s Participation Contracts | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	775.3	695.9
Investments, Excluding MediaAlpha | Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,060.6	898.1
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	416.6	373.6
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Elementum Holdings, L.P.
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Mandatorily redeemable preferred securities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	55.4	40.4
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	189.3	174.8
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	150.3	137.4
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Other
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	21.6	21
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Kudu’s Participation Contracts | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Investments, Excluding MediaAlpha | Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Kudu
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	780.7	695.9
Operating Segments | Kudu | PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Kudu | Elementum Holdings, L.P.
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Kudu | Mandatorily redeemable preferred securities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0
Operating Segments | Kudu | Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Kudu | Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Kudu | Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Kudu | ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Kudu | Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Kudu | Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	5.4	0
Operating Segments | Kudu | Other
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Kudu | Kudu’s Participation Contracts | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	775.3	695.9
Operating Segments | Kudu | Investments, Excluding MediaAlpha | Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	775.3	695.9
Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Equity method investments	188.8	168.6
Other long-term investments	698.5	418.5
Other Operations | PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	150	135
Other Operations | Elementum Holdings, L.P.
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	35	30
Other Operations | Mandatorily redeemable preferred securities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	58.5
Other Operations | Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	41.8	37.2
Other Operations | Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	243.9	157.4
Other Operations | Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Other Operations | ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	159.5	49.3
Other Operations | Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Other Operations | Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	9.4	9.6
Other Operations | Other
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.4	0
Other Operations | Kudu’s Participation Contracts | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Other Operations | Investments, Excluding MediaAlpha | Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	$ 285.3	$ 202.2

Investment Securities - Private

Investment Securities - Private Equity Funds and Hedge Funds (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Total private equity funds and hedge funds included in other long-term investments
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 299.3	$ 197.8
Unfunded Commitments	69.8	94.3
Private equity funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	239.2	140.6
Unfunded Commitments	69.8	94.3
Private equity funds | Aerospace/Defense/Government
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	146.4	59.4
Unfunded Commitments	25.5	37.5
Private equity funds | Financial services
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	88.6	77.1
Unfunded Commitments	41.8	54.3
Private equity funds | Real estate
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	4.2	4.1
Unfunded Commitments	2.5	2.5
Hedge funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	60.1	57.2
Unfunded Commitments	0	0
Hedge funds | Long/short equity financials and business services
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	51.1	49
Unfunded Commitments	0	0
Hedge funds | European small/mid cap
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	9	8.2
Unfunded Commitments	$ 0	$ 0

Investment Securities - Inves_2

Investment Securities - Investments In Private Equity Funds Subject to Lock-Up Periods (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Debt and Equity Securities, FV-NI [Line Items]
Total	$ 1,895.8	$ 1,488
Private equity funds
Debt and Equity Securities, FV-NI [Line Items]
1 – 3 years	4.4
3 – 5 years	72.1
5 – 10 years	148.8
>10 years	13.9
Total	$ 239.2

Investment Securities - Rollfor

Investment Securities - Rollforward of Fair Value Measurements by Level (Details) - Other long-term investments - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 911.6	$ 890.6
Net realized and unrealized gains	74	57.3
Purchases and contributions	200.1	127.9
Sales and distributions	(110.4)	(31.7)
Transfers in	0	0
Transfers out	0	0
Ending balance	$ 1,075.3	$ 1,044.1

Investment Securities - Signifi

Investment Securities - Significant Unobservable Inputs (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	$ 1,895.8	$ 1,488
Private debt instruments
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	14.8	9.6
Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
New and existing contracts deployed	108.3	99.8
Level 3 | Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	41.7	41.1
Level 3 | Discounted cash flow | Private debt instruments
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	14.8	9.6
Level 3 | Discounted cash flow | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	775.3	695.9
Level 3 | Discounted cash flow | PassportCard/DavidShield
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	150	135
Level 3 | Discounted cash flow | Elementum Holdings, L.P.
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	35	30
Level 3 | Discounted cash flow | Mandatorily redeemable preferred securities
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	$ 58.5
Level 3 | Probability weighted expected return method | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value		$ 189
Level 3 | Discount Rate | Discounted cash flow | Private debt instruments
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate		11%
Level 3 | Discount Rate | Discounted cash flow | PassportCard/DavidShield
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	25%	24%
Level 3 | Discount Rate | Discounted cash flow | Elementum Holdings, L.P.
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	25%	21%
Level 3 | Discount Rate | Discounted cash flow | Mandatorily redeemable preferred securities
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	15%
Level 3 | Discount Rate | Discounted cash flow | Minimum | Private debt instruments
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	12%
Level 3 | Discount Rate | Discounted cash flow | Minimum | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	19%	18%
Level 3 | Discount Rate | Discounted cash flow | Maximum | Private debt instruments
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	13%
Level 3 | Discount Rate | Discounted cash flow | Maximum | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	26%	25%
Level 3 | Terminal Cash Flow Exit Multiple | Discounted cash flow | Minimum | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	700%	700%
Level 3 | Terminal Cash Flow Exit Multiple | Discounted cash flow | Maximum | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	2,200%	1,600%
Level 3 | Terminal Revenue Growth Rate | Discounted cash flow | PassportCard/DavidShield
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	4%	4%
Level 3 | Terminal Revenue Growth Rate | Discounted cash flow | Elementum Holdings, L.P.
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	4%	4%
Level 3 | Measurement Input, Pre-Tax Discount Rate | Discounted cash flow | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	21%	21%
Level 3 | Measurement, Input, Pre-Tax Terminal Cash Flow Exit Multiple | Discounted cash flow | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	1,300%	1,200%

Goodwill and Other Intangible_3

Goodwill and Other Intangible Assets - Schedule of Acquisition Date Fair Values, Accumulated Amortization and Net Carrying Values for Other Intangibles Assets and Goodwill, By Company (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 168.8	$ 168.8	$ 176.5	$ 175.5	$ 201.8	$ 142.3
Other intangible assets, finite-lived, accumulated amortization	15.5		12.2
Total other intangible assets	218.8		228.1
Total other intangible assets	203.3	204.8	215.9	217.7	196.2	198.2
Total goodwill and other intangible assets	387.6		404.6
Goodwill and other intangible assets	372.1	$ 373.6	392.4	$ 393.2	$ 398	$ 340.5
Other
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, finite-lived, accumulated amortization	15.5		12.2
Total other intangible assets	203.3		215.9
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets	292.5		292.5
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	116.8		116.8
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Underwriting Capacity
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, indefinite-lived	175.7		175.7
Operating Segments | Kudu
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	7.6		7.6
Goodwill and other intangible assets	$ 8.4		8.6
Operating Segments | Kudu | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	7 years
Other intangible assets, finite-lived, acquisition date fair value	$ 2.2		2.2
Other intangible assets, finite-lived, accumulated amortization	1.4		1.2
Other intangible assets, finite-lived, net carrying value	0.8		1
Other Operations
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	44.4		52.1
Other intangible assets, finite-lived, acquisition date fair value	40.9		50.2
Other intangible assets, finite-lived, accumulated amortization	14.1		11
Other intangible assets, finite-lived, net carrying value	26.8		39.2
Goodwill and other intangible assets	$ 71.2		91.3
Other Operations | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	13 years 3 months 18 days
Other intangible assets, finite-lived, acquisition date fair value	$ 13.3		17.9
Other intangible assets, finite-lived, accumulated amortization	3.7		3
Other intangible assets, finite-lived, net carrying value	$ 9.6		14.9
Other Operations | Customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	10 years 10 months 24 days
Other intangible assets, finite-lived, acquisition date fair value	$ 24.8		29.5
Other intangible assets, finite-lived, accumulated amortization	9.4		7.5
Other intangible assets, finite-lived, net carrying value	$ 15.4		22
Other Operations | Other
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	12 years 1 month 6 days
Other intangible assets, finite-lived, acquisition date fair value	$ 2.8		2.8
Other intangible assets, finite-lived, accumulated amortization	1		0.5
Other intangible assets, finite-lived, net carrying value	$ 1.8		$ 2.3

Goodwill and Other Intangible_4

Goodwill and Other Intangible Assets - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Goodwill and Intangible Assets Disclosure [Abstract]
Impairments to goodwill and other intangibles	$ 0	$ 0	$ 0	$ 0

Goodwill and Other Intangible_5

Goodwill and Other Intangible Assets - Schedule of Goodwill and Other Intangible Assets Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Goodwill
Goodwill, beginning balance	$ 168.8	$ 201.8	$ 176.5	$ 142.3
Acquisition of businesses			0	59.5
Dispositions			(6.7)	0
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	0	(22.9)	0	(22.9)
Measurement period adjustments	0	(3.4)	(1)	(3.4)
Goodwill, ending balance	168.8	175.5	168.8	175.5
Other Intangible Assets
Other intangible assets (excluding goodwill), beginning balance	204.8	196.2	215.9	198.2
Acquisition of businesses			0	0
Dispositions			(6.9)	0
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	0	22.9	0	22.9
Amortization	(1.5)	(1.4)	(5.7)	(3.4)
Other intangible assets (excluding goodwill), ending balance	203.3	217.7	203.3	217.7
Total Goodwill and Other Intangible Assets
Total Goodwill and Other Intangible Assets, beginning balance	373.6	398	392.4	340.5
Acquisition of businesses			0	59.5
Dispositions			(13.6)	0
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	0	0	0	0
Measurement period adjustments	0	(3.4)	(1)	(3.4)
Amortization	(1.5)	(1.4)	(5.7)	(3.4)
Total Goodwill and Other Intangible Assets, ending balance	$ 372.1	$ 393.2	$ 372.1	$ 393.2

Loss and Loss Adjustment Expe_3

Loss and Loss Adjustment Expense Reserves (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Loss and LAE incurred relating to:
Net incurred losses and LAE	$ 265.8	$ 213.7	$ 581.1	$ 456.2
Syndicate - Ark Insurance Holdings Limited
Loss and LAE paid relating to:
Percent of underwriting capital provided by third parties					0%	42.80%	58.30%
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance	1,421	1,022.1	1,296.5	894.7
Less: beginning reinsurance recoverable on unpaid losses	(362.7)	(375.5)	(505)	(428.9)
Net loss and LAE reserves	1,058.3	646.6	791.5	465.8
Loss and LAE incurred relating to:
Current year losses	266.4	222.9	562.1	491.8
Prior year losses	(0.6)	(9.2)	19	(35.6)
Net incurred losses and LAE	265.8	213.7	581.1	456.2
Loss and LAE paid relating to:
Current year losses	(8.8)	(6.1)	(18.4)	(20.8)
Prior year losses	(90.4)	(37)	(281.1)	(130.8)
Net paid loss and LAE	(99.2)	(43.1)	(299.5)	(151.6)
Change in TPC providers’ participation	0	0	145.4	57.5
Foreign currency translation and other adjustments to loss and LAE reserves	(5.5)	(9.2)	0.9	(19.9)
Net ending balance	1,219.4	808	1,219.4	808
Plus: ending reinsurance recoverable on unpaid losses	352.4	521.4	352.4	521.4
Gross ending balance	1,571.8	1,329.4	1,571.8	1,329.4
P&C Insurance and Reinsurance (Ark/WM Outrigger) | TPC Providers
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Less: beginning reinsurance recoverable on unpaid losses	0	(182.4)	(145.4)	(276.8)
Loss and LAE paid relating to:
Reinsurance recoverables	$ 0	$ 168	$ 0	$ 168

Third-Party Reinsurance - Effec

Third-Party Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Earned premiums:
Net earned premiums	$ 506.8	$ 353.2	$ 1,070.6	$ 783.8
Losses and LAE:
Net incurred losses and LAE	265.8	213.7	581.1	456.2
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Written premiums:
Direct	189.4	118.6	727.1	563.8
Assumed	61.8	96.9	939.6	688.7
Gross written premiums	251.2	215.5	1,666.7	1,252.5
Ceded	(20.2)	(22.9)	(360.3)	(246)
Net written premiums	231	192.6	1,306.4	1,006.5
Earned premiums:
Direct	273.1	193.5	624.1	492.9
Assumed	370	261.4	684.6	473.7
Gross earned premiums	643.1	454.9	1,308.7	966.6
Ceded	(144.2)	(108.8)	(261.4)	(208.8)
Net earned premiums	498.9	346.1	1,047.3	757.8
Losses and LAE:
Gross	269.3	391.4	627.5	712.9
Ceded	(3.5)	(177.7)	(46.4)	(256.7)
Net incurred losses and LAE	265.8	$ 213.7	581.1	$ 456.2
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark
Written premiums:
Ceded	(6)		(108.4)
Earned premiums:
Ceded	(60.6)		(75.4)
Losses and LAE:
Ceded	$ (7.3)		$ (7.9)

Third-Party Reinsurance - Reins

Third-Party Reinsurance - Reinsurance Recoverables (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment Reporting Information [Line Items]
Reinsurance recoverables on unpaid losses	$ 352.4	$ 505
Reinsurance recoverables on paid losses	26.3	31.1
Ceded unearned premiums	159.7	59.2
Reinsurance recoverables	538.4	595.3
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark
Segment Reporting Information [Line Items]
Reinsurance recoverables on unpaid losses	(7.9)
Ceded unearned premiums	$ (33)
TPC Providers
Segment Reporting Information [Line Items]
Reinsurance recoverables on unpaid losses		$ 145.4

Third-Party Reinsurance - Remai

Third-Party Reinsurance - Remaining Gross and Net Reinsurance Recoverables (Details) - P&C Insurance and Reinsurance (Ark/WM Outrigger) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Reinsurance recoverables	$ 378.7
Collateral	538.4	$ 595.3
Net	292.8
Reinsurance recoverables on unpaid losses	352.4	$ 505
Ark
Segment Reporting Information [Line Items]
Reinsurance recoverables on unpaid losses	$ (7.9)
Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting Information [Line Items]
% of Total	100%
Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 85.9
A+ or better
Segment Reporting Information [Line Items]
Reinsurance recoverables	203.7
Net	$ 203.7
A+ or better | Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting Information [Line Items]
% of Total	69.60%
A+ or better | Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 0
A- to A
Segment Reporting Information [Line Items]
Reinsurance recoverables	78.8
Net	$ 78.8
A- to A | Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting Information [Line Items]
% of Total	26.90%
A- to A | Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 0
B++ or lower and not rated
Segment Reporting Information [Line Items]
Reinsurance recoverables	96.2
Net	$ 10.3
B++ or lower and not rated | Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting Information [Line Items]
% of Total	3.50%
B++ or lower and not rated | Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 85.9

Third-Party Reinsurance - Narra

Third-Party Reinsurance - Narrative (Details) - Third Point Re - P&C Insurance and Reinsurance (Ark/WM Outrigger) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Annual crediting rate	3%
Carrying value of investment	$ 20.3	$ 20.4

Debt - Summary of Debt Outstand

Debt - Summary of Debt Outstanding (Details) € in Millions, $ in Millions	Sep. 30, 2023 USD ($)	Sep. 30, 2023 EUR (€)	Jun. 30, 2023 USD ($)	Dec. 31, 2022 USD ($)	Sep. 30, 2022 USD ($)	Jun. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)
Debt Instrument
Total debt	$ 564.7			$ 575.2
HG Global Senior Notes
Debt Instrument
Total debt	146.8			146.5
Ark 2007 Subordinated Notes
Debt Instrument
Total debt	30			30
Ark 2021 Subordinated Notes
Debt Instrument
Total debt	154.6			153.7
Kudu Credit Facility
Debt Instrument
Total debt	203.7			208.3
Other Operations debt
Debt Instrument
Total debt	29.6			36.7
Senior Notes | HG Global Senior Notes
Debt Instrument
Amount borrowed	150			150
Unamortized discount and issuance cost	(3.2)			(3.5)
Total debt	$ 146.8			$ 146.5
Effective Rate	11.80%	11.80%		8.90%
Subordinated Debt | Ark
Debt Instrument
Total debt	$ 184.6			$ 183.7
Effective Rate	10.80%	10.80%		7.60%
Subordinated Debt | Ark 2007 Subordinated Notes
Debt Instrument
Amount borrowed	$ 30			$ 30
Subordinated Debt | Ark 2021 Subordinated Notes
Debt Instrument
Unamortized issuance cost	(4.3)			(4.6)
Total debt	154.6			153.7
Subordinated Debt | Ark 2021 Subordinated Notes Tranche 1
Debt Instrument
Amount borrowed	41.9	€ 39.1		41.3
Subordinated Debt | Ark 2021 Subordinated Notes Tranche 2
Debt Instrument
Amount borrowed	47			47
Subordinated Debt | Ark 2021 Subordinated Notes Tranche 3
Debt Instrument
Amount borrowed	70			70
Secured Debt | Kudu Credit Facility
Debt Instrument
Amount borrowed	210.3			215.2
Unamortized issuance cost	(6.6)			(6.9)
Total debt	$ 210.3		$ 210.3	$ 215.2	$ 260.4	$ 260.4	$ 225.4
Effective Rate	9.90%	9.90%		6.10%
Secured Debt | Other Operations debt
Debt Instrument
Amount borrowed	$ 30.1			$ 37.4
Unamortized issuance cost	$ (0.5)			$ (0.7)
Effective Rate	10.30%	10.30%		6.60%

Debt - Narrative (Details)

Debt - Narrative (Details) € in Millions

1 Months Ended

3 Months Ended

6 Months Ended

9 Months Ended

Jul. 01, 2023

Apr. 29, 2022 USD ($)

Sep. 08, 2021 USD ($)

Sep. 30, 2021 USD ($)

Aug. 31, 2021

Jul. 31, 2021

Mar. 31, 2007 USD ($)

Sep. 30, 2021 USD ($)

Jun. 30, 2023

Sep. 30, 2023 USD ($) credit_facility

Sep. 30, 2023 EUR (€) credit_facility

Dec. 31, 2022 USD ($)

Sep. 30, 2022 USD ($)

Jun. 16, 2022 USD ($)

Apr. 30, 2022 USD ($)

Dec. 31, 2021 USD ($) letterOfCredit

Aug. 11, 2021 USD ($)

Jul. 13, 2021 USD ($)

Jul. 13, 2021 EUR (€)

Interest Rate Cap | Subsidiary - HG Global

Debt Instrument

Notional amount

$ 150,000,000

HG Global Senior Notes | Senior Notes

Debt Instrument

Face value

$ 150,000,000

Anniversary of funding date

5 years

Annual payments

$ 15,000,000

Interest accrued multiplier

8

Interest reserve account, maximum

$ 29,200,000

Interest reserve account

$ 29,900,000

$ 31,200,000

Additional interest rate

1%

Outstanding balance

$ 150,000,000

150,000,000

HG Global Senior Notes | Senior Notes | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate

Debt Instrument

Basis spread on variable rate

6.30%

Ark 2007 Subordinated Notes | Junior Subordinated Debt

Debt Instrument

Face value

$ 30,000,000

Outstanding balance

30,000,000

Ark 2007 Subordinated Notes | Junior Subordinated Debt | London Interbank Offered Rate (LIBOR)

Debt Instrument

Basis spread on variable rate

4.60%

Ark 2007 Subordinated Notes | Subordinated Debt

Debt Instrument

Outstanding balance

30,000,000

30,000,000

Ark 2021 Subordinated Notes | Subordinated Debt

Debt Instrument

Face value

$ 163,300,000

$ 163,300,000

Proceeds from debt, net of debt issuance costs

$ 157,800,000

Ark 2021 Subordinated Notes Tranche 1 | Subordinated Debt

Debt Instrument

Face value

$ 46,300,000

€ 39.1

Outstanding balance

41,900,000

€ 39.1

41,300,000

Ark 2021 Subordinated Notes Tranche 1 | Subordinated Debt | Euro Interbank Offered Rate (EURIBOR)

Debt Instrument

Basis spread on variable rate

5.75%

Ark 2021 Subordinated Notes Tranche 2 | Subordinated Debt

Debt Instrument

Face value

$ 47,000,000

Outstanding balance

$ 47,000,000

47,000,000

Ark 2021 Subordinated Notes Tranche 2 | Subordinated Debt | London Interbank Offered Rate (LIBOR)

Debt Instrument

Basis spread on variable rate

5.75%

Ark 2021 Subordinated Notes Tranche 3 | Subordinated Debt

Debt Instrument

Face value

$ 70,000,000

Additional interest rate

1%

Outstanding balance

$ 70,000,000

70,000,000

Solvency capital requirement

120%

Debt to capital requirement

40%

Ark 2021 Subordinated Notes Tranche 3 | Subordinated Debt | London Interbank Offered Rate (LIBOR)

Debt Instrument

Basis spread on variable rate

6.10%

ING LOC Facility | Line of Credit | Letter of Credit

Debt Instrument

Number of letter of credit agreements | letterOfCredit

2

Total commitment under revolving credit facility

$ 50,000,000

Citibank LOC Facility | Line of Credit | Letter of Credit

Debt Instrument

Total commitment under revolving credit facility

$ 100,000,000

Letters of credit outstanding

93,200,000

Pledged collateral

108,600,000

Lloyds LOC Facility | Line of Credit | Letter of Credit

Debt Instrument

Total commitment under revolving credit facility

$ 50,000,000

Letters of credit outstanding

25,900,000

Pledged collateral

38,600,000

Kudu Credit Facility | Secured Debt

Debt Instrument

Face value

300,000,000

Interest reserve account

14,100,000

Outstanding balance

$ 210,300,000

215,200,000

Stated margin

4.30%

4.30%

Restricted cash

12,200,000

Debt covenant, maximum loan to value percent

27.90%

27.90%

Percentage of borrowing base

35%

35%

Remaining borrowing capacity under revolving credit facility

$ 74,100,000

Kudu Credit Facility | Secured Debt | Debt Covenant, Period 1

Debt Instrument

Debt covenant, loan to value percent

50%

50%

Kudu Credit Facility | Secured Debt | Debt Covenant, Period 2

Debt Instrument

Debt covenant, loan to value percent

40%

40%

Kudu Credit Facility | Secured Debt | Debt Covenant, Period 3

Debt Instrument

Debt covenant, loan to value percent

25%

25%

Kudu Credit Facility | Secured Debt | Debt Covenant, Period 4

Debt Instrument

Debt covenant, loan to value percent

15%

15%

Kudu Credit Facility | Secured Debt | Debt Covenant, Period 5

Debt Instrument

Debt covenant, loan to value percent

0%

0%

Kudu Credit Facility | Secured Debt | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate

Debt Instrument

Basis spread on variable rate

0.26%

Kudu Credit Facility | Secured Debt | London Interbank Offered Rate (LIBOR)

Debt Instrument

Basis spread on variable rate

0.25%

Other Operations debt | Secured Debt

Debt Instrument

Outstanding balance

$ 30,100,000

$ 37,400,000

Secured credit facilities | credit_facility

4

4

Debt - Schedule of the Change i

Debt - Schedule of the Change in Debt (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Debt Instrument
Beginning balance			$ 575.2
Borrowings			12.2	$ 211.8
Repayments			(24.3)	(10.5)
Ending balance	$ 564.7		564.7
Kudu Credit Facility
Debt Instrument
Beginning balance			208.3
Ending balance	203.7		203.7
Secured Debt | Kudu Credit Facility
Debt Instrument
Beginning balance	210.3	$ 260.4	215.2	225.4
Borrowings	0	0	12	35
Repayments	0	0	(16.9)	0
Ending balance	$ 210.3	$ 260.4	$ 210.3	$ 260.4

Income Taxes (Details)

Income Taxes (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Income Tax Disclosure [Abstract]
Effective tax rate	34.60%	23.90%	8.20%	9.40%

Derivatives (Details)

Derivatives (Details) - Interest Rate Cap - Subsidiary - HG Global - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022	Jun. 16, 2022
Derivative [Line Items]
Notional amount						$ 150,000,000
Premiums paid						$ 3,300,000
Proceeds from Periodic Settlement of Derivatives	$ 600,000		$ 1,200,000
Derivative, fair value, net	4,800,000		4,800,000		$ 4,100,000
Interest Expense
Derivative [Line Items]
Gain (loss) on fair value of derivative instruments	$ 0	$ 2,100,000	$ 700,000	$ 600,000
Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Derivative [Line Items]
Derivative, cap interest rate						3.50%

Municipal Bond Guarantee Insu_3

Municipal Bond Guarantee Insurance - Narrative (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2022	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2018	Jan. 01, 2025	Dec. 31, 2023
Guarantor Obligations [Line Items]
Assets held-in-trust	$ 856.9	$ 899.7		$ 899.7		$ 856.9
Collateral trusts, investments and cash	503.3	526.5		526.5		503.3
Surplus notes	340	340		340		340
Interest receivable	13.6	33.2		33.2		13.6
Debt	575.2	$ 564.7		564.7		575.2
Subsidiary - HG Global
Guarantor Obligations [Line Items]
Purchase of surplus notes				$ 503
Percentage of risk premium ceded		60%		60%
Renegotiation period				5 years
Percent over a target trust balance				102%
Subsidiary - HG Global | Regulation 114
Guarantor Obligations [Line Items]
Assets held-in-trust	288.6	$ 311.2		$ 311.2		288.6
Supplement trust target		603		603
Subsidiary - HG Global | Supplemental Trust
Guarantor Obligations [Line Items]
Assets held-in-trust	568.3	588.5		588.5		568.3
Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company
Guarantor Obligations [Line Items]
Guaranteed contracts, par value			$ 42.5		$ 42.5
Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Issuance of insurance linked securities						$ 150	$ 150	$ 100
Insurance-linked securities, term						12 years	12 years	12 years
Insurance-linked securities, callable term						7 years	5 years	5 years
Percentage of par value of policy reinsured						90%	90%	90%
Reinsurance, limit on total reimbursement						$ 150	$ 150	$ 100
Reinsured amount				$ 125
2018 Covered Portfolio | Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Percentage of financial guaranty policies covered				24%
2021 Covered Portfolio | Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Percentage of financial guaranty policies covered				29%
2022 Covered Portfolio | Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Percentage of financial guaranty policies covered				30%
BAM Surplus Notes | Surplus Notes
Guarantor Obligations [Line Items]
Repayments of unsecured debt	36	0	$ 0	$ 0	$ 0
Accrued interest payment held in trust	1					1
Repayment of principal	24.6
Accrued interest payment held outside of trust	10.4					10.4
Debt	340	340		340		340
Interest receivable	$ 157.9	$ 177.6		$ 177.6		157.9
US Treasury (UST) Interest Rate | BAM Surplus Notes | Surplus Notes
Guarantor Obligations [Line Items]
Basis spread on variable rate				3%
Forecast | US Treasury (UST) Interest Rate | BAM Surplus Notes | Surplus Notes
Guarantor Obligations [Line Items]
Interest rate									8%	7.70%
Maximum | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Reinsurance, maximum amount reinsured						276.7	301.7	276.1
Maximum | Subsidiary - HG Global
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured		15%		15%
Minimum | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Reinsurance obligation						$ 110	$ 135	$ 165

Municipal Bond Guarantee Insu_4

Municipal Bond Guarantee Insurance - Schedule of Municipal Bond Guarantee Insured Obligations (Details) - BAM $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2023 USD ($) contract	Dec. 31, 2022 USD ($) contract
Guarantor Obligations [Line Items]
Contracts outstanding | contract	14,084	13,382
Remaining weighted average contract period (in years)	11 years	10 years 9 months 18 days
Principal	$ 105,918.1	$ 99,996.9
Interest	52,628.8	48,880.6
Total debt service outstanding	158,546.9	148,877.5
Total gross unearned insurance premiums	$ 312.3	$ 298.3

Municipal Bond Guarantee Insu_5

Municipal Bond Guarantee Insurance - Schedule of Municipal Guarantee Insurance Contracts, Premium Received Over Contract Period (Details) - Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company $ in Millions	Sep. 30, 2023 USD ($)
Guarantor Obligations [Line Items]
October 1, 2023 - December 31, 2023	$ 7.3
January 1, 2024 - March 31, 2024	7.2
April 1, 2024 - June 30, 2024	7.1
July 1, 2024 - September 30, 2024	7
October 1, 2024 - December 31, 2024	6.9
Total 2024	28.2
2025	26.4
2026	24.8
2027	23.2
2028	21.4
2029 and thereafter	181
Total gross unearned insurance premiums	$ 312.3

Municipal Bond Guarantee Insu_6

Municipal Bond Guarantee Insurance - Schedule of Net Written Premiums (Details) - HG Global/BAM - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Written premiums:
Direct	$ 16.5	$ 18.4	$ 37.3	$ 44.9
Assumed	0	1.3	0	1.3
Gross written premiums	16.5	19.7	37.3	46.2
Earned premiums:
Direct	7.4	6.5	21.4	22
Assumed	0.5	0.6	1.9	4
Gross earned premiums	$ 7.9	$ 7.1	$ 23.3	$ 26

Earnings Per Share - Basic and

Earnings Per Share - Basic and Diluted Earnings Per Share from Continuing Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 23.6	$ 888.2	$ 222.7	$ 752.4
Less: total income (loss) from discontinued operations, net of tax	0	893.1	0	903.2
Net (income) loss from discontinued operations attributable to noncontrolling interests	0	(0.1)	0	(0.7)
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	23.6	(4.8)	222.7	(150.1)
Allocation of losses (earnings) to participating restricted common shares	(0.3)	0.1	(3.1)	1.8
Basic earnings (losses) per share numerators	23.3	(4.7)	219.6	(148.3)
Diluted earnings (losses) per share numerators	$ 23.3	$ (4.7)	$ 219.6	$ (148.3)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	2,560,500	2,893,800	2,564,200	2,959,300
Average unvested restricted common shares (in shares)	(37,600)	(38,300)	(35,500)	(35,500)
Basic earnings (losses) per share denominator (in shares)	2,522,900	2,855,500	2,528,700	2,923,800
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)	2,560,500	2,893,800	2,564,200	2,959,300
Average unvested restricted common shares (in shares)	(37,600)	(38,300)	(35,500)	(35,500)
Diluted earnings (losses) per share denominator (in shares)	2,522,900	2,855,500	2,528,700	2,923,800
Basic and diluted earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid (in usd per share)	$ 0	$ 0	$ 1	$ 1
Distributed earnings - dividends declared and paid (in usd per share)	0	0	1	1
Undistributed earnings (losses) (in usd per share)	9.19	(1.66)	85.82	(51.73)
Undistributed earnings (losses) (in usd per share)	9.19	(1.66)	85.82	(51.73)
Basic and diluted earnings (losses) per share (in usd per share)	9.19	(1.66)	86.82	(50.73)
Basic and diluted earnings (losses) per share (in usd per share)	$ 9.19	$ (1.66)	$ 86.82	$ (50.73)

Earnings Per Share - Undistribu

Earnings Per Share - Undistributed Net Earnings From Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Earnings Per Share [Abstract]
Basic earnings (losses) per share numerators	$ 23.3	$ (4.7)	$ 219.6	$ (148.3)
Dividends declared, net of restricted common share amounts	0	0	(2.5)	(3)
Total undistributed net earnings (losses), net of restricted common share amounts	$ 23.3	$ (4.7)	$ 217.1	$ (151.3)

Employee Share-Based Incentiv_3

Employee Share-Based Incentive Compensation Plans - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended		36 Months Ended
	Sep. 30, 2023 USD ($) shares	Sep. 30, 2022 USD ($) shares	Sep. 30, 2023 USD ($) shares	Sep. 30, 2022 USD ($) shares	Dec. 31, 2021 USD ($) shares	Jun. 30, 2023 USD ($)	Dec. 31, 2022 USD ($)	Jun. 30, 2022 USD ($)
Performance Share
Share-based compensation arrangement by share-based payment award
Vesting period			3 years
Performance cycle period			3 years
New grants (in shares) | shares					750
Additional compensation cost that would be recognized if all outstanding performance shares vested | $	$ 32.9		$ 32.9
Performance Share | Minimum
Share-based compensation arrangement by share-based payment award
Share-based compensation arrangement by share-based payment award, multiplier			0
Performance Share | Maximum
Share-based compensation arrangement by share-based payment award
Share-based compensation arrangement by share-based payment award, multiplier			2
Restricted Stock
Share-based compensation arrangement by share-based payment award
Vesting period			34 months
Additional compensation cost that would be recognized if all outstanding performance shares vested | $	$ 20.2	$ 19.2	$ 20.2	$ 19.2	$ 15.9	$ 24.3	$ 15.5	$ 23
Issued (in shares) | shares	0	0	10,895	13,225

Employee Share-Based Incentiv_4

Employee Share-Based Incentive Compensation Plans - Performance Shares (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended		9 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Target Performance Shares Outstanding
Beginning of period (in shares)			37,031	39,449	39,449	40,828
Shares paid (in shares)			0	0	(13,350)	(14,625)
New grants (in shares)			0	0	10,895	13,225
Forfeitures and cancellations (in shares)			0	0	37	21
End of period (in shares)			37,031	39,449	37,031	39,449
Accrued Expense
Beginning of period			$ 44.6	$ 41.2	$ 67.5	$ 42.2
Shares paid			0	0	(40.8)	(26.4)
New grants			0	0	0	0
Forfeitures and cancellations			0	(0.1)	0.2	(0.2)
Expense recognized			10.5	11.9	28.2	37.4
End of period			$ 55.1	$ 53	$ 55.1	$ 53
Performance Share
Accrued Expense
Percentage of target payout - minimum actual	200%	172%

Employee Share-Based Incentiv_5

Employee Share-Based Incentive Compensation Plans - Performance Shares Granted Under the WTM Incentive Plan (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	37,595
Assumed forfeitures (in shares)	(564)
Total target performance shares outstanding (in shares)	37,031	37,031	39,449	39,449	39,449	40,828
Accrued Expense	$ 55.9
Assumed forfeitures	(0.8)
Total	$ 55.1	$ 44.6	$ 67.5	$ 53	$ 41.2	$ 42.2
2021 – 2023
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,475
Accrued Expense	$ 30.4
2022 – 2024
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,225
Accrued Expense	$ 22.1
2023 – 2025
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	10,895
Accrued Expense	$ 3.4

Employee Share-Based Incentiv_6

Employee Share-Based Incentive Compensation Plans - Restricted Shares (Details) - Restricted Stock - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Restricted Shares
Beginning of period (in shares)	37,595	38,350	38,350	37,850
Issued (in shares)	0	0	10,895	13,225
Vested (in shares)	0	0	(11,650)	(12,725)
Forfeited (in shares)	0	0	0	0
End of period (in shares)	37,595	38,350	37,595	38,350
Unamortized Issue Date Fair Value
Beginning of period	$ 24.3	$ 23	$ 15.5	$ 15.9
Issued	0	0	16	13.8
Vested	0	0	0	0
Forfeited	0	0	0	0
Expense recognized	(4.1)	(3.8)	(11.3)	(10.5)
End of period	$ 20.2	$ 19.2	$ 20.2	$ 19.2

Noncontrolling Interests (Detai

Noncontrolling Interests (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Noncontrolling Interest [Line Items]
Noncontrolling Equity	$ 215.1	$ 188.1
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling Equity	(3.3)	(0.6)
Ark
Noncontrolling Interest [Line Items]
Noncontrolling Equity	279.8	247.9
Kudu
Noncontrolling Interest [Line Items]
Noncontrolling Equity	94.5	75.1
Other
Noncontrolling Interest [Line Items]
Noncontrolling Equity	10	20.4
Total, excluding BAM
Noncontrolling Interest [Line Items]
Noncontrolling Equity	381	342.8
Variable Interest Entity, Primary Beneficiary
Noncontrolling Interest [Line Items]
Noncontrolling Equity	$ (165.9)	$ (154.7)
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	3.10%	3.10%
Ark
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	28%	28%
Kudu
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	10.80%	10.80%
BAM
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	100%	100%

Segment Information - Narrative

Segment Information - Narrative (Details)	9 Months Ended
	Sep. 30, 2023 segment
Segment Reporting [Abstract]
Number of reportable segments	3

Segment Information - Financial

Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Earned insurance premiums	$ 506.8	$ 353.2	$ 1,070.6	$ 783.8
Net investment income	48.1	33.9	130.1	79.6
Net investment income (expense) - BAM Surplus Note interest	0	0	0	0
Net realized and unrealized investment gains (losses)	(58.3)	(48)	151.2	(255.2)
Commission revenues	3.5	3.2	10	8.7
Other revenues	19.5	40.9	68	103.4
Total revenues	519.6	383.2	1,429.9	720.3
Loss and loss adjustment expenses	265.8	213.7	581.1	456.2
Acquisition expenses	92.4	76.5	217.8	184.4
Cost of sales	8	25	33.5	68.8
General and administrative expenses	99.6	85.8	298.2	256.9
Change in fair value of contingent consideration	17	2.7	16.8	4.9
Interest expense	15.7	10.5	44.9	27.5
Total expenses	498.5	414.2	1,192.3	998.7
Pre-tax income (loss) from continuing operations	21.1	(31)	237.6	(278.4)
Other Operations
Segment Reporting Information [Line Items]
Earned insurance premiums	0	0	0	0
Net investment income	8	8.5	22	13.6
Net investment income (expense) - BAM Surplus Note interest	0	0	0	0
Commission revenues	3.5	3.2	10	8.7
Other revenues	15.2	33	67.1	89.6
Total revenues	(12.1)	8.8	186	1.4
Loss and loss adjustment expenses	0	0	0	0
Acquisition expenses	0	0	0	0
Cost of sales	8	25	33.5	68.8
General and administrative expenses	42.2	41.3	130.7	122.1
Change in fair value of contingent consideration	0	0	0	0
Interest expense	0.9	0.6	2.9	1.2
Total expenses	51.1	66.9	167.1	192.1
Pre-tax income (loss) from continuing operations	(63.2)	(58.1)	18.9	(190.7)
Investments, Excluding MediaAlpha
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(11.5)	(29.4)	190.1	(141.9)
Investments, Excluding MediaAlpha | Other Operations
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	8	(17.3)	125.8	2.8
MediaAlpha
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(46.8)	(18.6)	(38.9)	(113.3)
MediaAlpha | Other Operations
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(46.8)	(18.6)	(38.9)	(113.3)
HG Global/BAM | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	7.9	7.1	23.3	26
Net investment income	8.1	5.7	22.9	15.1
Net realized and unrealized investment gains (losses)	(24.1)	(38.8)	(17)	(114)
Other revenues	0.7	1.3	2	3.7
Total revenues	(7.4)	(24.7)	31.2	(69.2)
Acquisition expenses	2.1	1.7	6.3	9.5
General and administrative expenses	17.5	15.8	49.7	49.5
Interest expense	3.8	2	10.8	5.4
Total expenses	23.4	19.5	66.8	64.4
HG Global/BAM | HG Global | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	6.6	5.9	19.4	21.5
Net investment income	4.3	2.8	12.4	7.1
Net investment income (expense) - BAM Surplus Note interest	6.6	2.9	19.7	8.8
Commission revenues	0	0	0	0
Other revenues	0	0.1	0	0.3
Total revenues	3.9	(7.9)	40.1	(20.1)
Loss and loss adjustment expenses	0	0	0	0
Acquisition expenses	2	1.6	5.6	7.6
Cost of sales	0	0	0	0
General and administrative expenses	0.5	0.6	1.9	2.1
Change in fair value of contingent consideration	0	0	0	0
Interest expense	3.8	2	10.8	5.4
Total expenses	6.3	4.2	18.3	15.1
Pre-tax income (loss) from continuing operations	(2.4)	(12.1)	21.8	(35.2)
HG Global/BAM | HG Global | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(13.6)	(19.6)	(11.4)	(57.8)
HG Global/BAM | HG Global | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0	0	0	0
HG Global/BAM | BAM | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	1.3	1.2	3.9	4.5
Net investment income	3.8	2.9	10.5	8
Net investment income (expense) - BAM Surplus Note interest	(6.6)	(2.9)	(19.7)	(8.8)
Commission revenues	0	0	0	0
Other revenues	0.7	1.2	2	3.4
Total revenues	(11.3)	(16.8)	(8.9)	(49.1)
Loss and loss adjustment expenses	0	0	0	0
Acquisition expenses	0.1	0.1	0.7	1.9
Cost of sales	0	0	0	0
General and administrative expenses	17	15.2	47.8	47.4
Change in fair value of contingent consideration	0	0	0	0
Interest expense	0	0	0	0
Total expenses	17.1	15.3	48.5	49.3
Pre-tax income (loss) from continuing operations	(28.4)	(32.1)	(57.4)	(98.4)
HG Global/BAM | BAM | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(10.5)	(19.2)	(5.6)	(56.2)
HG Global/BAM | BAM | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0	0	0	0
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment Reporting Information [Line Items]
Loss and loss adjustment expenses	265.8	213.7	581.1	456.2
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	498.9	346.1	1,047.3	757.8
Net investment income	16.9	4.9	41.2	9.7
Net realized and unrealized investment gains (losses)	(6.6)	(14.4)	35.9	(76.5)
Other revenues	3.6	6.6	(1.1)	10.1
Total revenues	512.8	343.2	1,123.3	701.1
Loss and loss adjustment expenses	265.8	213.7	581.1	456.2
Acquisition expenses	90.3	74.8	211.5	174.9
General and administrative expenses	35.4	24.2	105.5	74.9
Change in fair value of contingent consideration	17	2.7	16.8	4.9
Interest expense	5.5	3.7	15.7	10.6
Total expenses	414	319.1	930.6	721.5
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	438.3	346.1	971.9	757.8
Net investment income	13.9	4.9	33.5	9.7
Net investment income (expense) - BAM Surplus Note interest	0	0	0	0
Commission revenues	0	0	0	0
Other revenues	3.6	6.6	(1.1)	10.1
Total revenues	449.2	343.2	1,040.2	701.1
Loss and loss adjustment expenses	258.5	213.7	573.2	456.2
Acquisition expenses	71	74.8	189.3	174.9
Cost of sales	0	0	0	0
General and administrative expenses	35.3	24.2	105.3	74.9
Change in fair value of contingent consideration	17	2.7	16.8	4.9
Interest expense	5.5	3.7	15.7	10.6
Total expenses	387.3	319.1	900.3	721.5
Pre-tax income (loss) from continuing operations	61.9	24.1	139.9	(20.4)
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(6.6)	(14.4)	35.9	(76.5)
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0	0	0	0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	60.6		75.4
Net investment income	3		7.7
Net investment income (expense) - BAM Surplus Note interest	0		0
Commission revenues	0		0
Other revenues	0		0
Total revenues	63.6		83.1
Loss and loss adjustment expenses	7.3		7.9
Acquisition expenses	19.3		22.2
Cost of sales	0		0
General and administrative expenses	0.1		0.2
Change in fair value of contingent consideration	0		0
Interest expense	0		0
Total expenses	26.7		30.3
Pre-tax income (loss) from continuing operations	36.9		52.8
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0		0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0		0
Kudu | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	0	0	0	0
Net investment income	15.1	14.8	44	41.2
Net investment income (expense) - BAM Surplus Note interest	0	0	0	0
Net realized and unrealized investment gains (losses)	11.2	41.1	45.4	45.8
Commission revenues	0	0	0	0
Other revenues	0	0	0	0
Total revenues	26.3	55.9	89.4	87
Loss and loss adjustment expenses	0	0	0	0
Acquisition expenses	0	0	0	0
Cost of sales	0	0	0	0
General and administrative expenses	4.5	4.5	12.3	10.4
Change in fair value of contingent consideration	0	0	0	0
Interest expense	5.5	4.2	15.5	10.3
Total expenses	10	8.7	27.8	20.7
Pre-tax income (loss) from continuing operations	16.3	47.2	61.6	66.3
Kudu | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	11.2	41.1	45.4	45.8
Kudu | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	$ 0	$ 0	$ 0	$ 0

Variable Interest Entities (Det

Variable Interest Entities (Details) - Variable Interest Entity, Not Primary Beneficiary $ in Millions	9 Months Ended
	Sep. 30, 2023 USD ($)
PassportCard/DavidShield
Variable Interest Entity [Line Items]
Ownership interest	53.80%
Partnership interest, carrying value	$ 159.4
Elementum Holdings, L.P.
Variable Interest Entity [Line Items]
Ownership interest	26.60%
Partnership interest, carrying value	$ 35
Limited Partnerships
Variable Interest Entity [Line Items]
Partnership interest, carrying value	$ 239.2

Equity Method Eligible Invest_3

Equity Method Eligible Investments - Ownership Interests and Carrying Values (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Kudu's Participation Contracts
Investments in unconsolidated affiliates
Carrying Value	$ 775.3	$ 695.9
Kudu's Participation Contracts | Minimum
Investments in unconsolidated affiliates
Ownership Interest	4.10%	4.10%
Kudu's Participation Contracts | Maximum
Investments in unconsolidated affiliates
Ownership Interest	30%	30%
MediaAlpha
Investments in unconsolidated affiliates
Ownership Interest	35.30%	27.10%
Carrying Value	$ 188.8	$ 168.6
PassportCard/DavidShield
Investments in unconsolidated affiliates
Ownership Interest	53.80%	53.80%
Carrying Value	$ 150	$ 135
Elementum Holdings, L.P.
Investments in unconsolidated affiliates
Ownership Interest	26.60%	29.70%
Carrying Value	$ 35	$ 30
Other equity method eligible investments, at fair value
Investments in unconsolidated affiliates
Carrying Value	279	84.4
Other equity method eligible investments, at fair value
Investments in unconsolidated affiliates
Carrying Value	$ 25.2	$ 0

Equity Method Eligible Invest_4

Equity Method Eligible Investments - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Equity Method Investments and Joint Ventures [Abstract]
Dividend and income distributions from equity method eligible investments	$ 13.2	$ 16.1	$ 42.1	$ 42.9

Fair Value of Financial Instr_3

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Balance Sheet Grouping, Financial Statement Captions
Carrying Value	$ 564.7	$ 575.2
HG Global Senior Notes
Balance Sheet Grouping, Financial Statement Captions
Fair Value	157.6	155.7
Carrying Value	146.8	146.5
Ark 2007 Subordinated Notes
Balance Sheet Grouping, Financial Statement Captions
Fair Value	29.1	28.4
Carrying Value	30	30
Ark 2021 Subordinated Notes
Balance Sheet Grouping, Financial Statement Captions
Fair Value	164.5	163.1
Carrying Value	154.6	153.7
Kudu Credit Facility
Balance Sheet Grouping, Financial Statement Captions
Fair Value	223	223.9
Carrying Value	203.7	208.3
Other Operations debt
Balance Sheet Grouping, Financial Statement Captions
Fair Value	31.1	38.2
Carrying Value	$ 29.6	$ 36.7

Held for Sale and Discontinue_3

Held for Sale and Discontinued Operations - Financial Statement Disclosure (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Aug. 01, 2022	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022	Dec. 31, 2021
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain (loss) from sale of discontinued operations, net of tax - NSM Group		$ 0	$ 886.8	$ 0	$ 886.8
Net income (loss) from discontinued operations, net of tax - NSM Group		0	6.3	0	16.4
Net (income) loss from discontinued operations attributable to noncontrolling interests		0	(0.1)	0	(0.7)
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders		0	893.1	0	903.2
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group		0	0.7	0	(5.2)
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group		0	2.9	0	2.9
Total comprehensive income (loss)		21.9	890.4	223.6	747.1
Other comprehensive (income) loss from discontinued operations attributable to noncontrolling interests		0.7	(0.3)	(0.1)	0.4
Comprehensive income (loss) from discontinued operations attributable to White Mountains’s common shareholders		22.6	890.1	223.5	747.5
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from (used for) operations				0	38.7
Net cash provided from (used for) investing activities				0	7.1
Net cash used from (used for) financing activities				0	(17.5)
Restricted cash balance		0	8.9	0	8.9	$ 12.2	$ 4.5
Discontinued Operations, Disposed of by Sale | NSM Insurance Group
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total revenues			33.6		225
General and administrative expenses	$ 14		15.7		126.8
Broker commission expenses			8.2		52.9
Change in fair value of contingent consideration			0		0.1
Amortization of other intangible assets			0		9.1
Interest expense			1.6		12.1
Total expenses			25.5		201
Pre-tax income (loss) from discontinued operations			8.1		24
Income tax (expense) benefit			(1.8)		(7.6)
Net income (loss) from discontinued operations, net of tax - NSM Group			6.3		16.4
Net gain (loss) from sale of discontinued operations, net of tax - NSM Group	886.8		886.8		886.8
Net income (loss) from discontinued operations, net of tax - NSM Group			893.1		903.2
Net (income) loss from discontinued operations attributable to noncontrolling interests			(0.2)		(0.8)
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders			892.9		902.4
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group			0.7		(5.2)
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	$ 2.9		2.9		2.9
Total comprehensive income (loss)			896.5		900.1
Other comprehensive (income) loss from discontinued operations attributable to noncontrolling interests			(0.1)		0.2
Comprehensive income (loss) from discontinued operations attributable to White Mountains’s common shareholders			896.4		900.3
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from (used for) operations					38.7
Net cash provided from (used for) investing activities					7.1
Net cash used from (used for) financing activities					(17.5)
Effect of exchange rate changes on cash					4
Net change in cash during the period					32.3
Restricted cash balance							$ 89.2
Cash balances at beginning of period (includes restricted cash of $89.2)				0	111.6
Cash sold as part of the sale of NSM Group			(143.9)		(143.9)
Cash balances at end of period		$ 0	0	$ 0	0
Interest paid					(12)
Net income tax payments					0
Discontinued Operations, Disposed of by Sale | NSM Insurance Group | Commission revenues
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total revenues			26.6		176.9
Discontinued Operations, Disposed of by Sale | NSM Insurance Group | Other revenues
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total revenues			$ 7		$ 48.1

Held for Sale and Discontinue_4

Held for Sale and Discontinued Operations - Earnings Per Share for Discontinued Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 23.6	$ 888.2	$ 222.7	$ 752.4
Net income (loss) from continuing operations	$ 13.8	(23.6)	$ 218.2	(252.3)
Less: net (income) loss from continuing operations attributable to noncontrolling interests		18.8		102.2
Total gain (loss) from discontinued operations attributable to White Mountains’s common shareholders		893		902.5
Allocation of earnings to participating restricted common shares		(11.8)		(10.8)
Basic earnings per share numerators		881.2		891.7
Diluted earnings per share numerators		$ 881.2		$ 891.7
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	2,560,500	2,893,800	2,564,200	2,959,300
Average unvested restricted common shares (in shares)	(37,600)	(38,300)	(35,500)	(35,500)
Basic earnings (losses) per share denominator (in shares)	2,522,900	2,855,500	2,528,700	2,923,800
Diluted earnings per share denominator (in thousands):
Average unvested restricted common shares (in shares)	(37,600)	(38,300)	(35,500)	(35,500)
Diluted earnings (losses) per share denominator (in shares)	2,522,900	2,855,500	2,528,700	2,923,800
Basic earnings (losses) per share (in dollars) - discontinued operations (in dollars per share)	$ 0	$ 308.59	$ 0	$ 304.97
Diluted earnings (losses) per share (in dollars) - discontinued operations (in dollars per share)	$ 0	$ 308.59	$ 0	$ 304.97

Subsequent Events (Details)

Subsequent Events (Details) - Subsequent Event - Bamboo $ in Millions	Oct. 19, 2023 USD ($)
Subsequent Event [Line Items]
Percentage of voting interests acquired	70%
Expected consideration	$ 285