White Mountains Insurance (WTM) 10-Q 9 May 24 2024 Q1 Quarterly report Financial data

Cover Page

Cover Page - shares	3 Months Ended
	Mar. 31, 2024	May 06, 2024
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2024
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)		2,565,733
Entity Central Index Key	0000776867
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	Q1
Amendment Flag	false

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Assets
Common equity securities, at fair value	$ 561.5	$ 538.4
Other long-term investments	1,991.9	1,998.2
Goodwill and other intangible assets	736.6	370.6
Total assets	9,590.3	8,385.9
Liabilities
Debt	532.1	534.6
Total liabilities	4,699.7	3,824.3
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 2,565,733 and 2,560,452 shares	2.6	2.6
Paid-in surplus	551.9	548.7
Retained earnings	3,917.7	3,690.8
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax	(1.8)	(1.6)
Accumulated other comprehensive income (loss), after-tax:
Total White Mountains’s common shareholders’ equity	4,470.4	4,240.5
Noncontrolling interests	420.2	321.1
Total equity	4,890.6	4,561.6
Total liabilities and equity	9,590.3	8,385.9
Other Operations
Assets
Fixed maturity investments, at fair value	267.4	230.2
Common equity securities, at fair value	151.6	137.8
Investment in MediaAlpha, at fair value	465.6	254.9
Short-term investments, at fair value	126.1	425.2
Other long-term investments	617	661
Total investments	1,627.7	1,709.1
Cash	23	23.8
Goodwill and other intangible assets	68.7	69.8
Other assets	81.7	73.2
Total assets	1,801.1	1,875.9
Liabilities
Debt	25.9	28.4
Accrued incentive compensation	47.6	87.7
Other liabilities	41.8	25
Total liabilities	115.3	141.1
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liabilities
Loss and loss adjustment expense reserves	1,744.5	1,605.1
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Assets
Fixed maturity investments, at fair value	946.9	866.8
Common equity securities, at fair value	409.9	400.6
Short-term investments, at fair value	835.3	962.8
Other long-term investments	485	440.9
Total investments	2,677.1	2,671.1
Cash	129.1	90.5
Reinsurance recoverables	695	442
Insurance premiums receivable	1,001.5	612.2
Deferred acquisition costs	213.3	145.3
Goodwill and other intangible assets	292.5	292.5
Other assets	140.9	125
Total assets	5,149.4	4,378.6
Liabilities
Loss and loss adjustment expense reserves	1,744.5	1,605.1
Unearned insurance premiums	1,234.7	743.6
Debt	155.4	185.5
Reinsurance payable	262.5	81.1
Contingent consideration	94	94
Other liabilities	190.1	166.8
Total liabilities	3,681.2	2,876.1
Financial Guarantee (HG Global/BAM) | Operating Segments
Assets
Fixed maturity investments, at fair value	1,015.7	1,012.3
Short-term investments, at fair value	54.7	70.6
Total investments	1,070.4	1,082.9
Cash	4.7	6.7
Insurance premiums receivable	5.8	5.5
Deferred acquisition costs	40.9	40.1
Other assets	36.8	36.8
Total assets	1,158.6	1,172
Liabilities
Unearned insurance premiums	328.5	325.8
Debt	147	146.9
Accrued incentive compensation	11.9	27.2
Other liabilities	31.9	31.8
Total liabilities	519.3	531.7
Asset Management (Kudu) | Operating Segments
Assets
Short-term investments, at fair value	19.8	29.3
Other long-term investments	889.9	896.3
Total investments	909.7	925.6
Cash	6.5	1.4
Accrued investment income	22.1	17.6
Goodwill and other intangible assets	8.2	8.3
Other assets	6.6	6.5
Total assets	953.1	959.4
Liabilities
Debt	203.8	203.8
Other liabilities	61.8	71.6
Total liabilities	265.6	275.4
P&C Insurance Distribution (Bamboo) | Operating Segments
Assets
Fixed maturity investments, at fair value	24.6	0
Short-term investments, at fair value	18.6	0
Total investments	43.2	0
Cash	62	0
Premiums, commissions and fees receivable	43.3	0
Goodwill and other intangible assets	367.2	0
Other assets	12.4	0
Total assets	528.1	0
Liabilities
Loss and loss adjustment expense reserves	12.3	0
Unearned insurance premiums	25.7	0
Premiums and commissions payable	61.7	0
Other liabilities	18.6	0
Total liabilities	$ 118.3	$ 0

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
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)	2,565,733	2,560,452
Common shares, outstanding shares (in shares)	2,565,733	2,560,452
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Restricted cash	$ 24.7	$ 0.7
P&C Insurance Distribution (Bamboo) | Operating Segments
Restricted cash	$ 58.3	$ 0

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Revenues:
Earned insurance premiums	$ 319	$ 262.8
Net investment income	57	39
Net realized and unrealized investment gains (losses)	226.8	198.1
Commission revenues	25.5	3.3
Other revenues	19	28.7
Total revenues	647.3	531.9
Expenses:
Loss and loss adjustment expenses	185.8	147.8
Acquisition expenses	71.6	62.5
Cost of sales	7.6	13.9
General and administrative expenses	125.2	96
Broker commission expenses	9.3
Change in fair value of contingent consideration	0	(2.4)
Interest expense	15.2	15
Total expenses	414.7	332.8
Pre-tax income (loss)	232.6	199.1
Income tax (expense) benefit	(10.8)	(11.9)
Net income (loss)	221.8	187.2
Net (income) loss attributable to noncontrolling interests	14.6	(7.7)
Net income (loss) attributable to White Mountains’s common shareholders	236.4	179.5
Other Operations
Revenues:
Earned insurance premiums	0	0
Net investment income	9.9	7
Commission revenues	3.6	3.3
Other revenues	14.4	30.6
Total revenues	260.8	167.9
Expenses:
Loss and loss adjustment expenses	0	0
Acquisition expenses	0	0
Cost of sales	7.6	13.9
General and administrative expenses	50.3	39.7
Broker commission expenses	0
Change in fair value of contingent consideration	0	0
Interest expense	0.7	0.8
Total expenses	58.6	54.4
Pre-tax income (loss)	202.2	113.5
Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	16.1	112.9
Investments, Excluding MediaAlpha | Other Operations
Revenues:
Net realized and unrealized investment gains (losses)	22.2	41.8
MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	210.7	85.2
MediaAlpha | Other Operations
Revenues:
Net realized and unrealized investment gains (losses)	210.7	85.2
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Expenses:
Loss and loss adjustment expenses	180	147.8
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Revenues:
Earned insurance premiums	302.8	255.1
Net investment income	19.9	10.6
Net realized and unrealized investment gains (losses)	10.6	24.5
Other revenues	3.5	(2.7)
Total revenues	336.8	287.5
Expenses:
Loss and loss adjustment expenses	180	147.8
Acquisition expenses	66.3	59.8
General and administrative expenses	42.2	35.2
Change in fair value of contingent consideration	0	(2.4)
Interest expense	5.4	5
Total expenses	293.9	245.4
Financial Guarantee (HG Global/BAM) | Operating Segments
Revenues:
Earned insurance premiums	7.8	7.7
Net investment income	9.7	7.2
Net realized and unrealized investment gains (losses)	(10.1)	17
Other revenues	0.5	0.8
Total revenues	7.9	32.7
Expenses:
Acquisition expenses	2.2	2.7
General and administrative expenses	17.3	17.3
Interest expense	3.5	4.5
Total expenses	23	24.5
Asset Management (Kudu) | Operating Segments
Revenues:
Earned insurance premiums	0	0
Net investment income	17.2	14.2
Net realized and unrealized investment gains (losses)	(6.5)	29.6
Commission revenues	0	0
Other revenues	0	0
Total revenues	10.7	43.8
Expenses:
Loss and loss adjustment expenses	0	0
Acquisition expenses	0	0
Cost of sales	0	0
General and administrative expenses	3.4	3.8
Broker commission expenses	0
Change in fair value of contingent consideration	0	0
Interest expense	5.6	4.7
Total expenses	9	8.5
Pre-tax income (loss)	1.7	35.3
Asset Management (Kudu) | Investments, Excluding MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	(6.5)	29.6
Asset Management (Kudu) | MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	0	0
P&C Insurance Distribution (Bamboo) | Operating Segments
Revenues:
Earned insurance premiums	8.4	0
Net investment income	0.3
Commission revenues	21.9	0
Other revenues	0.8	0
Total revenues	31.1	0
Expenses:
Loss and loss adjustment expenses	5.8	0
Acquisition expenses	3.1	0
Cost of sales	0
General and administrative expenses	12	0
Broker commission expenses	9.3	0
Change in fair value of contingent consideration	0
Interest expense	0
Total expenses	30.2	$ 0
Pre-tax income (loss)	0.9
P&C Insurance Distribution (Bamboo) | Investments, Excluding MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	(0.1)
P&C Insurance Distribution (Bamboo) | MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	$ 0

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Statement of Comprehensive Income [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ 236.4	$ 179.5
Other comprehensive income, net of tax:
Other comprehensive income (loss), net of tax	(0.3)	1.2
Comprehensive income (loss) from discontinued operations attributable to White Mountains’s common shareholders	236.1	180.7
Other comprehensive (income) loss attributable to noncontrolling interests	0.1	(0.4)
Total comprehensive income (loss)	$ 236.2	$ 180.3
Basic earnings (loss) per share
Basic and diluted earnings (losses) per share (in usd per share)	$ 92.33	$ 69.83
Discontinued operations (in usd per share)	0	0
Total consolidated operations (in usd per share)	92.33	69.83
Diluted earnings (loss) per share
Continuing operations (in usd per share)	92.33	69.83
Discontinued operations (in usd per share)	0	0
Total consolidated operations (in usd per share)	92.33	69.83
Dividends declared per White Mountains’s common share (in usd per share)	1	1
Dividends paid per White Mountains’s common share (in usd per share)	$ 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, 2022		$ 3,935	$ 3,746.9	$ 538.6	$ 3,211.8	$ (3.5)	$ 188.1
Increase (Decrease) in Shareholders' Equity
Net income (loss)	$ 179.5	187.2	179.5		179.5		7.7
Other comprehensive income (loss), net of tax		1.2	0.8			0.8	0.4
Total comprehensive income (loss)	180.3	188.4	180.3		179.5	0.8	8.1
Dividends declared on common shares		(2.6)	(2.6)		(2.6)
Dividends to noncontrolling interests		(7.2)					(7.2)
Issuances of common shares		0.1	0.1	0.1
Repurchases and retirements of common shares		(25.3)	(25.3)	(3.9)	(21.4)
BAM member surplus contribution, net of tax		11.8					11.8
Amortization of restricted share awards		3.1	3.1	3.1
Recognition of equity-based compensation expense of subsidiaries		0.6	0.4	0.4			0.2
Net contributions and dilution from other noncontrolling interests		(1.7)	(0.5)	(0.5)			(1.2)
Disposition of noncontrolling interests		(9.4)					(9.4)
Ending Balances at Mar. 31, 2023		4,092.8	3,902.4	537.8	3,367.3	(2.7)	190.4
Beginning Balances at Dec. 31, 2023	4,561.6	4,561.6	4,240.5	551.3	3,690.8	(1.6)	321.1
Increase (Decrease) in Shareholders' Equity
Net income (loss)	236.4	221.8	236.4		236.4		(14.6)
Other comprehensive income (loss), net of tax		(0.3)	(0.2)			(0.2)	(0.1)
Total comprehensive income (loss)	236.2	221.5	236.2		236.4	(0.2)	(14.7)
Dividends declared on common shares		(2.5)	(2.5)		(2.5)
Dividends to noncontrolling interests		(9.8)					(9.8)
Issuances of common shares		0.3	0.3	0.3
Repurchases and retirements of common shares		(8.1)	(8.1)	(1.1)	(7)
BAM member surplus contribution, net of tax		11.4					11.4
Amortization of restricted share awards		3.2	3.2	3.2
Recognition of equity-based compensation expense of subsidiaries		0.8	0.6	0.6			0.2
Net contributions and dilution from other noncontrolling interests		1.1	0.2	0.2			0.9
Acquisition of noncontrolling interests - Bamboo		111.1					111.1
Ending Balances at Mar. 31, 2024	$ 4,890.6	$ 4,890.6	$ 4,470.4	$ 554.5	$ 3,917.7	$ (1.8)	$ 420.2

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Cash flows from operations:
Net income (loss)	$ 221.8	$ 187.2
Adjustments to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses from investment in MediaAlpha	(226.8)	(198.1)
Deferred income tax expense (benefit)	(9.8)	(3.6)
Amortization of restricted share awards	3.2	3.1
Amortization (accretion) and depreciation	(2.1)	(2.4)
Change in fair value of contingent consideration	0	(2.4)
Other operating items:
Net change in reinsurance recoverables	(253)	24.7
Net change in insurance premiums, commissions and fees receivable	(394.9)	(365.3)
Net change in deferred acquisition costs	(68.8)	(82.2)
Net change in loss and loss adjustment expense reserves	142.4	49.1
Net change in unearned insurance premiums	499.4	501.8
Net change in reinsurance payable	181.4	(58.1)
Net change in premiums and commissions payable	12.5	0
Contributions to Kudu’s Participation Contracts	0	(30.9)
Proceeds from Kudu’s Participation Contracts sold	0	62.9
Net other operating activities	(68.7)	(0.2)
Net cash provided from (used for) operations	36.5	85.6
Cash flows from investing activities:
Net change in short-term investments	441.3	47.6
Sales of fixed maturity investments	185.8	54.6
Maturities, calls and paydowns of fixed maturity investments	81.8	66.8
Distributions and redemptions of other long-term investments	91.5	2
Proceeds from the sale of Other Operating Businesses, net of cash sold of $0.0 and $0.8	0	17.3
Purchases of consolidated subsidiaries, net of cash acquired of $44.9 and $0.0	(231.8)	0
Purchases of fixed maturity investments	(417.3)	(127.6)
Purchases of other long-term investments	(79.9)	(146.1)
Net other investing activities	29.2	(11.2)
Net cash provided from (used for) investing activities	100.6	(96.6)
Cash flows from financing activities:
Repayment of debt and revolving lines of credit	(32.6)	(20)
Cash dividends paid to common shareholders	(2.5)	(2.6)
Repurchases and retirements of common shares	(8.1)	(25.3)
BAM member surplus contributions	11.4	11.8
Net contributions from (distributions to) other noncontrolling interests	1.1	(1.7)
Fidus Re premium payments	(3.5)	(3.5)
Net cash provided from (used for) financing activities	(34.2)	(41.3)
Net change in cash during the period	102.9	(52.3)
Cash balances at beginning of period (includes restricted cash balances of $0.7 and $12.2)	122.4	255
Cash balances at end of period (includes restricted cash balances of $83.0 and $13.0)	225.3	202.7
Supplemental cash flows information:
Interest paid	(16.7)	(5.8)
Net income tax payments	(5.3)	(17.8)
Investments, Excluding MediaAlpha and Bamboo
Adjustments to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses from investment in MediaAlpha	(16.2)	(112.9)
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	$ (210.7)	$ (85.2)

CONSOLIDATED STATEMENTS OF CA_2

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Statement of Cash Flows [Abstract]
Cash sold in subsidiary transaction	$ 0	$ 0.8
Cash acquired from acquisition	44.9	0
Restricted cash balance	$ 83	$ 13	$ 0.7	$ 12.2

Basis of Presentation and Signi

Basis of Presentation and Significant Accounting Policies	3 Months Ended
	Mar. 31, 2024
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 2023 Annual Report on Form 10-K. Reportable Segments As of March 31, 2024, White Mountains conducted its operations through four reportable segments: (1) Ark/WM Outrigger, (2) HG Global/BAM, (3) Kudu and (4) Bamboo, 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 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, accident & health and casualty. 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 March 31, 2024 and December 31, 2023, White Mountains owned 72.0% of Ark on a basic shares outstanding basis (61.9% after taking account of management’s equity incentives). The remaining shares are owned by current and former employees of Ark. In the future, management rollover shareholders could earn additional shares in Ark if and to the extent that White Mountains achieves certain thresholds for its multiple of invested capital return. 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. 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. White Mountains consolidates the results of its segregated account, WM Outrigger Re, in its financial statements. See Note 2 — “Significant Transactions.” As of March 31, 2024 and December 31, 2023, White Mountains owned 100.0% of WM Outrigger Re’s preferred equity. 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 whose debt issuances are insured by BAM. 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 March 31, 2024 and December 31, 2023, 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 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 March 31, 2024 and December 31, 2023, White Mountains owned 89.4% and 89.6% of Kudu’s basic units outstanding (76.2% and 76.3% on a fully-diluted, fully-converted basis). The Bamboo segment consists of PM Holdings LLC (“Bamboo Holdings”), Bamboo Ide8 Insurance Services LLC (“Bamboo MGA”) and Ide8 Limited (“Bamboo Captive”) (collectively with Bamboo Holdings and Bamboo MGA, “Bamboo”). Bamboo is a capital-light, tech- and data-enabled insurance distribution platform providing homeowners’ insurance and related products to the residential property market in California. Bamboo operates primarily through Bamboo MGA, its full-service managing general agent (“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 it earns commissions based on the volume and profitability of the insurance that it places. It offers both admitted and non-admitted products. Bamboo also operates two separate but integrated businesses: (i) a retail agency, within Bamboo MGA, offering ancillary products on behalf of third parties and (ii) Bamboo Captive, a Bermuda-domiciled captive reinsurer that participates in the underwriting risk of Bamboo’s MGA programs to align interests with reinsurance partners. On January 2, 2024, White Mountains acquired a controlling interest in Bamboo. See Note 2 — “Significant Transactions” . As of March 31, 2024, White Mountains owned 72.8% of the basic units outstanding of Bamboo (63.7% on a fully-diluted, fully-converted basis). White Mountains’s other operations consist 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 (collectively, “Other Operations”). On October 25, 2023, White Mountains announced the launch of White Mountains Partners LLC (“WTM Partners”), which will acquire businesses in non-insurance, non-financial services sectors including essential services, light industrial and specialty consumer. White Mountains expects to deploy up to $500.0 million of equity capital through WTM Partners over time. WTM Partners is included within Other Operations. Significant Accounting Policies In addition to the following, refer to the Notes to Consolidated Financial Statements in the Company’s 2023 Annual Report on Form 10-K for a complete discussion regarding White Mountains’s significant accounting policies.

Significant Transactions

Significant Transactions	3 Months Ended
	Mar. 31, 2024
Significant Transactions [Abstract]
Significant Transactions	Significant Transactions Bamboo Transaction On October 19, 2023, White Mountains entered into an agreement and plan of merger (the “Bamboo Merger Agreement”) with Bamboo MGA and John Chu, as the unitholders’ representative. Under the terms of the Bamboo Merger Agreement, White Mountains’s wholly-owned subsidiary, WM Pierce Merger Sub LLC, agreed to merge with and into Bamboo MGA, with Bamboo MGA continuing as the surviving company (the “Bamboo Merger”). Concurrently with the execution of the Bamboo Merger Agreement, certain Bamboo management unitholders agreed to roll over the majority of their existing equity in Bamboo MGA into Bamboo Holdings. White Mountains also agreed to make an equity contribution to Bamboo immediately following the Bamboo Merger (together with the Bamboo Merger and the rollover transactions, the “Bamboo Transaction”). On January 2, 2024, White Mountains closed the Bamboo Transaction in accordance with the terms of the Bamboo Merger Agreement, investing $296.7 million of equity into Bamboo, which included the contribution of $36.0 million to retire Bamboo’s legacy credit facility and the contribution of $20.0 million of primary capital. The consideration is subject to customary purchase price adjustments. At closing, White Mountains owned 72.8% of Bamboo on a basic shares outstanding basis (63.7% on a fully-diluted/fully-converted basis, taking account of management’s equity incentives), while Bamboo management owned 16.1% of basic shares outstanding (26.6% on a fully-diluted/fully-converted basis). White Mountains recognized total assets acquired related to the Bamboo Transaction of $479.5 million, total liabilities assumed of $91.7 million and noncontrolling interest of $111.1 million reflecting acquisition date fair values. Total assets acquired included $371.4 million of goodwill and other intangible assets. In connection with the acquisition, White Mountains incurred transaction costs of $4.0 million in Other Operations, of which $0.3 million were expensed in the first quarter of 2024. The following presents additional details of the assets acquired and liabilities assumed as of the January 2, 2024 acquisition date: Millions As of January 2, 2024 Fixed maturity investments, at fair value $ 8.2 Short-term investments, at fair value 9.3 Cash (restricted $37.0) 44.9 (1) Premiums and commissions receivable 38.0 Other assets 7.7 Total tangible assets 108.1 Loss and loss adjustment expense reserves (9.3) Unearned insurance premiums (20.1) Premiums and commissions payable (49.2) Other liabilities (13.1) Total tangible liabilities (91.7) Net tangible assets acquired 16.4 Goodwill 270.4 Other intangible assets 101.0 Total goodwill and other intangible assets 371.4 Net assets acquired $ 387.8 (1) Cash excludes the White Mountains cash contribution of $20.0 as part of the Bamboo Transaction, which is not part of the purchase consideration. The values of net tangible assets acquired and the resulting goodwill and other intangible assets were recorded at fair value using Level 3 inputs. The majority of the tangible assets acquired and liabilities assumed were recorded at their carrying values, as their carrying values approximated their fair values due to their short-term nature. The fair values of other intangible assets were internally estimated based primarily on the income approach. The income approach estimates fair value based on the present value of the cash flows that the assets are expected to generate in the future. White Mountains developed internal estimates for the expected future cash flows and discount rates used in the present value calculations. See Note 4 — “Goodwill and Other Intangible Assets.” The value of the noncontrolling interest is recorded at the acquisition date fair value, based on the valuation implied in the Bamboo Transaction. Bamboo’s segment income and expenses for three months ended March 31, 2024 are presented in Note 14 — “Segment Information.” 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. Outrigger Re Ltd. was capitalized with $250.0 million of preference shares for business written in the 2023 underwriting year, of which White Mountains contributed $205.0 million. The remaining capital was provided by unrelated third-party investors. 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 the 2023 underwriting year. 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 of Outrigger Re Ltd. During the fourth quarter of 2023, Ark renewed Outrigger Re Ltd. for the 2024 underwriting year with $250.0 million of capital, of which White Mountains rolled over $130.0 million from its commitment to the 2023 underwriting year. The remaining capital was provided by new and continuing unrelated third-party investors. As of March 31, 2024 and December 31, 2023, short-term investments of $222.4 million and $265.3 million were held in a collateral trust account required to be maintained in relation to WM Outrigger Re’s reinsurance agreement with GAIL. White Mountains owns 100% of the preference shares linked to its segregated account, WM Outrigger Re. 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. During January 2024, White Mountains received an initial return of capital of $68.1 million from WM Outrigger Re for business written in the 2023 underwriting year. White Mountains expects to receive any additional return of capital and associated profits from business written in the 2023 underwriting year beginning in the third quarter of 2024.

Investment Securities

Investment Securities	3 Months Ended
	Mar. 31, 2024
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. White Mountains’s portfolio of fixed maturity investments, including those within short-term investments, are classified as trading securities. Trading securities are reported at fair value as of the balance sheet date. Short-term investments also include interest-bearing money market funds and certificates of deposit that are carried at fair value. White Mountains’s portfolio of common equity securities, its investment in MediaAlpha and other long-term investments are measured at fair value. 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, ILS funds and private debt instruments. White Mountains has taken the fair value option for its equity method eligible investments. See Note 16 — “ Equity Method Eligible Investments ”. Net realized and unrealized investment gains (losses) are reported in pre-tax revenues. 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 and distributions from other long-term investments. The following table presents pre-tax net investment income for the three months ended March 31, 2024 and 2023: Three Months Ended March 31, Millions 2024 2023 Fixed maturity investments $ 20.9 $ 14.5 Short-term investments 15.7 9.7 Common equity securities .5 1.3 Other long-term investments 20.6 14.1 Total investment income 57.7 39.6 Third-party investment expenses (.7) (.6) Net investment income, pre-tax $ 57.0 $ 39.0 Net Realized and Unrealized Investment Gains (Losses) The following table presents net realized and unrealized investment gains (losses) for the three months ended March 31, 2024 and 2023: Three Months Ended March 31, Millions 2024 2023 Realized investment gains (losses) Fixed maturity investments $ (5.8) $ (.1) Short-term investments (.2) (.4) Other long-term investments .3 47.2 Net realized investment gains (losses) (5.7) 46.7 Unrealized investment gains (losses) Fixed maturity investments (7.4) 29.7 Short-term investments (1.0) 1.2 Common equity securities 23.1 29.4 Investment in MediaAlpha 210.7 85.2 Other long-term investments 7.1 5.9 Net unrealized investment gains (losses) 232.5 151.4 Net realized and unrealized investment gains (losses) (1) $ 226.8 $ 198.1 Fixed maturity and short-term investments Net realized and unrealized investment gains (losses) $ (14.4) $ 30.4 Less: net realized and unrealized gains (losses) on investment (.7) 1.8 Net unrealized investment gains (losses) recognized during the period on $ (13.7) $ 28.6 Common equity securities and investment in MediaAlpha Net realized and unrealized investment gains (losses) on common $ 23.1 $ 29.4 Net realized and unrealized investment gains (losses) from 210.7 85.2 Total net realized and unrealized investment gains (losses) 233.8 114.6 Less: net realized and unrealized gains (losses) on investment — — Net unrealized investment gains (losses) recognized during the period on $ 233.8 $ 114.6 (1) For the three months ended March 31, 2024 and 2023, includes $(11.5) and $5.4 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 months ended March 31, 2024 and 2023 for investments still held at the end of the period: Three Months Ended March 31, Millions 2024 2023 Total net unrealized investment gains on other long-term investments held at the end of the period, pre-tax $ (6.2) $ 37.0 Investment Holdings The following tables present the cost or amortized cost, gross unrealized investment gains (losses), net foreign currency gains (losses) and carrying value of White Mountains’s fixed maturity investments as of March 31, 2024 and December 31, 2023: March 31, 2024 Millions Cost or Gross Gross Net Foreign Carrying U.S. Government and agency obligations $ 247.6 $ .1 $ (1.8) $ — $ 245.9 Debt securities issued by corporations 1,118.3 3.0 (46.0) (2.2) 1,073.1 Municipal obligations 251.4 .6 (13.0) — 239.0 Mortgage and asset-backed securities 484.8 .6 (35.3) — 450.1 Collateralized loan obligations 248.9 .7 (1.5) (1.6) 246.5 Total fixed maturity investments $ 2,351.0 $ 5.0 $ (97.6) $ (3.8) $ 2,254.6 December 31, 2023 Millions Cost or Gross Gross Net Foreign Carrying U.S. Government and agency obligations $ 209.0 $ .3 $ (4.4) $ — $ 204.9 Debt securities issued by corporations 1,085.9 5.6 (45.4) (1.1) 1,045.0 Municipal obligations 275.1 .9 (15.1) — 260.9 Mortgage and asset-backed securities 417.2 2.0 (29.8) — 389.4 Collateralized loan obligations 211.2 .4 (2.4) (.1) 209.1 Total fixed maturity investments $ 2,198.4 $ 9.2 $ (97.1) $ (1.2) $ 2,109.3 The following table presents the cost or amortized cost and carrying value of White Mountains’s fixed maturity investments by contractual maturity as of March 31, 2024. Actual maturities could differ from contractual maturities because borrowers may have the right to call or prepay certain obligations with or without penalties. March 31, 2024 Millions Cost or Amortized Cost Carrying Value Due in one year or less $ 300.5 $ 296.1 Due after one year through five years 886.3 856.8 Due after five years through ten years 295.7 277.8 Due after ten years 134.8 127.3 Mortgage and asset-backed securities and collateralized loan obligations 733.7 696.6 Total fixed maturity investments $ 2,351.0 $ 2,254.6 The following tables present the cost or amortized cost, gross unrealized investment gains (losses), net foreign currency gains (losses) and carrying value of common equity securities, White Mountains’s investment in MediaAlpha and other long-term investments as of March 31, 2024 and December 31, 2023: March 31, 2024 Millions Cost or Gross Gross Net Foreign Carrying Common equity securities $ 483.5 $ 86.0 $ (1.2) $ (6.8) $ 561.5 Investment in MediaAlpha $ 59.2 $ 406.4 $ — $ — $ 465.6 Other long-term investments $ 1,642.2 $ 466.7 $ (101.1) $ (15.9) $ 1,991.9 December 31, 2023 Millions Cost or Gross Gross Net Foreign Carrying Common equity securities $ 483.5 $ 62.3 $ (1.2) $ (6.2) $ 538.4 Investment in MediaAlpha $ 59.2 $ 195.7 $ — $ — $ 254.9 Other long-term investments $ 1,655.7 $ 446.3 $ (94.5) $ (9.3) $ 1,998.2 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.” Fair Value Measurement By Level The following tables present White Mountains’s fair value measurements for investments as of March 31, 2024 and December 31, 2023 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. March 31, 2024 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 245.9 $ 245.1 $ .8 $ — Debt securities issued by corporations: Financials 323.7 — 323.7 — Consumer 219.3 — 219.3 — Industrial 128.2 — 128.2 — Healthcare 121.9 — 121.9 — Technology 101.6 — 101.6 — Utilities 68.8 — 68.8 — Communications 41.3 — 41.3 — Energy 37.5 — 37.5 — Materials 30.8 — 30.8 — Total debt securities issued by corporations 1,073.1 — 1,073.1 — Municipal obligations 239.0 — 239.0 — Mortgage and asset-backed securities 450.1 — 450.1 — Collateralized loan obligations 246.5 — 246.5 — Total fixed maturity investments 2,254.6 245.1 2,009.5 — Short-term investments 1,054.5 1,031.1 23.4 — Common equity securities: Exchange-traded funds 151.6 151.6 — — Other (1) 409.9 — 409.9 — Total common equity securities 561.5 151.6 409.9 — Investment in MediaAlpha 465.6 465.6 — — Other long-term investments 1,155.9 — 27.1 1,128.8 Other long-term investments — NAV (2) 836.0 — — — Total other long-term investments 1,991.9 — 27.1 1,128.8 Total investments $ 6,328.1 $ 1,893.4 $ 2,469.9 $ 1,128.8 (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, 2023 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 204.9 $ 204.9 $ — $ — Debt securities issued by corporations: Financials 309.8 — 309.8 — Consumer 214.6 — 214.6 — Industrial 121.6 — 121.6 — Healthcare 121.3 — 121.3 — Technology 108.9 — 108.9 — Utilities 70.2 — 70.2 — Communications 46.3 — 46.3 — Energy 28.9 — 28.9 — Materials 23.4 — 23.4 — Total debt securities issued by corporations 1,045.0 — 1,045.0 — Municipal obligations 260.9 — 260.9 — Mortgage and asset-backed securities 389.4 — 389.4 — Collateralized loan obligations 209.1 — 209.1 — Total fixed maturity investments 2,109.3 204.9 1,904.4 — Short-term investments 1,487.9 1,457.6 30.3 — Common equity securities: Exchange-traded funds 137.8 137.8 — — Other (1) 400.6 — 400.6 — Total common equity securities 538.4 137.8 400.6 — Investment in MediaAlpha 254.9 254.9 — — Other long-term investments 1,164.4 — 26.2 1,138.2 Other long-term investments — NAV (2) 833.8 — — — Total other long-term investments 1,998.2 — 26.2 1,138.2 Total investments $ 6,388.7 $ 2,055.2 $ 2,361.5 $ 1,138.2 (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 March 31, 2024 and December 31, 2023, investments of $589.6 million and $585.6 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 March 31, 2024 and December 31, 2023, the fair value of the interest reserve account, which is included in short-term investments, was $30.5 million and $30.4 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 $4.7 million and $4.6 million as of March 31, 2024 and December 31, 2023. 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 March 31, 2024 and December 31, 2023, Ark held Lloyd’s trust deposits with a fair value of $150.5 million and $158.0 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 March 31, 2024 and December 31, 2023, the fair value of Ark’s Funds at Lloyd’s investment deposits totaled $345.6 million and $344.7 million. As of March 31, 2024 and December 31, 2023, Ark held additional investments on deposit or as collateral for insurance regulators and reinsurance counterparties of $204.6 million and $244.5 million. As of March 31, 2024 and December 31, 2023, Ark had $201.9 million and $198.9 million of short-term investments pledged as collateral under uncommitted standby letters of credit. See Note 7 — “Debt.” As of March 31, 2024 and December 31, 2023, short-term investments of $222.4 million and $265.3 million were held in a collateral trust account required to be maintained in relation to WM Outrigger Re’s reinsurance agreement with GAIL. As of March 31, 2024, fixed maturity and short-term investments of $20.7 million were held as collateral required to be maintained in relation to the Bamboo Captive’s reinsurance agreement. Kudu is required to maintain an interest reserve account in connection with its credit facility. As of March 31, 2024, the interest reserve account of $15.1 million was held in short term investments. As of December 31, 2023, the interest reserve account of $14.9 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 March 31, 2024 and December 31, 2023: Fair Value at Millions March 31, 2024 December 31, 2023 AAA $ 11.4 $ 11.5 AA 83.5 83.8 A 562.4 552.4 BBB 407.5 390.9 BB 1.8 — Other 6.5 6.4 Debt securities issued by corporations (1) $ 1,073.1 $ 1,045.0 (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 March 31, 2024 and December 31, 2023: March 31, 2024 December 31, 2023 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: FNMA $ 216.7 $ 216.7 $ — $ 198.8 $ 198.8 $ — FHLMC 147.8 147.8 — 137.2 137.2 — GNMA 39.9 39.9 — 30.2 30.2 — Total agency (1) 404.4 404.4 — 366.2 366.2 — Non-agency: Residential .1 .1 — .2 .2 — Total non-agency .1 .1 — .2 .2 — Total mortgage-backed securities 404.5 404.5 — 366.4 366.4 — Other asset-backed securities: Vehicle receivables 31.1 31.1 — 17.2 17.2 — Credit card receivables 3.3 3.3 — 3.3 3.3 — Other 11.2 11.2 — 2.5 2.5 — Total other asset-backed securities 45.6 45.6 — 23.0 23.0 — Total mortgage and asset-backed securities 450.1 450.1 — 389.4 389.4 — Collateralized loan obligations 246.5 246.5 — 209.1 209.1 — Total mortgage and asset-backed securities $ 696.6 $ 696.6 $ — $ 598.5 $ 598.5 $ — (1) Represents publicly traded mortgage-backed securities which carry the full faith and credit guaranty of the U.S. Government (i.e., GNMA) or are guaranteed by a government sponsored entity (i.e., FNMA, FHLMC). As of March 31, 2024 and December 31, 2023, White Mountains’s investment portfolio included $246.5 million and $209.1 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 March 31, 2024. 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 March 31, 2024, White Mountains owned 22.9 million shares, representing a 34.7% basic ownership interest (32.4% on a fully-diluted/fully-converted basis). See Note 16 — “Equity Method Eligible Investments.” At White Mountains’s March 31, 2024 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 March 28, 2024 closing price of $20.37 per share, the fair value of White Mountains’s investment in MediaAlpha was $465.6 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 March 31, 2024 and December 31, 2023: Fair Value at March 31, 2024 Millions Ark/ WM Outrigger Kudu Other Total Kudu’s Participation Contracts $ — $ 884.2 $ — $ 884.2 PassportCard/DavidShield — — 150.0 150.0 Elementum Holdings, L.P. — — 35.0 35.0 Other unconsolidated entities (1) — — 47.1 47.1 Total unconsolidated entities — 884.2 232.1 1,116.3 Private equity funds and hedge funds 52.5 — 265.3 317.8 Bank loan fund 249.6 — — 249.6 Lloyd’s trust deposits 150.5 — — 150.5 ILS funds — — 109.7 109.7 Private debt instruments — 5.7 9.9 15.6 Other 32.4 — — 32.4 Total other long-term investments $ 485.0 $ 889.9 $ 617.0 $ 1,991.9 (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, 2023 Millions Ark/ WM Outrigger Kudu Other Total Kudu’s Participation Contracts $ — $ 890.5 $ — $ 890.5 PassportCard/DavidShield — — 150.0 150.0 Elementum Holdings, L.P. — — 35.0 35.0 Other unconsolidated entities (1) — — 48.1 48.1 Total unconsolidated entities — 890.5 233.1 1,123.6 Private equity funds and hedge funds 55.5 — 257.4 312.9 Bank loan fund 194.4 — — 194.4 Lloyd’s trust deposits 158.0 — — 158.0 ILS funds — — 160.5 160.5 Private debt instruments — 5.8 10.0 15.8 Other 33.0 — — 33.0 Total other long-term investments $ 440.9 $ 896.3 $ 661.0 $ 1,998.2 (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 March 31, 2024, White Mountains held investments in sixteen private equity funds and two hedge funds. The largest investment in a single private equity fund or hedge fund was $59.7 million and $57.3 million as of March 31, 2024 and December 31, 2023. 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 March 31, 2024 and December 31, 2023: March 31, 2024 December 31, 2023 Millions Fair Value Unfunded Fair Value Unfunded Private equity funds Aerospace/Defense/Government $ 163.5 $ 14.8 $ 157.2 $ 17.4 Financial services 88.0 35.5 87.5 37.8 Real estate 3.7 2.4 3.9 2.5 Total private equity funds 255.2 52.7 248.6 57.7 Hedge funds Long/short equity financials and business services 57.7 — 54.4 — European small/mid cap 4.9 — 9.9 — Total hedge funds 62.6 — 64.3 — Total private equity funds and hedge funds included in other long-term investments $ 317.8 $ 52.7 $ 312.9 $ 57.7 Investments in private equity funds are generally subject to a lock-up period during which investors may not request a redemption. Distributions prior to the expected termination date of the fund may be limited to dividends or proceeds arising from the liquidation of the fund’s underlying investments. In addition, certain private equity funds have the option to extend the lock-up period. The following table presents investments in private equity funds that were subject to lock-up periods as of March 31, 2024: Millions 1 – 3 years 3 – 5 years 5 – 10 years >10 years Total Private equity funds — expected lock-up period remaining $14.4 $63.5 $161.0 $16.3 $255.2 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 March 31, 2024 and December 31, 2023, 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 $249.6 million and $194.4 million as of March 31, 2024 and December 31, 2023. 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 March 31, 2024 and December 31, 2023, White Mountains held Lloyd’s trust deposits with a fair value of $150.5 million and $158.0 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 March 31, 2024 and December 31, 2023, White Mountains held investments in ILS funds with a fair value of $109.7 million and $160.5 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 is generally at the end of the defined redemption period or when the underlying investment has fully matured or been commuted. Rollforward of Level 3 Investments Level 3 measurements as of March 31, 2024 and 2023 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 three months ended March 31, 2024 and 2023: Level 3 Investments Millions Other Long-term Other Long-term Balance at December 31, 2023 $ 1,138.2 Balance at December 31, 2022 $ 911.6 Net realized and unrealized gains (5.7) Net realized and unrealized gains 35.1 Purchases and contributions .1 Purchases and contributions 117.0 Sales and distributions (3.8) Sales and distributions (109.0) Transfers in — Transfers in — Transfers out — Transfers out — Balance at March 31, 2024 $ 1,128.8 Balance at March 31, 2023 $ 954.7 Fair Value Measurements — Transfers Between Levels - Three months ended March 31, 2024 and 2023 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 three months ended March 31, 2024 and 2023, 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 three months ended March 31, 2024 and 2023, 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 March 31, 2024 and December 31, 2023. The tables below exclude $15.7 million and $19.0 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 March 31, 2024 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 $884.2 18% - 25% 7x - 22x PassportCard/DavidShield Discounted cash flow $150.0 24% 4% Elementum Holdings, L.P. Discounted cash flow $35.0 21% 4% Preferred Securities Discounted cash flow $28.7 8% N/A Private debt instruments Discounted cash flow $15.2 11% - 12% 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 20% and 13x. (4) In the first three months of 2024, Kudu had negligible deployments into new and existing Participation Contracts. (5) As of March 31, 2024, one of Kudu’s Participation Contracts with a total fair value of $62.4 was 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. $ in Millions December 31, 2023 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 $890.5 18% - 25% 7x - 22x PassportCard/DavidShield Discounted cash flow $150.0 24% 4% Elementum Holdings, L.P. Discounted cash flow $35.0 21% 4% Preferred securities Discounted cash flow $28.2 8% N/A Private debt instruments Discounted cash flow $15.5 11% - 12% 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 20% and 13x. (4) In 2023, Kudu deployed a total of $163.8 into new and existing Participation Contracts. (5) As of December 31, 2023, one of Kudu’s Participation Contracts with a total fair value of $69.1 was 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 A

Goodwill and Other Intangible Assets	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024 and December 31, 2023: $ in Millions Weighted Average Economic March 31, 2024 December 31, 2023 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 Bamboo (1) N/A 270.4 — 270.4 — — — Other Operations N/A 44.4 — 44.4 44.4 — 44.4 Total goodwill 439.2 — 439.2 168.8 — 168.8 Other intangible assets: Ark Underwriting capacity N/A 175.7 — 175.7 175.7 — 175.7 Kudu Trade names 7.0 2.2 1.6 .6 2.2 1.5 .7 Bamboo (1) Trade names 10.0 23.5 .5 23.0 — — — Agency relationships 6.0 72.4 2.9 69.5 — — — Developed Technology 3.0 4.7 .4 4.3 — — — Other 0.3 .4 .4 — — — — Subtotal 101.0 4.2 96.8 — — — Other Operations Trade names 13.3 13.3 4.5 8.8 13.3 4.1 9.2 Customer relationships 10.9 24.8 11.1 13.7 24.8 10.3 14.5 Other 11.1 3.1 1.3 1.8 2.8 1.1 1.7 Subtotal 41.2 16.9 24.3 40.9 15.5 25.4 Total other intangible assets 320.1 22.7 297.4 218.8 17.0 201.8 Total goodwill and other intangible assets $ 759.3 $ 22.7 $ 736.6 $ 387.6 $ 17.0 $ 370.6 (1) The relative fair values of goodwill and other intangible assets recognized in connection with the acquisition of Bamboo had not yet been finalized as of March 31, 2024. 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, agency relationships 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, patents and certain information technology platforms. 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 non-compete agreements. 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 indicate an impairment may exist and, if necessary will perform an interim review for potential impairment. During the three months ended March 31, 2024 and 2023, 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 months ended March 31, 2024 and 2023: Three Months Ended March 31, 2024 2023 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 $ 201.8 $ 370.6 $ 176.5 $ 215.9 $ 392.4 Acquisition of businesses (1) 270.4 101.0 371.4 — — — Acquisitions of intangible assets (2) — .3 .3 — — — Dispositions (2) — — — (6.7) (6.9) (13.6) Measurement period adjustments (3) — — — (1.0) — (1.0) Amortization — (5.7) (5.7) — (1.6) (1.6) Ending balance $ 439.2 $ 297.4 $ 736.6 $ 168.8 $ 207.4 $ 376.2 (1) Amounts relate to the fair values of goodwill and other intangible assets recognized in connection with the acquisition of Bamboo, which had not yet been finalized as of March 31, 2024. (2) Relates to acquisitions and dispositions within Other Operations. (3) Measurement period adjustments relate to updated information about acquisition date fair values of assets acquired and liabilities assumed. During the three months ended March 31, 2023, adjustments relate to an acquisition within Other Operations.

Loss and Loss Adjustment Expens

Loss and Loss Adjustment Expense Reserves	3 Months Ended
	Mar. 31, 2024
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 months ended March 31, 2024 and 2023: Three Months Ended March 31, Millions 2024 2023 Gross beginning balance $ 1,605.1 $ 1,296.5 Less: beginning reinsurance recoverable on unpaid losses (1) (340.8) (505.0) Net loss and loss adjustment expense reserves 1,264.3 791.5 Loss and loss adjustment expenses incurred relating to: Current year losses 180.3 139.2 Prior year losses (.3) 8.6 Net incurred loss and loss adjustment expenses 180.0 147.8 Loss and loss adjustment expenses paid relating to: Current year losses (4.0) (4.0) Prior year losses (96.7) (87.7) Net paid loss and loss adjustment expenses (100.7) (91.7) Change in TPC Providers’ participation (2) — 145.4 Foreign currency translation and other adjustments to loss and loss adjustment (3.6) 3.7 Net ending balance 1,340.0 996.7 Plus: ending reinsurance recoverable on unpaid losses 404.5 348.9 Gross ending balance $ 1,744.5 $ 1,345.6 (1) The beginning reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $145.4 as of December 31, 2022. (2) Amount represents the impact to net loss and loss adjustment expense reserves due to a change in the TPC Providers’ participation related to the annual RITC process. For the three months ended March 31, 2024, the Ark/WM Outrigger segment recognized $0.3 million of net favorable prior year loss reserve development. For the three months ended March 31, 2023, the Ark/WM Outrigger segment recognized $8.6 million of net unfavorable prior year loss reserve development at Ark, driven primarily by Winter Storm Elliot. 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 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. 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%. Financial Guarantee HG Re and BAM do not have any outstanding loss and LAE reserves related to BAM’s municipal bond guarantee insurance business. P&C Insurance Distribution As of March 31, 2024, Bamboo has recorded loss and LAE reserves of $12.3 million. For the three months ended March 31, 2024, Bamboo recognized incurred loss and LAE of $5.8 million.

Third-Party Reinsurance

Third-Party Reinsurance	3 Months Ended
	Mar. 31, 2024
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 loss and LAE for the Ark/WM Outrigger segment for the three months ended March 31, 2024 and 2023: Millions Three Months Ended March 31, 2024 2023 Written premiums: Direct $ 285.0 $ 246.2 Assumed 587.1 563.2 Gross written premiums 872.1 809.4 Ceded (1) (274.1) (195.2) Net written premiums $ 598.0 $ 614.2 Earned premiums: Direct $ 194.4 $ 150.4 Assumed 192.4 162.1 Gross earned premiums 386.8 312.5 Ceded (2) (84.0) (57.4) Net earned premiums $ 302.8 $ 255.1 Loss and loss adjustment expenses: Gross $ 253.3 $ 162.4 Ceded (3) (73.3) (14.6) Net loss and loss adjustment expenses $ 180.0 $ 147.8 (1) The ceded written premiums exclude $34.3 and $44.1 ceded by Ark to WM Outrigger Re for the three months ended March 31, 2024 and 2023, which eliminate in White Mountains’s consolidated financial statements. (2) The ceded earned premiums exclude $10.3 and $5.2 ceded by Ark to WM Outrigger Re for the three months ended March 31, 2024 and 2023, which eliminate in White Mountains’s consolidated financial statements. (3) The ceded loss and loss adjustment expenses exclude $0.7 and $0.2 ceded by Ark to WM Outrigger Re for the three months ended March 31, 2024 and 2023, which eliminate in White Mountains’s consolidated financial statements. The following table presents the Ark/WM Outrigger segment’s reinsurance recoverables as of March 31, 2024 and December 31, 2023: Millions March 31, 2024 December 31, 2023 Reinsurance recoverables on unpaid losses (1) $ 404.5 $ 340.8 Reinsurance recoverables on paid losses 20.7 27.4 Ceded unearned premiums (2) 269.8 73.8 Reinsurance recoverables $ 695.0 $ 442.0 (1) The reinsurance recoverables on unpaid losses exclude $15.8 and $15.6 ceded by Ark to WM Outrigger Re as of March 31, 2024 and December 31, 2023, which eliminate in White Mountains’s consolidated financial statements. (2) The ceded unearned premiums exclude $24.0 and $5.7 ceded by Ark to WM Outrigger Re as of March 31, 2024 and December 31, 2023, which eliminate in White Mountains’s consolidated financial statements. 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 March 31, 2024: $ in Millions As of March 31, 2024 A.M. Best Rating (1) Gross Collateral Net % of Total A+ or better $ 202.2 $ — $ 202.2 58.1 % A- to A 139.1 — 139.1 40.0 B++ or lower and not rated 83.9 (2) 77.3 (2) 6.6 1.9 Total $ 425.2 $ 77.3 $ 347.9 100.0 % (1) A.M. Best ratings as detailed above are: “A+ or better” (Superior) “A- to A” (Excellent), “B++” (Good). (2) Excludes $15.8 ceded by Ark to WM Outrigger Re as of March 31, 2024, 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 March 31, 2024 and December 31, 2023, the carrying value of Ark’s deposit in SiriusPoint, including accrued interest, was $20.6 million and $20.4 million. See Note 10 — “Municipal Bond Guarantee Insurance” for reinsurance contracts accounted for as deposits related to White Mountains’s financial guarantee business.

Debt

Debt	3 Months Ended
	Mar. 31, 2024
Debt Disclosure [Abstract]
Debt	Debt The following table presents White Mountains’s debt outstanding as of March 31, 2024 and December 31, 2023: $ in Millions March 31, 2024 Effective (1) December 31, 2023 Effective (1) Ark 2007 Subordinated Notes, carrying value $ — $ 30.0 Ark 2021 Subordinated Notes Tranche 1 42.4 42.7 Ark 2021 Subordinated Notes Tranche 2 47.0 47.0 Ark 2021 Subordinated Notes Tranche 3 70.0 70.0 Unamortized issuance cost (4.0) (4.2) Ark 2021 Subordinated Notes, carrying value 155.4 155.5 Total Ark Subordinated Notes, carrying value 155.4 13.1% 185.5 11.0% HG Global Senior Notes (2) 150.0 11.9% 150.0 11.9% Unamortized discount and issuance cost (3.0) (3.1) HG Global Senior Notes, carrying value 147.0 146.9 Kudu Credit Facility 210.3 10.6% 210.3 10.1% Unamortized issuance cost (6.5) (6.5) Kudu Credit Facility, carrying value 203.8 203.8 Other Operations debt 26.4 10.2% 28.9 10.2% Unamortized issuance cost (.5) (.5) Other Operations debt, carrying value 25.9 28.4 Total debt $ 532.1 $ 534.6 (1) Effective rate includes the effect of the amortization of debt issuance costs and, where applicable, the original issue discount. (2) Effective rate excludes the effect of the interest rate cap. 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. During the three months ended March 31, 2024, Ark repaid the outstanding balance of $30.0 million and extinguished the Ark 2007 Subordinated Notes. 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, accrued interest at a floating rate equal to the three-month U.S. LIBOR plus 5.75% per annum until August 2023. Effective August 2023, the Ark 2021 Subordinated Notes Tranche 2 accrue interest at a floating rate equal to the three-month SOFR plus a credit adjustment spread of 0.26% and a stated margin of 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, accrued interest at a floating rate equal to the three-month U.S. LIBOR plus 6.1% per annum until September 2023. Effective September 2023, the Ark 2021 Subordinated Notes Tranche 3 accrue interest at a floating rate equal to the three-month SOFR plus a credit adjustment spread of 0.26% and a stated margin of 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 March 31, 2024, there were no premium load events. As of March 31, 2024, the Ark 2021 Subordinated Notes Tranche 1 had an outstanding balance of €39.1 million ($42.4 million based upon the foreign exchange spot rate as of March 31, 2024), 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 GAIL’s 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 $125.0 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 $100.0 million on a collateralized basis. As of January 1, 2024, the ING LOC Facility was undrawn and the availability period expired. Ark did not renew the credit facility. As of March 31, 2024, the Citibank LOC Facility had an outstanding balance of $111.5 million and short-term investments pledged as collateral of $136.9 million. As of March 31, 2024, the Lloyds LOC Facility had an outstanding balance of $35.4 million and short-term investments pledged as collateral of $65.0 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. 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 Overnight Financing Rate (“SOFR”) plus a credit adjustment spread of 0.26% and a stated margin of 6.0% 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 Global 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 $29.3 million. As of March 31, 2024, the fair value of the interest reserve account, which is included in short-term investments, was $30.5 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 March 31, 2024, the HG Global Senior Notes had an investment grade rating. As of March 31, 2024, the HG Global Senior Notes had an outstanding balance of $150.0 million. 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 2023, the Kudu Credit Facility accrues interest at a floating interest rate equal to the three-month 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 March 31, 2024 and December 31, 2023, the interest reserve account of $15.1 million and $14.9 million was held in short term investments. 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 March 31, 2024, Kudu had a 23.1% 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. The Kudu Credit Facility’s availability period expired as of March 31, 2024. Kudu is currently in discussions with Mass Mutual to renew the Kudu Credit Facility. The following table presents the change in debt under the Kudu Credit Facility for the three months ended March 31, 2024 and 2023: Three Months Ended March 31, Millions 2024 2023 Kudu Credit Facility Beginning balance $ 210.3 $ 215.2 Term loans Borrowings — — Repayments — (16.9) Ending balance $ 210.3 $ 198.3 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 March 31, 2024, White Mountains’s Other Operations had debt with an outstanding balance of $26.4 million, which consisted of four secured credit facilities (collectively, “Other Operations debt”). Compliance At March 31, 2024, White Mountains was in compliance, in all material respects, with all of the covenants under its debt instruments.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2024
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company and its Bermuda-domiciled subsidiaries are not subject to income tax in Bermuda in 2024 and prior years. On December 27, 2023, Bermuda enacted a 15.0% corporate income tax that will generally become effective on January 1, 2025. The Bermuda legislation defers the effective date for five years for Bermuda companies in consolidated groups that meet certain requirements. White Mountains expects to meet the requirements to be exempt from the Bermuda corporate income tax until January 1, 2030. The Bermuda legislation also provides for an economic transition adjustment that will reduce future years’ taxable income. Under GAAP, this economic transition adjustment was required to be recognized as a net deferred tax asset as of December 31, 2023. Accordingly, White Mountains’s net income for 2023 included a net deferred tax benefit of $68.0 million, of which $51.0 million was recorded at Ark and $17.0 million was recorded at HG Global. The deferred tax assets remained unchanged as of March 31, 2024. 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 March 31, 2024, the primary jurisdictions in which the Company’s subsidiaries and branches operate and were subject to tax include Israel, Luxembourg, the United Kingdom and the United States. On April 1, 2023, the U.K. corporate tax rate increased from 19.0% to 25.0%. White Mountains’s income tax expense related to pre-tax income from continuing operations for the three months ended March 31, 2024 and 2023 represented an effective tax rate of 4.6% and 6.0%. 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. In arriving at the effective tax rate for the three months ended March 31, 2024 and 2023, 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, 2024 and 2023. 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 2018.

Derivatives

Derivatives	3 Months Ended
	Mar. 31, 2024
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives	Derivatives HG Global Interest Rate Cap On June 16, 2022, HG Global 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 Global 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 on a quarterly determination date exceeds 3.5%, HG Global will receive a payment from the counterparty on the subsequent settlement date equal to the difference between the three-month SOFR on the determination date and 3.5%, multiplied by the notional amount of the cap and the number of days in the settlement period divided by 360. As of March 31, 2024, the three-month SOFR was 5.3%.

Municipal Bond Guarantee Insura

Municipal Bond Guarantee Insurance	3 Months Ended
	Mar. 31, 2024
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 21% of BAM’s gross par outstanding as of March 31, 2024. 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 27% of BAM’s gross par outstanding as of March 31, 2024. 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 28% of BAM’s gross par outstanding as of March 31, 2024. 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 March 31, 2024, 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 March 31, 2024 and December 31, 2023 was $330.7 million and $341.6 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 March 31, 2024 and December 31, 2023 was $602.4 million and $606.9 million. As of March 31, 2024 and December 31, 2023, the Collateral Trusts held assets of $933.1 million and $948.5 million, which included $592.8 million and $588.6 million of cash, investments and accrued investment income, $322.2 million and $322.2 million of BAM Surplus Notes and $18.1 million and $37.7 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. Beginning in 2025, the rate will be fixed through maturity at the higher of the 2024 variable rate or 8.0%. Accordingly, in 2024 and through maturity, the interest rate on the BAM Surplus Notes is 8.2%. 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 2023, BAM made a $27.4 million cash payment of principal and interest on the BAM Surplus Notes held by HG Global. Of this payment, $17.8 million was a repayment of principal held in the Supplemental Trust, $2.0 million was a payment of accrued interest held in the Supplemental Trust and $7.6 million was a payment of accrued interest held outside the Supplemental Trust. During the three months ended March 31, 2024 and 2023, BAM made no repayments of the BAM Surplus Notes or accrued interest. As of March 31, 2024 and December 31, 2023, the principal balance on the BAM Surplus Notes was $322.2 million for both periods, and total interest receivable on the BAM Surplus Notes was $181.1 million and $174.5 million. Insured Obligations and Premiums The following table presents a schedule of BAM’s insured obligations as of March 31, 2024 and December 31, 2023: March 31, 2024 December 31, 2023 Contracts outstanding 14,725 14,485 Remaining weighted average contract period (in years) 10.5 11.1 Contractual debt service outstanding (in millions): Principal $ 112,216.7 $ 109,673.8 Interest 57,545.8 56,012.2 Total debt service outstanding $ 169,762.5 $ 165,686.0 Gross unearned insurance premiums (in millions) $ 328.5 $ 325.8 The following table presents a schedule of BAM’s future premium revenues as of March 31, 2024: Millions March 31, 2024 April 1, 2024 - December 31, 2024 $ 22.4 January 1, 2025 - March 31, 2025 7.2 April 1, 2025 - June 30, 2025 7.2 July 1, 2025 - September 30, 2025 7.0 October 1, 2025 - December 31, 2025 6.9 Total 2025 28.3 2026 26.6 2027 24.9 2028 23.1 2029 21.4 2030 and thereafter 181.8 Total gross unearned insurance premiums $ 328.5 The following table presents a schedule of written premiums and earned premiums included in the HG Global/BAM segment for the three months ended March 31, 2024 and 2023: Three Months Ended March 31, Millions 2024 2023 Written premiums: Direct $ 10.5 $ 9.2 Assumed — — Gross written premiums (1) $ 10.5 $ 9.2 Earned premiums: Direct $ 7.3 $ 7.0 Assumed .5 .7 Gross earned premiums (1) $ 7.8 $ 7.7 (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

Earnings Per Share	3 Months Ended
	Mar. 31, 2024
Earnings Per Share [Abstract]
Earnings Per Share	Earnings Per Share White Mountains calculates earnings per share using the two-class method, which allocates earnings between common shares and unvested restricted common shares. Both classes of shares participate equally in dividends and earnings on a per share basis. Basic earnings per share amounts are based on the weighted average number of common shares outstanding adjusted for unvested restricted common shares. The following table presents the Company’s computation of earnings per share from continuing operations for the three months ended March 31, 2024 and 2023. Three Months Ended March 31, 2024 2023 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common $ 236.4 $ 179.5 Allocation of (earnings) losses to participating restricted common shares (1) (2.7) (2.2) Basic and diluted earnings (losses) per share numerators $ 233.7 $ 177.3 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 2,559.7 2,569.9 Average unvested restricted common shares (2) (28.6) (31.1) Basic earnings (losses) per share denominator 2,531.1 2,538.8 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 2,559.7 2,569.9 Average unvested restricted common shares (2) (28.6) (31.1) Diluted earnings (losses) per share denominator 2,531.1 2,538.8 Basic and diluted earnings per share (in dollars): Distributed earnings - dividends declared and paid $ 1.00 $ 1.00 Undistributed earnings (losses) 91.33 68.83 Basic and diluted earnings (losses) per share $ 92.33 $ 69.83 (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (2) 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) for the three months ended March 31, 2024 and 2023. Three Months Ended March 31, Millions 2024 2023 Undistributed net earnings: Net income (loss) attributable to White Mountains’s common $ 233.7 $ 177.3 Dividends declared, net of restricted common share amounts (1) (2.5) (2.5) Total undistributed net earnings (losses), net of restricted common share amounts $ 231.2 $ 174.8 (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	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024 and 2023, 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 months ended March 31, 2024 and 2023 for performance shares granted under the WTM Incentive Plan: Three Months Ended March 31, 2024 2023 $ in Millions Target Performance Accrued Target Performance Accrued Beginning of period 37,031 $ 69.4 39,449 $ 67.5 Shares paid (1) (13,475) (44.9) (13,350) (40.8) New grants 10,405 — 10,895 — Forfeitures and cancellations (2) 46 .3 37 .4 Expense recognized — 19.4 — 9.3 End of period 34,007 $ 44.2 37,031 $ 36.4 (1) WTM performance share payments for the 2021-2023 performance cycle were made in March 2024 at 188% of target. WTM performance share payments for the 2020-2022 performance cycle were made in March 2023 at 200% of target. (2) Amounts include changes in assumed forfeitures, as required under GAAP. During the three months ended March 31, 2024, White Mountains granted 10,405 performance shares for the 2023-2025 performance cycle. During the three months ended March 31, 2023, White Mountains granted 10,895 performance shares for the 2023-2025 performance cycle. For the 2021-2023 performance cycle, the Company issued common shares for 100 performance shares earned, and all other performance shares earned were settled in cash. For the 2020-2022 performance cycle, all performance shares earned were settled in cash. If all outstanding performance shares had vested on March 31, 2024, the total additional compensation cost to be recognized would have been $45.2 million, based on accrual factors as of March 31, 2024 (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 March 31, 2024 for each performance cycle: March 31, 2024 $ in Millions Target Performance Accrued Performance cycle: 2022 – 2024 13,225 $ 35.0 2023 – 2025 10,895 9.4 2024 – 2026 10,405 .6 Sub-total 34,525 45.0 Assumed forfeitures (518) (.8) Total 34,007 $ 44.2 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 months ended March 31, 2024 and 2023: Three Months Ended March 31, 2024 2023 $ in Millions Restricted Unamortized Issue Restricted Unamortized Issue Non-vested, Beginning of period 37,595 $ 16.2 38,350 $ 15.5 Issued 10,405 18.3 10,895 16.0 Vested (13,475) — (11,650) — Forfeited — — — — Expense recognized — (3.2) — (3.2) End of period 34,525 $ 31.3 37,595 $ 28.3

Noncontrolling Interests

Noncontrolling Interests	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024 and December 31, 2023: March 31, 2024 December 31, 2023 $ in Millions Noncontrolling Percentage (1) Noncontrolling Equity Noncontrolling Percentage (1) Noncontrolling Equity Noncontrolling interests, excluding BAM Ark 28.0 % $ 334.7 (2) 28.0 % $ 336.9 (2) HG Global 3.1 % (.4) 3.1 % .8 Kudu 10.6 % 114.4 10.5 % 113.8 Bamboo 27.2 % 111.3 — % — Other various 9.3 various 9.4 Total, excluding BAM 569.3 460.9 BAM 100.0 % (149.1) 100.0 % (139.8) Total noncontrolling interests $ 420.2 $ 321.1 (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. (2) As of March 31, 2024 and December 31, 2023, Ark’s noncontrolling equity includes $17.9 and $21.0 related to management’s equity incentives.

Segment Information

Segment Information	3 Months Ended
	Mar. 31, 2024
Segment Reporting [Abstract]
Segment Information	Segment Information As of March 31, 2024, White Mountains conducted its operations through four reportable segments: (1) Ark/WM Outrigger, (2) HG Global/BAM, (3) Kudu and (4) Bamboo, 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. As a result of the Bamboo Transaction, White Mountains began consolidating Bamboo in its financial statements as of January 2, 2024. See Note 2 — “Significant Transaction” . The following tables present the financial information for White Mountains’s segments for the three months ended March 31, 2024 and 2023: Ark/WM Outrigger HG Global/BAM Other Operations Millions Ark WM Outrigger Re HG Global BAM (2) Kudu Bamboo Total Three Months Ended March 31, 2024 Earned insurance premiums (1) $ 292.5 $ 10.3 $ 6.5 $ 1.3 $ — $ 8.4 $ — $ 319.0 Net investment income (3) 17.0 2.9 5.4 4.3 17.2 .3 9.9 57.0 Net investment income (expense) - — — 6.6 (6.6) — — — — Net realized and unrealized investment gains (losses) (3) 10.6 — (7.3) (2.8) (6.5) (.1) 22.2 16.1 Net realized and unrealized investment gains — — — — — — 210.7 210.7 Commission and fee revenues — — — — — 21.9 3.6 25.5 Other revenues 3.5 — — .5 — .6 14.4 19.0 Total revenues 323.6 13.2 11.2 (3.3) 10.7 31.1 260.8 647.3 Loss and loss adjustment expenses 179.3 .7 — — — 5.8 — 185.8 Acquisition expenses 63.7 2.6 1.8 .4 — 3.1 — 71.6 Cost of sales — — — — — — 7.6 7.6 Broker commission expenses — — — — — 9.3 — 9.3 General and administrative expenses 42.2 — .4 16.9 3.4 12.0 50.3 125.2 Change in fair value of contingent consideration — — — — — — — — Interest expense 5.4 — 3.5 — 5.6 — .7 15.2 Total expenses 290.6 3.3 5.7 17.3 9.0 30.2 58.6 414.7 Pre-tax income (loss) $ 33.0 $ 9.9 $ 5.5 $ (20.6) $ 1.7 $ .9 $ 202.2 $ 232.6 (1) Ark’s earned insurance premiums based on the location of Ark’s underwriting offices in the United Kingdom and Bermuda are $177.4 and $115.1. (2) 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. (3) Bamboo’s net investment income and net realized and unrealized investment gains (losses) are included in other revenues in the consolidated statement of operations. Ark/WM Outrigger HG Global/BAM Other Operations Millions Ark WM Outrigger Re HG Global BAM (2) Kudu Total Three Months Ended March 31, 2023 Earned insurance premiums (1) $ 249.9 $ 5.2 $ 6.4 $ 1.3 $ — $ — $ 262.8 Net investment income 8.4 2.2 4.0 3.2 14.2 7.0 39.0 Net investment income (expense) - — — 6.6 (6.6) — — — Net realized and unrealized investment 24.5 — 7.9 9.1 29.6 41.8 112.9 Net realized and unrealized investment gains — — — — — 85.2 85.2 Commission and fee revenues — — — — — 3.3 3.3 Other revenues (2.7) — — .8 — 30.6 28.7 Total revenues 280.1 7.4 24.9 7.8 43.8 167.9 531.9 Loss and loss adjustment expenses 147.6 .2 — — — — 147.8 Acquisition expenses 58.9 .9 1.8 .9 — — 62.5 Cost of sales — — — — — 13.9 13.9 General and administrative expenses 35.1 .1 1.1 16.2 3.8 39.7 96.0 Change in fair value of contingent consideration (2.4) — — — — — (2.4) Interest expense 5.0 — 4.5 — 4.7 .8 15.0 Total expenses 244.2 1.2 7.4 17.1 8.5 54.4 332.8 Pre-tax income (loss) $ 35.9 $ 6.2 $ 17.5 $ (9.3) $ 35.3 $ 113.5 $ 199.1 (1) Ark’s earned insurance premiums based on the location of Ark’s underwriting offices in the United Kingdom and Bermuda are $146.0 and $104.0. (2)

Variable Interest Entities

Variable Interest Entities	3 Months Ended
	Mar. 31, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Variable Interest Entities	Variable Interest Entities Under GAAP, White Mountains is required to consolidate any entity in which it holds a controlling financial interest. A controlling financial interest is usually in the form of an investment representing the majority of the subsidiary’s voting interests. However, a controlling financial interest may also arise from a financial interest in a VIE through arrangements that do not involve ownership of voting interests. A VIE is a legal entity that (i) does not have sufficient equity at risk to finance its activities without additional financial support; (ii) is structured such that equity investors, as a group, lack the power, through voting or similar rights, to direct the activities that most significantly impact the entity’s economic performance; (iii) is structured such that the equity investors lack the obligation to absorb losses of, or the right to receive returns from, the entity; or (iv) is structured with non-substantive voting rights. White Mountains determines whether an entity is a VIE at the inception of its variable interest in the entity and upon the occurrence of certain reconsideration events. White Mountains consolidates a VIE if it determines that it is the primary beneficiary. The primary beneficiary is defined as the entity that holds a variable interest that gives it both the power to direct the VIE’s activities that most significantly impact its economic performance and the obligation to absorb losses of, or the right to receive returns from, the VIE that could potentially be significant to the VIE. The identification of the primary beneficiary of a VIE may require significant assumptions and judgment. When White Mountains determines it has a variable interest in a VIE, it determines whether it is the primary beneficiary of that VIE by performing an analysis that principally considers: (i) the VIE’s purpose and design, including the risks the VIE was designed to create and pass through to its variable interest holders; (ii) the VIE’s capital structure; (iii) the identification of the activities that most significantly impact the VIE’s economic performance; (iv) the governance provisions and other contractual arrangements between the VIE and its variable interest holders and other parties involved with the VIE; and (v) related party relationships. At inception of its variable interest in the VIE as well as on an ongoing basis, White Mountains performs qualitative assessments of its VIEs to determine whether White Mountains is the primary beneficiary of a VIE. 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. BAM BAM is the first and only mutual municipal bond insurance company in the United States. HG Global, together with its subsidiaries, funded the initial capitalization of BAM through the purchase of $503.0 million of BAM Surplus Notes and, through its reinsurance subsidiary HG Re, provides up to 15%-of-par, first loss reinsurance protection for policies underwritten by BAM. As a mutual insurance company, BAM is owned by and operated for the benefit of its members, the municipalities whose debt issuances are insured by BAM. As of March 31, 2024, BAM’s members have contributed equity capital at risk through MSC of $556.5 million. However, the BAM Surplus Notes remain a significant portion of BAM’s statutory capital. Accordingly, White Mountains has determined that BAM is a VIE. BAM’s underwriting process was determined to be the activity that most significantly impacts BAM’s economic performance. BAM’s underwriting guidelines define the types of credits that BAM may insure. Pursuant to the FLRT, BAM’s underwriting guidelines may only be amended with the consent of HG Re. As a result, White Mountains concluded it has the power to direct BAM’s activities that most significantly impact BAM’s economic performance and it is the primary beneficiary. Accordingly, White Mountains 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. BAM’s assets can be used only to settle BAM’s obligations, and general creditors of BAM have no recourse to the Company or HG Global. HG Re’s obligations to BAM under the FLRT are subject to an aggregate limit equal to the assets in the Collateral Trusts at any point in time. PassportCard/DavidShield As of March 31, 2024, 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 March 31, 2024, 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 $159.5 million. Elementum As of March 31, 2024, 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 March 31, 2024, 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 March 31, 2024, White Mountains’s maximum exposure to loss on its investments in limited partnerships is the carrying value of $255.2 million.

Equity Method Eligible Investme

Equity Method Eligible Investments	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024 and December 31, 2023: March 31, 2024 December 31, 2023 $ in Millions Ownership Interest Carrying Value Ownership Interest Carrying Value Kudu’s Participation Contracts (1) 4.1% - 30.0% $ 884.2 4.1% - 30.0% $ 890.5 Investment in MediaAlpha 34.7% 465.6 34.9% 254.9 PassportCard/DavidShield 53.8% 150.0 53.8% 150.0 Elementum Holdings, L.P. 26.6% 35.0 26.6% 35.0 Other equity method eligible investments, at fair value Under 50.0% 247.4 Under 50.0% 291.7 Other equity method eligible investments, at fair value 50.0% and over 24.8 50.0% and over 24.8 (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 Instrum

Fair Value of Financial Instruments	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024 and December 31, 2023: March 31, 2024 December 31, 2023 Millions Fair Value Carrying Value Fair Value Carrying Value Ark 2007 Subordinated Notes $ — $ — $ 30.5 $ 30.0 Ark 2021 Subordinated Notes $ 171.8 $ 155.4 $ 171.8 $ 155.5 HG Global Senior Notes $ 158.5 $ 147.0 $ 158.7 $ 146.9 Kudu Credit Facility $ 224.6 $ 203.8 $ 225.6 $ 203.8 Other Operations debt $ 27.4 $ 25.9 $ 30.0 $ 28.4 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	3 Months Ended
	Mar. 31, 2024
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 — “Loss and Loss Adjustment Expense Reserves.”

Basis of Presentation and Sig_2

Basis of Presentation and Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024, White Mountains conducted its operations through four reportable segments: (1) Ark/WM Outrigger, (2) HG Global/BAM, (3) Kudu and (4) Bamboo, 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 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, accident & health and casualty. 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 March 31, 2024 and December 31, 2023, White Mountains owned 72.0% of Ark on a basic shares outstanding basis (61.9% after taking account of management’s equity incentives). The remaining shares are owned by current and former employees of Ark. In the future, management rollover shareholders could earn additional shares in Ark if and to the extent that White Mountains achieves certain thresholds for its multiple of invested capital return. 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. 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. White Mountains consolidates the results of its segregated account, WM Outrigger Re, in its financial statements. See Note 2 — “Significant Transactions.” As of March 31, 2024 and December 31, 2023, White Mountains owned 100.0% of WM Outrigger Re’s preferred equity. 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 whose debt issuances are insured by BAM. 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 March 31, 2024 and December 31, 2023, 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 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 March 31, 2024 and December 31, 2023, White Mountains owned 89.4% and 89.6% of Kudu’s basic units outstanding (76.2% and 76.3% on a fully-diluted, fully-converted basis). The Bamboo segment consists of PM Holdings LLC (“Bamboo Holdings”), Bamboo Ide8 Insurance Services LLC (“Bamboo MGA”) and Ide8 Limited (“Bamboo Captive”) (collectively with Bamboo Holdings and Bamboo MGA, “Bamboo”). Bamboo is a capital-light, tech- and data-enabled insurance distribution platform providing homeowners’ insurance and related products to the residential property market in California. Bamboo operates primarily through Bamboo MGA, its full-service managing general agent (“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 it earns commissions based on the volume and profitability of the insurance that it places. It offers both admitted and non-admitted products. Bamboo also operates two separate but integrated businesses: (i) a retail agency, within Bamboo MGA, offering ancillary products on behalf of third parties and (ii) Bamboo Captive, a Bermuda-domiciled captive reinsurer that participates in the underwriting risk of Bamboo’s MGA programs to align interests with reinsurance partners. On January 2, 2024, White Mountains acquired a controlling interest in Bamboo. See Note 2 — “Significant Transactions” . As of March 31, 2024, White Mountains owned 72.8% of the basic units outstanding of Bamboo (63.7% on a fully-diluted, fully-converted basis). White Mountains’s other operations consist 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 (collectively, “Other Operations”). On October 25, 2023, White Mountains announced the launch of White Mountains Partners LLC (“WTM Partners”), which will acquire businesses in non-insurance, non-financial services sectors including essential services, light industrial and specialty consumer. White Mountains expects to deploy up to $500.0 million of equity capital through WTM Partners over time. WTM Partners is included within Other Operations.
Revenue Recognition	Revenue Recognition Bamboo’s revenues consist primarily of commission and fee revenues for placement of insurance policies. Commission and fee revenues are measured based on the contractual rates with insurance carriers, net of any amounts expected to be uncollectible and any amounts associated with expected policy cancellations and adjustments, and are recognized when contractual performance obligations have been fulfilled. Bamboo’s primary contractual performance obligations are generally satisfied upon the issuance of an insurance policy. Where Bamboo has significant performance obligations beyond the policy effective date, Bamboo estimates the relative standalone selling price for the post-issuance services using the price charged for the service when sold separately in similar circumstances to similar customers in order to allocate the transaction price. Bamboo’s premiums, commissions and fees receivable consist of insurance premiums and fees receivable from customers and commissions receivable from insurance carriers, net of a provision for amounts estimated to be uncollectible and any amounts associated with expected policy cancellations and adjustments. Deferred revenues associated with unsatisfied performance obligations are recognized within other liabilities.
Broker Commission Expenses	Broker Commission Expenses Bamboo’s broker commission expense consists of commissions paid to sub-agents and brokers. Broker commission expense is measured in accordance with contractual terms and recognized when incurred, which is generally at the policy issuance date.

Significant Transactions (Table

Significant Transactions (Tables)	3 Months Ended
	Mar. 31, 2024
Significant Transactions [Abstract]
Schedule of assets acquired and liabilities assumed	The following presents additional details of the assets acquired and liabilities assumed as of the January 2, 2024 acquisition date: Millions As of January 2, 2024 Fixed maturity investments, at fair value $ 8.2 Short-term investments, at fair value 9.3 Cash (restricted $37.0) 44.9 (1) Premiums and commissions receivable 38.0 Other assets 7.7 Total tangible assets 108.1 Loss and loss adjustment expense reserves (9.3) Unearned insurance premiums (20.1) Premiums and commissions payable (49.2) Other liabilities (13.1) Total tangible liabilities (91.7) Net tangible assets acquired 16.4 Goodwill 270.4 Other intangible assets 101.0 Total goodwill and other intangible assets 371.4 Net assets acquired $ 387.8 (1) Cash excludes the White Mountains cash contribution of $20.0 as part of the Bamboo Transaction, which is not part of the purchase consideration.

Investment Securities (Tables)

Investment Securities (Tables)	3 Months Ended
	Mar. 31, 2024
Investments, Debt and Equity Securities [Abstract]
Pre-tax net investment income	The following table presents pre-tax net investment income for the three months ended March 31, 2024 and 2023: Three Months Ended March 31, Millions 2024 2023 Fixed maturity investments $ 20.9 $ 14.5 Short-term investments 15.7 9.7 Common equity securities .5 1.3 Other long-term investments 20.6 14.1 Total investment income 57.7 39.6 Third-party investment expenses (.7) (.6) Net investment income, pre-tax $ 57.0 $ 39.0
Net realized and unrealized investment gains (losses)	The following table presents net realized and unrealized investment gains (losses) for the three months ended March 31, 2024 and 2023: Three Months Ended March 31, Millions 2024 2023 Realized investment gains (losses) Fixed maturity investments $ (5.8) $ (.1) Short-term investments (.2) (.4) Other long-term investments .3 47.2 Net realized investment gains (losses) (5.7) 46.7 Unrealized investment gains (losses) Fixed maturity investments (7.4) 29.7 Short-term investments (1.0) 1.2 Common equity securities 23.1 29.4 Investment in MediaAlpha 210.7 85.2 Other long-term investments 7.1 5.9 Net unrealized investment gains (losses) 232.5 151.4 Net realized and unrealized investment gains (losses) (1) $ 226.8 $ 198.1 Fixed maturity and short-term investments Net realized and unrealized investment gains (losses) $ (14.4) $ 30.4 Less: net realized and unrealized gains (losses) on investment (.7) 1.8 Net unrealized investment gains (losses) recognized during the period on $ (13.7) $ 28.6 Common equity securities and investment in MediaAlpha Net realized and unrealized investment gains (losses) on common $ 23.1 $ 29.4 Net realized and unrealized investment gains (losses) from 210.7 85.2 Total net realized and unrealized investment gains (losses) 233.8 114.6 Less: net realized and unrealized gains (losses) on investment — — Net unrealized investment gains (losses) recognized during the period on $ 233.8 $ 114.6 (1) For the three months ended March 31, 2024 and 2023, includes $(11.5) and $5.4 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 months ended March 31, 2024 and 2023 for investments still held at the end of the period: Three Months Ended March 31, Millions 2024 2023 Total net unrealized investment gains on other long-term investments held at the end of the period, pre-tax $ (6.2) $ 37.0
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 value of White Mountains’s fixed maturity investments as of March 31, 2024 and December 31, 2023: March 31, 2024 Millions Cost or Gross Gross Net Foreign Carrying U.S. Government and agency obligations $ 247.6 $ .1 $ (1.8) $ — $ 245.9 Debt securities issued by corporations 1,118.3 3.0 (46.0) (2.2) 1,073.1 Municipal obligations 251.4 .6 (13.0) — 239.0 Mortgage and asset-backed securities 484.8 .6 (35.3) — 450.1 Collateralized loan obligations 248.9 .7 (1.5) (1.6) 246.5 Total fixed maturity investments $ 2,351.0 $ 5.0 $ (97.6) $ (3.8) $ 2,254.6 December 31, 2023 Millions Cost or Gross Gross Net Foreign Carrying U.S. Government and agency obligations $ 209.0 $ .3 $ (4.4) $ — $ 204.9 Debt securities issued by corporations 1,085.9 5.6 (45.4) (1.1) 1,045.0 Municipal obligations 275.1 .9 (15.1) — 260.9 Mortgage and asset-backed securities 417.2 2.0 (29.8) — 389.4 Collateralized loan obligations 211.2 .4 (2.4) (.1) 209.1 Total fixed maturity investments $ 2,198.4 $ 9.2 $ (97.1) $ (1.2) $ 2,109.3 The following tables present the cost or amortized cost, gross unrealized investment gains (losses), net foreign currency gains (losses) and carrying value of common equity securities, White Mountains’s investment in MediaAlpha and other long-term investments as of March 31, 2024 and December 31, 2023: March 31, 2024 Millions Cost or Gross Gross Net Foreign Carrying Common equity securities $ 483.5 $ 86.0 $ (1.2) $ (6.8) $ 561.5 Investment in MediaAlpha $ 59.2 $ 406.4 $ — $ — $ 465.6 Other long-term investments $ 1,642.2 $ 466.7 $ (101.1) $ (15.9) $ 1,991.9 December 31, 2023 Millions Cost or Gross Gross Net Foreign Carrying Common equity securities $ 483.5 $ 62.3 $ (1.2) $ (6.2) $ 538.4 Investment in MediaAlpha $ 59.2 $ 195.7 $ — $ — $ 254.9 Other long-term investments $ 1,655.7 $ 446.3 $ (94.5) $ (9.3) $ 1,998.2
Schedule of fixed maturity investment holdings	The following table presents the cost or amortized cost and carrying value of White Mountains’s fixed maturity investments by contractual maturity as of March 31, 2024. Actual maturities could differ from contractual maturities because borrowers may have the right to call or prepay certain obligations with or without penalties. March 31, 2024 Millions Cost or Amortized Cost Carrying Value Due in one year or less $ 300.5 $ 296.1 Due after one year through five years 886.3 856.8 Due after five years through ten years 295.7 277.8 Due after ten years 134.8 127.3 Mortgage and asset-backed securities and collateralized loan obligations 733.7 696.6 Total fixed maturity investments $ 2,351.0 $ 2,254.6
Fair value measurements by level, investment securities	The following tables present White Mountains’s fair value measurements for investments as of March 31, 2024 and December 31, 2023 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. March 31, 2024 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 245.9 $ 245.1 $ .8 $ — Debt securities issued by corporations: Financials 323.7 — 323.7 — Consumer 219.3 — 219.3 — Industrial 128.2 — 128.2 — Healthcare 121.9 — 121.9 — Technology 101.6 — 101.6 — Utilities 68.8 — 68.8 — Communications 41.3 — 41.3 — Energy 37.5 — 37.5 — Materials 30.8 — 30.8 — Total debt securities issued by corporations 1,073.1 — 1,073.1 — Municipal obligations 239.0 — 239.0 — Mortgage and asset-backed securities 450.1 — 450.1 — Collateralized loan obligations 246.5 — 246.5 — Total fixed maturity investments 2,254.6 245.1 2,009.5 — Short-term investments 1,054.5 1,031.1 23.4 — Common equity securities: Exchange-traded funds 151.6 151.6 — — Other (1) 409.9 — 409.9 — Total common equity securities 561.5 151.6 409.9 — Investment in MediaAlpha 465.6 465.6 — — Other long-term investments 1,155.9 — 27.1 1,128.8 Other long-term investments — NAV (2) 836.0 — — — Total other long-term investments 1,991.9 — 27.1 1,128.8 Total investments $ 6,328.1 $ 1,893.4 $ 2,469.9 $ 1,128.8 (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, 2023 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 204.9 $ 204.9 $ — $ — Debt securities issued by corporations: Financials 309.8 — 309.8 — Consumer 214.6 — 214.6 — Industrial 121.6 — 121.6 — Healthcare 121.3 — 121.3 — Technology 108.9 — 108.9 — Utilities 70.2 — 70.2 — Communications 46.3 — 46.3 — Energy 28.9 — 28.9 — Materials 23.4 — 23.4 — Total debt securities issued by corporations 1,045.0 — 1,045.0 — Municipal obligations 260.9 — 260.9 — Mortgage and asset-backed securities 389.4 — 389.4 — Collateralized loan obligations 209.1 — 209.1 — Total fixed maturity investments 2,109.3 204.9 1,904.4 — Short-term investments 1,487.9 1,457.6 30.3 — Common equity securities: Exchange-traded funds 137.8 137.8 — — Other (1) 400.6 — 400.6 — Total common equity securities 538.4 137.8 400.6 — Investment in MediaAlpha 254.9 254.9 — — Other long-term investments 1,164.4 — 26.2 1,138.2 Other long-term investments — NAV (2) 833.8 — — — Total other long-term investments 1,998.2 — 26.2 1,138.2 Total investments $ 6,388.7 $ 2,055.2 $ 2,361.5 $ 1,138.2 (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 March 31, 2024 and December 31, 2023: March 31, 2024 December 31, 2023 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: FNMA $ 216.7 $ 216.7 $ — $ 198.8 $ 198.8 $ — FHLMC 147.8 147.8 — 137.2 137.2 — GNMA 39.9 39.9 — 30.2 30.2 — Total agency (1) 404.4 404.4 — 366.2 366.2 — Non-agency: Residential .1 .1 — .2 .2 — Total non-agency .1 .1 — .2 .2 — Total mortgage-backed securities 404.5 404.5 — 366.4 366.4 — Other asset-backed securities: Vehicle receivables 31.1 31.1 — 17.2 17.2 — Credit card receivables 3.3 3.3 — 3.3 3.3 — Other 11.2 11.2 — 2.5 2.5 — Total other asset-backed securities 45.6 45.6 — 23.0 23.0 — Total mortgage and asset-backed securities 450.1 450.1 — 389.4 389.4 — Collateralized loan obligations 246.5 246.5 — 209.1 209.1 — Total mortgage and asset-backed securities $ 696.6 $ 696.6 $ — $ 598.5 $ 598.5 $ — (1) Represents publicly traded mortgage-backed securities which carry the full faith and credit guaranty of the U.S. Government (i.e., GNMA) or are guaranteed by a government sponsored entity (i.e., FNMA, FHLMC).
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 March 31, 2024 and December 31, 2023: Fair Value at Millions March 31, 2024 December 31, 2023 AAA $ 11.4 $ 11.5 AA 83.5 83.8 A 562.4 552.4 BBB 407.5 390.9 BB 1.8 — Other 6.5 6.4 Debt securities issued by corporations (1) $ 1,073.1 $ 1,045.0 (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 March 31, 2024 and December 31, 2023: Fair Value at March 31, 2024 Millions Ark/ WM Outrigger Kudu Other Total Kudu’s Participation Contracts $ — $ 884.2 $ — $ 884.2 PassportCard/DavidShield — — 150.0 150.0 Elementum Holdings, L.P. — — 35.0 35.0 Other unconsolidated entities (1) — — 47.1 47.1 Total unconsolidated entities — 884.2 232.1 1,116.3 Private equity funds and hedge funds 52.5 — 265.3 317.8 Bank loan fund 249.6 — — 249.6 Lloyd’s trust deposits 150.5 — — 150.5 ILS funds — — 109.7 109.7 Private debt instruments — 5.7 9.9 15.6 Other 32.4 — — 32.4 Total other long-term investments $ 485.0 $ 889.9 $ 617.0 $ 1,991.9 (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, 2023 Millions Ark/ WM Outrigger Kudu Other Total Kudu’s Participation Contracts $ — $ 890.5 $ — $ 890.5 PassportCard/DavidShield — — 150.0 150.0 Elementum Holdings, L.P. — — 35.0 35.0 Other unconsolidated entities (1) — — 48.1 48.1 Total unconsolidated entities — 890.5 233.1 1,123.6 Private equity funds and hedge funds 55.5 — 257.4 312.9 Bank loan fund 194.4 — — 194.4 Lloyd’s trust deposits 158.0 — — 158.0 ILS funds — — 160.5 160.5 Private debt instruments — 5.8 10.0 15.8 Other 33.0 — — 33.0 Total other long-term investments $ 440.9 $ 896.3 $ 661.0 $ 1,998.2 (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 March 31, 2024 and December 31, 2023: March 31, 2024 December 31, 2023 Millions Fair Value Unfunded Fair Value Unfunded Private equity funds Aerospace/Defense/Government $ 163.5 $ 14.8 $ 157.2 $ 17.4 Financial services 88.0 35.5 87.5 37.8 Real estate 3.7 2.4 3.9 2.5 Total private equity funds 255.2 52.7 248.6 57.7 Hedge funds Long/short equity financials and business services 57.7 — 54.4 — European small/mid cap 4.9 — 9.9 — Total hedge funds 62.6 — 64.3 — Total private equity funds and hedge funds included in other long-term investments $ 317.8 $ 52.7 $ 312.9 $ 57.7
Fair value of private equity funds subject to lock-up periods	The following table presents investments in private equity funds that were subject to lock-up periods as of March 31, 2024: Millions 1 – 3 years 3 – 5 years 5 – 10 years >10 years Total Private equity funds — expected lock-up period remaining $14.4 $63.5 $161.0 $16.3 $255.2
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 three months ended March 31, 2024 and 2023: Level 3 Investments Millions Other Long-term Other Long-term Balance at December 31, 2023 $ 1,138.2 Balance at December 31, 2022 $ 911.6 Net realized and unrealized gains (5.7) Net realized and unrealized gains 35.1 Purchases and contributions .1 Purchases and contributions 117.0 Sales and distributions (3.8) Sales and distributions (109.0) Transfers in — Transfers in — Transfers out — Transfers out — Balance at March 31, 2024 $ 1,128.8 Balance at March 31, 2023 $ 954.7
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 March 31, 2024 and December 31, 2023. The tables below exclude $15.7 million and $19.0 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 March 31, 2024 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 $884.2 18% - 25% 7x - 22x PassportCard/DavidShield Discounted cash flow $150.0 24% 4% Elementum Holdings, L.P. Discounted cash flow $35.0 21% 4% Preferred Securities Discounted cash flow $28.7 8% N/A Private debt instruments Discounted cash flow $15.2 11% - 12% 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 20% and 13x. (4) In the first three months of 2024, Kudu had negligible deployments into new and existing Participation Contracts. (5) As of March 31, 2024, one of Kudu’s Participation Contracts with a total fair value of $62.4 was 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. $ in Millions December 31, 2023 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 $890.5 18% - 25% 7x - 22x PassportCard/DavidShield Discounted cash flow $150.0 24% 4% Elementum Holdings, L.P. Discounted cash flow $35.0 21% 4% Preferred securities Discounted cash flow $28.2 8% N/A Private debt instruments Discounted cash flow $15.5 11% - 12% 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 20% and 13x. (4) In 2023, Kudu deployed a total of $163.8 into new and existing Participation Contracts. (5) As of December 31, 2023, one of Kudu’s Participation Contracts with a total fair value of $69.1 was 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)	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024 and December 31, 2023: $ in Millions Weighted Average Economic March 31, 2024 December 31, 2023 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 Bamboo (1) N/A 270.4 — 270.4 — — — Other Operations N/A 44.4 — 44.4 44.4 — 44.4 Total goodwill 439.2 — 439.2 168.8 — 168.8 Other intangible assets: Ark Underwriting capacity N/A 175.7 — 175.7 175.7 — 175.7 Kudu Trade names 7.0 2.2 1.6 .6 2.2 1.5 .7 Bamboo (1) Trade names 10.0 23.5 .5 23.0 — — — Agency relationships 6.0 72.4 2.9 69.5 — — — Developed Technology 3.0 4.7 .4 4.3 — — — Other 0.3 .4 .4 — — — — Subtotal 101.0 4.2 96.8 — — — Other Operations Trade names 13.3 13.3 4.5 8.8 13.3 4.1 9.2 Customer relationships 10.9 24.8 11.1 13.7 24.8 10.3 14.5 Other 11.1 3.1 1.3 1.8 2.8 1.1 1.7 Subtotal 41.2 16.9 24.3 40.9 15.5 25.4 Total other intangible assets 320.1 22.7 297.4 218.8 17.0 201.8 Total goodwill and other intangible assets $ 759.3 $ 22.7 $ 736.6 $ 387.6 $ 17.0 $ 370.6 (1) The relative fair values of goodwill and other intangible assets recognized in connection with the acquisition of Bamboo had not yet been finalized as of March 31, 2024. The following table presents the change in goodwill and other intangible assets for the three months ended March 31, 2024 and 2023: Three Months Ended March 31, 2024 2023 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 $ 201.8 $ 370.6 $ 176.5 $ 215.9 $ 392.4 Acquisition of businesses (1) 270.4 101.0 371.4 — — — Acquisitions of intangible assets (2) — .3 .3 — — — Dispositions (2) — — — (6.7) (6.9) (13.6) Measurement period adjustments (3) — — — (1.0) — (1.0) Amortization — (5.7) (5.7) — (1.6) (1.6) Ending balance $ 439.2 $ 297.4 $ 736.6 $ 168.8 $ 207.4 $ 376.2 (1) Amounts relate to the fair values of goodwill and other intangible assets recognized in connection with the acquisition of Bamboo, which had not yet been finalized as of March 31, 2024. (2) Relates to acquisitions and dispositions within Other Operations. (3) Measurement period adjustments relate to updated information about acquisition date fair values of assets acquired and liabilities assumed. During the three months ended March 31, 2023, adjustments relate to an acquisition within Other Operations.

Loss and Loss Adjustment Expe_2

Loss and Loss Adjustment Expense Reserves (Tables)	3 Months Ended
	Mar. 31, 2024
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 months ended March 31, 2024 and 2023: Three Months Ended March 31, Millions 2024 2023 Gross beginning balance $ 1,605.1 $ 1,296.5 Less: beginning reinsurance recoverable on unpaid losses (1) (340.8) (505.0) Net loss and loss adjustment expense reserves 1,264.3 791.5 Loss and loss adjustment expenses incurred relating to: Current year losses 180.3 139.2 Prior year losses (.3) 8.6 Net incurred loss and loss adjustment expenses 180.0 147.8 Loss and loss adjustment expenses paid relating to: Current year losses (4.0) (4.0) Prior year losses (96.7) (87.7) Net paid loss and loss adjustment expenses (100.7) (91.7) Change in TPC Providers’ participation (2) — 145.4 Foreign currency translation and other adjustments to loss and loss adjustment (3.6) 3.7 Net ending balance 1,340.0 996.7 Plus: ending reinsurance recoverable on unpaid losses 404.5 348.9 Gross ending balance $ 1,744.5 $ 1,345.6 (1) The beginning reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $145.4 as of December 31, 2022. (2) Amount represents the impact to net loss and loss adjustment expense reserves due to a change in the TPC Providers’ participation related to the annual RITC process.

Third-Party Reinsurance (Tables

Third-Party Reinsurance (Tables)	3 Months Ended
	Mar. 31, 2024
Reinsurance Disclosures [Abstract]
Effects of reinsurance	The following table summarizes the effects of reinsurance on written and earned premiums and on loss and LAE for the Ark/WM Outrigger segment for the three months ended March 31, 2024 and 2023: Millions Three Months Ended March 31, 2024 2023 Written premiums: Direct $ 285.0 $ 246.2 Assumed 587.1 563.2 Gross written premiums 872.1 809.4 Ceded (1) (274.1) (195.2) Net written premiums $ 598.0 $ 614.2 Earned premiums: Direct $ 194.4 $ 150.4 Assumed 192.4 162.1 Gross earned premiums 386.8 312.5 Ceded (2) (84.0) (57.4) Net earned premiums $ 302.8 $ 255.1 Loss and loss adjustment expenses: Gross $ 253.3 $ 162.4 Ceded (3) (73.3) (14.6) Net loss and loss adjustment expenses $ 180.0 $ 147.8 (1) The ceded written premiums exclude $34.3 and $44.1 ceded by Ark to WM Outrigger Re for the three months ended March 31, 2024 and 2023, which eliminate in White Mountains’s consolidated financial statements. (2) The ceded earned premiums exclude $10.3 and $5.2 ceded by Ark to WM Outrigger Re for the three months ended March 31, 2024 and 2023, which eliminate in White Mountains’s consolidated financial statements. (3) The ceded loss and loss adjustment expenses exclude $0.7 and $0.2 ceded by Ark to WM Outrigger Re for the three months ended March 31, 2024 and 2023, which eliminate in White Mountains’s consolidated financial statements. The following table presents the Ark/WM Outrigger segment’s reinsurance recoverables as of March 31, 2024 and December 31, 2023: Millions March 31, 2024 December 31, 2023 Reinsurance recoverables on unpaid losses (1) $ 404.5 $ 340.8 Reinsurance recoverables on paid losses 20.7 27.4 Ceded unearned premiums (2) 269.8 73.8 Reinsurance recoverables $ 695.0 $ 442.0 (1) The reinsurance recoverables on unpaid losses exclude $15.8 and $15.6 ceded by Ark to WM Outrigger Re as of March 31, 2024 and December 31, 2023, which eliminate in White Mountains’s consolidated financial statements. (2) The ceded unearned premiums exclude $24.0 and $5.7 ceded by Ark to WM Outrigger Re as of March 31, 2024 and December 31, 2023, which eliminate in White Mountains’s consolidated financial statements. The following table presents a schedule of written premiums and earned premiums included in the HG Global/BAM segment for the three months ended March 31, 2024 and 2023: Three Months Ended March 31, Millions 2024 2023 Written premiums: Direct $ 10.5 $ 9.2 Assumed — — Gross written premiums (1) $ 10.5 $ 9.2 Earned premiums: Direct $ 7.3 $ 7.0 Assumed .5 .7 Gross earned premiums (1) $ 7.8 $ 7.7 (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 March 31, 2024: $ in Millions As of March 31, 2024 A.M. Best Rating (1) Gross Collateral Net % of Total A+ or better $ 202.2 $ — $ 202.2 58.1 % A- to A 139.1 — 139.1 40.0 B++ or lower and not rated 83.9 (2) 77.3 (2) 6.6 1.9 Total $ 425.2 $ 77.3 $ 347.9 100.0 % (1) A.M. Best ratings as detailed above are: “A+ or better” (Superior) “A- to A” (Excellent), “B++” (Good). (2)

Debt (Tables)

Debt (Tables)	3 Months Ended
	Mar. 31, 2024
Debt Disclosure [Abstract]
Schedule of debt outstanding	The following table presents White Mountains’s debt outstanding as of March 31, 2024 and December 31, 2023: $ in Millions March 31, 2024 Effective (1) December 31, 2023 Effective (1) Ark 2007 Subordinated Notes, carrying value $ — $ 30.0 Ark 2021 Subordinated Notes Tranche 1 42.4 42.7 Ark 2021 Subordinated Notes Tranche 2 47.0 47.0 Ark 2021 Subordinated Notes Tranche 3 70.0 70.0 Unamortized issuance cost (4.0) (4.2) Ark 2021 Subordinated Notes, carrying value 155.4 155.5 Total Ark Subordinated Notes, carrying value 155.4 13.1% 185.5 11.0% HG Global Senior Notes (2) 150.0 11.9% 150.0 11.9% Unamortized discount and issuance cost (3.0) (3.1) HG Global Senior Notes, carrying value 147.0 146.9 Kudu Credit Facility 210.3 10.6% 210.3 10.1% Unamortized issuance cost (6.5) (6.5) Kudu Credit Facility, carrying value 203.8 203.8 Other Operations debt 26.4 10.2% 28.9 10.2% Unamortized issuance cost (.5) (.5) Other Operations debt, carrying value 25.9 28.4 Total debt $ 532.1 $ 534.6 (1) Effective rate includes the effect of the amortization of debt issuance costs and, where applicable, the original issue discount. (2) Effective rate excludes the effect of the interest rate cap. The following table presents the change in debt under the Kudu Credit Facility for the three months ended March 31, 2024 and 2023: Three Months Ended March 31, Millions 2024 2023 Kudu Credit Facility Beginning balance $ 210.3 $ 215.2 Term loans Borrowings — — Repayments — (16.9) Ending balance $ 210.3 $ 198.3

Municipal Bond Guarantee Insu_2

Municipal Bond Guarantee Insurance (Tables)	3 Months Ended
	Mar. 31, 2024
Guarantees [Abstract]
Schedule of municipal bond guarantee insured obligations	The following table presents a schedule of BAM’s insured obligations as of March 31, 2024 and December 31, 2023: March 31, 2024 December 31, 2023 Contracts outstanding 14,725 14,485 Remaining weighted average contract period (in years) 10.5 11.1 Contractual debt service outstanding (in millions): Principal $ 112,216.7 $ 109,673.8 Interest 57,545.8 56,012.2 Total debt service outstanding $ 169,762.5 $ 165,686.0 Gross unearned insurance premiums (in millions) $ 328.5 $ 325.8
Financial guarantee insurance contracts, premium received over contract period	The following table presents a schedule of BAM’s future premium revenues as of March 31, 2024: Millions March 31, 2024 April 1, 2024 - December 31, 2024 $ 22.4 January 1, 2025 - March 31, 2025 7.2 April 1, 2025 - June 30, 2025 7.2 July 1, 2025 - September 30, 2025 7.0 October 1, 2025 - December 31, 2025 6.9 Total 2025 28.3 2026 26.6 2027 24.9 2028 23.1 2029 21.4 2030 and thereafter 181.8 Total gross unearned insurance premiums $ 328.5
Effects of reinsurance	The following table summarizes the effects of reinsurance on written and earned premiums and on loss and LAE for the Ark/WM Outrigger segment for the three months ended March 31, 2024 and 2023: Millions Three Months Ended March 31, 2024 2023 Written premiums: Direct $ 285.0 $ 246.2 Assumed 587.1 563.2 Gross written premiums 872.1 809.4 Ceded (1) (274.1) (195.2) Net written premiums $ 598.0 $ 614.2 Earned premiums: Direct $ 194.4 $ 150.4 Assumed 192.4 162.1 Gross earned premiums 386.8 312.5 Ceded (2) (84.0) (57.4) Net earned premiums $ 302.8 $ 255.1 Loss and loss adjustment expenses: Gross $ 253.3 $ 162.4 Ceded (3) (73.3) (14.6) Net loss and loss adjustment expenses $ 180.0 $ 147.8 (1) The ceded written premiums exclude $34.3 and $44.1 ceded by Ark to WM Outrigger Re for the three months ended March 31, 2024 and 2023, which eliminate in White Mountains’s consolidated financial statements. (2) The ceded earned premiums exclude $10.3 and $5.2 ceded by Ark to WM Outrigger Re for the three months ended March 31, 2024 and 2023, which eliminate in White Mountains’s consolidated financial statements. (3) The ceded loss and loss adjustment expenses exclude $0.7 and $0.2 ceded by Ark to WM Outrigger Re for the three months ended March 31, 2024 and 2023, which eliminate in White Mountains’s consolidated financial statements. The following table presents the Ark/WM Outrigger segment’s reinsurance recoverables as of March 31, 2024 and December 31, 2023: Millions March 31, 2024 December 31, 2023 Reinsurance recoverables on unpaid losses (1) $ 404.5 $ 340.8 Reinsurance recoverables on paid losses 20.7 27.4 Ceded unearned premiums (2) 269.8 73.8 Reinsurance recoverables $ 695.0 $ 442.0 (1) The reinsurance recoverables on unpaid losses exclude $15.8 and $15.6 ceded by Ark to WM Outrigger Re as of March 31, 2024 and December 31, 2023, which eliminate in White Mountains’s consolidated financial statements. (2) The ceded unearned premiums exclude $24.0 and $5.7 ceded by Ark to WM Outrigger Re as of March 31, 2024 and December 31, 2023, which eliminate in White Mountains’s consolidated financial statements. The following table presents a schedule of written premiums and earned premiums included in the HG Global/BAM segment for the three months ended March 31, 2024 and 2023: Three Months Ended March 31, Millions 2024 2023 Written premiums: Direct $ 10.5 $ 9.2 Assumed — — Gross written premiums (1) $ 10.5 $ 9.2 Earned premiums: Direct $ 7.3 $ 7.0 Assumed .5 .7 Gross earned premiums (1) $ 7.8 $ 7.7 (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)	3 Months Ended
	Mar. 31, 2024
Earnings Per Share [Abstract]
Computation of earnings per share	The following table presents the Company’s computation of earnings per share from continuing operations for the three months ended March 31, 2024 and 2023. Three Months Ended March 31, 2024 2023 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common $ 236.4 $ 179.5 Allocation of (earnings) losses to participating restricted common shares (1) (2.7) (2.2) Basic and diluted earnings (losses) per share numerators $ 233.7 $ 177.3 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 2,559.7 2,569.9 Average unvested restricted common shares (2) (28.6) (31.1) Basic earnings (losses) per share denominator 2,531.1 2,538.8 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 2,559.7 2,569.9 Average unvested restricted common shares (2) (28.6) (31.1) Diluted earnings (losses) per share denominator 2,531.1 2,538.8 Basic and diluted earnings per share (in dollars): Distributed earnings - dividends declared and paid $ 1.00 $ 1.00 Undistributed earnings (losses) 91.33 68.83 Basic and diluted earnings (losses) per share $ 92.33 $ 69.83 (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (2) 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) for the three months ended March 31, 2024 and 2023. Three Months Ended March 31, Millions 2024 2023 Undistributed net earnings: Net income (loss) attributable to White Mountains’s common $ 233.7 $ 177.3 Dividends declared, net of restricted common share amounts (1) (2.5) (2.5) Total undistributed net earnings (losses), net of restricted common share amounts $ 231.2 $ 174.8 (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)	3 Months Ended
	Mar. 31, 2024
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 months ended March 31, 2024 and 2023 for performance shares granted under the WTM Incentive Plan: Three Months Ended March 31, 2024 2023 $ in Millions Target Performance Accrued Target Performance Accrued Beginning of period 37,031 $ 69.4 39,449 $ 67.5 Shares paid (1) (13,475) (44.9) (13,350) (40.8) New grants 10,405 — 10,895 — Forfeitures and cancellations (2) 46 .3 37 .4 Expense recognized — 19.4 — 9.3 End of period 34,007 $ 44.2 37,031 $ 36.4 (1) WTM performance share payments for the 2021-2023 performance cycle were made in March 2024 at 188% of target. WTM performance share payments for the 2020-2022 performance cycle were made in March 2023 at 200% of target. (2) Amounts include changes in assumed forfeitures, as required under GAAP.
Share-based compensation, performance shares award outstanding activity	The following table presents performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan as of March 31, 2024 for each performance cycle: March 31, 2024 $ in Millions Target Performance Accrued Performance cycle: 2022 – 2024 13,225 $ 35.0 2023 – 2025 10,895 9.4 2024 – 2026 10,405 .6 Sub-total 34,525 45.0 Assumed forfeitures (518) (.8) Total 34,007 $ 44.2
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 months ended March 31, 2024 and 2023: Three Months Ended March 31, 2024 2023 $ in Millions Restricted Unamortized Issue Restricted Unamortized Issue Non-vested, Beginning of period 37,595 $ 16.2 38,350 $ 15.5 Issued 10,405 18.3 10,895 16.0 Vested (13,475) — (11,650) — Forfeited — — — — Expense recognized — (3.2) — (3.2) End of period 34,525 $ 31.3 37,595 $ 28.3

Noncontrolling Interests (Table

Noncontrolling Interests (Tables)	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024 and December 31, 2023: March 31, 2024 December 31, 2023 $ in Millions Noncontrolling Percentage (1) Noncontrolling Equity Noncontrolling Percentage (1) Noncontrolling Equity Noncontrolling interests, excluding BAM Ark 28.0 % $ 334.7 (2) 28.0 % $ 336.9 (2) HG Global 3.1 % (.4) 3.1 % .8 Kudu 10.6 % 114.4 10.5 % 113.8 Bamboo 27.2 % 111.3 — % — Other various 9.3 various 9.4 Total, excluding BAM 569.3 460.9 BAM 100.0 % (149.1) 100.0 % (139.8) Total noncontrolling interests $ 420.2 $ 321.1 (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. (2) As of March 31, 2024 and December 31, 2023, Ark’s noncontrolling equity includes $17.9 and $21.0 related to management’s equity incentives.

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2024
Segment Reporting [Abstract]
Schedule of segments	The following tables present the financial information for White Mountains’s segments for the three months ended March 31, 2024 and 2023: Ark/WM Outrigger HG Global/BAM Other Operations Millions Ark WM Outrigger Re HG Global BAM (2) Kudu Bamboo Total Three Months Ended March 31, 2024 Earned insurance premiums (1) $ 292.5 $ 10.3 $ 6.5 $ 1.3 $ — $ 8.4 $ — $ 319.0 Net investment income (3) 17.0 2.9 5.4 4.3 17.2 .3 9.9 57.0 Net investment income (expense) - — — 6.6 (6.6) — — — — Net realized and unrealized investment gains (losses) (3) 10.6 — (7.3) (2.8) (6.5) (.1) 22.2 16.1 Net realized and unrealized investment gains — — — — — — 210.7 210.7 Commission and fee revenues — — — — — 21.9 3.6 25.5 Other revenues 3.5 — — .5 — .6 14.4 19.0 Total revenues 323.6 13.2 11.2 (3.3) 10.7 31.1 260.8 647.3 Loss and loss adjustment expenses 179.3 .7 — — — 5.8 — 185.8 Acquisition expenses 63.7 2.6 1.8 .4 — 3.1 — 71.6 Cost of sales — — — — — — 7.6 7.6 Broker commission expenses — — — — — 9.3 — 9.3 General and administrative expenses 42.2 — .4 16.9 3.4 12.0 50.3 125.2 Change in fair value of contingent consideration — — — — — — — — Interest expense 5.4 — 3.5 — 5.6 — .7 15.2 Total expenses 290.6 3.3 5.7 17.3 9.0 30.2 58.6 414.7 Pre-tax income (loss) $ 33.0 $ 9.9 $ 5.5 $ (20.6) $ 1.7 $ .9 $ 202.2 $ 232.6 (1) Ark’s earned insurance premiums based on the location of Ark’s underwriting offices in the United Kingdom and Bermuda are $177.4 and $115.1. (2) 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. (3) Bamboo’s net investment income and net realized and unrealized investment gains (losses) are included in other revenues in the consolidated statement of operations. Ark/WM Outrigger HG Global/BAM Other Operations Millions Ark WM Outrigger Re HG Global BAM (2) Kudu Total Three Months Ended March 31, 2023 Earned insurance premiums (1) $ 249.9 $ 5.2 $ 6.4 $ 1.3 $ — $ — $ 262.8 Net investment income 8.4 2.2 4.0 3.2 14.2 7.0 39.0 Net investment income (expense) - — — 6.6 (6.6) — — — Net realized and unrealized investment 24.5 — 7.9 9.1 29.6 41.8 112.9 Net realized and unrealized investment gains — — — — — 85.2 85.2 Commission and fee revenues — — — — — 3.3 3.3 Other revenues (2.7) — — .8 — 30.6 28.7 Total revenues 280.1 7.4 24.9 7.8 43.8 167.9 531.9 Loss and loss adjustment expenses 147.6 .2 — — — — 147.8 Acquisition expenses 58.9 .9 1.8 .9 — — 62.5 Cost of sales — — — — — 13.9 13.9 General and administrative expenses 35.1 .1 1.1 16.2 3.8 39.7 96.0 Change in fair value of contingent consideration (2.4) — — — — — (2.4) Interest expense 5.0 — 4.5 — 4.7 .8 15.0 Total expenses 244.2 1.2 7.4 17.1 8.5 54.4 332.8 Pre-tax income (loss) $ 35.9 $ 6.2 $ 17.5 $ (9.3) $ 35.3 $ 113.5 $ 199.1 (1) Ark’s earned insurance premiums based on the location of Ark’s underwriting offices in the United Kingdom and Bermuda are $146.0 and $104.0. (2)

Equity Method Eligible Invest_2

Equity Method Eligible Investments (Tables)	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024 and December 31, 2023: March 31, 2024 December 31, 2023 $ in Millions Ownership Interest Carrying Value Ownership Interest Carrying Value Kudu’s Participation Contracts (1) 4.1% - 30.0% $ 884.2 4.1% - 30.0% $ 890.5 Investment in MediaAlpha 34.7% 465.6 34.9% 254.9 PassportCard/DavidShield 53.8% 150.0 53.8% 150.0 Elementum Holdings, L.P. 26.6% 35.0 26.6% 35.0 Other equity method eligible investments, at fair value Under 50.0% 247.4 Under 50.0% 291.7 Other equity method eligible investments, at fair value 50.0% and over 24.8 50.0% and over 24.8 (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)	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024 and December 31, 2023: March 31, 2024 December 31, 2023 Millions Fair Value Carrying Value Fair Value Carrying Value Ark 2007 Subordinated Notes $ — $ — $ 30.5 $ 30.0 Ark 2021 Subordinated Notes $ 171.8 $ 155.4 $ 171.8 $ 155.5 HG Global Senior Notes $ 158.5 $ 147.0 $ 158.7 $ 146.9 Kudu Credit Facility $ 224.6 $ 203.8 $ 225.6 $ 203.8 Other Operations debt $ 27.4 $ 25.9 $ 30.0 $ 28.4

Basis of Presentation and Sig_3

Basis of Presentation and Significant Accounting Policies (Details) $ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($) segment	Dec. 31, 2023	Oct. 25, 2023 USD ($)
Basis of Presentation
Number of reportable segments | segment	4
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 - WTM Partners
Basis of Presentation
Expected capital contributions			$ 500
Subsidiary - Ark Insurance Holdings Limited
Basis of Presentation
Ownership percentage	72%	72%
Ownership percentage, after equity incentives	61.90%	61.90%
Subsidiary - WM Outrigger Re | Preferred Stock
Basis of Presentation
Ownership percentage	100%	100%
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 - Kudu Investment Management, LLC
Basis of Presentation
Ownership percentage		89.40%
Ownership percentage of basic units outstanding		89.60%
Ownership percentage, fully diluted	76.20%
Ownership percentage, fully converted		76.30%
Subsidiary - Bamboo Ide8 Insurance Services LLC
Basis of Presentation
Ownership percentage		72.80%
Ownership percentage, fully diluted	63.70%

Significant Transactions - Bamb

Significant Transactions - Bamboo (Details) - USD ($) $ in Millions		3 Months Ended
	Jan. 02, 2024	Dec. 31, 2022	Mar. 31, 2024
Bamboo Ide8 Insurance Services LLC
Other Significant Transactions [Line Items]
Assets and goodwill acquired	$ 479.5
Liabilities acquired	91.7
Noncontrolling interest acquired	111.1
Total goodwill and other intangible assets	371.4
Transaction costs	4		$ 0.3
Bamboo Ide8 Insurance Services LLC
Other Significant Transactions [Line Items]
Contributions to subsidiaries	296.7
Contribution of primary capital	36
Cash contributions for transaction	$ 20
Basic ownership	72.80%
Ownership percentage, after equity incentives	63.70%
Bamboo Ide8 Insurance Services LLC | Bamboo Management
Other Significant Transactions [Line Items]
Basic ownership	16.10%
Ownership percentage, after equity incentives	26.60%
Outrigger Re
Other Significant Transactions [Line Items]
Contributions to subsidiaries		$ 205

Significant Transactions - Sche

Significant Transactions - Schedule of Assets Acquired and Liabilities Assumed, Ark (Details) - USD ($) $ in Millions	Jan. 02, 2024	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Other Significant Transactions [Line Items]
Goodwill		$ 439.2	$ 168.8	$ 168.8	$ 176.5
Bamboo Ide8 Insurance Services LLC
Other Significant Transactions [Line Items]
Cash contributions for transaction	$ 20
Bamboo Ide8 Insurance Services LLC
Other Significant Transactions [Line Items]
Fixed maturity investments, at fair value	8.2
Short-term investments, at fair value	9.3
Restricted cash	37
Cash (restricted $37.0)	44.9
Premiums and commissions receivable	38
Other assets	7.7
Total tangible assets	108.1
Loss and loss adjustment expense reserves	(9.3)
Unearned insurance premiums	(20.1)
Premiums and commissions payable	(49.2)
Other liabilities	(13.1)
Total tangible liabilities	(91.7)
Net tangible assets acquired	16.4
Goodwill	270.4
Other intangible assets	101
Total goodwill and other intangible assets	371.4
Net assets acquired	$ 387.8

Significant Transactions - Outr

Significant Transactions - Outrigger Re (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Jan. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Mar. 31, 2024
Outrigger Re
Other Significant Transactions [Line Items]
Contributions to subsidiaries			$ 205
Outrigger Re
Other Significant Transactions [Line Items]
Ceded unearned premiums			$ 250
Rollover amount		$ 130
Short-term investments, at fair value		$ 265.3		$ 222.4
Return of capital	$ 68.1

Investment Securities - Schedul

Investment Securities - Schedule of Pre-Tax Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Net Investment Income [Line Items]
Total investment income	$ 57.7	$ 39.6
Third-party investment expenses	(0.7)	(0.6)
Net investment income	57	39
Fixed maturity investments
Net Investment Income [Line Items]
Total investment income	20.9	14.5
Short-term investments
Net Investment Income [Line Items]
Total investment income	15.7	9.7
Common equity securities
Net Investment Income [Line Items]
Total investment income	0.5	1.3
Other long-term investments
Net Investment Income [Line Items]
Total investment income	$ 20.6	$ 14.1

Investment Securities - Net Rea

Investment Securities - Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Gain (Loss) on Securities [Line Items]
Net realized and unrealized investment gains (losses) (1)	$ 226.8	$ 198.1
MediaAlpha
Gain (Loss) on Securities [Line Items]
Net realized and unrealized investment gains (losses) (1)	210.7	85.2
Net realized and unrealized investment (losses) gains
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	(5.7)	46.7
Net unrealized investment gains (losses)	232.5	151.4
Fixed maturity investments
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	(5.8)	(0.1)
Net unrealized investment gains (losses)	(7.4)	29.7
Short-term investments
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	(0.2)	(0.4)
Net unrealized investment gains (losses)	(1)	1.2
Common equity securities
Gain (Loss) on Securities [Line Items]
Net unrealized investment gains (losses)	23.1	29.4
Net realized and unrealized investment gains (losses) (1)	23.1	29.4
Equity Method Investments | MediaAlpha
Gain (Loss) on Securities [Line Items]
Net unrealized investment gains (losses)	210.7	85.2
Net realized and unrealized investment gains (losses) (1)	210.7	85.2
Other long-term investments
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	0.3	47.2
Net unrealized investment gains (losses)	7.1	5.9
Fixed maturity and short-term investments
Gain (Loss) on Securities [Line Items]
Net realized and unrealized investment gains (losses) (1)	(14.4)	30.4
Less: net realized and unrealized gains (losses) on investment securities sold during the period	(0.7)	1.8
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	(13.7)	28.6
Common equity securities and investment in MediaAlpha
Gain (Loss) on Securities [Line Items]
Net realized and unrealized investment gains (losses) (1)	233.8	114.6
Less: net realized and unrealized gains (losses) on investment securities sold during the period	0	0
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	233.8	114.6
Total Investments After TPC Providers - Investments
Gain (Loss) on Securities [Line Items]
Gain (loss) on investments, exchange rate effect	$ (11.5)	$ 5.4

Investment Securities - Investm

Investment Securities - Investment Gains (Losses) for Level 3 (Details) - Other long-term investments - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Gain (Loss) on Securities [Line Items]
Net unrealized investment gains (losses)	$ 7.1	$ 5.9
Level 3
Gain (Loss) on Securities [Line Items]
Net unrealized investment gains (losses)	$ (6.2)	$ 37

Investment Securities - Fixed M

Investment Securities - Fixed Maturity Investments (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	$ 2,351	$ 2,198.4
Gross Unrealized Gains	5	9.2
Gross Unrealized Losses	(97.6)	(97.1)
Net Foreign Currency Gains (Losses)	(3.8)	(1.2)
Carrying Value	2,254.6	2,109.3
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	247.6	209
Gross Unrealized Gains	0.1	0.3
Gross Unrealized Losses	(1.8)	(4.4)
Net Foreign Currency Gains (Losses)	0	0
Carrying Value	245.9	204.9
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	1,118.3	1,085.9
Gross Unrealized Gains	3	5.6
Gross Unrealized Losses	(46)	(45.4)
Net Foreign Currency Gains (Losses)	(2.2)	(1.1)
Carrying Value	1,073.1	1,045
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	251.4	275.1
Gross Unrealized Gains	0.6	0.9
Gross Unrealized Losses	(13)	(15.1)
Net Foreign Currency Gains (Losses)	0	0
Carrying Value	239	260.9
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	484.8	417.2
Gross Unrealized Gains	0.6	2
Gross Unrealized Losses	(35.3)	(29.8)
Net Foreign Currency Gains (Losses)	0	0
Carrying Value	450.1	389.4
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	248.9	211.2
Gross Unrealized Gains	0.7	0.4
Gross Unrealized Losses	(1.5)	(2.4)
Net Foreign Currency Gains (Losses)	(1.6)	(0.1)
Carrying Value	$ 246.5	$ 209.1

Investment Securities - Cost an

Investment Securities - Cost and Amortized Cost Maturity Schedule (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Fixed Maturities, Excluding Asset-Backed Securities And Collateralized Loan Obligations
Cost or Amortized Cost
Due in one year or less	$ 300.5
Due after one year through five years	886.3
Due after five years through ten years	295.7
Due after ten years	134.8
Carrying Value
Due in one year or less	296.1
Due after one year through five years	856.8
Due after five years through ten years	277.8
Due after ten years	127.3
Mortgage and asset-backed securities and collateralized loan obligations
Cost or Amortized Cost
Mortgage and asset-backed securities and collateralized loan obligations	733.7
Carrying Value
Mortgage and asset-backed securities and collateralized loan obligations	696.6
Fixed maturity investments
Cost or Amortized Cost
Total fixed maturity investments	2,351	$ 2,198.4
Carrying Value
Total fixed maturity investments	$ 2,254.6	$ 2,109.3

Investment Securities - Common

Investment Securities - Common Equity Securities and Other Long Term Investments (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Common equity securities
Cost or Amortized Cost	$ 483.5	$ 483.5
Gross Unrealized Gains	86	62.3
Gross Unrealized Losses	(1.2)	(1.2)
Net Foreign Currency Gains (Losses)	(6.8)	(6.2)
Carrying Value	561.5	538.4
Other long-term investments
Cost or Amortized Cost	1,642.2	1,655.7
Gross Unrealized Gains	466.7	446.3
Other Long-Term Investments, Unrealized Loss	(101.1)	(94.5)
Net Foreign Currency Gains (Losses)	(15.9)	(9.3)
Carrying Value	1,991.9	1,998.2
MediaAlpha
Investment in MediaAlpha
Cost or Amortized Cost	59.2	59.2
Gross Unrealized Gains	406.4	195.7
Gross Unrealized Losses	0	0
Net Foreign Currency Gains (Losses)	0	0
Carrying Value	$ 465.6	$ 254.9

Investment Securities - Narrati

Investment Securities - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended
	Mar. 28, 2024 USD ($) $ / shares	Mar. 31, 2024 USD ($) fund $ / shares shares	Jun. 30, 2023 USD ($) $ / shares	Dec. 31, 2023 USD ($)
Concentration Risk [Line Items]
Investments held in trusts		$ 589.6		$ 585.6
Assets held by insurance regulators		4.7		4.6
Other long-term investments		$ 1,991.9		1,998.2
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	$ 465.6	$ 465.6		254.9
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments		150.5		158
Lloyds Bank
Concentration Risk [Line Items]
Fair value investments		345.6		344.7
Outrigger Re
Concentration Risk [Line Items]
Short-term investments		222.4		$ 265.3
Bamboo Captive
Concentration Risk [Line Items]
Short-term investments		$ 20.7
MediaAlpha
Concentration Risk [Line Items]
Tender offer purchase price			$ 5.9
Purchase price (in dollars per share) | $ / shares	$ 20.37		$ 10
Investment owned (in shares) | shares		22.9
Ownership interest (as a percent)		34.70%		34.90%
Fully diluted ownership interest (as a percent)		32.40%
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		$ 59.7		$ 57.3
HG Global Senior Notes | Senior Notes
Concentration Risk [Line Items]
Interest reserve account		30.5		30.4
Kudu Credit Facility | Secured Debt
Concentration Risk [Line Items]
Restricted cash		15.1		14.9
Asset Pledged as Collateral
Concentration Risk [Line Items]
Short-term investments		201.9		198.9
Asset Pledged as Collateral | Additional Investments, Insurance Regulators and Reinsurance Counterparties
Concentration Risk [Line Items]
Fair value investments		204.6		244.5
Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value		246.5		209.1
Collateralized loan obligations | Fair value measured on a recurring basis
Concentration Risk [Line Items]
Fixed maturity investments, at fair value		246.5		209.1
Private equity funds
Concentration Risk [Line Items]
Other long-term investments		$ 255.2
Number of investments | fund		16
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		$ 249.6		194.4
Foreign regulatory deposits
Concentration Risk [Line Items]
Other long-term investments		150.5		158
ILS funds
Concentration Risk [Line Items]
Other long-term investments		$ 109.7		$ 160.5
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	Mar. 31, 2024	Mar. 28, 2024	Dec. 31, 2023
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	$ 561.5		$ 538.4
Other long-term investments	1,991.9		1,998.2
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	245.9		204.9
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	1,073.1		1,045
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	239		260.9
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	450.1		389.4
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	246.5		209.1
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	2,254.6		2,109.3
Level 2 | Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	450.1		389.4
Level 2 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	246.5		209.1
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	6,328.1		6,388.7
Fair value measured on a recurring basis | MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	465.6	$ 465.6	254.9
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	245.9		204.9
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	1,073.1		1,045
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	323.7		309.8
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	219.3		214.6
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	128.2		121.6
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	121.9		121.3
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	101.6		108.9
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	68.8		70.2
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	41.3		46.3
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	37.5		28.9
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	30.8		23.4
Fair value measured on a recurring basis | Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	239		260.9
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	450.1		389.4
Fair value measured on a recurring basis | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	246.5		209.1
Fair value measured on a recurring basis | Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	2,254.6		2,109.3
Fair value measured on a recurring basis | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	1,054.5		1,487.9
Fair value measured on a recurring basis | Exchange-traded funds
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	151.6		137.8
Fair value measured on a recurring basis | Common stock, other
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	409.9		400.6
Fair value measured on a recurring basis | Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	561.5		538.4
Fair value measured on a recurring basis | Other Long-Term Investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,991.9		1,998.2
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	1,155.9		1,164.4
Fair value measured on a recurring basis | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,893.4		2,055.2
Fair value measured on a recurring basis | Level 1 | MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	465.6		254.9
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	245.1		204.9
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 | 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 | 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 | 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	245.1		204.9
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	1,031.1		1,457.6
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	151.6		137.8
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	151.6		137.8
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	2,469.9		2,361.5
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	0.8		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	1,073.1		1,045
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	323.7		309.8
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	219.3		214.6
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	128.2		121.6
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	121.9		121.3
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	101.6		108.9
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	68.8		70.2
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	41.3		46.3
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	37.5		28.9
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	30.8		23.4
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	239		260.9
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	450.1		389.4
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	246.5		209.1
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	2,009.5		1,904.4
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	23.4		30.3
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	409.9		400.6
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	409.9		400.6
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	27.1		26.2
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	27.1		26.2
Fair value measured on a recurring basis | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,128.8		1,138.2
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 | 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 | 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 | 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	1,128.8		1,138.2
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	1,128.8		1,138.2
Fair value measured on a recurring basis | NAV | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	$ 836		$ 833.8

Investment Securities - Debt Se

Investment Securities - Debt Securities Issued by Corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	$ 1,073.1	$ 1,045
AAA
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	11.4	11.5
AA
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	83.5	83.8
A
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	562.4	552.4
BBB
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	407.5	390.9
BB
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	1.8	0
Other
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	$ 6.5	$ 6.4

Investment Securities - Mortgag

Investment Securities - Mortgage and Asset-Backed Securities and Collateralized Loan Obligations (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Total mortgage and asset-backed securities and collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	$ 696.6	$ 598.5
Total mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	450.1	389.4
Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	404.5	366.4
Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	45.6	23
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	246.5	209.1
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	696.6	598.5
Level 2 | Total mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	450.1	389.4
Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	404.5	366.4
Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	45.6	23
Level 2 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	246.5	209.1
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	0.1	0.2
Total non-agency | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.1	0.2
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	404.4	366.2
Total agency | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	404.4	366.2
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	216.7	198.8
FNMA | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	216.7	198.8
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	147.8	137.2
FHLMC | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	147.8	137.2
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	39.9	30.2
GNMA | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	39.9	30.2
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	0.1	0.2
Non-agency: Residential | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.1	0.2
Non-agency: Residential | Level 3 | Mortgage-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	31.1	17.2
Vehicle receivables | Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	31.1	17.2
Vehicle receivables | Level 3 | Other asset-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	3.3	3.3
Credit card receivables | Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	3.3	3.3
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
Other | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	11.2	2.5
Other | Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	11.2	2.5
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	Mar. 31, 2024	Dec. 31, 2023
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	$ 1,991.9	$ 1,998.2
PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	150	150
Elementum Holdings, L.P.
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	35	35
Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	47.1	48.1
Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	317.8	312.9
Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	249.6	194.4
Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	150.5	158
ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	109.7	160.5
Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	15.6	15.8
Other
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	32.4	33
Kudu’s Participation Contracts | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	884.2	890.5
Investments, Excluding MediaAlpha | Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,116.3	1,123.6
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	485	440.9
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) | 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	52.5	55.5
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	249.6	194.4
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	150.5	158
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) | 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	32.4	33
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 | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	889.9	896.3
Operating Segments | Asset Management (Kudu) | PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Asset Management (Kudu) | Elementum Holdings, L.P.
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Asset Management (Kudu) | Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Asset Management (Kudu) | Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Asset Management (Kudu) | Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Asset Management (Kudu) | Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Asset Management (Kudu) | ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Asset Management (Kudu) | Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	5.7	5.8
Operating Segments | Asset Management (Kudu) | Other
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Asset Management (Kudu) | Kudu’s Participation Contracts | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	884.2	890.5
Operating Segments | Asset Management (Kudu) | Investments, Excluding MediaAlpha | Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	884.2	890.5
Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	617	661
Other Operations | PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	150	150
Other Operations | Elementum Holdings, L.P.
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	35	35
Other Operations | Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	47.1	48.1
Other Operations | Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	265.3	257.4
Other Operations | Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Other Operations | Lloyd’s trust deposits
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	109.7	160.5
Other Operations | Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	9.9	10
Other Operations | Other
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	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	$ 232.1	$ 233.1

Investment Securities - Private

Investment Securities - Private Equity Funds and Hedge Funds (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
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	$ 317.8	$ 312.9
Unfunded Commitments	52.7	57.7
Private equity funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	255.2	248.6
Unfunded Commitments	52.7	57.7
Private equity funds | Aerospace/Defense/Government
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	163.5	157.2
Unfunded Commitments	14.8	17.4
Private equity funds | Financial services
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	88	87.5
Unfunded Commitments	35.5	37.8
Private equity funds | Real estate
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	3.7	3.9
Unfunded Commitments	2.4	2.5
Hedge funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	62.6	64.3
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	57.7	54.4
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	4.9	9.9
Unfunded Commitments	$ 0	$ 0

Investment Securities - Inves_2

Investment Securities - Investments In Private Equity Funds Subject to Lock-Up Periods (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Debt and Equity Securities, FV-NI [Line Items]
1 – 3 years	$ 14.4
3 – 5 years	63.5
5 – 10 years	161
>10 years	16.3
Total	1,991.9	$ 1,998.2
Private equity funds
Debt and Equity Securities, FV-NI [Line Items]
Total	$ 255.2

Investment Securities - Rollfor

Investment Securities - Rollforward of Fair Value Measurements by Level (Details) - Other long-term investments - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 1,138.2	$ 911.6
Net realized and unrealized gains	(5.7)	35.1
Purchases and contributions	0.1	117
Sales and distributions	(3.8)	(109)
Transfers in	0	0
Transfers out	0	0
Ending balance	$ 1,128.8	$ 954.7

Investment Securities - Signifi

Investment Securities - Significant Unobservable Inputs (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
	Mar. 31, 2024	Jun. 30, 2023	Dec. 31, 2023
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	$ 1,991.9		$ 1,998.2
Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
New and existing contracts deployed			163.8
Level 3 | Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	15.7		19
Level 3 | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	15.2		15.5
Level 3 | Discounted cash flow | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	884.2		890.5
Level 3 | Discounted cash flow | PassportCard/DavidShield
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	150		150
Level 3 | Discounted cash flow | Elementum Holdings, L.P.
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	35		35
Level 3 | Discounted cash flow | Preferred Securities
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	28.7		28.2
Level 3 | Probability weighted expected return method | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	$ 62.4		$ 69.1
Level 3 | Discount Rate | Discounted cash flow | PassportCard/DavidShield
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	24%		24%
Level 3 | Discount Rate | Discounted cash flow | Elementum Holdings, L.P.
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	21%		21%
Level 3 | Discount Rate | Discounted cash flow | Preferred Securities
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	8%		8%
Level 3 | Discount Rate | Discounted cash flow | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	11%		11%
Level 3 | Discount Rate | Discounted cash flow | Minimum | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	18%		18%
Level 3 | Discount Rate | Discounted cash flow | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	12%		12%
Level 3 | Discount Rate | Discounted cash flow | Maximum | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	25%		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%		2,200%
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	20%	20%
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%	1,300%

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	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 439.2	$ 168.8	$ 168.8	$ 176.5
Other intangible assets, finite-lived, accumulated amortization	22.7	17
Total other intangible assets	320.1	218.8
Total other intangible assets	297.4	201.8	207.4	215.9
Total goodwill and other intangible assets	759.3	387.6
Goodwill and other intangible assets	736.6	370.6	$ 376.2	$ 392.4
Other
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, finite-lived, accumulated amortization	22.7	17
Total other intangible assets	297.4	201.8
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 | Asset Management (Kudu)
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	7.6	7.6
Goodwill and other intangible assets	$ 8.2	8.3
Operating Segments | Asset Management (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	1.6	1.5
Other intangible assets, finite-lived, net carrying value	0.6	0.7
Operating Segments | P&C Insurance Distribution (Bamboo)
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	270.4	0
Other intangible assets, finite-lived, acquisition date fair value	101	0
Other intangible assets, finite-lived, accumulated amortization	4.2	0
Other intangible assets, finite-lived, net carrying value	96.8	0
Goodwill and other intangible assets	$ 367.2	0
Operating Segments | P&C Insurance Distribution (Bamboo) | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	10 years
Other intangible assets, finite-lived, acquisition date fair value	$ 23.5	0
Other intangible assets, finite-lived, accumulated amortization	0.5	0
Other intangible assets, finite-lived, net carrying value	$ 23	0
Operating Segments | P&C Insurance Distribution (Bamboo) | Agency relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	6 years
Other intangible assets, finite-lived, acquisition date fair value	$ 72.4	0
Other intangible assets, finite-lived, accumulated amortization	2.9	0
Other intangible assets, finite-lived, net carrying value	$ 69.5	0
Operating Segments | P&C Insurance Distribution (Bamboo) | Developed Technology
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	3 years
Other intangible assets, finite-lived, acquisition date fair value	$ 4.7	0
Other intangible assets, finite-lived, accumulated amortization	0.4	0
Other intangible assets, finite-lived, net carrying value	$ 4.3	0
Operating Segments | P&C Insurance Distribution (Bamboo) | Other
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	3 months 18 days
Other intangible assets, finite-lived, acquisition date fair value	$ 0.4	0
Other intangible assets, finite-lived, accumulated amortization	0.4	0
Other intangible assets, finite-lived, net carrying value	0	0
Other Operations
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	44.4	44.4
Other intangible assets, finite-lived, acquisition date fair value	41.2	40.9
Other intangible assets, finite-lived, accumulated amortization	16.9	15.5
Other intangible assets, finite-lived, net carrying value	24.3	25.4
Goodwill and other intangible assets	$ 68.7	69.8
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	13.3
Other intangible assets, finite-lived, accumulated amortization	4.5	4.1
Other intangible assets, finite-lived, net carrying value	$ 8.8	9.2
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	24.8
Other intangible assets, finite-lived, accumulated amortization	11.1	10.3
Other intangible assets, finite-lived, net carrying value	$ 13.7	14.5
Other Operations | Other
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	11 years 1 month 6 days
Other intangible assets, finite-lived, acquisition date fair value	$ 3.1	2.8
Other intangible assets, finite-lived, accumulated amortization	1.3	1.1
Other intangible assets, finite-lived, net carrying value	$ 1.8	$ 1.7

Goodwill and Other Intangible_4

Goodwill and Other Intangible Assets - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Impairments to goodwill and other intangibles	$ 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
	Mar. 31, 2024	Mar. 31, 2023
Goodwill
Goodwill, beginning balance	$ 168.8	$ 176.5
Acquisition of businesses	270.4	0
Dispositions	0	(6.7)
Measurement period adjustments	0	(1)
Goodwill, ending balance	439.2	168.8
Other Intangible Assets
Other intangible assets (excluding goodwill), beginning balance	201.8	215.9
Acquisition of businesses	101	0
Acquisition of intangible assets	0.3	0
Dispositions	0	(6.9)
Amortization	(5.7)	(1.6)
Other intangible assets (excluding goodwill), ending balance	297.4	207.4
Total Goodwill and Other Intangible Assets
Total Goodwill and Other Intangible Assets, beginning balance	370.6	392.4
Acquisition of businesses	371.4	0
Acquisition of intangible assets	0.3	0
Dispositions	0	(13.6)
Measurement period adjustments	0	(1)
Amortization	(5.7)	(1.6)
Total Goodwill and Other Intangible Assets, ending balance	$ 736.6	$ 376.2

Loss and Loss Adjustment Expe_3

Loss and Loss Adjustment Expense Reserves - Reserve Activity (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2021	Dec. 31, 2020
Loss and loss adjustment expenses incurred relating to:
Net incurred loss and loss adjustment expenses	$ 185.8	$ 147.8
Syndicate - Ark Insurance Holdings Limited
Loss and loss adjustment expenses paid relating to:
Percent of underwriting capital provided by third parties			0%	42.80%
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance	1,605.1	1,296.5
Less: beginning reinsurance recoverable on unpaid losses	(340.8)	(505)
Net loss and loss adjustment expense reserves	1,264.3	791.5
Loss and loss adjustment expenses incurred relating to:
Current year losses	180.3	139.2
Prior year losses	(0.3)	8.6
Net incurred loss and loss adjustment expenses	180	147.8
Loss and loss adjustment expenses paid relating to:
Current year losses	(4)	(4)
Prior year losses	(96.7)	(87.7)
Net paid loss and loss adjustment expenses	(100.7)	(91.7)
Change in TPC providers’ participation	0	145.4
Foreign currency translation and other adjustments to loss and loss adjustment expense reserves	(3.6)	3.7
Net ending balance	1,340	996.7
Plus: ending reinsurance recoverable on unpaid losses	(404.5)	(348.9)
Gross ending balance	1,744.5	1,345.6
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance	1,605.1
Loss and loss adjustment expenses incurred relating to:
Net incurred loss and loss adjustment expenses	180	147.8
Loss and loss adjustment expenses paid relating to:
Gross ending balance	1,744.5
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		(145.4)
P&C Insurance Distribution (Bamboo) | Operating Segments
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance	0
Loss and loss adjustment expenses incurred relating to:
Net incurred loss and loss adjustment expenses	5.8	$ 0
Loss and loss adjustment expenses paid relating to:
Gross ending balance	$ 12.3

Loss and Loss Adjustment Expe_4

Loss and Loss Adjustment Expense Reserves - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Net incurred loss and loss adjustment expenses	$ 185.8	$ 147.8
Syndicate - Ark Insurance Holdings Limited
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of underwriting capital provided by third parties					0%	42.80%
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year losses	(0.3)	8.6
Loss and loss adjustment expense reserves	1,744.5	1,345.6	$ 1,605.1	$ 1,296.5
Net incurred loss and loss adjustment expenses	180	147.8
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	1,744.5		1,605.1
Net incurred loss and loss adjustment expenses	180	147.8
P&C Insurance Distribution (Bamboo) | Operating Segments
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	12.3		$ 0
Net incurred loss and loss adjustment expenses	$ 5.8	$ 0

Third-Party Reinsurance - Effec

Third-Party Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Earned premiums:
Net earned premiums	$ 319	$ 262.8
Loss and loss adjustment expenses:
Net incurred loss and loss adjustment expenses	185.8	147.8
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Written premiums:
Direct	285	246.2
Assumed	587.1	563.2
Gross written premiums	872.1	809.4
Ceded	(274.1)	(195.2)
Net written premiums	598	614.2
Earned premiums:
Direct	194.4	150.4
Assumed	192.4	162.1
Gross earned premiums	386.8	312.5
Ceded	(84)	(57.4)
Net earned premiums	302.8	255.1
Loss and loss adjustment expenses:
Gross	253.3	162.4
Ceded	(73.3)	(14.6)
Net incurred loss and loss adjustment expenses	180	147.8
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark
Written premiums:
Ceded	(34.3)	44.1
Earned premiums:
Ceded	(10.3)	5.2
Loss and loss adjustment expenses:
Ceded	$ (0.7)	$ 0.2

Third-Party Reinsurance - Reins

Third-Party Reinsurance - Reinsurance Recoverables (Details) - P&C Insurance and Reinsurance (Ark/WM Outrigger) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Reinsurance recoverables on unpaid losses (1)	$ 404.5	$ 340.8
Reinsurance recoverables on paid losses	20.7	27.4
Ceded unearned premiums (2)	269.8	73.8
Reinsurance recoverables	695	442
Ark
Segment Reporting Information [Line Items]
Reinsurance recoverables on unpaid losses (1)	(15.8)	(15.6)
Ceded unearned premiums (2)	$ (24)	$ (5.7)

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	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Reinsurance recoverables	$ 425.2
Collateral	695	$ 442
Net	347.9
Reinsurance recoverables on unpaid losses (1)	404.5	340.8
Ark
Segment Reporting Information [Line Items]
Reinsurance recoverables on unpaid losses (1)	$ (15.8)	$ (15.6)
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	$ 77.3
A+ or better
Segment Reporting Information [Line Items]
Reinsurance recoverables	202.2
Net	$ 202.2
A+ or better | Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting Information [Line Items]
% of Total	58.10%
A+ or better | Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 0
A- to A
Segment Reporting Information [Line Items]
Reinsurance recoverables	139.1
Net	$ 139.1
A- to A | Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting Information [Line Items]
% of Total	40%
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	83.9
Net	$ 6.6
B++ or lower and not rated | Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting Information [Line Items]
% of Total	1.90%
B++ or lower and not rated | Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 77.3

Third-Party Reinsurance - Narra

Third-Party Reinsurance - Narrative (Details) - Third Point Re - P&C Insurance and Reinsurance (Ark/WM Outrigger) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Annual crediting rate	3%
Carrying value of investment	$ 20.6	$ 20.4

Debt - Summary of Debt Outstand

Debt - Summary of Debt Outstanding (Details) € in Millions, $ in Millions	Mar. 31, 2024 USD ($)	Mar. 31, 2024 EUR (€)	Dec. 31, 2023 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Debt Instrument
Total debt	$ 532.1		$ 534.6
Ark 2021 Subordinated Notes
Debt Instrument
Total debt	155.4		155.5
HG Global Senior Notes
Debt Instrument
Total debt	147		146.9
Kudu Credit Facility
Debt Instrument
Total debt	203.8		203.8
Other Operations debt
Debt Instrument
Total debt	25.9		28.4
Subordinated Debt | Ark
Debt Instrument
Total debt	$ 155.4		$ 185.5
Effective Rate	13.10%	13.10%	11%
Subordinated Debt | Ark 2007 Notes | Ark Subordinated Debt
Debt Instrument
Total debt	$ 0		$ 30
Subordinated Debt | Ark 2021 Subordinated Notes
Debt Instrument
Unamortized issuance cost	(4)		(4.2)
Total debt	155.4		155.5
Subordinated Debt | Ark 2021 Subordinated Notes Tranche 1
Debt Instrument
Amount borrowed	42.4	€ 39.1	42.7
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
Senior Notes | HG Global Senior Notes
Debt Instrument
Amount borrowed	150		150
Unamortized discount and issuance cost	(3)		(3.1)
Total debt	$ 147		$ 146.9
Effective Rate	11.90%	11.90%	11.90%
Secured Debt | Kudu Credit Facility
Debt Instrument
Amount borrowed	$ 210.3		$ 210.3
Unamortized issuance cost	(6.5)		(6.5)
Total debt	$ 210.3		$ 210.3	$ 198.3	$ 215.2
Effective Rate	10.60%	10.60%	10.10%
Secured Debt | Other Operations debt
Debt Instrument
Amount borrowed	$ 26.4		$ 28.9
Unamortized issuance cost	$ (0.5)		$ (0.5)
Effective Rate	10.20%	10.20%	10.20%

Debt - Narrative (Details)

Debt - Narrative (Details) € in Millions

1 Months Ended

3 Months Ended

6 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 ($)

Mar. 31, 2024 USD ($) credit_facility

Mar. 31, 2023 USD ($)

Sep. 30, 2021 USD ($)

Dec. 31, 2023 USD ($)

Jun. 30, 2023

Mar. 31, 2024 EUR (€) credit_facility

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 (€)

Debt Instrument

Repayments of notes

$ 32,600,000

$ 20,000,000

Interest Rate Cap | Subsidiary - HG Global

Debt Instrument

Notional amount

$ 150,000,000

Ark 2007 Subordinated Notes | Junior Subordinated Debt

Debt Instrument

Face value

$ 30,000,000

Repayments of notes

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 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

42,400,000

$ 42,700,000

€ 39.1

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%

Debt instrument, period for interest rate increase

10 years

Solvency capital requirement

120%

Debt to capital requirement

40%

Outstanding balance

$ 70,000,000

70,000,000

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

$ 125,000,000

Letters of credit outstanding

111,500,000

Pledged collateral

136,900,000

Lloyds LOC Facility | Line of Credit | Letter of Credit

Debt Instrument

Total commitment under revolving credit facility

$ 100,000,000

Letters of credit outstanding

35,400,000

Pledged collateral

$ 65,000,000

HG Global Senior Notes | Senior Notes

Debt Instrument

Face value

$ 150,000,000

Additional interest rate

1%

Outstanding balance

$ 150,000,000

150,000,000

Stated margin

6%

Anniversary of funding date

5 years

Annual payments

$ 15,000,000

Interest accrued multiplier

8

Interest reserve account, maximum

$ 29,300,000

Interest reserve account

30,500,000

30,400,000

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

Debt Instrument

Basis spread on variable rate

0.26%

Kudu Credit Facility | Secured Debt

Debt Instrument

Face value

300,000,000

Repayments of notes

0

$ 16,900,000

Outstanding balance

210,300,000

210,300,000

Stated margin

4.30%

4.30%

Restricted cash

$ 15,100,000

$ 14,900,000

Debt covenant, maximum loan to value percent

23.10%

23.10%

Percentage of borrowing base

35%

35%

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 | London Interbank Offered Rate (LIBOR)

Debt Instrument

Basis spread on variable rate

0.25%

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

Debt Instrument

Basis spread on variable rate

0.26%

0.26%

Other Operations debt | Secured Debt

Debt Instrument

Outstanding balance

$ 26,400,000

$ 28,900,000

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
	Mar. 31, 2024	Mar. 31, 2023
Debt Instrument
Beginning balance	$ 534.6
Repayments	(32.6)	$ (20)
Ending balance	532.1
Kudu Credit Facility
Debt Instrument
Beginning balance	203.8
Ending balance	203.8
Secured Debt | Kudu Credit Facility
Debt Instrument
Beginning balance	210.3	215.2
Borrowings	0	0
Repayments	0	(16.9)
Ending balance	$ 210.3	$ 198.3

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Investments, Owned, Federal Income Tax Note [Line Items]
Net deferred tax benefit	$ 68
Effective tax rate	4.60%	6%
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark
Investments, Owned, Federal Income Tax Note [Line Items]
Net deferred tax benefit	$ 51
Financial Guarantee (HG Global/BAM) | HG Global
Investments, Owned, Federal Income Tax Note [Line Items]
Net deferred tax benefit	$ 17

Derivatives (Details)

Derivatives (Details) - Interest Rate Cap - Subsidiary - HG Global - USD ($)	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Jun. 16, 2022
Derivative [Line Items]
Notional amount				$ 150,000,000
Premiums paid				$ 3,300,000
Proceeds from Periodic Settlement of Derivatives	$ 700,000	$ 200,000
Derivative, fair value, net	3,100,000		$ 2,900,000
Interest Expense
Derivative [Line Items]
Gain (loss) on fair value of derivative instruments	$ 200,000	$ (700,000)
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 ($)	1 Months Ended	3 Months Ended		12 Months Ended
	Dec. 31, 2023	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2018	Jan. 01, 2025	Dec. 31, 2024
Guarantor Obligations [Line Items]
Assets held-in-trust	$ 948,500,000	$ 933,100,000
Collateral trusts, investments and cash	588,600,000	592,800,000
Surplus notes	322,200,000	322,200,000
Interest receivable	37,700,000	18,100,000
Debt	534,600,000	532,100,000
Subsidiary - HG Global
Guarantor Obligations [Line Items]
Purchase of surplus notes		$ 503,000,000
Percentage of risk premium ceded		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	341,600,000	$ 330,700,000
Supplement trust target		603,000,000
Subsidiary - HG Global | Supplemental Trust
Guarantor Obligations [Line Items]
Assets held-in-trust	606,900,000	602,400,000
Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Issuance of insurance linked securities				$ 150,000,000	$ 150,000,000	$ 100,000,000
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				$ 150,000,000	$ 150,000,000	$ 100,000,000
Reinsured amount		$ 125,000,000
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		21%
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		27%
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		28%
BAM Surplus Notes | Surplus Notes
Guarantor Obligations [Line Items]
Repayments of unsecured debt	27,400,000	$ 0	$ 0
Accrued interest payment held in trust	2,000,000
Repayment of principal	17,800,000
Accrued interest payment held outside of trust	7,600,000
Debt	322,200,000	322,200,000
Interest receivable	$ 174,500,000	$ 181,100,000
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%	8.20%
Maximum | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Reinsurance, maximum amount reinsured				276,700,000	301,700,000	276,100,000
Maximum | Subsidiary - HG Global
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured		15%
Minimum | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Reinsurance obligation				$ 110,000,000	$ 135,000,000	$ 165,000,000

Municipal Bond Guarantee Insu_4

Municipal Bond Guarantee Insurance - Schedule of Municipal Bond Guarantee Insured Obligations (Details) - BAM $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024 USD ($) contract	Dec. 31, 2023 USD ($) contract
Guarantor Obligations [Line Items]
Contracts outstanding | contract	14,725	14,485
Remaining weighted average contract period (in years)	10 years 6 months	11 years 1 month 6 days
Principal	$ 112,216.7	$ 109,673.8
Interest	57,545.8	56,012.2
Total debt service outstanding	169,762.5	165,686
Total gross unearned insurance premiums	$ 328.5	$ 325.8

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	Mar. 31, 2024 USD ($)
Guarantor Obligations [Line Items]
April 1, 2024 - December 31, 2024	$ 22.4
January 1, 2025 - March 31, 2025	7.2
April 1, 2025 - June 30, 2025	7.2
July 1, 2025 - September 30, 2025	7
October 1, 2025 - December 31, 2025	6.9
Total 2025	28.3
2026	26.6
2027	24.9
2028	23.1
2029	21.4
2030 and thereafter	181.8
Total gross unearned insurance premiums	$ 328.5

Municipal Bond Guarantee Insu_6

Municipal Bond Guarantee Insurance - Schedule of Net Written Premiums (Details) - Financial Guarantee (HG Global/BAM) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Written premiums:
Direct	$ 10.5	$ 9.2
Assumed	0	0
Gross written premiums	10.5	9.2
Earned premiums:
Direct	7.3	7
Assumed	0.5	0.7
Gross earned premiums	$ 7.8	$ 7.7

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
	Mar. 31, 2024	Mar. 31, 2023
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 236.4	$ 179.5
Allocation of losses (earnings) to participating restricted common shares	(2.7)	(2.2)
Basic earnings (losses) per share numerators, Total	233.7	177.3
Diluted earnings (losses) per share numerators, Total	$ 233.7	$ 177.3
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	2,559,700	2,569,900
Average unvested restricted common shares (in shares)	(28,600)	(31,100)
Basic earnings (losses) per share denominator (in shares)	2,531,100	2,538,800
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)	2,559,700	2,569,900
Average unvested restricted common shares (in shares)	(28,600)	(31,100)
Diluted earnings (losses) per share denominator (in shares)	2,531,100	2,538,800
Basic and diluted earnings per share (in dollars):
Distributed earnings - dividends declared and paid (in usd per share)	$ 1	$ 1
Distributed earnings - dividends declared and paid (in usd per share)	1	1
Undistributed earnings (losses) (in usd per share)	91.33	68.83
Undistributed earnings (losses) (in usd per share)	91.33	68.83
Basic and diluted earnings (losses) per share (in usd per share)	92.33	69.83
Basic and diluted earnings (losses) per share (in usd per share)	$ 92.33	$ 69.83

Earnings Per Share - Undistribu

Earnings Per Share - Undistributed Net Earnings From Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 233.7	$ 177.3
Dividends declared, net of restricted common share amounts	(2.5)	(2.5)
Total undistributed net earnings (losses), net of restricted common share amounts	$ 231.2	$ 174.8

Employee Share-Based Incentiv_3

Employee Share-Based Incentive Compensation Plans - Narrative (Details) $ in Millions	3 Months Ended		36 Months Ended
	Mar. 31, 2024 USD ($) shares	Mar. 31, 2023 USD ($) shares	Dec. 31, 2023 USD ($) shares	Dec. 31, 2022 USD ($)
Share-based compensation arrangement by share-based payment award
New grants (in shares)	10,405	10,895
Performance Share
Share-based compensation arrangement by share-based payment award
Vesting period	3 years
Performance cycle period	3 years
New grants (in shares)			100
Additional compensation cost that would be recognized if all outstanding performance shares vested | $	$ 45.2
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 | $	$ 31.3	$ 28.3	$ 16.2	$ 15.5
Issued (in shares)	10,405	10,895

Employee Share-Based Incentiv_4

Employee Share-Based Incentive Compensation Plans - Performance Shares (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Target Performance Shares Outstanding
Beginning of period (in shares)			37,031	39,449
Shares paid (in shares)			(13,475)	(13,350)
New grants (in shares)			10,405	10,895
Forfeitures and cancellations (in shares)			(46)	(37)
End of period (in shares)	34,007	37,031	34,007	37,031
Accrued Expense
Beginning of period			$ 69.4	$ 67.5
Shares paid			(44.9)	(40.8)
New grants			0	0
Forfeitures and cancellations			(0.3)	(0.4)
Expense recognized			19.4	9.3
End of period	$ 44.2	$ 36.4	$ 44.2	$ 36.4
Performance Share
Accrued Expense
Percentage of target payout - minimum actual	188%	200%

Employee Share-Based Incentiv_5

Employee Share-Based Incentive Compensation Plans - Performance Shares Granted Under the WTM Incentive Plan (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	34,525
Assumed forfeitures (in shares)	(518)
Total target performance shares outstanding (in shares)	34,007	37,031	37,031	39,449
Accrued Expense	$ 45
Assumed forfeitures	(0.8)
Total	$ 44.2	$ 69.4	$ 36.4	$ 67.5
2022 – 2024
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,225
Accrued Expense	$ 35
2023 – 2025
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	10,895
Accrued Expense	$ 9.4
2024 – 2026
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	10,405
Accrued Expense	$ 0.6

Employee Share-Based Incentiv_6

Employee Share-Based Incentive Compensation Plans - Restricted Shares (Details) - Restricted Stock - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Restricted Shares
Beginning of period (in shares)	37,595	38,350
Issued (in shares)	10,405	10,895
Vested (in shares)	(13,475)	(11,650)
Forfeited (in shares)	0	0
End of period (in shares)	34,525	37,595
Unamortized Issue Date Fair Value
Beginning of period	$ 16.2	$ 15.5
Issued	18.3	16
Vested	0	0
Forfeited	0	0
Expense recognized	(3.2)	(3.2)
End of period	$ 31.3	$ 28.3

Noncontrolling Interests (Detai

Noncontrolling Interests (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Noncontrolling Interest [Line Items]
Noncontrolling Equity	$ 420.2	$ 321.1
Ark
Noncontrolling Interest [Line Items]
Noncontrolling Equity	334.7	336.9
Noncontrolling equity attributable to management's equity incentives	17.9	21
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling Equity	(0.4)	0.8
Kudu
Noncontrolling Interest [Line Items]
Noncontrolling Equity	114.4	113.8
Bamboo
Noncontrolling Interest [Line Items]
Noncontrolling Equity	111.3	0
Other
Noncontrolling Interest [Line Items]
Noncontrolling Equity	9.3	9.4
Total, excluding BAM
Noncontrolling Interest [Line Items]
Noncontrolling Equity	569.3	460.9
Variable Interest Entity, Primary Beneficiary
Noncontrolling Interest [Line Items]
Noncontrolling Equity	$ (149.1)	$ (139.8)
Ark
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	28%	28%
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	3.10%	3.10%
Kudu
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	10.60%	10.50%
Bamboo
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	27.20%	0%
BAM
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	100%	100%

Segment Information - Narrative

Segment Information - Narrative (Details)	3 Months Ended
	Mar. 31, 2024 segment
Segment Reporting [Abstract]
Number of reportable segments	4

Segment Information - Financial

Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Earned insurance premiums	$ 319	$ 262.8
Net investment income	57	39
Net investment income (expense) - BAM Surplus Note interest	0	0
Net realized and unrealized investment gains (losses)	226.8	198.1
Commission and fee revenues	25.5	3.3
Other revenues	19	28.7
Total revenues	647.3	531.9
Loss and loss adjustment expenses	185.8	147.8
Acquisition expenses	71.6	62.5
Cost of sales	7.6	13.9
Broker commission expenses	9.3
General and administrative expenses	125.2	96
Change in fair value of contingent consideration	0	(2.4)
Interest expense	15.2	15
Total expenses	414.7	332.8
Pre-tax income (loss)	232.6	199.1
Other Operations
Segment Reporting Information [Line Items]
Earned insurance premiums	0	0
Net investment income	9.9	7
Net investment income (expense) - BAM Surplus Note interest	0	0
Commission and fee revenues	3.6	3.3
Other revenues	14.4	30.6
Total revenues	260.8	167.9
Loss and loss adjustment expenses	0	0
Acquisition expenses	0	0
Cost of sales	7.6	13.9
Broker commission expenses	0
General and administrative expenses	50.3	39.7
Change in fair value of contingent consideration	0	0
Interest expense	0.7	0.8
Total expenses	58.6	54.4
Pre-tax income (loss)	202.2	113.5
Investments, Excluding MediaAlpha
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	16.1	112.9
Investments, Excluding MediaAlpha | Other Operations
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	22.2	41.8
MediaAlpha
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	210.7	85.2
MediaAlpha | Other Operations
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	210.7	85.2
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment Reporting Information [Line Items]
Loss and loss adjustment expenses	180	147.8
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	302.8	255.1
Net investment income	19.9	10.6
Net realized and unrealized investment gains (losses)	10.6	24.5
Other revenues	3.5	(2.7)
Total revenues	336.8	287.5
Loss and loss adjustment expenses	180	147.8
Acquisition expenses	66.3	59.8
General and administrative expenses	42.2	35.2
Change in fair value of contingent consideration	0	(2.4)
Interest expense	5.4	5
Total expenses	293.9	245.4
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | UNITED KINGDOM
Segment Reporting Information [Line Items]
Earned insurance premiums	177.4	146
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | BERMUDA
Segment Reporting Information [Line Items]
Earned insurance premiums	115.1	104
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	292.5	249.9
Net investment income	17	8.4
Net investment income (expense) - BAM Surplus Note interest	0	0
Commission and fee revenues	0	0
Other revenues	3.5	(2.7)
Total revenues	323.6	280.1
Loss and loss adjustment expenses	179.3	147.6
Acquisition expenses	63.7	58.9
Cost of sales	0	0
Broker commission expenses	0
General and administrative expenses	42.2	35.1
Change in fair value of contingent consideration	0	(2.4)
Interest expense	5.4	5
Total expenses	290.6	244.2
Pre-tax income (loss)	33	35.9
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)	10.6	24.5
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
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	10.3	5.2
Net investment income	2.9	2.2
Net investment income (expense) - BAM Surplus Note interest	0	0
Commission and fee revenues	0	0
Other revenues	0	0
Total revenues	13.2	7.4
Loss and loss adjustment expenses	0.7	0.2
Acquisition expenses	2.6	0.9
Cost of sales	0	0
Broker commission expenses	0
General and administrative expenses	0	0.1
Change in fair value of contingent consideration	0	0
Interest expense	0	0
Total expenses	3.3	1.2
Pre-tax income (loss)	9.9	6.2
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
Financial Guarantee (HG Global/BAM) | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	7.8	7.7
Net investment income	9.7	7.2
Net realized and unrealized investment gains (losses)	(10.1)	17
Other revenues	0.5	0.8
Total revenues	7.9	32.7
Acquisition expenses	2.2	2.7
General and administrative expenses	17.3	17.3
Interest expense	3.5	4.5
Total expenses	23	24.5
Financial Guarantee (HG Global/BAM) | HG Global | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	6.5	6.4
Net investment income	5.4	4
Net investment income (expense) - BAM Surplus Note interest	6.6	6.6
Commission and fee revenues	0	0
Other revenues	0	0
Total revenues	11.2	24.9
Loss and loss adjustment expenses	0	0
Acquisition expenses	1.8	1.8
Cost of sales	0	0
Broker commission expenses	0
General and administrative expenses	0.4	1.1
Change in fair value of contingent consideration	0	0
Interest expense	3.5	4.5
Total expenses	5.7	7.4
Pre-tax income (loss)	5.5	17.5
Financial Guarantee (HG Global/BAM) | HG Global | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(7.3)	7.9
Financial Guarantee (HG Global/BAM) | HG Global | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0	0
Financial Guarantee (HG Global/BAM) | BAM | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	1.3	1.3
Net investment income	4.3	3.2
Net investment income (expense) - BAM Surplus Note interest	(6.6)	(6.6)
Commission and fee revenues	0	0
Other revenues	0.5	0.8
Total revenues	(3.3)	7.8
Loss and loss adjustment expenses	0	0
Acquisition expenses	0.4	0.9
Cost of sales	0	0
Broker commission expenses	0
General and administrative expenses	16.9	16.2
Change in fair value of contingent consideration	0	0
Interest expense	0	0
Total expenses	17.3	17.1
Pre-tax income (loss)	(20.6)	(9.3)
Financial Guarantee (HG Global/BAM) | BAM | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(2.8)	9.1
Financial Guarantee (HG Global/BAM) | BAM | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0	0
Asset Management (Kudu) | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	0	0
Net investment income	17.2	14.2
Net investment income (expense) - BAM Surplus Note interest	0	0
Net realized and unrealized investment gains (losses)	(6.5)	29.6
Commission and fee revenues	0	0
Other revenues	0	0
Total revenues	10.7	43.8
Loss and loss adjustment expenses	0	0
Acquisition expenses	0	0
Cost of sales	0	0
Broker commission expenses	0
General and administrative expenses	3.4	3.8
Change in fair value of contingent consideration	0	0
Interest expense	5.6	4.7
Total expenses	9	8.5
Pre-tax income (loss)	1.7	35.3
Asset Management (Kudu) | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(6.5)	29.6
Asset Management (Kudu) | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0	0
Bamboo | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	8.4	0
Net investment income	0.3
Net investment income (expense) - BAM Surplus Note interest	0
Commission and fee revenues	21.9	0
Other revenues	0.8	0
Total revenues	31.1	0
Loss and loss adjustment expenses	5.8	0
Acquisition expenses	3.1	0
Cost of sales	0
Broker commission expenses	9.3	0
General and administrative expenses	12	0
Change in fair value of contingent consideration	0
Interest expense	0
Total expenses	30.2	$ 0
Pre-tax income (loss)	0.9
Bamboo | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(0.1)
Bamboo | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0
Bamboo Ide8 Services, LLC - Segment, Net Of Reconciling Items | Operating Segments
Segment Reporting Information [Line Items]
Other revenues	$ 0.6

Variable Interest Entities (Det

Variable Interest Entities (Details) $ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($)
Subsidiary - HG Global
Variable Interest Entity [Line Items]
Purchase of surplus notes	$ 503
Subsidiary - HG Global | Maximum
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured	15%
Subsidiary - BAM
Variable Interest Entity [Line Items]
BAM member surplus contributions	$ 556.5
PassportCard/DavidShield | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Ownership interest	53.80%
Partnership interest, carrying value	$ 159.5
Elementum Holdings, L.P. | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Ownership interest	26.60%
Partnership interest, carrying value	$ 35
Limited Partnerships | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Partnership interest, carrying value	$ 255.2

Equity Method Eligible Invest_3

Equity Method Eligible Investments - Ownership Interests and Carrying Values (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Kudu's Participation Contracts
Investments in unconsolidated affiliates
Carrying Value	$ 884.2	$ 890.5
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	34.70%	34.90%
Carrying Value	$ 465.6	$ 254.9
PassportCard/DavidShield
Investments in unconsolidated affiliates
Ownership Interest	53.80%	53.80%
Carrying Value	$ 150	$ 150
Elementum Holdings, L.P.
Investments in unconsolidated affiliates
Ownership Interest	26.60%	26.60%
Carrying Value	$ 35	$ 35
Other equity method eligible investments, at fair value
Investments in unconsolidated affiliates
Carrying Value	247.4	291.7
Other equity method eligible investments, at fair value
Investments in unconsolidated affiliates
Carrying Value	$ 24.8	$ 24.8

Equity Method Eligible Invest_4

Equity Method Eligible Investments - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Equity Method Investments and Joint Ventures [Abstract]
Dividend and income distributions from equity method eligible investments	$ 15.9	$ 7.7

Fair Value of Financial Instr_3

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Balance Sheet Grouping, Financial Statement Captions
Carrying Value	$ 532.1	$ 534.6
Ark 2007 Subordinated Notes
Balance Sheet Grouping, Financial Statement Captions
Fair Value	0	30.5
Carrying Value	0	30
Ark 2021 Subordinated Notes
Balance Sheet Grouping, Financial Statement Captions
Fair Value	171.8	171.8
Carrying Value	155.4	155.5
HG Global Senior Notes
Balance Sheet Grouping, Financial Statement Captions
Fair Value	158.5	158.7
Carrying Value	147	146.9
Kudu Credit Facility
Balance Sheet Grouping, Financial Statement Captions
Fair Value	224.6	225.6
Carrying Value	203.8	203.8
Other Operations debt
Balance Sheet Grouping, Financial Statement Captions
Fair Value	27.4	30
Carrying Value	$ 25.9	$ 28.4