White Mountains Insurance (WTM) 10-K 28 Feb 25 2024 FY Annual report Financial data

Cover Page

Cover Page - USD ($)	12 Months Ended
	Dec. 31, 2024	Feb. 24, 2025	Jun. 30, 2024
Document Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2024
Current Fiscal Year End Date	--12-31
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
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
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
ICFR Auditor Attestation Flag	true
Document Financial Statement Error Correction	false
Entity Shell Company	false
Entity Public Float			$ 4,526,288,344
Entity Common Stock, Shares Outstanding (in shares)		2,563,051
Documents Incorporated by Reference	Portions of the Registrant’s Definitive Proxy Statement to be filed with the Securities and Exchange Commission (“SEC”) pursuant to Regulation 14A under the Securities Exchange Act of 1934, as amended (the “Exchange Act”), relating to the Registrant’s Annual General Meeting of Members scheduled to be held May 22, 2025 are incorporated by reference into Part III of this Form 10-K. With the exception of the portions of the Proxy Statement specifically incorporated herein by reference, the Proxy Statement is not deemed to be filed as part of this Form 10-K.
Entity Central Index Key	0000776867
Amendment Flag	false
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	FY
New York Stock Exchange
Document Information [Line Items]
Trading Symbol	WTM
Security Exchange Name	NYSE

Audit Information

Audit Information	12 Months Ended
	Dec. 31, 2024
Audit Information [Abstract]
Auditor Firm ID	238
Auditor Name	PricewaterhouseCoopers LLP
Auditor Location	Atlanta, Georgia

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Assets
Common equity securities, at fair value	$ 650	$ 538.4
Other long-term investments	2,150.2	1,998.2
Total investments	6,477.6	6,388.7
Goodwill and other intangible assets	720.3	370.6
Total assets	9,925.6	8,385.9
Liabilities
Debt	562.5	564.6
Total liabilities	4,794.6	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 2,568,148 and 2,560,452 shares	2.6	2.6
Paid-in surplus	563.8	548.7
Retained earnings	3,919	3,690.8
Accumulated other comprehensive income (loss), after-tax:
Net unrealized gains (losses) from foreign currency translation	(1.7)	(1.6)
Total White Mountains’s common shareholders’ equity	4,483.7	4,240.5
Noncontrolling interests	647.3	321.1
Total equity	5,131	4,561.6
Total liabilities and equity	9,925.6	8,385.9
Other Operations
Assets
Fixed maturity investments, at fair value	293.7	230.2
Common equity securities, at fair value	224.6	137.8
Short-term investments, at fair value	262.1	425.2
Other long-term investments	588.4	661
Investment in MediaAlpha, at fair value	201.6	254.9
Total investments	1,570.4	1,709.1
Cash	38.6	23.8
Insurance premiums receivable	15.3	0
Goodwill and other intangible assets	64.8	69.8
Other assets	65.1	73.2
Total assets	1,754.2	1,875.9
Liabilities
Loss and loss adjustment expense reserves	12.1	0
Unearned insurance premiums	29	0
Debt	22	28.4
Accrued incentive compensation	79.3	87.7
Other liabilities	38.9	25
Total liabilities	181.3	141.1
Accumulated other comprehensive income (loss), after-tax:
Total White Mountains’s common shareholders’ equity	1,563.8	1,725.4
Noncontrolling interests	9.1	9.4
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liabilities
Loss and loss adjustment expense reserves	2,127.5	1,605.1
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Assets
Fixed maturity investments, at fair value	1,565.1	866.8
Common equity securities, at fair value	425.4	400.6
Short-term investments, at fair value	601.4	962.8
Other long-term investments	547.8	440.9
Total investments	3,139.7	2,671.1
Cash	141.2	90.5
Reinsurance recoverables	589	442
Insurance premiums receivable	768.6	612.2
Deferred acquisition costs	165.2	145.3
Goodwill and other intangible assets	292.5	292.5
Other assets	202.8	125
Total assets	5,299	4,378.6
Liabilities
Loss and loss adjustment expense reserves	2,127.5	1,605.1
Unearned insurance premiums	853.3	743.6
Debt	154.5	185.5
Reinsurance payable	149.5	81.1
Contingent consideration	155.3	94
Other liabilities	224.7	166.8
Total liabilities	3,664.8	2,876.1
Accumulated other comprehensive income (loss), after-tax:
Total White Mountains’s common shareholders’ equity	1,223.8	1,165.6
Noncontrolling interests	410.4	336.9
Financial Guarantee (HG Global) | Operating Segments
Assets
Fixed maturity investments, at fair value	612.1	1,012.3
Short-term investments, at fair value	55.5	70.6
Total investments	667.6	1,082.9
Cash	11.5	6.7
BAM Surplus Notes, at fair value	381.7	0
Insurance premiums receivable	4.4	5.5
Deferred acquisition costs	86.6	40.1
Other assets	27.6	36.8
Total assets	1,179.4	1,172
Liabilities
Unearned insurance premiums	297.3	325.8
Debt	147.4	146.9
Other liabilities	19.4	59
Total liabilities	464.1	531.7
Accumulated other comprehensive income (loss), after-tax:
Total White Mountains’s common shareholders’ equity	728.7	779.3
Noncontrolling interests	(13.4)	(139)
Asset Management (Kudu) | Operating Segments
Assets
Short-term investments, at fair value	27.9	29.3
Other long-term investments	1,014	896.3
Total investments	1,041.9	925.6
Cash	0.6	1.4
Accrued investment income	18	17.6
Goodwill and other intangible assets	8	8.3
Other assets	39.9	6.5
Total assets	1,108.4	959.4
Liabilities
Debt	238.6	203.8
Other liabilities	78.1	71.6
Total liabilities	316.7	275.4
Accumulated other comprehensive income (loss), after-tax:
Total White Mountains’s common shareholders’ equity	664.1	570.2
Noncontrolling interests	127.6	113.8
P&C Insurance Distribution (Bamboo) | Operating Segments
Assets
Fixed maturity investments, at fair value	40.7	0
Short-term investments, at fair value	17.3	0
Total investments	58	0
Cash	74.5	0
Premiums, commissions and fees receivable	70	0
Goodwill and other intangible assets	355	0
Other assets	27.1	0
Total assets	584.6	0
Liabilities
Loss and loss adjustment expense reserves	17.8	0
Unearned insurance premiums	31.5	0
Premiums and commissions payable	88.1	0
Other liabilities	30.3	0
Total liabilities	167.7	0
Accumulated other comprehensive income (loss), after-tax:
Total White Mountains’s common shareholders’ equity	303.3	0
Noncontrolling interests	$ 113.6	$ 0

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Dec. 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)	2,568,148	2,560,452
Common shares, outstanding shares (in shares)	2,568,148	2,560,452
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Restricted cash	$ 14.1	$ 0.7
Operating Segments | Bamboo
Restricted cash	$ 59.5	$ 0

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Revenues:
Earned insurance premiums	$ 1,691.6	$ 1,440.9	$ 1,076.7
Net investment income	227.4	194.2	124.4
Net realized and unrealized investment gains (losses)	184.9	434.2	(191.5)
Interest income from BAM Surplus Notes	15.8	0	0
Change in fair value of BAM Surplus Notes	0.5	0	0
Unrealized loss on deconsolidation of BAM	(114.5)	0	0
Commission and fees revenues	149.4	13.2	11.5
Other revenues	84.7	84.2	136.8
Total revenues	2,239.8	2,166.7	1,157.9
Expenses:
Broker commission expense	51.3
Loss and loss adjustment expenses	888.5	726.8	536.4
Acquisition expenses	341.5	290.1	250.6
Cost of sales	29.6	40.4	98.6
General and administrative expenses	490.1	432.6	364.1
Change in fair value of contingent consideration	61.3	48.7	17.3
Interest expense	60.8	62.7	40.3
Total expenses	1,923.1	1,601.3	1,307.3
Pre-tax income (loss) from continuing operations	316.7	565.4	(149.4)
Income tax (expense) benefit	(32.6)	15.5	(41.4)
Net income (loss) from continuing operations	284.1	580.9	(190.8)
Net income (loss) from discontinued operations, net of tax - NSM Group	0	0	16.4
Net income (loss)	284.1	580.9	712.4
Net (income) loss attributable to noncontrolling interests	(53.7)	(71.7)	80.4
Net income (loss) attributable to White Mountains’s common shareholders	230.4	509.2	792.8
NSM Group
Expenses:
Net income (loss) from discontinued operations, net of tax - NSM Group	0	0	16.4
Net gain (loss) from sale of discontinued operations, net of tax - NSM Group	0	0	886.8
Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	146.9	407.1	(98.5)
MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	38	27.1	(93)
Other Operations
Revenues:
Earned insurance premiums	32.7	0	0
Net investment income	35.6	30.1	32.2
Interest income from BAM Surplus Notes	0	0	0
Change in fair value of BAM Surplus Notes	0
Unrealized loss on deconsolidation of BAM	0
Commission and fees revenues	14.8	13.2	11.5
Other revenues	56.8	80.5	127.2
Total revenues	234.9	339.4	76.3
Expenses:
Broker commission expense	0
Loss and loss adjustment expenses	12.1	0	0
Acquisition expenses	12.1	0	0
Cost of sales	29.6	40.4	98.6
General and administrative expenses	169.5	182.3	174.1
Change in fair value of contingent consideration	0	0	0
Interest expense	2.5	3.7	1.9
Total expenses	225.8	226.4	274.6
Pre-tax income (loss) from continuing operations	9.1	113	(198.3)
Other Operations | Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	57	188.5	(1.6)
Other Operations | MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	38	27.1	(93)
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Revenues:
Earned insurance premiums	1,587.8	1,409.7	1,043.4
Expenses:
Loss and loss adjustment expenses	855.8	726.8	536.4
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Revenues:
Earned insurance premiums	1,587.8	1,409.7	1,043.4
Net investment income	90.7	61.4	16.3
Net realized and unrealized investment gains (losses)	50.1	85.9	(55.2)
Other revenues	22.3	0.8	5
Total revenues	1,750.9	1,557.8	1,009.5
Expenses:
Loss and loss adjustment expenses	855.8	726.8	536.4
Acquisition expenses	307.1	281.5	239.4
General and administrative expenses	208.4	162	106.2
Change in fair value of contingent consideration	61.3	48.7	17.3
Interest expense	19.5	21.3	15.1
Total expenses	1,452.1	1,240.3	914.4
Financial Guarantee (HG Global)
Revenues:
Earned insurance premiums	31.7	31.2	33.3
Financial Guarantee (HG Global) | Operating Segments
Revenues:
Earned insurance premiums	31.7	31.2	33.3
Net investment income	32.2	31.7	21.5
Net realized and unrealized investment gains (losses)	(11.5)	26.6	(105.8)
Interest income from BAM Surplus Notes	15.8	0	0
Change in fair value of BAM Surplus Notes	0.5	0	0
Unrealized loss on deconsolidation of BAM	(114.5)	0	0
Other revenues	1.2	2.9	4.6
Total revenues	(44.6)	92.4	(46.4)
Expenses:
Broker commission expense	0
Acquisition expenses	8.2	8.6	11.2
General and administrative expenses	35.7	68.9	69.1
Interest expense	16.7	16.5	8.3
Total expenses	60.6	94	88.6
Asset Management (Kudu) | Operating Segments
Revenues:
Earned insurance premiums	0	0	0
Net investment income	66.7	71	54.4
Net realized and unrealized investment gains (losses)	51.3	106.1	64.1
Interest income from BAM Surplus Notes	0	0	0
Change in fair value of BAM Surplus Notes	0
Unrealized loss on deconsolidation of BAM	0
Commission and fees revenues	0	0	0
Other revenues	0.8	0	0
Total revenues	118.8	177.1	118.5
Expenses:
Broker commission expense	0
Loss and loss adjustment expenses	0	0	0
Acquisition expenses	0	0	0
Cost of sales	0	0	0
General and administrative expenses	15.4	19.4	14.7
Change in fair value of contingent consideration	0	0	0
Interest expense	22.1	21.2	15
Total expenses	37.5	40.6	29.7
Pre-tax income (loss) from continuing operations	81.3	136.5	88.8
Asset Management (Kudu) | Operating Segments | Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	51.3	106.1	64.1
Asset Management (Kudu) | Operating Segments | MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	0	0	0
P&C Insurance Distribution (Bamboo)
Expenses:
Loss and loss adjustment expenses	20.6
P&C Insurance Distribution (Bamboo) | Operating Segments
Revenues:
Earned insurance premiums	39.4	0	0
Net investment income	2.2
Interest income from BAM Surplus Notes	0
Change in fair value of BAM Surplus Notes	0
Unrealized loss on deconsolidation of BAM	0
Commission and fees revenues	134.6	0	0
Other revenues	5.8	0	0
Total revenues	179.8	0	0
Expenses:
Broker commission expense	51.3	0	0
Loss and loss adjustment expenses	20.6	0	0
Acquisition expenses	14.1	0	0
Cost of sales	0
General and administrative expenses	61.1	0	0
Change in fair value of contingent consideration	0
Interest expense	0
Total expenses	147.1	$ 0	$ 0
Pre-tax income (loss) from continuing operations	32.7
P&C Insurance Distribution (Bamboo) | Operating Segments | Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	0
P&C Insurance Distribution (Bamboo) | Operating Segments | MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	$ 0

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Statement of Comprehensive Income [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ 230.4	$ 509.2	$ 792.8
Other comprehensive income (loss), net of tax	(0.1)	2.4	(3.8)
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	0	0	(5.2)
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	0	0	2.9
Comprehensive income (loss)	230.3	511.6	786.7
Other comprehensive (income) loss attributable to noncontrolling interests	0	(0.5)	0.9
Comprehensive income (loss) attributable to White Mountains’s common shareholders	$ 230.3	$ 511.1	$ 787.6
Basic earnings (loss) per share
Continuing operations (usd per share)	$ 89.79	$ 198.60	$ (38.34)
Discontinued operations (usd per share)	0	0	315.30
Total consolidated operations (usd per share)	89.79	198.60	276.96
Diluted earnings (loss) per share
Continuing operations (usd per share)	89.79	198.60	(38.34)
Discontinued operations (usd per share)	0	0	315.30
Total consolidated operations (usd per share)	89.79	198.60	276.96
Dividends declared per White Mountains’s common share (usd per share)	1	1	1
Dividends paid per White Mountains’s common share (usd per share)	$ 1	$ 1	$ 1

CONSOLIDATED STATEMENTS OF SHAR

CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Millions	Total	Total Equity	Common Shares and Paid-in Surplus	Retained Earnings	AOCL, After-tax	Total	Noncontrolling Interests
Beginning balance at Dec. 31, 2021		$ 3,704.7	$ 588.9	$ 2,957.5	$ 1.7	$ 3,548.1	$ 156.6
Increase (Decrease) in Shareholders' Equity
Net income (loss)	$ 792.8	712.4		792.8		792.8	(80.4)
Other comprehensive income (loss), net of tax	(3.8)	(3.8)			(2.9)	(2.9)	(0.9)
Other comprehensive income from discontinued operations, net of tax - NSM Group	(5.2)	(5.2)			(5.2)	(5.2)
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	2.9	2.9			2.9	2.9
Total comprehensive income (loss)		706.3		792.8	(5.2)	787.6	(81.3)
Dividends declared on common shares		(3)		(3)		(3)
Dividends to noncontrolling interests		(8)					(8)
Issuances of common shares		3	3			3
Issuances of shares of noncontrolling interest		74.6					74.6
Repurchases and retirements of common shares	(615.8)	(615.8)	(90)	(525.8)		(615.8)
BAM member surplus contributions, net of tax		81.4					81.4
Amortization of restricted share awards		14	14			14
Recognition of equity-based compensation expense of subsidiaries		9.5	8.6			8.6	0.9
Net contributions (distributions) and dilution from other noncontrolling interests		(14.2)	14.1	(9.7)		4.4	(18.6)
Acquisition of noncontrolling interests		(17.5)					(17.5)
Ending balance at Dec. 31, 2022		3,935	538.6	3,211.8	(3.5)	3,746.9	188.1
Increase (Decrease) in Shareholders' Equity
Net income (loss)	509.2	580.9		509.2		509.2	71.7
Other comprehensive income (loss), net of tax	2.4	2.4			1.9	1.9	0.5
Other comprehensive income from discontinued operations, net of tax - NSM Group	0
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	0
Total comprehensive income (loss)		583.3		509.2	1.9	511.1	72.2
Dividends declared on common shares		(2.6)		(2.6)		(2.6)
Dividends to noncontrolling interests		(8.9)					(8.9)
Issuances of common shares		2.1	2.1			2.1
Repurchases and retirements of common shares	(32.7)	(32.7)	(5.1)	(27.6)		(32.7)
BAM member surplus contributions, net of tax		72.8					72.8
Amortization of restricted share awards		15.3	15.3			15.3
Recognition of equity-based compensation expense of subsidiaries		2.4	1.7			1.7	0.7
Net contributions (distributions) and dilution from other noncontrolling interests		4.3	(1.3)			(1.3)	5.6
Acquisition of noncontrolling interests		(9.4)					(9.4)
Ending balance at Dec. 31, 2023	4,561.6	4,561.6	551.3	3,690.8	(1.6)	4,240.5	321.1
Increase (Decrease) in Shareholders' Equity
Net income (loss)	230.4	284.1		230.4		230.4	53.7
Other comprehensive income (loss), net of tax	(0.1)	(0.1)			(0.1)	(0.1)
Other comprehensive income from discontinued operations, net of tax - NSM Group	0
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	0
Total comprehensive income (loss)		284		230.4	(0.1)	230.3	53.7
Dividends declared on common shares		(2.5)		(2.5)		(2.5)
Dividends to noncontrolling interests		(16.2)					(16.2)
Issuances of common shares		2.9	2.9			2.9
Repurchases and retirements of common shares	(7.9)	(7.9)	(1.1)	(6.8)		(7.9)
BAM member surplus contributions, net of tax		26					26
Amortization of restricted share awards		16.3	16.3			16.3
Recognition of equity-based compensation expense of subsidiaries		4.4	3.2			3.2	1.2
Net contributions (distributions) and dilution from other noncontrolling interests		(2.2)	(6.2)	7.1		0.9	(3.1)
Impact to noncontrolling interests for the deconsolidation of BAM		153.5					153.5
Acquisition of noncontrolling interests		111.1					111.1
Ending balance at Dec. 31, 2024	$ 5,131	$ 5,131	$ 566.4	$ 3,919	$ (1.7)	$ 4,483.7	$ 647.3

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Cash flows from operations:
Net income (loss)	$ 284.1	$ 580.9	$ 712.4
Adjustments to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses	(184.9)	(434.2)	191.5
Change in fair value of contingent consideration - Ark	61.3	48.7	17.3
Change in fair value of BAM Surplus Notes	(0.5)	0	0
Unrealized loss on deconsolidation of BAM	114.5	0	0
Deferred income tax expense (benefit)	11.1	(46.9)	12.9
Amortization of restricted share awards	16.3	15.3	14
Amortization (accretion) and depreciation	(6.4)	(17.8)	4.3
Net (income) loss from discontinued operation, net of tax - NSM Group	0	0	(16.4)
Other operating items:
Net change in reinsurance recoverables	(147)	153.3	(79.8)
Net change in insurance premiums, commissions and fees receivable	(204.7)	(67)	(127.8)
Net change in deferred acquisition costs	(27.1)	(22.2)	(21.9)
Net change in loss and loss adjustment expense reserves	543	308.6	401.8
Net change in unearned insurance premiums	174.4	147.9	159.3
Net change in reinsurance payable	68.4	(170)	(173)
Net change in premiums and commissions payable	38.9	0	0
Contributions to Kudu’s Participation Contracts	(103.5)	(163.8)	(99.8)
Proceeds from Kudu’s Participation Contracts sold	9.4	75.2	137.5
Net other operating activities	(50.5)	(3.9)	80.8
Net cash provided from (used for) operations - continuing operations	586.8	404.1	326.3
Net cash provided from (used for) operations - NSM Group discontinued operations (Note 20)	0	0	38.7
Net cash provided from (used for) operations	586.8	404.1	365
Cash flows from investing activities:
Net change in short-term investments	548.4	(495.8)	(455.1)
Sales of fixed maturity investments	429.5	195.1	188.7
Maturities, calls and paydowns of fixed maturity investments	416.5	266.5	198.5
Sales of common equity securities and investment in MediaAlpha	162.4	261.6	0
Distributions and redemptions of other long-term investments	195.7	116.9	115.4
Proceeds from the sale of NSM Group and Other Operating Businesses, net of cash sold of $0.0, $0.8 and $144.4	0	17.3	1,392
Purchases of consolidated subsidiaries, net of cash acquired of $44.9, $0.0, and $0.3	(231.8)	0	(67.9)
Release of cash (pre-funding) for ILS funds managed by Elementum	0	70	(70)
Purchases of fixed maturity investments	(1,677.5)	(592.7)	(585.5)
Purchases of common equity securities and investment in MediaAlpha	(121.1)	(114.2)	(424.5)
Purchases of other long-term investments	(190.2)	(265.2)	(178.8)
Payments of principal received on the BAM Surplus Notes subsequent to deconsolidation	16.2	0	0
Impact to cash from deconsolidation of BAM	(4.2)	0	0
Net other investing activities	9.1	(3.4)	(4.4)
Net cash provided from (used for) investing activities - continuing operations	(447)	(543.9)	108.4
Net cash provided from (used for) investing activities - NSM Group discontinued operations (Note 20)	0	0	7.1
Net cash provided from (used for) investing activities	(447)	(543.9)	115.5
Cash flows from financing activities:
Draw down of debt and revolving lines of credit	36.6	12.4	213.8
Repayment of debt and revolving lines of credit	(38.1)	(25.7)	(56.7)
Cash dividends paid to common shareholders	(2.5)	(2.6)	(3)
Repurchases and retirements of common shares	(7.9)	(32.7)	(615.8)
BAM member surplus contributions prior to deconsolidation	26	72.8	81.4
Contributions from other noncontrolling interests	1	9	74.6
Distributions to other noncontrolling interests	(21.1)	(12)	(21.6)
Net (contributions to) distributions from discontinued operations	0	0	11.6
Net other financing activities	10.2	(14)	(11.7)
Net cash provided from (used for) financing activities - continuing operations	4.2	7.2	(327.4)
Net cash provided from (used for) financing activities - NSM Group discontinued operations (Note 20)	0	0	(17.5)
Net cash provided from (used for) financing activities	4.2	7.2	(344.9)
Net change in cash during the period - continuing operations	144	(132.6)	107.3
Cash balance at beginning of year	122.4	255	147.7
Cash balance at end of year	266.4	122.4	255
Operating Segments | Financial Guarantee (HG Global)
Adjustments to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses	11.5	(26.6)	105.8
Interest income from BAM Surplus Notes subsequent to deconsolidation, net of interest payments received of $5.8	(10)	0	0
Change in fair value of BAM Surplus Notes	(0.5)	0	0
Unrealized loss on deconsolidation of BAM	114.5	0	0
NSM
Adjustments to reconcile net income to net cash provided from (used for) operations:
Net (income) loss from discontinued operation, net of tax - NSM Group	0	0	(16.4)
Net (gain) loss from sale of discontinued operations, net of tax - NSM Group	0	0	(886.8)
Investments, Excluding MediaAlpha
Adjustments to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses	(146.9)	(407.1)	98.5
MediaAlpha
Adjustments to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses	$ (38)	$ (27.1)	$ 93

CONSOLIDATED STATEMENTS OF CA_2

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Interest income interest payments	$ 5.8
Cash sold in subsidiary transaction	4.2	$ 0	$ 0
Cash acquired from acquisition	44.9	0	0.3
Restricted cash balance	73.6	0.7	18.3	$ 4.5
Discontinued operations cash balance	0	0	0	$ 116.6
Other Operations
Cash sold in subsidiary transaction	$ 0	$ 0.8	$ 144.4

Basis of Presentation and Signi

Basis of Presentation and Significant Accounting Policies	12 Months Ended
	Dec. 31, 2024
Accounting Policies [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. 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. Consolidation Principles Under GAAP, the Company is required to consolidate any entity in which it holds a controlling financial interest. A controlling financial interest is usually in the form of an investment representing the majority of the subsidiary’s voting interests. However, a controlling financial interest may also arise from a financial interest in a variable interest entity (“VIE”) through arrangements that do not involve ownership of voting interests. The Company consolidates a VIE if it determines that it is the primary beneficiary. The primary beneficiary is defined as the entity who holds a variable interest that gives it both the power to direct the VIE’s activities that most significantly impact its economic performance and the obligation to absorb losses of, or the right to receive returns from, the VIE that could potentially be significant to the VIE. See Note 16 — “Variable Interest Entities.” Intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. Reportable Segments As of December 31, 2024, White Mountains conducted its operations through four reportable segments: (1) Ark/WM Outrigger, (2) HG Global, (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 15 — “ 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, casualty and accident & health. Ark underwrites select coverages through Lloyd’s Syndicates 4020 and 3902 and Additional Central Settlement Number (“ACSN”) 3832 (collectively, 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 December 31, 2024 and 2023, White Mountains owned 72.1% and 72.0% of Ark on a basic shares outstanding basis (61.9% and 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 (“MOIC”) return. If fully earned, these shares would represent an additional 12.3% of the shares outstanding as of December 31, 2024. 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 collateralized reinsurance protection on Ark’s Bermuda global property catastrophe excess of loss portfolio written in the 2023 underwriting year. Ark renewed its quota share reinsurance agreement with Outrigger Re Ltd. for the 2024 and 2025 underwriting years. White Mountains consolidates the results of its segregated account, WM Outrigger Re, in its financial statements. See Note 2 — “Significant Transactions.” As of December 31, 2024 and 2023, White Mountains owned 100.0% of WM Outrigger Re’s preferred equity. The HG Global segment consists of HG Global Ltd. and its wholly-owned subsidiaries (collectively, “HG Global”) and, prior to its deconsolidation on July 1, 2024, the consolidated results of Build America Mutual Assurance Company (“BAM”). See Note 2 — “Significant Transactions.” HG Global was established to fund the startup of BAM and, through its reinsurance subsidiary, HG Re Ltd. (“HG Re”), to provide first-loss reinsurance protection of up to 15%-of-par outstanding for each policy assumed from 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 December 31, 2024 and 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, through June 30, 2024, White Mountains was required to consolidate BAM’s results in its financial statements because BAM is a VIE for which White Mountains was the primary beneficiary. BAM’s results were all attributed to noncontrolling interests. On July 1, 2024, HG Re and BAM amended the terms of the first-loss reinsurance treaty (“FLRT”) with respect to certain governance rights held by HG Re. As a result, and in combination with other governance changes at BAM, White Mountains concluded that it no longer has the power to direct BAM’s activities that most significantly impact its economic performance and is no longer BAM’s primary beneficiary. Accordingly, as of July 1, 2024, White Mountains no longer consolidates BAM. Through June 30, 2024, BAM’s assets, liabilities and noncontrolling interests, as well as its results of operations, are presented within the HG Global segment. See Note 2 — “Significant Transactions.” 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 December 31, 2024 and 2023, White Mountains owned 90.4% and 89.6% of Kudu’s basic units outstanding (77.0% and 76.3% on a fully-diluted/fully-converted basis, taking account of management’s equity incentives). The Bamboo segment consists of PM Holdings LLC (“Bamboo Holdings”), Bamboo Ide8 Insurance Services LLC (“Bamboo MGA”) and Ide8 Re Inc. (“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 carrier partners (“Capacity Providers”), 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. Bamboo MGA offers both admitted and non-admitted products. Under its existing capacity agreements for the treaty year ending April 1, 2025, Bamboo MGA’s commission levels are based on a sliding scale tied primarily to its attritional loss ratio. Bamboo also operates two separate but integrated businesses: (i) a retail agency, within Bamboo MGA, offering ancillary products (e.g., flood, earthquake) on behalf of third parties and (ii) Bamboo Captive, a U.S.-domiciled captive reinsurer that participates in the underwriting risk of Bamboo’s MGA programs to align interests with Capacity Partners. During the fourth quarter of 2024, Bamboo Captive redomiciled from Bermuda to Arizona and changed its name from Ide8 Limited to Ide8 Re Inc. On January 2, 2024, White Mountains acquired a controlling interest in Bamboo. See Note 2 — “Significant Transactions.” As of December 31, 2024, White Mountains owned 72.8% of the basic units outstanding of Bamboo (63.7% on a fully-diluted/fully-converted basis, taking account of management’s equity incentives). 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”), DavidShield PassportCard Ltd. and its subsidiaries (collectively, “PassportCard/DavidShield”), Elementum Holdings LP (“Elementum”), White Mountains Partners LLC (“WTM Partners”), a Bermuda special purpose collateralized reinsurance vehicle that provides reinsurance capacity to Bamboo (“Bamboo CRV”), certain other consolidated and unconsolidated entities (“Other Operating Businesses”) and certain other assets (collectively, “Other Operations”). Held for Sale and Discontinued Operations White Mountains recognizes assets and liabilities classified as held for sale at the lower of carrying value on the date the asset is initially classified as held for sale or fair value less costs to sell. At the time of reclassification to held for sale, White Mountains ceases recognizing depreciation and amortization on assets held for sale. The results of operations of a business that has either been disposed of or are classified as held for sale are reported in discontinued operations if the disposal of business represents a strategic shift that has (or will have) a major effect on White Mountains’s operations and financial results. On August 1, 2022, White Mountains Holdings (Luxembourg) S.à r.l. (“WTM Holdings Seller”), an indirect wholly-owned subsidiary of White Mountains, completed the sale of White Mountains Catskill Holdings, Inc. and NSM Insurance HoldCo, LLC (“NSM”) (collectively with White Mountains Catskill Holdings, Inc., the “NSM Group”) to Riser Merger Sub, Inc., an affiliate of The Carlyle Group Inc. (the “NSM Transaction”), pursuant to the terms of the securities purchase agreement, dated as of May 9, 2022. See Note 2 — “Significant Transactions.” NSM is a full-service MGA and program administrator with delegated binding authorities for specialty property and casualty insurance. As a result of the NSM Transaction, the results of operations for NSM Group have been classified as discontinued operations in the statements of operations and comprehensive income through the closing of the transaction. See Note 20 — “Held for Sale and Discontinued Operations.” Significant Accounting Policies Investment Securities As of December 31, 2024 and 2023, White Mountains’s invested assets consisted of securities and other investments held for general investment purposes. 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. Short-term investments consist of interest-bearing money market funds, certificates of deposit and other fixed maturity securities, which at the time of purchase, mature or become available for use within one year. Short-term investments are carried at fair value, which approximated amortized or accreted cost as of December 31, 2024 and 2023. Other long-term investments consist primarily of unconsolidated entities, including noncontrolling equity interests in the form of revenue and earnings participation contracts held at Kudu (“Participation Contracts”), private equity funds and hedge funds, a bank loan fund, Lloyd’s of London (“Lloyd’s”) trust deposits, insurance-linked securities (“ILS”) funds and private debt instruments. 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. White Mountains’s portfolio of common equity securities, its investment in MediaAlpha and other long-term investments are measured at fair value. White Mountains has taken the fair value option for its equity method eligible investments. Changes in net unrealized investment gains (losses) are reported pre-tax in revenues. Realized investment gains (losses) are accounted for using the specific identification method and are reported pre-tax in revenues. Premiums and discounts on all fixed maturity investments are amortized and accreted to income over the anticipated life of the investment. Income on mortgage and asset-backed securities is recognized using an effective yield based on anticipated prepayments and the estimated economic life of the securities. When actual prepayments differ significantly from anticipated prepayments, the estimated economic life is recalculated, and the remaining unamortized premium or discount is amortized prospectively over the remaining economic life. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants (an exit price) at a particular measurement date. 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 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”). Assets and liabilities carried at fair value include White Mountains’s investment portfolio and derivative instruments. Valuation of assets and liabilities measured at fair value require management to make estimates and apply judgment to matters that may carry a significant degree of uncertainty. In determining its estimates of fair value, White Mountains uses a variety of valuation approaches and inputs. Whenever possible, White Mountains estimates fair value using valuation methods that maximize the use of quoted market prices or other observable inputs. Where appropriate, assets and liabilities measured at fair value have been adjusted for the effect of counterparty credit risk. White Mountains uses outside pricing services and brokers to assist in determining fair values. The outside pricing services White Mountains uses have indicated that they will only provide prices where observable inputs are available. Level 1 Measurements Investments valued using Level 1 inputs include White Mountains’s fixed maturity investments, primarily investments in U.S. Treasuries and short-term investments, which include U.S. Treasury Bills, common equity securities and its investment in MediaAlpha. For investments in active markets, White Mountains uses the quoted market prices provided by outside pricing services to determine fair value. Level 2 Measurements Investments valued using Level 2 inputs include fixed maturity investments which have been disaggregated into classes, including debt securities issued by corporations, municipal obligations, foreign government and agency obligations, mortgage and asset-backed securities and collateralized loan obligations. Investments valued using Level 2 inputs also include certain international listed common equity funds, which White Mountains values using the fund manager’s published net asset value (“NAV”) to account for the difference in market exchange close times, and certain derivative instruments. In circumstances where quoted market prices are unavailable or are not considered reasonable, White Mountains estimates the fair value using industry standard pricing methodologies and observable inputs such as benchmark yields, reported trades, broker-dealer quotes, issuer spreads, benchmark securities, bids, offers, credit ratings, prepayment speeds, reference data including research publications and other relevant inputs. Given that many fixed maturity investments do not trade on a daily basis, the outside pricing services evaluate a wide range of fixed maturity investments by regularly drawing parallels from recent trades and quotes of comparable securities with similar features. The characteristics used to identify comparable fixed maturity investments vary by asset type and take into account market convention. White Mountains’s process to assess the reasonableness of the market prices obtained from the outside pricing sources covers substantially all of its fixed maturity investments and includes, but is not limited to, the evaluation of pricing methodologies and a review of the pricing services’ quality control procedures on at least an annual basis, a comparison of invested asset prices obtained from alternate independent pricing vendors on at least a semi-annual basis, monthly analytical reviews of certain prices and a review of the underlying assumptions utilized by the pricing services for select measurements on an ad hoc basis throughout the year. White Mountains also performs back-testing of selected investment sales activity to determine whether there are any significant differences between the market price used to value the security prior to sale and the actual sale price of the security on an ad-hoc basis throughout the year. Prices provided by the pricing services that vary by more than $0.5 million and 5% from the expected price based on these assessment procedures are considered outliers, as are prices that have not changed from period to period and prices that have trended unusually compared to market conditions. In circumstances where the results of White Mountains’s review process does not appear to support the market price provided by the pricing services, White Mountains challenges the vendor provided price. If White Mountains cannot gain satisfactory evidence to support the challenged price, White Mountains will rely upon its own internal pricing methodologies to estimate the fair value of the security in question. The valuation process described above is generally applicable to all of White Mountains’s fixed maturity investments. The techniques and inputs specific to asset classes within White Mountains’s fixed maturity investments for Level 2 securities that use observable inputs are as follows: Debt Securities Issued by Corporations The fair value of debt securities issued by corporations is determined from a pricing evaluation technique that uses information from market sources and integrates relative credit information, observed market movements and sector news. Key inputs include benchmark yields, reported trades, broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers and reference data including sector, coupon, credit quality ratings, duration, credit enhancements, early redemption features and market research publications. Municipal Obligations The fair value of municipal obligations is determined from a pricing evaluation technique that uses information from market makers, brokers-dealers, buy-side firms and analysts along with general market information. Key inputs include benchmark yields, reported trades, issuer financial statements, material event notices and new issue data, as well as broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers and reference data including type, coupon, credit quality ratings, duration, credit enhancements, geographic location and market research publications. Foreign Government and Agency Obligations The fair value of foreign government and agency obligations is determined from a pricing evaluation technique that uses information from data sources in each respective country, including active market makers and inter-dealer brokers. Key inputs include benchmark yields, reported trades, broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, local exchange prices, foreign exchange rates and reference data including coupon, credit quality ratings, duration and market research publications. Mortgage and Asset-Backed Securities and Collateralized Loan Obligations The fair value of mortgage and asset-backed securities and collateralized loan obligations is determined from a pricing evaluation technique that uses information from market sources and leveraging similar securities. Key inputs include benchmark yields, reported trades, underlying tranche cash flow data, collateral performance, plus new issue data, as well as broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers and reference data including issuer, vintage, loan type, collateral attributes, prepayment speeds, default rates, recovery rates, cash flow stress testing, credit quality ratings and market research publications. Level 3 Measurements Fair value estimates for investments that trade infrequently and have few or no quoted market prices or other observable inputs are classified as Level 3 measurements. Investments valued using Level 3 fair value estimates are based upon unobservable inputs and include investments in certain fixed maturity investments, common equity securities and other long-term investments where quoted market prices or other observable inputs are unavailable or are not considered reliable or reasonable. Level 3 valuations are generated from techniques that use assumptions not observable in the market. These unobservable inputs reflect White Mountains’s assumptions of what market participants would use in valuing the investment. In certain circumstances, investment securities may start out as Level 3 when they are originally issued, but as observable inputs become available in the market, they may be reclassified to Level 2. Transfers of securities between levels are based on investments held as of the beginning of the period. Other Long-Term Investments As of December 31, 2024, $1,262.7 million of White Mountains’s other long-term investments, which consisted primarily of unconsolidated entities, including Kudu’s Participation Contracts and PassportCard/DavidShield were classified as Level 3 investments in the GAAP fair value hierarchy. The determination of the fair value of these securities involves significant management judgment, and the use of valuation analyses and assumptions that are inherently subjective and uncertain. White Mountains may use a variety of valuation techniques to determine fair value depending on the nature of the investment, including a discounted cash flow analysis, market multiple approach, cost approach and/or liquidation analysis. On an ongoing basis, White Mountains also considers qualitative changes in facts and circumstances, which may impact the valuation of its unconsolidated entities, including economic and market changes in relevant industries, changes to the entity’s capital structure, business strategy and key personnel and any recent transactions. On a quarterly basis, White Mountains evaluates the most recent qualitative and quantitative information of the business and completes a fair valuation analysis for all other long-term investments classified as Level 3 investments. Periodically, and at least on an annual basis, White Mountains uses a third-party valuation firm to complete an independent valuation analysis of significant unconsolidated entities. Other Long-term Investments - NAV As of December 31, 2024, $864.0 million of White Mountains’s other long-term investments, which consisted of private equity funds and hedge funds, a bank loan fund, Lloyd’s trust deposits and ILS funds, were valued at fair value using NAV as a practical expedient. Investments for which fair value is measured using NAV as a practical expedient are not classified within the fair value hierarchy. White Mountains employs a number of procedures to assess the reasonableness of the fair value measurements for other long-term investments measured at NAV, including obtaining and reviewing interim unaudited and annual audited financial statements as well as periodically discussing the valuations and methodologies used to value the underlying investments of each fund with fund managers. However, since fund managers do not provide sufficient information to evaluate the pricing methods and inputs for each underlying investment, White Mountains considers the valuation inputs to be unobservable. The fair value of White Mountains’s other long-term investments measured at NAV are generally determined using the fund manager’s NAV. Certain other long-term investments measured at NAV may be reported to White Mountains on a lag. In these circumstances, the NAV of such investments is adjusted to account for any applicable contributions, distributions, known transactions and/or pricing estimates provided by the fund manager through White Mountains’s reporting date. BAM Surplus Notes Prior to the deconsolidation of BAM on July 1, 2024, the BAM Surplus Notes, including accrued interest receivable, were classified as intercompany notes carried at nominal value, which eliminated in consolidation. Upon deconsolidation, White Mountains elected the fair value option for the BAM Surplus Notes. White Mountains values the BAM Surplus Notes each quarter using a discounted cash flow analysis. The BAM Surplus Notes are classified as a Level 3 measurement. Upon election of the fair value option, White Mountains made the accounting policy election to present the interest income from the BAM Surplus Notes separately from the change in fair value of the BAM Surplus Notes. See Note 10 — “Municipal Bond Guarantee Reinsurance.” Cash and Restricted Cash Cash includes amounts on hand and demand deposits with banks and other financial institutions. Amounts presented in the statement of cash flows are shown net of balances acquired and sold in the purchase or sale of the Company’s consolidated subsidiaries. Cash balances that are not immediately available for general corporate and operating purposes are classified as restricted. Derivatives From time to time, White Mountains holds derivative financial instruments for risk management purposes. White Mountains recognizes all derivatives as either assets or liabilities, measured at fair value, on its consolidated balance sheet. Changes in the fair value of derivative instruments that meet the criteria for hedge accounting are recognized in other comprehensive income (loss) and reclassified into current period pre-tax income (loss) when the hedged items are recognized therein. Changes in the fair value of derivative instruments that do not meet the criteria for hedge accounting are recognized in current period pre-tax income (loss). As of December 31, 2024, White Mountains holds interest rate cap derivative instruments that do not meet the criteria for hedge accounting. See Note 9 — “Derivatives.” Property and Casualty Insurance and Reinsurance Ark writes a diversified portfolio of reinsurance and insurance products, including property, specialty, marine & energy, casualty and accident & health. Ark accounts for insurance and reinsurance policies that it writes in accordance with ASC 944, Financial Services - Insurance . Ark’s premiums written comprise premiums on insurance contracts incepted during the year as well as premium adjustments related to prior underwriting years. Insurance premiums are recognized as revenues over the loss exposure or coverage period. In most cases, premiums are earned ratably over the term of the contract, with unearned premiums calculated on a monthly pro-rata basis. Catastrophe premiums are earned in proportion to the insurance protection provided. Premiums earned are presented net of amounts ceded to reinsurers. Insurance premiums receivable, representing amounts due from insureds, are presented net of an allowance for uncollectible premiums, including expected credit losses. The allowance is based upon Ark’s ongoing review of amounts outstanding, historical loss data, including delinquencies and write-offs, current and forecasted economic conditions and other relevant factors. Credit risk is partially mitigated by Ark’s ability to cancel the policy if the policyholder does not pay the premium. Deferred acquisition costs comprise commission and brokerage fees and taxes which are directly attributable to and vary with the production of business. These costs are deferred and amortized to the extent they relate to successful contract acquisitions over the applicable premium recognition period as insurance and reinsurance acquisition expenses. Deferred acquisition costs are limited to the amount expected to be recovered from future earned premiums and anticipated investment income. A premium deficiency is recognized if the sum of expected loss and loss adjustment expenses (“LAE”), expected dividends to policyholders, unamortized acquisition costs and maintenance costs exceeds related unearned premiums and anticipated investment income. A premium deficiency is recognized by charging any unamortized acquisition costs to expense to the extent required in order to eliminate the deficiency. If the premium deficiency exceeds unamortized acquisition costs, then a liability is accrued for the excess deficiency. Loss and LAE are charged against income as incurred. Loss and LAE reserves, including estimates for amounts incurred but not reported (“IBNR”), are based on management’s best estimate of the ultimate costs to settle claims, including the effects of inflation and other societal and economic factors. Loss and LAE reserves are presented net of estimated salvage and subrogation recoveries, if applicable. Such estimates are regularly reviewed and updated, and any resulting adjustments are reflected in current results of operations. The process of estimating loss and LAE reserves involves a considerable degree of judgment by management, and the ultima

Significant Transactions

Significant Transactions	12 Months Ended
	Dec. 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). 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 was not part of the purchase consideration. 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 the year ended December 31, 2024 are presented in Note 15 — “Segment Information.” BAM On July 1, 2024, HG Re and BAM amended the terms of the FLRT with respect to certain governance rights held by HG Re. As a result, and in combination with other governance changes at BAM, White Mountains concluded that it no longer has the power to direct BAM’s activities that most significantly impact its economic performance and is no longer BAM’s primary beneficiary. Accordingly, as of July 1, 2024, White Mountains no longer consolidates BAM. See Note 16 — “Variable Interest Entities.” Through June 30, 2024, BAM’s assets, liabilities and noncontrolling interests, as well as its results of operations, are presented within the HG Global segment. Upon deconsolidation, the BAM Surplus Notes met the criteria to be accounted for under the fair value option, which White Mountains elected. Accordingly, the BAM Surplus Notes, including accrued interest receivable, were carried at their fair value of $387.4 million as of July 1, 2024 resulting in an unrealized loss on deconsolidation of $114.5 million. This fair value includes the impact of a discount for the time value of money, which was previously included in adjusted book value per share as a non-GAAP adjustment to book value per share. See Note 10 — “Municipal Bond Guarantee Reinsurance” for the valuation techniques and inputs utilized to determine the fair value of the BAM Surplus Notes. NSM On August 1, 2022, the NSM Transaction closed. White Mountains received $1.4 billion in net cash proceeds at closing and recognized a net transaction gain of $875.7 million, which was comprised of $886.8 million of net gain from sale of discontinued operations and $2.9 million of comprehensive income related to the recognition of foreign currency translation gains (losses) from the sale, partially offset by $14.0 million of compensation and other costs related to the transaction recorded in Other Operations. WM Outrigger Re During the fourth quarter of 2022, Ark sponsored the formation of Outrigger Re Ltd., a Bermuda company registered as a special purpose insurer and segregated accounts company, to provide reinsurance capacity to Ark. Outrigger Re Ltd. was initially 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 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. White Mountains rolled over $130.0 million from its commitment to the 2023 underwriting year and received a return of capital of $75.0 million during 2024 as a result of its reduced capital commitment. The remaining capital was provided by third-party investors. During the fourth quarter of 2024, Ark renewed Outrigger Re Ltd. for the 2025 underwriting year with $230.0 million of capital. White Mountains’s total commitment was $150.0 million, of which $130.0 million was rolled over from its commitment to the 2024 underwriting year. The remaining capital was provided by third-party investors. The reduced capacity at Outrigger Re Ltd. was replaced by Ark through traditional quota share reinsurance agreements. 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 the year ended December 31, 2024, White Mountains received net distributions of $122.7 million from WM Outrigger Re, which included a net return of capital related to changes in White Mountains’s capital commitments for the 2024 and 2025 underwriting years and reinsurance profits for the 2023 underwriting year. During the year ended December 31, 2024, WM Outrigger Re commuted its reinsurance agreement with GAIL for the 2023 underwriting year. As of December 31, 2024 and 2023, investments of $203.7 million and $265.3 million were held in a collateral trust. Kudu On May 26, 2022, Kudu raised $114.5 million of equity capital (the “Kudu Transaction”) from Massachusetts Mutual Life Insurance Company (“Mass Mutual”), White Mountains and Kudu management. Mass Mutual, White Mountains and Kudu management contributed $64.1 million, $50.0 million and $0.4 million at a pre-money valuation of 1.3x book value, or $114.0 million, above the December 31, 2021 equity value of Kudu’s go-forward portfolio of Participation Contracts. The go-forward portfolio of Kudu’s Participation Contracts excluded $54.3 million of enterprise value as of December 31, 2021 relating to two portfolio companies that had announced sale transactions prior to the capital raise. As a result of the Kudu Transaction, White Mountains’s basic ownership of Kudu decreased from 99.1% to 89.3%.

Investment Securities

Investment Securities	12 Months Ended
	Dec. 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, is 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 17 — “Equity Method Eligible Investments.” Net realized and unrealized investment gains (losses) are reported in pre-tax revenues. Effective July 1, 2024, White Mountains no longer consolidates BAM. Through June 30, 2024, White Mountains’s consolidated financial statements included BAM’s fixed income portfolio and related investment results. See Note 2 — “Significant Transactions.” 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 years ended December 31, 2024, 2023 and 2022: Year Ended December 31, Millions 2024 2023 2022 Fixed maturity investments $ 93.2 $ 64.4 $ 41.4 Short-term investments 56.6 52.0 16.4 Common equity securities 2.5 4.1 1.6 Other long-term investments 77.9 76.1 68.1 Amount attributable to TPC Providers — — (1.0) Total investment income 230.2 196.6 126.5 Third-party investment expenses (2.8) (2.4) (2.1) Net investment income, pre-tax $ 227.4 $ 194.2 $ 124.4 Net Realized and Unrealized Investment Gains (Losses) The following table presents net realized and unrealized investment gains (losses) for the years ended December 31, 2024, 2023 and 2022: Year Ended December 31, Millions 2024 2023 2022 Realized investment gains (losses) Fixed maturity investments $ (10.1) $ (4.7) $ (11.9) Short-term investments (1.2) (.2) (1.4) Common equity securities 3.5 29.5 — Investment in MediaAlpha 91.2 — — Other long-term investments 34.6 72.2 128.4 Net realized investment gains (losses) 118.0 96.8 115.1 Unrealized investment gains (losses) Fixed maturity investments 3.9 66.1 (168.4) Short-term investments (.8) 1.9 (1.1) Common equity securities 58.1 47.1 (7.1) Investment in MediaAlpha (53.2) 27.1 (93.0) Other long-term investments 58.9 195.2 (43.8) Net unrealized investment gains (losses) 66.9 337.4 (313.4) Net realized and unrealized investment gains (losses), before amount attributable to TPC providers (1) 184.9 434.2 (198.3) Amount attributable to TPC Providers — — 6.8 Net realized and unrealized investment gains (losses) $ 184.9 $ 434.2 $ (191.5) Fixed maturity and short-term investments Net realized and unrealized investment gains (losses) $ (8.2) $ 63.1 $ (182.8) Less: net realized and unrealized gains (losses) on investment .9 4.7 (2.9) Net unrealized investment gains (losses) recognized during the $ (9.1) $ 58.4 $ (179.9) Common equity securities and investment in MediaAlpha Net realized and unrealized investment gains (losses) on $ 61.6 $ 76.6 $ (7.1) Net realized and unrealized investment gains (losses) from 38.0 27.1 (93.0) Total net realized and unrealized investment gains (losses) 99.6 103.7 (100.1) Less: net realized and unrealized gains (losses) on investment 35.4 28.4 — Net unrealized investment gains (losses) recognized during the $ 64.2 $ 75.3 $ (100.1) (1) For 2024, 2023 and 2022, includes $(31.5), $13.6 and $(29.3) of net realized and unrealized investment gains (losses) related to foreign currency exchange. For the years ended December 31, 2024, 2023 and 2022, all of White Mountains’s net realized and unrealized investment gains (losses) were recorded in the consolidated statements of operations. There were no investment gains (losses) recorded in other comprehensive income. White Mountains recognized gross realized investment gains of $139.1 million, $113.7 million and $129.9 million and gross realized investment losses of $21.1 million, $16.9 million and $14.8 million on sales of investment securities for the years ending December 31, 2024, 2023 and 2022. The following table presents total net unrealized gains (losses) attributable to Level 3 investments for the years ended December 31, 2024, 2023 and 2022 for investments still held at the end of the period: Year Ended December 31, Millions 2024 2023 2022 Total net unrealized investment gains on other long-term investments held at $ 57.2 $ 133.9 $ 56.5 Proceeds from the sales and maturities of investments, excluding short-term investments, totaled $1.2 billion, $0.8 billion and $0.5 billion for the years ended December 31, 2024, 2023 and 2022. 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 December 31, 2024 and 2023: December 31, 2024 Millions Cost or Gross Gross Net Foreign Carrying U.S. Government and agency obligations $ 461.8 $ 1.1 $ (.8) $ — $ 462.1 Debt securities issued by corporations 1,444.5 4.6 (31.4) (3.5) 1,414.2 Municipal obligations 3.2 — — — 3.2 Mortgage and asset-backed securities 400.2 .2 (26.5) — 373.9 Collateralized loan obligations 237.3 1.2 (.2) (1.6) 236.7 Foreign government and agency obligations 22.2 — (.1) (.6) 21.5 Total fixed maturity investments $ 2,569.2 $ 7.1 $ (59.0) $ (5.7) $ 2,511.6 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 weighted average duration of White Mountains’s fixed income portfolio, including short-term investments, was 1.9 years as of both December 31, 2024 and 2023. Excluding short-term investments, the weighted average duration of White Mountains’s fixed income portfolio was 2.6 years and 3.3 years as of December 31, 2024 and 2023. The following table presents the cost or amortized cost and carrying value of White Mountains’s fixed maturity investments by contractual maturity as of December 31, 2024 and 2023. Actual maturities could differ from contractual maturities because borrowers may have the right to call or prepay certain obligations with or without penalties. December 31, 2024 December 31, 2023 Millions Cost or Amortized Cost Carrying Value Cost or Amortized Cost Carrying Value Due in one year or less $ 205.8 $ 203.9 $ 319.9 $ 315.0 Due after one year through five years 1,494.5 1,478.7 845.0 813.9 Due after five years through ten years 206.1 193.2 303.4 287.0 Due after ten years 25.3 25.2 101.7 94.9 Mortgage and asset-backed securities and 637.5 610.6 628.4 598.5 Total fixed maturity investments $ 2,569.2 $ 2,511.6 $ 2,198.4 $ 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 December 31, 2024 and 2023: December 31, 2024 Millions Cost or Gross Unrealized Gross Unrealized Losses Net Foreign Carrying Common equity securities $ 537.0 $ 120.4 $ — $ (7.4) $ 650.0 Investment in MediaAlpha $ 59.2 $ 142.4 $ — $ — $ 201.6 Other long-term investments $ 1,748.7 $ 546.4 $ (116.7) $ (28.2) $ 2,150.2 December 31, 2023 Millions Cost or Gross Unrealized Gross Unrealized Losses 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 by Level The following tables present White Mountains’s fair value measurements for investments as of December 31, 2024 and 2023 by level. See Note 1 — “Basis of Presentation and Significant Accounting Policies.” The major security types were based on the legal form of the securities. White Mountains has disaggregated its fixed maturity investments based on the issuing entity type, which impacts credit quality, with debt securities issued by U.S. government entities carrying minimal credit risk, while the credit and other risks associated with other issuers, such as corporations, foreign governments and agencies, municipalities, entities issuing mortgage and asset-backed securities or entities issuing collateralized loan obligations 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 U.S. Intermediate Aggregate Index. December 31, 2024 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 462.1 $ 461.3 $ .8 $ — Debt securities issued by corporations: Financials 466.7 — 466.7 — Consumer 255.2 — 255.2 — Industrial 164.4 — 164.4 — Healthcare 153.1 — 153.1 — Technology 113.8 — 113.8 — Communications 71.6 — 71.6 — Materials 65.7 — 65.7 — Energy 62.8 — 62.8 — Utilities 60.9 — 60.9 — Total debt securities issued by corporations 1,414.2 — 1,414.2 — Municipal obligations 3.2 — 3.2 — Mortgage and asset-backed securities 373.9 — 373.9 — Collateralized loan obligations 236.7 — 236.7 — Foreign government and agency obligations 21.5 — 21.5 — Total fixed maturity investments 2,511.6 461.3 2,050.3 — Short-term investments 964.2 951.1 13.1 — Common equity securities: Exchange-traded funds 224.6 224.6 — — Other (1) 425.4 — 425.4 — Total common equity securities 650.0 224.6 425.4 — Investment in MediaAlpha 201.6 201.6 — — Other long-term investments 1,286.2 — 23.5 1,262.7 Other long-term investments — NAV (2) 864.0 — — — Total other long-term investments 2,150.2 — 23.5 1,262.7 Total investments $ 6,477.6 $ 1,838.6 $ 2,512.3 $ 1,262.7 (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 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 December 31, 2024 and 2023, Ark held Lloyd’s trust deposits with a fair value of $149.9 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 an annual capital adequacy determination by Lloyd’s. As of December 31, 2024 and 2023, the fair value of Ark’s Funds at Lloyd’s cash and investment deposits totaled $361.5 million and $345.2 million. As of December 31, 2024 and 2023, Ark held additional investments on deposit or as collateral for insurance regulators and reinsurance counterparties of $226.5 million and $244.5 million. As of December 31, 2024 and 2023, investments of $203.7 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. Ark is required to pledge collateral under its standby letters of credit. See Note 7 — “Debt.” HG Re is required to maintain assets, including investments, in collateral trusts under the FLRT with BAM. See Note 10 — “Municipal Bond Guarantee Reinsurance - Collateral Trusts.” HG Global is required to maintain an interest reserve account in connection with its senior notes issued in 2022. See Note 7 — “Debt.” Kudu is required to maintain an interest reserve account in connection with its credit facility. See Note 7 - “Debt.” As of December 31, 2024, investments of $32.2 million were held as collateral required to be maintained in relation to Bamboo Captive’s reinsurance agreements. As of December 31, 2024, investments of $46.7 million were held as collateral required to be maintained in relation to Bamboo CRV’s reinsurance agreements. Debt Securities Issued by Corporations The following table presents the fair values for credit ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of December 31, 2024 and 2023: December 31, Millions 2024 2023 AAA $ 14.6 $ 11.5 AA 91.2 83.8 A 607.2 552.4 BBB 688.7 390.9 BB 5.4 — Other 7.1 6.4 Debt securities issued by corporations (1) $ 1,414.2 $ 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 December 31, 2024 and 2023: December 31, 2024 December 31, 2023 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: FNMA $ 198.3 $ 198.3 $ — $ 198.8 $ 198.8 $ — FHLMC 147.1 147.1 — 137.2 137.2 — GNMA 24.2 24.2 — 30.2 30.2 — Total agency (1) 369.6 369.6 — 366.2 366.2 — Non-agency: Commercial .4 .4 — — — — Residential — — — .2 .2 — Total non-agency .4 .4 — .2 .2 — Total mortgage-backed securities 370.0 370.0 — 366.4 366.4 — Other asset-backed securities: Vehicle receivables 1.7 1.7 — 17.2 17.2 — Credit card receivables .2 .2 — 3.3 3.3 — Other 2.0 2.0 — 2.5 2.5 — Total other asset-backed securities 3.9 3.9 — 23.0 23.0 — Total mortgage and asset-backed securities 373.9 373.9 — 389.4 389.4 — Collateralized loan obligations: 236.7 236.7 — 209.1 209.1 — Total mortgage and asset-backed securities and $ 610.6 $ 610.6 $ — $ 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 December 31, 2024 and 2023, White Mountains’s investment portfolio included $236.7 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 December 31, 2024 and 2023. Investment in MediaAlpha 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 2024, MediaAlpha completed a secondary offering of 7.6 million shares at $19.00 per share ($18.24 per share net of underwriting fees). In the secondary offering, White Mountains sold 5.0 million shares for net proceeds of $91.2 million. 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 December 31, 2024, White Mountains owned 17.9 million shares of MediaAlpha, representing a 26.6% basic ownership interest (25.4% on a fully-diluted/fully-converted basis). See Note 17 — “Equity Method Eligible Investments.” At White Mountains’s December 31, 2024 level of ownership, each $1.00 per share increase or decrease in the share price of MediaAlpha will result in an approximate $7.00 per share increase or decrease in White Mountains’s book value per share. At the December 31, 2024 share price of $11.29, the fair value of White Mountains’s investment in MediaAlpha was $201.6 million. At the December 31, 2023 share price of $11.15, the fair value of White Mountains’s investment in MediaAlpha was $254.9 million. Other Long-Term Investments The following tables presents the carrying values of White Mountains’s other long-term investments by reportable segment as of December 31, 2024 and 2023: Fair Value as of December 31, 2024 Millions Ark/WM Outrigger Kudu Other Total Kudu’s Participation Contracts $ — $ 1,008.4 $ — $ 1,008.4 PassportCard/DavidShield — — 150.0 150.0 Elementum — — 35.0 35.0 Other unconsolidated entities (1) — — 63.6 63.6 Total unconsolidated entities — 1,008.4 248.6 1,257.0 Private equity funds and hedge funds 104.1 — 256.5 360.6 Bank loan fund 264.7 — — 264.7 Lloyd’s trust deposits 149.9 — — 149.9 ILS funds — — 74.0 74.0 Private debt instruments — 5.6 9.3 14.9 Other 29.1 — — 29.1 Total other long-term investments $ 547.8 $ 1,014.0 $ 588.4 $ 2,150.2 (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. Fair Value as of 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 — — 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 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 December 31, 2024, White Mountains held investments in seventeen private equity funds and two hedge funds. The largest investment in a single private equity fund or hedge fund was $59.2 million and $57.3 million as of December 31, 2024 and 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 December 31, 2024 and 2023: December 31, 2024 December 31, 2023 Millions Fair Value Unfunded Fair Value Unfunded Private equity funds Aerospace/Defense/Government $ 152.4 $ 49.3 $ 157.2 $ 17.4 Financial services 93.4 32.0 87.5 37.8 Real estate 3.7 2.4 3.9 2.5 Total private equity funds 249.5 83.7 248.6 57.7 Hedge funds Long/short equity financials and business services 59.2 — 54.4 — Long/short all cap global 52.0 — — — European small/mid cap — — 9.9 — Total hedge funds 111.2 — 64.3 — Total private equity funds and hedge funds included in other long-term investments $ 360.7 $ 83.7 $ 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 December 31, 2024: Millions 1 – 3 years 3 – 5 years 5 – 10 years >10 years Total Private equity funds — expected lock-up period remaining $ 18.0 $ 51.1 $ 159.4 $ 21.0 $ 249.5 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 December 31, 2024 and 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. White Mountains’s hedge fund investments are subject to monthly and quarterly restrictions on redemptions and advance written redemption notice period requirements that range between 45 and 90 calendar days. Bank Loan Fund White Mountains’s other long-term investments include a bank loan fund with a fair value of $264.7 million and $194.4 million as of December 31, 2024 and 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 December 31, 2024 and 2023, White Mountains held Lloyd’s trust deposits with a fair value of $149.9 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 December 31, 2024 and 2023, White Mountains held investments in ILS funds with a fair value of $74.0 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 calendar 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 December 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 years ended December 31, 2024 and 2023: Level 3 Investments Millions Other Long-term Other Long-term Balance as of December 31, 2023 $ 1,138.2 Balance as of December 31, 2022 $ 911.6 Net realized and unrealized gains 54.4 Net realized and unrealized gains 157.3 Purchases and contributions 111.5 Purchases and contributions 256.6 Sales and distributions (41.4) Sales and distributions (187.3) Transfers in — Transfers in — Transfers out — Transfers out — Balance as of December 31, 2024 $ 1,262.7 Balance as of December 31, 2023 $ 1,138.2 Fair Value Measurements — Transfers Between Levels - For Years Ended December 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 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 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 December 31, 2024 and 2023. The tables below exclude $23.4 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 December 31, 2024 Description Valuation Technique(s) (1) Fair Value (2) Unobservable Inputs Discount Rate (5) Terminal Cash Flow Exit Multiple (x) or Terminal Revenue Growth Rate (%) (5) Kudu’s Participation Contracts (3)(4) Discounted cash flow $1,008.4 17% - 25% 7x - 22x PassportCard/DavidShield Discounted cash flow $150.0 24% 4% Elementum Discounted cash flow $35.0 22% 4% Preferred securities Discounted cash flow $31.4 9% N/A Private debt instruments Discounted cash flow $14.5 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 19% and 14x. (4) In 2024, Kudu contributed total cash of $103.5 into new and existing Participation Contracts. (5) Increases (decreases) to the discount rates in isolation would result in lower (higher) fair value measurements, while increases (decreases) to the terminal cash flow exit multiples or terminal revenue growth rates in isolation would result in higher (lower) fair value measurements. $ in Millions December 31, 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 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 contributed total cash 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 sale 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	12 Months Ended
	Dec. 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, including other intangible assets, and liabilities assumed, including contingent consideration liabilities, at their estimated fair values as of the acquisition date. Goodwill represents the excess of the amount paid to acquire a business over the fair value of identifiable net assets at the acquisition date. The estimated acquisition date fair values, generally consisting of intangible assets and contingent consideration liabilities, 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 life, acquisition date fair value, accumulated amortization and net carrying value for other intangible assets and goodwill as of December 31, 2024 and 2023: $ in Millions Weighted Average Economic December 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 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 1.8 .4 2.2 1.5 .7 Bamboo Trade names 10.0 23.5 2.3 21.2 — — — Agency relationships 6.0 72.4 12.1 60.3 — — — Developed technology 3.0 4.7 1.6 3.1 — — — Other 0.3 .4 .4 — — — — Subtotal 101.0 16.4 84.6 — — — Other Operations Trade names 13.3 13.3 5.6 7.7 13.3 4.1 9.2 Customer relationships 11.0 24.8 13.6 11.2 24.8 10.3 14.5 Other 11.8 3.1 1.6 1.5 2.8 1.1 1.7 Subtotal 41.2 20.8 20.4 40.9 15.5 25.4 Total other intangible assets 320.1 39.0 281.1 218.8 17.0 201.8 Total goodwill and other intangible assets $ 759.3 $ 39.0 $ 720.3 $ 387.6 $ 17.0 $ 370.6 Rollforward of Goodwill and Other Intangible Assets The following table presents the change in goodwill and other intangible assets for the years ended December 31, 2024 and 2023: December 31, 2024 December 31, 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 Acquisitions 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 — (22.0) (22.0) — (7.2) (7.2) Ending balance $ 439.2 $ 281.1 $ 720.3 $ 168.8 $ 201.8 $ 370.6 (1) Relates to the Bamboo Transaction. See Note 2 — “Significant Transactions .” (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. Adjustments relate to acquisitions of certain consolidated Other Operating Businesses. During the years ended December 31, 2024 and 2023, White Mountains did not recognize any impairments to goodwill and other intangible assets. Amortization of Other Intangible Assets Amortization expense was $22.0 million, $7.2 million and $5.2 million for the years ended December 31, 2024, 2023 and 2022. The following table presents a schedule of White Mountains’s future amortization expense as of December 31, 2024: Millions Amortization Expense 2025 $ 20.8 2026 20.1 2027 17.9 2028 16.9 2029 16.4 2030 and years after 13.3 Total (1) $ 105.4 (1) Excludes Ark’s indefinite-lived intangible assets of $175.7.

Loss and Loss Adjustment Expens

Loss and Loss Adjustment Expense Reserves	12 Months Ended
	Dec. 31, 2024
Insurance Loss Reserves [Abstract]
Loss and Loss Adjustment Expense Reserves	Loss and Loss Adjustment Expense Reserves Loss and LAE reserves are typically comprised of case reserves for claims reported and reserves for losses that have occurred but for which claims have not yet been reported, referred to as IBNR reserves. IBNR reserves include a provision for expected future development on case reserves. Case reserves are estimated based on the experience and knowledge of claims staff regarding the nature and potential cost of each claim and are adjusted as additional information becomes known or payments are made. IBNR reserves are derived by subtracting paid loss and LAE and case reserves from estimates of ultimate loss and LAE. Actuaries estimate ultimate loss and LAE using various generally accepted actuarial methods applied to known losses and other relevant information. Like case reserves, IBNR reserves are adjusted as additional information becomes known or payments are made. Loss and LAE reserves are recognized within White Mountains’s P&C Insurance and Reinsurance, Financial Guarantee, P&C Insurance Distribution and certain Other Operations businesses. P&C Insurance and Reinsurance Ark establishes loss and LAE reserves that are estimates of amounts needed to pay claims and related expenses in the future for insured events that have already occurred. Ark’s process of estimating reserves involves a considerable degree of judgment by management and, as of any given date, is inherently uncertain. WM Outrigger Re entered into a quota share agreement with GAIL to provide collateralized reinsurance protection on Ark’s Bermuda global property catastrophe excess of loss portfolio written in the 2023 underwriting year. Ark renewed its quota share reinsurance agreement with WM Outrigger Re for the 2024 and 2025 underwriting years. WM Outrigger Re’s quota share reinsurance agreement with GAIL, including its loss and LAE reserves, eliminates in White Mountains’s consolidated financial statements. Loss and LAE are categorized by the year in which the policy is underwritten (the year of account, or underwriting year) for purposes of Ark’s claims management and estimation of the ultimate loss and LAE reserves. For purposes of Ark’s reporting under GAAP, loss and LAE are categorized by the year in which the claim is incurred (the accident year). Ultimate loss and LAE are generally determined by extrapolation of claims emergence and settlement patterns observed in the past that can reasonably be expected to persist into the future. In forecasting ultimate loss and LAE for any line of business, past experience with respect to that line of business is the primary resource, but cannot be relied upon in isolation. Ark’s own experience, particularly claims development experience, such as trends in case reserves, payments on and closings of claims, as well as changes in business mix and coverage limits, is the most important information for estimating its loss and LAE reserves. External data, available from organizations such as the Lloyd’s Market Association, consulting firms and other insurance and reinsurance companies, is used to supplement or corroborate Ark’s own experience. External data can be especially useful for estimating costs on newer lines of business. Ultimate loss and LAE for major losses and catastrophes are estimated based on the known and expected exposures to the loss event, rather than simply relying on the extrapolation of reported and settled claims. For some lines of business, such as long-tail coverages discussed below, claims data reported in the most recent years of account are often too limited to provide a meaningful basis for analysis due to the typical delay in reporting and settling of claims. For this type of business, Ark uses an expected loss ratio method for the initial years of account. This is a standard and accepted actuarial reserve estimation method in these circumstances in which the loss ratio is selected based upon information used in pricing policies for that line of business, as well as any publicly available industry data, such as industry pricing, experience and trends. Uncertainties in estimating ultimate loss and LAE are magnified by the time lag between when a claim actually occurs and when it is reported and eventually settled. This time lag is sometimes referred to as the “claim-tail.” The claim-tail for reinsurance and insurance obtained through brokers, MGAs and reinsurance intermediaries (collectively, the “insurance and reinsurance intermediaries”) is further extended because claims are first reported to either the original primary insurance company or the insurance and reinsurance intermediaries. The claim-tail for most property coverages is typically short (usually a few days up to a few months). Settlements for casualty/liability coverages can extend for long periods of time as claims are often reported and ultimately paid or settled years after the related loss events occur. During the long claims reporting and settlement period, additional facts regarding coverages written in prior years of account, as well as about actual claims and trends, may become known and, as a result, Ark may adjust its reserves. The inherent uncertainties of estimating loss and LAE reserves are increased by the diversity of loss development patterns among different types of reinsurance treaties, facultative contracts or direct insurance contracts, the necessary reliance on the ceding companies and insurance and reinsurance intermediaries for information regarding reported claims and the differing reserving practices among ceding companies and insurance and reinsurance intermediaries. If management determines that an adjustment is appropriate, the adjustment is booked in the accounting period in which such determination is made. Accordingly, should reserves need to be increased or decreased in the future from amounts currently established, future results of operations would be negatively or positively impacted. In determining ultimate loss and LAE, the cost to indemnify claimants, provide needed legal defense and other services for insureds and administer the investigation and adjustment of claims are considered. These claims costs are influenced by many factors that change over time, such as expanded coverage definitions as a result of new court decisions, inflation in costs to repair or replace damaged property, inflation in the cost of medical services and legislated changes in statutory benefits, as well as by the unique facts that pertain to each claim. As a result, the rate at which claims arose in the past and the costs to settle them may not always be representative of what will occur in the future. The factors influencing changes in claims costs are often difficult to isolate or quantify and developments in paid and incurred losses from historical trends are frequently subject to multiple and conflicting interpretations. Changes in coverage terms or claims handling practices may also cause future experience and/or development patterns to vary from the past. A key objective of actuaries in developing estimates of ultimate loss and LAE and resulting IBNR reserves is to identify aberrations and systemic changes occurring within historical experience and accurately adjust for them so that the future can be projected more reliably. Because of the factors previously discussed, this process requires the use of informed judgment and is inherently uncertain. Ark performs an actuarial review of its recorded loss and LAE reserves each quarter, using several generally accepted actuarial methods to evaluate its loss reserves, each of which has its own strengths and weaknesses. Management places more or less reliance on a particular method based on the facts and circumstances at the time the reserve estimates are made. These methods generally fall into one of the following categories or are hybrids of one or more of the following categories: • Historical paid loss development methods: These methods use historical loss payments over discrete periods of time to estimate future losses. Historical paid loss development methods assume that the ratio of losses paid in one period to losses paid in an earlier period will remain constant. These methods necessarily assume that factors that have affected paid losses in the past, such as inflation or the effects of litigation, will remain constant in the future. Because historical paid loss development methods do not use case reserves to estimate ultimate losses, they can be more reliable than the other methods discussed below that look to case reserves (such as actuarial methods that use incurred losses) in situations where there are significant changes in how case reserves are established by a company’s claims adjusters. However, historical paid loss development methods are more leveraged, meaning that small changes in payments have a larger impact on estimates of ultimate losses, than actuarial methods that use incurred losses because cumulative loss payments take much longer to approach the expected ultimate losses than cumulative incurred amounts. In addition, and for similar reasons, historical paid loss development methods are often slow to react to situations when new or different factors arise than those that have affected paid losses in the past. • Historical incurred loss development methods: These methods, like historical paid loss development methods, assume that the ratio of losses in one period to losses in an earlier period will remain constant in the future. However, instead of using paid losses, these methods use incurred losses (i.e., the sum of cumulative historical loss payments plus outstanding case reserves) over discrete periods of time to estimate future losses. Historical incurred loss development methods can be preferable to historical paid loss development methods because they explicitly take into account open cases and the claims adjusters’ evaluations of the cost to settle all known claims. However, historical incurred loss development methods necessarily assume that case reserving practices are consistently applied over time. Therefore, when there have been significant changes in how case reserves are established, using incurred loss data to project ultimate losses can be less reliable than other methods. • Expected loss ratio methods: These methods are based on the assumption that ultimate losses vary proportionately with premiums. Expected loss ratios are typically developed based upon the information used in pricing and are multiplied by the total amount of premiums earned to calculate ultimate losses. Expected loss ratio methods are useful for estimating ultimate losses in the early years of long-tail lines of business, when little or no paid or incurred loss information is available. • Bornhuetter-Ferguson methods: These methods are a blend of the expected loss ratio and loss development methods. The percent of incurred (or paid) loss to ultimate loss implied by the selected development pattern from the incurred (or paid) loss development method is used to determine the percentage of ultimate loss yet to be developed. Inception to date losses are added to losses yet to be developed, yielding an estimate of ultimate for each year of account. • Adjusted historical paid and incurred loss development methods: These methods take traditional historical paid and incurred loss development methods and adjust them for the estimated impact of changes from the past in factors such as inflation, the speed of claims payment or the adequacy of case reserves. Adjusted historical paid and incurred loss development methods are often more reliable methods of predicting ultimate losses in periods of significant change, provided the actuaries can develop methods to reasonably quantify the impact of changes. As part of Ark’s quarterly actuarial review, Ark compares the previous quarter’s projections of incurred, paid and case reserve activity, including amounts incurred but not reported, to actual amounts experienced in the quarter. Differences between previous estimates and actual experience are evaluated to determine whether a given actuarial method for estimating loss and LAE reserves should be relied upon to a greater or lesser extent than it has been in the past. While some variance is expected each quarter due to the inherent uncertainty in estimating loss and LAE reserves, persistent or large variances would indicate that prior assumptions and/or reliance on certain actuarial methods may need to be revised going forward. Upon completion of each quarterly review, Ark selects indicated loss and LAE reserve levels based on the results of the actuarial methods described previously, which are the primary consideration in determining management's best estimate of required loss and LAE reserves. However, in making its best estimate, management also considers other qualitative factors that may lead to a difference between held reserves and the actuarial central estimate of reserves. Typically, these qualitative factors are considered when management and Ark’s actuaries conclude that there is insufficient historical incurred and paid loss information or that trends included in the historical incurred and paid loss information are not likely to repeat in the future. Such qualitative factors include, among others, recent entry into new markets or new products, improvements in the claims department that are expected to lessen future ultimate loss costs, legal and regulatory developments or other uncertainties that may arise. Ark Reserve Estimation by Line of Business The process of establishing loss and LAE reserves, including amounts incurred but not reported, is complex and imprecise, as it must consider many variables that are subject to the outcome of future events. As a result, informed subjective estimates and judgments as to the ultimate exposure to losses are an integral component of the loss and LAE reserving process. Ark categorizes and tracks insurance and reinsurance reserves by “reserving class of business” for each underwriting office, London and Bermuda, and then aggregates the reserving classes by line of business, which are summarized herein as property and accident & health, specialty, marine & energy, casualty-active and casualty-runoff. Ark regularly reviews the appropriateness of its loss and LAE reserves at the reserving class of business level, considering a variety of trends that impact the ultimate settlement of claims for the subsets of claims in each particular reserving class. For loss and LAE reserves as of December 31, 2024, Ark considers that the impact of the various reserving factors, as described below, on future paid losses would be similar to the impact of those factors on historical paid losses. The major causes of material uncertainty (i.e., reserving factors) generally will vary for each line of business, as well as for each separately analyzed reserving class of business within the line of business. Also, reserving factors can have offsetting or compounding effects on estimated loss and LAE reserves. In most cases, it is not possible to measure the effect of a single reserving factor and construct a meaningful sensitivity expectation. Actual results will likely vary from expectations for each of these assumptions, resulting in an ultimate claims liability that is different from that being estimated currently. Additional causes of material uncertainty exist in most product lines and may impact the types of claims that could occur within a particular line of business or reserving class of business. Examples where reserving factors, within a line of business or reserving class of business, are subject to change include changing types of insureds (e.g., size of account, industry insured, jurisdiction), changing underwriting standards, or changing policy provisions (e.g., deductibles, policy limits, endorsements). Following is a detailed description of the reserve factors and consideration for each of Ark’s reserving lines of business. Property and Accident & Health Ark’s property and accident & health reserving line of business contains short-tail exposures. As such, reserving for these classes generally involves less uncertainty given the speed of settlement. For property reserving classes, the reserve risk is driven primarily by occasional catastrophe events, though the financial effect of these is mitigated by reinsurance and retrocessional purchases. Ark writes property business on both an insurance and reinsurance basis. The insurance business primarily consists of direct and facultative contracts. However, some business is written through line slips and MGA binding authorities, which could have a longer-tail due to the increased exposure period caused by underlying policies attaching to the binder contract. The reinsurance business can also have a longer-tail due to timing delays resulting from attachment points on excess of loss contracts. For accident & health reserving classes, the losses emanate from a wide range of personal accident, sickness, travel and medical insurance risks. The underlying business is a mix of direct and facultative contracts, as well as some MGA and reinsurance contracts, which are typically shorter-tail lines. Certain smaller components of the accident & health business can be longer-tail. The accident & health business is also exposed to occasional catastrophic events though not to the same degree as the property business. Marine & Energy Ark’s marine & energy reserving line of business is underwritten on both an insurance and reinsurance basis and can be broken down into physical damage on marine risks, physical damage on upstream energy platforms and marine & energy liabilities. The marine reserving classes consist primarily of marine hull, cargo and specie risks. These all generally have some element of transportability, which mitigates the catastrophe risk exposure; for example, having the ability to move out of the path of a hurricane if provided with sufficient notice. The marine reserving classes are generally shorter-tail. The energy platform reserving classes cover risks that are less transportable and therefore are exposed to catastrophe events similar to property reserving classes. Other energy reserving classes cover construction contracts, which often have considerably protracted exposure periods with the bulk of the risk towards the end of the coverage period. This can have the effect of increasing the tail on an otherwise short-tail reserving class. The marine & energy liability reserving classes, which represent a smaller portion of the marine & energy business, are typically longer-tail compared to physical damage reserving classes. Specialty Ark’s specialty portfolio is comprised of a diverse portfolio of insurance and reinsurance subclasses of business including aviation, space, political and credit, cyber, terrorism and political violence, nuclear, fine art & specie, surety and mortgage. Certain subclasses of business are exposed to both catastrophe events and man-made loss events; for example, terrorism, war and war-like actions, political violence and space. Although these subclasses have different coverages and exposures, they are all short-tail in nature and have similar reserving features. Casualty-Active and Casualty-Runoff Ark’s casualty reserving lines of business, which include casualty–active and casualty–runoff, are long-tail classes of business. Consequently, the ultimate liability may not be known at the date of loss, which results in greater uncertainty when reserving for casualty lines. The casualty–active line of business consists of U.S. reinsurance and insurance risks written on an excess of loss basis. The casualty–runoff line of business consists of international reinsurance risks and U.S. casualty insurance risks written through an MGA binding authority. The losses arising from these lines of business are primarily related to medical malpractice, professional liability and general liability coverages, which are long-tail lines of business. Casualty policies are generally written on either a claims-made or occurrence basis. On a claims-made basis, the trigger of loss is based on the date that the loss is reported. On an occurrence basis, the trigger of loss is the date that the loss occurred. Due to delays between loss occurrence and loss reporting, business written on an occurrence basis can be longer-tail than business written on a claims-made basis. There are a number of common reserving factors for casualty lines that can affect the estimated casualty reserves, including: • Changes in claims-handling practices, both in-house and through third-party claims administrators, • Changes in court interpretations of policy provisions, and • Trends in litigation or jury awards. Cumulative Number of Reported Claims Ark counts a claim for each unique combination of individual claimant, loss event and risk. A claim is still counted if the claim is closed with no payment. Bulk-coded losses are counted as one claim, as underlying claim counts are not available. Discounting Ark does not discount loss and LAE reserves. Impact of Third-Party Capital For the years of account prior to the Ark Transaction, a significant proportion of the Syndicates’ underwriting capital was provided by TPC Providers using whole account reinsurance contracts with Ark’s corporate member. For the years of account subsequent to the Ark Transaction, Ark is no longer using TPC Providers to provide underwriting capital for the Syndicates. A Reinsurance to Close (“RITC”) agreement is generally put in place after the third year of operations for a year of account such that the outstanding loss and LAE reserves, including future development thereon, are reinsured into the next year of account. As a result, and in combination with the changing participation provided by TPC Providers, Ark’s participation on outstanding loss and LAE reserves reinsured into the next year of account changes. After 2023, Ark is no longer subject to changes in TPC Providers’ participation. During 2023, an RITC agreement was executed such that the outstanding loss and LAE reserves for claims arising out of the 2020 year of account, for which the TPC Providers’ participation in the total net results of the Syndicates was 42.8%, were reinsured into the 2021 year of account, for which the TPC Providers’ participation in the total net results of the Syndicates was 0.0%. During 2022, an RITC agreement was executed such that the outstanding loss and LAE reserves for claims arising out of the 2019 year of account, for which the TPC Providers’ participation in the total net results of the Syndicates was 58.3%, were reinsured into the 2020 year of account, for which the TPC Providers’ participation in the total net results of the Syndicates was 42.8%. Loss and Loss Adjustment Expense Reserve Summary The following table summarizes the loss and LAE reserve activity of the Ark/WM Outrigger segment for the years ended December 31, 2024, 2023 and 2022: Year Ended December 31, Millions 2024 2023 2022 Gross beginning balance $ 1,605.1 $ 1,296.5 $ 894.7 Less: beginning reinsurance recoverable on unpaid losses (1) (340.8) (505.0) (428.9) Net loss and LAE reserves 1,264.3 791.5 465.8 Loss and LAE incurred relating to: Current year losses 914.4 706.9 588.1 Prior year losses (58.6) 19.9 (51.7) Net incurred loss and LAE 855.8 726.8 536.4 Loss and LAE paid relating to: Current year losses (109.2) (51.3) (98.9) Prior year losses (307.5) (356.6) (158.6) Net paid loss and LAE (416.7) (407.9) (257.5) Change in TPC Providers’ participation (2) — 145.4 57.5 Foreign currency translation and other adjustments to loss and LAE reserves (10.3) 8.5 (10.7) Net ending balance 1,693.1 1,264.3 791.5 Plus: ending reinsurance recoverable on unpaid losses (3) 434.4 340.8 505.0 Gross ending balance $ 2,127.5 $ 1,605.1 $ 1,296.5 (1) The beginning reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $145.4 and $276.8 as of January 1, 2023 and 2022. (2) Amount represents the impact to net loss and LAE reserves due to a change in the TPC Providers’ participation related to the annual RITC process. (3) The ending reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $145.4 as of December 31, 2022. During the year ended December 31, 2024, the Ark/WM Outrigger segment experienced $58.6 million of net favorable prior year loss reserve development. The net favorable prior year loss reserve development was driven primarily by the specialty ($34.3 million) and property and accident & health ($29.2 million) reserving lines of business, partially offset by net unfavorable development in the casualty-active ($4.5 million) reserving line of business. The net favorable prior year loss reserve development was driven primarily by positive claims experience in specialty for the 2023 and 2019 accident years and in property and accident & health for the 2023 accident year. During the year ended December 31, 2023, the Ark/WM Outrigger segment experienced $19.9 million of net unfavorable prior year loss reserve development. The net unfavorable prior year loss reserve development was driven primarily by the property and accident & health ($41.7 million) reserving line of business, partially offset by net favorable prior year loss reserve development within the specialty ($11.9 million) and casualty–runoff ($6.0 million) reserving lines of business. The net unfavorable prior year loss reserve development in the property and accident & health reserving line of business was driven primarily by Hurricane Ian and Winter Storm Elliott within the 2022 accident year. The net favorable prior year loss reserve development in specialty and casualty-runoff was driven primarily by positive claims experience for the 2021 and 2020 accident years. During the year ended December 31, 2022, The Ark/WM Outrigger segment experienced $51.7 million of net favorable prior year loss reserve development. The net favorable prior year loss reserve development was driven primarily by the property and accident & health ($20.8 million), marine & energy ($18.8 million) and specialty ($12.7 million) reserving lines of business. The net favorable prior year loss reserve development in property and accident & health, marine & energy and specialty was driven primarily by positive claims experience for the 2021 accident year. The following table summarizes the unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses, for each of the Ark/WM Outrigger segment’s major reserving lines of business as of December 31, 2024 and 2023: Year Ended December 31, Millions 2024 2023 Property and Accident & Health (1) $ 533.6 $ 358.7 Marine & Energy 463.1 331.7 Specialty 417.0 339.7 Casualty-Active 178.0 137.1 Casualty-Runoff 66.5 81.3 WM Outrigger Re 34.9 15.8 Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses (1) 1,693.1 1,264.3 Plus: Reinsurance recoverables on unpaid losses Property and Accident & Health (2) 121.5 126.9 Marine & Energy 180.1 59.1 Specialty 51.3 77.6 Casualty-Active 80.7 74.8 Casualty-Runoff .8 2.4 Total Reinsurance recoverables on unpaid losses (2) 434.4 340.8 Total unpaid loss and LAE reserves $ 2,127.5 $ 1,605.1 (1) The amounts exclude $3.1 ceded by Ark to WM Outrigger Re as of December 31, 2024, which eliminates in White Mountains’s consolidated financial statements. (2) The amounts exclude $31.8 and $15.6 ceded by Ark to WM Outrigger Re as of December 31, 2024 and 2023, which eliminate in White Mountains’s consolidated financial statements. The following five tables cover each of Ark’s property and accident & health, marine & energy, specialty, casualty-active and casualty-runoff reserving lines of business and are presented net of reinsurance, which includes the impact of whole-account quota-share reinsurance arrangements related to TPC Providers. Each of the five tables includes three sections as follows: The top section of the table presents, for each of the previous 10 accident years: (1) cumulative total undiscounted incurred loss and LAE as of each of the previous 10 year-end evaluations, (2) total IBNR plus expected development on reported claims as of December 31, 2024 and (3) the cumulative number of reported claims as of December 31, 2024. The middle section of the table presents cumulative paid loss and LAE for each of the previous 10 accident years as of each of the previous 10 year-end evaluations. Also included in this section is a calculation of the loss and LAE reserves as of December 31, 2024, which is then included in the reconciliation to the consolidated balance sheet presented above. The total unpaid loss and LAE reserves as of December 31, 2024 are calculated as the cumulative incurred loss and LAE from the top section less the cumulative paid loss and LAE from the middle section, plus any outstanding liabilities from accident years prior to 2014. The bottom section of the table is supplementary information about the average historical claims duration as of December 31, 2024. It shows the weighted average annual percentage payout of incurred loss and LAE by accident year as of each age. For example, the first column is calculated as the incremental paid loss and LAE in the first calendar year for each given accident year (e.g. calendar year 2024 for accident year 2024, calendar year 2023 for accident year 2023) divided by the cumulative incurred loss and LAE as of December 31, 2024 for that accident year. The resulting ratios are weighted using cumulative incurred loss and LAE as of December 31, 2024. Property and Accident & Health $ in Millions Incurred Loss and LAE, Net of Reinsurance For the Years Ended December 31, As of December 31, 2024 Accident Year 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Total IBNR plus expected development on reported claims Cumulative number of reported claims Unaudited 2015 $ 19.1 $ 18.1 $ 17.2 $ 16.2 $ 16.0 $ 16.0 $ 15.8 $ 15.7 $ 16.0 $ 16.1 $ .1 2,829 2016 22.2 17.5 18.2 18.4 18.3 18.5 18.5 18.5 18.5 .2 3,433 2017 31.1 37.7 $ 45.2 44.2 42.8 42.3 43.8 43.4 15.8 4,624 2018 40.7 $ 47.1 49.0 46.7 46.8 46.3 46.4 1.9 4,288 2019 33.9 31.2 27.0 23.8 23.2 22.8 .6 4,024 2020 76.9 75.1 74.5 77.6 79.5 9.9 4,646 2021 170.0 153.7 165.3 168.0 7.5 3,509 2022 241.5 266.9 277.3 12.9 4,044 2023 213.9 176.1 77.3 3,598 2024 359.5 202.7 3,614 Total $ 1,207.6 Property and Accident & Health Millions Cumulative Paid Loss and LAE, Net of Reinsurance For the Years Ended December 31, Accident Year 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Unaudited 2015 $ 6.9 $ 12.2 $ 13.4 $ 14.6 $ 14.5 $ 14.8 $ 15.0 $ 15.0 $ 15.4 $ 15.6 2016 8.5 13.0 16.3 16.7 16.8 17.1 17.7 17.9 18.1 2017 16.8 25.7 31.5 32.7 29.4 27.1 25.4 28.2 2018 15.6 32.2 40.1 40.0 40.8 42.8 43.6 2019 6.8 16.7 18.3 18.5 19.3 20.6 2020 11.2 33.9 46.9 55.6 66.5 2021 30.7 86.5 129.8 142.7 2022 69.4 191.9 229.4 2023 19.9 52.9 2024 54.8 Total 672.4 All outstanding liabilities before 2015, net of reinsurance 1.5 Loss and LAE reserves, net of reinsurance $ 536.7 Property and Accident & Health Average Annual Percentage Payout of Incurred Loss and LAE by Age, Net of Reinsurance Unaudited Years 1 2 3 4 5 6 7 8 9 10 23.8% 32.8% 18.3% 6.4% 2.8% 1.3% 0.5% 0.8% 0.1% 0.1% Marine & Energy $ in Millions Incurred Loss and LAE, Net of Reinsurance For the Years Ended December 31, As of December 31, 2024 Accident Year 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Total IBNR plus expected development on reported claims Cumulative number of reported claims Unaudited 2015 $ 21.7 $ 17.4 $ 16.1 $ 13.3 $ 12.7 $ 12.8 $ 12.7 $ 12.9 $ 12.7 $ 12.7 $ — 3,243 2016 23.4 19.5 15.6 14.6 14.3 14.8 14.1 13.6 13.3 — 3,772 2017 26.0 19.3 17.6 16.9 16.6 15.8 16.0 16.1 .4 4,139 2018 25.4 19.9 17.4 17.8 17.3 17.7 16.7 .2 3,238 2019 23.7 21.6 21.6 21.4 21.9 21.2 .5 2,393 2020 29.7 27.1 28.5 27.4 27.1 .8 1,582 2021 86.2 69.3 67.3 74.5 4.0 1,505 2022 149.7 153.6 156.0 31.9 1,968 2023 197.0 188.2 115.9 2,138 2024 239.9 182.6 1,485 Total $ 765.7 Marine & Energy Millions Cumulative Paid Loss and LAE, Net of Reinsurance For the Years Ended December 31, Accident Year 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Unaudited 2015 $ 4.0 $ 7.8 $ 9.6 $ 11.0 $ 10.4 $ 10.5 $ 10.9 $ 11.5 $ 11.6 $ 11.6 2016 5.5 10.0 12.6 13.0 13.1 13.7 13.4 13.4 13.4 2017 5.1 11.1 12.8 14.0 14.1 14.1 14.0 14.3 2018 2.6 12.4 13.9 14.6 15.3 15.3 15.4 2019 3.3 10.6 12.6 14.3 15.3 18.1 2020 3.1 12.7 16.0 18.5 21.9 2021 6.3 24.3 38.2 51.9 2022 12.2 66.2 97.7 2023 10.5 42.1 2024 20.9 Total 307.3 All outstanding liabilities before 2015, net of rein

Third-Party Reinsurance

Third-Party Reinsurance	12 Months Ended
	Dec. 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 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 years ended December 31, 2024, 2023 and 2022. Year Ended December 31, Millions 2024 2023 2022 Written premiums: Direct $ 1,101.5 $ 931.9 $ 760.4 Assumed 1,105.5 966.5 691.6 Gross written premiums 2,207.0 1,898.4 1,452.0 Ceded (527.9) (1) (377.5) (1) (256.8) Net written premiums $ 1,679.1 $ 1,520.9 $ 1,195.2 Earned premiums: Direct $ 1,004.2 $ 848.1 $ 655.5 Assumed 1,088.2 925.6 668.7 Gross earned premiums 2,092.4 1,773.7 1,324.2 Ceded (504.6) (2) (364.0) (2) (280.8) Net earned premiums $ 1,587.8 $ 1,409.7 $ 1,043.4 Loss and LAE: Gross $ 1,056.7 $ 782.5 $ 814.9 Ceded (200.9) (3) (55.7) (3) (278.5) Net loss and LAE $ 855.8 $ 726.8 $ 536.4 (1) The years ended December 31, 2024 and 2023 exclude $86.5 and $110.0 ceded by Ark to WM Outrigger Re, which eliminate in White Mountains’s consolidated financial statements. (2) The years ended December 31, 2024 and 2023 exclude $88.0 and $104.3 ceded by Ark to WM Outrigger Re, which eliminate in White Mountains’s consolidated financial statements. (3) The years ended December 31, 2024 and 2023 exclude $29.9 and $15.6 ceded by Ark to WM Outrigger Re, which eliminate in White Mountains’s consolidated financial statements. The following table presents the Ark/WM Outrigger segment’s reinsurance recoverables as of December 31, 2024 and December 31, 2023: Millions December 31, 2024 December 31, 2023 Reinsurance recoverables on unpaid losses (1) $ 434.4 $ 340.8 Reinsurance recoverables on paid losses (2) 57.5 27.4 Ceded unearned premiums (3) 97.1 73.8 Reinsurance recoverables $ 589.0 $ 442.0 (1) The reinsurance recoverables on unpaid losses exclude $31.8 and $15.6 ceded by Ark to WM Outrigger Re as of December 31, 2024 and 2023, which eliminate in White Mountains’s consolidated financial statements. (2) The reinsurance recoverables on paid losses exclude $3.1 ceded by Ark to WM Outrigger Re as of December 31, 2024, which eliminates in White Mountains’s consolidated financial statements. (3) The ceded unearned premiums exclude $4.3 and $5.7 ceded by Ark to WM Outrigger Re as of December 31, 2024 and 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 reinsurers’ A.M. Best Company, Inc (“A.M. Best”) ratings as of December 31, 2024: $ in Millions As of December 31, 2024 A.M. Best Rating (1) Gross Collateral Net % of Total A+ or better $ 310.3 $ — $ 310.3 74.3 % A - to A 98.5 — 98.5 23.6 B++ or lower and not rated 83.1 (2) 74.5 (2) 8.6 2.1 Total $ 491.9 $ 74.5 $ 417.4 100.0 % (1) A.M. Best financial strength ratings as detailed above are: “A+ or better” (Superior) “A- to A” (Excellent), “B++” (Good). (2) Excludes $34.9 ceded by Ark to WM Outrigger Re as of December 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, 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 December 31, 2024 and 2023, the carrying value of Ark’s deposit in SiriusPoint, including accrued interest, was $20.4 million. Effective in January 2025, the SiriusPoint contract was commuted.

Debt

Debt	12 Months Ended
	Dec. 31, 2024
Debt Disclosure [Abstract]
Debt	Debt The following table presents White Mountains’s debt outstanding as of December 31, 2024 and 2023: Effective Effective $ in Millions December 31, 2024 Rate (1) December 31, 2023 Rate (1) Ark 2007 Subordinated Notes, carrying value $ — $ 30.0 Ark 2021 Subordinated Notes Tranche 1 41.1 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 (3.6) (4.2) Ark 2021 Subordinated Notes, carrying value 154.5 155.5 Total Ark Subordinated Notes, carrying value 154.5 11.8% 185.5 11.0% HG Global Senior Notes (2) 150.0 150.0 Unamortized discount and issuance cost (2.6) (3.1) HG Global Senior Notes, carrying value 147.4 11.8% 146.9 11.9% Kudu Credit Facility (2) 245.3 210.3 Unamortized issuance cost (6.7) (6.5) Kudu Credit Facility, carrying value 238.6 10.3% 203.8 10.1% Other Operations debt 22.4 28.9 Unamortized issuance cost (.4) (.5) Other Operations debt, carrying value 22.0 10.2% 28.4 10.2% Total debt $ 562.5 $ 564.6 (1) Effective rate includes the effect of the amortization of debt issuance costs and the original issue discount. (2) Effective rate excludes the effect of the interest rate caps. See Note 9 — “Derivatives.” The following table presents a schedule of contractual repayments of White Mountains’s debt as of December 31, 2024: Millions December 31, 2024 Due in one year or less $ 6.7 Due in two to three years 27.0 Due in four to five years 30.0 Due after five years 512.1 Total $ 575.8 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 had a maturity date of June 2037, accrued interest at a floating rate equal to the three-month U.S. London Inter-Bank Offered Rate (“LIBOR”) plus 4.6% per annum. During 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 Secured Overnight Financing Rate (“SOFR”) plus a SOFR benchmark adjustment 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 SOFR benchmark adjustment 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 of GAIL to maintain 120% of its 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 December 31, 2024, there were no premium load events. As of December 31, 2024, the Ark 2021 Subordinated Notes Tranche 1 had an outstanding balance of €39.1 million ($41.1 million based upon the foreign exchange spot rate as of December 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 December 31, 2024, the Citibank LOC Facility had an outstanding balance of $101.7 million and cash and investments pledged as collateral of $141.1 million. As of December 31, 2024, the Lloyds LOC Facility had an outstanding balance of $42.7 million and cash and investments pledged as collateral of $67.1 million. Ark’s uncommitted secured standby letter of credit facility agreements contain various representations, warranties as well as affirmative and negative 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 SOFR plus a SOFR benchmark adjustment 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 “HG Global 2022 Interest Rate Cap”). Under the HG Global 2022 Interest Rate Cap, the notional amount is $150.0 million, the maximum interest rate is 9.76% per annum and the termination date is July 25, 2025. On August 22, 2024, HG Global entered into a new interest rate cap agreement, effective upon the termination of the prior interest rate cap (the “HG Global 2024 Interest Rate Cap”). Under the HG Global 2024 Interest Rate Cap, the initial notional amount is $150.0 million, the maximum interest rate is 10.76% per annum and the termination date is July 25, 2028. 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, subject to a maximum required balance of $29.3 million as of December 31, 2024. As of December 31, 2024 and 2023, the interest reserve account held short-term investments of $29.3 million and $30.4 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 December 31, 2024, the HG Global Senior Notes had an investment grade rating. As of December 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 was $300.0 million. On June 28, 2024, Kudu amended the Kudu Credit Facility, and the total commitment increased from $300.0 million to $350.0 million. The Kudu Credit Facility matures on March 23, 2036. Through June 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% per annum. Effective July 2023, the Kudu Credit Facility accrued interest at a floating interest rate equal to the three-month SOFR plus a SOFR benchmark adjustment of 0.26% and a stated margin of 4.30% per annum. Under the amended terms, effective July 2024, the Kudu Credit Facility will accrue interest at a floating interest rate equal to the three-month SOFR plus a stated margin of 4.45% per annum with no SOFR benchmark adjustment. On September 17, 2024, Kudu entered into an interest rate cap agreement, effective on September 30, 2024, to limit its exposure to the risk of interest rate increases on the Kudu Credit Facility (the “Kudu Interest Rate Cap”). Under the Kudu Interest Rate Cap, the notional amount is $150.0 million, the maximum interest rate is 8.95% per annum and the termination date is September 30, 2027. See Note 9 — “Derivatives.” The Kudu Credit Facility requires Kudu to maintain an interest reserve account of four times the interest accrued for the most recent quarterly interest period. As of December 31, 2024 and 2023, the interest reserve account held short-term investments of $15.1 million and $14.9 million. The Kudu Credit Facility requires Kudu to maintain a maximum ratio of the outstanding balance to the sum of the fair market value of Kudu’s other long-term investments and cash held in certain accounts (the “LTV Percentage”) for annual periods after the June 28, 2024 effective date of the amendment as follows: 50% in years 0-3, 40% in years 4-5, 25% in years 6-7, 15% in years 8-10 and 0% thereafter. As of December 31, 2024, Kudu had a 23.6% LTV Percentage. Kudu may borrow undrawn balances until June 28, 2027, 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 following table presents the change in debt under the Kudu Credit Facility for the years ended December 31, 2024, 2023 and 2022: Year Ended December 31, Millions 2024 2023 2022 Kudu Credit Facility Beginning balance $ 210.3 $ 215.2 $ 225.4 Term loans Borrowings 35.0 12.0 35.0 Repayments — (16.9) (45.2) Ending balance $ 245.3 $ 210.3 $ 215.2 The Kudu Credit Facility is secured by all property of the loan parties and contains various representations, warranties as well as affirmative and negative covenants that White Mountains considers to be customary for such borrowings. Other Operations Debt As of December 31, 2024, White Mountains’s Other Operations had debt with an outstanding balance of $22.4 million, which consisted of four secured credit facilities (collectively, “Other Operations debt”). Compliance As of December 31, 2024, White Mountains was in compliance, in all material respects, with all of the covenants under its debt instruments. Interest Total interest expense incurred by White Mountains for its indebtedness was $60.8 million, $62.7 million and $40.3 million for the years ended December 31, 2024, 2023 and 2022. Total interest paid by White Mountains for its indebtedness was $61.0 million, $42.2 million and $30.5 million for the years ended December 31, 2024, 2023 and 2022.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2024
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes 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 December 31, 2024, the primary jurisdictions in which the Company’s subsidiaries and branches operated and were subject to tax include Israel, Luxembourg, the United Kingdom and the United States. The Company and its Bermuda-domiciled subsidiaries were not subject to income tax in Bermuda in 2024 and prior years. On December 27, 2023, Bermuda enacted a 15% corporate income tax that became 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, 2024. 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. As of July 1, 2024, White Mountains no longer consolidates BAM. As a result of the deconsolidation, the BAM Surplus Notes are recorded at fair value, which resulted in the reversal of a $5.0 million deferred tax liability related to the economic transition adjustment, generating a $5.0 million tax benefit in the third quarter of 2024. For the year ended December 31, 2024, certain of the Company’s subsidiaries were subject to the global minimum tax regime of the Organization for Economic Cooperation and Development (“OECD”) Pillar Two initiative, as enacted by Luxembourg and the United Kingdom in their respective domestic laws. The Pillar Two initiative includes a set of model rules that are generally designed to impose a top-up tax on a large multinational enterprise group to the extent the group is not subject to an effective tax rate of at least 15% in each jurisdiction in which the group has a consolidated affiliate or permanent establishment. The Company and its subsidiaries did not incur a top-up tax for the year ended December 31, 2024. The following table presents the total income tax (expense) benefit for the years ended December 31, 2024, 2023 and 2022: Year Ended December 31, Millions 2024 2023 2022 Current income tax (expense) benefit: U.S. federal $ (6.3) $ (12.6) $ (16.9) State (.4) (2.2) (4.5) Non-U.S. (14.8) (16.6) (7.1) Total current income tax (expense) benefit (21.5) (31.4) (28.5) Deferred income tax (expense) benefit: U.S. federal (18.8) (13.2) (7.3) State 2.7 (3.8) (1.8) Non-U.S. 5.0 63.9 (3.8) Total deferred income tax (expense) benefit (11.1) 46.9 (12.9) Total income tax (expense) benefit $ (32.6) $ 15.5 $ (41.4) Effective Rate Reconciliation The following table presents a reconciliation of taxes calculated for 2024, 2023 and 2022 using the 21% U.S. federal statutory rate (the tax rate at which the majority of White Mountains’s worldwide operations are taxed) to the income tax (expense) benefit on pre-tax income (loss): Year Ended December 31, Millions 2024 2023 2022 Tax (expense) benefit at the U.S. statutory rate $ (66.5) $ (118.7) $ 31.4 Differences in taxes resulting from: Non-U.S. earnings, net of foreign taxes 40.2 103.7 (41.2) Change in valuation allowance (8.1) (30.2) (19.6) Bermuda corporate income tax 5.0 68.0 — Noncontrolling interest 3.9 6.8 3.4 Tax rate changes 2.9 (.6) (.4) BAM member surplus contributions prior to deconsolidation (2.8) (5.7) (6.2) Withholding tax (2.5) (4.1) (3.1) State taxes (1.8) (8.2) (2.8) Tax exempt interest and dividends .3 .2 .2 Officer compensation — (.2) (1.0) Other, net (3.2) 4.5 (2.1) Total income tax (expense) benefit on pre-tax income (loss) $ (32.6) $ 15.5 $ (41.4) The non-U.S. component of pre-tax income (loss) was $282.7 million, $450.0 million and $(94.4) million for the years ended December 31, 2024, 2023 and 2022. Tax Payments and Receipts Net income tax (refunds) payments totaled $34.3 million, $42.7 million and $10.3 million for the years ended December 31, 2024, 2023 and 2022. Deferred Tax Assets and Liabilities Deferred taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts for tax purposes. The following table presents an outline of the significant components of White Mountains’s U.S. federal, state and non-U.S. deferred tax assets and liabilities: December 31, Millions 2024 2023 Deferred tax assets related to: Non-U.S. net operating loss carryforwards $ 62.8 $ 68.2 Intangible assets 51.5 51.5 Incentive compensation 23.8 25.4 Unearned premiums 20.5 18.2 Loss reserves 8.5 8.4 U.S. federal and state net operating and capital 6.4 87.6 Tax credit carryforwards 2.8 3.3 Accrued interest .2 10.1 Deferred acquisition costs — 3.7 Other items .9 1.0 Total gross deferred tax assets 177.4 277.4 Less: valuation allowances 89.2 116.1 Total net deferred tax assets 88.2 161.3 Deferred tax liabilities related to: Purchase accounting 43.9 43.9 Investment basis difference 43.5 40.9 Deferred underwriting 24.3 18.3 Net unrealized investment gains 19.0 15.5 Deferred acquisition costs 6.5 — BAM member surplus contributions prior to deconsolidation — 82.2 Other items 1.2 1.2 Total deferred tax liabilities 138.4 202.0 Net deferred tax asset (liability) $ (50.2) $ (40.7) White Mountains’s deferred tax assets (liabilities) are net of U.S. federal, state and non-U.S. valuation allowances and, to the extent they relate to non-U.S. jurisdictions, are shown at year-end exchange rates. Valuation Allowance 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. It is possible that certain planning strategies or projected earnings in certain subsidiaries may not be sufficient to utilize the entire deferred tax asset, which could result in material changes to White Mountains’s deferred tax assets and tax expense. Of the $89.2 million valuation allowance as of December 31, 2024, $43.4 million related to deferred tax assets on net operating losses and net unrealized investment gains and losses in the Company’s Luxembourg subsidiary, $25.2 million related to deferred tax assets on net operating losses in U.S. subsidiaries and other federal and state deferred tax benefits, $20.6 million related to net operating losses and other deferred tax benefits in Israeli subsidiaries. Of the $116.1 million valuation allowance as of December 31, 2023, $47.6 million related to deferred tax assets on net operating losses and net unrealized investment gains and losses in the Luxembourg subsidiary, $46.9 million related to deferred tax assets on net operating losses in U.S. subsidiaries and other federal and state deferred tax benefits, $20.8 million related to net operating losses and other deferred tax benefits in Israeli subsidiaries and $0.8 million related to net operating losses in U.K. subsidiaries. United States For the year ended December 31, 2024, White Mountains recorded income tax expense of $8.6 million to reflect an increase in the valuation allowance on the net deferred tax assets for certain U.S. operations within Other Operations, as White Mountains does not currently anticipate sufficient taxable income to utilize the remaining deferred tax assets. For the year ended December 31, 2023, White Mountains recorded income tax benefit of $7.2 million to reflect a decrease in the valuation allowance on the net deferred tax assets for certain U.S. operations within Other Operations. The decrease is due to realized gains during the year which reduced the Company’s net operating loss carryforward and related deferred tax asset. As the deferred tax asset declined so did the corresponding valuation allowance. As of December 31, 2023, White Mountains did not anticipate sufficient taxable income to utilize the remaining deferred tax assets. As of July 1, 2024, White Mountains no longer consolidates BAM. Prior to the deconsolidation, White Mountains recorded income tax expense of $5.7 million and $6.3 million to reflect the increase in the valuation allowance on net deferred tax assets of BAM for the years ended December 31, 2024 and 2023. White Mountains records both the tax expense related to BAM’s member surplus contributions (“MSC”) and the related changes in valuation allowance on such taxes directly through noncontrolling interest equity. For the years ended December 31, 2024 and 2023, BAM had income included in equity due to MSC that was available to offset its loss from continuing operations. For the years ended December 31, 2024 and 2023, BAM recorded both the income tax benefit on MSC of $3.1 million and $8.7 million and the offsetting expense in paid-in surplus. As of December 31, 2023, BAM had a full valuation allowance recorded against its net deferred tax assets, as White Mountains was unsure it will generate sufficient taxable income to utilize the deferred tax assets. Non-U.S. Jurisdictions For the year ended December 31, 2024, White Mountains recognized income tax benefit of $4.2 million to reflect a decrease in deferred tax assets and the corresponding decrease in the full valuation allowance at the Luxembourg-domiciled subsidiary. The decrease in the deferred tax assets was driven primarily by (i) a partial utilization from income on intercompany notes and (ii) a decline in the Luxembourg tax rate. White Mountains does not currently anticipate sufficient income to utilize the remaining deferred tax assets. For the year ended December 31, 2023, White Mountains recognized income tax expense of $31.6 million to reflect an increase in the full valuation allowance against deferred tax assets which primarily related to losses on the write-down of foreign subsidiaries and investments held in the Luxembourg-domiciled subsidiary. As of December 31, 2023, White Mountains did not anticipate sufficient income to utilize the deferred tax assets. For the year ended December 31, 2024, White Mountains recognized income tax benefit of $0.2 million to reflect a decrease in the valuation allowance against the deferred tax assets at certain Israel-domiciled subsidiaries. The decrease is due to movement of foreign currency exchange rate gains during the year, which reduced the Company’s net operating loss carryforward and related deferred tax asset. White Mountains does not currently anticipate sufficient taxable income to utilize the deferred tax assets. For the year ended December 31, 2023, White Mountains recorded income tax expense of $0.4 million to reflect an increase in the valuation allowance against the deferred tax assets at certain Israel-domiciled subsidiaries, as White Mountains did not anticipate sufficient taxable income to utilize the deferred tax assets. Net Operating Loss and Capital Loss Carryforwards The following table presents net operating loss and capital loss carryforwards as of December 31, 2024, the expiration dates and the deferred tax assets thereon: December 31, 2024 Millions United States Luxembourg United Kingdom Israel Total 2025-2029 $ .4 $ — $ — $ — $ .4 2030-2034 .3 — — — .3 2035-2044 8.8 177.1 — — 185.9 No expiration date 3.5 — — 88.1 91.6 Total $ 13.0 $ 177.1 $ — $ 88.1 $ 278.2 Gross deferred tax asset $ 6.4 $ 42.3 $ — $ 20.3 $ 69.0 Valuation allowance (6.4) (42.3) — (20.3) (69.0) Net deferred tax asset $ — $ — $ — $ — $ — Included in the U.S. net operating loss carryforwards are losses of $7.8 million subject to an annual limitation on utilization under Internal Revenue Code Section 382 and $0.7 million subject to the limitation on utilization under the separate-return-limitation-year (SRLY) rules in the Internal Revenue Code. These loss carryforwards will begin to expire in 2026. As of December 31, 2024, there are U.K. foreign tax credit carryforwards available of $2.8 million, which do not have an expiration date. Uncertain Tax Positions Recognition of the benefit of a given tax position is based upon whether a company determines that it is more likely than not that a tax position will be sustained upon examination based upon the technical merits of the position. In evaluating the more likely than not recognition threshold, White Mountains must presume that the tax position will be subject to examination by a taxing authority with full knowledge of all relevant information. If the recognition threshold is met, then the tax position is measured at the largest amount of benefit that is more than 50% likely of being realized upon ultimate settlement. As of December 31, 2024 and 2023, White Mountains did not have any unrecognized tax benefits. White Mountains classifies all interest and penalties on unrecognized tax benefits as part of income tax expense. During the years ended December 31, 2024, 2023 and 2022, White Mountains did not recognize any net interest (income) expense. There was no accrued interest as of December 31, 2024, 2023 and 2022. Tax Examinations 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 2019.

Derivatives

Derivatives	12 Months Ended
	Dec. 31, 2024
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives	Derivatives HG Global Interest Rate Caps HG Global entered into two interest rate cap agreements to limit its exposure to the risk of interest rate increases on the HG Global Senior Notes. On June 16, 2022, HG Global entered into the HG Global 2022 Interest Rate Cap, effective on July 25, 2022. The notional amount of the HG Global 2022 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 HG Global 2022 Interest Rate Cap. On August 22, 2024, HG Global entered into the HG Global 2024 Interest Rate Cap, which is effective upon the termination of the HG Global 2022 Interest Rate Cap on July 25, 2025. The initial notional amount of the HG Global 2024 Interest Rate Cap is $150.0 million, and the termination date is July 25, 2028. For interest periods after April 26, 2027, the notional amount of the HG Global 2024 Interest Rate Cap will decrease as the outstanding principal of the HG Global Senior Notes is paid down. HG Global paid initial premiums of $1.3 million for the HG Global 2024 Interest Rate Cap. Under the interest rate caps, if the three-month SOFR on a quarterly determination date exceeds 3.5% through July 25, 2025 or 4.5% between July 25, 2025 and July 25, 2028, HG Global will receive a payment from the counterparty for the difference on the subsequent settlement date. As of December 31, 2024, the three-month SOFR was 4.3%. HG Global accounts for the interest rate caps as derivatives at fair value within other assets, with changes in fair value recognized in current period earnings within interest expense. For the year ended December 31, 2024, White Mountains recognized a net unrealized loss of $1.7 million related to the change in fair value on both interest rate caps within interest expense. For the year ended December 31, 2023, White Mountains recognized a net unrealized loss of $1.2 million related to the change in fair value on the HG Global 2022 Interest Rate Cap within interest expense. For the years ended December 31, 2024 and 2023, White Mountains received $2.8 million and $2.0 million related to the periodic settlements of the HG Global 2022 Interest Rate Cap. As of December 31, 2024, the total fair value of both interest rate caps was $2.5 million. As of December 31, 2023, the fair value of the HG Global 2022 Interest Rate Cap was $2.9 million. White Mountains classifies the interest rate caps as Level 2 measurements. Kudu Interest Rate Cap On September 17, 2024, Kudu entered into the Kudu Interest Rate Cap, effective on September 30, 2024, to limit its exposure to the risk of interest rate increases on the Kudu Credit Facility. The notional amount of the Kudu Interest Rate Cap is $150.0 million, and the termination date is September 30, 2027. Kudu paid initial premiums of $0.9 million for the Kudu Interest Rate Cap. Under the Kudu Interest Rate Cap, if the three-month SOFR on a quarterly determination date exceeds 4.5%, Kudu will receive a payment from the counterparty for the difference on the subsequent settlement date. As of December 31, 2024, the three-month SOFR was 4.3%.

Municipal Bond Guarantee Reinsu

Municipal Bond Guarantee Reinsurance	12 Months Ended
	Dec. 31, 2024
Insurance [Abstract]
Municipal Bond Guarantee Reinsurance	Municipal Bond Guarantee Reinsurance 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 HG Re is a party to the FLRT with BAM, under which HG Re provides first-loss reinsurance protection of up to 15%-of-par outstanding for each policy assumed from BAM. For capital appreciation bonds, par is adjusted to the estimated equivalent par value for current interest paying bonds. HG Re is required to provide reinsurance on policies that fall within the FLRT underwriting guidelines agreed upon by HG Re. BAM charges an insurance premium on each municipal bond insurance policy it underwrites. Historically, approximately 55% of the total insurance premium charged by BAM has been a MSC and the remainder is a risk premium. In return for the reinsurance provided, HG Re receives approximately 60% of the risk premium charged, which is net of a ceding commission. The FLRT is a perpetual agreement with terms that can be renegotiated every five years. For the next renegotiation period, either party may provide notice during 2028 to trigger a renegotiation that would take effect on January 1, 2030. Prior to the deconsolidation of BAM on July 1, 2024, HG Re’s reinsurance balances under the FLRT eliminated in White Mountains’s consolidated financial statements. Subsequent to the deconsolidation, White Mountains recognized gross written premiums of $31.9 million and earned premiums of $14.9 million for the period from July 1, 2024 to December 31, 2024. XOLT HG Re is party to the XOLT with BAM, under which 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 December 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 XOLT is accounted for using deposit accounting as the agreement does not meet the risk transfer requirements necessary to be accounted for as reinsurance. Accordingly, any financing revenues related to the XOLT are recorded in other revenues. Prior to the deconsolidation of BAM on July 1, 2024, HG Re’s reinsurance balances under the XOLT eliminated in White Mountains’s consolidated financial statements. Subsequent to the deconsolidation, other revenues recognized by White Mountains for period from July 1, 2024 to December 31, 2024 related to the XOLT were insignificant. 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 December 31, 2024 and 2023 was $352.1 million and $341.6 million, which consisted of cash, investments and accrued investment income. 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. For the year ended December 31, 2024, HG Re received a distribution out of the Supplemental Trust of $79.7 million, which was comprised of the assignment of $58.5 million of accrued interest on the BAM Surplus Notes and a cash distribution of $21.2 million. For the year ended December 31, 2023, HG Re did not receive any distributions out of the Supplemental Trust. 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 December 31, 2024 and 2023 was $598.0 million and $606.9 million, which included $289.4 million and $247.0 million of cash, investments and accrued investment income, $300.9 million and $322.2 million of BAM Surplus Notes at nominal value and $7.7 million and $37.7 million of accrued interest receivable on the BAM Surplus Notes at nominal value. As of December 31, 2024 and 2023, the Collateral Trusts held total assets of $950.1 million and $948.5 million. BAM Surplus Notes Through June 30, 2024, the interest rate on the BAM Surplus Notes was a variable rate equal to the one-year U.S. Treasury rate plus 300 basis points, set annually, with each payment applied pro rata between outstanding principal and interest. Accordingly, in 2024 the interest rate on the BAM Surplus Notes was 8.2% through June 30, 2024. Effective July 1, 2024 and through maturity, HG Global and BAM amended the interest rate on the BAM Surplus Notes to be 10.0%, with a higher proportion of each payment to be applied to outstanding principal. 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. During 2024, HG Global received cash payments of principal and interest on the BAM Surplus Notes totaling $30.0 million. Of these payments, $21.3 million was a repayment of principal held in the Supplemental Trust, $0.5 million was a payment of accrued interest held in the Supplemental Trust and $8.2 million was a payment of accrued interest held outside the Supplemental Trust. During 2023, HG Global received a cash payment of principal and interest on the BAM Surplus Notes of $27.4 million. 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. As of December 31, 2024 and 2023, the principal balance on the BAM Surplus Notes was $300.9 million and $322.2 million and total interest receivable on the BAM Surplus Notes was $194.8 million and $174.5 million, all at nominal value. Prior to the deconsolidation of BAM on July 1, 2024, the BAM Surplus Notes, including accrued interest receivable, were classified as intercompany notes carried at nominal value, which eliminated in consolidation. Upon deconsolidation, White Mountains elected the fair value option for the BAM Surplus Notes. As of July 1, 2024 and December 31, 2024, the fair value of the BAM Surplus Notes was $387.4 million and $381.7 million. The difference between the nominal value of the BAM Surplus Notes and the fair value as of July 1, 2024 was recorded as an unrealized loss on deconsolidation in 2024. This loss includes the impact of a discount for the time value of money, which was previously included in adjusted book value per share as a non-GAAP adjustment to book value per share. Subsequent to the deconsolidation, White Mountains values the BAM Surplus Notes each quarter using a discounted cash flow analysis. The BAM Surplus Notes are classified as a Level 3 measurement. The discounted cash flow analysis used to value the BAM Surplus Notes depends on key inputs, such as projections of future revenues and earnings for BAM, expected payments on the BAM Surplus Notes through maturity and a discount rate to reflect time value and related uncertainty of the repayment pattern. The expected payments on the BAM Surplus Notes are based on management judgment, considering current performance, budgets and projected future results. These expected payments depend on BAM’s ability to generate excess cash flows from its operations, driven primarily by assumptions regarding future trends for the issuance of municipal bonds, interest rates, credit spreads, insured market penetration, competitive activity in the market for municipal bond insurance and other factors affecting the demand for and pricing of BAM’s municipal bond insurance as well as BAM’s investment returns. The discount rate considers comparably-rated companies and instruments, adjusted for risks specific to BAM and the BAM Surplus Notes. As of July 1, 2024 and December 31, 2024, White Mountains concluded that a discount rate, which is a significant unobservable input used in estimating the fair value of the BAM Surplus Notes, of 8.15% and 8.10% was appropriate. The change in the discount rate in the period from July 1, 2024 to December 31, 2024 was driven by a decline in market interest rates. Subsequent to the deconsolidation of BAM, White Mountains recognized an unrealized gain of $0.5 million related to the change in fair value of the BAM Surplus Notes for the period from July 1, 2024 to December 31, 2024. Insured Obligations and Premiums The following table presents the HG Global segment’s insured obligations as of December 31, 2024 and 2023: December 31, 2024 December 31, 2023 Contracts outstanding 15,884 14,485 Remaining weighted average contract period (in years) (1) 11.2 11.1 Outstanding par value of policies assumed (in millions) (2) $ 18,503.3 $ 16,575.2 (3) Gross unearned insurance premiums (in millions) $ 297.3 $ 273.9 (4) (1) The remaining weighted average contract period was calculated using total contractual debt service outstanding, including principal and interest. (2) Under the FLRT, HG Re provides first-loss reinsurance protection of up to 15% -of-par outstanding for each policy assumed from BAM. For capital appreciation bonds, par is adjusted to the estimated equivalent par value for current interest paying bonds. (3) As of December 31, 2023, which was prior to BAM’s deconsolidation, BAM’s total contractual debt service outstanding was $165,686.0, split between principal of $109,673.8 and interest of $56,012.2. (4) As of December 31, 2023, which was prior to BAM’s deconsolidation, BAM’s gross unearned insurance premiums were $325.8. The following table presents the HG Global segment’s future premium revenues as of December 31, 2024: Millions December 31, 2024 January 1, 2025 - March 31, 2025 $ 6.8 April 1, 2025 - June 30, 2025 6.6 July 1, 2025 - September 30, 2025 6.5 October 1, 2025 - December 31, 2025 6.4 Total 2025 26.3 2026 24.7 2027 23.3 2028 21.8 2029 20.2 2030 and thereafter 181.0 Total gross unearned insurance premiums $ 297.3 The following tables present gross written premiums and gross earned premiums included in the HG Global segment for the years ended December 31, 2024, 2023 and 2022: Year Ended December 31, 2024 Millions HG Global BAM Eliminations Total Written premiums: Direct $ — $ 24.1 $ — $ 24.1 Assumed 52.4 — (20.5) 31.9 Gross written premiums (1) $ 52.4 $ 24.1 $ (20.5) $ 56.0 Earned premiums: Direct $ — $ 15.8 $ — $ 15.8 Assumed 28.9 1.0 (14.0) 15.9 Gross earned premiums (1) $ 28.9 $ 16.8 $ (14.0) $ 31.7 (1) Subsequent to the deconsolidation of BAM, there are no ceded premiums, such that gross written premiums and gross earned premiums are equivalent to net written premiums and net earned premiums, respectively. For the six months ended June 30, 2024, BAM ceded written premiums of $20.5 and ceded earned premiums of $14.0 to HG Global, which eliminated in consolidation. Year Ended December 31, 2023 Millions HG Global BAM Eliminations Total Written premiums: Direct $ — $ 58.6 $ — $ 58.6 Assumed 50.1 — (50.1) — Gross written premiums (1) $ 50.1 $ 58.6 $ (50.1) $ 58.6 Earned premiums: Direct $ — $ 28.7 $ — $ 28.7 Assumed 26.0 2.5 (26.0) 2.5 Gross earned premiums (1) $ 26.0 $ 31.2 $ (26.0) $ 31.2 (1) For the year ended December 31, 2023, BAM ceded written premiums of $50.1 and ceded earned premiums of $26.0 to HG Global, which eliminated in consolidation. Year Ended December 31, 2022 Millions HG Global BAM Eliminations Total Written premiums: Direct $ — $ 63.8 $ — $ 63.8 Assumed 55.9 1.3 (55.9) 1.3 Gross written premiums (1) $ 55.9 $ 65.1 $ (55.9) $ 65.1 Earned premiums: Direct $ — $ 28.6 $ — $ 28.6 Assumed 27.5 4.7 (27.5) 4.7 Gross earned premiums (1) $ 27.5 $ 33.3 $ (27.5) $ 33.3 (1) For the year ended December 31, 2022, BAM ceded written premiums of $55.9 and ceded earned premiums of $27.5 to HG Global, which eliminated in consolidation.

Earnings Per Share

Earnings Per Share	12 Months Ended
	Dec. 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 years ended December 31, 2024, 2023 and 2022. See Note 20 — “Held for Sale and Discontinued Operations.” Year Ended December 31, 2024 2023 2022 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s $ 230.4 $ 509.2 $ 792.8 Less: total income (loss) from discontinued operations, net of tax (1) — — 903.2 Less: net (income) loss from discontinued operations attributable — — (.7) Net income (loss) from continuing operations attributable to 230.4 509.2 (109.7) Allocation of (earnings) losses to participating restricted common shares (2) (3.0) (7.2) 1.3 Basic and diluted earnings (losses) per share numerators $ 227.4 $ 502.0 $ (108.4) Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 2,565.7 2,563.3 2,862.4 Average unvested restricted common shares (3) (33.5) (36.0) (36.2) Basic earnings (losses) per share denominator 2,532.2 2,527.3 2,826.2 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 2,565.7 2,563.3 2,862.4 Average unvested restricted common shares (3) (33.5) (36.0) (36.2) Diluted earnings (losses) per share denominator 2,532.2 2,527.3 2,826.2 Basic and diluted earnings per share (in dollars) - continuing operations: Distributed earnings - dividends declared and paid $ 1.00 $ 1.00 $ 1.00 Undistributed earnings (losses) 88.79 197.60 (39.34) Basic and diluted earnings (losses) per share $ 89.79 $ 198.60 $ (38.34) (1) Includes net income (loss) from discontinued operations, net of tax - NSM Group and net gain (loss) from sale of discontinued operations, net of tax - NSM Group. See Note 20 — “Held for Sale and Discontinued Operations.” (2) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (3) Restricted shares outstanding vest upon a stated date. See Note 12 — “Employee Share-Based Incentive Compensation Plans.” The following table presents the undistributed net earnings (losses) from continuing operations for the years ended December 31, 2024, 2023 and 2022. See Note 20 — “Held for Sale and Discontinued Operations.” Year Ended December 31, Millions 2024 2023 2022 Undistributed net earnings - continuing operations: Net income (loss) attributable to White Mountains’s common shareholders, $ 227.4 $ 502.0 $ (108.4) Dividends declared, net of restricted common share amounts (1) (2.5) (2.5) (3.0) Total undistributed net earnings (losses), net of restricted common share amounts $ 224.9 $ 499.5 $ (111.4) (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	12 Months Ended
	Dec. 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 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. 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 December 31, 2024 and 2023, White Mountains’ 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 measure used for determining performance share payouts is the growth in compensation value per share (“CVPS”), which is the average of the growth in adjusted book value per share and the growth in intrinsic value per share. Intrinsic value per share is calculated by adjusting White Mountains’s 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 value. The following table presents performance share activity for the years ended December 31, 2024, 2023 and 2022 for performance shares granted under the WTM Incentive Plan: Year Ended December 31, 2024 2023 2022 $ in Millions Target Accrued Target Accrued Target Accrued Beginning of period 37,031 $ 69.4 39,449 $ 67.5 40,828 $ 42.2 Shares paid or expired (1) (13,475) (44.9) (13,350) (40.8) (14,625) (26.4) New grants 11,405 — 10,895 — 13,225 — Forfeitures and cancellations (2) (102) — 37 (.1) 21 (.4) Expense recognized — 46.6 — 42.8 — 52.1 End of period 34,859 $ 71.1 37,031 $ 69.4 39,449 $ 67.5 (1) WTM performance share payments in 2024 for the 2021-2023 performance cycle, which were paid in March 2024 at 188% of target. WTM performance share payments in 2023 for the 2020-2022 performance cycle, which were paid in March 2023 at 200% of target. WTM performance share payments in 2022 for the 2019-2021 performance cycle, which were paid in March 2022 at 172% of target. (2) Amounts include changes in assumed forfeitures, as required under GAAP. During 2024, White Mountains granted 11,405 performance shares for the 2024-2026 performance cycle. During 2023, White Mountains granted 10,895 performance shares for the 2023-2025 performance cycle. During 2022, White Mountains granted 13,225 performance shares for the 2022-2024 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. For the 2019-2021 performance cycle, the Company issued common shares for 750 performance shares earned, and all other performance shares earned were settled in cash. If all outstanding performance shares had vested on December 31, 2024, the total additional compensation cost to be recognized would have been $23.1 million, based on accrual factors as of December 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 December 31, 2024 for each performance cycle: December 31, 2024 $ in Millions Target Performance Accrued Expense Performance cycle: 2024 – 2026 11,405 $ 6.5 2023 – 2025 10,835 14.4 2022 – 2024 13,150 51.3 Sub-total 35,390 72.2 Assumed forfeitures (531) (1.1) Total 34,859 $ 71.1 For the 2024-2026 performance cycle, the targeted performance goal for full payment of outstanding performance shares granted under the WTM Incentive Plan is 11% average annual growth in CVPS. Average annual growth of 6% or less would result in no payout, and average annual growth of 16% or more would result in a payout of 200%. For the 2023-2025 performance cycle, the targeted performance goal for full payment of outstanding performance shares granted under the WTM Incentive Plan is 11% average annual growth in CVPS. Average annual growth of 6% or less would result in no payout, and average annual growth of 16% or more would result in a payout of 200%. For the 2022-2024 performance cycle, the targeted performance goal for full payment of outstanding performance shares granted under the WTM Incentive Plan is 9% average annual growth in CVPS. Average annual growth of 4% or less would result in no payout, and average annual growth of 14% or more would result in a payout of 200%. 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 years ended December 31, 2024, 2023 and 2022: Year Ended December 31, 2024 2023 2022 $ in Millions Restricted Unamortized Restricted Unamortized Restricted Unamortized Non-vested: Beginning of period 37,595 $ 16.2 38,350 $ 15.5 37,850 $ 15.9 Issued 11,405 20.0 10,895 16.0 13,225 13.8 Vested (13,475) — (11,650) — (12,725) — Forfeited (135) — — — — — Expense recognized — (16.3) — (15.3) — (14.2) End of period 35,390 $ 19.9 37,595 $ 16.2 38,350 $ 15.5

Common Shareholders_ Equity and

Common Shareholders’ Equity and Noncontrolling Interests	12 Months Ended
	Dec. 31, 2024
Equity [Abstract]
Common Shareholders’ Equity and Noncontrolling Interests	Common Shareholders’ Equity and Noncontrolling Interests Common Shares Repurchased and Retired The Company’s Board of Directors has authorized it to repurchase its common shares from time to time, subject to market conditions. Shares may be repurchased on the open market or through privately negotiated transactions. The repurchase authorizations do not have a stated expiration date. As of December 31, 2024, White Mountains may repurchase an additional 301,014 shares under these board authorizations. In addition, from time to time White Mountains has also repurchased its common shares through self-tender offers that were separately authorized by its Board of Directors. During 2024, the Company repurchased 5,269 common shares for $7.9 million at an average share price of $1,505.01. The common shares were repurchased to satisfy employee income tax withholding pursuant to employee benefit plans. During 2023, the Company repurchased 24,165 common shares for $32.7 million at an average share price of $1,354.88, which included 19,536 common shares repurchased under the board authorizations for $26.2 million at an average share price of $1,340.79 and 4,629 common shares repurchased to satisfy employee income tax withholding pursuant to employee benefit plans. During 2022, the Company repurchased 461,256 common shares for $615.8 million at an average share price of $1,335.11, which included 129,450 common shares repurchased under the board authorizations for $150.9 million at an average share price of $1,165.84 and 4,011 common shares repurchased to satisfy employee income tax withholding pursuant to employee benefit plans. In addition, on September 26, 2022, the Company completed a self-tender offer, through which it repurchased 327,795 common shares at a purchase price of $1,400.00 per share for a total cost of approximately $460.8 million, including expenses. Common Shares Issued During 2024, the Company issued a total of 13,100 common shares, which consisted of 11,405 restricted shares issued to key personnel and 1,695 shares issued to directors of the Company. During 2023, the Company issued a total of 12,461 common shares, which consisted of 10,895 restricted shares issued to key personnel and 1,566 shares issued to directors of the Company. During 2022, the Company issued a total of 15,640 common shares, which consisted of 13,225 restricted shares issued to key personnel and 2,415 shares issued to directors of the Company. Dividends on Common Shares For the years ended December 31, 2024, 2023 and 2022, the Company declared and paid cash dividends totaling $2.5 million, $2.6 million and $3.0 million (or $1.00 per common share). Noncontrolling Interests Noncontrolling interests consist of the ownership interests of noncontrolling shareholders in consolidated entities and are presented separately on the balance sheet. Effective July 1, 2024, White Mountains no longer consolidates BAM. Through June 30, 2024, BAM’s results are attributed to noncontrolling interests in White Mountains’s consolidated financial statements. See Note 2 — “Significant Transactions.” 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 December 31, 2024 and 2023: December 31, 2024 December 31, 2023 $ in Millions Noncontrolling Percentage (1) Noncontrolling Equity Noncontrolling Percentage (1) Noncontrolling Equity Noncontrolling interests: Ark 27.9 % $ 410.4 (2) 28.0 % $ 336.9 (2) HG Global 3.1 % (13.4) 3.1 % .8 Kudu 9.6 % 127.6 (3) 10.5 % 113.8 (3) Bamboo 27.2 % 113.6 — — Other various 9.1 various 9.4 Total, excluding BAM 647.3 460.9 BAM (4) 100.0 % — 100.0 % (139.8) Total noncontrolling interests $ 647.3 $ 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 December 31, 2024 and 2023, Ark’s noncontrolling equity includes $42.9 and $21.0 related to management’s equity incentives. (3) As of December 31, 2024 and 2023, Kudu’s noncontrolling equity includes $47.9 and $38.7 related to management’s equity incentives. (4) Effective July 1, 2024, White Mountains no longer consolidates BAM. Through June 30, 2024, BAM’s results are attributed to noncontrolling interests in White Mountains’s consolidated financial statements.

Statutory Capital and Surplus

Statutory Capital and Surplus	12 Months Ended
	Dec. 31, 2024
Insurance [Abstract]
Statutory Capital and Surplus	Loss and Loss Adjustment Expense Reserves Loss and LAE reserves are typically comprised of case reserves for claims reported and reserves for losses that have occurred but for which claims have not yet been reported, referred to as IBNR reserves. IBNR reserves include a provision for expected future development on case reserves. Case reserves are estimated based on the experience and knowledge of claims staff regarding the nature and potential cost of each claim and are adjusted as additional information becomes known or payments are made. IBNR reserves are derived by subtracting paid loss and LAE and case reserves from estimates of ultimate loss and LAE. Actuaries estimate ultimate loss and LAE using various generally accepted actuarial methods applied to known losses and other relevant information. Like case reserves, IBNR reserves are adjusted as additional information becomes known or payments are made. Loss and LAE reserves are recognized within White Mountains’s P&C Insurance and Reinsurance, Financial Guarantee, P&C Insurance Distribution and certain Other Operations businesses. P&C Insurance and Reinsurance Ark establishes loss and LAE reserves that are estimates of amounts needed to pay claims and related expenses in the future for insured events that have already occurred. Ark’s process of estimating reserves involves a considerable degree of judgment by management and, as of any given date, is inherently uncertain. WM Outrigger Re entered into a quota share agreement with GAIL to provide collateralized reinsurance protection on Ark’s Bermuda global property catastrophe excess of loss portfolio written in the 2023 underwriting year. Ark renewed its quota share reinsurance agreement with WM Outrigger Re for the 2024 and 2025 underwriting years. WM Outrigger Re’s quota share reinsurance agreement with GAIL, including its loss and LAE reserves, eliminates in White Mountains’s consolidated financial statements. Loss and LAE are categorized by the year in which the policy is underwritten (the year of account, or underwriting year) for purposes of Ark’s claims management and estimation of the ultimate loss and LAE reserves. For purposes of Ark’s reporting under GAAP, loss and LAE are categorized by the year in which the claim is incurred (the accident year). Ultimate loss and LAE are generally determined by extrapolation of claims emergence and settlement patterns observed in the past that can reasonably be expected to persist into the future. In forecasting ultimate loss and LAE for any line of business, past experience with respect to that line of business is the primary resource, but cannot be relied upon in isolation. Ark’s own experience, particularly claims development experience, such as trends in case reserves, payments on and closings of claims, as well as changes in business mix and coverage limits, is the most important information for estimating its loss and LAE reserves. External data, available from organizations such as the Lloyd’s Market Association, consulting firms and other insurance and reinsurance companies, is used to supplement or corroborate Ark’s own experience. External data can be especially useful for estimating costs on newer lines of business. Ultimate loss and LAE for major losses and catastrophes are estimated based on the known and expected exposures to the loss event, rather than simply relying on the extrapolation of reported and settled claims. For some lines of business, such as long-tail coverages discussed below, claims data reported in the most recent years of account are often too limited to provide a meaningful basis for analysis due to the typical delay in reporting and settling of claims. For this type of business, Ark uses an expected loss ratio method for the initial years of account. This is a standard and accepted actuarial reserve estimation method in these circumstances in which the loss ratio is selected based upon information used in pricing policies for that line of business, as well as any publicly available industry data, such as industry pricing, experience and trends. Uncertainties in estimating ultimate loss and LAE are magnified by the time lag between when a claim actually occurs and when it is reported and eventually settled. This time lag is sometimes referred to as the “claim-tail.” The claim-tail for reinsurance and insurance obtained through brokers, MGAs and reinsurance intermediaries (collectively, the “insurance and reinsurance intermediaries”) is further extended because claims are first reported to either the original primary insurance company or the insurance and reinsurance intermediaries. The claim-tail for most property coverages is typically short (usually a few days up to a few months). Settlements for casualty/liability coverages can extend for long periods of time as claims are often reported and ultimately paid or settled years after the related loss events occur. During the long claims reporting and settlement period, additional facts regarding coverages written in prior years of account, as well as about actual claims and trends, may become known and, as a result, Ark may adjust its reserves. The inherent uncertainties of estimating loss and LAE reserves are increased by the diversity of loss development patterns among different types of reinsurance treaties, facultative contracts or direct insurance contracts, the necessary reliance on the ceding companies and insurance and reinsurance intermediaries for information regarding reported claims and the differing reserving practices among ceding companies and insurance and reinsurance intermediaries. If management determines that an adjustment is appropriate, the adjustment is booked in the accounting period in which such determination is made. Accordingly, should reserves need to be increased or decreased in the future from amounts currently established, future results of operations would be negatively or positively impacted. In determining ultimate loss and LAE, the cost to indemnify claimants, provide needed legal defense and other services for insureds and administer the investigation and adjustment of claims are considered. These claims costs are influenced by many factors that change over time, such as expanded coverage definitions as a result of new court decisions, inflation in costs to repair or replace damaged property, inflation in the cost of medical services and legislated changes in statutory benefits, as well as by the unique facts that pertain to each claim. As a result, the rate at which claims arose in the past and the costs to settle them may not always be representative of what will occur in the future. The factors influencing changes in claims costs are often difficult to isolate or quantify and developments in paid and incurred losses from historical trends are frequently subject to multiple and conflicting interpretations. Changes in coverage terms or claims handling practices may also cause future experience and/or development patterns to vary from the past. A key objective of actuaries in developing estimates of ultimate loss and LAE and resulting IBNR reserves is to identify aberrations and systemic changes occurring within historical experience and accurately adjust for them so that the future can be projected more reliably. Because of the factors previously discussed, this process requires the use of informed judgment and is inherently uncertain. Ark performs an actuarial review of its recorded loss and LAE reserves each quarter, using several generally accepted actuarial methods to evaluate its loss reserves, each of which has its own strengths and weaknesses. Management places more or less reliance on a particular method based on the facts and circumstances at the time the reserve estimates are made. These methods generally fall into one of the following categories or are hybrids of one or more of the following categories: • Historical paid loss development methods: These methods use historical loss payments over discrete periods of time to estimate future losses. Historical paid loss development methods assume that the ratio of losses paid in one period to losses paid in an earlier period will remain constant. These methods necessarily assume that factors that have affected paid losses in the past, such as inflation or the effects of litigation, will remain constant in the future. Because historical paid loss development methods do not use case reserves to estimate ultimate losses, they can be more reliable than the other methods discussed below that look to case reserves (such as actuarial methods that use incurred losses) in situations where there are significant changes in how case reserves are established by a company’s claims adjusters. However, historical paid loss development methods are more leveraged, meaning that small changes in payments have a larger impact on estimates of ultimate losses, than actuarial methods that use incurred losses because cumulative loss payments take much longer to approach the expected ultimate losses than cumulative incurred amounts. In addition, and for similar reasons, historical paid loss development methods are often slow to react to situations when new or different factors arise than those that have affected paid losses in the past. • Historical incurred loss development methods: These methods, like historical paid loss development methods, assume that the ratio of losses in one period to losses in an earlier period will remain constant in the future. However, instead of using paid losses, these methods use incurred losses (i.e., the sum of cumulative historical loss payments plus outstanding case reserves) over discrete periods of time to estimate future losses. Historical incurred loss development methods can be preferable to historical paid loss development methods because they explicitly take into account open cases and the claims adjusters’ evaluations of the cost to settle all known claims. However, historical incurred loss development methods necessarily assume that case reserving practices are consistently applied over time. Therefore, when there have been significant changes in how case reserves are established, using incurred loss data to project ultimate losses can be less reliable than other methods. • Expected loss ratio methods: These methods are based on the assumption that ultimate losses vary proportionately with premiums. Expected loss ratios are typically developed based upon the information used in pricing and are multiplied by the total amount of premiums earned to calculate ultimate losses. Expected loss ratio methods are useful for estimating ultimate losses in the early years of long-tail lines of business, when little or no paid or incurred loss information is available. • Bornhuetter-Ferguson methods: These methods are a blend of the expected loss ratio and loss development methods. The percent of incurred (or paid) loss to ultimate loss implied by the selected development pattern from the incurred (or paid) loss development method is used to determine the percentage of ultimate loss yet to be developed. Inception to date losses are added to losses yet to be developed, yielding an estimate of ultimate for each year of account. • Adjusted historical paid and incurred loss development methods: These methods take traditional historical paid and incurred loss development methods and adjust them for the estimated impact of changes from the past in factors such as inflation, the speed of claims payment or the adequacy of case reserves. Adjusted historical paid and incurred loss development methods are often more reliable methods of predicting ultimate losses in periods of significant change, provided the actuaries can develop methods to reasonably quantify the impact of changes. As part of Ark’s quarterly actuarial review, Ark compares the previous quarter’s projections of incurred, paid and case reserve activity, including amounts incurred but not reported, to actual amounts experienced in the quarter. Differences between previous estimates and actual experience are evaluated to determine whether a given actuarial method for estimating loss and LAE reserves should be relied upon to a greater or lesser extent than it has been in the past. While some variance is expected each quarter due to the inherent uncertainty in estimating loss and LAE reserves, persistent or large variances would indicate that prior assumptions and/or reliance on certain actuarial methods may need to be revised going forward. Upon completion of each quarterly review, Ark selects indicated loss and LAE reserve levels based on the results of the actuarial methods described previously, which are the primary consideration in determining management's best estimate of required loss and LAE reserves. However, in making its best estimate, management also considers other qualitative factors that may lead to a difference between held reserves and the actuarial central estimate of reserves. Typically, these qualitative factors are considered when management and Ark’s actuaries conclude that there is insufficient historical incurred and paid loss information or that trends included in the historical incurred and paid loss information are not likely to repeat in the future. Such qualitative factors include, among others, recent entry into new markets or new products, improvements in the claims department that are expected to lessen future ultimate loss costs, legal and regulatory developments or other uncertainties that may arise. Ark Reserve Estimation by Line of Business The process of establishing loss and LAE reserves, including amounts incurred but not reported, is complex and imprecise, as it must consider many variables that are subject to the outcome of future events. As a result, informed subjective estimates and judgments as to the ultimate exposure to losses are an integral component of the loss and LAE reserving process. Ark categorizes and tracks insurance and reinsurance reserves by “reserving class of business” for each underwriting office, London and Bermuda, and then aggregates the reserving classes by line of business, which are summarized herein as property and accident & health, specialty, marine & energy, casualty-active and casualty-runoff. Ark regularly reviews the appropriateness of its loss and LAE reserves at the reserving class of business level, considering a variety of trends that impact the ultimate settlement of claims for the subsets of claims in each particular reserving class. For loss and LAE reserves as of December 31, 2024, Ark considers that the impact of the various reserving factors, as described below, on future paid losses would be similar to the impact of those factors on historical paid losses. The major causes of material uncertainty (i.e., reserving factors) generally will vary for each line of business, as well as for each separately analyzed reserving class of business within the line of business. Also, reserving factors can have offsetting or compounding effects on estimated loss and LAE reserves. In most cases, it is not possible to measure the effect of a single reserving factor and construct a meaningful sensitivity expectation. Actual results will likely vary from expectations for each of these assumptions, resulting in an ultimate claims liability that is different from that being estimated currently. Additional causes of material uncertainty exist in most product lines and may impact the types of claims that could occur within a particular line of business or reserving class of business. Examples where reserving factors, within a line of business or reserving class of business, are subject to change include changing types of insureds (e.g., size of account, industry insured, jurisdiction), changing underwriting standards, or changing policy provisions (e.g., deductibles, policy limits, endorsements). Following is a detailed description of the reserve factors and consideration for each of Ark’s reserving lines of business. Property and Accident & Health Ark’s property and accident & health reserving line of business contains short-tail exposures. As such, reserving for these classes generally involves less uncertainty given the speed of settlement. For property reserving classes, the reserve risk is driven primarily by occasional catastrophe events, though the financial effect of these is mitigated by reinsurance and retrocessional purchases. Ark writes property business on both an insurance and reinsurance basis. The insurance business primarily consists of direct and facultative contracts. However, some business is written through line slips and MGA binding authorities, which could have a longer-tail due to the increased exposure period caused by underlying policies attaching to the binder contract. The reinsurance business can also have a longer-tail due to timing delays resulting from attachment points on excess of loss contracts. For accident & health reserving classes, the losses emanate from a wide range of personal accident, sickness, travel and medical insurance risks. The underlying business is a mix of direct and facultative contracts, as well as some MGA and reinsurance contracts, which are typically shorter-tail lines. Certain smaller components of the accident & health business can be longer-tail. The accident & health business is also exposed to occasional catastrophic events though not to the same degree as the property business. Marine & Energy Ark’s marine & energy reserving line of business is underwritten on both an insurance and reinsurance basis and can be broken down into physical damage on marine risks, physical damage on upstream energy platforms and marine & energy liabilities. The marine reserving classes consist primarily of marine hull, cargo and specie risks. These all generally have some element of transportability, which mitigates the catastrophe risk exposure; for example, having the ability to move out of the path of a hurricane if provided with sufficient notice. The marine reserving classes are generally shorter-tail. The energy platform reserving classes cover risks that are less transportable and therefore are exposed to catastrophe events similar to property reserving classes. Other energy reserving classes cover construction contracts, which often have considerably protracted exposure periods with the bulk of the risk towards the end of the coverage period. This can have the effect of increasing the tail on an otherwise short-tail reserving class. The marine & energy liability reserving classes, which represent a smaller portion of the marine & energy business, are typically longer-tail compared to physical damage reserving classes. Specialty Ark’s specialty portfolio is comprised of a diverse portfolio of insurance and reinsurance subclasses of business including aviation, space, political and credit, cyber, terrorism and political violence, nuclear, fine art & specie, surety and mortgage. Certain subclasses of business are exposed to both catastrophe events and man-made loss events; for example, terrorism, war and war-like actions, political violence and space. Although these subclasses have different coverages and exposures, they are all short-tail in nature and have similar reserving features. Casualty-Active and Casualty-Runoff Ark’s casualty reserving lines of business, which include casualty–active and casualty–runoff, are long-tail classes of business. Consequently, the ultimate liability may not be known at the date of loss, which results in greater uncertainty when reserving for casualty lines. The casualty–active line of business consists of U.S. reinsurance and insurance risks written on an excess of loss basis. The casualty–runoff line of business consists of international reinsurance risks and U.S. casualty insurance risks written through an MGA binding authority. The losses arising from these lines of business are primarily related to medical malpractice, professional liability and general liability coverages, which are long-tail lines of business. Casualty policies are generally written on either a claims-made or occurrence basis. On a claims-made basis, the trigger of loss is based on the date that the loss is reported. On an occurrence basis, the trigger of loss is the date that the loss occurred. Due to delays between loss occurrence and loss reporting, business written on an occurrence basis can be longer-tail than business written on a claims-made basis. There are a number of common reserving factors for casualty lines that can affect the estimated casualty reserves, including: • Changes in claims-handling practices, both in-house and through third-party claims administrators, • Changes in court interpretations of policy provisions, and • Trends in litigation or jury awards. Cumulative Number of Reported Claims Ark counts a claim for each unique combination of individual claimant, loss event and risk. A claim is still counted if the claim is closed with no payment. Bulk-coded losses are counted as one claim, as underlying claim counts are not available. Discounting Ark does not discount loss and LAE reserves. Impact of Third-Party Capital For the years of account prior to the Ark Transaction, a significant proportion of the Syndicates’ underwriting capital was provided by TPC Providers using whole account reinsurance contracts with Ark’s corporate member. For the years of account subsequent to the Ark Transaction, Ark is no longer using TPC Providers to provide underwriting capital for the Syndicates. A Reinsurance to Close (“RITC”) agreement is generally put in place after the third year of operations for a year of account such that the outstanding loss and LAE reserves, including future development thereon, are reinsured into the next year of account. As a result, and in combination with the changing participation provided by TPC Providers, Ark’s participation on outstanding loss and LAE reserves reinsured into the next year of account changes. After 2023, Ark is no longer subject to changes in TPC Providers’ participation. During 2023, an RITC agreement was executed such that the outstanding loss and LAE reserves for claims arising out of the 2020 year of account, for which the TPC Providers’ participation in the total net results of the Syndicates was 42.8%, were reinsured into the 2021 year of account, for which the TPC Providers’ participation in the total net results of the Syndicates was 0.0%. During 2022, an RITC agreement was executed such that the outstanding loss and LAE reserves for claims arising out of the 2019 year of account, for which the TPC Providers’ participation in the total net results of the Syndicates was 58.3%, were reinsured into the 2020 year of account, for which the TPC Providers’ participation in the total net results of the Syndicates was 42.8%. Loss and Loss Adjustment Expense Reserve Summary The following table summarizes the loss and LAE reserve activity of the Ark/WM Outrigger segment for the years ended December 31, 2024, 2023 and 2022: Year Ended December 31, Millions 2024 2023 2022 Gross beginning balance $ 1,605.1 $ 1,296.5 $ 894.7 Less: beginning reinsurance recoverable on unpaid losses (1) (340.8) (505.0) (428.9) Net loss and LAE reserves 1,264.3 791.5 465.8 Loss and LAE incurred relating to: Current year losses 914.4 706.9 588.1 Prior year losses (58.6) 19.9 (51.7) Net incurred loss and LAE 855.8 726.8 536.4 Loss and LAE paid relating to: Current year losses (109.2) (51.3) (98.9) Prior year losses (307.5) (356.6) (158.6) Net paid loss and LAE (416.7) (407.9) (257.5) Change in TPC Providers’ participation (2) — 145.4 57.5 Foreign currency translation and other adjustments to loss and LAE reserves (10.3) 8.5 (10.7) Net ending balance 1,693.1 1,264.3 791.5 Plus: ending reinsurance recoverable on unpaid losses (3) 434.4 340.8 505.0 Gross ending balance $ 2,127.5 $ 1,605.1 $ 1,296.5 (1) The beginning reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $145.4 and $276.8 as of January 1, 2023 and 2022. (2) Amount represents the impact to net loss and LAE reserves due to a change in the TPC Providers’ participation related to the annual RITC process. (3) The ending reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $145.4 as of December 31, 2022. During the year ended December 31, 2024, the Ark/WM Outrigger segment experienced $58.6 million of net favorable prior year loss reserve development. The net favorable prior year loss reserve development was driven primarily by the specialty ($34.3 million) and property and accident & health ($29.2 million) reserving lines of business, partially offset by net unfavorable development in the casualty-active ($4.5 million) reserving line of business. The net favorable prior year loss reserve development was driven primarily by positive claims experience in specialty for the 2023 and 2019 accident years and in property and accident & health for the 2023 accident year. During the year ended December 31, 2023, the Ark/WM Outrigger segment experienced $19.9 million of net unfavorable prior year loss reserve development. The net unfavorable prior year loss reserve development was driven primarily by the property and accident & health ($41.7 million) reserving line of business, partially offset by net favorable prior year loss reserve development within the specialty ($11.9 million) and casualty–runoff ($6.0 million) reserving lines of business. The net unfavorable prior year loss reserve development in the property and accident & health reserving line of business was driven primarily by Hurricane Ian and Winter Storm Elliott within the 2022 accident year. The net favorable prior year loss reserve development in specialty and casualty-runoff was driven primarily by positive claims experience for the 2021 and 2020 accident years. During the year ended December 31, 2022, The Ark/WM Outrigger segment experienced $51.7 million of net favorable prior year loss reserve development. The net favorable prior year loss reserve development was driven primarily by the property and accident & health ($20.8 million), marine & energy ($18.8 million) and specialty ($12.7 million) reserving lines of business. The net favorable prior year loss reserve development in property and accident & health, marine & energy and specialty was driven primarily by positive claims experience for the 2021 accident year. The following table summarizes the unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses, for each of the Ark/WM Outrigger segment’s major reserving lines of business as of December 31, 2024 and 2023: Year Ended December 31, Millions 2024 2023 Property and Accident & Health (1) $ 533.6 $ 358.7 Marine & Energy 463.1 331.7 Specialty 417.0 339.7 Casualty-Active 178.0 137.1 Casualty-Runoff 66.5 81.3 WM Outrigger Re 34.9 15.8 Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses (1) 1,693.1 1,264.3 Plus: Reinsurance recoverables on unpaid losses Property and Accident & Health (2) 121.5 126.9 Marine & Energy 180.1 59.1 Specialty 51.3 77.6 Casualty-Active 80.7 74.8 Casualty-Runoff .8 2.4 Total Reinsurance recoverables on unpaid losses (2) 434.4 340.8 Total unpaid loss and LAE reserves $ 2,127.5 $ 1,605.1 (1) The amounts exclude $3.1 ceded by Ark to WM Outrigger Re as of December 31, 2024, which eliminates in White Mountains’s consolidated financial statements. (2) The amounts exclude $31.8 and $15.6 ceded by Ark to WM Outrigger Re as of December 31, 2024 and 2023, which eliminate in White Mountains’s consolidated financial statements. The following five tables cover each of Ark’s property and accident & health, marine & energy, specialty, casualty-active and casualty-runoff reserving lines of business and are presented net of reinsurance, which includes the impact of whole-account quota-share reinsurance arrangements related to TPC Providers. Each of the five tables includes three sections as follows: The top section of the table presents, for each of the previous 10 accident years: (1) cumulative total undiscounted incurred loss and LAE as of each of the previous 10 year-end evaluations, (2) total IBNR plus expected development on reported claims as of December 31, 2024 and (3) the cumulative number of reported claims as of December 31, 2024. The middle section of the table presents cumulative paid loss and LAE for each of the previous 10 accident years as of each of the previous 10 year-end evaluations. Also included in this section is a calculation of the loss and LAE reserves as of December 31, 2024, which is then included in the reconciliation to the consolidated balance sheet presented above. The total unpaid loss and LAE reserves as of December 31, 2024 are calculated as the cumulative incurred loss and LAE from the top section less the cumulative paid loss and LAE from the middle section, plus any outstanding liabilities from accident years prior to 2014. The bottom section of the table is supplementary information about the average historical claims duration as of December 31, 2024. It shows the weighted average annual percentage payout of incurred loss and LAE by accident year as of each age. For example, the first column is calculated as the incremental paid loss and LAE in the first calendar year for each given accident year (e.g. calendar year 2024 for accident year 2024, calendar year 2023 for accident year 2023) divided by the cumulative incurred loss and LAE as of December 31, 2024 for that accident year. The resulting ratios are weighted using cumulative incurred loss and LAE as of December 31, 2024. Property and Accident & Health $ in Millions Incurred Loss and LAE, Net of Reinsurance For the Years Ended December 31, As of December 31, 2024 Accident Year 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Total IBNR plus expected development on reported claims Cumulative number of reported claims Unaudited 2015 $ 19.1 $ 18.1 $ 17.2 $ 16.2 $ 16.0 $ 16.0 $ 15.8 $ 15.7 $ 16.0 $ 16.1 $ .1 2,829 2016 22.2 17.5 18.2 18.4 18.3 18.5 18.5 18.5 18.5 .2 3,433 2017 31.1 37.7 $ 45.2 44.2 42.8 42.3 43.8 43.4 15.8 4,624 2018 40.7 $ 47.1 49.0 46.7 46.8 46.3 46.4 1.9 4,288 2019 33.9 31.2 27.0 23.8 23.2 22.8 .6 4,024 2020 76.9 75.1 74.5 77.6 79.5 9.9 4,646 2021 170.0 153.7 165.3 168.0 7.5 3,509 2022 241.5 266.9 277.3 12.9 4,044 2023 213.9 176.1 77.3 3,598 2024 359.5 202.7 3,614 Total $ 1,207.6 Property and Accident & Health Millions Cumulative Paid Loss and LAE, Net of Reinsurance For the Years Ended December 31, Accident Year 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Unaudited 2015 $ 6.9 $ 12.2 $ 13.4 $ 14.6 $ 14.5 $ 14.8 $ 15.0 $ 15.0 $ 15.4 $ 15.6 2016 8.5 13.0 16.3 16.7 16.8 17.1 17.7 17.9 18.1 2017 16.8 25.7 31.5 32.7 29.4 27.1 25.4 28.2 2018 15.6 32.2 40.1 40.0 40.8 42.8 43.6 2019 6.8 16.7 18.3 18.5 19.3 20.6 2020 11.2 33.9 46.9 55.6 66.5 2021 30.7 86.5 129.8 142.7 2022 69.4 191.9 229.4 2023 19.9 52.9 2024 54.8 Total 672.4 All outstanding liabilities before 2015, net of reinsurance 1.5 Loss and LAE reserves, net of reinsurance $ 536.7 Property and Accident & Health Average Annual Percentage Payout of Incurred Loss and LAE by Age, Net of Reinsurance Unaudited Years 1 2 3 4 5 6 7 8 9 10 23.8% 32.8% 18.3% 6.4% 2.8% 1.3% 0.5% 0.8% 0.1% 0.1% Marine & Energy $ in Millions Incurred Loss and LAE, Net of Reinsurance For the Years Ended December 31, As of December 31, 2024 Accident Year 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Total IBNR plus expected development on reported claims Cumulative number of reported claims Unaudited 2015 $ 21.7 $ 17.4 $ 16.1 $ 13.3 $ 12.7 $ 12.8 $ 12.7 $ 12.9 $ 12.7 $ 12.7 $ — 3,243 2016 23.4 19.5 15.6 14.6 14.3 14.8 14.1 13.6 13.3 — 3,772 2017 26.0 19.3 17.6 16.9 16.6 15.8 16.0 16.1 .4 4,139 2018 25.4 19.9 17.4 17.8 17.3 17.7 16.7 .2 3,238 2019 23.7 21.6 21.6 21.4 21.9 21.2 .5 2,393 2020 29.7 27.1 28.5 27.4 27.1 .8 1,582 2021 86.2 69.3 67.3 74.5 4.0 1,505 2022 149.7 153.6 156.0 31.9 1,968 2023 197.0 188.2 115.9 2,138 2024 239.9 182.6 1,485 Total $ 765.7 Marine & Energy Millions Cumulative Paid Loss and LAE, Net of Reinsurance For the Years Ended December 31, Accident Year 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Unaudited 2015 $ 4.0 $ 7.8 $ 9.6 $ 11.0 $ 10.4 $ 10.5 $ 10.9 $ 11.5 $ 11.6 $ 11.6 2016 5.5 10.0 12.6 13.0 13.1 13.7 13.4 13.4 13.4 2017 5.1 11.1 12.8 14.0 14.1 14.1 14.0 14.3 2018 2.6 12.4 13.9 14.6 15.3 15.3 15.4 2019 3.3 10.6 12.6 14.3 15.3 18.1 2020 3.1 12.7 16.0 18.5 21.9 2021 6.3 24.3 38.2 51.9 2022 12.2 66.2 97.7 2023 10.5 42.1 2024 20.9 Total 307.3 All outstanding liabilities before 2015, net of rein

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2024
Segment Reporting [Abstract]
Segment Information	Segment Information As of December 31, 2024, White Mountains conducted its operations through four reportable segments: (1) Ark/WM Outrigger, (2) HG Global, (3) Kudu and (4) Bamboo, with its 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 CODMs and the Board of Directors. The Company’s CODMs are its Chief Executive Officer and its President and Chief Financial Officer. The CODMs utilize each segment's pre-tax income (loss) in assessing each segment's performance and allocating resources. Other measures of segment profitability are also reviewed by the CODMs. Significant intercompany transactions among White Mountains’s segments have been eliminated herein. During the fourth quarter of 2022, Ark sponsored the formation of Outrigger Re Ltd. to provide collateralized reinsurance protection on Ark’s Bermuda global property catastrophe excess of loss portfolio written in the 2023 underwriting year. Ark renewed its quota share reinsurance agreements with Outrigger Re Ltd. for the 2024 and 2025 underwriting years. White Mountains consolidates its segregated account of Outrigger Re Ltd., WM Outrigger Re, in its financial statements. WM Outrigger Re’s quota share reinsurance agreement with GAIL eliminates in White Mountains’s consolidated financial statements. WM Outrigger Re exclusively provides reinsurance protection to Ark. As a result, WM Outrigger Re was aggregated with Ark within the Ark/WM Outrigger segment starting in 2023. See Note 2 — “Significant Transactions.” Effective July 1, 2024, White Mountains no longer consolidates BAM. Through June 30, 2024, BAM’s assets, liabilities and noncontrolling interests, as well as its results of operations, are presented within the HG Global segment. See Note 2 — “Significant Transactions.” 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 Transactions.” As a result of the NSM Transaction, the results of operations for NSM, previously reported as a segment, have been classified as discontinued operations in the statements of operations and comprehensive income through the closing of the transaction. See Note 20 — “Held for Sale and Discontinued Operations.” The following tables present White Mountains’s pre-tax financial results by segment for the years ended December 31, 2024, 2023 and 2022: Ark/WM Outrigger HG Global Millions Ark WM Outrigger Re HG Global BAM (1)(2) Kudu Bamboo Other Total Year Ended December 31, 2024 Earned insurance premiums $ 1,499.8 $ 88.0 $ 28.9 $ 2.8 $ — $ 39.4 $ 32.7 $ 1,691.6 Net investment income (3) 79.4 11.3 23.4 8.8 66.7 2.2 35.6 227.4 Net realized and unrealized investment gains (losses) (3) 50.1 — (6.4) (5.1) 51.3 — 57.0 146.9 Net realized and unrealized investment gains — — — — — — 38.0 38.0 Interest income (expense) from BAM Surplus Notes — — 29.0 (13.2) — — — 15.8 Change in fair value of BAM Surplus Notes — — .5 — — — — .5 Unrealized loss on deconsolidation of BAM — — (114.5) — — — — (114.5) Commission and fee revenues — — — — — 134.6 14.8 149.4 Other revenues 22.3 — .1 1.1 .8 3.6 56.8 84.7 Total revenues 1,651.6 99.3 (39.0) (5.6) 118.8 179.8 234.9 2,239.8 Loss and loss adjustment expenses 825.9 29.9 — — — 20.6 12.1 888.5 Acquisition expenses 283.9 23.2 7.8 .4 — 14.1 12.1 341.5 Cost of sales — — — — — — 29.6 29.6 Broker commission expenses — — — — — — 51.3 — 51.3 General and administrative expenses (4) (5) 208.3 .1 2.2 33.5 15.4 61.1 169.5 490.1 Change in fair value of contingent consideration 61.3 — — — — — — 61.3 Interest expense 19.5 — 16.7 — 22.1 — 2.5 60.8 Total expenses 1,398.9 53.2 26.7 33.9 37.5 147.1 225.8 1,923.1 Pre-tax income (loss) $ 252.7 $ 46.1 $ (65.7) $ (39.5) $ 81.3 $ 32.7 $ 9.1 $ 316.7 (1) Effective July 1, 2024, White Mountains no longer consolidates BAM. For the period from January 1, 2024 through June 30, 2024, BAM’s results of operations are presented within the HG Global segment. (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. (4) Ark’s general and administrative expenses include $136.1 of other underwriting expenses. (5) Bamboo’s general and administrative expenses include $16.4 of amortization of other intangible assets. Ark/WM Outrigger HG Global Other Millions Ark WM Outrigger Re HG Global BAM (1) Kudu Total Year Ended December 31, 2023 Earned insurance premiums $ 1,305.4 $ 104.3 $ 26.0 $ 5.2 $ — $ — $ 1,440.9 Net investment income 50.4 11.0 17.1 14.6 71.0 30.1 194.2 Net realized and unrealized investment gains (losses) 85.9 — 13.6 13.0 106.1 188.5 407.1 Net realized and unrealized investment gains — — — — — 27.1 27.1 Interest income (expense) from BAM Surplus Notes — — 26.2 (26.2) — — — Commission and fee revenues — — — — — 13.2 13.2 Other revenues .8 — — 2.9 — 80.5 84.2 Total revenues 1,442.5 115.3 82.9 9.5 177.1 339.4 2,166.7 Losses and loss adjustment expenses 711.2 15.6 — — — — 726.8 Acquisition expenses 251.0 30.5 7.4 1.2 — — 290.1 Cost of sales — — — — — 40.4 40.4 General and administrative expenses (2) 161.7 .3 2.8 66.1 19.4 182.3 432.6 Change in fair value of contingent consideration 48.7 — — — — — 48.7 Interest expense 21.3 — 16.5 — 21.2 3.7 62.7 Total expenses 1,193.9 46.4 26.7 67.3 40.6 226.4 1,601.3 Pre-tax income (loss) $ 248.6 $ 68.9 $ 56.2 $ (57.8) $ 136.5 $ 113.0 $ 565.4 (1) BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the BAM Surplus Notes and is not reduced by accruals of interest expense on the BAM Surplus Notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the NYDFS. (2) Ark’s general and administrative expenses include $113.6 of other underwriting expenses. Ark/WM Outrigger HG Global Other Millions Ark HG Global BAM (1) Kudu Total Year Ended December 31, 2022 Earned insurance premiums $ 1,043.4 $ 27.5 $ 5.8 $ — $ — $ 1,076.7 Net investment income 16.3 10.3 11.2 54.4 32.2 124.4 Net realized and unrealized investment gains (losses) (55.2) (52.5) (53.3) 64.1 (1.6) (98.5) Net realized and unrealized investment gains — — — — (93.0) (93.0) Interest income (expense) from BAM Surplus Notes — 11.7 (11.7) — — — Commission and fee revenues — — — — 11.5 11.5 Other revenues 5.0 .5 4.1 — 127.2 136.8 Total revenues 1,009.5 (2.5) (43.9) 118.5 76.3 1,157.9 Loss and loss adjustment expenses 536.4 — — — — 536.4 Acquisition expenses 239.4 9.3 1.9 — — 250.6 Cost of sales — — — — 98.6 98.6 General and administrative expenses (2) 106.2 2.8 66.3 14.7 174.1 364.1 Change in fair value of contingent consideration 17.3 — — — — 17.3 Interest expense 15.1 8.3 — 15.0 1.9 40.3 Total expenses 914.4 20.4 68.2 29.7 274.6 1,307.3 Pre-tax income (loss) $ 95.1 $ (22.9) $ (112.1) $ 88.8 $ (198.3) $ (149.4) (1) BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the BAM Surplus Notes and is not reduced by accruals of interest expense on the BAM Surplus Notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the NYDFS. (2) Ark’s general and administrative expenses includes $78.7 of other underwriting expenses. The following tables present White Mountains’s revenues from external customers by country for the years ended December 31, 2024, 2023 and 2022: December 31, 2024 Millions United States United Kingdom Bermuda Other Total Earned insurance premiums $ 2.8 $ 956.8 $ 732.0 $ — $ 1,691.6 Commission and fee revenues 134.6 — — 14.8 149.4 Other revenues (1) 55.8 — — — 55.8 Total $ 193.2 $ 956.8 $ 732.0 $ 14.8 $ 1,896.8 (1) Amounts include revenues from external customers related to certain consolidated Other Operating Businesses. December 31, 2023 Millions United States United Kingdom Bermuda Other Total Earned insurance premiums $ 5.2 $ 791.7 $ 644.0 $ — $ 1,440.9 Commission and fee revenues — — — 13.2 13.2 Other revenues (1) 69.0 — — — 69.0 Total $ 74.2 $ 791.7 $ 644.0 $ 13.2 $ 1,523.1 (1) Amounts include revenues from external customers related to certain consolidated Other Operating Businesses. December 31, 2022 Millions United States United Kingdom Bermuda Other Total Earned insurance premiums $ 5.8 $ 638.5 $ 432.4 $ — $ 1,076.7 Commission and fee revenues — — — 11.5 11.5 Other revenues (1) 124.0 — — — 124.0 Total $ 129.8 $ 638.5 $ 432.4 $ 11.5 $ 1,212.2 (1) Amounts include revenues from external customers related to certain consolidated Other Operating Businesses. The following table presents White Mountains’s balance sheet information by segment as of December 31, 2024 and 2023: Millions Ark/WM Outrigger HG Global Kudu Bamboo Other Total December 31, 2024: Total investments $ 3,139.7 $ 667.6 $ 1,041.9 $ 58.0 $ 1,570.4 $ 6,477.6 Total assets $ 5,299.0 $ 1,179.4 $ 1,108.4 $ 584.6 $ 1,754.2 (1) $ 9,925.6 Total liabilities $ 3,664.8 $ 464.1 (1) $ 316.7 $ 167.7 $ 181.3 $ 4,794.6 Total White Mountains’s common $ 1,223.8 $ 728.7 (1) $ 664.1 $ 303.3 $ 1,563.8 (1) $ 4,483.7 Noncontrolling interests $ 410.4 $ (13.4) $ 127.6 $ 113.6 $ 9.1 $ 647.3 December 31, 2023: Total investments $ 2,671.1 $ 1,082.9 $ 925.6 $ — $ 1,709.1 $ 6,388.7 Total assets $ 4,378.6 $ 1,172.0 (2) $ 959.4 $ — $ 1,875.9 (1) $ 8,385.9 Total liabilities $ 2,876.1 $ 531.7 (1) $ 275.4 $ — $ 141.1 $ 3,824.3 Total White Mountains’s common $ 1,165.6 $ 779.3 (1) $ 570.2 $ — $ 1,725.4 (1) $ 4,240.5 Noncontrolling interests $ 336.9 $ (139.0) $ 113.8 $ — $ 9.4 $ 321.1 (1) HG Global preferred dividends payable to White Mountains’s subsidiaries is eliminated in White Mountains’s consolidated financial statements. For segment reporting, the HG Global preferred dividends payable to White Mountains’s subsidiaries included within the HG Global segment are eliminated against the offsetting receivable included within Other Operations and therefore added back to White Mountains’s common shareholders’ equity within the HG Global segment. As of December 31, 2024 and 2023, the HG Global preferred dividends payable to White Mountains’s subsidiaries was $462.1 and $399.8. (2) As of December 31, 2023, total assets in the HG Global segment reflected the elimination of $322.2 of BAM Surplus Notes issued to HG Global and its subsidiaries and $174.5 in accrued interest related to the BAM Surplus Notes.

Equity Method Eligible Investme

Equity Method Eligible Investments	12 Months Ended
	Dec. 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, 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. Under GAAP, equity method eligible investments are considered related parties. The following table presents the ownership interests and carrying values of White Mountains’s equity method eligible investments as of December 31, 2024 and 2023: December 31, 2024 December 31, 2023 $ in Millions Ownership Interest Carrying Value Ownership Interest Carrying Value Kudu’s Participation Contracts (1) 4.1% - 30.0% $ 1,008.4 4.1% - 30.0% $ 890.5 Investment in MediaAlpha 26.6% 201.6 34.9% 254.9 PassportCard/DavidShield 53.8% 150.0 53.8% 150.0 Elementum 26.6% 35.0 26.6% 35.0 Other equity method eligible investments, at fair value Under 50.0% 243.7 Under 50.0% 291.7 Other equity method eligible investments, at fair value 50.0% and over — 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. For the years ended December 31, 2024, 2023 and 2022, White Mountains received dividend and income distributions from equity method eligible investments of $71.4 million, $67.0 million and $68.9 million, which were recorded within net investment income in the consolidated statements of operations. The following tables present aggregated summarized financial information for White Mountains’s investments in equity method eligible unconsolidated entities: December 31, Millions 2024 2023 Balance sheet data (1) : Total assets $ 3,719.1 $ 3,203.2 Total liabilities $ 1,063.9 $ 905.6 (1) Financial data for White Mountains’s equity method eligible investees is generally reported on a one-quarter lag. Year Ended December 31, Millions 2024 2023 2022 Income statement data (1) : Total revenues $ 2,011.8 $ 1,575.4 $ 1,194.4 Total expenses $ 1,634.9 $ 1,126.2 $ 1,047.4 Net income (loss) $ 376.9 $ 449.2 $ 147.0 (1) Financial data for White Mountains’s equity method eligible investees is generally reported on a one-quarter lag.

Variable Interest Entities

Variable Interest Entities	12 Months Ended
	Dec. 31, 2024
Variable Interest Entity, Not Primary Beneficiary, Disclosures [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 its segregated accounts, including WM Outrigger Re, are VIEs. White Mountains is not the primary beneficiary of Outrigger Re Ltd. or its third-party 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. WM Outrigger Re’s obligations under its reinsurance agreement with GAIL is subject to an aggregate limit equal to the assets in its collateral trust at any point in time. As of December 31, 2024, investments of $203.7 million were held in its collateral trust account. 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 first-loss reinsurance protection of up to 15%-of-par outstanding for each policy assumed from 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. 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 original FLRT, BAM’s underwriting guidelines could only be amended with the consent of HG Re. As a result, White Mountains concluded at inception and through June 30, 2024 that it had the power to direct BAM’s activities that most significantly impacted BAM’s economic performance and it was the primary beneficiary. Accordingly, White Mountains was required to consolidate BAM’s results in its financial statements. Since BAM is owned by its members, its equity and results of operations were included in noncontrolling interests. On July 1, 2024, HG Re and BAM amended the terms of the FLRT with respect to certain governance rights held by HG Re. Under the amended FLRT, HG Re no longer has approval rights over changes to BAM’s underwriting guidelines; however, HG Re is only required to provide reinsurance on policies that fall within the FLRT underwriting guidelines agreed upon by HG Re. In conjunction with the amendments to the FLRT, HG Global and BAM increased the interest rate on the BAM Surplus Notes to 10.0%, with a higher proportion of each payment to be applied to outstanding principal prospectively. As a result, and in combination with other governance changes at BAM, White Mountains concluded that it no longer has the power to direct BAM’s activities that most significantly impact its economic performance and is no longer BAM’s primary beneficiary. Accordingly, as of July 1, 2024, White Mountains no longer consolidates BAM. BAM’s assets can only be used 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. As of December 31, 2024, the Collateral Trusts held assets of $950.1 million. PassportCard/DavidShield As of December 31, 2024, White Mountains’s ownership interest in PassportCard/DavidShield was 53.8%. In 2024, PassportCard/DavidShield restructured its organizational structure such that White Mountains and its co-investor no longer have a direct investment in PassportCard Limited. The restructuring did not change PassportCard/DavidShield’s governance structure or White Mountains’s economic rights with respect to its investment in PassportCard/DavidShield. White Mountains has determined that PassportCard/DavidShield is a VIE but that White Mountains is not the primary beneficiary and therefore does not consolidate PassportCard/DavidShield. The governance structure for PassportCard/DavidShield was designed to give White Mountains and its co-investor equal power to make the decisions that most significantly impact its operations. White Mountains does not have the unilateral power to direct the operations of PassportCard/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 December 31, 2024, White Mountains’s maximum exposure to loss on its equity investment in PassportCard/DavidShield and the non-interest-bearing loan to its co-investor is the total carrying value of $158.9 million. Elementum As of December 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 December 31, 2024, White Mountains’s maximum exposure to loss on its limited partnership interest in Elementum is the carrying value of $35.0 million. Bamboo CRV White Mountains has determined that Bamboo CRV is a VIE. White Mountains is the primary beneficiary of Bamboo CRV, as it has both the power to direct the activities that most significantly impact Bamboo CRV’s economic performance and the obligation to absorb losses, or the right to receive returns, that could potentially be significant to Bamboo CRV. As a result, White Mountains consolidates Bamboo CRV’s results in its financial statements. The assets of Bamboo CRV can only be used to settle the liabilities of Bamboo CRV, and there is no recourse to the Company for any creditors of Bamboo CRV. As of December 31, 2024, Bamboo CRV’s obligations under its reinsurance agreements are subject to an aggregate limit equal to White Mountains’s total capital commitment of $30.0 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 December 31, 2024, White Mountains’s maximum exposure to loss on its investments in limited partnerships is the carrying value of $249.5 million.

Fair Value of Financial Instrum

Fair Value of Financial Instruments	12 Months Ended
	Dec. 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 debt issuance costs and original issue discount. See Note 7 — “Debt.” The following table presents the fair value and carrying value of these financial instruments as of December 31, 2024 and 2023: December 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 $ 173.9 $ 154.5 $ 171.8 $ 155.5 HG Global Senior Notes $ 157.2 $ 147.4 $ 158.7 $ 146.9 Kudu Credit Facility $ 253.3 $ 238.6 $ 225.6 $ 203.8 Other Operations debt $ 23.1 $ 22.0 $ 30.0 $ 28.4 The fair value estimates for the Ark 2007 Subordinated Notes, Ark 2021 Subordinated Notes, HG Global Senior Notes, Kudu Credit Facility and Other Operations debt have been determined based on discounted cash flow analyses and are considered to be Level 3 measurements. For the fair value measurements associated with White Mountains’s investment securities see Note 3 — “Investment Securities.” For the fair value measurements associated with White Mountains’s derivative instruments see Note 9 — “Derivatives.” For the fair value measurements associated with the BAM Surplus Notes see Note 10 — “Municipal Bond Guarantee Reinsurance.”

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 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.”

Held for Sale and Discontinued

Held for Sale and Discontinued Operations	12 Months Ended
	Dec. 31, 2024
Discontinued Operations and Disposal Groups [Abstract]
Held for Sale and Discontinued Operations	Held for Sale and Discontinued Operations NSM On August 1, 2022, White Mountains closed the NSM Transaction. See Note 2 — “Significant Transactions.” As a result of the NSM Transaction, the results of operations for NSM Group have been classified as discontinued operations in the statements of operations and comprehensive income through the closing of the transaction. Net Income (Loss) from Discontinued Operations The following summarizes the results of operations, including related income taxes associated with the businesses classified as discontinued operations for the year ended December 31, 2022: December 31, 2022 (1) Millions Revenues Commission revenues $ 176.9 Other revenues 48.1 Total revenues - NSM Group 225.0 Expenses General and administrative expenses 126.8 Broker commission expenses 52.9 Change in fair value of contingent consideration .1 Amortization of other intangible assets 9.1 Interest expense 12.1 Total expenses - NSM Group 201.0 Pre-tax income (loss) from discontinued operations - NSM Group 24.0 Income tax (expense) benefit (7.6) Net income (loss) from discontinued operations, net tax - NSM Group 16.4 Net gain (loss) from sale of discontinued operations, net of 886.8 Total income (loss) from discontinued operations, net of tax 903.2 Net (income) loss from discontinued operations (.7) Total income (loss) from discontinued operations attributable to 902.5 Other comprehensive income (loss) from discontinued operations, (5.2) Net gain (loss) from foreign currency translation from sale of discontinued 2.9 Comprehensive income (loss) from discontinued operations 900.2 Other comprehensive (income) loss from discontinued operations attributable .2 Comprehensive income (loss) from discontinued operations attributable $ 900.4 (1) As a result of the NSM Transaction, the results of operations for NSM Group are presented for the period from January 1, 2022 to August 1, 2022. Net Change in Cash from Discontinued Operations The following summarizes the net change in cash associated with the businesses classified as discontinued operations for the years ended December 31, 2022: Year Ended Millions Net cash provided from (used for) operations $ 38.7 Net cash provided from (used for) investing activities 7.1 Net cash used from (used for) financing activities (17.5) Effect of exchange rate changes on cash 4.0 Net change in cash during the period 32.3 Cash balances at beginning of period (includes restricted cash of $89.2) 111.6 Cash sold as part of the sale of NSM Group (includes restricted cash of $105.1) (143.9) Cash balances at end of period (includes restricted cash of $0.0) $ — Supplemental cash flows information: Interest paid $ (12.0) Net income tax payments $ — Earnings Per Share from Discontinued Operations White Mountains calculates earnings per share using the two-class method, which allocates earnings between common and unvested restricted common shares. Both classes of shares participate equally in earnings on a per share basis. Basic earnings per share amounts are based on the weighted average number of common shares outstanding adjusted for unvested restricted common shares. Diluted earnings per share amounts are also impacted by the net effect of potentially dilutive common shares outstanding. The following table presents the Company’s computation of earnings per share for discontinued operations for the years ended December 31, 2022: Year Ended Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 792.8 Less: net income (loss) from continuing operations (190.8) Less: net (income) loss from continuing operations attributable to noncontrolling interest 81.1 Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders (1) 902.5 Allocation of (earnings) losses to participating restricted common shares (2) (11.4) Basic and diluted (loss) earnings per share numerators $ 891.1 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 2,862.4 Average unvested restricted common shares (3) (36.2) Basic earnings (loss) per share denominator 2,826.2 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 2,862.4 Average unvested restricted common shares (3) (36.2) Diluted earnings (loss) per share denominator 2,826.2 Basic (loss) earnings per share (in dollars) - discontinued operations: $ 315.30 Diluted (loss) earnings per share (in dollars) - discontinued operations: $ 315.30 (1) Includes net income (loss) from discontinued operations, net of tax - NSM Group, net gain (loss) from sale of discontinued operations, net of tax - NSM Group and net (income) loss from discontinued operations attributable to noncontrolling interests. (2) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities. (3) Restricted shares outstanding vest upon a stated date. See Note 12 — “Employee Share-Based Incentive Compensation Plans.”

SCHEDULE I - SUMMARY OF INVESTM

SCHEDULE I - SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES	12 Months Ended
	Dec. 31, 2024
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Abstract]
SCHEDULE I - SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES	WHITE MOUNTAINS INSURANCE GROUP, LTD. SUMMARY OF INVESTMENTS—OTHER THAN INVESTMENTS IN RELATED PARTIES As of December 31, 2024 Millions Cost Carrying Fair Fixed maturity investments: U.S. Government and agency obligations $ 461.8 $ 462.1 $ 462.1 Debt securities issued by corporations 1,444.5 1,414.2 1,414.2 Municipal obligations 3.2 3.2 3.2 Mortgage and asset-backed securities 400.2 373.9 373.9 Collateralized loan obligations 237.3 236.7 236.7 Foreign government and agency obligations 22.2 21.5 21.5 Total fixed maturity investments 2,569.2 2,511.6 2,511.6 Short-term investments 964.2 964.2 964.2 Investment in MediaAlpha (1) 59.2 201.6 201.6 Common equity securities: Common equity securities - Industrial, Miscellaneous and Other 373.7 425.4 425.4 Common equity securities - Exchange traded funds 163.3 224.6 224.6 Total common equity securities 537.0 650.0 650.0 Other long-term investments (2) 1,748.7 2,150.2 2,150.2 Total investments $ 5,878.3 $ 6,477.6 $ 6,477.6 (1) Under GAAP, equity method eligible investments are considered related parties. White Mountains’s Investment in MediaAlpha is equity method eligible. See Note 17 — “Equity Method Eligible Investments.” (2)

SCHEDULE II - CONDENSED FINANCI

SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT	12 Months Ended
	Dec. 31, 2024
Condensed Financial Information Disclosure [Abstract]
SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT	CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT CONDENSED BALANCE SHEETS (1) December 31, Millions 2024 2023 Assets: Cash $ .1 $ .3 Fixed maturity investments, at fair value 70.0 — Common equity securities, at fair value 234.7 147.8 Short-term investments, at fair value 62.4 198.8 Other long-term investments 74.0 160.5 Net receivable due from subsidiaries 118.3 89.1 Other assets 4.6 4.0 Investments in consolidated and unconsolidated subsidiaries 3,944.5 3,666.9 Total assets $ 4,508.6 $ 4,267.4 Liabilities: Other liabilities $ 24.9 $ 26.9 Total liabilities 24.9 26.9 Equity: Total White Mountains’s common shareholders’ equity 4,483.7 4,240.5 Total liabilities and equity $ 4,508.6 $ 4,267.4 (1) These condensed unconsolidated financial statements reflect the results of operations, financial condition and cash flows for the Company. Investments in which White Mountains holds a controlling financial interest are accounted for using the equity method. Under the equity method, investments in subsidiaries are recorded on the condensed balance sheets at the amount of the Company’s ownership percentage of the subsidiary’s GAAP book value. The income from subsidiaries is reported on a net of tax basis as equity in earnings from consolidated and unconsolidated subsidiaries on the condensed statements of operations and comprehensive income (loss). Capital contributions to consolidated subsidiaries are presented within investing activities on the condensed statements of cash flows. Distributions from consolidated subsidiaries are presented within operating or investing activities on the condensed statements of cash flows based on the nature of the distribution. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes. SCHEDULE II (continued) CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS) (1) Year Ended December 31, Millions 2024 2023 2022 Revenues (loss) (including realized and unrealized gains and losses) $ 50.7 $ 81.7 $ (.4) Expenses 51.8 60.5 67.1 Pre-tax income (loss) (1.1) 21.2 (67.5) Income tax (expense) benefit (.8) (1.2) (.9) Net income (loss) (1.9) 20.0 (68.4) Equity in earnings (losses) from consolidated and unconsolidated subsidiaries, 232.3 489.2 (42.0) Equity in earnings from discontinued operations, net of tax - NSM Group (2) — — 16.4 Net gain (loss) from sale of discontinued operations, net of tax - NSM Group (2) — — 886.8 Net income (loss) attributable to White Mountains’s 230.4 509.2 792.8 Equity in other comprehensive income (loss) from consolidated and (.1) 1.9 (2.9) Equity in other comprehensive income (loss) from discontinued operations, — — (5.2) Net gain (loss) from foreign currency translation from sale of — — 2.9 Comprehensive income (loss) attributable to White Mountains’s common $ 230.3 $ 511.1 $ 787.6 (1) These condensed unconsolidated financial statements reflect the results of operations, financial condition and cash flows for the Company. Investments in which White Mountains holds a controlling financial interest are accounted for using the equity method. Under the equity method, investments in subsidiaries are recorded on the condensed balance sheets at the amount of the Company’s ownership percentage of the subsidiary’s GAAP book value. The income from subsidiaries is reported on a net of tax basis as equity in earnings from consolidated and unconsolidated subsidiaries on the condensed statements of operations and comprehensive income (loss). Capital contributions to consolidated subsidiaries are presented within investing activities on the condensed statements of cash flows. Distributions from consolidated subsidiaries are presented within operating or investing activities on the condensed statements of cash flows based on the nature of the distribution. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation . (2) On August 1, 2022, White Mountains closed the NSM Transaction. See Note 2 — “Significant Transactions.” As a result of the NSM Transaction, the results of operations for NSM Group have been classified as discontinued operations in the statements of operations and comprehensive income through the closing of the transaction. See Note 20 — “Held for Sale and Discontinued Operations.” Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes. SCHEDULE II (continued) CONDENSED STATEMENTS OF CASH FLOWS (1)(2) Year Ended December 31, Millions 2024 2023 2022 Net income (loss) attributable to White Mountains’s common shareholders $ 230.4 $ 509.2 $ 792.8 Charges (credits) to reconcile net income to net cash from operations: Net realized and unrealized investment (gains) losses (50.5) (78.9) 5.6 Equity in (earnings) losses from consolidated and unconsolidated subsidiaries, net of tax (232.3) (489.2) 42.0 Equity in (earnings) losses from discontinued operation, net of tax - NSM Group (2) — — (16.4) Net (gain) loss from sale of discontinued operations, net of tax - NSM Group (2) — — (886.8) Amortization of restricted share awards 16.3 15.3 14.0 Net other non-cash reconciling items (1.1) (2.6) (7.0) Distributions of accumulated earnings from consolidated subsidiaries (3) 24.2 17.3 7.0 Net other operating activities (3)(4) (17.1) 9.2 17.2 Net cash provided from (used for) operations (30.1) (19.7) (31.6) Cash flows from investing activities: Net change in short-term investments (3)(5)(6) 133.9 (63.8) 1,165.6 Purchases of investment securities (107.9) (125.0) (359.2) Maturities, calls, paydowns and sales of investment securities 100.0 335.4 15.7 Purchases of investment securities from subsidiaries (7) (12.0) — (14.0) Release of cash (pre-funding) for ILS funds managed by Elementum — 70.0 (70.0) Net issuance of debt (to) from subsidiaries (8) (224.1) (132.5) (142.0) Net repayment of debt (to) from subsidiaries (6)(8) 209.2 31.4 (15.0) Net distributions from (contributions to) subsidiaries (5)(6) (58.8) (60.3) 49.3 Proceeds from the sale of Other Operating Businesses, net of cash sold of $0.0, $0.0 and $0.5 — — 19.5 Net cash provided from (used for) investing activities 40.3 55.2 649.9 Cash flows from financing activities: Repurchases and retirement of common shares (7.9) (32.7) (615.8) Cash dividends paid to common shareholders (2.5) (2.6) (3.0) Net cash provided from (used for) financing activities (10.4) (35.3) (618.8) Net change in cash during the year (.2) .2 (.5) Cash balance at beginning of year .3 .1 .6 Cash balance at end of year $ .1 $ .3 $ .1 (1) These condensed unconsolidated financial statements reflect the results of operations, financial condition and cash flows for the Company. Investments in which White Mountains holds a controlling financial interest are accounted for using the equity method. Under the equity method, investments in subsidiaries are recorded on the condensed balance sheets at the amount of the Company’s ownership percentage of the subsidiary’s GAAP book value. The income from subsidiaries is reported on a net of tax basis as equity in earnings from consolidated and unconsolidated subsidiaries on the condensed statements of operations and comprehensive income (loss). Capital contributions to consolidated subsidiaries are presented within investing activities on the condensed statements of cash flows. Distributions from consolidated subsidiaries are presented within operating or investing activities on the condensed statements of cash flows based on the nature of the distribution. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. (2) See Note 20 — “Held for Sale and Discontinued Operations.” (3) During 2022, Bridge Holdings, Ltd. (“Bridge”), a wholly-owned subsidiary of the Company, merged into the Company. The merger was treated as a liquidation for financial statement purposes. As part of the liquidation, Bridge transferred cash of $7.0 and non-cash balances to the Company including ending net equity of $3,540.6, intercompany note receivable of $76.4, investments in its subsidiaries of $2,003.6, fixed maturity investments of $28.6, common equity securities of $8.1, other long-term investments of $52.2, short-term investments of $1,358.7 and other assets of $6.0. (4) For 2024, 2023 and 2022, net other operating activities also included $(14.4), $8.4 and $3.3 of net changes in (receivables) payables to the Company’s subsidiaries. During 2023, the Company made a non-cash payment in fixed maturity investments of $10.0 for a payable due to its wholly-owned subsidiary, White Mountains Lafayette Holdings, Inc. (5) During 2024, the Company made cash contributions to its wholly-owned subsidiaries White Mountains Investments (Bermuda), Ltd. (“WMIB”) of $4.3 and non-cash contributions of $18.7 in short-term investments and cash contributions to its wholly-owned subsidiaries WM Hinson (Bermuda) Ltd. (“WM Hinson”) of $47.0. Also during 2024, the Company made cash contributions to its subsidiary WM Webster Holdings, Inc (“WM Webster”) of $7.5 and non-cash contributions of $4.9 in short-term investments. During 2024, the Company received a $319.5 distribution from its wholly-owned subsidiary WM Adams Holdings, Inc (“WM Adams”), which included cash of $0.2, short-term investments of $17.2, 3.5% ownership of its subsidiary WM Madison, Inc (“WM Madison”) of $2.1 and 94.2% ownership of its subsidiary WM Taylor Holdings, Inc (“WM Taylor”) of $300.0. Subsequently, the Company contributed its wholly-owned subsidiary WM Webster to WM Taylor. During 2022, the Company made cash contributions to its wholly-owned subsidiaries White Mountains Investments (Bermuda), Ltd. of $51.0 and WM Adams of $25.0. Also, during 2022, the Company made non-cash contributions of $100.0 in short-term investments to WM Adams. During 2022, the Company received cash distributions of $116.3 from HG Global, $7.7 from Ark and $1.3 from its wholly-owned subsidiary, PSC Holdings, Ltd. (6) During 2024, the Company had issuances of debt of $208.9 to WM Hinson and $15.2 to HG Global. During 2024, the Company received repayments of debt of $190.0 from WM Hinson and $19.2 from HG Global. During 2023, the Company had issuances of debt of $20.0 to WM Hinson, and $7.5 to HG Global. During 2023, the Company received repayments of debt of $42.4 in cash and $107.6 in investments ($45.9 in fixed maturity investments and $61.7 short-term investments) from WM Hinson and $9.5 from HG Global. Also, during 2023, the Company made repayments of debt of $20.5 in cash and $73.8 in investments ($36.9 in fixed maturity investments and $36.9 in short-term investments) to WM Birkdale Ltd. (“WM Birkdale”), a wholly-owned subsidiaries of the Company. Also, in 2023, the Company had issuances of debt of $105.0 in cash to White Mountains Investments (Luxembourg) S.a.r.l. (“WMIL”) under a new promissory note. Subsequently, the Company contributed the promissory note to WM Birkdale. During 2022, prior to the merger of Bridge into the Company, Bridge had an issuance of debt of $69.0 to the Company. Also, during 2022, the Company had issuances of debt of $205.0 to WM Hinson and $6.0 to HG Global. (7) During 2024, the Company purchased $12.0 of fixed maturity investments, which included $8.0 from its subsidiary WMIL and $4.0 from WM (8) During 2022, prior to the merger of Bridge into the Company, the Company had a repayment of debt to Bridge of $15.0. Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

SCHEDULE III - SUPPLEMENTARY IN

SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION	12 Months Ended
	Dec. 31, 2024
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Abstract]
SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION	WHITE MOUNTAINS INSURANCE GROUP, LTD. SUPPLEMENTARY INSURANCE INFORMATION Column Column Column Column Column E Column Column Column Column Column Column Millions Deferred Future Unearned Other Policy Premiums Net Benefits, Amortization Other Operating Premiums Years ended: December 31, 2024 Ark/WM Outrigger $ 165.2 $ 2,127.5 $ 853.3 $ — $ 1,587.8 $ 90.7 $ 855.8 $ 294.1 $ 136.1 $ 1,679.1 HG Global 86.6 — 297.3 — 31.7 32.2 — 8.2 .2 56.0 Bamboo Captive 10.2 17.8 31.5 — 39.4 1.4 20.6 14.1 — 50.0 Bamboo CRV 5.8 12.1 29.0 — 32.7 .4 12.1 12.1 — 61.7 December 31, 2023 Ark/WM Outrigger 145.3 1,605.1 743.6 — 1,409.7 61.4 726.8 273.1 113.6 1,520.9 HG Global 40.1 — 325.8 — 31.2 31.7 — 8.6 .4 58.6 December 31, 2022 Ark/WM Outrigger 127.2 1,296.5 623.2 — 1,043.4 16.3 536.4 238.3 78.7 1,195.2 HG Global 36.0 — 298.3 — 33.3 21.5 — 11.2 .4 65.1 Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

SCHEDULE IV - REINSURANCE

SCHEDULE IV - REINSURANCE	12 Months Ended
	Dec. 31, 2024
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Abstract]
SCHEDULE IV - REINSURANCE	WHITE MOUNTAINS INSURANCE GROUP, LTD. REINSURANCE Column A Column B Column C Column D Column E Column F $ in Millions Premiums Earned Gross Amount Ceded to Other Assumed from Net Amount Percentage of Year ended: December 31, 2024 Ark/WM Outrigger $ 1,004.2 $ (504.6) $ 1,088.2 $ 1,587.8 68.5 % HG Global 15.8 — 15.9 31.7 50.2 Bamboo Captive — (4.0) 43.4 39.4 110.2 Bamboo CRV — — 32.7 32.7 100.0 December 31, 2023 Ark/WM Outrigger 848.1 (364.0) 925.6 1,409.7 65.7 HG Global 28.7 — 2.5 31.2 8.0 December 31, 2022 Ark/WM Outrigger 655.5 (280.8) 668.7 1,043.4 64.1 HG Global 28.6 — 4.7 33.3 14.1 Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

SCHEDULE V - VALUATION AND QUAL

SCHEDULE V - VALUATION AND QUALIFYING ACCOUNTS	12 Months Ended
	Dec. 31, 2024
SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract]
SCHEDULE V - VALUATION AND QUALIFYING ACCOUNTS	WHITE MOUNTAINS INSURANCE GROUP, LTD. VALUATION AND QUALIFYING ACCOUNTS Column A Column B Column C Column D Column E Additions (subtractions) Millions Balance at beginning of Charged to costs Charged to other Deductions described (1) Balance at end of Years ended: Ark/WM Outrigger December 31, 2024 Reinsurance recoverables: Allowance for uncollectible balances $ 7.7 $ 2.5 $ — $ — $ 10.2 December 31, 2023 Reinsurance recoverables: Allowance for uncollectible balances 7.8 (.2) — .1 7.7 December 31, 2022 Reinsurance recoverables: Allowance for uncollectible balances 3.6 4.6 — (.4) 7.8 (1) Represents net collections (charge-offs) of balances receivable and foreign currency translation.

SCHEDULE VI - SUPPLEMENTAL INFO

SCHEDULE VI - SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS	12 Months Ended
	Dec. 31, 2024
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Abstract]
SCHEDULE VI - SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS	WHITE MOUNTAINS INSURANCE GROUP, LTD. SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS (Millions) Column A Column B Column C Column D Column E Column F Column G Column H Column I Column J Column K Deferred acquisition Reserves Discount, if any, deducted in Unearned Earned Net investment Claims and Claims Amortization Paid Net Premiums Affiliation Current Prior Ark/WM Outrigger 2024 $ 165.2 $ 2,127.5 $ — $ 853.3 $ 1,587.8 $ 90.7 $ 914.4 $ (58.6) $ 294.1 $ 416.7 $ 1,679.1 2023 145.3 1,605.1 — 743.6 1,409.7 61.4 706.9 19.9 273.1 407.9 1,520.9 2022 127.2 1,296.5 — 623.2 1,043.4 16.3 588.1 (51.7) 238.3 257.5 1,195.2 Bamboo Captive 2024 $ 10.2 $ 17.8 $ — $ 31.5 $ 39.4 $ 1.4 $ 17.3 $ 3.3 $ 14.1 $ 12.0 $ 50.0 2023 — — — — — — — — — — — 2022 — — — — — — — — — — — Bamboo CRV 2024 $ 5.8 $ 12.1 $ — $ 29.0 $ 32.7 $ .4 $ 12.1 $ — $ 12.1 $ — $ 61.7 2023 — — — — — — — — — — — 2022 — — — — — — — — — — —

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Pay vs Performance Disclosure
Net income (loss) attributable to White Mountains’s common shareholders	$ 230.4	$ 509.2	$ 792.8

Insider Trading Arrangements

Insider Trading Arrangements	3 Months Ended
	Dec. 31, 2024
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false

Insider Trading Policies and Pr

Insider Trading Policies and Procedures	12 Months Ended
	Dec. 31, 2024
Insider Trading Policies and Procedures [Line Items]
Insider Trading Policies and Procedures Adopted	true

Cybersecurity Risk Management a

Cybersecurity Risk Management and Strategy Disclosure	12 Months Ended
	Dec. 31, 2024
Cybersecurity Risk Management, Strategy, and Governance [Line Items]
Cybersecurity Risk Management Processes for Assessing, Identifying, and Managing Threats [Text Block]	Identifying, assessing and managing cybersecurity risks is an important component of White Mountains’s overall enterprise risk management program. As with the management of risks generally, given our holding company structure, the management of cybersecurity risks involves coordination between the Company and its consolidated subsidiaries/affiliates. The Company and each of its consolidated subsidiaries/affiliates are responsible for developing a cybersecurity program appropriate for their respective businesses. The design of these cybersecurity programs is informed by the ISO 27001 standards and the Center for Internet Security Critical Security Controls framework (“CISCSC”). This does not imply that these programs meet all specifications of ISO 27001 and CISCSC, but rather that we use them as a guide to help us identify, assess and manage cybersecurity risks relevant to our business. The cybersecurity programs developed by the Company and its consolidated subsidiaries/affiliates include, among other things, (i) advanced threat protection and detection systems; (ii) vulnerability scanning and testing of network defenses; (iii) user authentication, role-based access and privileged access management; (iv) data encryption, loss prevention, backup and recovery mechanisms; (v) employee testing and training; (vi) technical and business team-focused incident response tabletop exercises; (vii) disaster recovery testing and (viii) security assessments of third-party service providers. White Mountains engages both its internal auditors and third-party information security experts to assist management in assessing the effectiveness of these cybersecurity programs. Risks from cybersecurity threats may cause material disruptions to our operations and reputational harm, which could materially adversely affect our results of operations and financial condition. See Item 1.A Risk Factors, “We may be unable to adequately maintain our systems and safeguard the security of our data, which could adversely impact our ability to operate our business and cause reputational harm and, consequently, could materially adversely affect our results of operations and financial condition.” on page 41
Cybersecurity Risk Management Processes Integrated [Flag]	true
Cybersecurity Risk Management Processes Integrated [Text Block]	Identifying, assessing and managing cybersecurity risks is an important component of White Mountains’s overall enterprise risk management program. As with the management of risks generally, given our holding company structure, the management of cybersecurity risks involves coordination between the Company and its consolidated subsidiaries/affiliates.
Cybersecurity Risk Management Third Party Engaged [Flag]	true
Cybersecurity Risk Third Party Oversight and Identification Processes [Flag]	true
Cybersecurity Risk Materially Affected or Reasonably Likely to Materially Affect Registrant [Flag]	false
Cybersecurity Risk Board of Directors Oversight [Text Block]	The Company’s Board of Directors has assigned oversight of White Mountains’s cybersecurity risk management to the Audit Committee. The Audit Committee receives periodic reports on White Mountains’s cybersecurity risks and any material cybersecurity incidents at the direction of White Mountains’s senior management. In addition, the Audit Committee receives reports addressing cybersecurity risks as part of the Company’s overall enterprise risk management program.
Cybersecurity Risk Board Committee or Subcommittee Responsible for Oversight [Text Block]	The Company’s Board of Directors has assigned oversight of White Mountains’s cybersecurity risk management to the Audit Committee. The Audit Committee receives periodic reports on White Mountains’s cybersecurity risks and any material cybersecurity incidents at the direction of White Mountains’s senior management. In addition, the Audit Committee receives reports addressing cybersecurity risks as part of the Company’s overall enterprise risk management program.
Cybersecurity Risk Process for Informing Board Committee or Subcommittee Responsible for Oversight [Text Block]	The Audit Committee receives periodic reports on White Mountains’s cybersecurity risks and any material cybersecurity incidents at the direction of White Mountains’s senior management. In addition, the Audit Committee receives reports addressing cybersecurity risks as part of the Company’s overall enterprise risk management program.
Cybersecurity Risk Role of Management [Text Block]	White Mountains’s Information Technology (“IT”) Steering Committee, which includes its Chief Information Security Officer, Chief Technology Officer and various members of senior management, as well as the senior IT leadership at each of its consolidated subsidiaries/affiliates are responsible for assessing and managing White Mountains’s cybersecurity risk. These individuals include IT and cybersecurity professionals with relevant education, including degrees and/or certifications, and prior work experience. These individuals also monitor the prevention, detection, mitigation and remediation of cybersecurity incidents as part of the cybersecurity programs described above.
Cybersecurity Risk Management Positions or Committees Responsible [Flag]	true
Cybersecurity Risk Management Positions or Committees Responsible [Text Block]	White Mountains’s Information Technology (“IT”) Steering Committee, which includes its Chief Information Security Officer, Chief Technology Officer and various members of senior management, as well as the senior IT leadership at each of its consolidated subsidiaries/affiliates are responsible for assessing and managing White Mountains’s cybersecurity risk. These individuals include IT and cybersecurity professionals with relevant education, including degrees and/or certifications, and prior work experience. These individuals also monitor the prevention, detection, mitigation and remediation of cybersecurity incidents as part of the cybersecurity programs described above.
Cybersecurity Risk Management Expertise of Management Responsible [Text Block]	White Mountains’s Information Technology (“IT”) Steering Committee, which includes its Chief Information Security Officer, Chief Technology Officer and various members of senior management, as well as the senior IT leadership at each of its consolidated subsidiaries/affiliates are responsible for assessing and managing White Mountains’s cybersecurity risk. These individuals include IT and cybersecurity professionals with relevant education, including degrees and/or certifications, and prior work experience.
Cybersecurity Risk Process for Informing Management or Committees Responsible [Text Block]	The Audit Committee receives periodic reports on White Mountains’s cybersecurity risks and any material cybersecurity incidents at the direction of White Mountains’s senior management. In addition, the Audit Committee receives reports addressing cybersecurity risks as part of the Company’s overall enterprise risk management program.
Cybersecurity Risk Management Positions or Committees Responsible Report to Board [Flag]	true

Basis of Presentation and Sig_2

Basis of Presentation and Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2024
Accounting Policies [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. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation.
Consolidation Principles	Consolidation Principles Under GAAP, the Company is required to consolidate any entity in which it holds a controlling financial interest. A controlling financial interest is usually in the form of an investment representing the majority of the subsidiary’s voting interests. However, a controlling financial interest may also arise from a financial interest in a variable interest entity (“VIE”) through arrangements that do not involve ownership of voting interests. The Company consolidates a VIE if it determines that it is the primary beneficiary. The primary beneficiary is defined as the entity who holds a variable interest that gives it both the power to direct the VIE’s activities that most significantly impact its economic performance and the obligation to absorb losses of, or the right to receive returns from, the VIE that could potentially be significant to the VIE. See Note 16 — “Variable Interest Entities.”
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Reportable Segments	Reportable Segments As of December 31, 2024, White Mountains conducted its operations through four reportable segments: (1) Ark/WM Outrigger, (2) HG Global, (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 15 — “ 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, casualty and accident & health. Ark underwrites select coverages through Lloyd’s Syndicates 4020 and 3902 and Additional Central Settlement Number (“ACSN”) 3832 (collectively, 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 December 31, 2024 and 2023, White Mountains owned 72.1% and 72.0% of Ark on a basic shares outstanding basis (61.9% and 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 (“MOIC”) return. If fully earned, these shares would represent an additional 12.3% of the shares outstanding as of December 31, 2024. 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 collateralized reinsurance protection on Ark’s Bermuda global property catastrophe excess of loss portfolio written in the 2023 underwriting year. Ark renewed its quota share reinsurance agreement with Outrigger Re Ltd. for the 2024 and 2025 underwriting years. White Mountains consolidates the results of its segregated account, WM Outrigger Re, in its financial statements. See Note 2 — “Significant Transactions.” As of December 31, 2024 and 2023, White Mountains owned 100.0% of WM Outrigger Re’s preferred equity. The HG Global segment consists of HG Global Ltd. and its wholly-owned subsidiaries (collectively, “HG Global”) and, prior to its deconsolidation on July 1, 2024, the consolidated results of Build America Mutual Assurance Company (“BAM”). See Note 2 — “Significant Transactions.” HG Global was established to fund the startup of BAM and, through its reinsurance subsidiary, HG Re Ltd. (“HG Re”), to provide first-loss reinsurance protection of up to 15%-of-par outstanding for each policy assumed from 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 December 31, 2024 and 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, through June 30, 2024, White Mountains was required to consolidate BAM’s results in its financial statements because BAM is a VIE for which White Mountains was the primary beneficiary. BAM’s results were all attributed to noncontrolling interests. On July 1, 2024, HG Re and BAM amended the terms of the first-loss reinsurance treaty (“FLRT”) with respect to certain governance rights held by HG Re. As a result, and in combination with other governance changes at BAM, White Mountains concluded that it no longer has the power to direct BAM’s activities that most significantly impact its economic performance and is no longer BAM’s primary beneficiary. Accordingly, as of July 1, 2024, White Mountains no longer consolidates BAM. Through June 30, 2024, BAM’s assets, liabilities and noncontrolling interests, as well as its results of operations, are presented within the HG Global segment. See Note 2 — “Significant Transactions.” 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 December 31, 2024 and 2023, White Mountains owned 90.4% and 89.6% of Kudu’s basic units outstanding (77.0% and 76.3% on a fully-diluted/fully-converted basis, taking account of management’s equity incentives). The Bamboo segment consists of PM Holdings LLC (“Bamboo Holdings”), Bamboo Ide8 Insurance Services LLC (“Bamboo MGA”) and Ide8 Re Inc. (“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 carrier partners (“Capacity Providers”), 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. Bamboo MGA offers both admitted and non-admitted products. Under its existing capacity agreements for the treaty year ending April 1, 2025, Bamboo MGA’s commission levels are based on a sliding scale tied primarily to its attritional loss ratio. Bamboo also operates two separate but integrated businesses: (i) a retail agency, within Bamboo MGA, offering ancillary products (e.g., flood, earthquake) on behalf of third parties and (ii) Bamboo Captive, a U.S.-domiciled captive reinsurer that participates in the underwriting risk of Bamboo’s MGA programs to align interests with Capacity Partners. During the fourth quarter of 2024, Bamboo Captive redomiciled from Bermuda to Arizona and changed its name from Ide8 Limited to Ide8 Re Inc. On January 2, 2024, White Mountains acquired a controlling interest in Bamboo. See Note 2 — “Significant Transactions.” As of December 31, 2024, White Mountains owned 72.8% of the basic units outstanding of Bamboo (63.7% on a fully-diluted/fully-converted basis, taking account of management’s equity incentives).
Held for Sale and Discontinued Operations	Held for Sale and Discontinued Operations White Mountains recognizes assets and liabilities classified as held for sale at the lower of carrying value on the date the asset is initially classified as held for sale or fair value less costs to sell. At the time of reclassification to held for sale, White Mountains ceases recognizing depreciation and amortization on assets held for sale. The results of operations of a business that has either been disposed of or are classified as held for sale are reported in discontinued operations if the disposal of business represents a strategic shift that has (or will have) a major effect on White Mountains’s operations and financial results. On August 1, 2022, White Mountains Holdings (Luxembourg) S.à r.l. (“WTM Holdings Seller”), an indirect wholly-owned subsidiary of White Mountains, completed the sale of White Mountains Catskill Holdings, Inc. and NSM Insurance HoldCo, LLC (“NSM”) (collectively with White Mountains Catskill Holdings, Inc., the “NSM Group”) to Riser Merger Sub, Inc., an affiliate of The Carlyle Group Inc. (the “NSM Transaction”), pursuant to the terms of the securities purchase agreement, dated as of May 9, 2022. See Note 2 — “Significant Transactions.” NSM is a full-service MGA and program administrator with delegated binding authorities for specialty property and casualty insurance. As a result of the NSM Transaction, the results of operations for NSM Group have been classified as discontinued operations in the statements of operations and comprehensive income through the closing of the transaction. See Note 20 — “Held for Sale and Discontinued Operations.”
Investment Securities and Other Long-Term Investments	Investment Securities As of December 31, 2024 and 2023, White Mountains’s invested assets consisted of securities and other investments held for general investment purposes. 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. Short-term investments consist of interest-bearing money market funds, certificates of deposit and other fixed maturity securities, which at the time of purchase, mature or become available for use within one year. Short-term investments are carried at fair value, which approximated amortized or accreted cost as of December 31, 2024 and 2023. Other long-term investments consist primarily of unconsolidated entities, including noncontrolling equity interests in the form of revenue and earnings participation contracts held at Kudu (“Participation Contracts”), private equity funds and hedge funds, a bank loan fund, Lloyd’s of London (“Lloyd’s”) trust deposits, insurance-linked securities (“ILS”) funds and private debt instruments. 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. White Mountains’s portfolio of common equity securities, its investment in MediaAlpha and other long-term investments are measured at fair value. White Mountains has taken the fair value option for its equity method eligible investments. Changes in net unrealized investment gains (losses) are reported pre-tax in revenues. Realized investment gains (losses) are accounted for using the specific identification method and are reported pre-tax in revenues. Premiums and discounts on all fixed maturity investments are amortized and accreted to income over the anticipated life of the investment. Income on mortgage and asset-backed securities is recognized using an effective yield based on anticipated prepayments and the estimated economic life of the securities. When actual prepayments differ significantly from anticipated prepayments, the estimated economic life is recalculated, and the remaining unamortized premium or discount is amortized prospectively over the remaining economic life. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants (an exit price) at a particular measurement date. 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 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”). Assets and liabilities carried at fair value include White Mountains’s investment portfolio and derivative instruments. Valuation of assets and liabilities measured at fair value require management to make estimates and apply judgment to matters that may carry a significant degree of uncertainty. In determining its estimates of fair value, White Mountains uses a variety of valuation approaches and inputs. Whenever possible, White Mountains estimates fair value using valuation methods that maximize the use of quoted market prices or other observable inputs. Where appropriate, assets and liabilities measured at fair value have been adjusted for the effect of counterparty credit risk. White Mountains uses outside pricing services and brokers to assist in determining fair values. The outside pricing services White Mountains uses have indicated that they will only provide prices where observable inputs are available. Level 1 Measurements Investments valued using Level 1 inputs include White Mountains’s fixed maturity investments, primarily investments in U.S. Treasuries and short-term investments, which include U.S. Treasury Bills, common equity securities and its investment in MediaAlpha. For investments in active markets, White Mountains uses the quoted market prices provided by outside pricing services to determine fair value. Level 2 Measurements Investments valued using Level 2 inputs include fixed maturity investments which have been disaggregated into classes, including debt securities issued by corporations, municipal obligations, foreign government and agency obligations, mortgage and asset-backed securities and collateralized loan obligations. Investments valued using Level 2 inputs also include certain international listed common equity funds, which White Mountains values using the fund manager’s published net asset value (“NAV”) to account for the difference in market exchange close times, and certain derivative instruments. In circumstances where quoted market prices are unavailable or are not considered reasonable, White Mountains estimates the fair value using industry standard pricing methodologies and observable inputs such as benchmark yields, reported trades, broker-dealer quotes, issuer spreads, benchmark securities, bids, offers, credit ratings, prepayment speeds, reference data including research publications and other relevant inputs. Given that many fixed maturity investments do not trade on a daily basis, the outside pricing services evaluate a wide range of fixed maturity investments by regularly drawing parallels from recent trades and quotes of comparable securities with similar features. The characteristics used to identify comparable fixed maturity investments vary by asset type and take into account market convention. White Mountains’s process to assess the reasonableness of the market prices obtained from the outside pricing sources covers substantially all of its fixed maturity investments and includes, but is not limited to, the evaluation of pricing methodologies and a review of the pricing services’ quality control procedures on at least an annual basis, a comparison of invested asset prices obtained from alternate independent pricing vendors on at least a semi-annual basis, monthly analytical reviews of certain prices and a review of the underlying assumptions utilized by the pricing services for select measurements on an ad hoc basis throughout the year. White Mountains also performs back-testing of selected investment sales activity to determine whether there are any significant differences between the market price used to value the security prior to sale and the actual sale price of the security on an ad-hoc basis throughout the year. Prices provided by the pricing services that vary by more than $0.5 million and 5% from the expected price based on these assessment procedures are considered outliers, as are prices that have not changed from period to period and prices that have trended unusually compared to market conditions. In circumstances where the results of White Mountains’s review process does not appear to support the market price provided by the pricing services, White Mountains challenges the vendor provided price. If White Mountains cannot gain satisfactory evidence to support the challenged price, White Mountains will rely upon its own internal pricing methodologies to estimate the fair value of the security in question. The valuation process described above is generally applicable to all of White Mountains’s fixed maturity investments. The techniques and inputs specific to asset classes within White Mountains’s fixed maturity investments for Level 2 securities that use observable inputs are as follows: Debt Securities Issued by Corporations The fair value of debt securities issued by corporations is determined from a pricing evaluation technique that uses information from market sources and integrates relative credit information, observed market movements and sector news. Key inputs include benchmark yields, reported trades, broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers and reference data including sector, coupon, credit quality ratings, duration, credit enhancements, early redemption features and market research publications. Municipal Obligations The fair value of municipal obligations is determined from a pricing evaluation technique that uses information from market makers, brokers-dealers, buy-side firms and analysts along with general market information. Key inputs include benchmark yields, reported trades, issuer financial statements, material event notices and new issue data, as well as broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers and reference data including type, coupon, credit quality ratings, duration, credit enhancements, geographic location and market research publications. Foreign Government and Agency Obligations The fair value of foreign government and agency obligations is determined from a pricing evaluation technique that uses information from data sources in each respective country, including active market makers and inter-dealer brokers. Key inputs include benchmark yields, reported trades, broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, local exchange prices, foreign exchange rates and reference data including coupon, credit quality ratings, duration and market research publications. Mortgage and Asset-Backed Securities and Collateralized Loan Obligations The fair value of mortgage and asset-backed securities and collateralized loan obligations is determined from a pricing evaluation technique that uses information from market sources and leveraging similar securities. Key inputs include benchmark yields, reported trades, underlying tranche cash flow data, collateral performance, plus new issue data, as well as broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers and reference data including issuer, vintage, loan type, collateral attributes, prepayment speeds, default rates, recovery rates, cash flow stress testing, credit quality ratings and market research publications. Level 3 Measurements Fair value estimates for investments that trade infrequently and have few or no quoted market prices or other observable inputs are classified as Level 3 measurements. Investments valued using Level 3 fair value estimates are based upon unobservable inputs and include investments in certain fixed maturity investments, common equity securities and other long-term investments where quoted market prices or other observable inputs are unavailable or are not considered reliable or reasonable. Level 3 valuations are generated from techniques that use assumptions not observable in the market. These unobservable inputs reflect White Mountains’s assumptions of what market participants would use in valuing the investment. In certain circumstances, investment securities may start out as Level 3 when they are originally issued, but as observable inputs become available in the market, they may be reclassified to Level 2. Transfers of securities between levels are based on investments held as of the beginning of the period. Other Long-Term Investments As of December 31, 2024, $1,262.7 million of White Mountains’s other long-term investments, which consisted primarily of unconsolidated entities, including Kudu’s Participation Contracts and PassportCard/DavidShield were classified as Level 3 investments in the GAAP fair value hierarchy. The determination of the fair value of these securities involves significant management judgment, and the use of valuation analyses and assumptions that are inherently subjective and uncertain. White Mountains may use a variety of valuation techniques to determine fair value depending on the nature of the investment, including a discounted cash flow analysis, market multiple approach, cost approach and/or liquidation analysis. On an ongoing basis, White Mountains also considers qualitative changes in facts and circumstances, which may impact the valuation of its unconsolidated entities, including economic and market changes in relevant industries, changes to the entity’s capital structure, business strategy and key personnel and any recent transactions. On a quarterly basis, White Mountains evaluates the most recent qualitative and quantitative information of the business and completes a fair valuation analysis for all other long-term investments classified as Level 3 investments. Periodically, and at least on an annual basis, White Mountains uses a third-party valuation firm to complete an independent valuation analysis of significant unconsolidated entities. Other Long-term Investments - NAV As of December 31, 2024, $864.0 million of White Mountains’s other long-term investments, which consisted of private equity funds and hedge funds, a bank loan fund, Lloyd’s trust deposits and ILS funds, were valued at fair value using NAV as a practical expedient. Investments for which fair value is measured using NAV as a practical expedient are not classified within the fair value hierarchy. White Mountains employs a number of procedures to assess the reasonableness of the fair value measurements for other long-term investments measured at NAV, including obtaining and reviewing interim unaudited and annual audited financial statements as well as periodically discussing the valuations and methodologies used to value the underlying investments of each fund with fund managers. However, since fund managers do not provide sufficient information to evaluate the pricing methods and inputs for each underlying investment, White Mountains considers the valuation inputs to be unobservable. The fair value of White Mountains’s other long-term investments measured at NAV are generally determined using the fund manager’s NAV. Certain other long-term investments measured at NAV may be reported to White Mountains on a lag. In these circumstances, the NAV of such investments is adjusted to account for any applicable contributions, distributions, known transactions and/or pricing estimates provided by the fund manager through White Mountains’s reporting date.
BAM Surplus Notes	BAM Surplus Notes Prior to the deconsolidation of BAM on July 1, 2024, the BAM Surplus Notes, including accrued interest receivable, were classified as intercompany notes carried at nominal value, which eliminated in consolidation. Upon deconsolidation, White Mountains elected the fair value option for the BAM Surplus Notes. White Mountains values the BAM Surplus Notes each quarter using a discounted cash flow analysis. The BAM Surplus Notes are classified as a Level 3 measurement. Upon election of the fair value option, White Mountains made the accounting policy election to present the interest income from the BAM Surplus Notes separately from the change in fair value of the BAM Surplus Notes. See Note 10 — “Municipal Bond Guarantee Reinsurance.”
Cash and Restricted Cash	Cash and Restricted Cash Cash includes amounts on hand and demand deposits with banks and other financial institutions. Amounts presented in the statement of cash flows are shown net of balances acquired and sold in the purchase or sale of the Company’s consolidated subsidiaries. Cash balances that are not immediately available for general corporate and operating purposes are classified as restricted.
Derivatives	Derivatives From time to time, White Mountains holds derivative financial instruments for risk management purposes. White Mountains recognizes all derivatives as either assets or liabilities, measured at fair value, on its consolidated balance sheet. Changes in the fair value of derivative instruments that meet the criteria for hedge accounting are recognized in other comprehensive income (loss) and reclassified into current period pre-tax income (loss) when the hedged items are recognized therein. Changes in the fair value of derivative instruments that do not meet the criteria for hedge accounting are recognized in current period pre-tax income (loss). As of December 31, 2024, White Mountains holds interest rate cap derivative instruments that do not meet the criteria for hedge accounting. See Note 9 — “Derivatives.”
Property and Casualty Insurance and Reinsurance	Property and Casualty Insurance and Reinsurance Ark writes a diversified portfolio of reinsurance and insurance products, including property, specialty, marine & energy, casualty and accident & health. Ark accounts for insurance and reinsurance policies that it writes in accordance with ASC 944, Financial Services - Insurance . Ark’s premiums written comprise premiums on insurance contracts incepted during the year as well as premium adjustments related to prior underwriting years. Insurance premiums are recognized as revenues over the loss exposure or coverage period. In most cases, premiums are earned ratably over the term of the contract, with unearned premiums calculated on a monthly pro-rata basis. Catastrophe premiums are earned in proportion to the insurance protection provided. Premiums earned are presented net of amounts ceded to reinsurers. Insurance premiums receivable, representing amounts due from insureds, are presented net of an allowance for uncollectible premiums, including expected credit losses. The allowance is based upon Ark’s ongoing review of amounts outstanding, historical loss data, including delinquencies and write-offs, current and forecasted economic conditions and other relevant factors. Credit risk is partially mitigated by Ark’s ability to cancel the policy if the policyholder does not pay the premium. Deferred acquisition costs comprise commission and brokerage fees and taxes which are directly attributable to and vary with the production of business. These costs are deferred and amortized to the extent they relate to successful contract acquisitions over the applicable premium recognition period as insurance and reinsurance acquisition expenses. Deferred acquisition costs are limited to the amount expected to be recovered from future earned premiums and anticipated investment income. A premium deficiency is recognized if the sum of expected loss and loss adjustment expenses (“LAE”), expected dividends to policyholders, unamortized acquisition costs and maintenance costs exceeds related unearned premiums and anticipated investment income. A premium deficiency is recognized by charging any unamortized acquisition costs to expense to the extent required in order to eliminate the deficiency. If the premium deficiency exceeds unamortized acquisition costs, then a liability is accrued for the excess deficiency. Loss and LAE are charged against income as incurred. Loss and LAE reserves, including estimates for amounts incurred but not reported (“IBNR”), are based on management’s best estimate of the ultimate costs to settle claims, including the effects of inflation and other societal and economic factors. Loss and LAE reserves are presented net of estimated salvage and subrogation recoveries, if applicable. Such estimates are regularly reviewed and updated, and any resulting adjustments are reflected in current results of operations. The process of estimating loss and LAE reserves involves a considerable degree of judgment by management, and the ultimate amount of expense to be incurred could be considerably greater than or less than the amounts currently reflected in the financial statements. See Note 5 — “Loss and Loss Adjustment Expense Reserves.” As part of its enterprise risk management function, Ark purchases reinsurance for risk mitigation purposes. Ark utilizes reinsurance and retrocessional agreements to reduce earnings volatility, protect capital, limit concentration and accumulation risk and manage within its overall internal risk tolerances and/or those set and agreed by regulators, ratings agencies and Lloyd’s. Ark also enters into reinsurance and retrocessional agreements to reduce its exposures on individual risks and enable it to underwrite policies with higher limits where Ark believes this has a broader business benefit. Ark seeks to protect its downside risk from catastrophes and large loss events by purchasing reinsurance, including excess of loss protections, aggregate covers and industry loss warranties. Ark also considers alternative structures such as collateralized reinsurance, retrocessional reinsurance and catastrophe bonds. The purchase of reinsurance does not discharge Ark from its primary liability for the full value of its policies, and thus the collectability of balances due from Ark’s reinsurers is critical to its financial strength. Ark monitors the financial strength and ratings of its reinsurers on an ongoing basis. See Note 6 — “Third-Party Reinsurance.” Reinsurance recoverables represent paid loss and LAE, case reserves and IBNR reserves ceded to reinsurers under reinsurance treaties. Amounts recoverable from reinsurers are estimated in a manner consistent with the associated claims liability. Ark reports its reinsurance recoverables net of an allowance for estimated uncollectible reinsurance, including expected credit losses and coverage disputes. The allowance is based upon Ark’s ongoing review of amounts outstanding, length of collection periods, changes in reinsurer credit standing, disputes, applicable coverage defenses and other relevant factors. For the years of account prior to the Ark Transaction, a significant proportion of the Syndicates’ underwriting capital was provided by third-party insurance and reinsurance groups (“TPC Providers”) using whole account reinsurance contracts with Ark’s corporate member. For the years of account subsequent to the Ark Transaction, Ark is no longer using TPC Providers to provide underwriting capital for the Syndicates. Captions within results of operations and other comprehensive income are shown net of amounts relating to the TPC Providers share of the Syndicates’ results, including investment results.
Municipal Bond Guarantee Reinsurance	Municipal Bond Guarantee Reinsurance As of July 1, 2024, White Mountains no longer consolidates BAM. Through June 30, 2024, BAM’s assets, liabilities and noncontrolling interests, as well as its results of operations, are presented within the HG Global segment. The FLRT and all the contracts issued by BAM are accounted for as insurance contracts under ASC 944-605, Financial Guarantee Insurance Contracts. For capital appreciation bonds, par is adjusted to the estimated equivalent par value for current interest paying bonds. Prior to the deconsolidation of BAM, HG Re’s reinsurance balances under the FLRT eliminated in White Mountains’s consolidated financial statements. See Note 10 — “Municipal Bond Guarantee Reinsurance.” Premiums are generally received upfront and an unearned premium reserve, equal to the amount of the premium received, is established at contract inception. Premiums are recognized in revenue over the period of the contracts in proportion to the amount of insurance protection provided using a constant rate. The constant rate is calculated based on the relationship between the par outstanding in a given reporting period compared with the sum of each of the par amounts outstanding for all periods. HG Re’s insurance premiums receivable represents amounts due from BAM. BAM’s insurance premiums receivable represents amounts due from customers for municipal bond insurance policies. Installment premiums are measured at the present value of contractual premiums, discounted at the risk-free rate, which is set at the inception of the insurance contract. HG Re’s deferred acquisition costs represent the ceding commission paid to BAM. BAM’s deferred acquisition costs represent commissions, premium taxes, excise taxes and other costs which are directly attributable to and vary with the production of business. These costs are deferred and amortized to the extent they relate to successful contract acquisitions over the applicable premium recognition period as acquisition expenses. Deferred acquisition costs are limited to the amount expected to be recovered from future earned premiums and anticipated investment income. A premium deficiency is recognized if the sum of expected loss and LAE, expected dividends to policyholders, unamortized acquisition costs and maintenance costs exceeds related unearned premiums and anticipated investment income. A premium deficiency is recognized by charging any unamortized acquisition costs to expense to the extent required in order to eliminate the deficiency. If the premium deficiency exceeds unamortized acquisition costs, then a liability is accrued for the excess deficiency. Loss reserves are recorded only to the extent that the present value of any payments projected to be made, net of any expected recoveries, exceeds the associated unearned premium reserve. See Note 5 — “Loss and Loss Adjustment Expense Reserves.”
Reinsurance Contracts Accounted for as Deposits	Reinsurance Contracts Accounted for as Deposits Reinsurance contracts that do not meet the risk transfer requirements necessary to be accounted for as reinsurance are accounted for using the deposit method. Under the deposit method, ceded premiums paid are not recognized through income but rather treated as a deposit. 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. Effective in January 2025, the SiriusPoint contract was commuted. See Note 6 — “Third-Party Reinsurance. HG Re is party to an excess of loss reinsurance agreement (the “XOLT”) with BAM. The XOLT is accounted for using deposit accounting as the agreement does not meet the risk transfer requirements necessary to be accounted for as reinsurance. Accordingly, any financing revenues related to the XOLT are recorded in other revenues. See Note 10 — “Municipal Bond Guarantee Reinsurance.”
Revenue Recognition	Revenue Recognition Kudu’s revenues are primarily generated from Participation Contracts, which are noncontrolling equity interests in revenue and earnings participation contracts with asset and wealth management firms. Kudu’s Participation Contracts are measured at fair value with the change therein recognized within net realized and unrealized investment gains (losses). Distributions from Kudu’s investees are recognized through investment income when Kudu’s right to receive payment has been established and can be reliably measured, which generally occurs on a quarterly basis in accordance with the terms of the Participation Contracts. 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. 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. White Mountains’s Other Operations recognizes agent commissions and other revenues when it has satisfied its performance obligations.
Broker Commission Expenses and Cost of Sales	Broker Commission Expenses and Cost of Sales Bamboo’s broker commission expenses consist 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. White Mountains’s Other Operations’ cost of sales consists of salaries and related expenses, professional services and marketing and advertising expenses directly related to sales generation. These expenses are recognized as incurred.
Incentive Compensation	Incentive Compensation White Mountains’s Long-Term Incentive Plan (the “WTM Incentive Plan”) provides for grants of various types of share-based and non-share-based incentive awards to key employees of White Mountains. Non-share-based awards are recognized over the related service periods based on management’s best estimate of the amounts at which the awards are expected to be paid. Share-based compensation which is typically settled in cash, such as performance shares, is classified as a liability-type award. The compensation cost for liability-classified awards is measured initially at the grant date fair value and remeasured each reporting period until settlement. The compensation cost for equity-classified awards expected to be settled in shares, such as options and restricted shares, is measured at the original grant date fair value of the award. The compensation cost for all awards is recognized for the vested portion of the awards over the related service periods. See Note 12 — “Employee Share-Based Incentive Compensation Plans.”
Income Taxes	Income Taxes White Mountains has subsidiaries and branches that operate in various jurisdictions around the world and are subject to tax in the jurisdictions in which they operate. As of December 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. Income earned or losses generated by companies outside the United States are generally subject to an overall effective tax rate lower than that imposed by the United States. Deferred tax assets and liabilities are recorded when a difference between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts for tax purposes exists, or there are other temporary differences. The deferred tax asset or liability is recorded based on tax rates expected to be in effect when the difference reverses. Deferred tax assets represent amounts available to reduce income taxes payable in future periods. 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. See Note 8 — “Income Taxes.”
Leases	Leases White Mountains has entered into lease agreements, primarily for office space and equipment. These leases are classified as operating leases, with lease expense recognized on a straight-line basis over the term of the lease. Lease assets and liabilities are not recorded for leases with a term at inception of one year or less. Lease incentives, such as free rent or landlord reimbursements for leasehold improvements, are recognized at lease inception and amortized on a straight-line basis over the term of the lease. Lease expense and the amortization of leasehold improvements are recognized within general and administrative expenses. Lease payments related to options to extend or renew the lease term are excluded from the calculation of lease liabilities unless White Mountains is reasonably certain of exercising those options. right-of-use (“ROU”) assets lease liabilities
Noncontrolling Interests	Noncontrolling Interests Noncontrolling interests consist of the ownership interests of noncontrolling shareholders in consolidated subsidiaries and are presented separately on the balance sheet. The portion of comprehensive income (loss) attributable to noncontrolling interests is presented net of related income taxes in the statement of operations and comprehensive income (loss). See Note 13 — “Common Shareholders’ Equity and Noncontrolling Interests.”
Foreign Currency Exchange	Foreign Currency Exchange The functional currencies for White Mountains’s non-U.S. based subsidiaries are measured, in most instances, using currencies other than the U.S. dollar. Net foreign exchange gains and losses arising from the translation of functional currencies are generally reported in shareholders’ equity, in accumulated other comprehensive income (loss). White Mountains also invests in securities denominated in foreign currencies. Assets and liabilities recorded in these foreign currencies are translated into U.S. dollars at exchange rates in effect at the balance sheet date, and revenues and expenses are converted using the weighted average exchange rates for the period. As of December 31, 2024 and 2023, White Mountains had unrealized foreign currency translation losses of $1.7 million and $1.6 million recorded in accumulated other comprehensive income (loss) on its consolidated balance sheet.
Fair Value Measurements	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 18 — “Fair Value of Financial Instruments.”
Business Combinations	Business Combinations White Mountains accounts for purchases of businesses using the acquisition method, which requires the measurement of assets acquired, including other intangible assets, liabilities assumed, including contingent consideration liabilities, and any noncontrolling interest in the acquired entities at their estimated fair values as of the acquisition date. The acquisition date fair values represent management’s best estimates and are based upon established valuation techniques, reasonable assumptions and, where appropriate, valuations performed by independent third parties. In circumstances where additional information is required in order to determine the acquisition date fair value of balance sheet amounts, provisional amounts may be recorded as of the acquisition date and may be subject to subsequent adjustment throughout the measurement period, which is up to one year from the acquisition date. Measurement period adjustments are recognized in the period in which they are determined. The results of operations and cash flows of businesses acquired are included in the consolidated financial statements from the acquisition date. White Mountains accounts for purchases of other intangible assets that do not meet the definition of a business as asset acquisitions. Asset acquisitions are recognized at the amount of consideration paid, which is deemed to equal fair value.
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets Under the acquisition method, White Mountains recognizes and measures the assets acquired, including other intangible assets, at their acquisition date fair values. Goodwill represents the excess of the amount paid to acquire a business over the fair value of identifiable net assets at the acquisition date. Other intangible assets consist primarily of underwriting capacity, agency relationships, customer relationships and trade names. The acquisition date fair values of other intangible assets 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. Goodwill and other intangible assets with indefinite lives are not amortized, but rather are evaluated for impairment on an annual basis, or whenever indications of potential impairment exist. In the absence of any indications of potential impairment, the evaluation of goodwill and indefinite-lived intangible assets is performed no later than the interim period in which the anniversary of the acquisition date falls. White Mountains initially evaluates goodwill and indefinite-lived intangible assets using a qualitative approach (step zero) to determine whether it is more likely than not that the implied fair value is greater than the carrying value. If the results of the qualitative evaluation indicate that it is more likely than not that the carrying value of goodwill or the indefinite-lived intangible assets exceeds the implied fair value, White Mountains performs a quantitative analysis to compare the fair value with the carrying value. If the carrying value exceeds the estimated fair value, then an impairment charge is recognized through current period pre-tax income (loss). Other intangible assets with finite lives are initially measured at their acquisition date fair values and subsequently amortized over their economic lives. Finite-lived intangible assets are presented net of accumulated amortization on the balance sheet. Finite-lived intangible assets are reviewed for impairment when events occur or there are changes in circumstances indicating that their carrying value may exceed fair value. An impairment exists when the carrying value of a finite-lived intangible asset exceeds the fair value. See Note 4 — “Goodwill and Other Intangible Assets.”
Contingent Consideration Liabilities	Contingent Consideration Liabilities The consideration for the Ark Transaction included potential future payments to management rollover shareholders, which are payable in a variable number of shares, if and to the extent that White Mountains achieves certain MOIC return thresholds. This additional consideration is generally payable in three equal tranches at MOIC thresholds of 2.0x, 2.5x and 3.0x. If fully earned, these shares would represent an additional 12.3% of the shares outstanding as of December 31, 2024. The liability related to these additional shares is recorded as contingent consideration. Contingent consideration in a business combination that is classified as a liability is measured at fair value on the acquisition date and remeasured to fair value each subsequent reporting period with changes in the fair value recognized through pre-tax income (loss). As a result of the Ark Transaction, White Mountains recognized a contingent consideration liability of $22.5 million as of January 1, 2021. During the years ended December 31, 2024, 2023 and 2022, White Mountains recognized expense of $61.3 million, $48.7 million and $17.3 million for the change in the fair value of its contingent consideration liability. As of December 31, 2024 and 2023, White Mountains’s contingent consideration liability was $155.3 million and $94.0 million. The contingent consideration liability is a Level 3 measurement.
Recently Adopted Accounting Standards and Recently Issued Accounting Standards	Recently Adopted Accounting Standards Segment Reporting White Mountains adopted ASU 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures (“ASU 2023-07") effective for its annual disclosures for the year ended December 31, 2024. ASU 2023-07 requires disclosures of significant segment expenses for each reportable segment, as well as certain other disclosures to help users of financial statements understand how the chief operating decision makers (“CODMs”) assess each segment's performance and allocate resources. ASU 2023-07 does not change how an entity identifies its operating segments. ASU 2023-07 was adopted retrospectively, including the recasting of previously reported segment information. The adoption of ASU 2023-07 did not have a material impact on White Mountains's financial statements. See Note 15 — “Segment Information.” Recently Issued Accounting Standards Income Taxes In December 2023, the Financial Accounting Standards Board (“FASB”) issued ASU 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures (“ASU 2023-09”), which requires disaggregated information about a reporting entity’s effective tax rate reconciliation as well as information on income taxes paid. For public business entities, ASU 2023-09 is effective for annual periods beginning after December 15, 2024. The new disclosure requirements in ASU 2023-09 should be presented on a prospective basis with the option to apply the standard retrospectively. Early adoption is permitted for annual financial statements not yet issued or made available for issuance. Accordingly, the new disclosure requirements in ASU 2023-09 are initially required within White Mountains’ Annual Report on Form 10-K for the year ending December 31, 2025. White Mountains is in the process of evaluating the potential impact of adoption on its financial statements. Disaggregation of Income Statement Expenses In November 2024, the FASB issued ASU 2024-03, Income Statement—Reporting Comprehensive Income—Expense Disaggregation Disclosures (Subtopic 220-40): Disaggregation of Income Statement Expenses (“ASU 2024-03”), which requires additional disclosure of the nature of expenses included in the income statement. ASU 2024-03 requires disclosures about specific types of expenses (i.e., (a) purchases of inventory, (b) employee compensation, (c) depreciation and (d) intangible asset amortization) included in the expense captions presented on the face of the income statement as well as disclosures about selling expenses. ASU 2024-03 does not change the requirements for the presentation of expenses on the income statement. ASU 2024-03 is effective for annual reporting periods beginning after December 15, 2026 and interim reporting periods beginning after December 15, 2027. Early adoption is permitted. Although ASU 2024-03 requires comparative disclosures for all periods presented, reporting entities will be permitted to begin applying the guidance prospectively with the option for retrospective application for all prior periods presented. Accordingly, the new disclosure requirements in ASU 2024-03 are initially required within White Mountains’s Annual Report on Form 10-K for the year ending December 31, 2027. White Mountains is in the process of evaluating the potential impact of adoption on its financial statements.

Significant Transactions (Table

Significant Transactions (Tables)	12 Months Ended
	Dec. 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 was not part of the purchase consideration.

Investment Securities (Tables)

Investment Securities (Tables)	12 Months Ended
	Dec. 31, 2024
Investments, Debt and Equity Securities [Abstract]
Schedule of pre-tax net investment income	The following table presents pre-tax net investment income for the years ended December 31, 2024, 2023 and 2022: Year Ended December 31, Millions 2024 2023 2022 Fixed maturity investments $ 93.2 $ 64.4 $ 41.4 Short-term investments 56.6 52.0 16.4 Common equity securities 2.5 4.1 1.6 Other long-term investments 77.9 76.1 68.1 Amount attributable to TPC Providers — — (1.0) Total investment income 230.2 196.6 126.5 Third-party investment expenses (2.8) (2.4) (2.1) Net investment income, pre-tax $ 227.4 $ 194.2 $ 124.4
Schedule of net realized and unrealized investment gains and losses	The following table presents net realized and unrealized investment gains (losses) for the years ended December 31, 2024, 2023 and 2022: Year Ended December 31, Millions 2024 2023 2022 Realized investment gains (losses) Fixed maturity investments $ (10.1) $ (4.7) $ (11.9) Short-term investments (1.2) (.2) (1.4) Common equity securities 3.5 29.5 — Investment in MediaAlpha 91.2 — — Other long-term investments 34.6 72.2 128.4 Net realized investment gains (losses) 118.0 96.8 115.1 Unrealized investment gains (losses) Fixed maturity investments 3.9 66.1 (168.4) Short-term investments (.8) 1.9 (1.1) Common equity securities 58.1 47.1 (7.1) Investment in MediaAlpha (53.2) 27.1 (93.0) Other long-term investments 58.9 195.2 (43.8) Net unrealized investment gains (losses) 66.9 337.4 (313.4) Net realized and unrealized investment gains (losses), before amount attributable to TPC providers (1) 184.9 434.2 (198.3) Amount attributable to TPC Providers — — 6.8 Net realized and unrealized investment gains (losses) $ 184.9 $ 434.2 $ (191.5) Fixed maturity and short-term investments Net realized and unrealized investment gains (losses) $ (8.2) $ 63.1 $ (182.8) Less: net realized and unrealized gains (losses) on investment .9 4.7 (2.9) Net unrealized investment gains (losses) recognized during the $ (9.1) $ 58.4 $ (179.9) Common equity securities and investment in MediaAlpha Net realized and unrealized investment gains (losses) on $ 61.6 $ 76.6 $ (7.1) Net realized and unrealized investment gains (losses) from 38.0 27.1 (93.0) Total net realized and unrealized investment gains (losses) 99.6 103.7 (100.1) Less: net realized and unrealized gains (losses) on investment 35.4 28.4 — Net unrealized investment gains (losses) recognized during the $ 64.2 $ 75.3 $ (100.1) (1) For 2024, 2023 and 2022, includes $(31.5), $13.6 and $(29.3) 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 years ended December 31, 2024, 2023 and 2022 for investments still held at the end of the period: Year Ended December 31, Millions 2024 2023 2022 Total net unrealized investment gains on other long-term investments held at $ 57.2 $ 133.9 $ 56.5
Schedule of equity securities 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 December 31, 2024 and 2023: December 31, 2024 Millions Cost or Gross Gross Net Foreign Carrying U.S. Government and agency obligations $ 461.8 $ 1.1 $ (.8) $ — $ 462.1 Debt securities issued by corporations 1,444.5 4.6 (31.4) (3.5) 1,414.2 Municipal obligations 3.2 — — — 3.2 Mortgage and asset-backed securities 400.2 .2 (26.5) — 373.9 Collateralized loan obligations 237.3 1.2 (.2) (1.6) 236.7 Foreign government and agency obligations 22.2 — (.1) (.6) 21.5 Total fixed maturity investments $ 2,569.2 $ 7.1 $ (59.0) $ (5.7) $ 2,511.6 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 December 31, 2024 and 2023: December 31, 2024 Millions Cost or Gross Unrealized Gross Unrealized Losses Net Foreign Carrying Common equity securities $ 537.0 $ 120.4 $ — $ (7.4) $ 650.0 Investment in MediaAlpha $ 59.2 $ 142.4 $ — $ — $ 201.6 Other long-term investments $ 1,748.7 $ 546.4 $ (116.7) $ (28.2) $ 2,150.2 December 31, 2023 Millions Cost or Gross Unrealized Gross Unrealized Losses 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 contractual maturities of fixed maturities	The following table presents the cost or amortized cost and carrying value of White Mountains’s fixed maturity investments by contractual maturity as of December 31, 2024 and 2023. Actual maturities could differ from contractual maturities because borrowers may have the right to call or prepay certain obligations with or without penalties. December 31, 2024 December 31, 2023 Millions Cost or Amortized Cost Carrying Value Cost or Amortized Cost Carrying Value Due in one year or less $ 205.8 $ 203.9 $ 319.9 $ 315.0 Due after one year through five years 1,494.5 1,478.7 845.0 813.9 Due after five years through ten years 206.1 193.2 303.4 287.0 Due after ten years 25.3 25.2 101.7 94.9 Mortgage and asset-backed securities and 637.5 610.6 628.4 598.5 Total fixed maturity investments $ 2,569.2 $ 2,511.6 $ 2,198.4 $ 2,109.3
Schedule of fair value measurements for investment securities by level	The following tables present White Mountains’s fair value measurements for investments as of December 31, 2024 and 2023 by level. See Note 1 — “Basis of Presentation and Significant Accounting Policies.” The major security types were based on the legal form of the securities. White Mountains has disaggregated its fixed maturity investments based on the issuing entity type, which impacts credit quality, with debt securities issued by U.S. government entities carrying minimal credit risk, while the credit and other risks associated with other issuers, such as corporations, foreign governments and agencies, municipalities, entities issuing mortgage and asset-backed securities or entities issuing collateralized loan obligations 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 U.S. Intermediate Aggregate Index. December 31, 2024 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 462.1 $ 461.3 $ .8 $ — Debt securities issued by corporations: Financials 466.7 — 466.7 — Consumer 255.2 — 255.2 — Industrial 164.4 — 164.4 — Healthcare 153.1 — 153.1 — Technology 113.8 — 113.8 — Communications 71.6 — 71.6 — Materials 65.7 — 65.7 — Energy 62.8 — 62.8 — Utilities 60.9 — 60.9 — Total debt securities issued by corporations 1,414.2 — 1,414.2 — Municipal obligations 3.2 — 3.2 — Mortgage and asset-backed securities 373.9 — 373.9 — Collateralized loan obligations 236.7 — 236.7 — Foreign government and agency obligations 21.5 — 21.5 — Total fixed maturity investments 2,511.6 461.3 2,050.3 — Short-term investments 964.2 951.1 13.1 — Common equity securities: Exchange-traded funds 224.6 224.6 — — Other (1) 425.4 — 425.4 — Total common equity securities 650.0 224.6 425.4 — Investment in MediaAlpha 201.6 201.6 — — Other long-term investments 1,286.2 — 23.5 1,262.7 Other long-term investments — NAV (2) 864.0 — — — Total other long-term investments 2,150.2 — 23.5 1,262.7 Total investments $ 6,477.6 $ 1,838.6 $ 2,512.3 $ 1,262.7 (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.
Schedule of credit ratings of debt securities issued	The following table presents the fair values for credit ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of December 31, 2024 and 2023: December 31, Millions 2024 2023 AAA $ 14.6 $ 11.5 AA 91.2 83.8 A 607.2 552.4 BBB 688.7 390.9 BB 5.4 — Other 7.1 6.4 Debt securities issued by corporations (1) $ 1,414.2 $ 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 mortgage and asset-backed securities	The following table presents the fair value of White Mountains’s mortgage and asset-backed securities and collateralized loan obligations as of December 31, 2024 and 2023: December 31, 2024 December 31, 2023 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: FNMA $ 198.3 $ 198.3 $ — $ 198.8 $ 198.8 $ — FHLMC 147.1 147.1 — 137.2 137.2 — GNMA 24.2 24.2 — 30.2 30.2 — Total agency (1) 369.6 369.6 — 366.2 366.2 — Non-agency: Commercial .4 .4 — — — — Residential — — — .2 .2 — Total non-agency .4 .4 — .2 .2 — Total mortgage-backed securities 370.0 370.0 — 366.4 366.4 — Other asset-backed securities: Vehicle receivables 1.7 1.7 — 17.2 17.2 — Credit card receivables .2 .2 — 3.3 3.3 — Other 2.0 2.0 — 2.5 2.5 — Total other asset-backed securities 3.9 3.9 — 23.0 23.0 — Total mortgage and asset-backed securities 373.9 373.9 — 389.4 389.4 — Collateralized loan obligations: 236.7 236.7 — 209.1 209.1 — Total mortgage and asset-backed securities and $ 610.6 $ 610.6 $ — $ 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).
Schedule of carrying values for other long-term investments	The following tables presents the carrying values of White Mountains’s other long-term investments by reportable segment as of December 31, 2024 and 2023: Fair Value as of December 31, 2024 Millions Ark/WM Outrigger Kudu Other Total Kudu’s Participation Contracts $ — $ 1,008.4 $ — $ 1,008.4 PassportCard/DavidShield — — 150.0 150.0 Elementum — — 35.0 35.0 Other unconsolidated entities (1) — — 63.6 63.6 Total unconsolidated entities — 1,008.4 248.6 1,257.0 Private equity funds and hedge funds 104.1 — 256.5 360.6 Bank loan fund 264.7 — — 264.7 Lloyd’s trust deposits 149.9 — — 149.9 ILS funds — — 74.0 74.0 Private debt instruments — 5.6 9.3 14.9 Other 29.1 — — 29.1 Total other long-term investments $ 547.8 $ 1,014.0 $ 588.4 $ 2,150.2 (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. Fair Value as of 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 — — 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 SAFE investments.
Schedule of investments and unfunded commitments by investment objective and sector	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 December 31, 2024 and 2023: December 31, 2024 December 31, 2023 Millions Fair Value Unfunded Fair Value Unfunded Private equity funds Aerospace/Defense/Government $ 152.4 $ 49.3 $ 157.2 $ 17.4 Financial services 93.4 32.0 87.5 37.8 Real estate 3.7 2.4 3.9 2.5 Total private equity funds 249.5 83.7 248.6 57.7 Hedge funds Long/short equity financials and business services 59.2 — 54.4 — Long/short all cap global 52.0 — — — European small/mid cap — — 9.9 — Total hedge funds 111.2 — 64.3 — Total private equity funds and hedge funds included in other long-term investments $ 360.7 $ 83.7 $ 312.9 $ 57.7
Schedule investments in 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 December 31, 2024: Millions 1 – 3 years 3 – 5 years 5 – 10 years >10 years Total Private equity funds — expected lock-up period remaining $ 18.0 $ 51.1 $ 159.4 $ 21.0 $ 249.5
Schedule 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 years ended December 31, 2024 and 2023: Level 3 Investments Millions Other Long-term Other Long-term Balance as of December 31, 2023 $ 1,138.2 Balance as of December 31, 2022 $ 911.6 Net realized and unrealized gains 54.4 Net realized and unrealized gains 157.3 Purchases and contributions 111.5 Purchases and contributions 256.6 Sales and distributions (41.4) Sales and distributions (187.3) Transfers in — Transfers in — Transfers out — Transfers out — Balance as of December 31, 2024 $ 1,262.7 Balance as of December 31, 2023 $ 1,138.2
Schedule of significant unobservable inputs used in estimating the fair value of other long-term investments	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 December 31, 2024 and 2023. The tables below exclude $23.4 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 December 31, 2024 Description Valuation Technique(s) (1) Fair Value (2) Unobservable Inputs Discount Rate (5) Terminal Cash Flow Exit Multiple (x) or Terminal Revenue Growth Rate (%) (5) Kudu’s Participation Contracts (3)(4) Discounted cash flow $1,008.4 17% - 25% 7x - 22x PassportCard/DavidShield Discounted cash flow $150.0 24% 4% Elementum Discounted cash flow $35.0 22% 4% Preferred securities Discounted cash flow $31.4 9% N/A Private debt instruments Discounted cash flow $14.5 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 19% and 14x. (4) In 2024, Kudu contributed total cash of $103.5 into new and existing Participation Contracts. (5) Increases (decreases) to the discount rates in isolation would result in lower (higher) fair value measurements, while increases (decreases) to the terminal cash flow exit multiples or terminal revenue growth rates in isolation would result in higher (lower) fair value measurements. $ in Millions December 31, 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 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 contributed total cash 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 sale 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)	12 Months Ended
	Dec. 31, 2024
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of intangible assets and goodwill	The following table presents the economic life, acquisition date fair value, accumulated amortization and net carrying value for other intangible assets and goodwill as of December 31, 2024 and 2023: $ in Millions Weighted Average Economic December 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 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 1.8 .4 2.2 1.5 .7 Bamboo Trade names 10.0 23.5 2.3 21.2 — — — Agency relationships 6.0 72.4 12.1 60.3 — — — Developed technology 3.0 4.7 1.6 3.1 — — — Other 0.3 .4 .4 — — — — Subtotal 101.0 16.4 84.6 — — — Other Operations Trade names 13.3 13.3 5.6 7.7 13.3 4.1 9.2 Customer relationships 11.0 24.8 13.6 11.2 24.8 10.3 14.5 Other 11.8 3.1 1.6 1.5 2.8 1.1 1.7 Subtotal 41.2 20.8 20.4 40.9 15.5 25.4 Total other intangible assets 320.1 39.0 281.1 218.8 17.0 201.8 Total goodwill and other intangible assets $ 759.3 $ 39.0 $ 720.3 $ 387.6 $ 17.0 $ 370.6 The following table presents the change in goodwill and other intangible assets for the years ended December 31, 2024 and 2023: December 31, 2024 December 31, 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 Acquisitions 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 — (22.0) (22.0) — (7.2) (7.2) Ending balance $ 439.2 $ 281.1 $ 720.3 $ 168.8 $ 201.8 $ 370.6 (1) Relates to the Bamboo Transaction. See Note 2 — “Significant Transactions .” (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. Adjustments relate to acquisitions of certain consolidated Other Operating Businesses.
Schedule of finite-lived intangible assets, future amortization expense	The following table presents a schedule of White Mountains’s future amortization expense as of December 31, 2024: Millions Amortization Expense 2025 $ 20.8 2026 20.1 2027 17.9 2028 16.9 2029 16.4 2030 and years after 13.3 Total (1) $ 105.4 (1) Excludes Ark’s indefinite-lived intangible assets of $175.7.

Loss and Loss Adjustment Expe_2

Loss and Loss Adjustment Expense Reserves (Tables)	12 Months Ended
	Dec. 31, 2024
Insurance Loss Reserves [Abstract]
Summary of loss and LAE reserve activity	The following table summarizes the loss and LAE reserve activity of the Ark/WM Outrigger segment for the years ended December 31, 2024, 2023 and 2022: Year Ended December 31, Millions 2024 2023 2022 Gross beginning balance $ 1,605.1 $ 1,296.5 $ 894.7 Less: beginning reinsurance recoverable on unpaid losses (1) (340.8) (505.0) (428.9) Net loss and LAE reserves 1,264.3 791.5 465.8 Loss and LAE incurred relating to: Current year losses 914.4 706.9 588.1 Prior year losses (58.6) 19.9 (51.7) Net incurred loss and LAE 855.8 726.8 536.4 Loss and LAE paid relating to: Current year losses (109.2) (51.3) (98.9) Prior year losses (307.5) (356.6) (158.6) Net paid loss and LAE (416.7) (407.9) (257.5) Change in TPC Providers’ participation (2) — 145.4 57.5 Foreign currency translation and other adjustments to loss and LAE reserves (10.3) 8.5 (10.7) Net ending balance 1,693.1 1,264.3 791.5 Plus: ending reinsurance recoverable on unpaid losses (3) 434.4 340.8 505.0 Gross ending balance $ 2,127.5 $ 1,605.1 $ 1,296.5 (1) The beginning reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $145.4 and $276.8 as of January 1, 2023 and 2022. (2) Amount represents the impact to net loss and LAE reserves due to a change in the TPC Providers’ participation related to the annual RITC process. (3) The ending reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $145.4 as of December 31, 2022. The following table summarizes the unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses, for each of the Ark/WM Outrigger segment’s major reserving lines of business as of December 31, 2024 and 2023: Year Ended December 31, Millions 2024 2023 Property and Accident & Health (1) $ 533.6 $ 358.7 Marine & Energy 463.1 331.7 Specialty 417.0 339.7 Casualty-Active 178.0 137.1 Casualty-Runoff 66.5 81.3 WM Outrigger Re 34.9 15.8 Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses (1) 1,693.1 1,264.3 Plus: Reinsurance recoverables on unpaid losses Property and Accident & Health (2) 121.5 126.9 Marine & Energy 180.1 59.1 Specialty 51.3 77.6 Casualty-Active 80.7 74.8 Casualty-Runoff .8 2.4 Total Reinsurance recoverables on unpaid losses (2) 434.4 340.8 Total unpaid loss and LAE reserves $ 2,127.5 $ 1,605.1 (1) The amounts exclude $3.1 ceded by Ark to WM Outrigger Re as of December 31, 2024, which eliminates in White Mountains’s consolidated financial statements. (2) The amounts exclude $31.8 and $15.6 ceded by Ark to WM Outrigger Re as of December 31, 2024 and 2023, which eliminate in White Mountains’s consolidated financial statements.
Short-duration insurance contracts, claims development	Property and Accident & Health $ in Millions Incurred Loss and LAE, Net of Reinsurance For the Years Ended December 31, As of December 31, 2024 Accident Year 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Total IBNR plus expected development on reported claims Cumulative number of reported claims Unaudited 2015 $ 19.1 $ 18.1 $ 17.2 $ 16.2 $ 16.0 $ 16.0 $ 15.8 $ 15.7 $ 16.0 $ 16.1 $ .1 2,829 2016 22.2 17.5 18.2 18.4 18.3 18.5 18.5 18.5 18.5 .2 3,433 2017 31.1 37.7 $ 45.2 44.2 42.8 42.3 43.8 43.4 15.8 4,624 2018 40.7 $ 47.1 49.0 46.7 46.8 46.3 46.4 1.9 4,288 2019 33.9 31.2 27.0 23.8 23.2 22.8 .6 4,024 2020 76.9 75.1 74.5 77.6 79.5 9.9 4,646 2021 170.0 153.7 165.3 168.0 7.5 3,509 2022 241.5 266.9 277.3 12.9 4,044 2023 213.9 176.1 77.3 3,598 2024 359.5 202.7 3,614 Total $ 1,207.6 Property and Accident & Health Millions Cumulative Paid Loss and LAE, Net of Reinsurance For the Years Ended December 31, Accident Year 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Unaudited 2015 $ 6.9 $ 12.2 $ 13.4 $ 14.6 $ 14.5 $ 14.8 $ 15.0 $ 15.0 $ 15.4 $ 15.6 2016 8.5 13.0 16.3 16.7 16.8 17.1 17.7 17.9 18.1 2017 16.8 25.7 31.5 32.7 29.4 27.1 25.4 28.2 2018 15.6 32.2 40.1 40.0 40.8 42.8 43.6 2019 6.8 16.7 18.3 18.5 19.3 20.6 2020 11.2 33.9 46.9 55.6 66.5 2021 30.7 86.5 129.8 142.7 2022 69.4 191.9 229.4 2023 19.9 52.9 2024 54.8 Total 672.4 All outstanding liabilities before 2015, net of reinsurance 1.5 Loss and LAE reserves, net of reinsurance $ 536.7 Marine & Energy $ in Millions Incurred Loss and LAE, Net of Reinsurance For the Years Ended December 31, As of December 31, 2024 Accident Year 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Total IBNR plus expected development on reported claims Cumulative number of reported claims Unaudited 2015 $ 21.7 $ 17.4 $ 16.1 $ 13.3 $ 12.7 $ 12.8 $ 12.7 $ 12.9 $ 12.7 $ 12.7 $ — 3,243 2016 23.4 19.5 15.6 14.6 14.3 14.8 14.1 13.6 13.3 — 3,772 2017 26.0 19.3 17.6 16.9 16.6 15.8 16.0 16.1 .4 4,139 2018 25.4 19.9 17.4 17.8 17.3 17.7 16.7 .2 3,238 2019 23.7 21.6 21.6 21.4 21.9 21.2 .5 2,393 2020 29.7 27.1 28.5 27.4 27.1 .8 1,582 2021 86.2 69.3 67.3 74.5 4.0 1,505 2022 149.7 153.6 156.0 31.9 1,968 2023 197.0 188.2 115.9 2,138 2024 239.9 182.6 1,485 Total $ 765.7 Marine & Energy Millions Cumulative Paid Loss and LAE, Net of Reinsurance For the Years Ended December 31, Accident Year 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Unaudited 2015 $ 4.0 $ 7.8 $ 9.6 $ 11.0 $ 10.4 $ 10.5 $ 10.9 $ 11.5 $ 11.6 $ 11.6 2016 5.5 10.0 12.6 13.0 13.1 13.7 13.4 13.4 13.4 2017 5.1 11.1 12.8 14.0 14.1 14.1 14.0 14.3 2018 2.6 12.4 13.9 14.6 15.3 15.3 15.4 2019 3.3 10.6 12.6 14.3 15.3 18.1 2020 3.1 12.7 16.0 18.5 21.9 2021 6.3 24.3 38.2 51.9 2022 12.2 66.2 97.7 2023 10.5 42.1 2024 20.9 Total 307.3 All outstanding liabilities before 2015, net of reinsurance 4.7 Loss and LAE reserves, net of reinsurance $ 463.1 Specialty $ in Millions Incurred Loss and LAE, Net of Reinsurance For the Years Ended December 31, As of December 31, 2024 Accident Year 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Total IBNR plus expected development on reported claims Cumulative number of reported claims Unaudited 2015 $ 17.3 $ 14.6 $ 12.3 $ 10.7 $ 11.0 $ 11.2 $ 11.1 $ 8.9 $ 8.1 $ 11.3 $ .3 1,841 2016 18.2 14.3 10.9 11.3 11.8 11.8 8.9 8.5 12.3 .4 1,936 2017 17.9 12.8 11.9 11.4 11.6 10.6 10.3 10.9 .4 2,195 2018 14.4 16.2 16.6 15.9 14.8 15.7 16.6 .4 2,122 2019 21.6 19.4 18.6 25.6 30.1 19.9 .3 2,387 2020 23.7 22.8 18.6 19.5 16.7 .9 2,017 2021 70.3 62.1 51.4 43.8 10.0 1,725 2022 180.3 174.8 168.4 77.0 1,496 2023 214.5 197.4 94.4 1,641 2024 218.7 141.6 1,292 Total $ 716.0 Specialty Millions Cumulative Paid Loss and LAE, Net of Reinsurance For the Years Ended December 31, Accident Year 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Unaudited 2015 $ 4.0 $ 7.0 $ 7.6 $ 8.0 $ 8.0 $ 8.1 $ 8.1 $ 6.4 $ 6.2 $ 9.3 2016 3.2 7.9 9.1 9.9 10.3 10.3 8.5 8.3 11.7 2017 3.1 6.5 8.3 8.5 8.5 9.2 8.9 9.6 2018 2.7 8.2 9.9 10.4 11.8 13.0 14.1 2019 4.8 6.9 7.4 18.2 25.1 17.6 2020 5.0 10.5 12.9 17.7 17.7 2021 5.0 23.9 35.5 34.6 2022 16.0 61.7 82.5 2023 18.4 75.2 2024 27.3 Total 299.6 All outstanding liabilities before 2015, net of reinsurance .6 Loss and LAE reserves, net of reinsurance $ 417.0 Casualty-Active $ in Millions Incurred Loss and LAE, Net of Reinsurance For the Years Ended December 31, As of December 31, 2024 Accident Year 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Total IBNR plus expected development on reported claims Cumulative number of reported claims Unaudited 2015 $ 9.6 $ 9.7 $ 8.2 $ 8.1 $ 7.4 $ 7.1 $ 7.0 $ 7.3 $ 7.5 $ 7.4 $ .5 1,306 2016 8.8 8.3 8.9 9.0 9.1 9.2 9.2 10.1 11.8 .5 1,588 2017 11.5 11.7 10.8 9.3 9.0 10.5 10.6 10.9 .8 1,667 2018 12.9 13.3 11.1 10.8 8.6 9.1 9.4 1.1 1,147 2019 14.8 13.7 12.3 10.6 11.4 13.0 1.7 1,019 2020 13.5 12.0 10.8 9.2 8.8 2.0 665 2021 21.4 22.4 16.6 16.3 6.4 961 2022 32.9 38.0 34.6 27.9 1,558 2023 60.9 65.9 57.4 1,792 2024 59.5 54.9 1,170 Total $ 237.6 Casualty-Active Millions Cumulative Paid Loss and LAE, Net of Reinsurance For the Years Ended December 31, Accident Year 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Unaudited 2015 $ 1.8 $ 2.4 $ 3.2 $ 4.4 $ 4.7 $ 4.9 $ 5.1 $ 5.5 $ 6.1 $ 6.3 2016 .2 1.0 2.3 4.0 4.6 5.3 6.5 8.1 9.7 2017 .8 1.7 2.7 3.4 4.2 5.7 7.5 8.3 2018 .3 1.4 3.5 4.3 4.3 6.2 7.1 2019 .3 1.4 2.3 3.0 5.7 8.3 2020 .5 1.0 2.0 3.3 5.3 2021 .5 .9 3.1 9.6 2022 .4 1.5 2.4 2023 .9 5.6 2024 1.9 Total 64.5 All outstanding liabilities before 2015, net of reinsurance 4.9 Loss and LAE reserves, net of reinsurance $ 178.0 Casualty-Runoff $ in Millions Incurred Loss and LAE, Net of Reinsurance For the Years Ended December 31, As of December 31, 2024 Accident Year 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Total IBNR plus expected development on reported claims Cumulative number of reported claims Unaudited 2015 $ 36.4 $ 31.9 $ 33.1 $ 36.6 $ 36.3 $ 37.3 $ 36.7 $ 39.1 $ 40.2 $ 39.4 $ 1.9 1,950 2016 32.4 32.1 40.3 38.4 38.7 38.4 37.6 37.3 37.4 2.0 2,150 2017 30.5 33.8 31.3 32.0 31.5 29.8 28.2 28.4 2.3 1,604 2018 33.5 28.1 27.2 26.5 26.1 27.9 27.7 3.4 1,280 2019 26.4 23.2 23.3 24.8 23.5 23.6 5.2 973 2020 15.8 12.2 13.8 10.9 9.5 2.9 567 2021 10.4 7.0 5.4 4.5 1.8 283 2022 .8 2.6 2.5 1.6 80 2023 2.7 3.6 2.4 40 2024 1.3 .7 22 Total $ 177.9 Casualty-Runoff Millions Cumulative Paid Loss and LAE, Net of Reinsurance For the Years Ended December 31, Accident Year 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Unaudited 2015 $ 4.3 $ 8.2 $ 14.5 $ 21.4 $ 24.6 $ 27.3 $ 28.9 $ 33.0 $ 35.3 $ 35.8 2016 3.9 10.1 17.7 22.7 25.3 27.8 28.7 30.9 32.4 2017 3.2 9.4 14.6 18.4 21.3 22.5 22.8 23.5 2018 3.4 7.4 12.6 14.9 16.2 18.2 21.3 2019 3.3 5.8 7.8 12.1 15.1 15.8 2020 .8 1.3 3.1 6.0 6.3 2021 .5 1.7 1.8 2.3 2022 .3 .5 .7 2023 .9 1.0 2024 .5 Total 139.6 All outstanding liabilities before 2015, net of reinsurance 28.2 Loss and LAE reserves, net of reinsurance $ 66.5
Short-duration insurance contracts, schedule of historical claims duration	Property and Accident & Health Average Annual Percentage Payout of Incurred Loss and LAE by Age, Net of Reinsurance Unaudited Years 1 2 3 4 5 6 7 8 9 10 23.8% 32.8% 18.3% 6.4% 2.8% 1.3% 0.5% 0.8% 0.1% 0.1% Marine & Energy Average Annual Percentage Payout of Incurred Loss and LAE by Age, Net of Reinsurance Unaudited Years 1 2 3 4 5 6 7 8 9 10 12.9% 30.5% 19.5% 8.0% 4.9% 6.9% 0.3% 0.4% (0.3)% 0.1% Specialty Average Annual Percentage Payout of Incurred Loss and LAE by Age, Net of Reinsurance Unaudited Years 1 2 3 4 5 6 7 8 9 10 19.3% 30.8% 10.9% 4.9% 7.3% 2.5% 1.5% 1.8% (1.3)% 1.1% Casualty-Active Average Annual Percentage Payout of Incurred Loss and LAE by Age, Net of Reinsurance Unaudited Years 1 2 3 4 5 6 7 8 9 10 4.4% 8.4% 11.6% 14.4% 9.5% 11.7% 6.3% 4.8% 3.7% 3.1% Casualty-Runoff Average Annual Percentage Payout of Incurred Loss and LAE by Age, Net of Reinsurance Unaudited Years 1 2 3 4 5 6 7 8 9 10 8.8% 14.1% 15.6% 14.8% 8.5% 6.8% 5.9% 4.1% 3.1% 2.0%

Third-Party Reinsurance (Tables

Third-Party Reinsurance (Tables)	12 Months Ended
	Dec. 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 years ended December 31, 2024, 2023 and 2022. Year Ended December 31, Millions 2024 2023 2022 Written premiums: Direct $ 1,101.5 $ 931.9 $ 760.4 Assumed 1,105.5 966.5 691.6 Gross written premiums 2,207.0 1,898.4 1,452.0 Ceded (527.9) (1) (377.5) (1) (256.8) Net written premiums $ 1,679.1 $ 1,520.9 $ 1,195.2 Earned premiums: Direct $ 1,004.2 $ 848.1 $ 655.5 Assumed 1,088.2 925.6 668.7 Gross earned premiums 2,092.4 1,773.7 1,324.2 Ceded (504.6) (2) (364.0) (2) (280.8) Net earned premiums $ 1,587.8 $ 1,409.7 $ 1,043.4 Loss and LAE: Gross $ 1,056.7 $ 782.5 $ 814.9 Ceded (200.9) (3) (55.7) (3) (278.5) Net loss and LAE $ 855.8 $ 726.8 $ 536.4 (1) The years ended December 31, 2024 and 2023 exclude $86.5 and $110.0 ceded by Ark to WM Outrigger Re, which eliminate in White Mountains’s consolidated financial statements. (2) The years ended December 31, 2024 and 2023 exclude $88.0 and $104.3 ceded by Ark to WM Outrigger Re, which eliminate in White Mountains’s consolidated financial statements. (3) The years ended December 31, 2024 and 2023 exclude $29.9 and $15.6 ceded by Ark to WM Outrigger Re, which eliminate in White Mountains’s consolidated financial statements. The following table presents the Ark/WM Outrigger segment’s reinsurance recoverables as of December 31, 2024 and December 31, 2023: Millions December 31, 2024 December 31, 2023 Reinsurance recoverables on unpaid losses (1) $ 434.4 $ 340.8 Reinsurance recoverables on paid losses (2) 57.5 27.4 Ceded unearned premiums (3) 97.1 73.8 Reinsurance recoverables $ 589.0 $ 442.0 (1) The reinsurance recoverables on unpaid losses exclude $31.8 and $15.6 ceded by Ark to WM Outrigger Re as of December 31, 2024 and 2023, which eliminate in White Mountains’s consolidated financial statements. (2) The reinsurance recoverables on paid losses exclude $3.1 ceded by Ark to WM Outrigger Re as of December 31, 2024, which eliminates in White Mountains’s consolidated financial statements. (3) The ceded unearned premiums exclude $4.3 and $5.7 ceded by Ark to WM Outrigger Re as of December 31, 2024 and 2023, which eliminate in White Mountains’s consolidated financial statements. The following tables present gross written premiums and gross earned premiums included in the HG Global segment for the years ended December 31, 2024, 2023 and 2022: Year Ended December 31, 2024 Millions HG Global BAM Eliminations Total Written premiums: Direct $ — $ 24.1 $ — $ 24.1 Assumed 52.4 — (20.5) 31.9 Gross written premiums (1) $ 52.4 $ 24.1 $ (20.5) $ 56.0 Earned premiums: Direct $ — $ 15.8 $ — $ 15.8 Assumed 28.9 1.0 (14.0) 15.9 Gross earned premiums (1) $ 28.9 $ 16.8 $ (14.0) $ 31.7 (1) Subsequent to the deconsolidation of BAM, there are no ceded premiums, such that gross written premiums and gross earned premiums are equivalent to net written premiums and net earned premiums, respectively. For the six months ended June 30, 2024, BAM ceded written premiums of $20.5 and ceded earned premiums of $14.0 to HG Global, which eliminated in consolidation. Year Ended December 31, 2023 Millions HG Global BAM Eliminations Total Written premiums: Direct $ — $ 58.6 $ — $ 58.6 Assumed 50.1 — (50.1) — Gross written premiums (1) $ 50.1 $ 58.6 $ (50.1) $ 58.6 Earned premiums: Direct $ — $ 28.7 $ — $ 28.7 Assumed 26.0 2.5 (26.0) 2.5 Gross earned premiums (1) $ 26.0 $ 31.2 $ (26.0) $ 31.2 (1) For the year ended December 31, 2023, BAM ceded written premiums of $50.1 and ceded earned premiums of $26.0 to HG Global, which eliminated in consolidation. Year Ended December 31, 2022 Millions HG Global BAM Eliminations Total Written premiums: Direct $ — $ 63.8 $ — $ 63.8 Assumed 55.9 1.3 (55.9) 1.3 Gross written premiums (1) $ 55.9 $ 65.1 $ (55.9) $ 65.1 Earned premiums: Direct $ — $ 28.6 $ — $ 28.6 Assumed 27.5 4.7 (27.5) 4.7 Gross earned premiums (1) $ 27.5 $ 33.3 $ (27.5) $ 33.3 (1) For the year ended December 31, 2022, BAM ceded written premiums of $55.9 and ceded earned premiums of $27.5 to HG Global, which eliminated in consolidation.
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 reinsurers’ A.M. Best Company, Inc (“A.M. Best”) ratings as of December 31, 2024: $ in Millions As of December 31, 2024 A.M. Best Rating (1) Gross Collateral Net % of Total A+ or better $ 310.3 $ — $ 310.3 74.3 % A - to A 98.5 — 98.5 23.6 B++ or lower and not rated 83.1 (2) 74.5 (2) 8.6 2.1 Total $ 491.9 $ 74.5 $ 417.4 100.0 % (1) A.M. Best financial strength ratings as detailed above are: “A+ or better” (Superior) “A- to A” (Excellent), “B++” (Good). (2) Excludes $34.9 ceded by Ark to WM Outrigger Re as of December 31, 2024, which eliminates in White Mountains’s consolidated financial statements.

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2024
Debt Disclosure [Abstract]
Schedule of debt outstanding	The following table presents White Mountains’s debt outstanding as of December 31, 2024 and 2023: Effective Effective $ in Millions December 31, 2024 Rate (1) December 31, 2023 Rate (1) Ark 2007 Subordinated Notes, carrying value $ — $ 30.0 Ark 2021 Subordinated Notes Tranche 1 41.1 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 (3.6) (4.2) Ark 2021 Subordinated Notes, carrying value 154.5 155.5 Total Ark Subordinated Notes, carrying value 154.5 11.8% 185.5 11.0% HG Global Senior Notes (2) 150.0 150.0 Unamortized discount and issuance cost (2.6) (3.1) HG Global Senior Notes, carrying value 147.4 11.8% 146.9 11.9% Kudu Credit Facility (2) 245.3 210.3 Unamortized issuance cost (6.7) (6.5) Kudu Credit Facility, carrying value 238.6 10.3% 203.8 10.1% Other Operations debt 22.4 28.9 Unamortized issuance cost (.4) (.5) Other Operations debt, carrying value 22.0 10.2% 28.4 10.2% Total debt $ 562.5 $ 564.6 (1) Effective rate includes the effect of the amortization of debt issuance costs and the original issue discount. (2) Effective rate excludes the effect of the interest rate caps. See Note 9 — “Derivatives.” The following table presents the change in debt under the Kudu Credit Facility for the years ended December 31, 2024, 2023 and 2022: Year Ended December 31, Millions 2024 2023 2022 Kudu Credit Facility Beginning balance $ 210.3 $ 215.2 $ 225.4 Term loans Borrowings 35.0 12.0 35.0 Repayments — (16.9) (45.2) Ending balance $ 245.3 $ 210.3 $ 215.2
Schedule of contractual repayments of debt	The following table presents a schedule of contractual repayments of White Mountains’s debt as of December 31, 2024: Millions December 31, 2024 Due in one year or less $ 6.7 Due in two to three years 27.0 Due in four to five years 30.0 Due after five years 512.1 Total $ 575.8

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2024
Income Tax Disclosure [Abstract]
Schedule of total income tax benefit (expense)	The following table presents the total income tax (expense) benefit for the years ended December 31, 2024, 2023 and 2022: Year Ended December 31, Millions 2024 2023 2022 Current income tax (expense) benefit: U.S. federal $ (6.3) $ (12.6) $ (16.9) State (.4) (2.2) (4.5) Non-U.S. (14.8) (16.6) (7.1) Total current income tax (expense) benefit (21.5) (31.4) (28.5) Deferred income tax (expense) benefit: U.S. federal (18.8) (13.2) (7.3) State 2.7 (3.8) (1.8) Non-U.S. 5.0 63.9 (3.8) Total deferred income tax (expense) benefit (11.1) 46.9 (12.9) Total income tax (expense) benefit $ (32.6) $ 15.5 $ (41.4)
Schedule of U.S. federal statutory income tax rate and actual effective tax rate on pre-tax income	The following table presents a reconciliation of taxes calculated for 2024, 2023 and 2022 using the 21% U.S. federal statutory rate (the tax rate at which the majority of White Mountains’s worldwide operations are taxed) to the income tax (expense) benefit on pre-tax income (loss): Year Ended December 31, Millions 2024 2023 2022 Tax (expense) benefit at the U.S. statutory rate $ (66.5) $ (118.7) $ 31.4 Differences in taxes resulting from: Non-U.S. earnings, net of foreign taxes 40.2 103.7 (41.2) Change in valuation allowance (8.1) (30.2) (19.6) Bermuda corporate income tax 5.0 68.0 — Noncontrolling interest 3.9 6.8 3.4 Tax rate changes 2.9 (.6) (.4) BAM member surplus contributions prior to deconsolidation (2.8) (5.7) (6.2) Withholding tax (2.5) (4.1) (3.1) State taxes (1.8) (8.2) (2.8) Tax exempt interest and dividends .3 .2 .2 Officer compensation — (.2) (1.0) Other, net (3.2) 4.5 (2.1) Total income tax (expense) benefit on pre-tax income (loss) $ (32.6) $ 15.5 $ (41.4)
Schedule of components of deferred income tax assets and liabilities	The following table presents an outline of the significant components of White Mountains’s U.S. federal, state and non-U.S. deferred tax assets and liabilities: December 31, Millions 2024 2023 Deferred tax assets related to: Non-U.S. net operating loss carryforwards $ 62.8 $ 68.2 Intangible assets 51.5 51.5 Incentive compensation 23.8 25.4 Unearned premiums 20.5 18.2 Loss reserves 8.5 8.4 U.S. federal and state net operating and capital 6.4 87.6 Tax credit carryforwards 2.8 3.3 Accrued interest .2 10.1 Deferred acquisition costs — 3.7 Other items .9 1.0 Total gross deferred tax assets 177.4 277.4 Less: valuation allowances 89.2 116.1 Total net deferred tax assets 88.2 161.3 Deferred tax liabilities related to: Purchase accounting 43.9 43.9 Investment basis difference 43.5 40.9 Deferred underwriting 24.3 18.3 Net unrealized investment gains 19.0 15.5 Deferred acquisition costs 6.5 — BAM member surplus contributions prior to deconsolidation — 82.2 Other items 1.2 1.2 Total deferred tax liabilities 138.4 202.0 Net deferred tax asset (liability) $ (50.2) $ (40.7)
Schedule of net operating and capital loss carryforwards	The following table presents net operating loss and capital loss carryforwards as of December 31, 2024, the expiration dates and the deferred tax assets thereon: December 31, 2024 Millions United States Luxembourg United Kingdom Israel Total 2025-2029 $ .4 $ — $ — $ — $ .4 2030-2034 .3 — — — .3 2035-2044 8.8 177.1 — — 185.9 No expiration date 3.5 — — 88.1 91.6 Total $ 13.0 $ 177.1 $ — $ 88.1 $ 278.2 Gross deferred tax asset $ 6.4 $ 42.3 $ — $ 20.3 $ 69.0 Valuation allowance (6.4) (42.3) — (20.3) (69.0) Net deferred tax asset $ — $ — $ — $ — $ —

Municipal Bond Guarantee Rein_2

Municipal Bond Guarantee Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2024
Insurance [Abstract]
Schedule of insured obligations	The following table presents the HG Global segment’s insured obligations as of December 31, 2024 and 2023: December 31, 2024 December 31, 2023 Contracts outstanding 15,884 14,485 Remaining weighted average contract period (in years) (1) 11.2 11.1 Outstanding par value of policies assumed (in millions) (2) $ 18,503.3 $ 16,575.2 (3) Gross unearned insurance premiums (in millions) $ 297.3 $ 273.9 (4) (1) The remaining weighted average contract period was calculated using total contractual debt service outstanding, including principal and interest. (2) Under the FLRT, HG Re provides first-loss reinsurance protection of up to 15% -of-par outstanding for each policy assumed from BAM. For capital appreciation bonds, par is adjusted to the estimated equivalent par value for current interest paying bonds. (3) As of December 31, 2023, which was prior to BAM’s deconsolidation, BAM’s total contractual debt service outstanding was $165,686.0, split between principal of $109,673.8 and interest of $56,012.2. (4) As of December 31, 2023, which was prior to BAM’s deconsolidation, BAM’s gross unearned insurance premiums were $325.8.
Schedule of BAM’s future premium revenues	The following table presents the HG Global segment’s future premium revenues as of December 31, 2024: Millions December 31, 2024 January 1, 2025 - March 31, 2025 $ 6.8 April 1, 2025 - June 30, 2025 6.6 July 1, 2025 - September 30, 2025 6.5 October 1, 2025 - December 31, 2025 6.4 Total 2025 26.3 2026 24.7 2027 23.3 2028 21.8 2029 20.2 2030 and thereafter 181.0 Total gross unearned insurance premiums $ 297.3
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 years ended December 31, 2024, 2023 and 2022. Year Ended December 31, Millions 2024 2023 2022 Written premiums: Direct $ 1,101.5 $ 931.9 $ 760.4 Assumed 1,105.5 966.5 691.6 Gross written premiums 2,207.0 1,898.4 1,452.0 Ceded (527.9) (1) (377.5) (1) (256.8) Net written premiums $ 1,679.1 $ 1,520.9 $ 1,195.2 Earned premiums: Direct $ 1,004.2 $ 848.1 $ 655.5 Assumed 1,088.2 925.6 668.7 Gross earned premiums 2,092.4 1,773.7 1,324.2 Ceded (504.6) (2) (364.0) (2) (280.8) Net earned premiums $ 1,587.8 $ 1,409.7 $ 1,043.4 Loss and LAE: Gross $ 1,056.7 $ 782.5 $ 814.9 Ceded (200.9) (3) (55.7) (3) (278.5) Net loss and LAE $ 855.8 $ 726.8 $ 536.4 (1) The years ended December 31, 2024 and 2023 exclude $86.5 and $110.0 ceded by Ark to WM Outrigger Re, which eliminate in White Mountains’s consolidated financial statements. (2) The years ended December 31, 2024 and 2023 exclude $88.0 and $104.3 ceded by Ark to WM Outrigger Re, which eliminate in White Mountains’s consolidated financial statements. (3) The years ended December 31, 2024 and 2023 exclude $29.9 and $15.6 ceded by Ark to WM Outrigger Re, which eliminate in White Mountains’s consolidated financial statements. The following table presents the Ark/WM Outrigger segment’s reinsurance recoverables as of December 31, 2024 and December 31, 2023: Millions December 31, 2024 December 31, 2023 Reinsurance recoverables on unpaid losses (1) $ 434.4 $ 340.8 Reinsurance recoverables on paid losses (2) 57.5 27.4 Ceded unearned premiums (3) 97.1 73.8 Reinsurance recoverables $ 589.0 $ 442.0 (1) The reinsurance recoverables on unpaid losses exclude $31.8 and $15.6 ceded by Ark to WM Outrigger Re as of December 31, 2024 and 2023, which eliminate in White Mountains’s consolidated financial statements. (2) The reinsurance recoverables on paid losses exclude $3.1 ceded by Ark to WM Outrigger Re as of December 31, 2024, which eliminates in White Mountains’s consolidated financial statements. (3) The ceded unearned premiums exclude $4.3 and $5.7 ceded by Ark to WM Outrigger Re as of December 31, 2024 and 2023, which eliminate in White Mountains’s consolidated financial statements. The following tables present gross written premiums and gross earned premiums included in the HG Global segment for the years ended December 31, 2024, 2023 and 2022: Year Ended December 31, 2024 Millions HG Global BAM Eliminations Total Written premiums: Direct $ — $ 24.1 $ — $ 24.1 Assumed 52.4 — (20.5) 31.9 Gross written premiums (1) $ 52.4 $ 24.1 $ (20.5) $ 56.0 Earned premiums: Direct $ — $ 15.8 $ — $ 15.8 Assumed 28.9 1.0 (14.0) 15.9 Gross earned premiums (1) $ 28.9 $ 16.8 $ (14.0) $ 31.7 (1) Subsequent to the deconsolidation of BAM, there are no ceded premiums, such that gross written premiums and gross earned premiums are equivalent to net written premiums and net earned premiums, respectively. For the six months ended June 30, 2024, BAM ceded written premiums of $20.5 and ceded earned premiums of $14.0 to HG Global, which eliminated in consolidation. Year Ended December 31, 2023 Millions HG Global BAM Eliminations Total Written premiums: Direct $ — $ 58.6 $ — $ 58.6 Assumed 50.1 — (50.1) — Gross written premiums (1) $ 50.1 $ 58.6 $ (50.1) $ 58.6 Earned premiums: Direct $ — $ 28.7 $ — $ 28.7 Assumed 26.0 2.5 (26.0) 2.5 Gross earned premiums (1) $ 26.0 $ 31.2 $ (26.0) $ 31.2 (1) For the year ended December 31, 2023, BAM ceded written premiums of $50.1 and ceded earned premiums of $26.0 to HG Global, which eliminated in consolidation. Year Ended December 31, 2022 Millions HG Global BAM Eliminations Total Written premiums: Direct $ — $ 63.8 $ — $ 63.8 Assumed 55.9 1.3 (55.9) 1.3 Gross written premiums (1) $ 55.9 $ 65.1 $ (55.9) $ 65.1 Earned premiums: Direct $ — $ 28.6 $ — $ 28.6 Assumed 27.5 4.7 (27.5) 4.7 Gross earned premiums (1) $ 27.5 $ 33.3 $ (27.5) $ 33.3 (1) For the year ended December 31, 2022, BAM ceded written premiums of $55.9 and ceded earned premiums of $27.5 to HG Global, which eliminated in consolidation.

Earnings Per Share (Tables)

Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2024
Earnings Per Share [Abstract]
Schedule of earnings per share from continuing operations	The following table presents the Company’s computation of earnings per share from continuing operations for the years ended December 31, 2024, 2023 and 2022. See Note 20 — “Held for Sale and Discontinued Operations.” Year Ended December 31, 2024 2023 2022 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s $ 230.4 $ 509.2 $ 792.8 Less: total income (loss) from discontinued operations, net of tax (1) — — 903.2 Less: net (income) loss from discontinued operations attributable — — (.7) Net income (loss) from continuing operations attributable to 230.4 509.2 (109.7) Allocation of (earnings) losses to participating restricted common shares (2) (3.0) (7.2) 1.3 Basic and diluted earnings (losses) per share numerators $ 227.4 $ 502.0 $ (108.4) Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 2,565.7 2,563.3 2,862.4 Average unvested restricted common shares (3) (33.5) (36.0) (36.2) Basic earnings (losses) per share denominator 2,532.2 2,527.3 2,826.2 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 2,565.7 2,563.3 2,862.4 Average unvested restricted common shares (3) (33.5) (36.0) (36.2) Diluted earnings (losses) per share denominator 2,532.2 2,527.3 2,826.2 Basic and diluted earnings per share (in dollars) - continuing operations: Distributed earnings - dividends declared and paid $ 1.00 $ 1.00 $ 1.00 Undistributed earnings (losses) 88.79 197.60 (39.34) Basic and diluted earnings (losses) per share $ 89.79 $ 198.60 $ (38.34) (1) Includes net income (loss) from discontinued operations, net of tax - NSM Group and net gain (loss) from sale of discontinued operations, net of tax - NSM Group. See Note 20 — “Held for Sale and Discontinued Operations.” (2) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (3) Restricted shares outstanding vest upon a stated date. See Note 12 — “Employee Share-Based Incentive Compensation Plans.” Year Ended Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 792.8 Less: net income (loss) from continuing operations (190.8) Less: net (income) loss from continuing operations attributable to noncontrolling interest 81.1 Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders (1) 902.5 Allocation of (earnings) losses to participating restricted common shares (2) (11.4) Basic and diluted (loss) earnings per share numerators $ 891.1 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 2,862.4 Average unvested restricted common shares (3) (36.2) Basic earnings (loss) per share denominator 2,826.2 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 2,862.4 Average unvested restricted common shares (3) (36.2) Diluted earnings (loss) per share denominator 2,826.2 Basic (loss) earnings per share (in dollars) - discontinued operations: $ 315.30 Diluted (loss) earnings per share (in dollars) - discontinued operations: $ 315.30 (1) Includes net income (loss) from discontinued operations, net of tax - NSM Group, net gain (loss) from sale of discontinued operations, net of tax - NSM Group and net (income) loss from discontinued operations attributable to noncontrolling interests. (2) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities. (3) Restricted shares outstanding vest upon a stated date. See Note 12 — “Employee Share-Based Incentive Compensation Plans.”
Schedule of undistributed earnings	The following table presents the undistributed net earnings (losses) from continuing operations for the years ended December 31, 2024, 2023 and 2022. See Note 20 — “Held for Sale and Discontinued Operations.” Year Ended December 31, Millions 2024 2023 2022 Undistributed net earnings - continuing operations: Net income (loss) attributable to White Mountains’s common shareholders, $ 227.4 $ 502.0 $ (108.4) Dividends declared, net of restricted common share amounts (1) (2.5) (2.5) (3.0) Total undistributed net earnings (losses), net of restricted common share amounts $ 224.9 $ 499.5 $ (111.4) (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)	12 Months Ended
	Dec. 31, 2024
Share-Based Payment Arrangement [Abstract]
Schedule of performance share activity	The following table presents performance share activity for the years ended December 31, 2024, 2023 and 2022 for performance shares granted under the WTM Incentive Plan: Year Ended December 31, 2024 2023 2022 $ in Millions Target Accrued Target Accrued Target Accrued Beginning of period 37,031 $ 69.4 39,449 $ 67.5 40,828 $ 42.2 Shares paid or expired (1) (13,475) (44.9) (13,350) (40.8) (14,625) (26.4) New grants 11,405 — 10,895 — 13,225 — Forfeitures and cancellations (2) (102) — 37 (.1) 21 (.4) Expense recognized — 46.6 — 42.8 — 52.1 End of period 34,859 $ 71.1 37,031 $ 69.4 39,449 $ 67.5 (1) WTM performance share payments in 2024 for the 2021-2023 performance cycle, which were paid in March 2024 at 188% of target. WTM performance share payments in 2023 for the 2020-2022 performance cycle, which were paid in March 2023 at 200% of target. WTM performance share payments in 2022 for the 2019-2021 performance cycle, which were paid in March 2022 at 172% of target. (2) Amounts include changes in assumed forfeitures, as required under GAAP.
Schedule of performance shares outstanding and accrued expense for performance shares awarded	The following table presents performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan as of December 31, 2024 for each performance cycle: December 31, 2024 $ in Millions Target Performance Accrued Expense Performance cycle: 2024 – 2026 11,405 $ 6.5 2023 – 2025 10,835 14.4 2022 – 2024 13,150 51.3 Sub-total 35,390 72.2 Assumed forfeitures (531) (1.1) Total 34,859 $ 71.1
Schedule of unrecognized compensation costs associated with outstanding 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 years ended December 31, 2024, 2023 and 2022: Year Ended December 31, 2024 2023 2022 $ in Millions Restricted Unamortized Restricted Unamortized Restricted Unamortized Non-vested: Beginning of period 37,595 $ 16.2 38,350 $ 15.5 37,850 $ 15.9 Issued 11,405 20.0 10,895 16.0 13,225 13.8 Vested (13,475) — (11,650) — (12,725) — Forfeited (135) — — — — — Expense recognized — (16.3) — (15.3) — (14.2) End of period 35,390 $ 19.9 37,595 $ 16.2 38,350 $ 15.5

Common Shareholders_ Equity a_2

Common Shareholders’ Equity and Noncontrolling Interests (Tables)	12 Months Ended
	Dec. 31, 2024
Equity [Abstract]
Schedule of noncontrolling interest	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 December 31, 2024 and 2023: December 31, 2024 December 31, 2023 $ in Millions Noncontrolling Percentage (1) Noncontrolling Equity Noncontrolling Percentage (1) Noncontrolling Equity Noncontrolling interests: Ark 27.9 % $ 410.4 (2) 28.0 % $ 336.9 (2) HG Global 3.1 % (13.4) 3.1 % .8 Kudu 9.6 % 127.6 (3) 10.5 % 113.8 (3) Bamboo 27.2 % 113.6 — — Other various 9.1 various 9.4 Total, excluding BAM 647.3 460.9 BAM (4) 100.0 % — 100.0 % (139.8) Total noncontrolling interests $ 647.3 $ 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 December 31, 2024 and 2023, Ark’s noncontrolling equity includes $42.9 and $21.0 related to management’s equity incentives. (3) As of December 31, 2024 and 2023, Kudu’s noncontrolling equity includes $47.9 and $38.7 related to management’s equity incentives. (4) Effective July 1, 2024, White Mountains no longer consolidates BAM. Through June 30, 2024, BAM’s results are attributed to noncontrolling interests in White Mountains’s consolidated financial statements.

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2024
Segment Reporting [Abstract]
Schedule of financial information for segments	The following tables present White Mountains’s pre-tax financial results by segment for the years ended December 31, 2024, 2023 and 2022: Ark/WM Outrigger HG Global Millions Ark WM Outrigger Re HG Global BAM (1)(2) Kudu Bamboo Other Total Year Ended December 31, 2024 Earned insurance premiums $ 1,499.8 $ 88.0 $ 28.9 $ 2.8 $ — $ 39.4 $ 32.7 $ 1,691.6 Net investment income (3) 79.4 11.3 23.4 8.8 66.7 2.2 35.6 227.4 Net realized and unrealized investment gains (losses) (3) 50.1 — (6.4) (5.1) 51.3 — 57.0 146.9 Net realized and unrealized investment gains — — — — — — 38.0 38.0 Interest income (expense) from BAM Surplus Notes — — 29.0 (13.2) — — — 15.8 Change in fair value of BAM Surplus Notes — — .5 — — — — .5 Unrealized loss on deconsolidation of BAM — — (114.5) — — — — (114.5) Commission and fee revenues — — — — — 134.6 14.8 149.4 Other revenues 22.3 — .1 1.1 .8 3.6 56.8 84.7 Total revenues 1,651.6 99.3 (39.0) (5.6) 118.8 179.8 234.9 2,239.8 Loss and loss adjustment expenses 825.9 29.9 — — — 20.6 12.1 888.5 Acquisition expenses 283.9 23.2 7.8 .4 — 14.1 12.1 341.5 Cost of sales — — — — — — 29.6 29.6 Broker commission expenses — — — — — — 51.3 — 51.3 General and administrative expenses (4) (5) 208.3 .1 2.2 33.5 15.4 61.1 169.5 490.1 Change in fair value of contingent consideration 61.3 — — — — — — 61.3 Interest expense 19.5 — 16.7 — 22.1 — 2.5 60.8 Total expenses 1,398.9 53.2 26.7 33.9 37.5 147.1 225.8 1,923.1 Pre-tax income (loss) $ 252.7 $ 46.1 $ (65.7) $ (39.5) $ 81.3 $ 32.7 $ 9.1 $ 316.7 (1) Effective July 1, 2024, White Mountains no longer consolidates BAM. For the period from January 1, 2024 through June 30, 2024, BAM’s results of operations are presented within the HG Global segment. (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. (4) Ark’s general and administrative expenses include $136.1 of other underwriting expenses. (5) Bamboo’s general and administrative expenses include $16.4 of amortization of other intangible assets. Ark/WM Outrigger HG Global Other Millions Ark WM Outrigger Re HG Global BAM (1) Kudu Total Year Ended December 31, 2023 Earned insurance premiums $ 1,305.4 $ 104.3 $ 26.0 $ 5.2 $ — $ — $ 1,440.9 Net investment income 50.4 11.0 17.1 14.6 71.0 30.1 194.2 Net realized and unrealized investment gains (losses) 85.9 — 13.6 13.0 106.1 188.5 407.1 Net realized and unrealized investment gains — — — — — 27.1 27.1 Interest income (expense) from BAM Surplus Notes — — 26.2 (26.2) — — — Commission and fee revenues — — — — — 13.2 13.2 Other revenues .8 — — 2.9 — 80.5 84.2 Total revenues 1,442.5 115.3 82.9 9.5 177.1 339.4 2,166.7 Losses and loss adjustment expenses 711.2 15.6 — — — — 726.8 Acquisition expenses 251.0 30.5 7.4 1.2 — — 290.1 Cost of sales — — — — — 40.4 40.4 General and administrative expenses (2) 161.7 .3 2.8 66.1 19.4 182.3 432.6 Change in fair value of contingent consideration 48.7 — — — — — 48.7 Interest expense 21.3 — 16.5 — 21.2 3.7 62.7 Total expenses 1,193.9 46.4 26.7 67.3 40.6 226.4 1,601.3 Pre-tax income (loss) $ 248.6 $ 68.9 $ 56.2 $ (57.8) $ 136.5 $ 113.0 $ 565.4 (1) BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the BAM Surplus Notes and is not reduced by accruals of interest expense on the BAM Surplus Notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the NYDFS. (2) Ark’s general and administrative expenses include $113.6 of other underwriting expenses. Ark/WM Outrigger HG Global Other Millions Ark HG Global BAM (1) Kudu Total Year Ended December 31, 2022 Earned insurance premiums $ 1,043.4 $ 27.5 $ 5.8 $ — $ — $ 1,076.7 Net investment income 16.3 10.3 11.2 54.4 32.2 124.4 Net realized and unrealized investment gains (losses) (55.2) (52.5) (53.3) 64.1 (1.6) (98.5) Net realized and unrealized investment gains — — — — (93.0) (93.0) Interest income (expense) from BAM Surplus Notes — 11.7 (11.7) — — — Commission and fee revenues — — — — 11.5 11.5 Other revenues 5.0 .5 4.1 — 127.2 136.8 Total revenues 1,009.5 (2.5) (43.9) 118.5 76.3 1,157.9 Loss and loss adjustment expenses 536.4 — — — — 536.4 Acquisition expenses 239.4 9.3 1.9 — — 250.6 Cost of sales — — — — 98.6 98.6 General and administrative expenses (2) 106.2 2.8 66.3 14.7 174.1 364.1 Change in fair value of contingent consideration 17.3 — — — — 17.3 Interest expense 15.1 8.3 — 15.0 1.9 40.3 Total expenses 914.4 20.4 68.2 29.7 274.6 1,307.3 Pre-tax income (loss) $ 95.1 $ (22.9) $ (112.1) $ 88.8 $ (198.3) $ (149.4) (1) BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the BAM Surplus Notes and is not reduced by accruals of interest expense on the BAM Surplus Notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the NYDFS. (2) Ark’s general and administrative expenses includes $78.7 of other underwriting expenses. The following tables present White Mountains’s revenues from external customers by country for the years ended December 31, 2024, 2023 and 2022: December 31, 2024 Millions United States United Kingdom Bermuda Other Total Earned insurance premiums $ 2.8 $ 956.8 $ 732.0 $ — $ 1,691.6 Commission and fee revenues 134.6 — — 14.8 149.4 Other revenues (1) 55.8 — — — 55.8 Total $ 193.2 $ 956.8 $ 732.0 $ 14.8 $ 1,896.8 (1) Amounts include revenues from external customers related to certain consolidated Other Operating Businesses. December 31, 2023 Millions United States United Kingdom Bermuda Other Total Earned insurance premiums $ 5.2 $ 791.7 $ 644.0 $ — $ 1,440.9 Commission and fee revenues — — — 13.2 13.2 Other revenues (1) 69.0 — — — 69.0 Total $ 74.2 $ 791.7 $ 644.0 $ 13.2 $ 1,523.1 (1) Amounts include revenues from external customers related to certain consolidated Other Operating Businesses. December 31, 2022 Millions United States United Kingdom Bermuda Other Total Earned insurance premiums $ 5.8 $ 638.5 $ 432.4 $ — $ 1,076.7 Commission and fee revenues — — — 11.5 11.5 Other revenues (1) 124.0 — — — 124.0 Total $ 129.8 $ 638.5 $ 432.4 $ 11.5 $ 1,212.2 (1) Amounts include revenues from external customers related to certain consolidated Other Operating Businesses. The following table presents White Mountains’s balance sheet information by segment as of December 31, 2024 and 2023: Millions Ark/WM Outrigger HG Global Kudu Bamboo Other Total December 31, 2024: Total investments $ 3,139.7 $ 667.6 $ 1,041.9 $ 58.0 $ 1,570.4 $ 6,477.6 Total assets $ 5,299.0 $ 1,179.4 $ 1,108.4 $ 584.6 $ 1,754.2 (1) $ 9,925.6 Total liabilities $ 3,664.8 $ 464.1 (1) $ 316.7 $ 167.7 $ 181.3 $ 4,794.6 Total White Mountains’s common $ 1,223.8 $ 728.7 (1) $ 664.1 $ 303.3 $ 1,563.8 (1) $ 4,483.7 Noncontrolling interests $ 410.4 $ (13.4) $ 127.6 $ 113.6 $ 9.1 $ 647.3 December 31, 2023: Total investments $ 2,671.1 $ 1,082.9 $ 925.6 $ — $ 1,709.1 $ 6,388.7 Total assets $ 4,378.6 $ 1,172.0 (2) $ 959.4 $ — $ 1,875.9 (1) $ 8,385.9 Total liabilities $ 2,876.1 $ 531.7 (1) $ 275.4 $ — $ 141.1 $ 3,824.3 Total White Mountains’s common $ 1,165.6 $ 779.3 (1) $ 570.2 $ — $ 1,725.4 (1) $ 4,240.5 Noncontrolling interests $ 336.9 $ (139.0) $ 113.8 $ — $ 9.4 $ 321.1 (1) HG Global preferred dividends payable to White Mountains’s subsidiaries is eliminated in White Mountains’s consolidated financial statements. For segment reporting, the HG Global preferred dividends payable to White Mountains’s subsidiaries included within the HG Global segment are eliminated against the offsetting receivable included within Other Operations and therefore added back to White Mountains’s common shareholders’ equity within the HG Global segment. As of December 31, 2024 and 2023, the HG Global preferred dividends payable to White Mountains’s subsidiaries was $462.1 and $399.8. (2) As of December 31, 2023, total assets in the HG Global segment reflected the elimination of $322.2 of BAM Surplus Notes issued to HG Global and its subsidiaries and $174.5 in accrued interest related to the BAM Surplus Notes.

Equity Method Eligible Invest_2

Equity Method Eligible Investments (Tables)	12 Months Ended
	Dec. 31, 2024
Equity Method Investments and Joint Ventures [Abstract]
Schedule of significant equity method eligible investments	The following table presents the ownership interests and carrying values of White Mountains’s equity method eligible investments as of December 31, 2024 and 2023: December 31, 2024 December 31, 2023 $ in Millions Ownership Interest Carrying Value Ownership Interest Carrying Value Kudu’s Participation Contracts (1) 4.1% - 30.0% $ 1,008.4 4.1% - 30.0% $ 890.5 Investment in MediaAlpha 26.6% 201.6 34.9% 254.9 PassportCard/DavidShield 53.8% 150.0 53.8% 150.0 Elementum 26.6% 35.0 26.6% 35.0 Other equity method eligible investments, at fair value Under 50.0% 243.7 Under 50.0% 291.7 Other equity method eligible investments, at fair value 50.0% and over — 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. The following tables present aggregated summarized financial information for White Mountains’s investments in equity method eligible unconsolidated entities: December 31, Millions 2024 2023 Balance sheet data (1) : Total assets $ 3,719.1 $ 3,203.2 Total liabilities $ 1,063.9 $ 905.6 (1) Financial data for White Mountains’s equity method eligible investees is generally reported on a one-quarter lag. Year Ended December 31, Millions 2024 2023 2022 Income statement data (1) : Total revenues $ 2,011.8 $ 1,575.4 $ 1,194.4 Total expenses $ 1,634.9 $ 1,126.2 $ 1,047.4 Net income (loss) $ 376.9 $ 449.2 $ 147.0 (1) Financial data for White Mountains’s equity method eligible investees is generally reported on a one-quarter lag.

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2024
Fair Value Disclosures [Abstract]
Schedule 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 December 31, 2024 and 2023: December 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 $ 173.9 $ 154.5 $ 171.8 $ 155.5 HG Global Senior Notes $ 157.2 $ 147.4 $ 158.7 $ 146.9 Kudu Credit Facility $ 253.3 $ 238.6 $ 225.6 $ 203.8 Other Operations debt $ 23.1 $ 22.0 $ 30.0 $ 28.4

Held for Sale and Discontinue_2

Held for Sale and Discontinued Operations (Tables)	12 Months Ended
	Dec. 31, 2024
Discontinued Operations and Disposal Groups [Abstract]
Schedule of discontinued operations balance sheet, income statement, and cash flows	The following summarizes the results of operations, including related income taxes associated with the businesses classified as discontinued operations for the year ended December 31, 2022: December 31, 2022 (1) Millions Revenues Commission revenues $ 176.9 Other revenues 48.1 Total revenues - NSM Group 225.0 Expenses General and administrative expenses 126.8 Broker commission expenses 52.9 Change in fair value of contingent consideration .1 Amortization of other intangible assets 9.1 Interest expense 12.1 Total expenses - NSM Group 201.0 Pre-tax income (loss) from discontinued operations - NSM Group 24.0 Income tax (expense) benefit (7.6) Net income (loss) from discontinued operations, net tax - NSM Group 16.4 Net gain (loss) from sale of discontinued operations, net of 886.8 Total income (loss) from discontinued operations, net of tax 903.2 Net (income) loss from discontinued operations (.7) Total income (loss) from discontinued operations attributable to 902.5 Other comprehensive income (loss) from discontinued operations, (5.2) Net gain (loss) from foreign currency translation from sale of discontinued 2.9 Comprehensive income (loss) from discontinued operations 900.2 Other comprehensive (income) loss from discontinued operations attributable .2 Comprehensive income (loss) from discontinued operations attributable $ 900.4 (1) As a result of the NSM Transaction, the results of operations for NSM Group are presented for the period from January 1, 2022 to August 1, 2022. Net Change in Cash from Discontinued Operations The following summarizes the net change in cash associated with the businesses classified as discontinued operations for the years ended December 31, 2022: Year Ended Millions Net cash provided from (used for) operations $ 38.7 Net cash provided from (used for) investing activities 7.1 Net cash used from (used for) financing activities (17.5) Effect of exchange rate changes on cash 4.0 Net change in cash during the period 32.3 Cash balances at beginning of period (includes restricted cash of $89.2) 111.6 Cash sold as part of the sale of NSM Group (includes restricted cash of $105.1) (143.9) Cash balances at end of period (includes restricted cash of $0.0) $ — Supplemental cash flows information: Interest paid $ (12.0) Net income tax payments $ —
Schedule of earnings per share from continuing operations	The following table presents the Company’s computation of earnings per share from continuing operations for the years ended December 31, 2024, 2023 and 2022. See Note 20 — “Held for Sale and Discontinued Operations.” Year Ended December 31, 2024 2023 2022 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s $ 230.4 $ 509.2 $ 792.8 Less: total income (loss) from discontinued operations, net of tax (1) — — 903.2 Less: net (income) loss from discontinued operations attributable — — (.7) Net income (loss) from continuing operations attributable to 230.4 509.2 (109.7) Allocation of (earnings) losses to participating restricted common shares (2) (3.0) (7.2) 1.3 Basic and diluted earnings (losses) per share numerators $ 227.4 $ 502.0 $ (108.4) Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 2,565.7 2,563.3 2,862.4 Average unvested restricted common shares (3) (33.5) (36.0) (36.2) Basic earnings (losses) per share denominator 2,532.2 2,527.3 2,826.2 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 2,565.7 2,563.3 2,862.4 Average unvested restricted common shares (3) (33.5) (36.0) (36.2) Diluted earnings (losses) per share denominator 2,532.2 2,527.3 2,826.2 Basic and diluted earnings per share (in dollars) - continuing operations: Distributed earnings - dividends declared and paid $ 1.00 $ 1.00 $ 1.00 Undistributed earnings (losses) 88.79 197.60 (39.34) Basic and diluted earnings (losses) per share $ 89.79 $ 198.60 $ (38.34) (1) Includes net income (loss) from discontinued operations, net of tax - NSM Group and net gain (loss) from sale of discontinued operations, net of tax - NSM Group. See Note 20 — “Held for Sale and Discontinued Operations.” (2) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (3) Restricted shares outstanding vest upon a stated date. See Note 12 — “Employee Share-Based Incentive Compensation Plans.” Year Ended Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s common shareholders $ 792.8 Less: net income (loss) from continuing operations (190.8) Less: net (income) loss from continuing operations attributable to noncontrolling interest 81.1 Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders (1) 902.5 Allocation of (earnings) losses to participating restricted common shares (2) (11.4) Basic and diluted (loss) earnings per share numerators $ 891.1 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 2,862.4 Average unvested restricted common shares (3) (36.2) Basic earnings (loss) per share denominator 2,826.2 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 2,862.4 Average unvested restricted common shares (3) (36.2) Diluted earnings (loss) per share denominator 2,826.2 Basic (loss) earnings per share (in dollars) - discontinued operations: $ 315.30 Diluted (loss) earnings per share (in dollars) - discontinued operations: $ 315.30 (1) Includes net income (loss) from discontinued operations, net of tax - NSM Group, net gain (loss) from sale of discontinued operations, net of tax - NSM Group and net (income) loss from discontinued operations attributable to noncontrolling interests. (2) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities. (3) Restricted shares outstanding vest upon a stated date. See Note 12 — “Employee Share-Based Incentive Compensation Plans.”

Basis of Presentation and Sig_3

Basis of Presentation and Significant Accounting Policies (Details) $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($) segment	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	May 26, 2022	May 25, 2022	Jan. 01, 2021 USD ($)
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of reportable segments | segment	4
Other long-term investments	$ 2,150.2	$ 1,998.2
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Other assets	Other assets
Operating lease, right-of-use asset	$ 32.6	$ 20.3
Operating Lease, Liability, Statement of Financial Position [Extensible List]	Other liabilities	Other liabilities
Lease liabilities	$ 35.5	$ 22
Net unrealized gains (losses) from foreign currency translation	(1.7)	(1.6)
Change in fair value of contingent consideration - Ark	61.3	48.7	$ 17.3
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Other long-term investments	547.8	440.9
Contingent consideration	155.3	94				$ 22.5
Change in fair value of contingent consideration - Ark	$ 61.3	48.7	$ 17.3
Third Point Re | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Annual crediting rate	3%
Level 2
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Variance threshold for outliers	$ 0.5
Variance threshold for outliers (as a percent)	5%
Level 2 | Fair value measured on a recurring basis | Other long-term investments
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Other long-term investments	$ 23.5	26.2
Level 3 | Fair value measured on a recurring basis | Other long-term investments
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Other long-term investments	1,262.7	1,138.2
NAV | Fair value measured on a recurring basis | Other long-term investments
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Other long-term investments	864	$ 833.8
Subsidiary - HG Global
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Purchase of surplus notes	$ 503
Subsidiary - HG Global | Maximum
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Percentage of par value of policy reinsured	15%
Ark
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Percent of shares outstanding	12.30%
Subsidiary - Ark Insurance Holdings Limited
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Basic ownership	72.10%	72%
Ownership percentage, after equity incentives	61.90%	61.90%
Subsidiary - WM Outrigger Re | Preferred stocks
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Basic ownership	100%	100%
Subsidiary - HG Global | Preferred stocks
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Basic ownership	96.90%	96.90%
Subsidiary - HG Global | Common Stock
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Basic ownership	88.40%	88.40%
Kudu
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Basic ownership	90.40%
Ownership percentage basic units outstanding		89.60%		89.30%	99.10%
Ownership percentage, fully diluted	77%
Ownership percentage, fully converted		76.30%
Bamboo
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Basic ownership	72.80%
Ownership percentage, fully diluted	63.70%

Significant Transactions - Bamb

Significant Transactions - Bamboo (Details) - USD ($) $ in Millions	Jan. 02, 2024	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
Contributions to retire credit facility	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%

Significant Transactions - Sche

Significant Transactions - Schedule of Assets Acquired and Liabilities Assumed, Bamboo (Details) - USD ($) $ in Millions	Jan. 02, 2024	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Other Significant Transactions [Line Items]
Goodwill		$ 439.2	$ 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 - BAM

Significant Transactions - BAM (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Jul. 01, 2024
Other Significant Transactions [Line Items]
Unrealized loss on deconsolidation of BAM	$ 114.5	$ 0	$ 0
Operating Segments | Financial Guarantee (HG Global)
Other Significant Transactions [Line Items]
Notes receivable, fair value disclosure	381.7	0		$ 387.4
Unrealized loss on deconsolidation of BAM	$ 114.5	$ 0	$ 0

Significant Transactions - NSM

Significant Transactions - NSM (Details) - USD ($) $ in Millions		12 Months Ended
	Aug. 01, 2022	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Other Significant Noncash Transactions [Line Items]
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group		$ 0	$ 0	$ 2.9
NSM
Other Significant Noncash Transactions [Line Items]
Net gain (loss) from sale of discontinued operations, net of tax - NSM Group		$ 0	$ 0	886.8
Discontinued Operations, Disposed of by Sale | NSM
Other Significant Noncash Transactions [Line Items]
Disposal group, consideration	$ 1,400
Net transaction gain	875.7
Net gain (loss) from sale of discontinued operations, net of tax - NSM Group	886.8			886.8
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	2.9			2.9
General and administrative expenses	$ 14			$ 126.8

Significant Transactions - WM O

Significant Transactions - WM Outrigger Re (Details) - USD ($) $ in Millions		3 Months Ended			12 Months Ended
	Dec. 20, 2022	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2024
Outrigger Re
Other Significant Transactions [Line Items]
Ceded unearned premiums		$ 230	$ 250		$ 230
Rollover amount		130	130
Net distributions			75		122.7
Commitment amount		150
Short-term investments, at fair value		$ 203.7	$ 265.3		$ 203.7
Outrigger Re
Other Significant Transactions [Line Items]
Business combination, consideration transferred				$ 205
Redeemable Preferred Stock | Outrigger Re Ltd
Other Significant Transactions [Line Items]
Issuances of common shares	$ 250

Significant Transactions - Kudu

Significant Transactions - Kudu (Details) $ in Millions	May 26, 2022 USD ($)	Dec. 31, 2023	May 25, 2022	Dec. 31, 2021 USD ($) portfolioCompany
Other Significant Noncash Transactions [Line Items]
Number of portfolio companies | portfolioCompany				2
Kudu
Other Significant Noncash Transactions [Line Items]
Ownership percentage basic units outstanding	89.30%	89.60%	99.10%
Kudu
Other Significant Noncash Transactions [Line Items]
Equity capital raised	$ 114.5
Pre-money valuation ratio	1.3
Pre-money valuation amount	$ 114
Kudu | Kudu’s Participation Contracts
Other Significant Noncash Transactions [Line Items]
Enterprise value excluded				$ 54.3
Kudu | Massachusetts Mutual Life Insurance Company
Other Significant Noncash Transactions [Line Items]
Equity capital raised	64.1
Kudu | White Mountains
Other Significant Noncash Transactions [Line Items]
Equity capital raised	50
Kudu | Kudu
Other Significant Noncash Transactions [Line Items]
Equity capital raised	$ 0.4

Investment Securities - Schedul

Investment Securities - Schedule of Pre-Tax Net Investment Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Net Investment Income [Line Items]
Total investment income	$ 230.2	$ 196.6	$ 126.5
Third-party investment expenses	(2.8)	(2.4)	(2.1)
Net investment income, pre-tax	227.4	194.2	124.4
Fixed maturity investments
Net Investment Income [Line Items]
Total investment income	93.2	64.4	41.4
Short-term investments
Net Investment Income [Line Items]
Total investment income	56.6	52	16.4
Common equity securities
Net Investment Income [Line Items]
Total investment income	2.5	4.1	1.6
Other long-term investments
Net Investment Income [Line Items]
Total investment income	77.9	76.1	68.1
Amount attributable to TPC Providers
Net Investment Income [Line Items]
Amount attributable to TPC Providers	$ 0	$ 0	$ (1)

Investment Securities - Net Rea

Investment Securities - Net Realized and Unrealized Investment Gains and Losses - (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	$ 184.9	$ 434.2	$ (191.5)
Amount attributable to TPC Providers	0	0	6.8
Investment in MediaAlpha
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	38	27.1	(93)
Net realized and unrealized investment (losses) gains
Debt Securities, Available-for-sale [Line Items]
Net realized investment gains (losses)	118	96.8	115.1
Net unrealized investment gains (losses)	66.9	337.4	(313.4)
Net realized and unrealized investment gains (losses)	184.9	434.2	(198.3)
Fixed maturity investments
Debt Securities, Available-for-sale [Line Items]
Net realized investment gains (losses)	(10.1)	(4.7)	(11.9)
Net unrealized investment gains (losses)	3.9	66.1	(168.4)
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Net realized investment gains (losses)	(1.2)	(0.2)	(1.4)
Net unrealized investment gains (losses)	(0.8)	1.9	(1.1)
Common equity securities
Debt Securities, Available-for-sale [Line Items]
Net realized investment gains (losses)	3.5	29.5	0
Net unrealized investment gains (losses)	58.1	47.1	(7.1)
Net realized and unrealized investment gains (losses)	61.6	76.6	(7.1)
Equity Method Investments | Investment in MediaAlpha
Debt Securities, Available-for-sale [Line Items]
Net realized investment gains (losses)	91.2	0	0
Net unrealized investment gains (losses)	(53.2)	27.1	(93)
Net realized and unrealized investment gains (losses)	38	27.1	(93)
Other long-term investments
Debt Securities, Available-for-sale [Line Items]
Net realized investment gains (losses)	34.6	72.2	128.4
Net unrealized investment gains (losses)	58.9	195.2	(43.8)
Fixed Maturities And Other Short-Term Investments
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	(8.2)	63.1	(182.8)
Less: net realized and unrealized gains (losses) on investment securities sold during the period	0.9	4.7	(2.9)
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	(9.1)	58.4	(179.9)
Common Stock And Equity Method Investments
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	99.6	103.7	(100.1)
Less: net realized and unrealized gains (losses) on investment securities sold during the period	35.4	28.4	0
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	64.2	75.3	(100.1)
Total Investments After TPC Providers - Investments
Debt Securities, Available-for-sale [Line Items]
Gain (loss) on investments, exchange rate effect	$ (31.5)	$ 13.6	$ (29.3)

Investment Securities - Narrati

Investment Securities - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		12 Months Ended
	Jun. 30, 2024 USD ($) $ / shares shares	Jun. 30, 2023 $ / shares shares	Dec. 31, 2024 USD ($) fund $ / shares shares	Dec. 31, 2023 USD ($) $ / shares	Dec. 31, 2022 USD ($)
Concentration Risk [Line Items]
Gross realized investment gains			$ 139.1	$ 113.7	$ 129.9
Loss on sale of investment securities			21.1	16.9	14.8
Proceeds from sale and maturity of investments			$ 1,200	$ 800	$ 500
Weighted average portfolio period			1 year 10 months 24 days	1 year 10 months 24 days
Weighted average portfolio period, excluding short-term invetments			2 years 7 months 6 days	3 years 3 months 18 days
Other long-term investments			$ 2,150.2	$ 1,998.2
Total investments			6,477.6	6,388.7
Level 3 | Recent transaction
Concentration Risk [Line Items]
Other long-term investments			23.4	19
Private equity funds
Concentration Risk [Line Items]
Other long-term investments			$ 249.5
Number of investments | fund			17
Hedge funds
Concentration Risk [Line Items]
Number of investments | fund			2
Hedge funds | Minimum
Concentration Risk [Line Items]
Notice period			45 days
Bank loan fund
Concentration Risk [Line Items]
Other long-term investments			$ 264.7	194.4
Other long-term investments, redemption, written notice period			15 days
Foreign Regulatory Deposits
Concentration Risk [Line Items]
Other long-term investments			$ 149.9	158
ILS funds
Concentration Risk [Line Items]
Other long-term investments			$ 74	160.5
ILS funds | Minimum
Concentration Risk [Line Items]
Notice period			30 days
ILS funds | Maximum
Concentration Risk [Line Items]
Notice period			90 days
IPO | MediaAlpha
Concentration Risk [Line Items]
Increase in book value per share (in dollars per share) | $ / shares			$ 7
Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value			$ 236.7	209.1
Collateralized loan obligations | Level 3
Concentration Risk [Line Items]
Fixed maturity investments, at fair value			0	0
Fair value measured on a recurring basis | MediaAlpha
Concentration Risk [Line Items]
Other long-term investments			201.6	254.9
Fair value measured on a recurring basis | MediaAlpha | Level 3
Concentration Risk [Line Items]
Other long-term investments			0	0
Fair value measured on a recurring basis | Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value			236.7	209.1
Fair value measured on a recurring basis | Collateralized loan obligations | Level 3
Concentration Risk [Line Items]
Fixed maturity investments, at fair value			0	0
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments			149.9	158
Lloyds Bank
Concentration Risk [Line Items]
Total investments			361.5	345.2
Additional Investments, Insurance Regulators and Reinsurance Counterparties | Asset Pledged as Collateral
Concentration Risk [Line Items]
Total investments			226.5	244.5
Outrigger Re
Concentration Risk [Line Items]
Short-term investments, at fair value			203.7	$ 265.3
Bamboo CRV
Concentration Risk [Line Items]
Restricted cash			32.2
Pledged collateral			$ 46.7
MediaAlpha
Concentration Risk [Line Items]
Shares purchased (in shares) | shares	7.6	5.9
Purchase price (in dollars per share) | $ / shares	$ 19	$ 10	$ 11.29	$ 11.15
Net of underwriting fees (in dollars per share) | $ / shares	$ 18.24
Investment owned (in shares) | shares			17.9
Ownership interest (as a percent)			26.60%	34.90%
Fully diluted ownership interest (as a percent)			25.40%
Increase in investment owned (in dollars per share) | $ / shares			$ 1
MediaAlpha | Secondary Offering
Concentration Risk [Line Items]
Number of shares issued (in shares) | shares	5
Proceeds from sale of business	$ 91.2
Largest Investment | Investments | Private Equity and Hedge Fund Concentration Risk
Concentration Risk [Line Items]
Other long-term investments			$ 59.2	$ 57.3

Investment Securities - Investm

Investment Securities - Investment Gains and Losses For Level 3 (Details) - Other long-term investments - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Investment securities
Net unrealized investment gains (losses)	$ 58.9	$ 195.2	$ (43.8)
Level 3
Investment securities
Net unrealized investment gains (losses)	$ 57.2	$ 133.9	$ 56.5

Investment Securities - Inves_2

Investment Securities - Investment Holdings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
U.S. Government and agency obligations
Investment securities
Cost or Amortized Cost	$ 461.8	$ 209
Gross Unrealized Gains	1.1	0.3
Gross Unrealized Losses	(0.8)	(4.4)
Net Foreign Currency Gains (Losses)	0	0
Carrying Value	462.1	204.9
Debt securities issued by corporations
Investment securities
Cost or Amortized Cost	1,444.5	1,085.9
Gross Unrealized Gains	4.6	5.6
Gross Unrealized Losses	(31.4)	(45.4)
Net Foreign Currency Gains (Losses)	(3.5)	(1.1)
Carrying Value	1,414.2	1,045
Municipal obligations
Investment securities
Cost or Amortized Cost	3.2	275.1
Gross Unrealized Gains	0	0.9
Gross Unrealized Losses	0	(15.1)
Net Foreign Currency Gains (Losses)	0	0
Carrying Value	3.2	260.9
Mortgage and asset-backed securities
Investment securities
Cost or Amortized Cost	400.2	417.2
Gross Unrealized Gains	0.2	2
Gross Unrealized Losses	(26.5)	(29.8)
Net Foreign Currency Gains (Losses)	0	0
Carrying Value	373.9	389.4
Collateralized loan obligations
Investment securities
Cost or Amortized Cost	237.3	211.2
Gross Unrealized Gains	1.2	0.4
Gross Unrealized Losses	(0.2)	(2.4)
Net Foreign Currency Gains (Losses)	(1.6)	(0.1)
Carrying Value	236.7	209.1
Foreign government and agency obligations
Investment securities
Cost or Amortized Cost	22.2
Gross Unrealized Gains	0
Gross Unrealized Losses	(0.1)
Net Foreign Currency Gains (Losses)	(0.6)
Carrying Value	21.5
Fixed maturity investments
Investment securities
Cost or Amortized Cost	2,569.2	2,198.4
Gross Unrealized Gains	7.1	9.2
Gross Unrealized Losses	(59)	(97.1)
Net Foreign Currency Gains (Losses)	(5.7)	(1.2)
Carrying Value	$ 2,511.6	$ 2,109.3

Investment Securities - Cost an

Investment Securities - Cost and Amortized Cost Maturity Schedule (Details) - USD ($) $ in Millions	Dec. 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	$ 205.8	$ 319.9
Due after one year through five years	1,494.5	845
Due after five years through ten years	206.1	303.4
Due after ten years	25.3	101.7
Carrying Value
Due in one year or less	203.9	315
Due after one year through five years	1,478.7	813.9
Due after five years through ten years	193.2	287
Due after ten years	25.2	94.9
Mortgage and asset-backed securities and collateralized loan obligations
Cost or Amortized Cost
Mortgage and asset-backed securities and collateralized loan obligations	637.5	628.4
Carrying Value
Mortgage and asset-backed securities and collateralized loan obligations	610.6	598.5
Fixed maturity investments
Cost or Amortized Cost
Total fixed maturity investments	2,569.2	2,198.4
Carrying Value
Total fixed maturity investments	$ 2,511.6	$ 2,109.3

Investment Securities - Common

Investment Securities - Common Equity Securities and Other Long Term Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Common equity securities
Cost or Amortized Cost	$ 537	$ 483.5
Gross Unrealized Gains	120.4	62.3
Gross Unrealized Losses	0	(1.2)
Net Foreign Currency Gains (Losses)	(7.4)	(6.2)
Carrying Value	650	538.4
Other long-term investments
Cost or Amortized Cost	1,748.7	1,655.7
Gross Unrealized Gains	546.4	446.3
Gross Unrealized Losses	(116.7)	(94.5)
Net Foreign Currency Gains (Losses)	(28.2)	(9.3)
Carrying Value	2,150.2	1,998.2
MediaAlpha
Investment in MediaAlpha
Cost or Amortized Cost	59.2	59.2
Gross Unrealized Gains	142.4	195.7
Gross Unrealized Losses	0	0
Net Foreign Currency Gains (Losses)	0	0
Carrying Value	$ 201.6	$ 254.9

Investment Securities - Fair Va

Investment Securities - Fair Value Measurement by Level (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Investment securities
Common equity securities, at fair value	$ 650	$ 538.4
Other long-term investments	2,150.2	1,998.2
U.S. Government and agency obligations
Investment securities
Fixed maturity investments, at fair value	462.1	204.9
Debt securities issued by corporations
Investment securities
Fixed maturity investments, at fair value	1,414.2	1,045
Municipal obligations
Investment securities
Fixed maturity investments, at fair value	3.2	260.9
Mortgage and asset-backed securities
Investment securities
Fixed maturity investments, at fair value	373.9	389.4
Mortgage and asset-backed securities | Level 2
Investment securities
Fixed maturity investments, at fair value	373.9	389.4
Mortgage and asset-backed securities | Level 3
Investment securities
Fixed maturity investments, at fair value	0	0
Collateralized loan obligations
Investment securities
Fixed maturity investments, at fair value	236.7	209.1
Collateralized loan obligations | Level 2
Investment securities
Fixed maturity investments, at fair value	236.7	209.1
Collateralized loan obligations | Level 3
Investment securities
Fixed maturity investments, at fair value	0	0
Foreign government and agency obligations
Investment securities
Fixed maturity investments, at fair value	21.5
Fixed maturity investments
Investment securities
Fixed maturity investments, at fair value	2,511.6	2,109.3
Fair value measured on a recurring basis
Investment securities
Fair value investments	6,477.6	6,388.7
Fair value measured on a recurring basis | MediaAlpha
Investment securities
Other long-term investments	201.6	254.9
Fair value measured on a recurring basis | Level 1
Investment securities
Fair value investments	1,838.6	2,055.2
Fair value measured on a recurring basis | Level 1 | MediaAlpha
Investment securities
Other long-term investments	201.6	254.9
Fair value measured on a recurring basis | Level 2
Investment securities
Fair value investments	2,512.3	2,361.5
Fair value measured on a recurring basis | Level 2 | MediaAlpha
Investment securities
Other long-term investments	0	0
Fair value measured on a recurring basis | Level 3
Investment securities
Fair value investments	1,262.7	1,138.2
Fair value measured on a recurring basis | Level 3 | MediaAlpha
Investment securities
Other long-term investments	0	0
Fair value measured on a recurring basis | U.S. Government and agency obligations
Investment securities
Fixed maturity investments, at fair value	462.1	204.9
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 1
Investment securities
Fixed maturity investments, at fair value	461.3	204.9
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 2
Investment securities
Fixed maturity investments, at fair value	0.8	0
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 3
Investment securities
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fixed maturity investments, at fair value	1,414.2	1,045
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1
Investment securities
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2
Investment securities
Fixed maturity investments, at fair value	1,414.2	1,045
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3
Investment securities
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Municipal obligations
Investment securities
Fixed maturity investments, at fair value	3.2	260.9
Fair value measured on a recurring basis | Municipal obligations | Level 1
Investment securities
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Municipal obligations | Level 2
Investment securities
Fixed maturity investments, at fair value	3.2	260.9
Fair value measured on a recurring basis | Municipal obligations | Level 3
Investment securities
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Mortgage and asset-backed securities
Investment securities
Fixed maturity investments, at fair value	373.9	389.4
Fair value measured on a recurring basis | Mortgage and asset-backed securities | Level 1
Investment securities
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Mortgage and asset-backed securities | Level 2
Investment securities
Fixed maturity investments, at fair value	373.9	389.4
Fair value measured on a recurring basis | Mortgage and asset-backed securities | Level 3
Investment securities
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Collateralized loan obligations
Investment securities
Fixed maturity investments, at fair value	236.7	209.1
Fair value measured on a recurring basis | Collateralized loan obligations | Level 1
Investment securities
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Collateralized loan obligations | Level 2
Investment securities
Fixed maturity investments, at fair value	236.7	209.1
Fair value measured on a recurring basis | Collateralized loan obligations | Level 3
Investment securities
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Foreign government and agency obligations
Investment securities
Fixed maturity investments, at fair value	21.5
Fair value measured on a recurring basis | Foreign government and agency obligations | Level 1
Investment securities
Fixed maturity investments, at fair value	0
Fair value measured on a recurring basis | Foreign government and agency obligations | Level 2
Investment securities
Fixed maturity investments, at fair value	21.5
Fair value measured on a recurring basis | Foreign government and agency obligations | Level 3
Investment securities
Fixed maturity investments, at fair value	0
Fair value measured on a recurring basis | Fixed maturity investments
Investment securities
Fixed maturity investments, at fair value	2,511.6	2,109.3
Fair value measured on a recurring basis | Fixed maturity investments | Level 1
Investment securities
Fixed maturity investments, at fair value	461.3	204.9
Fair value measured on a recurring basis | Fixed maturity investments | Level 2
Investment securities
Fixed maturity investments, at fair value	2,050.3	1,904.4
Fair value measured on a recurring basis | Fixed maturity investments | Level 3
Investment securities
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Short-term investments
Investment securities
Short-term investments, at fair value	964.2	1,487.9
Fair value measured on a recurring basis | Short-term investments | Level 1
Investment securities
Short-term investments, at fair value	951.1	1,457.6
Fair value measured on a recurring basis | Short-term investments | Level 2
Investment securities
Short-term investments, at fair value	13.1	30.3
Fair value measured on a recurring basis | Short-term investments | Level 3
Investment securities
Short-term investments, at fair value	0	0
Fair value measured on a recurring basis | Exchange-traded funds
Investment securities
Common equity securities, at fair value	224.6	137.8
Fair value measured on a recurring basis | Exchange-traded funds | Level 1
Investment securities
Common equity securities, at fair value	224.6	137.8
Fair value measured on a recurring basis | Exchange-traded funds | Level 2
Investment securities
Common equity securities, at fair value	0	0
Fair value measured on a recurring basis | Exchange-traded funds | Level 3
Investment securities
Common equity securities, at fair value	0	0
Fair value measured on a recurring basis | Common stock, other
Investment securities
Common equity securities, at fair value	425.4	400.6
Fair value measured on a recurring basis | Common stock, other | Level 1
Investment securities
Common equity securities, at fair value	0	0
Fair value measured on a recurring basis | Common stock, other | Level 2
Investment securities
Common equity securities, at fair value	425.4	400.6
Fair value measured on a recurring basis | Common stock, other | Level 3
Investment securities
Common equity securities, at fair value	0	0
Fair value measured on a recurring basis | Common equity securities
Investment securities
Common equity securities, at fair value	650	538.4
Fair value measured on a recurring basis | Common equity securities | Level 1
Investment securities
Common equity securities, at fair value	224.6	137.8
Fair value measured on a recurring basis | Common equity securities | Level 2
Investment securities
Common equity securities, at fair value	425.4	400.6
Fair value measured on a recurring basis | Common equity securities | Level 3
Investment securities
Common equity securities, at fair value	0	0
Fair value measured on a recurring basis | Other long-term investments | Fair Value, Inputs, Level 1, Level 2, and Level 3
Investment securities
Other long-term investments	1,286.2	1,164.4
Fair value measured on a recurring basis | Other long-term investments | Level 1
Investment securities
Other long-term investments	0	0
Fair value measured on a recurring basis | Other long-term investments | Level 2
Investment securities
Other long-term investments	23.5	26.2
Fair value measured on a recurring basis | Other long-term investments | Level 3
Investment securities
Other long-term investments	1,262.7	1,138.2
Fair value measured on a recurring basis | Other long-term investments | NAV
Investment securities
Other long-term investments	864	833.8
Fair value measured on a recurring basis | Other Long-Term Investments
Investment securities
Other long-term investments	2,150.2	1,998.2
Fair value measured on a recurring basis | Other Long-Term Investments | Level 1
Investment securities
Other long-term investments	0	0
Fair value measured on a recurring basis | Other Long-Term Investments | Level 2
Investment securities
Other long-term investments	23.5	26.2
Fair value measured on a recurring basis | Other Long-Term Investments | Level 3
Investment securities
Other long-term investments	1,262.7	1,138.2
Financials | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fixed maturity investments, at fair value	466.7	309.8
Financials | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1
Investment securities
Fixed maturity investments, at fair value	0	0
Financials | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2
Investment securities
Fixed maturity investments, at fair value	466.7	309.8
Financials | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3
Investment securities
Fixed maturity investments, at fair value	0	0
Consumer | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fixed maturity investments, at fair value	255.2	214.6
Consumer | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1
Investment securities
Fixed maturity investments, at fair value	0	0
Consumer | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2
Investment securities
Fixed maturity investments, at fair value	255.2	214.6
Consumer | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3
Investment securities
Fixed maturity investments, at fair value	0	0
Industrial | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fixed maturity investments, at fair value	164.4	121.6
Industrial | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1
Investment securities
Fixed maturity investments, at fair value	0	0
Industrial | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2
Investment securities
Fixed maturity investments, at fair value	164.4	121.6
Industrial | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3
Investment securities
Fixed maturity investments, at fair value	0	0
Healthcare | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fixed maturity investments, at fair value	153.1	121.3
Healthcare | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1
Investment securities
Fixed maturity investments, at fair value	0	0
Healthcare | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2
Investment securities
Fixed maturity investments, at fair value	153.1	121.3
Healthcare | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3
Investment securities
Fixed maturity investments, at fair value	0	0
Technology | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fixed maturity investments, at fair value	113.8	108.9
Technology | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1
Investment securities
Fixed maturity investments, at fair value	0	0
Technology | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2
Investment securities
Fixed maturity investments, at fair value	113.8	108.9
Technology | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3
Investment securities
Fixed maturity investments, at fair value	0	0
Communications | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fixed maturity investments, at fair value	71.6	46.3
Communications | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1
Investment securities
Fixed maturity investments, at fair value	0	0
Communications | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2
Investment securities
Fixed maturity investments, at fair value	71.6	46.3
Communications | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3
Investment securities
Fixed maturity investments, at fair value	0	0
Materials | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fixed maturity investments, at fair value	65.7	23.4
Materials | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1
Investment securities
Fixed maturity investments, at fair value	0	0
Materials | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2
Investment securities
Fixed maturity investments, at fair value	65.7	23.4
Materials | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3
Investment securities
Fixed maturity investments, at fair value	0	0
Energy | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fixed maturity investments, at fair value	62.8	28.9
Energy | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1
Investment securities
Fixed maturity investments, at fair value	0	0
Energy | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2
Investment securities
Fixed maturity investments, at fair value	62.8	28.9
Energy | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3
Investment securities
Fixed maturity investments, at fair value	0	0
Utilities | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fixed maturity investments, at fair value	60.9	70.2
Utilities | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1
Investment securities
Fixed maturity investments, at fair value	0	0
Utilities | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2
Investment securities
Fixed maturity investments, at fair value	60.9	70.2
Utilities | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3
Investment securities
Fixed maturity investments, at fair value	$ 0	$ 0

Investment Securities - Debt Se

Investment Securities - Debt Securities Issued By Corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Investment securities
Fixed maturity investments, at fair value	$ 1,414.2	$ 1,045
AAA
Investment securities
Fixed maturity investments, at fair value	14.6	11.5
AA
Investment securities
Fixed maturity investments, at fair value	91.2	83.8
A
Investment securities
Fixed maturity investments, at fair value	607.2	552.4
BBB
Investment securities
Fixed maturity investments, at fair value	688.7	390.9
BB
Investment securities
Fixed maturity investments, at fair value	5.4	0
Other
Investment securities
Fixed maturity investments, at fair value	$ 7.1	$ 6.4

Investment Securities - Mortgag

Investment Securities - Mortgage-backed Asset Securities (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Total mortgage and asset-backed securities and collateralized loan obligations
Investment securities
Fixed maturity investments, at fair value	$ 610.6	$ 598.5
Mortgage and asset-backed securities
Investment securities
Fixed maturity investments, at fair value	373.9	389.4
Total mortgage-backed securities
Investment securities
Fixed maturity investments, at fair value	370	366.4
Total mortgage-backed securities | Total non-agency
Investment securities
Fixed maturity investments, at fair value	0.4	0.2
Total mortgage-backed securities | Total agency
Investment securities
Fixed maturity investments, at fair value	369.6	366.2
Total mortgage-backed securities | FNMA
Investment securities
Fixed maturity investments, at fair value	198.3	198.8
Total mortgage-backed securities | FHLMC
Investment securities
Fixed maturity investments, at fair value	147.1	137.2
Total mortgage-backed securities | GNMA
Investment securities
Fixed maturity investments, at fair value	24.2	30.2
Total mortgage-backed securities | Commercial
Investment securities
Fixed maturity investments, at fair value	0.4	0
Total mortgage-backed securities | Residential
Investment securities
Fixed maturity investments, at fair value	0	0.2
Total other asset-backed securities
Investment securities
Fixed maturity investments, at fair value	3.9	23
Total other asset-backed securities | Vehicle receivables
Investment securities
Fixed maturity investments, at fair value	1.7	17.2
Total other asset-backed securities | Credit card receivables
Investment securities
Fixed maturity investments, at fair value	0.2	3.3
Total other asset-backed securities | Other
Investment securities
Fixed maturity investments, at fair value	2	2.5
Collateralized loan obligations
Investment securities
Fixed maturity investments, at fair value	236.7	209.1
Level 2 | Total mortgage and asset-backed securities and collateralized loan obligations
Investment securities
Fixed maturity investments, at fair value	610.6	598.5
Level 2 | Mortgage and asset-backed securities
Investment securities
Fixed maturity investments, at fair value	373.9	389.4
Level 2 | Total mortgage-backed securities
Investment securities
Fixed maturity investments, at fair value	370	366.4
Level 2 | Total mortgage-backed securities | Total non-agency
Investment securities
Fixed maturity investments, at fair value	0.4	0.2
Level 2 | Total mortgage-backed securities | Total agency
Investment securities
Fixed maturity investments, at fair value	369.6	366.2
Level 2 | Total mortgage-backed securities | FNMA
Investment securities
Fixed maturity investments, at fair value	198.3	198.8
Level 2 | Total mortgage-backed securities | FHLMC
Investment securities
Fixed maturity investments, at fair value	147.1	137.2
Level 2 | Total mortgage-backed securities | GNMA
Investment securities
Fixed maturity investments, at fair value	24.2	30.2
Level 2 | Total mortgage-backed securities | Commercial
Investment securities
Fixed maturity investments, at fair value	0.4	0
Level 2 | Total mortgage-backed securities | Residential
Investment securities
Fixed maturity investments, at fair value	0	0.2
Level 2 | Total other asset-backed securities
Investment securities
Fixed maturity investments, at fair value	3.9	23
Level 2 | Total other asset-backed securities | Vehicle receivables
Investment securities
Fixed maturity investments, at fair value	1.7	17.2
Level 2 | Total other asset-backed securities | Credit card receivables
Investment securities
Fixed maturity investments, at fair value	0.2	3.3
Level 2 | Total other asset-backed securities | Other
Investment securities
Fixed maturity investments, at fair value	2	2.5
Level 2 | Collateralized loan obligations
Investment securities
Fixed maturity investments, at fair value	236.7	209.1
Level 3 | Total mortgage and asset-backed securities and collateralized loan obligations
Investment securities
Fixed maturity investments, at fair value	0	0
Level 3 | Mortgage and asset-backed securities
Investment securities
Fixed maturity investments, at fair value	0	0
Level 3 | Total mortgage-backed securities
Investment securities
Fixed maturity investments, at fair value	0	0
Level 3 | Total mortgage-backed securities | Total non-agency
Investment securities
Fixed maturity investments, at fair value	0	0
Level 3 | Total mortgage-backed securities | Total agency
Investment securities
Fixed maturity investments, at fair value	0	0
Level 3 | Total mortgage-backed securities | FNMA
Investment securities
Fixed maturity investments, at fair value	0	0
Level 3 | Total mortgage-backed securities | FHLMC
Investment securities
Fixed maturity investments, at fair value	0	0
Level 3 | Total mortgage-backed securities | GNMA
Investment securities
Fixed maturity investments, at fair value	0	0
Level 3 | Total mortgage-backed securities | Commercial
Investment securities
Fixed maturity investments, at fair value	0	0
Level 3 | Total mortgage-backed securities | Residential
Investment securities
Fixed maturity investments, at fair value	0	0
Level 3 | Total other asset-backed securities
Investment securities
Fixed maturity investments, at fair value	0	0
Level 3 | Total other asset-backed securities | Vehicle receivables
Investment securities
Fixed maturity investments, at fair value	0	0
Level 3 | Total other asset-backed securities | Credit card receivables
Investment securities
Fixed maturity investments, at fair value	0	0
Level 3 | Total other asset-backed securities | Other
Investment securities
Fixed maturity investments, at fair value	0	0
Level 3 | Collateralized loan obligations
Investment securities
Fixed maturity investments, at fair value	$ 0	$ 0

Investment Securities - Other L

Investment Securities - Other Long-Term Investments (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Investment securities
Other long-term investments	$ 2,150.2	$ 1,998.2
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	547.8	440.9
Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	1,014	896.3
Other Operations
Investment securities
Other long-term investments	588.4	661
PassportCard/DavidShield
Investment securities
Other long-term investments	150	150
PassportCard/DavidShield | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	0	0
PassportCard/DavidShield | Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	0	0
PassportCard/DavidShield | Other Operations
Investment securities
Other long-term investments	150	150
Elementum
Investment securities
Other long-term investments	35	35
Elementum | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	0	0
Elementum | Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	0	0
Elementum | Other Operations
Investment securities
Other long-term investments	35	35
Other unconsolidated entities
Investment securities
Other long-term investments	63.6	48.1
Other unconsolidated entities | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	0	0
Other unconsolidated entities | Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	0	0
Other unconsolidated entities | Other Operations
Investment securities
Other long-term investments	63.6	48.1
Private equity funds and hedge funds
Investment securities
Other long-term investments	360.6	312.9
Private equity funds and hedge funds | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	104.1	55.5
Private equity funds and hedge funds | Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	0	0
Private equity funds and hedge funds | Other Operations
Investment securities
Other long-term investments	256.5	257.4
Bank loan fund
Investment securities
Other long-term investments	264.7	194.4
Bank loan fund | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	264.7	194.4
Bank loan fund | Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	0	0
Bank loan fund | Other Operations
Investment securities
Other long-term investments	0	0
Lloyd’s trust deposits
Investment securities
Other long-term investments	149.9	158
Lloyd’s trust deposits | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	149.9	158
Lloyd’s trust deposits | Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	0	0
Lloyd’s trust deposits | Other Operations
Investment securities
Other long-term investments	0	0
ILS funds
Investment securities
Other long-term investments	74	160.5
ILS funds | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	0	0
ILS funds | Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	0	0
ILS funds | Other Operations
Investment securities
Other long-term investments	74	160.5
Private debt instruments
Investment securities
Other long-term investments	14.9	15.8
Private debt instruments | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	0	0
Private debt instruments | Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	5.6	5.8
Private debt instruments | Other Operations
Investment securities
Other long-term investments	9.3	10
Other
Investment securities
Other long-term investments	29.1	33
Other | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	29.1	33
Other | Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	0	0
Other | Other Operations
Investment securities
Other long-term investments	0	0
Kudu’s Participation Contracts | Kudu
Investment securities
Other long-term investments	1,008.4	890.5
Kudu’s Participation Contracts | Kudu | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	0	0
Kudu’s Participation Contracts | Kudu | Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	1,008.4	890.5
Kudu’s Participation Contracts | Kudu | Other Operations
Investment securities
Other long-term investments	0	0
Investments, Excluding MediaAlpha | Total unconsolidated entities
Investment securities
Other long-term investments	1,257	1,123.6
Investments, Excluding MediaAlpha | Total unconsolidated entities | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	0	0
Investments, Excluding MediaAlpha | Total unconsolidated entities | Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	1,008.4	890.5
Investments, Excluding MediaAlpha | Total unconsolidated entities | Other Operations
Investment securities
Other long-term investments	$ 248.6	$ 233.1

Investment Securities - Fair _2

Investment Securities - Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Private equity funds
Investment securities
Fair Value	$ 249.5	$ 248.6
Unfunded Commitments	83.7	57.7
Hedge funds
Investment securities
Fair Value	111.2	64.3
Unfunded Commitments	0	0
Aerospace/Defense/Government | Private equity funds
Investment securities
Fair Value	152.4	157.2
Unfunded Commitments	49.3	17.4
Financial services | Private equity funds
Investment securities
Fair Value	93.4	87.5
Unfunded Commitments	32	37.8
Real estate | Private equity funds
Investment securities
Fair Value	3.7	3.9
Unfunded Commitments	2.4	2.5
Long/short equity financials and business services | Hedge funds
Investment securities
Fair Value	59.2	54.4
Unfunded Commitments	0	0
Long/short all cap global | Hedge funds
Investment securities
Fair Value	52	0
Unfunded Commitments	0	0
European small/mid cap | Hedge funds
Investment securities
Fair Value	0	9.9
Unfunded Commitments	0	0
Private equity funds and hedge funds
Investment securities
Fair Value	360.7	312.9
Unfunded Commitments	$ 83.7	$ 57.7

Investment Securities - Inves_3

Investment Securities - Investments In Private Equity Funds Subject to Lock-Up Periods (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Investment securities
1 – 3 years	$ 18
3 – 5 years	51.1
5 – 10 years	159.4
>10 years	21
Total	2,150.2	$ 1,998.2
Private equity funds
Investment securities
Total	$ 249.5

Investment Securities - Rollfor

Investment Securities - Rollforward of Fair Value Measurements by Level (Details) - Other long-term investments - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 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	54.4	157.3
Purchases and contributions	111.5	256.6
Sales and distributions	(41.4)	(187.3)
Transfers in	0	0
Transfers out	0	0
Ending balance	$ 1,262.7	$ 1,138.2

Investment Securities - Signifi

Investment Securities - Significant Unobservable Inputs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	$ 2,150.2	$ 1,998.2
Level 3 | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	$ 14.5	$ 15.5
Level 3 | Discounted cash flow | Discount Rate | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	11%	11%
Level 3 | Discounted cash flow | Discount Rate | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	12%	12%
Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
New and existing contracts deployed	$ 103.5	$ 163.8
Kudu’s Participation Contracts | Level 3 | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	$ 1,008.4	$ 890.5
Kudu’s Participation Contracts | Level 3 | Discounted cash flow | Discount Rate | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	17%	18%
Kudu’s Participation Contracts | Level 3 | Discounted cash flow | Discount Rate | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	25%	25%
Kudu’s Participation Contracts | Level 3 | Discounted cash flow | Terminal Cash Flow Exit Multiple | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	700%	700%
Kudu’s Participation Contracts | Level 3 | Discounted cash flow | Terminal Cash Flow Exit Multiple | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	2,200%	2,200%
Kudu’s Participation Contracts | Level 3 | Discounted cash flow | Measurement Input, Pre-Tax Discount Rate
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	19%	20%
Kudu’s Participation Contracts | Level 3 | Discounted cash flow | Measurement, Input, Pre-Tax Terminal Cash Flow Exit Multiple
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	1,400%	1,300%
Kudu’s Participation Contracts | Level 3 | Probability weighted expected return method
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value		$ 69.1
PassportCard/DavidShield | Level 3 | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	$ 150	$ 150
PassportCard/DavidShield | Level 3 | Discounted cash flow | Discount Rate
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	24%	24%
PassportCard/DavidShield | Level 3 | Discounted cash flow | Terminal Revenue Growth Rate
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	4%	4%
Elementum | Level 3 | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	$ 35	$ 35
Elementum | Level 3 | Discounted cash flow | Discount Rate
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	22%	21%
Elementum | Level 3 | Discounted cash flow | Terminal Revenue Growth Rate
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	4%	4%
Preferred securities | Level 3 | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	$ 31.4	$ 28.2
Preferred securities | Level 3 | Discounted cash flow | Discount Rate
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	9%	8%

Goodwill and Other Intangible_3

Goodwill and Other Intangible Assets - Summary of Goodwill and Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Goodwill [Line Items]
Goodwill	$ 439.2	$ 168.8	$ 176.5
Indefinite-lived intangible assets	175.7
Other intangible assets, accumulated amortization	39	17
Other intangible assets, finite-lived, net carrying value	105.4
Total other intangible assets	320.1	218.8
Total other intangible assets	281.1	201.8	215.9
Total goodwill and other intangible assets	759.3	387.6
Goodwill and other intangible assets	720.3	370.6	$ 392.4
Other Operations
Goodwill [Line Items]
Goodwill	44.4	44.4
Other intangible assets, finite-lived, acquisition date fair value	41.2	40.9
Other intangible assets, accumulated amortization	20.8	15.5
Other intangible assets, finite-lived, net carrying value	20.4	25.4
Goodwill and other intangible assets	64.8	69.8
Ark | Operating Segments
Goodwill [Line Items]
Goodwill and other intangible assets	292.5	292.5
Ark | Operating Segments | Ark Insurance Holdings Limited - Segment
Goodwill [Line Items]
Goodwill	116.8	116.8
Ark | Operating Segments | Ark Insurance Holdings Limited - Segment | Underwriting Capacity
Goodwill [Line Items]
Indefinite-lived intangible assets	175.7	175.7
Kudu | Operating Segments
Goodwill [Line Items]
Goodwill	7.6	7.6
Goodwill and other intangible assets	8	8.3
Bamboo | Operating Segments
Goodwill [Line Items]
Goodwill	270.4	0
Other intangible assets, finite-lived, acquisition date fair value	101	0
Other intangible assets, accumulated amortization	16.4	0
Other intangible assets, finite-lived, net carrying value	84.6	0
Goodwill and other intangible assets	$ 355	0
Trade names | Other Operations
Goodwill [Line Items]
Acquired 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, accumulated amortization	5.6	4.1
Other intangible assets, finite-lived, net carrying value	$ 7.7	9.2
Trade names | Kudu | Operating Segments
Goodwill [Line Items]
Acquired 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, accumulated amortization	1.8	1.5
Other intangible assets, finite-lived, net carrying value	$ 0.4	0.7
Trade names | Bamboo | Operating Segments
Goodwill [Line Items]
Acquired 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, accumulated amortization	2.3	0
Other intangible assets, finite-lived, net carrying value	$ 21.2	0
Agency relationships | Bamboo | Operating Segments
Goodwill [Line Items]
Acquired 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, accumulated amortization	12.1	0
Other intangible assets, finite-lived, net carrying value	$ 60.3	0
Developed technology | Bamboo | Operating Segments
Goodwill [Line Items]
Acquired 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, accumulated amortization	1.6	0
Other intangible assets, finite-lived, net carrying value	$ 3.1	0
Customer relationships | Other Operations
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life (in years)	11 years
Other intangible assets, finite-lived, acquisition date fair value	$ 24.8	24.8
Other intangible assets, accumulated amortization	13.6	10.3
Other intangible assets, finite-lived, net carrying value	11.2	14.5
Total other intangible assets
Goodwill [Line Items]
Other intangible assets, accumulated amortization	39	17
Total other intangible assets	$ 281.1	201.8
Total other intangible assets | Other Operations
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life (in years)	11 years 9 months 18 days
Other intangible assets, finite-lived, acquisition date fair value	$ 3.1	2.8
Other intangible assets, accumulated amortization	1.6	1.1
Other intangible assets, finite-lived, net carrying value	$ 1.5	1.7
Total other intangible assets | Bamboo | Operating Segments
Goodwill [Line Items]
Acquired 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, accumulated amortization	0.4	0
Other intangible assets, finite-lived, net carrying value	$ 0	$ 0

Goodwill and Other Intangible_4

Goodwill and Other Intangible Assets - Schedule of Goodwill and Other Intangible Assets Rollforward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Goodwill
Goodwill, beginning balance	$ 168.8	$ 176.5
Acquisitions of businesses	270.4	0
Finite-Lived Intangible Assets Acquired	0.3	0
Dispositions	0	(6.7)
Measurement period adjustments	0	(1)
Goodwill, ending balance	439.2	168.8	$ 176.5
Other Intangible Assets
Other intangible assets (excluding goodwill), beginning balance	201.8	215.9
Acquisition of businesses	101	0
Dispositions of businesses	0	(6.9)
Amortization	(22)	(7.2)	(5.2)
Other intangible assets (excluding goodwill), ending balance	281.1	201.8	215.9
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
Disposition of businesses	0	(13.6)
Measurement period adjustments	0	(1)
Amortization	(22)	(7.2)	(5.2)
Total Goodwill and Other Intangible Assets, ending balance	$ 720.3	$ 370.6	$ 392.4

Goodwill and Other Intangible_5

Goodwill and Other Intangible Assets - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization of other intangible assets	$ 22	$ 7.2	$ 5.2

Goodwill and Other Intangible_6

Goodwill and Other Intangible Assets - Amortization Expense (Details) $ in Millions	Dec. 31, 2024 USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
2025	$ 20.8
2026	20.1
2027	17.9
2028	16.9
2029	16.4
2030 and years after	13.3
Other intangible assets, finite-lived, net carrying value	105.4
Indefinite-lived intangible assets	$ 175.7

Loss and Loss Adjustment Expe_3

Loss and Loss Adjustment Expense Reserves - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss adjustment expense reserves, loss on delinquent payment	$ 0.3
Loss and loss adjustment expenses	888.5	$ 726.8	$ 536.4
Other Operations
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	12.1	0
Loss and loss adjustment expenses	12.1	0	0
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development	(58.6)	19.9	(51.7)
Loss and loss adjustment expense reserves	2,127.5	1,605.1	1,296.5	$ 894.7
Loss and loss adjustment expenses	855.8	726.8	536.4
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	2,127.5	1,605.1
Loss and loss adjustment expenses	855.8	726.8	536.4
P&C Insurance Distribution (Bamboo)
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expenses	20.6
P&C Insurance Distribution (Bamboo) | Operating Segments
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	17.8	0
Loss and loss adjustment expenses	20.6	0	0
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%	58.30%
Property and Accident & Health | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development	(29.2)	41.7	(20.8)
Specialty | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development	(34.3)	11.9	(12.7)
Casualty-Runoff | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development		$ 6
Marine & Energy | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development			$ (18.8)
Casualty-Active | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development	$ 4.5

Loss and Loss Adjustment Expe_4

Loss and Loss Adjustment Expense Reserves - LAE Reserve Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Loss and LAE incurred relating to:
Net incurred loss and LAE	$ 888.5	$ 726.8	$ 536.4
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	894.7
Less: beginning reinsurance recoverable on unpaid losses	(340.8)	(505)	(428.9)
Net loss and LAE reserves	1,264.3	791.5	465.8
Loss and LAE incurred relating to:
Current Year	914.4	706.9	588.1
Prior Year	(58.6)	19.9	(51.7)
Net incurred loss and LAE	855.8	726.8	536.4
Loss and LAE paid relating to:
Current year losses	(109.2)	(51.3)	(98.9)
Prior year losses	(307.5)	(356.6)	(158.6)
Net paid loss and LAE	(416.7)	(407.9)	(257.5)
Change in TPC providers’ participation	0	145.4	57.5
Foreign currency translation and other adjustments to loss and LAE reserves	(10.3)	8.5	(10.7)
Net ending balance	1,693.1	1,264.3	791.5
Plus: ending reinsurance recoverable on unpaid losses	(434.4)	(340.8)	(505)
Gross ending balance	$ 2,127.5	1,605.1	1,296.5
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Amount attributable to TPC Providers
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Less: beginning reinsurance recoverable on unpaid losses		$ (145.4)	(276.8)
Loss and LAE paid relating to:
Plus: ending reinsurance recoverable on unpaid losses			$ (145.4)

Loss and Loss Adjustment Expe_5

Loss and Loss Adjustment Expense Reserves - Schedule of Ending Liabilities for Unpaid Loss and LAE, Net of Reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	$ 1,693.1	$ 1,264.3	$ 791.5	$ 465.8
Reinsurance recoverable on unpaid losses	(434.4)	(340.8)	(505)	(428.9)
Loss and loss adjustment expense reserves	2,127.5	1,605.1	1,296.5	894.7
Ark Insurance Holdings Limited/Outrigger
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Reinsurance recoverable on unpaid losses	(434.4)	(340.8)
Ceded	200.9	55.7	278.5
Ark Insurance Holdings Limited/Outrigger | Ark Insurance Holdings Limited - Segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Reinsurance recoverable on unpaid losses	31.8	15.6
Ceded	(29.9)	(15.6)
Amount attributable to TPC Providers | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Reinsurance recoverable on unpaid losses			$ (145.4)	$ (276.8)
Property and Accident & Health | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	533.6	358.7
Total Reinsurance recoverables on unpaid losses	121.5	126.9
Marine & Energy | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	463.1	331.7
Total Reinsurance recoverables on unpaid losses	180.1	59.1
Specialty | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	417	339.7
Total Reinsurance recoverables on unpaid losses	51.3	77.6
Casualty-Active | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	178	137.1
Total Reinsurance recoverables on unpaid losses	80.7	74.8
Casualty-Runoff | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	66.5	81.3
Total Reinsurance recoverables on unpaid losses	0.8	2.4
Other | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	$ 34.9	$ 15.8

Loss and Loss Adjustment Expe_6

Loss and Loss Adjustment Expense Reserves - TPC Participation (Details)	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
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%	58.30%

Loss and Loss Adjustment Expe_7

Loss and Loss Adjustment Expense Reserves - Incurred Loss and Allocated LAE, Net of Reinsurance (Details) - P&C Insurance and Reinsurance (Ark/WM Outrigger) $ in Millions	Dec. 31, 2024 USD ($) claim	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	$ 1,207.6
Cumulative Paid Loss and LAE, Net of Reinsurance	672.4
All outstanding liabilities before 2015, net of reinsurance	1.5
Loss and LAE reserves, net of reinsurance	536.7
Marine & Energy
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	765.7	$ 307.3
All outstanding liabilities before 2015, net of reinsurance	4.7
Loss and LAE reserves, net of reinsurance	463.1
Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	716
Cumulative Paid Loss and LAE, Net of Reinsurance	299.6
All outstanding liabilities before 2015, net of reinsurance	0.6
Loss and LAE reserves, net of reinsurance	417
Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	237.6
Cumulative Paid Loss and LAE, Net of Reinsurance	64.5
All outstanding liabilities before 2015, net of reinsurance	4.9
Loss and LAE reserves, net of reinsurance	178
Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	177.9
Cumulative Paid Loss and LAE, Net of Reinsurance	139.6
All outstanding liabilities before 2015, net of reinsurance	28.2
Loss and LAE reserves, net of reinsurance	66.5
2015 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	16.1	16	$ 15.7	$ 15.8	$ 16	$ 16	$ 16.2	$ 17.2	$ 18.1	$ 19.1
Total IBNR plus expected development on reported claims	$ 0.1
Cumulative number of reported claims | claim	2,829
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 15.6	15.4	15	15	14.8	14.5	14.6	13.4	12.2	6.9
2015 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	12.7	12.7	12.9	12.7	12.8	12.7	13.3	16.1	17.4	21.7
Total IBNR plus expected development on reported claims	$ 0
Cumulative number of reported claims | claim	3,243
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 11.6	11.6	11.5	10.9	10.5	10.4	11	9.6	7.8	4
2015 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	11.3	8.1	8.9	11.1	11.2	11	10.7	12.3	14.6	17.3
Total IBNR plus expected development on reported claims	$ 0.3
Cumulative number of reported claims | claim	1,841
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 9.3	6.2	6.4	8.1	8.1	8	8	7.6	7	4
2015 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	7.4	7.5	7.3	7	7.1	7.4	8.1	8.2	9.7	9.6
Total IBNR plus expected development on reported claims	$ 0.5
Cumulative number of reported claims | claim	1,306
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 6.3	6.1	5.5	5.1	4.9	4.7	4.4	3.2	2.4	1.8
2015 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	39.4	40.2	39.1	36.7	37.3	36.3	36.6	33.1	31.9	36.4
Total IBNR plus expected development on reported claims	$ 1.9
Cumulative number of reported claims | claim	1,950
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 35.8	35.3	33	28.9	27.3	24.6	21.4	14.5	8.2	$ 4.3
2016 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	18.5	18.5	18.5	18.5	18.3	18.4	18.2	17.5	22.2
Total IBNR plus expected development on reported claims	$ 0.2
Cumulative number of reported claims | claim	3,433
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 18.1	17.9	17.7	17.1	16.8	16.7	16.3	13	8.5
2016 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	13.3	13.6	14.1	14.8	14.3	14.6	15.6	19.5	23.4
Total IBNR plus expected development on reported claims	$ 0
Cumulative number of reported claims | claim	3,772
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 13.4	13.4	13.4	13.7	13.1	13	12.6	10	5.5
2016 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	12.3	8.5	8.9	11.8	11.8	11.3	10.9	14.3	18.2
Total IBNR plus expected development on reported claims	$ 0.4
Cumulative number of reported claims | claim	1,936
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 11.7	8.3	8.5	10.3	10.3	9.9	9.1	7.9	3.2
2016 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	11.8	10.1	9.2	9.2	9.1	9	8.9	8.3	8.8
Total IBNR plus expected development on reported claims	$ 0.5
Cumulative number of reported claims | claim	1,588
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 9.7	8.1	6.5	5.3	4.6	4	2.3	1	0.2
2016 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	37.4	37.3	37.6	38.4	38.7	38.4	40.3	32.1	32.4
Total IBNR plus expected development on reported claims	$ 2
Cumulative number of reported claims | claim	2,150
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 32.4	30.9	28.7	27.8	25.3	22.7	17.7	10.1	$ 3.9
2017 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	43.4	43.8	42.3	42.8	44.2	45.2	37.7	31.1
Total IBNR plus expected development on reported claims	$ 15.8
Cumulative number of reported claims | claim	4,624
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 28.2	25.4	27.1	29.4	32.7	31.5	25.7	16.8
2017 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	16.1	16	15.8	16.6	16.9	17.6	19.3	26
Total IBNR plus expected development on reported claims	$ 0.4
Cumulative number of reported claims | claim	4,139
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 14.3	14	14.1	14.1	14	12.8	11.1	5.1
2017 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	10.9	10.3	10.6	11.6	11.4	11.9	12.8	17.9
Total IBNR plus expected development on reported claims	$ 0.4
Cumulative number of reported claims | claim	2,195
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 9.6	8.9	9.2	8.5	8.5	8.3	6.5	3.1
2017 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	10.9	10.6	10.5	9	9.3	10.8	11.7	11.5
Total IBNR plus expected development on reported claims	$ 0.8
Cumulative number of reported claims | claim	1,667
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 8.3	7.5	5.7	4.2	3.4	2.7	1.7	0.8
2017 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	28.4	28.2	29.8	31.5	32	31.3	33.8	30.5
Total IBNR plus expected development on reported claims	$ 2.3
Cumulative number of reported claims | claim	1,604
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 23.5	22.8	22.5	21.3	18.4	14.6	9.4	$ 3.2
2018 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	46.4	46.3	46.8	46.7	49	47.1	40.7
Total IBNR plus expected development on reported claims	$ 1.9
Cumulative number of reported claims | claim	4,288
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 43.6	42.8	40.8	40	40.1	32.2	15.6
2018 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	16.7	17.7	17.3	17.8	17.4	19.9	25.4
Total IBNR plus expected development on reported claims	$ 0.2
Cumulative number of reported claims | claim	3,238
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 15.4	15.3	15.3	14.6	13.9	12.4	2.6
2018 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	16.6	15.7	14.8	15.9	16.6	16.2	14.4
Total IBNR plus expected development on reported claims	$ 0.4
Cumulative number of reported claims | claim	2,122
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 14.1	13	11.8	10.4	9.9	8.2	2.7
2018 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	9.4	9.1	8.6	10.8	11.1	13.3	12.9
Total IBNR plus expected development on reported claims	$ 1.1
Cumulative number of reported claims | claim	1,147
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 7.1	6.2	4.3	4.3	3.5	1.4	0.3
2018 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	27.7	27.9	26.1	26.5	27.2	28.1	33.5
Total IBNR plus expected development on reported claims	$ 3.4
Cumulative number of reported claims | claim	1,280
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 21.3	18.2	16.2	14.9	12.6	7.4	$ 3.4
2019 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	22.8	23.2	23.8	27	31.2	33.9
Total IBNR plus expected development on reported claims	$ 0.6
Cumulative number of reported claims | claim	4,024
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 20.6	19.3	18.5	18.3	16.7	6.8
2019 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	21.2	21.9	21.4	21.6	21.6	23.7
Total IBNR plus expected development on reported claims	$ 0.5
Cumulative number of reported claims | claim	2,393
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 18.1	15.3	14.3	12.6	10.6	3.3
2019 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	19.9	30.1	25.6	18.6	19.4	21.6
Total IBNR plus expected development on reported claims	$ 0.3
Cumulative number of reported claims | claim	2,387
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 17.6	25.1	18.2	7.4	6.9	4.8
2019 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	13	11.4	10.6	12.3	13.7	14.8
Total IBNR plus expected development on reported claims	$ 1.7
Cumulative number of reported claims | claim	1,019
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 8.3	5.7	3	2.3	1.4	0.3
2019 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	23.6	23.5	24.8	23.3	23.2	26.4
Total IBNR plus expected development on reported claims	$ 5.2
Cumulative number of reported claims | claim	973
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 15.8	15.1	12.1	7.8	5.8	$ 3.3
2020 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	79.5	77.6	74.5	75.1	76.9
Total IBNR plus expected development on reported claims	$ 9.9
Cumulative number of reported claims | claim	4,646
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 66.5	55.6	46.9	33.9	11.2
2020 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	27.1	27.4	28.5	27.1	29.7
Total IBNR plus expected development on reported claims	$ 0.8
Cumulative number of reported claims | claim	1,582
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 21.9	18.5	16	12.7	3.1
2020 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	16.7	19.5	18.6	22.8	23.7
Total IBNR plus expected development on reported claims	$ 0.9
Cumulative number of reported claims | claim	2,017
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 17.7	17.7	12.9	10.5	5
2020 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	8.8	9.2	10.8	12	13.5
Total IBNR plus expected development on reported claims	$ 2
Cumulative number of reported claims | claim	665
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 5.3	3.3	2	1	0.5
2020 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	9.5	10.9	13.8	12.2	15.8
Total IBNR plus expected development on reported claims	$ 2.9
Cumulative number of reported claims | claim	567
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 6.3	6	3.1	1.3	$ 0.8
2021 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	168	165.3	153.7	170
Total IBNR plus expected development on reported claims	$ 7.5
Cumulative number of reported claims | claim	3,509
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 142.7	129.8	86.5	30.7
2021 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	74.5	67.3	69.3	86.2
Total IBNR plus expected development on reported claims	$ 4
Cumulative number of reported claims | claim	1,505
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 51.9	38.2	24.3	6.3
2021 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	43.8	51.4	62.1	70.3
Total IBNR plus expected development on reported claims	$ 10
Cumulative number of reported claims | claim	1,725
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 34.6	35.5	23.9	5
2021 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	16.3	16.6	22.4	21.4
Total IBNR plus expected development on reported claims	$ 6.4
Cumulative number of reported claims | claim	961
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 9.6	3.1	0.9	0.5
2021 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	4.5	5.4	7	10.4
Total IBNR plus expected development on reported claims	$ 1.8
Cumulative number of reported claims | claim	283
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 2.3	1.8	1.7	$ 0.5
2022 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	277.3	266.9	241.5
Total IBNR plus expected development on reported claims	$ 12.9
Cumulative number of reported claims | claim	4,044
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 229.4	191.9	69.4
2022 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	156	153.6	149.7
Total IBNR plus expected development on reported claims	$ 31.9
Cumulative number of reported claims | claim	1,968
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 97.7	66.2	12.2
2022 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	168.4	174.8	180.3
Total IBNR plus expected development on reported claims	$ 77
Cumulative number of reported claims | claim	1,496
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 82.5	61.7	16
2022 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	34.6	38	32.9
Total IBNR plus expected development on reported claims	$ 27.9
Cumulative number of reported claims | claim	1,558
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 2.4	1.5	0.4
2022 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	2.5	2.6	0.8
Total IBNR plus expected development on reported claims	$ 1.6
Cumulative number of reported claims | claim	80
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 0.7	0.5	$ 0.3
2023 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	176.1	213.9
Total IBNR plus expected development on reported claims	$ 77.3
Cumulative number of reported claims | claim	3,598
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 52.9	19.9
2023 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	188.2	197
Total IBNR plus expected development on reported claims	$ 115.9
Cumulative number of reported claims | claim	2,138
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 42.1	10.5
2023 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	197.4	214.5
Total IBNR plus expected development on reported claims	$ 94.4
Cumulative number of reported claims | claim	1,641
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 75.2	18.4
2023 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	65.9	60.9
Total IBNR plus expected development on reported claims	$ 57.4
Cumulative number of reported claims | claim	1,792
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 5.6	0.9
2023 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	3.6	2.7
Total IBNR plus expected development on reported claims	$ 2.4
Cumulative number of reported claims | claim	40
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 1	$ 0.9
2024 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	359.5
Total IBNR plus expected development on reported claims	$ 202.7
Cumulative number of reported claims | claim	3,614
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 54.8
2024 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	239.9
Total IBNR plus expected development on reported claims	$ 182.6
Cumulative number of reported claims | claim	1,485
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 20.9
2024 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	218.7
Total IBNR plus expected development on reported claims	$ 141.6
Cumulative number of reported claims | claim	1,292
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 27.3
2024 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	59.5
Total IBNR plus expected development on reported claims	$ 54.9
Cumulative number of reported claims | claim	1,170
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 1.9
2024 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	1.3
Total IBNR plus expected development on reported claims	$ 0.7
Cumulative number of reported claims | claim	22
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 0.5

Loss and Loss Adjustment Expe_8

Loss and Loss Adjustment Expense Reserves - Average Annual Percentage Payout of Incurred Losses and Allocated LAE by Age, Net of Reinsurance (Details) - P&C Insurance and Reinsurance (Ark/WM Outrigger)	Dec. 31, 2024
Property and Accident & Health
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
1	23.80%
2	32.80%
3	18.30%
4	6.40%
5	2.80%
6	1.30%
7	0.50%
8	0.80%
9	0.10%
10	0.10%
Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
1	19.30%
2	30.80%
3	10.90%
4	4.90%
5	7.30%
6	2.50%
7	1.50%
8	1.80%
9	1.30%
10	(1.10%)
Marine & Energy
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
1	12.90%
2	30.50%
3	19.50%
4	8%
5	4.90%
6	6.90%
7	0.30%
8	0.40%
9	0.30%
10	0.10%
Casualty-Active
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
1	4.40%
2	8.40%
3	11.60%
4	14.40%
5	9.50%
6	11.70%
7	6.30%
8	4.80%
9	3.70%
10	3.10%
Casualty-Runoff
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
1	8.80%
2	14.10%
3	15.60%
4	14.80%
5	8.50%
6	6.80%
7	5.90%
8	4.10%
9	3.10%
10	2%

Third-Party Reinsurance - Effec

Third-Party Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Written premiums:
Direct	$ 24.1	$ 58.6	$ 63.8
Assumed	31.9	0	1.3
Gross written premiums	56	58.6	65.1
Earned premiums:
Direct	15.8	28.7	28.6
Assumed	15.9	2.5	4.7
Gross earned premiums	31.7	31.2	33.3
Net earned premiums	1,691.6	1,440.9	1,076.7
Loss and LAE:
Net incurred loss and LAE	888.5	726.8	536.4
Ark Insurance Holdings Limited/Outrigger
Written premiums:
Direct	1,101.5	931.9	760.4
Assumed	1,105.5	966.5	691.6
Gross written premiums	2,207	1,898.4	1,452
Ceded	(527.9)	(377.5)	(256.8)
Net written premiums	1,679.1	1,520.9	1,195.2
Earned premiums:
Direct	1,004.2	848.1	655.5
Assumed	1,088.2	925.6	668.7
Gross earned premiums	2,092.4	1,773.7	1,324.2
Ceded	(504.6)	(364)	(280.8)
Net earned premiums	1,587.8	1,409.7	1,043.4
Loss and LAE:
Gross	1,056.7	782.5	814.9
Ceded	(200.9)	(55.7)	(278.5)
Net incurred loss and LAE	855.8	726.8	$ 536.4
Ark Insurance Holdings Limited/Outrigger | Ark Insurance Holdings Limited - Segment
Written premiums:
Ceded	86.5	(110)
Earned premiums:
Ceded	88	104.3
Loss and LAE:
Ceded	$ 29.9	$ 15.6

Third-Party Reinsurance - Reins

Third-Party Reinsurance - Reinsurance Recoverables (Details) - Ark Insurance Holdings Limited/Outrigger - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Segment reporting information
Reinsurance recoverable on unpaid losses	$ 434.4	$ 340.8
Reinsurance recoverables on paid losses	57.5	27.4
Ceded unearned premiums	97.1	73.8
Reinsurance recoverables	589	442
Ark Insurance Holdings Limited - Segment
Segment reporting information
Reinsurance recoverable on unpaid losses	(31.8)	(15.6)
Reinsurance recoverables on paid losses	(3.1)
Ceded unearned premiums	$ (4.3)	$ (5.7)

Third-Party Reinsurance - Narra

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

Third-Party Reinsurance - Remai

Third-Party Reinsurance - Remaining Gross and Net Reinsurance Recoverables (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment Reporting Information [Line Items]
Gross	$ 491.9
Net	417.4
Reinsurance recoverable on unpaid losses	$ 434.4	$ 340.8	$ 505	$ 428.9
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Reinsurance Recoverable Including Reinsurance Premium Paid | Reinsurer Concentration Risk
Segment Reporting Information [Line Items]
% of Total	100%
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 74.5
Ark Insurance Holdings Limited/Outrigger
Segment Reporting Information [Line Items]
Collateral	589	442
Reinsurance recoverable on unpaid losses	434.4	340.8
Ark Insurance Holdings Limited/Outrigger | Ark Insurance Holdings Limited - Segment
Segment Reporting Information [Line Items]
Reinsurance recoverable on unpaid losses	(31.8)	$ (15.6)
A+ or better | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment Reporting Information [Line Items]
Gross	310.3
Net	$ 310.3
A+ or better | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Reinsurance Recoverable Including Reinsurance Premium Paid | Reinsurer Concentration Risk
Segment Reporting Information [Line Items]
% of Total	74.30%
A+ or better | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 0
A- to A | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment Reporting Information [Line Items]
Gross	98.5
Net	$ 98.5
A- to A | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Reinsurance Recoverable Including Reinsurance Premium Paid | Reinsurer Concentration Risk
Segment Reporting Information [Line Items]
% of Total	23.60%
A- to A | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 0
B++ or lower and not rated | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment Reporting Information [Line Items]
Gross	83.1
Net	$ 8.6
B++ or lower and not rated | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Reinsurance Recoverable Including Reinsurance Premium Paid | Reinsurer Concentration Risk
Segment Reporting Information [Line Items]
% of Total	2.10%
B++ or lower and not rated | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 74.5
B++ or lower and not rated | Ark Insurance Holdings Limited/Outrigger | Ark Insurance Holdings Limited - Segment
Segment Reporting Information [Line Items]
Gross	(34.9)
Collateral	$ (34.9)

Debt - Debt Outstanding (Detail

Debt - Debt Outstanding (Details) € in Millions, $ in Millions	Dec. 31, 2024 USD ($)	Dec. 31, 2024 EUR (€)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Debt Instrument
Amount borrowed	$ 575.8
Carrying Value	562.5		$ 564.6
HG Global Senior Notes
Debt Instrument
Carrying Value	147.4		146.9
Ark 2007 Subordinated Notes
Debt Instrument
Carrying Value	0		30
Ark 2021 Subordinated Notes
Debt Instrument
Carrying Value	154.5		155.5
Kudu Credit Facility
Debt Instrument
Carrying Value	238.6		203.8
Other Operations debt
Debt Instrument
Carrying Value	22		28.4
Senior Notes | HG Global Senior Notes
Debt Instrument
Amount borrowed	150		150
Unamortized discount and issuance cost	(2.6)		(3.1)
Carrying Value	$ 147.4		$ 146.9
Effective yield (as a percent)	11.80%	11.80%	11.90%
Subordinated Debt | Ark
Debt Instrument
Carrying Value	$ 154.5		$ 185.5
Effective yield (as a percent)	11.80%	11.80%	11%
Subordinated Debt | Ark 2007 Subordinated Notes
Debt Instrument
Amount borrowed	$ 0		$ 30
Subordinated Debt | Ark 2021 Subordinated Notes
Debt Instrument
Unamortized issuance cost	(3.6)		(4.2)
Carrying Value	154.5		155.5
Subordinated Debt | Ark 2021 Subordinated Notes, Tranche 1
Debt Instrument
Amount borrowed	41.1	€ 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
Secured Debt | Kudu Credit Facility
Debt Instrument
Amount borrowed	245.3		210.3
Unamortized issuance cost	(6.7)		(6.5)
Carrying Value	$ 245.3		$ 210.3	$ 215.2	$ 225.4
Effective yield (as a percent)	10.30%	10.30%	10.10%
Secured Debt | Other Operations debt
Debt Instrument
Amount borrowed	$ 22.4		$ 28.9
Unamortized issuance cost	$ (0.4)		$ (0.5)
Effective yield (as a percent)	10.20%	10.20%	10.20%

Debt - Schedule of Contractual

Debt - Schedule of Contractual Repayments (Details) $ in Millions	Dec. 31, 2024 USD ($)
Debt Disclosure [Abstract]
Due in one year or less	$ 6.7
Due in two to three years	27
Due in four to five years	30
Due after five years	512.1
Total	$ 575.8

Debt - Narrative (Details)

Debt - Narrative (Details) € in Millions

1 Months Ended

3 Months Ended

6 Months Ended

12 Months Ended

Apr. 29, 2022 USD ($)

Sep. 08, 2021 USD ($)

Aug. 11, 2021 USD ($)

Jul. 13, 2021 USD ($)

Sep. 30, 2021 USD ($)

Mar. 31, 2007 USD ($)

Sep. 30, 2021 USD ($)

Jun. 30, 2023

Dec. 31, 2024 USD ($) credit_facility

Dec. 31, 2023 USD ($)

Dec. 31, 2022 USD ($)

Dec. 31, 2024 EUR (€) credit_facility

Sep. 17, 2024 USD ($)

Aug. 22, 2024 USD ($)

Jul. 31, 2024

Jun. 28, 2024 USD ($)

Sep. 30, 2022 USD ($)

Jun. 16, 2022 USD ($)

Dec. 31, 2021 USD ($) letterOfCredit

Jul. 13, 2021 EUR (€)

Mar. 23, 2021 USD ($)

Debt Instrument

Repayments of notes

$ 38,100,000

$ 25,700,000

$ 56,700,000

Amount borrowed

575,800,000

Interest expense

60,800,000

62,700,000

40,300,000

Interest paid

61,000,000

42,200,000

30,500,000

Subsidiary - HG Global | Interest Rate Cap

Debt Instrument

Notational amount

$ 150,000,000

$ 150,000,000

Interest rate

10.76%

9.76%

Kudu | Interest Rate Cap

Debt Instrument

Notational amount

$ 150,000,000

Interest rate

8.95%

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 2007 Subordinated Notes | Subordinated Debt

Debt Instrument

Amount borrowed

0

30,000,000

Ark 2021 Subordinated Notes | Subordinated Debt

Debt Instrument

Face value

$ 163,300,000

$ 163,300,000

Proceeds from debt, net of debt issuance costs

$ 157,800,000

Ark 2021 Subordinated Notes, Tranche 1 | Subordinated Debt

Debt Instrument

Face value

$ 46,300,000

€ 39.1

Amount borrowed

41,100,000

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

Amount borrowed

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%

Kudu Credit Facility | Secured Debt

Debt Instrument

Face value

300,000,000

$ 350,000,000

$ 300,000,000

Repayments of notes

0

16,900,000

$ 45,200,000

Amount borrowed

245,300,000

$ 210,300,000

Interest rate

4.30%

4.30%

4.45%

Interest reserve account

$ 15,100,000

$ 14,900,000

Debt covenant, maximum loan to value percent

23.60%

23.60%

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)

Debt Instrument

Basis spread on variable rate

0.26%

0.26%

Ark 2021 Subordinated Notes, Tranche 3 | Subordinated Debt

Debt Instrument

Face value

$ 70,000,000

Debt instrument, period for interest rate increase

10 years

Additional interest rate

1%

Solvency capital requirement

120%

Debt to capital requirement

40%

Amount borrowed

$ 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

101,700,000

Pledged collateral

141,100,000

Lloyds LOC Facility | Line of Credit | Letter of Credit

Debt Instrument

Total commitment under revolving credit facility

$ 100,000,000

Letters of credit outstanding

42,700,000

Pledged collateral

$ 67,100,000

HG Global Senior Notes | Senior Notes

Debt Instrument

Face value

$ 150,000,000

Additional interest rate

1%

Amount borrowed

$ 150,000,000

150,000,000

Interest rate

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

29,300,000

30,400,000

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

Debt Instrument

Basis spread on variable rate

0.26%

Other Operations debt | Secured Debt

Debt Instrument

Amount borrowed

$ 22,400,000

$ 28,900,000

Secured credit facilities | credit_facility

4

4

Debt - Schedule of the Change i

Debt - Schedule of the Change in Debt under the Kudu Facility (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Debt Repayment [Roll Forward]
Beginning balance	$ 564.6
Borrowings	36.6	$ 12.4	$ 213.8
Repayments	(38.1)	(25.7)	(56.7)
Ending balance	562.5	564.6
Kudu Credit Facility
Debt Repayment [Roll Forward]
Beginning balance	203.8
Ending balance	238.6	203.8
Kudu Credit Facility | Secured Debt
Debt Repayment [Roll Forward]
Beginning balance	210.3	215.2	225.4
Borrowings	35	12	35
Repayments	0	(16.9)	(45.2)
Ending balance	$ 245.3	$ 210.3	$ 215.2

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2024	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Income Tax Contingency [Line Items]
Net deferred tax benefit		$ 68
Non-U.S. pre-tax income (loss)		282.7	$ 450	$ (94.4)
Net income tax payments to US		34.3	$ 42.7	$ 10.3
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark Insurance Holdings Limited - Segment
Income Tax Contingency [Line Items]
Net deferred tax benefit		51
Financial Guarantee (HG Global) | HG Global
Income Tax Contingency [Line Items]
Net deferred tax benefit		17
Deferred tax liabilities		5
Tax benefit	$ 5
Domestic Tax Jurisdiction
Income Tax Contingency [Line Items]
Separate return limitation year operating loss carryforwards		7.8
Separate return limitation year internal revenue		0.7
Foreign Tax Jurisdiction
Income Tax Contingency [Line Items]
Tax credit carryforwards		$ 2.8

Income Taxes - Income Tax (Bene

Income Taxes - Income Tax (Benefit) Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Current income tax (expense) benefit:
U.S. federal	$ (6.3)	$ (12.6)	$ (16.9)
State	(0.4)	(2.2)	(4.5)
Non-U.S.	(14.8)	(16.6)	(7.1)
Total current income tax (expense) benefit	(21.5)	(31.4)	(28.5)
Deferred income tax (expense) benefit:
U.S. federal	(18.8)	(13.2)	(7.3)
State	2.7	(3.8)	(1.8)
Non-U.S.	5	63.9	(3.8)
Total deferred income tax (expense) benefit	(11.1)	46.9	(12.9)
Total income tax (expense) benefit	$ (32.6)	$ 15.5	$ (41.4)

Income Taxes - Effective Rate R

Income Taxes - Effective Rate Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Income Tax Disclosure [Abstract]
Tax (expense) benefit at the U.S. statutory rate	$ (66.5)	$ (118.7)	$ 31.4
Differences in taxes resulting from:
Non-U.S. earnings, net of foreign taxes	40.2	103.7	(41.2)
Change in valuation allowance	(8.1)	(30.2)	(19.6)
Bermuda corporate income tax	5	68	0
Noncontrolling interest	3.9	6.8	3.4
Tax rate changes	2.9	(0.6)	(0.4)
BAM member surplus contributions prior to deconsolidation	(2.8)	(5.7)	(6.2)
Withholding tax	(2.5)	(4.1)	(3.1)
State taxes	(1.8)	(8.2)	(2.8)
Tax exempt interest and dividends	0.3	0.2	0.2
Officer compensation	0	(0.2)	(1)
Other, net	(3.2)	4.5	(2.1)
Total income tax (expense) benefit	$ (32.6)	$ 15.5	$ (41.4)

Income Taxes - Deferred Tax Ass

Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Deferred tax assets related to:
Non-U.S. net operating loss carryforwards	$ 62.8	$ 68.2
Intangible assets	51.5	51.5
Incentive compensation	23.8	25.4
Unearned premiums	20.5	18.2
Loss reserves	8.5	8.4
U.S. federal and state net operating and capital loss carryforwards	6.4	87.6
Tax credit carryforwards	2.8	3.3
Accrued interest	0.2	10.1
Deferred acquisition costs	0	3.7
Other items	0.9	1
Total gross deferred tax assets	177.4	277.4
Less: valuation allowances	89.2	116.1
Total net deferred tax assets	88.2	161.3
Deferred tax liabilities related to:
Purchase accounting	43.9	43.9
Investment basis difference	43.5	40.9
Deferred underwriting	24.3	18.3
Net unrealized investment gains	19	15.5
Deferred acquisition costs	6.5	0
BAM member surplus contributions prior to deconsolidation	0	82.2
Other items	1.2	1.2
Total deferred tax liabilities	138.4	202
Net deferred tax asset (liability)	$ (50.2)	$ (40.7)

Income Taxes - Valuation Allowa

Income Taxes - Valuation Allowance (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Valuation Allowance [Line Items]
Valuation allowance	$ 89.2	$ 116.1
Financial Guarantee (HG Global) | BAM
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	5.7	6.3
Net Operating Losses in Luxembourg Subsidiaries
Valuation Allowance [Line Items]
Valuation allowance	43.4	20.8
U S Losses and Other Federal Deferred Tax Benefits
Valuation Allowance [Line Items]
Valuation allowance	25.2	47.6
Net Operating Losses in Israel Subsidiaries
Valuation Allowance [Line Items]
Valuation allowance	20.6	46.9
Net Operating Losses in United Kingdom Subsidiaries
Valuation Allowance [Line Items]
Valuation allowance		0.8
Net Deferred Tax Assets For Certain U.S. Operations - Other Operations
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	(8.6)	7.2
Net Deferred Tax Assets For BAM
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	(3.1)	(8.7)
Investments Held in Luxembourg-Domiciled Subsidiaries
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	4.2	31.6
Investments Held in Israel-Domiciled Subsidiaries
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	$ 0.2	$ (0.4)

Income Taxes - Net Operating an

Income Taxes - Net Operating and Capital Loss Carryforwards (Details) $ in Millions	Dec. 31, 2024 USD ($)
Net deferred tax assets
No expiration date	$ 91.6
Total	278.2
Gross deferred tax asset	69
Valuation allowance	(69)
Total net deferred tax assets	0
2025-2029
Net deferred tax assets
2024-2043	0.4
2030-2034
Net deferred tax assets
2024-2043	0.3
2035-2044
Net deferred tax assets
2024-2043	185.9
United States
Net deferred tax assets
No expiration date	3.5
Total	13
Gross deferred tax asset	6.4
Valuation allowance	(6.4)
Total net deferred tax assets	0
United States | 2025-2029
Net deferred tax assets
2024-2043	0.4
United States | 2030-2034
Net deferred tax assets
2024-2043	0.3
United States | 2035-2044
Net deferred tax assets
2024-2043	8.8
Luxembourg
Net deferred tax assets
No expiration date	0
Total	177.1
Gross deferred tax asset	42.3
Valuation allowance	(42.3)
Total net deferred tax assets	0
Luxembourg | 2025-2029
Net deferred tax assets
2024-2043	0
Luxembourg | 2030-2034
Net deferred tax assets
2024-2043	0
Luxembourg | 2035-2044
Net deferred tax assets
2024-2043	177.1
United Kingdom
Net deferred tax assets
No expiration date	0
Total	0
Gross deferred tax asset	0
Valuation allowance	0
Total net deferred tax assets	0
United Kingdom | 2025-2029
Net deferred tax assets
2024-2043	0
United Kingdom | 2030-2034
Net deferred tax assets
2024-2043	0
United Kingdom | 2035-2044
Net deferred tax assets
2024-2043	0
Israel
Net deferred tax assets
No expiration date	88.1
Total	88.1
Gross deferred tax asset	20.3
Valuation allowance	(20.3)
Total net deferred tax assets	0
Israel | 2025-2029
Net deferred tax assets
2024-2043	0
Israel | 2030-2034
Net deferred tax assets
2024-2043	0
Israel | 2035-2044
Net deferred tax assets
2024-2043	$ 0

Derivatives (Details)

Derivatives (Details) - Interest Rate Cap	12 Months Ended
	Dec. 31, 2024 USD ($) agreement	Dec. 31, 2023 USD ($)	Jul. 25, 2028	Jul. 25, 2025	Sep. 17, 2024 USD ($)	Aug. 22, 2024 USD ($)	Jun. 16, 2022 USD ($)
Derivative [Line Items]
Number of instruments held | agreement	2
Subsidiary - HG Global
Derivative [Line Items]
Notational amount						$ 150,000,000	$ 150,000,000
Premiums paid						$ 1,300,000	$ 3,300,000
Periodic settlement of interest cap	$ 2,800,000	$ 2,000,000
Derivative, fair value, net	2,500,000	2,900,000
Subsidiary - HG Global | Forecast
Derivative [Line Items]
Derivative, cap interest rate			4.50%	3.50%
Subsidiary - HG Global | Interest Expense
Derivative [Line Items]
Gain (loss) on fair value of derivative instruments	(1,700,000)	$ (1,200,000)
Kudu
Derivative [Line Items]
Notational amount					$ 150,000,000
Premiums paid					$ 900,000
Derivative, cap interest rate					4.50%
Periodic settlement of interest cap	100,000
Derivative, fair value, net	900,000
Kudu | Interest Expense
Derivative [Line Items]
Gain (loss) on fair value of derivative instruments	$ 400,000

Municipal Bond Guarantee Rein_3

Municipal Bond Guarantee Reinsurance - Narrative (Details) $ in Millions	6 Months Ended	12 Months Ended
	Dec. 31, 2024 USD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Jul. 01, 2024 USD ($)	Jun. 30, 2024
Guarantor Obligations [Line Items]
Member surplus contributions, insurance premium charges, percentage	55%	55%
Direct written premiums		$ 56	$ 58.6	$ 65.1
Gross earned premiums		31.7	31.2	33.3
Assets held-in-trust	$ 950.1	950.1	948.5
Distribution received		79.7
Surplus notes distribution received		58.5
Cash distribution received		21.2
Collateral trusts, investments and cash	289.4	289.4	247
Interest receivable	7.7	7.7	37.7
Surplus notes	300.9	300.9	322.2
Debt	562.5	562.5	564.6
Change in fair value of BAM Surplus Notes		0.5	0	0
Operating Segments | Financial Guarantee (HG Global)
Guarantor Obligations [Line Items]
Debt	147.4	147.4	146.9
Notes receivable, fair value disclosure	381.7	381.7	0		$ 387.4
Change in fair value of BAM Surplus Notes		$ 0.5	0	$ 0
BAM Surplus Notes | Surplus Notes
Guarantor Obligations [Line Items]
Basis spread on variable rate		3%
Interest rate					10%	8.20%
Cash repayments on notes		$ 30	27.4
Repayment of principal		21.3	17.8
Accrued interest payment held in trust		0.5	2
Accrued interest payment held outside of trust		8.2	7.6
Debt	300.9	300.9	322.2
Interest receivable	$ 194.8	$ 194.8	174.5
BAM Surplus Notes | Surplus Notes | Discount Rate
Guarantor Obligations [Line Items]
Debt instrument, measurement input	0.0810	0.0810			0.0815
Subsidiary - HG Global
Guarantor Obligations [Line Items]
Purchase of surplus notes		$ 503
Percentage of risk premium ceded	60%	60%
Renegotiation period		5 years
Direct written premiums	$ 31.9
Gross earned premiums	14.9
Percent over a target trust balance		102%
Subsidiary - HG Global | Regulation 114
Guarantor Obligations [Line Items]
Assets held-in-trust	352.1	$ 352.1	341.6
Supplement trust target	603	603
Subsidiary - HG Global | Supplemental Trust
Guarantor Obligations [Line Items]
Assets held-in-trust	$ 598	$ 598	$ 606.9
Subsidiary - HG Global | Maximum
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured	15%	15%
Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Reinsured amount		$ 125

Municipal Bond Guarantee Rein_4

Municipal Bond Guarantee Reinsurance - Schedule of Municipal Bond Guarantee Insured Obligations (Details) $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($) contract	Dec. 31, 2023 USD ($) contract
Subsidiary - HG Global | Maximum
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured	15%
HG Global
Guarantor Obligations [Line Items]
Contracts outstanding | contract	15,884	14,485
Remaining weighted average contract period(in years)	11 years 2 months 12 days	11 years 1 month 6 days
Outstanding par value of policies assumed (in millions)	$ 18,503.3	$ 16,575.2
Gross unearned insurance premiums (in millions)	$ 297.3	273.9
HG/BAM
Guarantor Obligations [Line Items]
Outstanding par value of policies assumed (in millions)		109,673.8
Gross unearned insurance premiums (in millions)		325.8
Total debt service outstanding		165,686
Interest		$ 56,012.2

Municipal Bond Guarantee Rein_5

Municipal Bond Guarantee Reinsurance - Schedule of Municipal Guarantee Insurance Contracts, Premium Received Over Contract Period (Details) - Variable Interest Entity, Primary Beneficiary - HG Global $ in Millions	Dec. 31, 2024 USD ($)
Guarantor Obligations [Line Items]
January 1, 2025 - March 31, 2025	$ 6.8
April 1, 2025 - June 30, 2025	6.6
July 1, 2025 - September 30, 2025	6.5
October 1, 2025 - December 31, 2025	6.4
Total 2025	26.3
2026	24.7
2027	23.3
2028	21.8
2029	20.2
2030 and thereafter	181
Total gross unearned insurance premiums	$ 297.3

Municipal Bond Guarantee Rein_6

Municipal Bond Guarantee Reinsurance - Schedule of Net Written Premiums (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Written premiums:
Direct		$ 24.1	$ 58.6	$ 63.8
Assumed		31.9	0	1.3
Gross written premiums		56	58.6	65.1
Earned premiums:
Direct		15.8	28.7	28.6
Assumed Premiums Earned		15.9	2.5	4.7
Gross earned premiums		31.7	31.2	33.3
HG Global
Written premiums:
Direct		0	0	0
Assumed		52.4	50.1	55.9
Gross written premiums		52.4	50.1	55.9
Earned premiums:
Direct		0	0	0
Assumed Premiums Earned		28.9	26	27.5
Gross earned premiums		28.9	26	27.5
BAM
Written premiums:
Direct		24.1	58.6	63.8
Assumed		0	0	1.3
Gross written premiums		24.1	58.6	65.1
Earned premiums:
Direct		15.8	28.7	28.6
Assumed Premiums Earned		1	2.5	4.7
Gross earned premiums		16.8	31.2	33.3
Ceded Premiums Written	$ (20.5)		(50.1)	(55.9)
Ceded to Other Companies	$ (14)		(26)	(27.5)
Eliminations
Written premiums:
Direct		0	0	0
Assumed		(20.5)	(50.1)	(55.9)
Gross written premiums		(20.5)	(50.1)	(55.9)
Earned premiums:
Direct		0	0	0
Assumed Premiums Earned		(14)	(26)	(27.5)
Gross earned premiums		$ (14)	$ (26)	$ (27.5)

Earnings Per Share - Schedule o

Earnings Per Share - Schedule of Earnings Per Share from Continuing Operations (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 230.4	$ 509.2	$ 792.8
Less: total income (loss) from discontinued operations, net of tax	0	0	903.2
Less: net (income) loss from discontinued operations attributable to noncontrolling interests	0	0	(0.7)
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	230.4	509.2	(109.7)
Allocation of (earnings) losses to participating restricted common shares	(3)	(7.2)	1.3
Basic earnings (losses) per share numerators	227.4	502	(108.4)
Diluted earnings (losses) per share numerators	$ 227.4	$ 502	$ (108.4)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	2,565,700	2,563,300	2,862,400
Average unvested restricted common shares (in shares)	(33,500)	(36,000)	(36,200)
Basic earnings (losses) per share denominator (in shares)	2,532,200	2,527,300	2,826,200
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)	2,565,700	2,563,300	2,862,400
Average unvested restricted common shares (in shares)	(33,500)	(36,000)	(36,200)
Diluted earnings (loss) per share denominator (in shares)	2,532,200	2,527,300	2,826,200
Basic and diluted earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid (in usd per share)	$ 1	$ 1	$ 1
Distributed earnings - dividends declared and paid (in usd per share)	1	1	1
Undistributed earnings (losses) (in usd per share)	88.79	197.60	(39.34)
Undistributed earnings (losses) (in usd per share)	88.79	197.60	(39.34)
Basic and diluted earnings (losses) per share (in usd per share)	89.79	198.60	(38.34)
Basic and diluted earnings (losses) per share (in usd per share)	$ 89.79	$ 198.60	$ (38.34)

Earnings Per Share - Schedule_2

Earnings Per Share - Schedule of Undistributed Net Earnings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 227.4	$ 502	$ (108.4)
Dividends declared, net of restricted common share amounts	(2.5)	(2.5)	(3)
Total undistributed net earnings (losses), net of restricted common share amounts	$ 224.9	$ 499.5	$ (111.4)

Employee Share-Based Incentiv_3

Employee Share-Based Incentive Compensation Plans - Narrative (Details) $ in Millions	12 Months Ended			36 Months Ended
	Dec. 31, 2024 USD ($) shares	Dec. 31, 2023 USD ($) shares	Dec. 31, 2022 USD ($) shares	Dec. 31, 2023 USD ($) shares	Dec. 31, 2021 USD ($) shares
Share-based compensation arrangement by share-based payment award
Grants in period (in shares)	11,405	10,895	13,225
Performance Shares
Share-based compensation arrangement by share-based payment award
Vesting period	3 years
Performance cycle period	3 years
New grants (in shares)				100	750
Additional compensation cost that would be recognized if all outstanding performance shares vested | $	$ 23.1
Performance Shares | 2024 – 2026
Share-based compensation arrangement by share-based payment award
Targeted performance goal (as a percent)	11%
Payout maximum growth target percentage	200%
Performance Shares | 2023 – 2025
Share-based compensation arrangement by share-based payment award
Targeted performance goal (as a percent)	11%
Payout maximum growth target percentage	200%
Performance Shares | 2022 – 2024
Share-based compensation arrangement by share-based payment award
Targeted performance goal (as a percent)	9%
Payout maximum growth target percentage	200%
Performance Shares | Minimum
Share-based compensation arrangement by share-based payment award
Share-based compensation arrangement by share-based payment award, multiplier	0
Performance Shares | Minimum | 2024 – 2026
Share-based compensation arrangement by share-based payment award
Payout range for growth target percentage	6%
Performance Shares | Minimum | 2023 – 2025
Share-based compensation arrangement by share-based payment award
Payout range for growth target percentage	6%
Performance Shares | Minimum | 2022 – 2024
Share-based compensation arrangement by share-based payment award
Payout range for growth target percentage	4%
Performance Shares | Maximum
Share-based compensation arrangement by share-based payment award
Share-based compensation arrangement by share-based payment award, multiplier	2
Performance Shares | Maximum | 2024 – 2026
Share-based compensation arrangement by share-based payment award
Payout range for growth target percentage	16%
Performance Shares | Maximum | 2023 – 2025
Share-based compensation arrangement by share-based payment award
Payout range for growth target percentage	16%
Performance Shares | Maximum | 2022 – 2024
Share-based compensation arrangement by share-based payment award
Payout range for growth target percentage	14%
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 | $	$ 19.9	$ 16.2	$ 15.5	$ 16.2	$ 15.9
Issued (in shares)	11,405	10,895	13,225

Employee Share-Based Incentiv_4

Employee Share-Based Incentive Compensation Plans - Performance Shares (Details) - USD ($) $ in Millions	1 Months Ended			12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Target Performance Shares Outstanding
Beginning of period (in shares)				37,031	39,449	40,828
Shares paid or expired (in shares)				(13,475)	(13,350)	(14,625)
Grants in period (in shares)				11,405	10,895	13,225
Forfeitures and cancellations (in shares)				(102)	37	21
End of period (in shares)				34,859	37,031	39,449
Accrued Expense
Beginning of period				$ 69.4	$ 67.5	$ 42.2
Shares paid or expired				(44.9)	(40.8)	(26.4)
New grants				0	0	0
Forfeitures and cancellations				0	(0.1)	(0.4)
Expense recognized				46.6	42.8	52.1
End of period				$ 71.1	$ 69.4	$ 67.5
Performance Shares
Accrued Expense
Percentage of target payout - minimum actual	188%	200%	172%

Employee Share-Based Incentiv_5

Employee Share-Based Incentive Compensation Plans - Performance Shares Granted Under the WTM Incentive Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	35,390
Accrued Expense	$ 72.2
Assumed forfeitures (in shares)	(531)
Assumed forfeitures	$ (1.1)
Total target performance shares outstanding (in shares)	34,859	37,031	39,449	40,828
Total	$ 71.1	$ 69.4	$ 67.5	$ 42.2
2024 – 2026
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	11,405
Accrued Expense	$ 6.5
2023 – 2025
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	10,835
Accrued Expense	$ 14.4
2022 – 2024
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,150
Accrued Expense	$ 51.3

Employee Share-Based Incentiv_6

Employee Share-Based Incentive Compensation Plans - Restricted Shares (Details) - Restricted Stock - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Restricted Shares
Beginning of period (in shares)	37,595	38,350	37,850
Issued (in shares)	11,405	10,895	13,225
Vested (in shares)	(13,475)	(11,650)	(12,725)
Forfeited (in shares)	(135)	0	0
End of period (in shares)	35,390	37,595	38,350
Accrued Expense
Beginning of period	$ 16.2	$ 15.5	$ 15.9
Issued	20	16	13.8
Vested	0	0	0
Forfeited	0	0	0
Expense recognized	(16.3)	(15.3)	(14.2)
End of period	$ 19.9	$ 16.2	$ 15.5

Common Shareholders_ Equity a_3

Common Shareholders’ Equity and Noncontrolling Interests - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	Sep. 26, 2022	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Common Shares Repurchased and Retired
Additional authorized repurchase of common shares (in shares)		301,014
Common shares repurchased and retired (in shares)	327,795	5,269	24,165	461,256
Value of common shares repurchased and retired	$ 460.8	$ 7.9	$ 32.7	$ 615.8
Value of each common share repurchased and retired (in usd per share)	$ 1,400	$ 1,505.01	$ 1,354.88	$ 1,335.11
Shares repurchased to satisfy employee income tax withholding			4,629	4,011
Common Shares Issued
Stock issued (in shares)		13,100	12,461	15,640
Dividends on Common Shares
Cash dividends declared and paid		$ 2.5	$ 2.6	$ 3
Dividends declared and paid (in usd per share)		$ 1	$ 1	$ 1
General Board Authorization
Common Shares Repurchased and Retired
Common shares repurchased and retired (in shares)			19,536	129,450
Value of common shares repurchased and retired			$ 26.2	$ 150.9
Value of each common share repurchased and retired (in usd per share)			$ 1,340.79	$ 1,165.84
Key Personnel
Common Shares Issued
Stock issued (in shares)		11,405	10,895	13,225
Directors
Common Shares Issued
Stock issued (in shares)		1,695	1,566	2,415

Common Shareholders_ Equity a_4

Common Shareholders’ Equity and Noncontrolling Interests - Noncontrolling Interests (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Noncontrolling Interest [Line Items]
Noncontrolling Equity	$ 647.3	$ 321.1
Ark
Noncontrolling Interest [Line Items]
Noncontrolling Equity	410.4	336.9
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling Equity	(13.4)	0.8
Kudu
Noncontrolling Interest [Line Items]
Noncontrolling Equity	127.6	113.8
Bamboo
Noncontrolling Interest [Line Items]
Noncontrolling Equity	113.6	0
Other
Noncontrolling Interest [Line Items]
Noncontrolling Equity	9.1	9.4
Total, excluding BAM
Noncontrolling Interest [Line Items]
Noncontrolling Equity	647.3	460.9
BAM
Noncontrolling Interest [Line Items]
Noncontrolling Equity	0	(139.8)
Ark - Management Equity Incentive
Noncontrolling Interest [Line Items]
Noncontrolling Equity	42.9	21
Kudu - Management Equity Incentive
Noncontrolling Interest [Line Items]
Noncontrolling Equity	$ 47.9	$ 38.7
Ark
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	27.90%	28%
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	3.10%	3.10%
Kudu
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	9.60%	10.50%
Bamboo
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	27.20%	0%
BAM
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	100%	100%

Statutory Capital and Surplus (

Statutory Capital and Surplus (Details) - USD ($)	Dec. 31, 2024	Dec. 31, 2023
Statutory Accounting Practices [Line Items]
Assets held-in-trust	$ 950,100,000	$ 948,500,000
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re - Segment
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	196,300,000
Minimum regulatory capital requirement	$ 1
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark Insurance Holdings Limited - Segment
Statutory Accounting Practices [Line Items]
Percent of statutory capital balance	15%
Percent of statutory capital and surplus balance	25%
Maximum dividend payable	$ 336,700,000
Financial Guarantee (HG Global) | HG Global
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	718,300,000
Minimum regulatory capital requirement	1
Net unrestricted cash	6,500,000
Accrued interest	157,500,000
Bamboo Captive
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	6,800,000
Minimum regulatory capital requirement	500,000
Net unrestricted cash and short-term investments	11,400,000
Ark
Statutory Accounting Practices [Line Items]
Funds provided	361,500,000
Statutory capital and surplus	1,346,700,000
Minimum regulatory capital requirement	$ 577,700,000

Segment Information - Narrative

Segment Information - Narrative (Details)	12 Months Ended
	Dec. 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	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment reporting information
Earned insurance premiums	$ 1,691.6	$ 1,440.9	$ 1,076.7
Net investment income	227.4	194.2	124.4
Net realized and unrealized investment gains (losses)	184.9	434.2	(191.5)
Interest income (expense) from BAM Surplus Notes	15.8	0	0
Change in fair value of BAM Surplus Notes	0.5	0	0
Unrealized loss on deconsolidation of BAM	(114.5)	0	0
Commission and fees revenues	149.4	13.2	11.5
Other revenues	84.7	84.2	136.8
Total revenues	2,239.8	2,166.7	1,157.9
Loss and loss adjustment expenses	888.5	726.8	536.4
Acquisition expenses	341.5	290.1	250.6
Cost of sales	29.6	40.4	98.6
Broker commission expense	51.3
General and administrative expenses	490.1	432.6	364.1
Change in fair value of contingent consideration	61.3	48.7	17.3
Interest expense	60.8	62.7	40.3
Total expenses	1,923.1	1,601.3	1,307.3
Pre-tax income (loss) from continuing operations	316.7	565.4	(149.4)
Other Operations
Segment reporting information
Earned insurance premiums	32.7	0	0
Net investment income	35.6	30.1	32.2
Interest income (expense) from BAM Surplus Notes	0	0	0
Change in fair value of BAM Surplus Notes	0
Unrealized loss on deconsolidation of BAM	0
Commission and fees revenues	14.8	13.2	11.5
Other revenues	56.8	80.5	127.2
Total revenues	234.9	339.4	76.3
Loss and loss adjustment expenses	12.1	0	0
Acquisition expenses	12.1	0	0
Cost of sales	29.6	40.4	98.6
Broker commission expense	0
General and administrative expenses	169.5	182.3	174.1
Change in fair value of contingent consideration	0	0	0
Interest expense	2.5	3.7	1.9
Total expenses	225.8	226.4	274.6
Pre-tax income (loss) from continuing operations	9.1	113	(198.3)
Investments, Excluding MediaAlpha
Segment reporting information
Net realized and unrealized investment gains (losses)	146.9	407.1	(98.5)
Investments, Excluding MediaAlpha | Other Operations
Segment reporting information
Net realized and unrealized investment gains (losses)	57	188.5	(1.6)
MediaAlpha
Segment reporting information
Net realized and unrealized investment gains (losses)	38	27.1	(93)
MediaAlpha | Other Operations
Segment reporting information
Net realized and unrealized investment gains (losses)	38	27.1	(93)
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment reporting information
Earned insurance premiums	1,587.8	1,409.7	1,043.4
Loss and loss adjustment expenses	855.8	726.8	536.4
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Segment reporting information
Earned insurance premiums	1,587.8	1,409.7	1,043.4
Net investment income	90.7	61.4	16.3
Net realized and unrealized investment gains (losses)	50.1	85.9	(55.2)
Other revenues	22.3	0.8	5
Loss and loss adjustment expenses	855.8	726.8	536.4
Acquisition expenses	307.1	281.5	239.4
General and administrative expenses	208.4	162	106.2
Change in fair value of contingent consideration	61.3	48.7	17.3
Interest expense	19.5	21.3	15.1
Total expenses	1,452.1	1,240.3	914.4
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark Insurance Holdings Limited - Segment | Operating Segments
Segment reporting information
Earned insurance premiums	1,499.8	1,305.4	1,043.4
Net investment income	79.4	50.4	16.3
Interest income (expense) from BAM Surplus Notes	0	0	0
Change in fair value of BAM Surplus Notes	0
Unrealized loss on deconsolidation of BAM	0
Commission and fees revenues	0	0	0
Other revenues	22.3	0.8	5
Total revenues	1,651.6	1,442.5	1,009.5
Loss and loss adjustment expenses	825.9	711.2	536.4
Acquisition expenses	283.9	251	239.4
Cost of sales	0	0	0
Broker commission expense	0
General and administrative expenses	208.3	161.7	106.2
Change in fair value of contingent consideration	61.3	48.7	17.3
Interest expense	19.5	21.3	15.1
Total expenses	1,398.9	1,193.9	914.4
Pre-tax income (loss) from continuing operations	252.7	248.6	95.1
Other underwriting expenses	136.1	113.6	78.7
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark Insurance Holdings Limited - Segment | Investments, Excluding MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	50.1	85.9	(55.2)
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark Insurance Holdings Limited - Segment | MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	0	0	0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re - Segment | Operating Segments
Segment reporting information
Earned insurance premiums	88	104.3
Net investment income	11.3	11
Interest income (expense) from BAM Surplus Notes	0	0
Change in fair value of BAM Surplus Notes	0
Unrealized loss on deconsolidation of BAM	0
Commission and fees revenues	0	0
Other revenues	0	0
Total revenues	99.3	115.3
Loss and loss adjustment expenses	29.9	15.6
Acquisition expenses	23.2	30.5
Cost of sales	0	0
Broker commission expense	0
General and administrative expenses	0.1	0.3
Change in fair value of contingent consideration	0	0
Interest expense	0	0
Total expenses	53.2	46.4
Pre-tax income (loss) from continuing operations	46.1	68.9
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re - Segment | Investments, Excluding MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	0	0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re - Segment | MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	0	0
Financial Guarantee (HG Global)
Segment reporting information
Earned insurance premiums	31.7	31.2	33.3
Financial Guarantee (HG Global) | Operating Segments
Segment reporting information
Earned insurance premiums	31.7	31.2	33.3
Net investment income	32.2	31.7	21.5
Net realized and unrealized investment gains (losses)	(11.5)	26.6	(105.8)
Interest income (expense) from BAM Surplus Notes	15.8	0	0
Change in fair value of BAM Surplus Notes	0.5	0	0
Unrealized loss on deconsolidation of BAM	(114.5)	0	0
Other revenues	1.2	2.9	4.6
Acquisition expenses	8.2	8.6	11.2
Broker commission expense	0
General and administrative expenses	35.7	68.9	69.1
Interest expense	16.7	16.5	8.3
Total expenses	60.6	94	88.6
Financial Guarantee (HG Global) | HG Global | Operating Segments
Segment reporting information
Earned insurance premiums	28.9	26	27.5
Net investment income	23.4	17.1	10.3
Interest income (expense) from BAM Surplus Notes	29	26.2	11.7
Change in fair value of BAM Surplus Notes	0.5
Unrealized loss on deconsolidation of BAM	(114.5)
Commission and fees revenues	0	0	0
Other revenues	0.1	0	0.5
Total revenues	(39)	82.9	(2.5)
Loss and loss adjustment expenses	0	0	0
Acquisition expenses	7.8	7.4	9.3
Cost of sales	0	0	0
Broker commission expense	0
General and administrative expenses	2.2	2.8	2.8
Change in fair value of contingent consideration	0	0	0
Interest expense	16.7	16.5	8.3
Total expenses	26.7	26.7	20.4
Pre-tax income (loss) from continuing operations	(65.7)	56.2	(22.9)
Financial Guarantee (HG Global) | HG Global | Investments, Excluding MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	(6.4)	13.6	(52.5)
Financial Guarantee (HG Global) | HG Global | MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	0	0	0
Financial Guarantee (HG Global) | BAM | Operating Segments
Segment reporting information
Earned insurance premiums	2.8	5.2	5.8
Net investment income	8.8	14.6	11.2
Interest income (expense) from BAM Surplus Notes	(13.2)	(26.2)	(11.7)
Change in fair value of BAM Surplus Notes	0
Unrealized loss on deconsolidation of BAM	0
Commission and fees revenues	0	0	0
Other revenues	1.1	2.9	4.1
Total revenues	(5.6)	9.5	(43.9)
Loss and loss adjustment expenses	0	0	0
Acquisition expenses	0.4	1.2	1.9
Cost of sales	0	0	0
General and administrative expenses	33.5	66.1	66.3
Change in fair value of contingent consideration	0	0	0
Interest expense	0	0	0
Total expenses	33.9	67.3	68.2
Pre-tax income (loss) from continuing operations	(39.5)	(57.8)	(112.1)
Financial Guarantee (HG Global) | BAM | Investments, Excluding MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	(5.1)	13	(53.3)
Financial Guarantee (HG Global) | BAM | MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	0	0	0
Asset Management (Kudu) | Operating Segments
Segment reporting information
Earned insurance premiums	0	0	0
Net investment income	66.7	71	54.4
Net realized and unrealized investment gains (losses)	51.3	106.1	64.1
Interest income (expense) from BAM Surplus Notes	0	0	0
Change in fair value of BAM Surplus Notes	0
Unrealized loss on deconsolidation of BAM	0
Commission and fees revenues	0	0	0
Other revenues	0.8	0	0
Total revenues	118.8	177.1	118.5
Loss and loss adjustment expenses	0	0	0
Acquisition expenses	0	0	0
Cost of sales	0	0	0
Broker commission expense	0
General and administrative expenses	15.4	19.4	14.7
Change in fair value of contingent consideration	0	0	0
Interest expense	22.1	21.2	15
Total expenses	37.5	40.6	29.7
Pre-tax income (loss) from continuing operations	81.3	136.5	88.8
Asset Management (Kudu) | Investments, Excluding MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	51.3	106.1	64.1
Asset Management (Kudu) | MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	0	0	0
P&C Insurance Distribution (Bamboo)
Segment reporting information
Loss and loss adjustment expenses	20.6
P&C Insurance Distribution (Bamboo) | Operating Segments
Segment reporting information
Earned insurance premiums	39.4	0	0
Net investment income	2.2
Interest income (expense) from BAM Surplus Notes	0
Change in fair value of BAM Surplus Notes	0
Unrealized loss on deconsolidation of BAM	0
Commission and fees revenues	134.6	0	0
Other revenues	5.8	0	0
Total revenues	179.8
Loss and loss adjustment expenses	20.6	0	0
Acquisition expenses	14.1	0	0
Cost of sales	0
Broker commission expense	51.3	0	0
General and administrative expenses	61.1	0	0
Change in fair value of contingent consideration	0
Interest expense	0
Total expenses	147.1	$ 0	$ 0
Pre-tax income (loss) from continuing operations	32.7
Amortization of other intangibles	16.4
P&C Insurance Distribution (Bamboo) | Investments, Excluding MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	0
P&C Insurance Distribution (Bamboo) | MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	0
Bamboo Ide8 Services, LLC - Segment, Net Of Reconciling Items | Operating Segments
Segment reporting information
Other revenues	$ 3.6

Segment Information - Revenues

Segment Information - Revenues (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment reporting information
Earned insurance premiums	$ 1,691.6	$ 1,440.9	$ 1,076.7
Commission and fees revenues	149.4	13.2	11.5
Other revenues	55.8	69	124
Total	1,896.8	1,523.1	1,212.2
United States
Segment reporting information
Earned insurance premiums	2.8	5.2	5.8
Commission and fees revenues	134.6	0	0
Other revenues	55.8	69	124
Total	193.2	74.2	129.8
United Kingdom
Segment reporting information
Earned insurance premiums	956.8	791.7	638.5
Commission and fees revenues	0	0	0
Other revenues	0	0	0
Total	956.8	791.7	638.5
Bermuda
Segment reporting information
Earned insurance premiums	732	644	432.4
Commission and fees revenues	0	0	0
Other revenues	0	0	0
Total	732	644	432.4
Other
Segment reporting information
Earned insurance premiums	0	0	0
Commission and fees revenues	14.8	13.2	11.5
Other revenues	0	0	0
Total	$ 14.8	$ 13.2	$ 11.5

Segment Information - Assets (D

Segment Information - Assets (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Segment reporting information
Total investments	$ 6,477.6	$ 6,388.7
Total assets	9,925.6	8,385.9
Total liabilities	4,794.6	3,824.3
Total White Mountains’s common shareholders’ equity	4,483.7	4,240.5
Noncontrolling interests	647.3	321.1
Debt	562.5	564.6
Subsidiary - Ark Insurance Holdings Limited
Segment reporting information
Noncontrolling interests	410.4	336.9
Subsidiary - HG Global
Segment reporting information
Noncontrolling interests	(13.4)	0.8
Kudu
Segment reporting information
Noncontrolling interests	127.6	113.8
Bamboo
Segment reporting information
Noncontrolling interests	113.6	0
Other
Segment reporting information
Noncontrolling interests	9.1	9.4
BAM Surplus Notes | Surplus Notes
Segment reporting information
Debt	300.9	322.2
Interest receivable	194.8	174.5
Other Operations
Segment reporting information
Total investments	1,570.4	1,709.1
Total assets	1,754.2	1,875.9
Total liabilities	181.3	141.1
Total White Mountains’s common shareholders’ equity	1,563.8	1,725.4
Noncontrolling interests	9.1	9.4
Debt	22	28.4
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Segment reporting information
Total investments	3,139.7	2,671.1
Total assets	5,299	4,378.6
Total liabilities	3,664.8	2,876.1
Total White Mountains’s common shareholders’ equity	1,223.8	1,165.6
Noncontrolling interests	410.4	336.9
Debt	154.5	185.5
Financial Guarantee (HG Global) | Operating Segments
Segment reporting information
Total investments	667.6	1,082.9
Total assets	1,179.4	1,172
Total liabilities	464.1	531.7
Total White Mountains’s common shareholders’ equity	728.7	779.3
Noncontrolling interests	(13.4)	(139)
Dividends payable	462.1	399.8
Debt	147.4	146.9
Asset Management (Kudu) | Operating Segments
Segment reporting information
Total investments	1,041.9	925.6
Total assets	1,108.4	959.4
Total liabilities	316.7	275.4
Total White Mountains’s common shareholders’ equity	664.1	570.2
Noncontrolling interests	127.6	113.8
Debt	238.6	203.8
P&C Insurance Distribution (Bamboo) | Operating Segments
Segment reporting information
Total investments	58	0
Total assets	584.6	0
Total liabilities	167.7	0
Total White Mountains’s common shareholders’ equity	303.3	0
Noncontrolling interests	$ 113.6	$ 0

Equity Method Eligible Invest_3

Equity Method Eligible Investments - Carrying Value and Ownership Percentages (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Kudu’s Participation Contracts
Investments in and Advances to Affiliates [Line Items]
Carrying Value	$ 1,008.4	$ 890.5
Investment in MediaAlpha
Investments in and Advances to Affiliates [Line Items]
Ownership Interest	26.60%	34.90%
Carrying Value	$ 201.6	$ 254.9
PassportCard/DavidShield
Investments in and Advances to Affiliates [Line Items]
Ownership Interest	53.80%	53.80%
Carrying Value	$ 150	$ 150
Elementum
Investments in and Advances to Affiliates [Line Items]
Ownership Interest	26.60%	26.60%
Carrying Value	$ 35	$ 35
Other equity method eligible investments, at fair value
Investments in and Advances to Affiliates [Line Items]
Carrying Value	243.7	291.7
Other equity method eligible investments, at fair value
Investments in and Advances to Affiliates [Line Items]
Carrying Value	$ 0	$ 24.8
Minimum | Kudu’s Participation Contracts
Investments in and Advances to Affiliates [Line Items]
Ownership Interest	4.10%	4.10%
Maximum | Kudu’s Participation Contracts
Investments in and Advances to Affiliates [Line Items]
Ownership Interest	30%	30%

Equity Method Eligible Invest_4

Equity Method Eligible Investments - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Equity Method Investments and Joint Ventures [Abstract]
Dividend and income distributions from equity method eligible investments	$ 71.4	$ 67	$ 68.9

Equity Method Eligible Invest_5

Equity Method Eligible Investments - Balance Sheet and Income Statement Tables (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Investments in unconsolidated affiliates
Total assets	$ 9,925.6	$ 8,385.9
Total liabilities	4,794.6	3,824.3
Total revenues	2,239.8	2,166.7	$ 1,157.9
Total expenses	1,923.1	1,601.3	1,307.3
Net income (loss)	284.1	580.9	712.4
Equity Method Investment, Nonconsolidated Investee, Excluding MediaAlpha
Investments in unconsolidated affiliates
Total assets	3,719.1	3,203.2
Total liabilities	1,063.9	905.6
Total revenues	2,011.8	1,575.4	1,194.4
Total expenses	1,634.9	1,126.2	1,047.4
Net income (loss)	$ 376.9	$ 449.2	$ 147

Variable Interest Entities (Det

Variable Interest Entities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Jul. 01, 2024	Jun. 30, 2024	Dec. 31, 2023
Variable Interest Entity [Line Items]
Assets held-in-trust	$ 950.1			$ 948.5
Bamboo CRV
Variable Interest Entity [Line Items]
Expected capital contribution	30
Outrigger Re
Variable Interest Entity [Line Items]
Short-term investments, at fair value	203.7			$ 265.3
BAM Surplus Notes | Surplus Notes
Variable Interest Entity [Line Items]
Interest rate		10%	8.20%
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%
Variable Interest Entity, Not Primary Beneficiary | PassportCard/DavidShield
Variable Interest Entity [Line Items]
Ownership interest	53.80%
Partnership interest, carrying value	$ 158.9
Variable Interest Entity, Not Primary Beneficiary | Elementum
Variable Interest Entity [Line Items]
Ownership interest	26.60%
Partnership interest, carrying value	$ 35
Variable Interest Entity, Not Primary Beneficiary | Limited Partnerships
Variable Interest Entity [Line Items]
Partnership interest, carrying value	$ 249.5

Fair Value of Financial Instr_3

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	$ 562.5	$ 564.6
Ark 2007 Subordinated Notes
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	30.5
Debt	0	30
Ark 2021 Subordinated Notes
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	173.9	171.8
Debt	154.5	155.5
HG Global Senior Notes
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	157.2	158.7
Debt	147.4	146.9
Kudu Credit Facility
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	253.3	225.6
Debt	238.6	203.8
Other Operations debt
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	23.1	30
Debt	$ 22	$ 28.4

Held for Sale and Discontinue_3

Held for Sale and Discontinued Operations - Financial Statement Disclosure (Details) - USD ($) $ in Millions		12 Months Ended
	Aug. 01, 2022	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders		$ 0	$ 0	$ 903.2
Net (income) loss from discontinued operations attributable to noncontrolling interests		0	0	(0.7)
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders				902.5
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group		0	0	(5.2)
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group		0	0	2.9
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest		0	(0.5)	0.9
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from (used for) operations		0	0	38.7
Net cash provided from (used for) investing activities		0	0	7.1
Net cash used from (used for) financing activities		0	0	(17.5)
Cash balances at beginning of period (includes restricted cash of $89200000)		0	0	116.6
Cash balances at end of period (includes restricted cash of $0.0)		0	0	0
NSM
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain (loss) from sale of discontinued operations, net of tax - NSM Group		$ 0	0	886.8
Discontinued Operations, Disposed of by Sale | NSM
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total revenues - NSM Group				225
General and administrative expenses	$ 14			126.8
Broker commission expenses				52.9
Change in fair value of contingent consideration				0.1
Amortization of other intangible assets				9.1
Interest expense				12.1
Total expenses - NSM Group				201
Pre-tax income (loss) from discontinued operations - NSM Group				24
Income tax (expense) benefit				(7.6)
Net income (loss) from discontinued operations, net tax - NSM Group				16.4
Net gain (loss) from sale of discontinued operations, net of tax - NSM Group	886.8			886.8
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders				903.2
Net (income) loss from discontinued operations attributable to noncontrolling interests				(0.7)
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders				902.5
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group				(5.2)
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	$ 2.9			2.9
Comprehensive income (loss) attributable to White Mountains’s common shareholders				900.2
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest				0.2
Comprehensive income (loss) attributable to White Mountains’s common shareholders				900.4
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from (used for) operations				38.7
Net cash provided from (used for) investing activities				7.1
Net cash used from (used for) financing activities				(17.5)
Effect of exchange rate changes on cash				4
Net change in cash during the period				32.3
Restricted cash balance				0	$ 89.2
Cash balances at beginning of period (includes restricted cash of $89200000)			$ 0	111.6
Cash sold as part of the sale of NSM Group (includes restricted cash of $105100000)				(143.9)
Cash balances at end of period (includes restricted cash of $0.0)				0
Restricted cash				105.1
Interest paid				(12)
Net income tax payments				0
Discontinued Operations, Disposed of by Sale | NSM | Commission revenues
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total revenues - NSM Group				176.9
Discontinued Operations, Disposed of by Sale | NSM | Other revenues
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total revenues - NSM Group				$ 48.1

Held for Sale and Discontinue_4

Held for Sale and Discontinued Operations - Earnings Per Share for Discontinued Operations (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ 230.4	$ 509.2	$ 792.8
Less: net income (loss) from continuing operations	$ 284.1	$ 580.9	(190.8)
Less: net (income) loss from continuing operations attributable to noncontrolling interest			81.1
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders			902.5
Allocation of (earnings) losses to participating restricted common shares			(11.4)
Basic earnings (loss) per share numerators			891.1
Diluted earnings (loss) per share numerators			$ 891.1
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	2,565,700	2,563,300	2,862,400
Average unvested restricted common shares (in shares)	(33,500)	(36,000)	(36,200)
Basic earnings (losses) per share denominator (in shares)	2,532,200	2,527,300	2,826,200
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)	2,565,700	2,563,300	2,862,400
Average unvested restricted common shares (in shares)	(33,500)	(36,000)	(36,200)
Diluted earnings (loss) per share denominator (in shares)	2,532,200	2,527,300	2,826,200
Basic (loss) earnings per share (in dollars) - discontinued operations (in dollars per share)	$ 0	$ 0	$ 315.30
Diluted (loss) earnings per share (in dollars) - discontinued operations (in dollars per share)	$ 0	$ 0	$ 315.30

SCHEDULE I - SUMMARY OF INVES_2

SCHEDULE I - SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Summary of investments other than investments in related parties
Cost	$ 5,878.3
Carrying Value	6,477.6
Fair Value	6,477.6
Dividend and income distributions from equity method eligible investments	71.4	$ 67	$ 68.9
Fixed maturity investments
Summary of investments other than investments in related parties
Cost	2,569.2
Carrying Value	2,511.6
Fair Value	2,511.6
U.S. Government and agency obligations
Summary of investments other than investments in related parties
Cost	461.8
Carrying Value	462.1
Fair Value	462.1
Debt securities issued by corporations
Summary of investments other than investments in related parties
Cost	1,444.5
Carrying Value	1,414.2
Fair Value	1,414.2
Municipal obligations
Summary of investments other than investments in related parties
Cost	3.2
Carrying Value	3.2
Fair Value	3.2
Mortgage and asset-backed securities
Summary of investments other than investments in related parties
Cost	400.2
Carrying Value	373.9
Fair Value	373.9
Collateralized loan obligations
Summary of investments other than investments in related parties
Cost	237.3
Carrying Value	236.7
Fair Value	236.7
Foreign government and agency obligations
Summary of investments other than investments in related parties
Cost	22.2
Carrying Value	21.5
Fair Value	21.5
Short-term investments
Summary of investments other than investments in related parties
Cost	964.2
Carrying Value	964.2
Fair Value	964.2
Equity Method Investments
Summary of investments other than investments in related parties
Cost	1,083.8
Carrying Value	1,423.3
Equity Method Investments | Investment in MediaAlpha
Summary of investments other than investments in related parties
Cost	59.2
Carrying Value	201.6
Fair Value	201.6
Common equity securities - Industrial, Miscellaneous and Other
Summary of investments other than investments in related parties
Cost	373.7
Carrying Value	425.4
Fair Value	425.4
Exchange-traded funds
Summary of investments other than investments in related parties
Cost	163.3
Carrying Value	224.6
Fair Value	224.6
Total common equity securities
Summary of investments other than investments in related parties
Cost	537
Carrying Value	650
Fair Value	650
Other long-term investments
Summary of investments other than investments in related parties
Cost	1,748.7
Carrying Value	2,150.2
Fair Value	$ 2,150.2

SCHEDULE II - CONDENSED FINAN_2

SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - CONDENSED BALANCE SHEETS (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Assets
Common equity securities, at fair value	$ 650	$ 538.4
Other long-term investments	2,150.2	1,998.2
Total assets	9,925.6	8,385.9
Liabilities
Total liabilities	4,794.6	3,824.3
Total White Mountains’s common shareholders’ equity	4,483.7	4,240.5
Total liabilities and equity	9,925.6	8,385.9
White Mountains Insurance Group Ltd.
Assets
Cash	0.1	0.3
Fixed maturity investments, at fair value	70	0
Common equity securities, at fair value	234.7	147.8
Short-term investments, at fair value	62.4	198.8
Other long-term investments	74	160.5
Net receivable due from subsidiaries	118.3	89.1
Other assets	4.6	4
Investments in consolidated subsidiaries	3,944.5	3,666.9
Total assets	4,508.6	4,267.4
Liabilities
Other liabilities	24.9	26.9
Total liabilities	24.9	26.9
Total White Mountains’s common shareholders’ equity	4,483.7	4,240.5
Total liabilities and equity	$ 4,508.6	$ 4,267.4

SCHEDULE II - CONDENSED FINAN_3

SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (loss) (including realized and unrealized gains and losses)	$ 2,239.8	$ 2,166.7	$ 1,157.9
Expenses	1,923.1	1,601.3	1,307.3
Pre-tax income (loss) from continuing operations	316.7	565.4	(149.4)
Income tax (expense) benefit	(32.6)	15.5	(41.4)
Net income (loss) from continuing operations	284.1	580.9	(190.8)
Equity in earnings from discontinued operations, net of tax - NSM Group			902.5
Net income (loss) attributable to White Mountains’s common shareholders	230.4	509.2	792.8
Other comprehensive income (loss), net of tax	(0.1)	2.4	(3.8)
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	0	0	(5.2)
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	0	0	(2.9)
Comprehensive income (loss) attributable to White Mountains’s common shareholders	230.3	511.1	787.6
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (loss) (including realized and unrealized gains and losses)	50.7	81.7	(0.4)
Expenses	51.8	60.5	67.1
Pre-tax income (loss) from continuing operations	(1.1)	21.2	(67.5)
Income tax (expense) benefit	(0.8)	(1.2)	(0.9)
Net income (loss) from continuing operations	(1.9)	20	(68.4)
Equity in earnings (losses) from consolidated and unconsolidated subsidiaries, net of tax	232.3	489.2	(42)
Equity in earnings from discontinued operations, net of tax - NSM Group	0	0	16.4
Net income (loss) attributable to White Mountains’s common shareholders	230.4	509.2	792.8
Other comprehensive income (loss), net of tax	(0.1)	1.9	(2.9)
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	0	0	(5.2)
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	0	0	2.9
Comprehensive income (loss) attributable to White Mountains’s common shareholders	230.3	511.1	787.6
NSM
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net gain (loss) from sale of discontinued operations, net of tax - Sirius Group	0	0	886.8
NSM | White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net gain (loss) from sale of discontinued operations, net of tax - Sirius Group	$ 0	$ 0	$ 886.8

SCHEDULE II - CONDENSED FINAN_4

SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - CONDENSED STATEMENTS OF CASH FLOWS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income (loss)	$ 284.1	$ 580.9	$ 712.4
Charges (credits) to reconcile net income to net cash from operations:
Net realized and unrealized investment (gains) losses	(184.9)	(434.2)	191.5
Net (income) loss from discontinued operation, net of tax - NSM Group			(902.5)
Amortization of restricted share awards	16.3	15.3	14
Net other operating activities	(50.5)	(3.9)	80.8
Net cash provided from (used for) operations	586.8	404.1	365
Cash flows from investing activities:
Net change in short-term investments	548.4	(495.8)	(455.1)
Release of cash (pre-funding) for ILS funds managed by Elementum	0	70	(70)
Proceeds from the sale of Other Operating Businesses, net of cash sold of $0.0, $0.0 and $0.5	0	17.3	1,392
Cash sold in subsidiary transaction	4.2	0	0
Net cash provided from (used for) investing activities	(447)	(543.9)	115.5
Cash flows from financing activities:
Repurchases and retirement of common shares	(7.9)	(32.7)	(615.8)
Cash dividends paid to common shareholders	(2.5)	(2.6)	(3)
Net cash provided from (used for) financing activities	4.2	7.2	(344.9)
Cash balance at beginning of year	122.4	255	147.7
Cash balance at end of year	266.4	122.4	255
Total White Mountains’s common shareholders’ equity	4,483.7	4,240.5
Common equity securities, at fair value	650	538.4
Other long-term investments	2,150.2	1,998.2
Net change in (receivables) payables to subsidiaries	(14.4)	8.4	3.3
Purchases of fixed maturity investments	1,677.5	592.7	585.5
White Mountains Adams, Inc.
Cash flows from financing activities:
Distributions, cash			25
Bridge Holdings, Ltd.
Cash flows from financing activities:
Total White Mountains’s common shareholders’ equity			3,540.6
Intercompany note receivable			76.4
Investments in consolidated subsidiaries			2,003.6
Fixed maturity investments, at fair value			28.6
Common equity securities, at fair value			8.1
Other long-term investments			52.2
Short-term investments, at fair value			1,358.7
Other assets			6
Proceeds from borrowings			69
WM Hinson
Cash flows from financing activities:
Distributions, cash	47
Repayments of debt in short-term investments		61.7
Proceeds from borrowings	208.9	20
Repayments of principal debt	190	42.4
Repayments of debt in investments		107.6
Repayments of debt in fixed maturity investments		45.9
WM Webster
Cash flows from financing activities:
Distributions, cash	7.5
Distributions, short-term investments	4.9
WM Madison
Cash flows from financing activities:
Distributions, cash	$ 2.1
Ownership subsidiary	3.50%
Purchases of fixed maturity investments			14
WM Taylor
Cash flows from financing activities:
Distributions, cash	$ 300
Ownership subsidiary	94.20%
HG Global
Cash flows from financing activities:
Proceeds from borrowings	$ 15.2	7.5
Repayments of principal debt	19.2	9.5
White Mountain Investments Luxembourg
Cash flows from financing activities:
Proceeds from borrowings		105
Purchases of fixed maturity investments	8
Subsidiary - Bridge Holdings, Ltd.
Charges (credits) to reconcile net income to net cash from operations:
Accumulated earnings distributed from subsidiary in cash			7
Cash flows from financing activities:
Repayments of principal debt			15
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income (loss)	230.4	509.2	792.8
Charges (credits) to reconcile net income to net cash from operations:
Net realized and unrealized investment (gains) losses	(50.5)	(78.9)	5.6
Equity in (earnings) losses from consolidated and unconsolidated subsidiaries, net of tax	(232.3)	(489.2)	42
Net (income) loss from discontinued operation, net of tax - NSM Group	0	0	(16.4)
Other non-cash reconciling items	(1.1)	(2.6)	(7)
Accumulated earnings distributed from subsidiary in cash	24.2	17.3	7
Net other operating activities	(17.1)	9.2	17.2
Net cash provided from (used for) operations	(30.1)	(19.7)	(31.6)
Cash flows from investing activities:
Net change in short-term investments	133.9	(63.8)	1,165.6
Purchases of investment securities	(107.9)	(125)	(359.2)
Maturities, calls, paydowns and sales of investment securities	100	335.4	15.7
Purchases of investment securities from subsidiaries (7)	(12)	0	(14)
Release of cash (pre-funding) for ILS funds managed by Elementum	0	70	(70)
Net issuance of debt (to) from subsidiaries	(224.1)	(132.5)	(142)
Net repayment of debt to (from) subsidiaries	209.2	31.4	(15)
Net distributions from (contributions to) subsidiaries	(58.8)	(60.3)	49.3
Proceeds from the sale of Other Operating Businesses, net of cash sold of $0.0, $0.0 and $0.5	0	0	19.5
Cash sold in subsidiary transaction	0	0	0.5
Net cash provided from (used for) investing activities	40.3	55.2	649.9
Cash flows from financing activities:
Repurchases and retirement of common shares	(7.9)	(32.7)	(615.8)
Cash dividends paid to common shareholders	(2.5)	(2.6)	(3)
Net cash provided from (used for) financing activities	(10.4)	(35.3)	(618.8)
Net change in cash during the year	(0.2)	0.2	(0.5)
Cash balance at beginning of year	0.3	0.1	0.6
Cash balance at end of year	0.1	0.3	0.1
Total White Mountains’s common shareholders’ equity	4,483.7	4,240.5
Investments in consolidated subsidiaries	3,944.5	3,666.9
Fixed maturity investments, at fair value	70	0
Common equity securities, at fair value	234.7	147.8
Other long-term investments	74	160.5
Short-term investments, at fair value	62.4	198.8
Other assets	4.6	4
Noncash payment in short-term investments		10
Purchases of fixed maturity investments	12
Subsidiary - White Mountains Investments Bermuda
Cash flows from financing activities:
Distributions, cash	4.3		51
Distributions, short-term investments	18.7
Subsidiary - White Mountain Adams, Inc.
Cash flows from financing activities:
Distributions, short-term investments			100
Contributions, short-term investments and cash	319.5
Repayments of debt in short-term investments	17.2
Contributions, cash	0.2
Subsidiary - HG Global
Cash flows from financing activities:
Contributions, cash			116.3
Proceeds from borrowings			6
Subsidiary - Ark Insurance Holdings Limited
Cash flows from financing activities:
Contributions, cash			7.7
Subsidiary - PSC Holdings, Ltd.
Cash flows from financing activities:
Contributions, cash			1.3
Subsidiary - WM Birkdale
Cash flows from financing activities:
Repayments of debt in short-term investments		36.9
Repayments of principal debt		20.5
Repayments of debt in investments		73.8
Repayments of debt in fixed maturity investments		$ 36.9
Purchases of fixed maturity investments	$ 4
Subsidiary - White Mountain Hinson Bermuda
Cash flows from financing activities:
Proceeds from borrowings			$ 205

SCHEDULE III - SUPPLEMENTARY _2

SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
P&C Insurance and Reinsurance (Ark/WM Outrigger)
SUPPLEMENTARY INSURANCE INFORMATION
Deferred Acquisition Costs	$ 165.2	$ 145.3	$ 127.2
Future Policy Benefits, Losses, Claims and Loss Expenses	2,127.5	1,605.1	1,296.5
Unearned Premiums	853.3	743.6	623.2
Other Policy Claims and Benefits Payable	0	0	0
Premiums Earned	1,587.8	1,409.7	1,043.4
Net Investment Income	90.7	61.4	16.3
Benefits, Claims, Losses and Settlement Expenses	855.8	726.8	536.4
Amortization of Deferred Acquisition Costs	294.1	273.1	238.3
Other Operating Expenses	136.1	113.6	78.7
Premiums Written	1,679.1	1,520.9	1,195.2
Financial Guarantee (HG Global)
SUPPLEMENTARY INSURANCE INFORMATION
Deferred Acquisition Costs	86.6	40.1	36
Future Policy Benefits, Losses, Claims and Loss Expenses	0	0	0
Unearned Premiums	297.3	325.8	298.3
Other Policy Claims and Benefits Payable	0	0	0
Premiums Earned	31.7	31.2	33.3
Net Investment Income	32.2	31.7	21.5
Benefits, Claims, Losses and Settlement Expenses	0	0	0
Amortization of Deferred Acquisition Costs	8.2	8.6	11.2
Other Operating Expenses	0.2	0.4	0.4
Premiums Written	56	$ 58.6	$ 65.1
Bamboo Captive
SUPPLEMENTARY INSURANCE INFORMATION
Deferred Acquisition Costs	10.2
Future Policy Benefits, Losses, Claims and Loss Expenses	17.8
Unearned Premiums	31.5
Other Policy Claims and Benefits Payable	0
Premiums Earned	39.4
Net Investment Income	1.4
Benefits, Claims, Losses and Settlement Expenses	20.6
Amortization of Deferred Acquisition Costs	14.1
Other Operating Expenses	0
Premiums Written	50
Bamboo CRV
SUPPLEMENTARY INSURANCE INFORMATION
Deferred Acquisition Costs	5.8
Future Policy Benefits, Losses, Claims and Loss Expenses	12.1
Unearned Premiums	29
Other Policy Claims and Benefits Payable	0
Premiums Earned	32.7
Net Investment Income	0.4
Benefits, Claims, Losses and Settlement Expenses	12.1
Amortization of Deferred Acquisition Costs	12.1
Other Operating Expenses	0
Premiums Written	$ 61.7

SCHEDULE IV - REINSURANCE (Deta

SCHEDULE IV - REINSURANCE (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Premiums earned
Gross Amount	$ 15.8	$ 28.7	$ 28.6
Assumed	15.9	2.5	4.7
Net earned premiums	1,691.6	1,440.9	1,076.7
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Premiums earned
Gross Amount	1,004.2	848.1	655.5
Ceded	(504.6)	(364)	(280.8)
Assumed	1,088.2	925.6	668.7
Net earned premiums	$ 1,587.8	$ 1,409.7	$ 1,043.4
Percentage of Amount Assumed to Net	68.50%	65.70%	64.10%
Financial Guarantee (HG Global)
Premiums earned
Gross Amount	$ 15.8	$ 28.7	$ 28.6
Ceded	0	0	0
Assumed	15.9	2.5	4.7
Net earned premiums	$ 31.7	$ 31.2	$ 33.3
Percentage of Amount Assumed to Net	50.20%	8%	14.10%
Bamboo Captive
Premiums earned
Gross Amount	$ 0
Ceded	(4)
Assumed	43.4
Net earned premiums	$ 39.4
Percentage of Amount Assumed to Net	110.20%
Bamboo CRV
Premiums earned
Gross Amount	$ 0
Ceded	0
Assumed	32.7
Net earned premiums	$ 32.7
Percentage of Amount Assumed to Net	100%

SCHEDULE V - VALUATION AND QU_2

SCHEDULE V - VALUATION AND QUALIFYING ACCOUNTS (Details) - Reinsurance recoverables: Allowance for uncollectible balances - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
VALUATION AND QUALIFYING ACCOUNTS
Balance at beginning of period	$ 7.7	$ 7.8	$ 3.6
Charged to costs and expenses	2.5	(0.2)	4.6
Charged to other accounts	0	0	0
Deductions described	0	0.1	(0.4)
Balance at end of period	$ 10.2	$ 7.7	$ 7.8

SCHEDULE VI - SUPPLEMENTAL IN_2

SCHEDULE VI - SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
P&C Insurance and Reinsurance (Ark/WM Outrigger)
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Current Year	$ 914.4	$ 706.9	$ 588.1
Prior Year	(58.6)	19.9	(51.7)
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters, Consolidated Property and Casualty Insurance Entity | P&C Insurance and Reinsurance (Ark/WM Outrigger)
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Deferred acquisition costs	165.2	145.3	127.2
Reserves for Unpaid Claims and Claims Adjustment Expenses	2,127.5	1,605.1	1,296.5
Discount, if any, deducted in Column C	0	0	0
Unearned Premiums	853.3	743.6	623.2
Earned Premiums	1,587.8	1,409.7	1,043.4
Net investment income	90.7	61.4	16.3
Current Year	914.4	706.9	588.1
Prior Year	(58.6)	19.9	(51.7)
Amortization of deferred policy acquisition costs	294.1	273.1	238.3
Paid Claims and Claims Adjustment Expenses	416.7	407.9	257.5
Net Premiums written	1,679.1	1,520.9	1,195.2
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters, Consolidated Property and Casualty Insurance Entity | Bamboo Captive
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Deferred acquisition costs	10.2	0	0
Reserves for Unpaid Claims and Claims Adjustment Expenses	17.8	0	0
Discount, if any, deducted in Column C	0	0	0
Unearned Premiums	31.5	0	0
Earned Premiums	39.4	0	0
Net investment income	1.4	0	0
Current Year	17.3	0	0
Prior Year	3.3	0	0
Amortization of deferred policy acquisition costs	14.1	0	0
Paid Claims and Claims Adjustment Expenses	12	0	0
Net Premiums written	50	0	0
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters, Consolidated Property and Casualty Insurance Entity | Bamboo CRV
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Deferred acquisition costs	5.8	0	0
Reserves for Unpaid Claims and Claims Adjustment Expenses	12.1	0	0
Discount, if any, deducted in Column C	0	0	0
Unearned Premiums	29	0	0
Earned Premiums	32.7	0	0
Net investment income	0.4	0	0
Current Year	12.1	0	0
Prior Year	0	0	0
Amortization of deferred policy acquisition costs	12.1	0	0
Paid Claims and Claims Adjustment Expenses	0	0	0
Net Premiums written	$ 61.7	$ 0	$ 0