White Mountains Insurance (WTM) 10-Q 6 Nov 24 2024 Q3 Quarterly report Financial data

Cover Page

Cover Page - shares	9 Months Ended
	Sep. 30, 2024	Nov. 01, 2024
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2024
Document Transition Report	false
Entity File Number	1-8993
Entity Registrant Name	WHITE MOUNTAINS INSURANCE GROUP, LTD
Entity Incorporation, State or Country Code	D0
Entity Tax Identification Number	94-2708455
Entity Address, Address Line One	23 South Main Street, Suite 3B
Entity Address, City or Town	Hanover,
Entity Address, State or Province	NH
Entity Address, Postal Zip Code	03755-2053
City Area Code	603
Local Phone Number	640-2200
Title of 12(b) Security	Common Shares, par value $1.00 per share
Trading Symbol	WTM
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding (in shares)		2,568,148
Entity Central Index Key	0000776867
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	Q3
Amendment Flag	false

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Assets
Common equity securities, at fair value	$ 655.4	$ 538.4
Other long-term investments	2,071.1	1,998.2
Goodwill and other intangible assets	725.7	370.6
Total assets	10,337.5	8,385.9
Liabilities
Debt	544.9	564.6
Total liabilities	5,077.7	3,824.3
White Mountains’s common shareholders’ equity
White Mountains’s common shares at $1 par value per share—authorized 50,000,000 shares; issued and outstanding 2,568,148 and 2,560,452 shares	2.6	2.6
Paid-in surplus	560.7	548.7
Retained earnings	4,048.3	3,690.8
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax	(1)	(1.6)
Accumulated other comprehensive income (loss), after-tax:
Total White Mountains’s common shareholders’ equity	4,610.6	4,240.5
Noncontrolling interests	649.2	321.1
Total equity	5,259.8	4,561.6
Total liabilities and equity	10,337.5	8,385.9
Other Operations
Assets
Fixed maturity investments, at fair value	286.1	230.2
Common equity securities, at fair value	219.8	137.8
Investment in MediaAlpha, at fair value	323.4	254.9
Short-term investments, at fair value	241.7	425.2
Other long-term investments	589.5	661
Total investments	1,660.5	1,709.1
Cash	28.2	23.8
Insurance premiums receivable	34.3	0
Goodwill and other intangible assets	66.2	69.8
Other assets	74.2	73.2
Total assets	1,863.4	1,875.9
Liabilities
Loss and loss adjustment expense reserves	8.1	0
Unearned insurance premiums	28.5	0
Debt	22.3	28.4
Accrued incentive compensation	64.5	87.7
Other liabilities	33.8	25
Total liabilities	157.2	141.1
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liabilities
Loss and loss adjustment expense reserves	2,133	1,605.1
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Assets
Fixed maturity investments, at fair value	1,461.2	866.8
Common equity securities, at fair value	435.6	400.6
Short-term investments, at fair value	579.1	962.8
Other long-term investments	544.8	440.9
Total investments	3,020.7	2,671.1
Cash	148.8	90.5
Reinsurance recoverables	748	442
Insurance premiums receivable	993	612.2
Deferred acquisition costs	199.7	145.3
Goodwill and other intangible assets	292.5	292.5
Other assets	175.8	125
Total assets	5,578.5	4,378.6
Liabilities
Loss and loss adjustment expense reserves	2,133	1,605.1
Unearned insurance premiums	1,149.8	743.6
Debt	156.8	185.5
Reinsurance payable	230.2	81.1
Contingent consideration	141.5	94
Other liabilities	188.9	166.8
Total liabilities	4,000.2	2,876.1
Financial Guarantee (HG Global) | Operating Segments
Assets
Fixed maturity investments, at fair value	636.1	1,012.3
Short-term investments, at fair value	34.6	70.6
Total investments	670.7	1,082.9
Cash	1.2	6.7
BAM Surplus Notes, at fair value	411.1	0
Insurance premiums receivable	7.9	5.5
Deferred acquisition costs	83.2	40.1
Other assets	27.8	36.8
Total assets	1,201.9	1,172
Liabilities
Unearned insurance premiums	287	325.8
Debt	147.3	146.9
Other liabilities	19.6	59
Total liabilities	453.9	531.7
Asset Management (Kudu) | Operating Segments
Assets
Short-term investments, at fair value	20.3	29.3
Other long-term investments	936.8	896.3
Total investments	957.1	925.6
Cash	76.7	1.4
Accrued investment income	15.7	17.6
Goodwill and other intangible assets	8	8.3
Other assets	36.6	6.5
Total assets	1,094.1	959.4
Liabilities
Debt	218.5	203.8
Other liabilities	75.9	71.6
Total liabilities	294.4	275.4
P&C Insurance Distribution (Bamboo) | Operating Segments
Assets
Fixed maturity investments, at fair value	38.9	0
Short-term investments, at fair value	14.6	0
Total investments	53.5	0
Cash	91.1	0
Premiums, commissions and fees receivable	72.7	0
Goodwill and other intangible assets	359	0
Other assets	23.3	0
Total assets	599.6	0
Liabilities
Loss and loss adjustment expense reserves	17.7	0
Unearned insurance premiums	29.9	0
Premiums and commissions payable	93.7	0
Other liabilities	30.7	0
Total liabilities	$ 172	$ 0

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Sep. 30, 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
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Restricted cash	$ 3.3	$ 0.7
P&C Insurance Distribution (Bamboo) | Operating Segments
Restricted cash	$ 67.7	$ 0

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Revenues:
Earned insurance premiums	$ 581.5	$ 506.8	$ 1,244.4	$ 1,070.6
Net investment income	57.7	48.1	172.1	130.1
Net realized and unrealized investment gains (losses)	223.8	(58.3)	390.4	151.2
Interest Income, Operating	(7.9)
Change in fair value of BAM Surplus Notes	(15.8)		(15.8)	0
Unrealized loss on deconsolidation of BAM	(114.5)		(114.5)	0
Commission and fees revenues	46.8	3.5	108.4	10
Other revenues	20.1	19.5	57.3	68
Total revenues	839.1	519.6	1,881.8	1,429.9
Expenses:
Broker commission expenses	15.9		37.9
Loss and loss adjustment expenses	296.9	265.8	666.6	581.1
Acquisition expenses	106.4	92.4	253.9	217.8
Cost of sales	7.6	8	22.2	33.5
General and administrative expenses	121.9	99.6	369.6	298.2
Change in fair value of contingent consideration	34.2	17	47.5	16.8
Interest expense	16.7	15.7	46.7	44.9
Total expenses	599.6	498.5	1,444.4	1,192.3
Pre-tax income (loss)	239.5	21.1	437.4	237.6
Income tax (expense) benefit	(11.6)	(7.3)	(28.9)	(19.4)
Net income (loss)	227.9	13.8	408.5	218.2
Net (income) loss attributable to noncontrolling interests	(48.9)	9.8	(47.7)	4.5
Net income (loss) attributable to White Mountains’s common shareholders	179	23.6	360.8	222.7
Other Operations
Revenues:
Earned insurance premiums	11.2	0	19.8	0
Net investment income	9.6	8	27.9	22
Interest Income, Operating	0
Change in fair value of BAM Surplus Notes	0		0
Unrealized loss on deconsolidation of BAM	0		0
Commission and fees revenues	4.1	3.5	11.1	10
Other revenues	14.8	15.2	43.7	67.1
Total revenues	157.6	(12.1)	322.6	186
Expenses:
Broker commission expenses	0		0
Loss and loss adjustment expenses	4.2	0	8.1	0
Acquisition expenses	4.5	0	7.1	0
Cost of sales	7.6	8	22.2	33.5
General and administrative expenses	32.9	42.2	126.4	130.7
Change in fair value of contingent consideration	0	0	0	0
Interest expense	0.3	0.9	1.6	2.9
Total expenses	49.5	51.1	165.4	167.1
Pre-tax income (loss)	108.1	(63.2)	157.2	18.9
Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	135.6	(11.5)	230.7	190.1
Investments, Excluding MediaAlpha | Other Operations
Revenues:
Net realized and unrealized investment gains (losses)	29.7	8	60.4	125.8
MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	88.2	(46.8)	159.7	(38.9)
MediaAlpha | Other Operations
Revenues:
Net realized and unrealized investment gains (losses)	88.2	(46.8)	159.7	(38.9)
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Expenses:
Loss and loss adjustment expenses	288.3	265.8	644	581.1
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Revenues:
Earned insurance premiums	552.2	498.9	1,173.3	1,047.3
Net investment income	24.3	16.9	66.5	41.2
Net realized and unrealized investment gains (losses)	53.2	(6.6)	84.1	35.9
Other revenues	3.7	3.6	9.6	(1.1)
Total revenues	633.4	512.8	1,333.5	1,123.3
Expenses:
Loss and loss adjustment expenses	288.3	265.8	644	581.1
Acquisition expenses	96.3	90.3	230.8	211.5
General and administrative expenses	68.9	35.4	154	105.5
Change in fair value of contingent consideration	34.2	17	47.5	16.8
Interest expense	4.9	5.5	15	15.7
Total expenses	492.6	414	1,091.3	930.6
Financial Guarantee (HG Global) | Operating Segments
Revenues:
Earned insurance premiums	7.5	7.9	24.3	23.3
Net investment income	6	8.1	26.1	22.9
Net realized and unrealized investment gains (losses)	22.5	(24.1)	8.1	(17)
Interest Income, Operating	(7.9)	0	(7.9)	0
Change in fair value of BAM Surplus Notes	(15.8)	0	(15.8)	0
Unrealized loss on deconsolidation of BAM	(114.5)	0	(114.5)	0
Other revenues	0	0.7	1.1	2
Total revenues	(54.8)	(7.4)	(31.2)	31.2
Expenses:
Acquisition expenses	1.9	2.1	6.3	6.3
General and administrative expenses	0.3	17.5	34.8	49.7
Interest expense	5.8	3.8	13.4	10.8
Total expenses	8	23.4	54.5	66.8
Asset Management (Kudu) | Operating Segments
Revenues:
Earned insurance premiums	0	0	0	0
Net investment income	17.2	15.1	50.1	44
Net realized and unrealized investment gains (losses)	29.5	11.2	77.5	45.4
Interest Income, Operating	0
Change in fair value of BAM Surplus Notes	0		0
Unrealized loss on deconsolidation of BAM	0		0
Commission and fees revenues	0	0	0	0
Other revenues	0.5	0	0.5	0
Total revenues	47.2	26.3	128.1	89.4
Expenses:
Broker commission expenses	0		0
Loss and loss adjustment expenses	0	0	0	0
Acquisition expenses	0	0	0	0
Cost of sales	0	0	0	0
General and administrative expenses	3.6	4.5	10.5	12.3
Change in fair value of contingent consideration	0	0	0	0
Interest expense	5.7	5.5	16.7	15.5
Total expenses	9.3	10	27.2	27.8
Pre-tax income (loss)	37.9	16.3	100.9	61.6
Asset Management (Kudu) | Investments, Excluding MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	29.5	11.2	77.5	45.4
Asset Management (Kudu) | MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	0	0	0	0
P&C Insurance Distribution (Bamboo) | Operating Segments
Revenues:
Earned insurance premiums	10.6	0	27	0
Net investment income	0.6		1.5
Interest Income, Operating	0
Change in fair value of BAM Surplus Notes	0		0
Unrealized loss on deconsolidation of BAM	0		0
Commission and fees revenues	42.7	0	97.3	0
Other revenues	2.4	0	4.5	0
Total revenues	55.7	0	128.8	0
Expenses:
Broker commission expenses	15.9	0	37.9	0
Loss and loss adjustment expenses	4.4	0	14.5	0
Acquisition expenses	3.7	0	9.7	0
Cost of sales	0		0
General and administrative expenses	16.2	0	43.9	0
Change in fair value of contingent consideration	0		0
Interest expense	0		0
Total expenses	40.2	$ 0	106	$ 0
Pre-tax income (loss)	15.5		22.8
P&C Insurance Distribution (Bamboo) | Investments, Excluding MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	0.7		0.6
P&C Insurance Distribution (Bamboo) | MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	$ 0		$ 0

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Statement of Comprehensive Income [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ 179	$ 23.6	$ 360.8	$ 222.7
Other comprehensive income (loss), net of tax	1.3	(1.7)	1.1	0.9
Comprehensive income (loss)	180.3	21.9	361.9	223.6
Other comprehensive (income) loss attributable to noncontrolling interests	(0.5)	0.7	(0.5)	(0.1)
Comprehensive income (loss) attributable to White Mountains’s common shareholders	$ 179.8	$ 22.6	$ 361.4	$ 223.5
Earnings (loss) per share attributable to White Mountains’s common shareholders:
Basic earnings (loss) per share (in dollars per share)	$ 69.68	$ 9.19	$ 140.66	$ 86.82
Diluted earnings (loss) per share (in dollars per share)	69.68	9.19	140.66	86.82
Dividends declared per White Mountains’s common share (in usd per share)	0	0	1	1
Dividends paid per White Mountains’s common share (in usd per share)	$ 0	$ 0	$ 1	$ 1

CONSOLIDATED STATEMENTS OF CHAN

CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Millions	Total	Total Equity	Total	Common shares and paid-in surplus	Retained earnings	AOCI, after tax	Non-controlling interest
Beginning Balances at Dec. 31, 2022		$ 3,935	$ 3,746.9	$ 538.6	$ 3,211.8	$ (3.5)	$ 188.1
Increase (Decrease) in Shareholders' Equity
Net income (loss)	$ 218.2	218.2	222.7		222.7		(4.5)
Other comprehensive income (loss), net of tax		0.9	0.8			0.8	0.1
Total comprehensive income (loss)	223.6	219.1	223.5		222.7	0.8	(4.4)
Dividends declared on common shares		(2.6)	(2.6)		(2.6)
Dividends to noncontrolling interests		(8.3)					(8.3)
Issuances of common shares		2.1	2.1	2.1
Repurchases and retirements of common shares		(32.7)	(32.7)	(5.1)	(27.6)
BAM member surplus contribution, net of tax		46.4					46.4
Amortization of restricted share awards		11.2	11.2	11.2
Recognition of equity-based compensation expense of subsidiaries		1.8	1.3	1.3			0.5
Impact to noncontrolling interests for the deconsolidation of BAM		(9.4)					(9.4)
Net contributions (distributions) and dilution from other noncontrolling interests		1.6	(0.6)	(0.6)			2.2
Ending Balances at Sep. 30, 2023		4,164.2	3,949.1	547.5	3,404.3	(2.7)	215.1
Beginning Balances at Jun. 30, 2023		4,125.9	3,922.2	543.2	3,380.7	(1.7)	203.7
Increase (Decrease) in Shareholders' Equity
Net income (loss)	13.8	13.8	23.6		23.6		(9.8)
Other comprehensive income (loss), net of tax		(1.7)	(1)			(1)	(0.7)
Total comprehensive income (loss)	21.9	12.1	22.6		23.6	(1)	(10.5)
Dividends to noncontrolling interests		(0.5)					(0.5)
BAM member surplus contribution, net of tax		19.9					19.9
Amortization of restricted share awards		4	4	4
Recognition of equity-based compensation expense of subsidiaries		0.6	0.4	0.4			0.2
Net contributions (distributions) and dilution from other noncontrolling interests		2.2	(0.1)	(0.1)			2.3
Ending Balances at Sep. 30, 2023		4,164.2	3,949.1	547.5	3,404.3	(2.7)	215.1
Beginning Balances at Dec. 31, 2023	4,561.6	4,561.6	4,240.5	551.3	3,690.8	(1.6)	321.1
Increase (Decrease) in Shareholders' Equity
Net income (loss)	408.5	408.5	360.8		360.8		47.7
Other comprehensive income (loss), net of tax		1.1	0.6			0.6	0.5
Total comprehensive income (loss)	361.9	409.6	361.4		360.8	0.6	48.2
Dividends declared on common shares		(2.5)	(2.5)		(2.5)
Dividends to noncontrolling interests		(9.3)					(9.3)
Issuances of common shares		2.9	2.9	2.9
Repurchases and retirements of common shares		(8.1)	(8.1)	(1.1)	(7)
BAM member surplus contribution, net of tax		26					26
Amortization of restricted share awards		11.8	11.8	11.8
Recognition of equity-based compensation expense of subsidiaries		3.1	2.3	2.3			0.8
Impact to noncontrolling interests for the deconsolidation of BAM		153.5					153.5
Net contributions (distributions) and dilution from other noncontrolling interests		0.1	2.3	(3.9)	6.2		(2.2)
Acquisition of noncontrolling interests - Bamboo		111.1					111.1
Ending Balances at Sep. 30, 2024	5,259.8	5,259.8	4,610.6	563.3	4,048.3	(1)	649.2
Beginning Balances at Jun. 30, 2024		4,872.2	4,422.6	561.3	3,863.1	(1.8)	449.6
Increase (Decrease) in Shareholders' Equity
Net income (loss)	227.9	227.9	179		179		48.9
Other comprehensive income (loss), net of tax		1.3	0.8			0.8	0.5
Total comprehensive income (loss)	180.3	229.2	179.8		179	0.8	49.4
Amortization of restricted share awards		4.3	4.3	4.3
Recognition of equity-based compensation expense of subsidiaries		1	0.8	0.8			0.2
Impact to noncontrolling interests for the deconsolidation of BAM		153.5					153.5
Net contributions (distributions) and dilution from other noncontrolling interests		(0.4)	3.1	(3.1)	6.2		(3.5)
Ending Balances at Sep. 30, 2024	$ 5,259.8	$ 5,259.8	$ 4,610.6	$ 563.3	$ 4,048.3	$ (1)	$ 649.2

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2024	Sep. 30, 2023
Cash flows from operations:
Net income (loss)	$ 227.9	$ 408.5	$ 218.2
Adjustments to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses	(223.8)	(390.4)	(151.2)
Change in fair value of contingent consideration - Ark	34.2	47.5	16.8
Interest income from BAM Surplus Notes	7.9
Change in fair value of BAM Surplus Notes	(15.8)	(15.8)	0
Unrealized loss on deconsolidation of BAM	114.5	114.5	0
Deferred income tax expense (benefit)		3.7	1.9
Amortization of restricted share awards		11.8	11.2
Amortization (accretion) and depreciation		(7.5)	(10.3)
Other operating items:
Net change in reinsurance recoverables		(306)	56.9
Net change in insurance premiums, commissions and fees receivable		(454.3)	(236.4)
Net change in deferred acquisition costs		(58.2)	(54)
Net change in loss and loss adjustment expense reserves		544.4	275.3
Net change in unearned insurance premiums		458.5	375.4
Net change in reinsurance payable		149.1	(104.8)
Net change in premiums and commissions payable		44.5	0
Contributions to Kudu’s Participation Contracts		(0.2)	(108.3)
Proceeds from Kudu’s Participation Contracts sold		9.4	74.6
Net other operating activities		(28.9)	14.3
Net cash provided from (used for) operations		522.7	379.6
Cash flows from investing activities:
Net change in short-term investments		610.8	(194.9)
Sales of fixed maturity investments		320.9	107.9
Maturities, calls and paydowns of fixed maturity investments		323.3	212.6
Sales of common equity securities and investment in MediaAlpha		162.4	129.9
Distributions and redemptions of other long-term investments		169.9	26.8
Proceeds from the sale of Other Operating Businesses, net of cash sold of $0.0 and $0.8		0	17.3
Purchases of consolidated subsidiaries, net of cash acquired of $44.9 and $0.0		(231.8)	0
Purchases of fixed maturity investments		(1,342)	(392.9)
Purchases of common equity securities and investment in MediaAlpha		(121.1)	(114.2)
Purchases of other long-term investments		(158.2)	(237.7)
Cash sold in subsidiary transaction		(4.2)	0
Net other investing activities		(13.8)	(9.8)
Net cash provided from (used for) investing activities		(283.8)	(455)
Cash flows from financing activities:
Draw down of debt and revolving lines of credit		15.5	12.2
Repayment of debt and revolving lines of credit		(36.7)	(24.3)
Cash dividends paid to common shareholders		(2.5)	(2.6)
Repurchases and retirements of common shares		(8.1)	(32.7)
BAM member surplus contributions prior to the deconsolidation of BAM		26	46.4
Net contributions from (distributions to) other noncontrolling interests		(11.5)	(5.3)
Net other financing activities		2	(10.5)
Net cash provided from (used for) financing activities		(15.3)	(16.8)
Net change in cash during the period		223.6	(92.2)
Cash balances at beginning of period (includes restricted cash balances of $0.7 and $12.2)		122.4	255
Cash balances at end of period (includes restricted cash balances of $71.0 and $0.0)	346	346	162.8
Supplemental cash flows information:
Interest paid		(46.4)	(18)
Net income tax payments		(19.2)	(29.9)
Investments, Excluding MediaAlpha and Bamboo
Adjustments to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses		(230.7)	(190.1)
MediaAlpha
Adjustments to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses	(88.2)	(159.7)	38.9
Financial Guarantee (HG Global) | Operating Segments
Adjustments to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses	(22.5)	(8.1)	17
Interest income from BAM Surplus Notes	7.9	7.9	0
Change in fair value of BAM Surplus Notes	(15.8)	(15.8)	0
Unrealized loss on deconsolidation of BAM	$ 114.5	$ 114.5	$ 0

CONSOLIDATED STATEMENTS OF CA_2

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023	Dec. 31, 2022
Cash sold in subsidiary transaction		$ 4.2	$ 0
Cash acquired from acquisition		44.9	0
Restricted cash balance	$ 71	71	0	$ 0.7	$ 12.2
Gain (Loss) on Fair Value Adjustment of Surplus Notes	(15.8)	(15.8)	0
Other Operations
Cash sold in subsidiary transaction		0	$ 0.8
Gain (Loss) on Fair Value Adjustment of Surplus Notes	$ 0	$ 0

Basis of Presentation and Signi

Basis of Presentation and Significant Accounting Policies	9 Months Ended
	Sep. 30, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation and Significant Accounting Policies	Basis of Presentation and Significant Accounting Policies Basis of Presentation White Mountains Insurance Group, Ltd. (the “Company” or the “Registrant”) is an exempted Bermuda limited liability company whose principal businesses are conducted through its subsidiaries and other affiliates. The Company’s headquarters is located at 26 Reid Street, Hamilton, Bermuda HM 11, its principal executive office is located at 23 South Main Street, Suite 3B, Hanover, New Hampshire 03755-2053 and its registered office is located at Clarendon House, 2 Church Street, Hamilton, Bermuda HM 11. The Company’s website is www.whitemountains.com. The information contained on White Mountains’s website is not incorporated by reference into, and is not a part of, this report. The accompanying consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) and include the accounts of the Company, its subsidiaries (collectively with the Company, “White Mountains”) and other entities required to be consolidated under GAAP. Intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. These interim financial statements include all adjustments considered necessary by management to fairly state the financial position, results of operations and cash flows of White Mountains. These interim financial statements may not be indicative of financial results for the full year and should be read in conjunction with the Company’s 2023 Annual Report on Form 10-K. Reportable Segments As of September 30, 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 chief operating decision makers and its Board of Directors. See Note 14 — “Segment Information.” The Ark/WM Outrigger segment consists of Ark Insurance Holdings Limited and its subsidiaries (collectively, “Ark”) and Outrigger Re Ltd. Segregated Account 2023-1 (“WM Outrigger Re”) (collectively with Ark, “Ark/WM Outrigger”). Ark is a specialty property and casualty insurance and reinsurance company that offers a wide range of niche insurance and reinsurance products, including property, specialty, marine & energy, accident & health and casualty. Ark underwrites select coverages through Lloyd’s Syndicates 4020 and 3902 (the “Syndicates”) and its wholly-owned subsidiary Group Ark Insurance Limited (“GAIL”). White Mountains acquired a controlling ownership interest in Ark on January 1, 2021 (the “Ark Transaction”). As of September 30, 2024 and December 31, 2023, White Mountains owned 72.0% of Ark on a basic shares outstanding basis (61.9% after taking account of management’s equity incentives). The remaining shares are owned by current and former employees of Ark. In the future, management rollover shareholders could earn additional shares in Ark if and to the extent that White Mountains achieves certain thresholds for its multiple of invested capital return. If fully earned, these additional shares would represent 12.3% of the shares outstanding as of September 30, 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 reinsurance capacity to Ark. White Mountains consolidates the results of its segregated account, WM Outrigger Re, in its financial statements. See Note 2 — “Significant Transactions.” As of September 30, 2024 and December 31, 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”) (collectively with HG Global, “HG Global/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 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 September 30, 2024 and December 31, 2023, White Mountains owned 96.9% of HG Global’s preferred equity and 88.4% of its common equity. White Mountains does not have an ownership interest in BAM. However, through June 30, 2024, White Mountains was required to consolidate BAM’s results in its financial statements because BAM is a variable interest entity (“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 September 30, 2024 and December 31, 2023, White Mountains owned 90.1% and 89.6% of Kudu’s basic units outstanding (76.8% and 76.3% on a fully-diluted, fully-converted basis). The Bamboo segment consists of PM Holdings LLC (“Bamboo Holdings”), Bamboo Ide8 Insurance Services LLC (“Bamboo MGA”) and Ide8 Limited (“Bamboo Captive”) (collectively with Bamboo Holdings and Bamboo MGA, “Bamboo”). Bamboo is a capital-light, tech- and data-enabled insurance distribution platform providing homeowners’ insurance and related products to the residential property market in California. Bamboo operates primarily through Bamboo MGA, its full-service managing general agent (“MGA”) business, where the company manages all aspects of the placement process on behalf of its fronting and reinsurance partners, including product development, marketing, underwriting, policy issuance and claims oversight, and it earns commissions based on the volume and profitability of the insurance that it places. It offers both admitted and non-admitted products. Bamboo also operates two separate but integrated businesses: (i) a retail agency, within Bamboo MGA, offering ancillary products on behalf of third parties and (ii) Bamboo Captive, a Bermuda-domiciled captive reinsurer that participates in the underwriting risk of Bamboo’s MGA programs to align interests with reinsurance partners. On January 2, 2024, White Mountains acquired a controlling interest in Bamboo. See Note 2 — “Significant Transactions.” As of September 30, 2024, White Mountains owned 72.8% of the basic units outstanding of Bamboo (63.7% on a fully-diluted, fully-converted basis). White Mountains’s other operations consist of the Company and its wholly-owned subsidiary, White Mountains Capital, LLC (“WM Capital”), its other intermediate holding companies, its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), investment assets managed by WM Advisors, its interests in MediaAlpha, Inc. (“MediaAlpha”), DavidShield PassportCard Ltd. and its subsidiaries (collectively, “PassportCard/DavidShield”), Elementum Holdings LP (“Elementum”), certain other consolidated and unconsolidated entities (“Other Operating Businesses”) and certain other assets (collectively, “Other Operations”). On October 25, 2023, White Mountains announced the launch of White Mountains Partners LLC (“WTM Partners”), which will acquire businesses in non-insurance, non-financial services sectors including essential services, light industrial and specialty consumer. White Mountains expects to deploy up to $500.0 million of equity capital through WTM Partners over time. WTM Partners is included within Other Operations. In April 2024, White Mountains committed to provide up to $30.0 million to a Bermuda special purpose collateralized reinsurance vehicle that provides reinsurance capacity to Bamboo (“Bamboo CRV”). During the second quarter of 2024, White Mountains capitalized Bamboo CRV by purchasing $12.1 million of preference shares that were deposited into a collateral trust account. Bamboo CRV entered into a collateralized quota share agreement with one of Bamboo’s fronting partners to provide reinsurance protection on Bamboo’s admitted and non-admitted business written in the 2024 treaty year. Bamboo CRV is included within Other Operations. Significant Accounting Policies In addition to the following, refer to the Notes to Consolidated Financial Statements in the Company’s 2023 Annual Report on Form 10-K for a complete discussion regarding White Mountains’s significant accounting policies. BAM Surplus Notes 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. Revenue Recognition Bamboo’s revenues consist primarily of commission and fee revenues for placement of insurance policies. Commission and fee revenues are measured based on the contractual rates with insurance carriers, net of any amounts expected to be uncollectible and any amounts associated with expected policy cancellations and adjustments, and are recognized when contractual performance obligations have been fulfilled. Bamboo’s primary contractual performance obligations are generally satisfied upon the issuance of an insurance policy. Bamboo’s premiums, commissions and fees receivable consist of insurance premiums and fees receivable from customers and commissions receivable from insurance carriers, net of a provision for amounts estimated to be uncollectible and any amounts associated with expected policy cancellations and adjustments. Deferred revenues associated with unsatisfied performance obligations are recognized within other liabilities. Broker Commission Expenses Bamboo’s broker commission expense consists of commissions paid to sub-agents and brokers. Broker commission expense is measured in accordance with contractual terms and recognized when incurred, which is generally at the policy issuance date.

Significant Transactions

Significant Transactions	9 Months Ended
	Sep. 30, 2024
Significant Transactions [Abstract]
Significant Transactions	Significant Transactions Bamboo Transaction On October 19, 2023, White Mountains entered into an agreement and plan of merger (the “Bamboo Merger Agreement”) with Bamboo MGA and John Chu, as the unitholders’ representative. Under the terms of the Bamboo Merger Agreement, White Mountains’s wholly-owned subsidiary, WM Pierce Merger Sub LLC, agreed to merge with and into Bamboo MGA, with Bamboo MGA continuing as the surviving company (the “Bamboo Merger”). Concurrently with the execution of the Bamboo Merger Agreement, certain Bamboo management unitholders agreed to roll over the majority of their existing equity in Bamboo MGA into Bamboo Holdings. White Mountains also agreed to make an equity contribution to Bamboo immediately following the Bamboo Merger (together with the Bamboo Merger and the rollover transactions, the “Bamboo Transaction”). On January 2, 2024, White Mountains closed the Bamboo Transaction in accordance with the terms of the Bamboo Merger Agreement, investing $296.7 million of equity into Bamboo, which included the contribution of $36.0 million to retire Bamboo’s legacy credit facility and the contribution of $20.0 million of primary capital. The consideration is subject to customary purchase price adjustments. At closing, White Mountains owned 72.8% of Bamboo on a basic shares outstanding basis (63.7% on a fully-diluted/fully-converted basis, taking account of management’s equity incentives), while Bamboo management owned 16.1% of basic shares outstanding (26.6% on a fully-diluted/fully-converted basis). White Mountains recognized total assets acquired related to the Bamboo Transaction of $479.5 million, total liabilities assumed of $91.7 million and noncontrolling interest of $111.1 million reflecting acquisition date fair values. Total assets acquired included $371.4 million of goodwill and other intangible assets. In connection with the acquisition, White Mountains incurred transaction costs of $4.0 million in Other Operations, of which $0.3 million were expensed in the first quarter of 2024. The following presents additional details of the assets acquired and liabilities assumed as of the January 2, 2024 acquisition date: Millions As of January 2, 2024 Fixed maturity investments, at fair value $ 8.2 Short-term investments, at fair value 9.3 Cash (restricted $37.0) 44.9 (1) Premiums and commissions receivable 38.0 Other assets 7.7 Total tangible assets 108.1 Loss and loss adjustment expense reserves (9.3) Unearned insurance premiums (20.1) Premiums and commissions payable (49.2) Other liabilities (13.1) Total tangible liabilities (91.7) Net tangible assets acquired 16.4 Goodwill 270.4 Other intangible assets 101.0 Total goodwill and other intangible assets 371.4 Net assets acquired $ 387.8 (1) Cash excludes the White Mountains cash contribution of $20.0 as part of the Bamboo Transaction, which 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 three and nine months ended September 30, 2024 are presented in Note 14 — “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 15 — “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 measured at an estimated 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. WM Outrigger Re During the fourth quarter of 2022, Ark sponsored the formation of Outrigger Re Ltd., a Bermuda company registered as a special purpose insurer and segregated accounts company, to provide reinsurance capacity to Ark. Outrigger Re Ltd. was capitalized with $250.0 million of preference shares for business written in the 2023 underwriting year, of which White Mountains contributed $205.0 million. The remaining capital was provided by unrelated third-party investors. Outrigger Re Ltd. entered into collateralized quota share agreements with GAIL to provide reinsurance protection on Ark’s Bermuda global property catastrophe excess of loss portfolio written in the 2023 underwriting year. The proceeds from the issuance of the preference shares were deposited into collateral trust accounts to fund any potential obligations under the reinsurance agreements with GAIL. Outrigger Re Ltd.’s obligations under the reinsurance agreements with GAIL are subject to an aggregate limit equal to the assets in the collateral trusts at any point in time. The terms of the reinsurance agreements are renewable upon the mutual agreement of Ark and the applicable preference shareholder of Outrigger Re Ltd. During the fourth quarter of 2023, Ark renewed Outrigger Re Ltd. for the 2024 underwriting year with $250.0 million of capital, of which White Mountains rolled over $130.0 million from its commitment to the 2023 underwriting year. The remaining capital was provided by new and continuing unrelated third-party investors. As of September 30, 2024 and December 31, 2023, short-term investments of $199.9 million and $265.3 million were held in a collateral trust account required to be maintained in relation to WM Outrigger Re’s reinsurance agreements with GAIL. White Mountains owns 100% of the preference shares linked to its segregated account, WM Outrigger Re. White Mountains consolidates WM Outrigger Re’s results in its financial statements. WM Outrigger Re’s quota share reinsurance agreements with GAIL eliminate in White Mountains’s consolidated financial statements.

Investment Securities

Investment Securities	9 Months Ended
	Sep. 30, 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, insurance-linked securities (“ILS”) funds and private debt instruments. White Mountains has taken the fair value option for its equity method eligible investments. See Note 16 — “ Equity Method Eligible Investments ”. Net realized and unrealized investment gains (losses) are reported in pre-tax revenues. 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 three and nine months ended September 30, 2024 and 2023: Three Months Ended September 30, Nine Months Ended September 30, Millions 2024 2023 2024 2023 Fixed maturity investments $ 23.3 $ 16.6 $ 67.2 $ 46.6 Short-term investments 13.2 13.8 45.3 34.8 Common equity securities .7 1.0 1.8 3.6 Other long-term investments 21.2 17.4 60.0 46.8 Total investment income 58.4 48.8 174.3 131.8 Third-party investment expenses (.7) (.7) (2.2) (1.7) Net investment income, pre-tax $ 57.7 $ 48.1 $ 172.1 $ 130.1 Net Realized and Unrealized Investment Gains (Losses) The following table presents net realized and unrealized investment gains (losses) for the three and nine months ended September 30, 2024 and 2023: Three Months Ended September 30, Nine Months Ended September 30, Millions 2024 2023 2024 2023 Realized investment gains (losses) Fixed maturity investments $ (.5) $ (.3) $ (9.2) $ (1.7) Short-term investments — (.2) (.3) (.3) Common equity securities — 1.2 3.5 13.8 Investment in MediaAlpha — — 91.2 — Other long-term investments 4.8 .6 34.0 48.9 Net realized investment gains (losses) 4.3 1.3 119.2 60.7 Unrealized investment gains (losses) Fixed maturity investments 56.9 (27.9) 49.7 (9.7) Short-term investments .5 (.6) (.5) .7 Common equity securities 24.7 (18.5) 63.6 29.8 Investment in MediaAlpha 88.2 (46.8) 68.5 (38.9) Other long-term investments 49.2 34.2 89.9 108.6 Net unrealized investment gains (losses) 219.5 (59.6) 271.2 90.5 Net realized and unrealized investment gains (losses) (1) $ 223.8 $ (58.3) $ 390.4 $ 151.2 Fixed maturity and short-term investments Net realized and unrealized investment gains (losses) $ 56.9 $ (29.0) $ 39.7 $ (11.0) Less: net realized and unrealized gains (losses) on investment 1.2 2.5 .8 4.7 Net unrealized investment gains (losses) recognized during the period on $ 55.7 $ (31.5) $ 38.9 $ (15.7) Common equity securities and investment in MediaAlpha Net realized and unrealized investment gains (losses) on common $ 24.7 $ (17.3) $ 67.1 $ 43.6 Net realized and unrealized investment gains (losses) from 88.2 (46.8) 159.7 (38.9) Total net realized and unrealized investment gains (losses) 112.9 (64.1) 226.8 4.7 Less: net realized and unrealized gains (losses) on investment — (.1) 35.4 3.7 Net unrealized investment gains (losses) recognized during the period on $ 112.9 $ (64.0) $ 191.4 $ 1.0 (1) For the three months ended September 30, 2024 and 2023, includes $18.9 and $(12.8) of net realized and unrealized investment gains (losses) related to foreign currency exchange. For the nine months ended September 30, 2024 and 2023, includes $7.1 and $(1.9) of net realized and unrealized investment gains (losses) related to foreign currency exchange. The following table presents total net unrealized gains (losses) attributable to Level 3 investments for the three and nine months ended September 30, 2024 and 2023 for investments still held at the end of the period: Three Months Ended September 30, Nine Months Ended September 30, Millions 2024 2023 2024 2023 Total net unrealized investment gains on other long-term investments held at the end of the period, pre-tax $ 31.3 $ 24.6 $ 82.6 $ 76.0 Investment Holdings The following tables present the cost or amortized cost, gross unrealized investment gains (losses), net foreign currency gains (losses) and carrying value of White Mountains’s fixed maturity investments as of September 30, 2024 and December 31, 2023: September 30, 2024 Millions Cost or Gross Gross Net Foreign Carrying U.S. Government and agency obligations $ 446.0 $ 4.8 $ (.6) $ — $ 450.2 Debt securities issued by corporations 1,318.3 16.1 (24.4) .9 1,310.9 Municipal obligations 3.2 .1 — — 3.3 Mortgage and asset-backed securities 379.0 5.2 (16.1) — 368.1 Collateralized loan obligations 287.6 1.2 (.4) 1.4 289.8 Total fixed maturity investments $ 2,434.1 $ 27.4 $ (41.5) $ 2.3 $ 2,422.3 December 31, 2023 Millions Cost or Gross Gross Net Foreign Carrying U.S. Government and agency obligations $ 209.0 $ .3 $ (4.4) $ — $ 204.9 Debt securities issued by corporations 1,085.9 5.6 (45.4) (1.1) 1,045.0 Municipal obligations 275.1 .9 (15.1) — 260.9 Mortgage and asset-backed securities 417.2 2.0 (29.8) — 389.4 Collateralized loan obligations 211.2 .4 (2.4) (.1) 209.1 Total fixed maturity investments $ 2,198.4 $ 9.2 $ (97.1) $ (1.2) $ 2,109.3 The following table presents the cost or amortized cost and carrying value of White Mountains’s fixed maturity investments by contractual maturity as of September 30, 2024. Actual maturities could differ from contractual maturities because borrowers may have the right to call or prepay certain obligations with or without penalties. September 30, 2024 Millions Cost or Amortized Cost Carrying Value Due in one year or less $ 237.0 $ 235.1 Due after one year through five years 1,277.8 1,280.7 Due after five years through ten years 228.4 223.3 Due after ten years 24.2 25.3 Mortgage and asset-backed securities and collateralized loan obligations 666.7 657.9 Total fixed maturity investments $ 2,434.1 $ 2,422.3 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 September 30, 2024 and December 31, 2023: September 30, 2024 Millions Cost or Gross Gross Net Foreign Carrying Common equity securities $ 537.0 $ 120.2 $ — $ (1.8) $ 655.4 Investment in MediaAlpha $ 59.2 $ 264.2 $ — $ — $ 323.4 Other long-term investments $ 1,638.6 $ 547.9 $ (108.3) $ (7.1) $ 2,071.1 December 31, 2023 Millions Cost or Gross Gross Net Foreign Carrying Common equity securities $ 483.5 $ 62.3 $ (1.2) $ (6.2) $ 538.4 Investment in MediaAlpha $ 59.2 $ 195.7 $ — $ — $ 254.9 Other long-term investments $ 1,655.7 $ 446.3 $ (94.5) $ (9.3) $ 1,998.2 Fair Value Measurements Fair value measurements are categorized into a hierarchy that distinguishes between inputs based on market data from independent sources (observable inputs) and a reporting entity’s internal assumptions based upon the best information available when external market data is limited or unavailable (unobservable inputs). Quoted prices in active markets for identical assets or liabilities have the highest priority (Level 1), followed by observable inputs other than quoted prices, including prices for similar but not identical assets or liabilities (Level 2) and unobservable inputs, including the reporting entity’s estimates of the assumptions that market participants would use, having the lowest priority (Level 3). See Note 17 — “Fair Value of Financial Instruments.” Fair Value Measurement By Level The following tables present White Mountains’s fair value measurements for investments as of September 30, 2024 and December 31, 2023 by level. The major security types were based on the legal form of the securities. White Mountains has disaggregated its fixed maturity investments based on the issuing entity type, which impacts credit quality, with debt securities issued by U.S. government entities carrying minimal credit risk, while the credit and other risks associated with other issuers, such as corporations, municipalities or entities issuing mortgage and asset-backed securities vary depending on the nature of the issuing entity type. White Mountains further disaggregates debt securities issued by corporations by industry sector because investors often reference commonly used benchmarks and their subsectors to monitor risk and performance. Accordingly, White Mountains has further disaggregated this asset class into subclasses based on the similar sectors and industry classifications it uses to evaluate investment risk and performance against commonly used benchmarks, such as the Bloomberg Barclays U.S. Intermediate Aggregate. September 30, 2024 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 450.2 $ 449.4 $ .8 $ — Debt securities issued by corporations: Financials 399.0 — 399.0 — Consumer 253.5 — 253.5 — Industrial 149.2 — 149.2 — Healthcare 148.0 — 148.0 — Technology 109.8 — 109.8 — Utilities 73.0 — 73.0 — Communications 60.9 — 60.9 — Energy 62.9 — 62.9 — Materials 54.6 — 54.6 — Total debt securities issued by corporations 1,310.9 — 1,310.9 — Municipal obligations 3.3 — 3.3 — Mortgage and asset-backed securities 368.1 — 368.1 — Collateralized loan obligations 289.8 — 289.8 — Total fixed maturity investments 2,422.3 449.4 1,972.9 — Short-term investments 890.3 880.8 9.5 — Common equity securities: Exchange-traded funds 219.8 219.8 — — Other (1) 435.6 — 435.6 — Total common equity securities 655.4 219.8 435.6 — Investment in MediaAlpha 323.4 323.4 — — Other long-term investments 1,205.3 — 25.5 1,179.8 Other long-term investments — net asset value (2) 865.8 — — — Total other long-term investments 2,071.1 — 25.5 1,179.8 Total investments $ 6,362.5 $ 1,873.4 $ 2,443.5 $ 1,179.8 (1) Consists of investments in listed funds that predominantly invest in international equities. (2) Consists of private equity funds and hedge funds, a bank loan fund, Lloyd’s trust deposits and ILS funds for which fair value is measured using net asset value (“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 September 30, 2024 and December 31, 2023, Ark held Lloyd’s trust deposits with a fair value of $172.7 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 September 30, 2024 and December 31, 2023, the fair value of Ark’s Funds at Lloyd’s investment deposits totaled $356.8 million and $344.7 million. As of September 30, 2024 and December 31, 2023, Ark held additional investments on deposit or as collateral for insurance regulators and reinsurance counterparties of $215.8 million and $244.5 million. As of September 30, 2024 and December 31, 2023, Ark had $208.0 million and $198.9 million of fixed maturity and short-term investments pledged as collateral under uncommitted standby letters of credit. See Note 7 — “Debt.” As of September 30, 2024 and December 31, 2023, short-term investments of $199.9 million and $265.3 million were held in a collateral trust account required to be maintained in relation to WM Outrigger Re’s reinsurance agreements with GAIL. As of September 30, 2024 and December 31, 2023, investments of $641.4 million and $585.6 million were held in trusts required to be maintained in relation to HG Re’s reinsurance agreements with BAM. HG Global is required to maintain an interest reserve account in connection with its senior notes issued in 2022. As of September 30, 2024 and December 31, 2023, the fair value of the interest reserve account, which is included in short-term investments, was $29.3 million and $30.4 million. See Note 7 — “Debt.” Kudu is required to maintain an interest reserve account in connection with its credit facility. As of September 30, 2024 and December 31, 2023, the interest reserve account of $14.8 million and $14.9 million was held in short-term investments. See Note 7 - “Debt.” As of September 30, 2024, fixed maturity and short-term investments of $32.9 million were held as collateral required to be maintained in relation to Bamboo Captive’s reinsurance agreements. As of September 30, 2024, short-term investments of $12.1 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 credit ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of September 30, 2024 and December 31, 2023: Fair Value at Millions September 30, 2024 December 31, 2023 AAA $ 15.0 $ 11.5 AA 82.3 83.8 A 577.6 552.4 BBB 625.5 390.9 BB 3.2 — Other 7.3 6.4 Debt securities issued by corporations (1) $ 1,310.9 $ 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 September 30, 2024 and December 31, 2023: September 30, 2024 December 31, 2023 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: FNMA $ 194.8 $ 194.8 $ — $ 198.8 $ 198.8 $ — FHLMC 142.1 142.1 — 137.2 137.2 — GNMA 26.0 26.0 — 30.2 30.2 — Total agency (1) 362.9 362.9 — 366.2 366.2 — Non-agency: Commercial .4 .4 — — — — Residential .1 .1 — .2 .2 — Total non-agency .5 .5 — .2 .2 — Total mortgage-backed securities 363.4 363.4 — 366.4 366.4 — Other asset-backed securities: Vehicle receivables 1.7 1.7 — 17.2 17.2 — Credit card receivables 1.3 1.3 — 3.3 3.3 — Other 1.7 1.7 — 2.5 2.5 — Total other asset-backed securities 4.7 4.7 — 23.0 23.0 — Total mortgage and asset-backed securities 368.1 368.1 — 389.4 389.4 — Collateralized loan obligations 289.8 289.8 — 209.1 209.1 — Total mortgage and asset-backed securities $ 657.9 $ 657.9 $ — $ 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 September 30, 2024 and December 31, 2023, White Mountains’s investment portfolio included $289.8 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 September 30, 2024. 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 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. 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. As of September 30, 2024, White Mountains owned 17.9 million shares of MediaAlpha, representing a 26.8% basic ownership interest (25.4% on a fully-diluted/fully-converted basis). See Note 16 — “Equity Method Eligible Investments.” At White Mountains’s September 30, 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 September 30, 2024 share price of $18.11 per share, the fair value of White Mountains’s investment in MediaAlpha was $323.4 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 September 30, 2024 and December 31, 2023: Fair Value as of September 30, 2024 Millions Ark/ WM Outrigger Kudu Other Total Kudu’s Participation Contracts $ — $ 930.6 $ — $ 930.6 PassportCard/DavidShield — — 150.0 150.0 Elementum — — 35.0 35.0 Other unconsolidated entities (1) — — 56.6 56.6 Total unconsolidated entities — 930.6 241.6 1,172.2 Private equity funds and hedge funds 81.9 — 267.1 349.0 Bank loan fund 258.9 — — 258.9 Lloyd’s trust deposits 172.7 — — 172.7 ILS funds — — 71.2 71.2 Private debt instruments — 6.2 9.6 15.8 Other 31.3 — — 31.3 Total other long-term investments $ 544.8 $ 936.8 $ 589.5 $ 2,071.1 (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 Simple Agreement for Future Equity (“SAFE”) investments. Private Equity Funds and Hedge Funds White Mountains invests in private equity funds and hedge funds, which are included in other long-term investments. The fair value of these investments is generally estimated using the NAV of the funds. As of September 30, 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 $56.6 million and $57.3 million as of September 30, 2024 and December 31, 2023. The following table presents the fair value of investments and unfunded commitments in private equity funds and hedge funds by investment objective and sector as of September 30, 2024 and December 31, 2023: September 30, 2024 December 31, 2023 Millions Fair Value Unfunded Fair Value Unfunded Private equity funds Aerospace/Defense/Government $ 169.0 $ 46.8 $ 157.2 $ 17.4 Financial services 89.3 36.1 87.5 37.8 Real estate 3.7 2.4 3.9 2.5 Total private equity funds 262.0 85.3 248.6 57.7 Hedge funds Long/short equity financials and business services 56.5 — 54.4 — Long/short all cap global 30.5 — — — European small/mid cap — — 9.9 — Total hedge funds 87.0 — 64.3 — Total private equity funds and hedge funds included in other long-term investments $ 349.0 $ 85.3 $ 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 September 30, 2024: Millions 1 – 3 years 3 – 5 years 5 – 10 years >10 years Total Private equity funds — expected lock-up period remaining $18.5 $55.9 $184.5 $3.1 $262.0 Investors in private equity funds are generally subject to indemnification obligations outside of the capital commitment period and prior to the winding up of the fund. As of September 30, 2024 and December 31, 2023, White Mountains is not aware of any indemnification claims relating to its investments in private equity funds. Redemption of investments in most hedge funds is subject to restrictions, including lock-up periods where no redemptions or withdrawals are allowed, restrictions on redemption frequency and advance notice periods for redemptions. Amounts requested for redemptions remain subject to market fluctuations until the redemption effective date, which generally falls at the end of the defined redemption period. 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 $258.9 million and $194.4 million as of September 30, 2024 and December 31, 2023. The fair value of this investment is estimated using the NAV of the fund. The bank loan fund’s investment objective is to provide, on an unleveraged basis, high current income consistent with preservation of capital and low duration. The bank loan fund primarily invests in a broad portfolio of U.S. dollar-denominated, non-investment grade, floating-rate senior secured loans and may invest in other financial instruments, such as secured and unsecured corporate debt, credit default swaps, reverse repurchase agreements, synthetic indices and cash and cash equivalents. The investment in the bank loan fund is subject to restrictions on redemption frequency and advance notice periods for redemptions. Amounts requested for redemptions remain subject to market fluctuations until the redemption effective date, which generally falls at the end of the defined redemption period. White Mountains may redeem all or a portion of its bank loan fund investment as of any calendar month-end upon 15 calendar days advanced written notice. Lloyd’s Trust Deposits White Mountains’s other long-term investments include Lloyd’s trust deposits, which consist of non-U.K. deposits and Canadian comingled pooled funds. The Lloyd’s trust deposits invest primarily in short-term government securities, agency securities and corporate bonds held in trusts that are managed by Lloyd's of London. These investments are generally required of Lloyd's syndicates to protect policyholders in non-U.K. markets and are pledged into Lloyd’s trust accounts to provide a portion of the capital needed to support obligations at Lloyd’s. The fair value of the Lloyd’s trust deposits is generally estimated using the NAV of the funds. As of September 30, 2024 and December 31, 2023, White Mountains held Lloyd’s trust deposits with a fair value of $172.7 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 September 30, 2024 and December 31, 2023, White Mountains held investments in ILS funds with a fair value of $71.2 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 September 30, 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 nine months ended September 30, 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 79.9 Net realized and unrealized gains 74.0 Purchases and contributions 3.1 Purchases and contributions 200.1 Sales and distributions (41.4) Sales and distributions (110.4) Transfers in — Transfers in — Transfers out — Transfers out — Balance as of September 30, 2024 $ 1,179.8 Balance as of September 30, 2023 $ 1,075.3 Fair Value Measurements — Transfers Between Levels - Three and nine months ended September 30, 2024 and 2023 Transfers between levels are recorded using the fair value measurement as of the end of the quarterly period in which the event or change in circumstance giving rise to the transfer occurred. During the nine months ended September 30, 2024 and 2023, there were no fixed maturity investments or other long-term investments classified as Level 3 measurements in the prior period that were transferred to Level 2 measurements. During the nine months ended September 30, 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 September 30, 2024 and December 31, 2023. The tables below exclude $18.7 million and $19.0 million of Level 3 other long-term investments generally valued based on recent or expected transaction prices. The fair value of investments in private equity funds and hedge funds, bank loan funds, Lloyd’s trust deposits and ILS funds are generally estimated using the NAV of the funds. $ in Millions September 30, 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 $930.6 16% - 24% 7x - 22x PassportCard/DavidShield Discounted cash flow $150.0 24% 4% Elementum Discounted cash flow $35.0 21% 4% Preferred Securities Discounted cash flow $30.1 7% N/A Private debt instruments Discounted cash flow $15.4 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 the first nine months of 2024, Kudu deployed a total of $0.2 into 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 deployed a total of $163.8 into new and existing Participation Contracts. (5) As of December 31, 2023, one of Kudu’s Participation Contracts with a total fair value of $69.1 was valued using a probability weighted expected return method, which takes into account factors such as a discounted cash flow analysis, the expected value to be received in a pending sale transaction and the likelihood that a sales transaction will take place. (6) Increases (decreases) to the discount rates in isolation would result in lower (higher) fair value measurements, while increases (decreases) to th

Goodwill and Other Intangible A

Goodwill and Other Intangible Assets	9 Months Ended
	Sep. 30, 2024
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets White Mountains accounts for business combinations using the acquisition method. Under the acquisition method, White Mountains recognizes and measures the assets acquired, liabilities assumed and any noncontrolling interest in the acquired entities at their acquisition date fair values. Goodwill represents the excess of the amount paid to acquire a business over the fair value of identifiable net assets at the date of acquisition. The estimated acquisition date fair values, generally consisting of intangible assets and liabilities for contingent consideration, may be recorded at provisional amounts in circumstances where the information necessary to complete the acquisition accounting is not available at the reporting date. Any such provisional amounts are finalized as measurement period adjustments within one year of the acquisition date. The following table presents the economic lives, acquisition date fair values, accumulated amortization and net carrying values for other intangible assets and goodwill as of September 30, 2024 and December 31, 2023: $ in Millions Weighted Average Economic September 30, 2024 December 31, 2023 Acquisition Date Fair Value Accumulated Amortization Net Carrying Value Acquisition Date Fair Value Accumulated Amortization Net Carrying Value Goodwill: Ark N/A $ 116.8 $ — $ 116.8 $ 116.8 $ — $ 116.8 Kudu N/A 7.6 — 7.6 7.6 — 7.6 Bamboo (1) N/A 270.4 — 270.4 — — — Other Operations N/A 44.4 — 44.4 44.4 — 44.4 Total goodwill 439.2 — 439.2 168.8 — 168.8 Other intangible assets: Ark Underwriting capacity N/A 175.7 — 175.7 175.7 — 175.7 Kudu Trade names 7.0 2.2 1.8 .4 2.2 1.5 .7 Bamboo (1) Trade names 10.0 23.5 1.8 21.7 — — — Agency relationships 6.0 72.4 9.1 63.3 — — — Developed technology 3.0 4.7 1.1 3.6 — — — Other 0.3 .4 .4 — — — — Subtotal 101.0 12.4 88.6 — — — Other Operations Trade names 13.3 13.3 5.3 8.0 13.3 4.1 9.2 Customer relationships 11.0 24.8 12.7 12.1 24.8 10.3 14.5 Other 11.8 3.1 1.4 1.7 2.8 1.1 1.7 Subtotal 41.2 19.4 21.8 40.9 15.5 25.4 Total other intangible assets 320.1 33.6 286.5 218.8 17.0 201.8 Total goodwill and other intangible assets $ 759.3 $ 33.6 $ 725.7 $ 387.6 $ 17.0 $ 370.6 (1) The relative fair values of goodwill and other intangible assets recognized in connection with the acquisition of Bamboo had not yet been finalized as of September 30, 2024. Intangible Assets Valuation Methods The goodwill recognized for the entities shown above is attributed to expected future cash flows. The acquisition date fair values of other intangible assets with finite lives are estimated using income approach techniques, which use future expected cash flows to develop a discounted present value amount. The multi-period-excess-earnings method estimates fair value using the present value of the incremental after-tax cash flows attributable solely to the other intangible asset over its remaining life. This approach was used to estimate the fair value of other intangible assets associated with the underwriting capacity, agency relationships and customer relationships. The relief-from-royalty method was used to estimate fair value for other intangible assets that relate to rights that could be obtained via a license from a third-party owner. Under this method, the fair value is estimated using the present value of license fees avoided by owning rather than leasing the asset. This technique was used to estimate the fair value of trade names, patents and certain information technology platforms. The with-or-without method estimates the fair value of other intangible assets that provide an incremental benefit. Under this method, the fair value of the other intangible asset is calculated by comparing the value of the entity with and without the other intangible asset. This approach was used to estimate the fair value of non-compete agreements. On at least an annual basis beginning no later than the interim period included in the one-year anniversary of an acquisition, White Mountains evaluates goodwill and other intangible assets for potential impairment. Between annual evaluations, White Mountains considers changes in circumstances or events subsequent to the most recent evaluation that indicate an impairment may exist and, if necessary will perform an interim review for potential impairment. During the three and nine months ended September 30, 2024 and 2023, White Mountains did not recognize any impairments to goodwill and other intangible assets. Rollforward of Goodwill and Other Intangible Assets The following tables present the change in goodwill and other intangible assets for the three and nine months ended September 30, 2024 and 2023: Three Months Ended September 30, 2024 2023 Millions Goodwill Other Intangible Assets Total Goodwill and Other Intangible Assets Goodwill Other Intangible Assets Total Goodwill and Other Intangible Assets Beginning balance $ 439.2 $ 291.8 $ 731.0 $ 168.8 $ 204.8 $ 373.6 Amortization — (5.3) (5.3) — (1.5) (1.5) Ending balance $ 439.2 $ 286.5 $ 725.7 $ 168.8 $ 203.3 $ 372.1 Nine Months Ended September 30, 2024 2023 Millions Goodwill Other Intangible Assets Total Goodwill and Other Intangible Assets Goodwill Other Intangible Assets Total Goodwill and Other Intangible Assets Beginning balance $ 168.8 $ 201.8 $ 370.6 $ 176.5 $ 215.9 $ 392.4 Acquisition of businesses (1) 270.4 101.0 371.4 — — — Acquisitions of intangible assets (2) — .3 .3 — — — Dispositions (2) — — — (6.7) (6.9) (13.6) Measurement period adjustments (3) — — — (1.0) — (1.0) Amortization — (16.6) (16.6) — (5.7) (5.7) Ending balance $ 439.2 $ 286.5 $ 725.7 $ 168.8 $ 203.3 $ 372.1 (1) Amounts relate to the fair values of goodwill and other intangible assets recognized in connection with the acquisition of Bamboo, which had not yet been finalized as of September 30, 2024. (2) Relates to acquisitions and dispositions within Other Operations. (3) Measurement period adjustments relate to updated information about acquisition date fair values of assets acquired and liabilities assumed. During the nine months ended September 30, 2023, adjustments relate to an acquisition within Other Operations.

Loss and Loss Adjustment Expens

Loss and Loss Adjustment Expense Reserves	9 Months Ended
	Sep. 30, 2024
Insurance [Abstract]
Loss and Loss Adjustment Expense Reserves	Loss and Loss Adjustment Expense Reserves P&C Insurance and Reinsurance The following table summarizes the loss and loss adjustment expense (“LAE”) reserve activity of the Ark/WM Outrigger segment for the three and nine months ended September 30, 2024 and 2023: Three Months Ended September 30, Nine Months Ended September 30, Millions 2024 2023 2024 2023 Gross beginning balance $ 1,890.1 $ 1,421.0 $ 1,605.1 $ 1,296.5 Less: beginning reinsurance recoverable on unpaid losses (1) (458.5) (362.7) (340.8) (505.0) Net loss and loss adjustment expense reserves 1,431.6 1,058.3 1,264.3 791.5 Loss and loss adjustment expenses incurred relating to: Current year losses 313.1 266.4 676.0 562.1 Prior year losses (24.8) (.6) (32.0) 19.0 Net incurred loss and loss adjustment expenses 288.3 265.8 644.0 581.1 Loss and loss adjustment expenses paid relating to: Current year losses (19.1) (8.8) (27.4) (18.4) Prior year losses (86.3) (90.4) (261.8) (281.1) Net paid loss and loss adjustment expenses (105.4) (99.2) (289.2) (299.5) Change in TPC Providers’ participation (2) — — — 145.4 Foreign currency translation and other adjustments to loss 11.3 (5.5) 6.7 .9 Net ending balance 1,625.8 1,219.4 1,625.8 1,219.4 Plus: ending reinsurance recoverable on unpaid losses 507.2 352.4 507.2 352.4 Gross ending balance $ 2,133.0 $ 1,571.8 $ 2,133.0 $ 1,571.8 (1) The beginning reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $145.4 as of December 31, 2022. (2) Amount represents the impact to net loss and loss adjustment expense reserves due to a change in the TPC Providers’ participation related to the annual RITC process. For the three and nine months ended September 30, 2024, the Ark/WM Outrigger segment recognized $24.8 million and $32.0 million of net favorable prior year loss reserve development, primarily due to property lines of business. For the three and nine months ended September 30, 2023, the Ark/WM Outrigger segment recognized $0.6 million of net favorable prior year loss reserve development and $19.0 million of net unfavorable prior year loss reserve development. The net unfavorable prior year loss reserve development for the nine months ended September 30, 2023 was driven primarily by Winter Storm Elliot and three large claims in the property and marine & energy lines of business. Impact of Third-Party Capital For the years of account prior to the Ark Transaction, a significant proportion of the Syndicates’ underwriting capital was provided by third-party insurance and reinsurance groups (“TPC Providers”) using whole account reinsurance contracts with Ark’s corporate member. For the years of account subsequent to the Ark Transaction, Ark is no longer using TPC Providers to provide underwriting capital for the Syndicates. During the first quarter of 2023, a reinsurance to close (“RITC”) agreement was executed such that the outstanding loss and LAE reserves for claims arising out of the 2020 year of account, for which the TPC Providers’ participation in the total net results of the Syndicates was 42.8%, were reinsured into the 2021 year of account, for which the TPC Providers’ participation in the total net results of the Syndicates was 0.0%. Financial Guarantee HG Re does not have any outstanding loss and LAE reserves. For the three and nine months ended September 30, 2024, HG Re recognized gross incurred loss and LAE of $0.3 million related to a delinquent payment by a reinsured BAM policyholder. Net of estimated recoveries, HG Re recognized no incurred loss and LAE. P&C Insurance Distribution As of September 30, 2024, Bamboo Captive has recorded loss and LAE reserves of $17.7 million. For the three and nine months ended September 30, 2024, Bamboo Captive recognized incurred loss and LAE of $4.4 million and $14.5 million. Other Operations As of September 30, 2024, Bamboo CRV has recorded loss and LAE reserves of $8.1 million. For the three and nine months ended September 30, 2024, Bamboo CRV recognized incurred loss and LAE of $4.2 million and $8.1 million.

Third-Party Reinsurance

Third-Party Reinsurance	9 Months Ended
	Sep. 30, 2024
Reinsurance Disclosures [Abstract]
Third-Party Reinsurance	Third-Party Reinsurance P&C Insurance and Reinsurance In the normal course of business, Ark may seek to limit losses that may arise from catastrophes or other events by reinsuring certain risks with third-party reinsurers. Ark remains liable for risks reinsured in the event that the reinsurer does not honor its obligations under reinsurance contracts. The following table summarizes the effects of reinsurance on written and earned premiums and on loss and LAE for the Ark/WM Outrigger segment for the three and nine months ended September 30, 2024 and 2023: Millions Three Months Ended September 30, Nine Months Ended September 30, 2024 2023 2024 2023 Written premiums: Direct $ 239.0 $ 189.4 $ 863.4 $ 727.1 Assumed 134.6 61.8 1,079.3 939.6 Gross written premiums 373.6 251.2 1,942.7 1,666.7 Ceded (1) (34.2) (20.2) (502.5) (360.3) Net written premiums $ 339.4 $ 231.0 $ 1,440.2 $ 1,306.4 Earned premiums: Direct $ 317.6 $ 273.1 $ 731.5 $ 624.1 Assumed 432.6 370.0 805.3 684.6 Gross earned premiums 750.2 643.1 1,536.8 1,308.7 Ceded (2) (198.0) (144.2) (363.5) (261.4) Net earned premiums $ 552.2 $ 498.9 $ 1,173.3 $ 1,047.3 Loss and loss adjustment expenses: Gross $ 350.7 $ 269.3 $ 852.7 $ 627.5 Ceded (3) (62.4) (3.5) (208.7) (46.4) Net loss and loss adjustment expenses $ 288.3 $ 265.8 $ 644.0 $ 581.1 (1) The three months ended September 30, 2024 and 2023 exclude $8.8 and $6.0, and the nine months ended September 30, 2024 and 2023 exclude $82.0 and $108.4 ceded by Ark to WM Outrigger Re, which eliminate in White Mountains’s consolidated financial statements. (2) The three months ended September 30, 2024 and 2023 exclude $45.0 and $60.6, and the nine months ended September 30, 2024 and 2023 exclude $62.8 and $75.4, ceded by Ark to WM Outrigger Re, which eliminate in White Mountains’s consolidated financial statements. (3) The three months ended September 30, 2024 and 2023 exclude $13.2 and $7.3, and the nine months ended September 30, 2024 and 2023 exclude $13.6 and $7.9, 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 September 30, 2024 and December 31, 2023: Millions September 30, 2024 December 31, 2023 Reinsurance recoverables on unpaid losses (1) $ 507.2 $ 340.8 Reinsurance recoverables on paid losses 27.9 27.4 Ceded unearned premiums (2) 212.9 73.8 Reinsurance recoverables $ 748.0 $ 442.0 (1) The reinsurance recoverables on unpaid losses exclude $28.0 and $15.6 ceded by Ark to WM Outrigger Re as of September 30, 2024 and December 31, 2023, which eliminate in White Mountains’s consolidated financial statements. (2) The ceded unearned premiums exclude $25.0 and $5.7 ceded by Ark to WM Outrigger Re as of September 30, 2024 and December 31, 2023, which eliminate in White Mountains’s consolidated financial statements. As reinsurance contracts do not relieve Ark of its obligation to its policyholders, Ark seeks to reduce the credit risk associated with reinsurance balances by avoiding over-reliance on specific reinsurers through the application of concentration limits and thresholds. Ark is selective with its reinsurers, placing reinsurance with only those reinsurers having a strong financial condition. Ark monitors the financial strength of its reinsurers on an ongoing basis. The following table presents the Ark/WM Outrigger segment’s gross and net reinsurance recoverables by the reinsurer’s A.M. Best Company, Inc (“A.M. Best”) ratings as of September 30, 2024: $ in Millions As of September 30, 2024 A.M. Best Rating (1) Gross Collateral Net % of Total A+ or better $ 358.1 $ — $ 358.1 78.9 % A- to A 84.2 — 84.2 18.5 B++ or lower and not rated 92.8 (2) 81.1 (2) 11.7 2.6 Total $ 535.1 $ 81.1 $ 454.0 100.0 % (1) A.M. Best financial strength ratings as detailed above are: “A+ or better” (Superior) “A- to A” (Excellent), “B++” (Good). (2) Excludes $28.0 ceded by Ark to WM Outrigger Re as of September 30, 2024, which eliminates in White Mountains’s consolidated financial statements. Reinsurance Contracts Accounted for as Deposits Ark has an aggregate excess of loss contract with SiriusPoint Ltd. (“SiriusPoint”), which is accounted for using the deposit method and recorded within other assets. Ark earns an annual crediting rate of 3.0%, which is recorded within other revenue. As of September 30, 2024 and December 31, 2023, the carrying value of Ark’s deposit in SiriusPoint, including accrued interest, was $20.3 million and $20.4 million. Financial Guarantee See Note 10 — “Municipal Bond Guarantee Reinsurance” for third-party reinsurance balances and reinsurance contracts accounted for as deposits related to BAM’s municipal bond guarantee business prior to the deconsolidation of BAM on July 1, 2024.

Debt

Debt	9 Months Ended
	Sep. 30, 2024
Debt Disclosure [Abstract]
Debt	Debt The following table presents White Mountains’s debt outstanding as of September 30, 2024 and December 31, 2023: $ in Millions September 30, 2024 Effective (1) December 31, 2023 Effective (1) Ark 2007 Subordinated Notes, carrying value $ — $ 30.0 Ark 2021 Subordinated Notes Tranche 1 43.6 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 (3.8) (4.2) Ark 2021 Subordinated Notes, carrying value 156.8 155.5 Total Ark Subordinated Notes, carrying value 156.8 12.1% 185.5 11.0% HG Global Senior Notes (2) 150.0 11.8% 150.0 11.9% Unamortized discount and issuance cost (2.7) (3.1) HG Global Senior Notes, carrying value 147.3 146.9 Kudu Credit Facility (2) 225.3 10.5% 210.3 10.1% Unamortized issuance cost (6.8) (6.5) Kudu Credit Facility, carrying value 218.5 203.8 Other Operations debt 22.6 10.1% 28.9 10.2% Unamortized issuance cost (.3) (.5) Other Operations debt, carrying value 22.3 28.4 Total debt $ 544.9 $ 564.6 (1) Effective rate includes the effect of the amortization of debt issuance costs and, where applicable, the original issue discount. (2) Effective rate excludes the effect of the interest rate caps. 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 the first quarter of 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 to maintain 120% of GAIL’s Bermuda solvency capital requirement, failure of GAIL to maintain a debt to capital ratio below 40%, late filing of GAIL’s or Ark’s financial information, and making a restricted payment or distribution on GAIL’s common stock or other securities that rank junior or pari passu with the Ark 2021 Subordinated Notes Tranche 3 when a different premium load event exists or will be caused by the restricted payment. As of September 30, 2024, there were no premium load events. As of September 30, 2024, the Ark 2021 Subordinated Notes Tranche 1 had an outstanding balance of €39.1 million ($43.6 million based upon the foreign exchange spot rate as of September 30, 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 September 30, 2024, the Citibank LOC Facility had an outstanding balance of $107.9 million and short-term investments and cash pledged as collateral of $141.3 million. As of September 30, 2024, the Lloyds LOC Facility had an outstanding balance of $35.5 million and short-term investments and cash pledged as collateral of $66.9 million. Ark’s uncommitted secured standby letter of credit facility agreements contain various representations, warranties and covenants that White Mountains considers to be customary for such borrowings. HG Global Senior Notes On April 29, 2022, HG Global received the proceeds from the issuance of its $150.0 million face value floating rate secured senior notes (the “HG Global Senior Notes”). The HG Global Senior Notes, which mature in April 2032, accrue interest at a floating rate equal to the three-month 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 September 30, 2024. The fair value of the interest reserve account, which is included in short-term investments, was $29.3 million as of September 30, 2024. The HG Global Senior Notes are secured by the capital stock and other equity interests of HG Global’s subsidiaries, the interest reserve account, and all cash and non-cash proceeds from such collateral. The HG Global Senior Notes contain various affirmative and negative covenants that White Mountains considers to be customary for such borrowings. If the payment of principal and interest under the HG Global Senior Notes becomes subject to tax withholding on behalf of a relevant governmental authority for certain indemnified taxes, the HG Global Senior Notes require the payment of additional amounts such that the amount received by the noteholders is the same as would have been received absent the tax withholding being imposed. The HG Global Senior Notes require the payment of additional interest of 1.0% per annum if the HG Global Senior Notes receive a non-investment grade rating or are no longer rated. As of September 30, 2024, the HG Global Senior Notes had an investment grade rating. As of September 30, 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 30, 2023, interest on the Kudu Credit Facility accrued at a floating interest rate equal to the greater of the three-month LIBOR or 0.25% plus, in each case, 4.30% 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 September 30, 2024 and December 31, 2023, the interest reserve account of $14.8 million and $14.9 million was held in short term investments. 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 September 30, 2024, Kudu had a 23.7% 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 three and nine months ended September 30, 2024 and 2023: Three Months Ended September 30, Nine Months Ended September 30, Millions 2024 2023 2024 2023 Kudu Credit Facility Beginning balance $ 210.3 $ 210.3 $ 210.3 $ 215.2 Term loans Borrowings 15.0 — 15.0 12.0 Repayments — — — (16.9) Ending balance $ 225.3 $ 210.3 $ 225.3 $ 210.3 The Kudu Credit Facility is secured by all property of the loan parties and contains various affirmative and negative covenants that White Mountains considers to be customary for such borrowings. Other Operations Debt As of September 30, 2024, White Mountains’s Other Operations had debt with an outstanding balance of $22.6 million, which consisted of four secured credit facilities (collectively, “Other Operations debt”). Compliance As of September 30, 2024, White Mountains was in compliance, in all material respects, with all of the covenants under its debt instruments.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2024
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company and its Bermuda-domiciled subsidiaries are not subject to income tax in Bermuda in 2024 and prior years. On December 27, 2023, Bermuda enacted a 15.0% corporate income tax that will generally become effective on January 1, 2025. The Bermuda legislation defers the effective date for five years for Bermuda companies in consolidated groups that meet certain requirements. White Mountains expects to meet the requirements to be exempt from the Bermuda corporate income tax until January 1, 2030. The Bermuda legislation also provides for an economic transition adjustment that will reduce future years’ taxable income. Under GAAP, this economic transition adjustment was required to be recognized as a net deferred tax asset as of December 31, 2023. Accordingly, White Mountains’s net income for 2023 included a net deferred tax benefit of $68.0 million, of which $51.0 million was recorded at Ark and $17.0 million was recorded at HG Global. 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, generating a $5.0 million tax benefit in the third quarter of 2024. The Company has subsidiaries and branches that operate in various other jurisdictions around the world and are subject to tax in the jurisdictions in which they operate. As of September 30, 2024, the primary jurisdictions in which the Company’s subsidiaries and branches operate and were subject to tax include Israel, Luxembourg, the United Kingdom and the United States. On April 1, 2023, the U.K. corporate tax rate increased from 19.0% to 25.0%. White Mountains’s income tax expense related to pre-tax income from continuing operations for the three and nine months ended September 30, 2024 represented an effective tax rate of 4.8% and 6.6%. The effective tax rate was different from the U.S. statutory rate of 21.0%, driven primarily by full year forecasted income in jurisdictions with lower tax rates than the United States. White Mountains’s income tax expense related to pre-tax income from continuing operations for the three months ended September 30, 2023 represented an effective tax rate of 34.6%. The effective tax rate was different from the U.S. statutory rate of 21.0%, driven primarily by an increase in the full valuation allowance at BAM, as well as withholding taxes and state income taxes, partially offset by full year forecasted income in jurisdictions with lower tax rates than the United States. White Mountains’s income tax expense related to pre-tax income from continuing operations for the nine months ended September 30, 2023 represented an effective tax rate of 8.2%. The effective tax rate was different from the U.S. statutory rate of 21.0%, driven primarily by full year forecasted income in jurisdictions with lower tax rates than the United States. In arriving at the effective tax rate for the three and nine months ended September 30, 2024 and 2023, White Mountains forecasted all income and expense items including the change in net unrealized investment gains (losses) and net realized investment gains (losses) for the years ending December 31, 2024 and 2023. White Mountains records a valuation allowance against deferred tax assets if it becomes more likely than not that all or a portion of a deferred tax asset will not be realized. Changes in valuation allowances from period to period are included in income tax expense in the period of change. In determining whether or not a valuation allowance, or change therein, is warranted, White Mountains considers factors such as prior earnings history, expected future earnings, carryback and carryforward periods and strategies that if executed would result in the realization of a deferred tax asset. With few exceptions, White Mountains is no longer subject to U.S. federal, state, or non-U.S. income tax examinations by tax authorities for years before 2018.

Derivatives

Derivatives	9 Months Ended
	Sep. 30, 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 September 30, 2024, the three-month SOFR was 4.6%. 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 September 30, 2024, the three-month SOFR was 4.6%.

Municipal Bond Guarantee Insura

Municipal Bond Guarantee Insurance	9 Months Ended
	Sep. 30, 2024
Guarantees [Abstract]
Municipal Bond Guarantee Insurance	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 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. 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 member surplus contribution (“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 $14.0 million and earned premiums of $7.5 million for the three and nine months ended September 30, 2024. XOLT HG Re is party to an excess of loss reinsurance agreement (the “XOLT”) with BAM. Under the XOLT, HG Re provides last dollar protection for exposures on municipal bonds insured by BAM in excess of the New York State Department of Financial Services (“NYDFS”) single issuer limits. As of September 30, 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, and any related financing revenues are recorded in other revenues, as the agreement does not meet the risk transfer requirements necessary to be accounted for as reinsurance. 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 the three and nine months ended September 30, 2024 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 September 30, 2024 and December 31, 2023 was $363.2 million and $341.6 million. The Supplemental Trust target balance is $603.0 million, less the amount of cash and securities in the Regulation 114 Trust in excess of its target balance (the “Supplemental Trust Target Balance”). If, at the end of any quarter, the Supplemental Trust balance exceeds the Supplemental Trust Target Balance, such excess may be distributed to HG Re. The distribution will be made first as an assignment of accrued interest on the BAM Surplus Notes and second in cash and/or fixed income securities. As the BAM Surplus Notes are repaid over time, the BAM Surplus Notes will be replaced in the Supplemental Trust by cash and fixed income securities. The Supplemental Trust balance as of September 30, 2024 and December 31, 2023 was $613.2 million and $606.9 million. As of September 30, 2024 and December 31, 2023, the Collateral Trusts held assets of $976.4 million and $948.5 million, which included $644.7 million and $588.6 million of cash, investments and accrued investment income, $317.1 million and $322.2 million of BAM Surplus Notes and $14.6 million and $37.7 million of accrued interest receivable on the BAM Surplus Notes. 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. In May 2024, BAM made a $8.0 million cash payment of principal and interest on the BAM Surplus Notes held by HG Global. Of this payment, $5.1 million was a repayment of principal held in the Supplemental Trust, $0.3 million was a payment of accrued interest held in the Supplemental Trust and $2.6 million was a payment of accrued interest held outside the Supplemental Trust. In December 2023, BAM made a $27.4 million cash payment of principal and interest on the BAM Surplus Notes held by HG Global. Of this payment, $17.8 million was a repayment of principal held in the Supplemental Trust, $2.0 million was a payment of accrued interest held in the Supplemental Trust and $7.6 million was a payment of accrued interest held outside the Supplemental Trust. During the three and nine months ended September 30, 2023, BAM made no repayments of the BAM Surplus Notes or accrued interest. As of September 30, 2024 and December 31, 2023, the principal balance on the BAM Surplus Notes was $317.1 million and $322.2 million and total interest receivable on the BAM Surplus Notes was $192.7 million and $174.5 million. 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 September 30, 2024, the estimated fair value of the BAM Surplus Notes was $387.4 million and $411.1 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 the third quarter of 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 Level 3 measurements. 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. 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 September 30, 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 7.50% was appropriate. The change in the discount rate in the quarter was driven by a decline in market interest rates. Subsequent to the deconsolidation of BAM on July 1, 2024, White Mountains recognized a gain of $15.8 million related to the change in fair value of the BAM Surplus Notes for the three and nine months ended September 30, 2024. Insured Obligations and Premiums The following table presents a schedule of the HG Global segment’s insured obligations as of September 30, 2024 and December 31, 2023: September 30, 2024 December 31, 2023 Contracts outstanding 15,524 14,485 Remaining weighted average contract period (in years) (1) 12.3 12.2 (2) Outstanding first loss exposure from policies assumed (in millions) (1) $ 17,667.2 $ 16,341.0 (3) Gross unearned insurance premiums (in millions) $ 287.0 $ 273.9 (4) (1) Under the FLRT, HG Re provides first loss 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. Accordingly, HG Re has no exposure to future interest outstanding and HG Re’s remaining weighted average contract period is calculated using its outstanding first loss exposure from policies assumed. (2) As of December 31, 2023, which was prior to the deconsolidation of BAM, HG Global/BAM’s remaining weighted average contract period was 11.1 years, calculated using HG Global/BAM’s total contractual debt service outstanding, including principal and interest. (3) As of December 31, 2023, which was prior to the deconsolidation of BAM, HG Global/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 the deconsolidation of BAM, HG Global/BAM’s gross unearned insurance premiums was $325.8. The following table presents a schedule of HG Global’s future premium revenues as of September 30, 2024: Millions September 30, 2024 October 1, 2024 - December 31, 2024 $ 6.6 January 1, 2025 - March 31, 2025 6.4 April 1, 2025 - June 30, 2025 6.4 July 1, 2025 - September 30, 2025 6.2 October 1, 2025 - December 31, 2025 6.1 Total 2025 25.1 2026 23.7 2027 22.3 2028 20.8 2029 19.3 2030 and thereafter 169.2 Total gross unearned insurance premiums $ 287.0 The following tables present schedules of written premiums and earned premiums included in the HG Global segment for the three and nine months ended September 30, 2024 and 2023: Three Months Ended September 30, 2024 Millions HG Global Written premiums: Direct $ — Assumed 14.0 Gross written premiums (1) $ 14.0 Earned premiums: Direct $ — Assumed 7.5 Gross earned premiums (1) $ 7.5 (1) There are no ceded premium amounts in the periods presented, such that gross written premiums and gross earned premiums are equivalent to net written premiums and net earned premiums, respectively. Three Months Ended September 30, 2023 Millions HG Global BAM Eliminations Total Written premiums: Direct $ — $ 16.5 $ — $ 16.5 Assumed 14.2 — (14.2) — Gross written premiums (1) $ 14.2 $ 16.5 $ (14.2) $ 16.5 Earned premiums: Direct $ — $ 7.4 $ — $ 7.4 Assumed 6.6 .5 (6.6) .5 Gross earned premiums (1) $ 6.6 $ 7.9 $ (6.6) $ 7.9 (1) There are no ceded premium amounts in the periods presented, such that gross written premiums and gross earned premiums are equivalent to net written premiums and net earned premiums, respectively. Nine Months Ended September 30, 2024 Millions HG Global BAM Eliminations Total Written premiums: Direct $ — $ 24.1 $ — $ 24.1 Assumed 34.5 — (20.5) 14.0 Gross written premiums (1) $ 34.5 $ 24.1 $ (20.5) $ 38.1 Earned premiums: Direct $ — $ 15.8 $ — $ 15.8 Assumed 21.5 1.0 (14.0) 8.5 Gross earned premiums (1) $ 21.5 $ 16.8 $ (14.0) $ 24.3 (1) There are no ceded premium amounts in the periods presented, such that gross written premiums and gross earned premiums are equivalent to net written premiums and net earned premiums, respectively. Nine Months Ended September 30, 2023 Millions HG Global BAM Eliminations Total Written premiums: Direct $ — $ 37.3 $ — $ 37.3 Assumed 31.9 — (31.9) — Gross written premiums (1) $ 31.9 $ 37.3 $ (31.9) $ 37.3 Earned premiums: Direct $ — $ 21.4 $ — $ 21.4 Assumed 19.4 1.9 (19.4) 1.9 Gross earned premiums (1) $ 19.4 $ 23.3 $ (19.4) $ 23.3 (1) There are no ceded premium amounts in the periods presented, such that gross written premiums and gross earned premiums are equivalent to net written premiums and net earned premiums, respectively.

Earnings Per Share

Earnings Per Share	9 Months Ended
	Sep. 30, 2024
Earnings Per Share [Abstract]
Earnings Per Share	Earnings Per Share White Mountains calculates earnings per share using the two-class method, which allocates earnings between common shares and unvested restricted common shares. Both classes of shares participate equally in dividends and earnings on a per share basis. Basic earnings per share amounts are based on the weighted average number of common shares outstanding adjusted for unvested restricted common shares. The following table presents the Company’s computation of earnings per share from continuing operations for the three and nine months ended September 30, 2024 and 2023. Three Months Ended September 30, Nine Months Ended September 30, 2024 2023 2024 2023 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s $ 179.0 $ 23.6 $ 360.8 $ 222.7 Allocation of (earnings) losses to participating restricted common shares (1) (2.5) (.3) (4.6) (3.1) Basic and diluted earnings (losses) per share numerators $ 176.5 $ 23.3 $ 356.2 $ 219.6 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 2,568.1 2,560.5 2,564.8 2,564.2 Average unvested restricted common shares (2) (35.4) (37.6) (32.9) (35.5) Basic earnings (losses) per share denominator 2,532.7 2,522.9 2,531.9 2,528.7 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 2,568.1 2,560.5 2,564.8 2,564.2 Average unvested restricted common shares (2) (35.4) (37.6) (32.9) (35.5) Diluted earnings (losses) per share denominator 2,532.7 2,522.9 2,531.9 2,528.7 Basic and diluted earnings per share (in dollars): Distributed earnings - dividends declared and paid $ — $ — $ 1.00 $ 1.00 Undistributed earnings (losses) 69.68 9.19 139.66 85.82 Basic and diluted earnings (losses) per share $ 69.68 $ 9.19 $ 140.66 $ 86.82 (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (2) Restricted shares outstanding vest upon a stated date. See Note 12 — “Employee Share-Based Incentive Compensation Plans.” The following table presents the undistributed net earnings (losses) for the three and nine months ended September 30, 2024 and 2023. Three Months Ended September 30, Nine Months Ended September 30, Millions 2024 2023 2024 2023 Undistributed net earnings: Net income (loss) attributable to White Mountains’s common $ 176.5 $ 23.3 $ 356.2 $ 219.6 Dividends declared, net of restricted common share amounts (1) — — (2.5) (2.5) Total undistributed net earnings (losses), net of restricted $ 176.5 $ 23.3 $ 353.7 $ 217.1 (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.

Employee Share-Based Incentive

Employee Share-Based Incentive Compensation Plans	9 Months Ended
	Sep. 30, 2024
Share-Based Payment Arrangement [Abstract]
Employee Share-Based Incentive Compensation Plans	Employee Share-Based Incentive Compensation Plans White Mountains’s share-based incentive compensation plans are designed to incentivize key employees to maximize shareholder value over long periods of time. White Mountains believes that this is best pursued by utilizing a pay-for-performance program that closely aligns the financial interests of management with those of its shareholders while rewarding appropriate risk taking. White Mountains accomplishes this by emphasizing variable long-term compensation that is contingent on performance over a number of years rather than fixed entitlements. White Mountains expenses all of its share-based compensation. As a result, White Mountains’s calculation of its owners’ returns includes the expense of all outstanding share-based compensation awards. White Mountains’s Long-Term Incentive Plan (the “WTM Incentive Plan”) provides for grants of various types of share-based and non-share-based incentive awards to key employees and directors of White Mountains. As of September 30, 2024 and 2023, White Mountains’s share-based incentive compensation awards consist of performance shares and restricted shares. Performance Shares Performance shares are designed to reward employees for meeting company-wide performance targets. Performance shares are conditional grants of a specified maximum number of common shares or an equivalent amount of cash. Awards generally vest at the end of a three-year service period, are subject to the attainment of pre-specified performance goals, and are valued based on the market value of common shares at the time awards are paid. Performance shares earned under the WTM Incentive Plan are typically paid in cash but may be paid in common shares. Compensation expense is recognized for the vested portion of the awards over the related service periods. The level of payout ranges from zero to two times the number of shares initially granted, depending on White Mountains’s financial performance. Performance shares become payable at the conclusion of a performance cycle (typically three years) if pre-defined financial targets are met. The performance measures used for determining performance share payouts are growth in White Mountains’s adjusted book value per share and intrinsic value per share. Intrinsic value per share is generally calculated by adjusting adjusted book value per share for differences between the adjusted book value of certain assets and liabilities and White Mountains’s estimate of their underlying intrinsic values. The following table presents the performance share activity for the three and nine months ended September 30, 2024 and 2023 for performance shares granted under the WTM Incentive Plan: Three Months Ended September 30, Nine Months Ended September 30, 2024 2023 2024 2023 $ in Millions Target Performance Accrued Target Performance Accrued Target Performance Accrued Target Performance Accrued Beginning of period 34,992 $ 52.0 37,031 $ 44.6 37,031 $ 69.4 39,449 $ 67.5 Shares paid (1) (2) — — — — (13,475) (44.9) (13,350) (40.8) New grants — — — — 11,405 — 10,895 — Forfeitures and cancellations (3) (133) — — — (102) .3 37 .2 Expense recognized — 3.7 — 10.5 — 30.9 — 28.2 End of period 34,859 $ 55.7 37,031 $ 55.1 34,859 $ 55.7 37,031 $ 55.1 (1) WTM performance share payments for the 2021-2023 performance cycle were made in March 2024 at 188% of target. (2) WTM performance share payments for the 2020-2022 performance cycle were made in March 2023 at 200% of target. (3) Amounts include changes in assumed forfeitures, as required under GAAP. During the nine months ended September 30, 2024, White Mountains granted 11,405 performance shares for the 2024-2026 performance cycle. During the nine months ended September 30, 2023, White Mountains granted 10,895 performance shares for the 2023-2025 performance cycle. For the 2021-2023 performance cycle, the Company issued common shares for 100 performance shares earned, and all other performance shares earned were settled in cash. For the 2020-2022 performance cycle, all performance shares earned were settled in cash. If all outstanding performance shares had vested on September 30, 2024, the total additional compensation cost to be recognized would have been $28.1 million, based on accrual factors as of September 30, 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 September 30, 2024 for each performance cycle: September 30, 2024 $ in Millions Target Performance Accrued Performance cycle: 2022 – 2024 13,150 $ 40.7 2023 – 2025 10,835 11.8 2024 – 2026 11,405 4.0 Sub-total 35,390 56.5 Assumed forfeitures (531) (.8) Total 34,859 $ 55.7 Restricted Shares Restricted shares are grants of a specified number of common shares that generally vest at the end of a 34-month service period. The following table presents the unrecognized compensation cost associated with the outstanding restricted share awards under the WTM Incentive Plan for the three and nine months ended September 30, 2024 and 2023: Three Months Ended September 30, Nine Months Ended September 30, 2024 2023 2024 2023 $ in Millions Restricted Unamortized Restricted Unamortized Restricted Unamortized Issue Date Fair Value Restricted Unamortized Issue Date Fair Value Non-vested, Beginning of period 35,525 $ 28.8 37,595 $ 24.3 37,595 $ 16.2 38,350 $ 15.5 Issued — — — — 11,405 20.0 10,895 16.0 Vested — — — — (13,475) — (11,650) — Forfeited (135) — — — (135) — — — Expense recognized — (4.5) — (4.1) — (11.9) — (11.3) End of period 35,390 $ 24.3 37,595 $ 20.2 35,390 $ 24.3 37,595 $ 20.2

Noncontrolling Interests

Noncontrolling Interests	9 Months Ended
	Sep. 30, 2024
Noncontrolling Interest [Abstract]
Noncontrolling Interests	Noncontrolling Interests Noncontrolling interests consist of the ownership interests of noncontrolling shareholders in consolidated entities and are presented separately on the balance sheet. 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 September 30, 2024 and December 31, 2023: September 30, 2024 December 31, 2023 $ in Millions Noncontrolling Percentage (1) Noncontrolling Equity Noncontrolling Percentage (1) Noncontrolling Equity Noncontrolling interests, excluding BAM Ark 28.0 % $ 397.9 (2) 28.0 % $ 336.9 (2) HG Global 3.1 % (9.3) 3.1 % .8 Kudu 9.9 % 133.7 10.5 % 113.8 Bamboo 27.2 % 117.6 — % — Other various 9.3 various 9.4 Total, excluding BAM 649.2 460.9 BAM (3) — % — 100.0 % (139.8) Total noncontrolling interests $ 649.2 $ 321.1 (1) The noncontrolling percentage represents the basic ownership interests held by noncontrolling shareholders with the exception of HG Global, for which the noncontrolling percentage represents the preferred share ownership held by noncontrolling shareholders. (2) As of September 30, 2024 and December 31, 2023, Ark’s noncontrolling equity includes $40.3 and $21.0 related to management’s equity incentives. (3) 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

Segment Information	9 Months Ended
	Sep. 30, 2024
Segment Reporting [Abstract]
Segment Information	Segment Information As of September 30, 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 chief operating decision makers and the Board of Directors. Significant intercompany transactions among White Mountains’s segments have been eliminated herein. As a result of the Bamboo Transaction, White Mountains began consolidating Bamboo in its financial statements as of January 2, 2024. 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.” The following tables present the financial information for White Mountains’s segments for the three and nine months ended September 30, 2024 and 2023: Ark/WM Outrigger Other Operations Total Millions Ark WM Outrigger Re HG Global Kudu Bamboo Three Months Ended September 30, 2024 Earned insurance premiums (1) $ 507.2 $ 45.0 $ 7.5 $ — $ 10.6 $ 11.2 $ 581.5 Net investment income (2) 21.3 3.0 6.0 17.2 .6 9.6 57.7 Net realized and unrealized investment gains (losses) (2) 53.2 — 22.5 29.5 .7 29.7 135.6 Net realized and unrealized investment gains — — — — — 88.2 88.2 Interest income from BAM Surplus Notes — — 7.9 — — — 7.9 Change in fair value of BAM Surplus Notes — — 15.8 — — — 15.8 Unrealized loss on deconsolidation of BAM — — (114.5) — — — (114.5) Commission and fee revenues — — — — 42.7 4.1 46.8 Other revenues 3.7 — — .5 1.1 14.8 20.1 Total revenues 585.4 48.0 (54.8) 47.2 55.7 157.6 839.1 Loss and loss adjustment expenses 275.1 13.2 — — 4.4 4.2 296.9 Acquisition expenses 83.3 13.0 1.9 — 3.7 4.5 106.4 Cost of sales — — — — — 7.6 7.6 Broker commission expenses — — — — 15.9 — 15.9 General and administrative expenses 68.9 — .3 3.6 16.2 32.9 121.9 Change in fair value of contingent consideration 34.2 — — — — — 34.2 Interest expense 4.9 — 5.8 5.7 — .3 16.7 Total expenses 466.4 26.2 8.0 9.3 40.2 49.5 599.6 Pre-tax income (loss) $ 119.0 $ 21.8 $ (62.8) $ 37.9 $ 15.5 $ 108.1 $ 239.5 (1) Ark’s earned insurance premiums based on the location of Ark’s underwriting offices in the United Kingdom and Bermuda are $326.4 and $180.8. (2) Bamboo’s net investment income and net realized and unrealized investment gains (losses) are included in other revenues in the consolidated statement of operations. Ark/WM Outrigger HG Global Other Operations Millions Ark WM Outrigger Re HG Global BAM (1) Kudu Total Three Months Ended September 30, 2023 Earned insurance premiums (2) $ 438.3 $ 60.6 $ 6.6 $ 1.3 $ — $ — $ 506.8 Net investment income 13.9 3.0 4.3 3.8 15.1 8.0 48.1 Net realized and unrealized investment gains (losses) (6.6) — (13.6) (10.5) 11.2 8.0 (11.5) Net realized and unrealized investment gains — — — — — (46.8) (46.8) Interest income (expense) from — — 6.6 (6.6) — — — Commission and fee revenues — — — — — 3.5 3.5 Other revenues 3.6 — — .7 — 15.2 19.5 Total revenues 449.2 63.6 3.9 (11.3) 26.3 (12.1) 519.6 Loss and loss adjustment expenses 258.5 7.3 — — — — 265.8 Acquisition expenses 71.0 19.3 2.0 .1 — — 92.4 Cost of sales — — — — — 8.0 8.0 General and administrative expenses 35.3 .1 .5 17.0 4.5 42.2 99.6 Change in fair value of contingent 17.0 — — — — — 17.0 Interest expense 5.5 — 3.8 — 5.5 .9 15.7 Total expenses 387.3 26.7 6.3 17.1 10.0 51.1 498.5 Pre-tax income (loss) $ 61.9 $ 36.9 $ (2.4) $ (28.4) $ 16.3 $ (63.2) $ 21.1 (1) BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the BAM Surplus Notes and is not reduced by accruals of interest expense on the BAM Surplus Notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the NYDFS. (2) Ark’s earned insurance premiums based on the location of Ark’s underwriting offices in the United Kingdom and Bermuda are $273.6 and $164.7. Ark/WM Outrigger HG Global Other Operations Millions Ark WM Outrigger Re HG Global BAM (1) (2) Kudu Bamboo Total Nine Months Ended September 30, 2024 Earned insurance premiums (3) $ 1,110.5 $ 62.8 $ 21.5 $ 2.8 $ — $ 27.0 $ 19.8 $ 1,244.4 Net investment income (4) 57.6 8.9 17.3 8.8 50.1 1.5 27.9 172.1 Net realized and unrealized investment gains (losses) (4) 84.1 — 13.2 (5.1) 77.5 .6 60.4 230.7 Net realized and unrealized investment gains — — — — — — 159.7 159.7 Interest income (expense) from — — 21.1 (13.2) — — — 7.9 Change in fair value of BAM Surplus Notes — — 15.8 — — — — 15.8 Unrealized loss on deconsolidation of BAM — — (114.5) — — — — (114.5) Commission and fee revenues — — — — — 97.3 11.1 108.4 Other revenues 9.6 — — 1.1 .5 2.4 43.7 57.3 Total revenues 1,261.8 71.7 (25.6) (5.6) 128.1 128.8 322.6 1,881.8 Loss and loss adjustment expenses 630.4 13.6 — — — 14.5 8.1 666.6 Acquisition expenses 212.9 17.9 5.9 .4 — 9.7 7.1 253.9 Cost of sales — — — — — — 22.2 22.2 Broker commission expenses — — — — — 37.9 — 37.9 General and administrative expenses 153.9 .1 1.3 33.5 10.5 43.9 126.4 369.6 Change in fair value of contingent consideration 47.5 — — — — — — 47.5 Interest expense 15.0 — 13.4 — 16.7 — 1.6 46.7 Total expenses 1,059.7 31.6 20.6 33.9 27.2 106.0 165.4 1,444.4 Pre-tax income (loss) $ 202.1 $ 40.1 $ (46.2) $ (39.5) $ 100.9 $ 22.8 $ 157.2 $ 437.4 (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) Ark’s earned insurance premiums based on the location of Ark’s underwriting offices in the United Kingdom and Bermuda are $697.5 and $413.0. (4) Bamboo’s net investment income and net realized and unrealized investment gains (losses) are included in other revenues in the consolidated statement of operations. Ark/WM Outrigger HG Global Other Operations Millions Ark WM Outrigger Re HG Global BAM (1) Kudu Total Nine Months Ended September 30, 2023 Earned insurance premiums (2) $ 971.9 $ 75.4 $ 19.4 $ 3.9 $ — $ — $ 1,070.6 Net investment income 33.5 7.7 12.4 10.5 44.0 22.0 130.1 Net realized and unrealized 35.9 — (11.4) (5.6) 45.4 125.8 190.1 Net realized and unrealized investment gains — — — — — (38.9) (38.9) Interest income (expense) from — — 19.7 (19.7) — — — Commission and fee revenues — — — — — 10.0 10.0 Other revenues (1.1) — — 2.0 — 67.1 68.0 Total revenues 1,040.2 83.1 40.1 (8.9) 89.4 186.0 1,429.9 Loss and loss adjustment expenses 573.2 7.9 — — — — 581.1 Acquisition expenses 189.3 22.2 5.6 .7 — — 217.8 Cost of sales — — — — — 33.5 33.5 General and administrative expenses 105.3 .2 1.9 47.8 12.3 130.7 298.2 Change in fair value of contingent 16.8 — — — — — 16.8 Interest expense 15.7 — 10.8 — 15.5 2.9 44.9 Total expenses 900.3 30.3 18.3 48.5 27.8 167.1 1,192.3 Pre-tax income (loss) $ 139.9 $ 52.8 $ 21.8 $ (57.4) $ 61.6 $ 18.9 $ 237.6 (1) BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the BAM Surplus Notes and is not reduced by accruals of interest expense on the BAM Surplus Notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the NYDFS. (2) Ark’s earned insurance premiums based on the location of Ark’s underwriting offices in the United Kingdom and Bermuda are $588.3 and $383.6.

Variable Interest Entities

Variable Interest Entities	9 Months Ended
	Sep. 30, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Variable Interest Entities	Variable Interest Entities Under GAAP, White Mountains is required to consolidate any entity in which it holds a controlling financial interest. A controlling financial interest is usually in the form of an investment representing the majority of the subsidiary’s voting interests. However, a controlling financial interest may also arise from a financial interest in a VIE through arrangements that do not involve ownership of voting interests. A VIE is a legal entity that (i) does not have sufficient equity at risk to finance its activities without additional financial support; (ii) is structured such that equity investors, as a group, lack the power, through voting or similar rights, to direct the activities that most significantly impact the entity’s economic performance; (iii) is structured such that the equity investors lack the obligation to absorb losses of, or the right to receive returns from, the entity; or (iv) is structured with non-substantive voting rights. White Mountains determines whether an entity is a VIE at the inception of its variable interest in the entity and upon the occurrence of certain reconsideration events. White Mountains consolidates a VIE if it determines that it is the primary beneficiary. The primary beneficiary is defined as the entity that holds a variable interest that gives it both the power to direct the VIE’s activities that most significantly impact its economic performance and the obligation to absorb losses of, or the right to receive returns from, the VIE that could potentially be significant to the VIE. The identification of the primary beneficiary of a VIE may require significant assumptions and judgment. When White Mountains determines it has a variable interest in a VIE, it determines whether it is the primary beneficiary of that VIE by performing an analysis that principally considers: (i) the VIE’s purpose and design, including the risks the VIE was designed to create and pass through to its variable interest holders; (ii) the VIE’s capital structure; (iii) the identification of the activities that most significantly impact the VIE’s economic performance; (iv) the governance provisions and other contractual arrangements between the VIE and its variable interest holders and other parties involved with the VIE; and (v) related party relationships. At inception of its variable interest in the VIE, as well as on an ongoing basis, White Mountains performs qualitative assessments of its VIEs to determine whether White Mountains is the primary beneficiary of a VIE. WM Outrigger Re White Mountains has determined that Outrigger Re Ltd. and WM Outrigger Re are VIEs. White Mountains is not the primary beneficiary of Outrigger Re Ltd. or its other segregated accounts. White Mountains is the primary beneficiary of WM Outrigger Re, as it has both the power to direct the activities that most significantly impact WM Outrigger Re’s economic performance and the obligation to absorb losses, or the right to receive returns, that could potentially be significant to WM Outrigger Re. As a result, White Mountains consolidates WM Outrigger Re’s results in its financial statements. The assets of WM Outrigger Re can only be used to settle the liabilities of WM Outrigger Re, and there is no recourse to the Company for any creditors of WM Outrigger Re. WM Outrigger Re’s obligations under the reinsurance agreements with GAIL are subject to an aggregate limit equal to the assets in its collateral trust at any point in time. As of September 30, 2024, short-term investments of $199.9 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 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 September 30, 2024, the Collateral Trusts held assets of $976.4 million. PassportCard/DavidShield As of September 30, 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 September 30, 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 $159.2 million. Elementum As of September 30, 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 September 30, 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 September 30, 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 million. Limited Partnerships White Mountains’s investments in limited partnerships are generally considered VIEs because the limited partnership interests do not have substantive kick-out rights or participating rights. White Mountains does not have the unilateral power to direct the operations of these limited partnerships, and therefore White Mountains is not the primary beneficiary and does not consolidate the limited partnerships. White Mountains has taken the fair value option for its investments in limited partnerships, which are generally measured at NAV as a practical expedient. As of September 30, 2024, White Mountains’s maximum exposure to loss on its investments in limited partnerships is the carrying value of $262.0 million.

Equity Method Eligible Investme

Equity Method Eligible Investments	9 Months Ended
	Sep. 30, 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. The following table presents the ownership interests and carrying values of White Mountains’s equity method eligible investments as of September 30, 2024 and December 31, 2023: September 30, 2024 December 31, 2023 $ in Millions Ownership Interest Carrying Value Ownership Interest Carrying Value Kudu’s Participation Contracts (1) 4.1% - 30.0% $ 930.6 4.1% - 30.0% $ 890.5 Investment in MediaAlpha 26.8% 323.4 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% 243.9 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 three and nine months ended September 30, 2024, White Mountains received dividend and income distributions from equity method eligible investments of $19.7 million and $57.9 million, which were recorded within net investment income in the consolidated statements of operations. For the three and nine months ended September 30, 2023, White Mountains received dividend and income distributions from equity method eligible investments of $13.2 million and $42.1 million.

Fair Value of Financial Instrum

Fair Value of Financial Instruments	9 Months Ended
	Sep. 30, 2024
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	Fair Value of Financial Instruments White Mountains records its financial instruments at fair value with the exception of debt obligations, which are recorded as debt at face value less unamortized original issue discount. See Note 7 — “Debt.” The following table presents the fair value and carrying value of these financial instruments as of September 30, 2024 and December 31, 2023: September 30, 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 $ 173.7 $ 156.8 $ 171.8 $ 155.5 HG Global Senior Notes $ 158.1 $ 147.3 $ 158.7 $ 146.9 Kudu Credit Facility $ 232.6 $ 218.5 $ 225.6 $ 203.8 Other Operations debt $ 23.4 $ 22.3 $ 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 a discounted cash flow approach and are considered to be Level 3 measurements. For the fair value level measurements associated with White Mountains’s investment securities see Note 3 — “Investment Securities.” For the fair value level measurements associated with White Mountains’s derivative instruments see Note 9 — “Derivatives.” For the fair value measurements associated with the BAM Surplus Notes, see Note 10 — “Municipal Bond Guarantee Reinsurance.”

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2024
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Legal Contingencies White Mountains, and the insurance industry in general, is routinely subject to claims related litigation and arbitration in the normal course of business, as well as litigation and arbitration that do not arise from, nor are directly related to, claims activity. White Mountains’s estimates of the costs of settling matters routinely encountered in claims activity are reflected in the reserves for unpaid loss and LAE. See Note 5 — “Loss and Loss Adjustment Expense Reserves.”

Basis of Presentation and Sig_2

Basis of Presentation and Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation White Mountains Insurance Group, Ltd. (the “Company” or the “Registrant”) is an exempted Bermuda limited liability company whose principal businesses are conducted through its subsidiaries and other affiliates. The Company’s headquarters is located at 26 Reid Street, Hamilton, Bermuda HM 11, its principal executive office is located at 23 South Main Street, Suite 3B, Hanover, New Hampshire 03755-2053 and its registered office is located at Clarendon House, 2 Church Street, Hamilton, Bermuda HM 11. The Company’s website is www.whitemountains.com. The information contained on White Mountains’s website is not incorporated by reference into, and is not a part of, this report. The accompanying consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) and include the accounts of the Company, its subsidiaries (collectively with the Company, “White Mountains”) and other entities required to be consolidated under GAAP. Intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Reportable Segments	Reportable Segments As of September 30, 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 chief operating decision makers and its Board of Directors. See Note 14 — “Segment Information.” The Ark/WM Outrigger segment consists of Ark Insurance Holdings Limited and its subsidiaries (collectively, “Ark”) and Outrigger Re Ltd. Segregated Account 2023-1 (“WM Outrigger Re”) (collectively with Ark, “Ark/WM Outrigger”). Ark is a specialty property and casualty insurance and reinsurance company that offers a wide range of niche insurance and reinsurance products, including property, specialty, marine & energy, accident & health and casualty. Ark underwrites select coverages through Lloyd’s Syndicates 4020 and 3902 (the “Syndicates”) and its wholly-owned subsidiary Group Ark Insurance Limited (“GAIL”). White Mountains acquired a controlling ownership interest in Ark on January 1, 2021 (the “Ark Transaction”). As of September 30, 2024 and December 31, 2023, White Mountains owned 72.0% of Ark on a basic shares outstanding basis (61.9% after taking account of management’s equity incentives). The remaining shares are owned by current and former employees of Ark. In the future, management rollover shareholders could earn additional shares in Ark if and to the extent that White Mountains achieves certain thresholds for its multiple of invested capital return. If fully earned, these additional shares would represent 12.3% of the shares outstanding as of September 30, 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 reinsurance capacity to Ark. White Mountains consolidates the results of its segregated account, WM Outrigger Re, in its financial statements. See Note 2 — “Significant Transactions.” As of September 30, 2024 and December 31, 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”) (collectively with HG Global, “HG Global/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 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 September 30, 2024 and December 31, 2023, White Mountains owned 96.9% of HG Global’s preferred equity and 88.4% of its common equity. White Mountains does not have an ownership interest in BAM. However, through June 30, 2024, White Mountains was required to consolidate BAM’s results in its financial statements because BAM is a variable interest entity (“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 September 30, 2024 and December 31, 2023, White Mountains owned 90.1% and 89.6% of Kudu’s basic units outstanding (76.8% and 76.3% on a fully-diluted, fully-converted basis). The Bamboo segment consists of PM Holdings LLC (“Bamboo Holdings”), Bamboo Ide8 Insurance Services LLC (“Bamboo MGA”) and Ide8 Limited (“Bamboo Captive”) (collectively with Bamboo Holdings and Bamboo MGA, “Bamboo”). Bamboo is a capital-light, tech- and data-enabled insurance distribution platform providing homeowners’ insurance and related products to the residential property market in California. Bamboo operates primarily through Bamboo MGA, its full-service managing general agent (“MGA”) business, where the company manages all aspects of the placement process on behalf of its fronting and reinsurance partners, including product development, marketing, underwriting, policy issuance and claims oversight, and it earns commissions based on the volume and profitability of the insurance that it places. It offers both admitted and non-admitted products. Bamboo also operates two separate but integrated businesses: (i) a retail agency, within Bamboo MGA, offering ancillary products on behalf of third parties and (ii) Bamboo Captive, a Bermuda-domiciled captive reinsurer that participates in the underwriting risk of Bamboo’s MGA programs to align interests with reinsurance partners. On January 2, 2024, White Mountains acquired a controlling interest in Bamboo. See Note 2 — “Significant Transactions.” As of September 30, 2024, White Mountains owned 72.8% of the basic units outstanding of Bamboo (63.7% on a fully-diluted, fully-converted basis). White Mountains’s other operations consist of the Company and its wholly-owned subsidiary, White Mountains Capital, LLC (“WM Capital”), its other intermediate holding companies, its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), investment assets managed by WM Advisors, its interests in MediaAlpha, Inc. (“MediaAlpha”), DavidShield PassportCard Ltd. and its subsidiaries (collectively, “PassportCard/DavidShield”), Elementum Holdings LP (“Elementum”), certain other consolidated and unconsolidated entities (“Other Operating Businesses”) and certain other assets (collectively, “Other Operations”). On October 25, 2023, White Mountains announced the launch of White Mountains Partners LLC (“WTM Partners”), which will acquire businesses in non-insurance, non-financial services sectors including essential services, light industrial and specialty consumer. White Mountains expects to deploy up to $500.0 million of equity capital through WTM Partners over time. WTM Partners is included within Other Operations. In April 2024, White Mountains committed to provide up to $30.0 million to a Bermuda special purpose collateralized reinsurance vehicle that provides reinsurance capacity to Bamboo (“Bamboo CRV”). During the second quarter of 2024, White Mountains capitalized Bamboo CRV by purchasing $12.1 million of preference shares that were deposited into a collateral trust account. Bamboo CRV entered into a collateralized quota share agreement with one of Bamboo’s fronting partners to provide reinsurance protection on Bamboo’s admitted and non-admitted business written in the 2024 treaty year. Bamboo CRV is included within Other Operations.
BAM Surplus Notes	BAM Surplus Notes 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.
Revenue Recognition	Revenue Recognition Bamboo’s revenues consist primarily of commission and fee revenues for placement of insurance policies. Commission and fee revenues are measured based on the contractual rates with insurance carriers, net of any amounts expected to be uncollectible and any amounts associated with expected policy cancellations and adjustments, and are recognized when contractual performance obligations have been fulfilled. Bamboo’s primary contractual performance obligations are generally satisfied upon the issuance of an insurance policy. Bamboo’s premiums, commissions and fees receivable consist of insurance premiums and fees receivable from customers and commissions receivable from insurance carriers, net of a provision for amounts estimated to be uncollectible and any amounts associated with expected policy cancellations and adjustments. Deferred revenues associated with unsatisfied performance obligations are recognized within other liabilities.
Broker Commission Expenses	Broker Commission Expenses Bamboo’s broker commission expense consists of commissions paid to sub-agents and brokers. Broker commission expense is measured in accordance with contractual terms and recognized when incurred, which is generally at the policy issuance date.

Significant Transactions (Table

Significant Transactions (Tables)	9 Months Ended
	Sep. 30, 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)	9 Months Ended
	Sep. 30, 2024
Investments, Debt and Equity Securities [Abstract]
Pre-tax net investment income	The following table presents pre-tax net investment income for the three and nine months ended September 30, 2024 and 2023: Three Months Ended September 30, Nine Months Ended September 30, Millions 2024 2023 2024 2023 Fixed maturity investments $ 23.3 $ 16.6 $ 67.2 $ 46.6 Short-term investments 13.2 13.8 45.3 34.8 Common equity securities .7 1.0 1.8 3.6 Other long-term investments 21.2 17.4 60.0 46.8 Total investment income 58.4 48.8 174.3 131.8 Third-party investment expenses (.7) (.7) (2.2) (1.7) Net investment income, pre-tax $ 57.7 $ 48.1 $ 172.1 $ 130.1
Net realized and unrealized investment gains (losses)	The following table presents net realized and unrealized investment gains (losses) for the three and nine months ended September 30, 2024 and 2023: Three Months Ended September 30, Nine Months Ended September 30, Millions 2024 2023 2024 2023 Realized investment gains (losses) Fixed maturity investments $ (.5) $ (.3) $ (9.2) $ (1.7) Short-term investments — (.2) (.3) (.3) Common equity securities — 1.2 3.5 13.8 Investment in MediaAlpha — — 91.2 — Other long-term investments 4.8 .6 34.0 48.9 Net realized investment gains (losses) 4.3 1.3 119.2 60.7 Unrealized investment gains (losses) Fixed maturity investments 56.9 (27.9) 49.7 (9.7) Short-term investments .5 (.6) (.5) .7 Common equity securities 24.7 (18.5) 63.6 29.8 Investment in MediaAlpha 88.2 (46.8) 68.5 (38.9) Other long-term investments 49.2 34.2 89.9 108.6 Net unrealized investment gains (losses) 219.5 (59.6) 271.2 90.5 Net realized and unrealized investment gains (losses) (1) $ 223.8 $ (58.3) $ 390.4 $ 151.2 Fixed maturity and short-term investments Net realized and unrealized investment gains (losses) $ 56.9 $ (29.0) $ 39.7 $ (11.0) Less: net realized and unrealized gains (losses) on investment 1.2 2.5 .8 4.7 Net unrealized investment gains (losses) recognized during the period on $ 55.7 $ (31.5) $ 38.9 $ (15.7) Common equity securities and investment in MediaAlpha Net realized and unrealized investment gains (losses) on common $ 24.7 $ (17.3) $ 67.1 $ 43.6 Net realized and unrealized investment gains (losses) from 88.2 (46.8) 159.7 (38.9) Total net realized and unrealized investment gains (losses) 112.9 (64.1) 226.8 4.7 Less: net realized and unrealized gains (losses) on investment — (.1) 35.4 3.7 Net unrealized investment gains (losses) recognized during the period on $ 112.9 $ (64.0) $ 191.4 $ 1.0 (1) For the three months ended September 30, 2024 and 2023, includes $18.9 and $(12.8) of net realized and unrealized investment gains (losses) related to foreign currency exchange. For the nine months ended September 30, 2024 and 2023, includes $7.1 and $(1.9) of net realized and unrealized investment gains (losses) related to foreign currency exchange. The following table presents total net unrealized gains (losses) attributable to Level 3 investments for the three and nine months ended September 30, 2024 and 2023 for investments still held at the end of the period: Three Months Ended September 30, Nine Months Ended September 30, Millions 2024 2023 2024 2023 Total net unrealized investment gains on other long-term investments held at the end of the period, pre-tax $ 31.3 $ 24.6 $ 82.6 $ 76.0
Investment holdings, equity securities, convertible fixed maturities and other long-term investments	The following tables present the cost or amortized cost, gross unrealized investment gains (losses), net foreign currency gains (losses) and carrying value of White Mountains’s fixed maturity investments as of September 30, 2024 and December 31, 2023: September 30, 2024 Millions Cost or Gross Gross Net Foreign Carrying U.S. Government and agency obligations $ 446.0 $ 4.8 $ (.6) $ — $ 450.2 Debt securities issued by corporations 1,318.3 16.1 (24.4) .9 1,310.9 Municipal obligations 3.2 .1 — — 3.3 Mortgage and asset-backed securities 379.0 5.2 (16.1) — 368.1 Collateralized loan obligations 287.6 1.2 (.4) 1.4 289.8 Total fixed maturity investments $ 2,434.1 $ 27.4 $ (41.5) $ 2.3 $ 2,422.3 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 September 30, 2024 and December 31, 2023: September 30, 2024 Millions Cost or Gross Gross Net Foreign Carrying Common equity securities $ 537.0 $ 120.2 $ — $ (1.8) $ 655.4 Investment in MediaAlpha $ 59.2 $ 264.2 $ — $ — $ 323.4 Other long-term investments $ 1,638.6 $ 547.9 $ (108.3) $ (7.1) $ 2,071.1 December 31, 2023 Millions Cost or Gross Gross Net Foreign Carrying Common equity securities $ 483.5 $ 62.3 $ (1.2) $ (6.2) $ 538.4 Investment in MediaAlpha $ 59.2 $ 195.7 $ — $ — $ 254.9 Other long-term investments $ 1,655.7 $ 446.3 $ (94.5) $ (9.3) $ 1,998.2
Schedule of fixed maturity investment holdings	The following table presents the cost or amortized cost and carrying value of White Mountains’s fixed maturity investments by contractual maturity as of September 30, 2024. Actual maturities could differ from contractual maturities because borrowers may have the right to call or prepay certain obligations with or without penalties. September 30, 2024 Millions Cost or Amortized Cost Carrying Value Due in one year or less $ 237.0 $ 235.1 Due after one year through five years 1,277.8 1,280.7 Due after five years through ten years 228.4 223.3 Due after ten years 24.2 25.3 Mortgage and asset-backed securities and collateralized loan obligations 666.7 657.9 Total fixed maturity investments $ 2,434.1 $ 2,422.3
Fair value measurements by level, investment securities	The following tables present White Mountains’s fair value measurements for investments as of September 30, 2024 and December 31, 2023 by level. The major security types were based on the legal form of the securities. White Mountains has disaggregated its fixed maturity investments based on the issuing entity type, which impacts credit quality, with debt securities issued by U.S. government entities carrying minimal credit risk, while the credit and other risks associated with other issuers, such as corporations, municipalities or entities issuing mortgage and asset-backed securities vary depending on the nature of the issuing entity type. White Mountains further disaggregates debt securities issued by corporations by industry sector because investors often reference commonly used benchmarks and their subsectors to monitor risk and performance. Accordingly, White Mountains has further disaggregated this asset class into subclasses based on the similar sectors and industry classifications it uses to evaluate investment risk and performance against commonly used benchmarks, such as the Bloomberg Barclays U.S. Intermediate Aggregate. September 30, 2024 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 450.2 $ 449.4 $ .8 $ — Debt securities issued by corporations: Financials 399.0 — 399.0 — Consumer 253.5 — 253.5 — Industrial 149.2 — 149.2 — Healthcare 148.0 — 148.0 — Technology 109.8 — 109.8 — Utilities 73.0 — 73.0 — Communications 60.9 — 60.9 — Energy 62.9 — 62.9 — Materials 54.6 — 54.6 — Total debt securities issued by corporations 1,310.9 — 1,310.9 — Municipal obligations 3.3 — 3.3 — Mortgage and asset-backed securities 368.1 — 368.1 — Collateralized loan obligations 289.8 — 289.8 — Total fixed maturity investments 2,422.3 449.4 1,972.9 — Short-term investments 890.3 880.8 9.5 — Common equity securities: Exchange-traded funds 219.8 219.8 — — Other (1) 435.6 — 435.6 — Total common equity securities 655.4 219.8 435.6 — Investment in MediaAlpha 323.4 323.4 — — Other long-term investments 1,205.3 — 25.5 1,179.8 Other long-term investments — net asset value (2) 865.8 — — — Total other long-term investments 2,071.1 — 25.5 1,179.8 Total investments $ 6,362.5 $ 1,873.4 $ 2,443.5 $ 1,179.8 (1) Consists of investments in listed funds that predominantly invest in international equities. (2) Consists of private equity funds and hedge funds, a bank loan fund, Lloyd’s trust deposits and ILS funds for which fair value is measured using net asset value (“NAV”) as a practical expedient. Investments for which fair value is measured at NAV are not classified within the fair value hierarchy. December 31, 2023 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 204.9 $ 204.9 $ — $ — Debt securities issued by corporations: Financials 309.8 — 309.8 — Consumer 214.6 — 214.6 — Industrial 121.6 — 121.6 — Healthcare 121.3 — 121.3 — Technology 108.9 — 108.9 — Utilities 70.2 — 70.2 — Communications 46.3 — 46.3 — Energy 28.9 — 28.9 — Materials 23.4 — 23.4 — Total debt securities issued by corporations 1,045.0 — 1,045.0 — Municipal obligations 260.9 — 260.9 — Mortgage and asset-backed securities 389.4 — 389.4 — Collateralized loan obligations 209.1 — 209.1 — Total fixed maturity investments 2,109.3 204.9 1,904.4 — Short-term investments 1,487.9 1,457.6 30.3 — Common equity securities: Exchange-traded funds 137.8 137.8 — — Other (1) 400.6 — 400.6 — Total common equity securities 538.4 137.8 400.6 — Investment in MediaAlpha 254.9 254.9 — — Other long-term investments 1,164.4 — 26.2 1,138.2 Other long-term investments — NAV (2) 833.8 — — — Total other long-term investments 1,998.2 — 26.2 1,138.2 Total investments $ 6,388.7 $ 2,055.2 $ 2,361.5 $ 1,138.2 (1) Consists of investments in listed funds that predominantly invest in international equities. (2) Consists of private equity funds and hedge funds, a bank loan fund, Lloyd’s trust deposits and ILS funds for which fair value is measured using NAV as a practical expedient. Investments for which fair value is measured at NAV are not classified within the fair value hierarchy. The following table presents the fair value of White Mountains’s mortgage and asset-backed securities and collateralized loan obligations as of September 30, 2024 and December 31, 2023: September 30, 2024 December 31, 2023 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: FNMA $ 194.8 $ 194.8 $ — $ 198.8 $ 198.8 $ — FHLMC 142.1 142.1 — 137.2 137.2 — GNMA 26.0 26.0 — 30.2 30.2 — Total agency (1) 362.9 362.9 — 366.2 366.2 — Non-agency: Commercial .4 .4 — — — — Residential .1 .1 — .2 .2 — Total non-agency .5 .5 — .2 .2 — Total mortgage-backed securities 363.4 363.4 — 366.4 366.4 — Other asset-backed securities: Vehicle receivables 1.7 1.7 — 17.2 17.2 — Credit card receivables 1.3 1.3 — 3.3 3.3 — Other 1.7 1.7 — 2.5 2.5 — Total other asset-backed securities 4.7 4.7 — 23.0 23.0 — Total mortgage and asset-backed securities 368.1 368.1 — 389.4 389.4 — Collateralized loan obligations 289.8 289.8 — 209.1 209.1 — Total mortgage and asset-backed securities $ 657.9 $ 657.9 $ — $ 598.5 $ 598.5 $ — (1) Represents publicly traded mortgage-backed securities which carry the full faith and credit guaranty of the U.S. Government (i.e., GNMA) or are guaranteed by a government sponsored entity (i.e., FNMA, FHLMC).
Debt securities issued by corporations, credit ratings	The following table presents the credit ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of September 30, 2024 and December 31, 2023: Fair Value at Millions September 30, 2024 December 31, 2023 AAA $ 15.0 $ 11.5 AA 82.3 83.8 A 577.6 552.4 BBB 625.5 390.9 BB 3.2 — Other 7.3 6.4 Debt securities issued by corporations (1) $ 1,310.9 $ 1,045.0 (1) Credit ratings are based upon issuer credit ratings provided by Standard & Poor’s Financial Services LLC (“Standard & Poor’s”), or if unrated by Standard & Poor’s, long-term obligation ratings provided by Moody’s Investor Service, Inc.
Schedule of carrying values for other long-term investments	The following tables presents the carrying values of White Mountains’s other long-term investments by reportable segment as of September 30, 2024 and December 31, 2023: Fair Value as of September 30, 2024 Millions Ark/ WM Outrigger Kudu Other Total Kudu’s Participation Contracts $ — $ 930.6 $ — $ 930.6 PassportCard/DavidShield — — 150.0 150.0 Elementum — — 35.0 35.0 Other unconsolidated entities (1) — — 56.6 56.6 Total unconsolidated entities — 930.6 241.6 1,172.2 Private equity funds and hedge funds 81.9 — 267.1 349.0 Bank loan fund 258.9 — — 258.9 Lloyd’s trust deposits 172.7 — — 172.7 ILS funds — — 71.2 71.2 Private debt instruments — 6.2 9.6 15.8 Other 31.3 — — 31.3 Total other long-term investments $ 544.8 $ 936.8 $ 589.5 $ 2,071.1 (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 Simple Agreement for Future Equity (“SAFE”) investments.
Other long-term investments	The following table presents the fair value of investments and unfunded commitments in private equity funds and hedge funds by investment objective and sector as of September 30, 2024 and December 31, 2023: September 30, 2024 December 31, 2023 Millions Fair Value Unfunded Fair Value Unfunded Private equity funds Aerospace/Defense/Government $ 169.0 $ 46.8 $ 157.2 $ 17.4 Financial services 89.3 36.1 87.5 37.8 Real estate 3.7 2.4 3.9 2.5 Total private equity funds 262.0 85.3 248.6 57.7 Hedge funds Long/short equity financials and business services 56.5 — 54.4 — Long/short all cap global 30.5 — — — European small/mid cap — — 9.9 — Total hedge funds 87.0 — 64.3 — Total private equity funds and hedge funds included in other long-term investments $ 349.0 $ 85.3 $ 312.9 $ 57.7
Fair value of private equity funds subject to lock-up periods	The following table presents investments in private equity funds that were subject to lock-up periods as of September 30, 2024: Millions 1 – 3 years 3 – 5 years 5 – 10 years >10 years Total Private equity funds — expected lock-up period remaining $18.5 $55.9 $184.5 $3.1 $262.0
Rollforward of fair value investments by level	The following table presents the changes in White Mountains’s fair value measurements for Level 3 investments for the nine months ended September 30, 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 79.9 Net realized and unrealized gains 74.0 Purchases and contributions 3.1 Purchases and contributions 200.1 Sales and distributions (41.4) Sales and distributions (110.4) Transfers in — Transfers in — Transfers out — Transfers out — Balance as of September 30, 2024 $ 1,179.8 Balance as of September 30, 2023 $ 1,075.3
Schedule of significant unobservable inputs used in estimating the fair value of investment securities	The following tables present significant unobservable inputs used in estimating the fair value of White Mountains’s other long-term investments, classified within Level 3 as of September 30, 2024 and December 31, 2023. The tables below exclude $18.7 million and $19.0 million of Level 3 other long-term investments generally valued based on recent or expected transaction prices. The fair value of investments in private equity funds and hedge funds, bank loan funds, Lloyd’s trust deposits and ILS funds are generally estimated using the NAV of the funds. $ in Millions September 30, 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 $930.6 16% - 24% 7x - 22x PassportCard/DavidShield Discounted cash flow $150.0 24% 4% Elementum Discounted cash flow $35.0 21% 4% Preferred Securities Discounted cash flow $30.1 7% N/A Private debt instruments Discounted cash flow $15.4 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 the first nine months of 2024, Kudu deployed a total of $0.2 into 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 deployed a total of $163.8 into new and existing Participation Contracts. (5) As of December 31, 2023, one of Kudu’s Participation Contracts with a total fair value of $69.1 was valued using a probability weighted expected return method, which takes into account factors such as a discounted cash flow analysis, the expected value to be received in a pending sale transaction and the likelihood that a sales transaction will take place. (6) Increases (decreases) to the discount rates in isolation would result in lower (higher) fair value measurements, while increases (decreases) to the terminal cash flow exit multiples or terminal revenue growth rates in isolation would result in higher (lower) fair value measurements.

Goodwill and Other Intangible_2

Goodwill and Other Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 2024
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of intangible assets and goodwill	The following table presents the economic lives, acquisition date fair values, accumulated amortization and net carrying values for other intangible assets and goodwill as of September 30, 2024 and December 31, 2023: $ in Millions Weighted Average Economic September 30, 2024 December 31, 2023 Acquisition Date Fair Value Accumulated Amortization Net Carrying Value Acquisition Date Fair Value Accumulated Amortization Net Carrying Value Goodwill: Ark N/A $ 116.8 $ — $ 116.8 $ 116.8 $ — $ 116.8 Kudu N/A 7.6 — 7.6 7.6 — 7.6 Bamboo (1) N/A 270.4 — 270.4 — — — Other Operations N/A 44.4 — 44.4 44.4 — 44.4 Total goodwill 439.2 — 439.2 168.8 — 168.8 Other intangible assets: Ark Underwriting capacity N/A 175.7 — 175.7 175.7 — 175.7 Kudu Trade names 7.0 2.2 1.8 .4 2.2 1.5 .7 Bamboo (1) Trade names 10.0 23.5 1.8 21.7 — — — Agency relationships 6.0 72.4 9.1 63.3 — — — Developed technology 3.0 4.7 1.1 3.6 — — — Other 0.3 .4 .4 — — — — Subtotal 101.0 12.4 88.6 — — — Other Operations Trade names 13.3 13.3 5.3 8.0 13.3 4.1 9.2 Customer relationships 11.0 24.8 12.7 12.1 24.8 10.3 14.5 Other 11.8 3.1 1.4 1.7 2.8 1.1 1.7 Subtotal 41.2 19.4 21.8 40.9 15.5 25.4 Total other intangible assets 320.1 33.6 286.5 218.8 17.0 201.8 Total goodwill and other intangible assets $ 759.3 $ 33.6 $ 725.7 $ 387.6 $ 17.0 $ 370.6 (1) The relative fair values of goodwill and other intangible assets recognized in connection with the acquisition of Bamboo had not yet been finalized as of September 30, 2024. The following tables present the change in goodwill and other intangible assets for the three and nine months ended September 30, 2024 and 2023: Three Months Ended September 30, 2024 2023 Millions Goodwill Other Intangible Assets Total Goodwill and Other Intangible Assets Goodwill Other Intangible Assets Total Goodwill and Other Intangible Assets Beginning balance $ 439.2 $ 291.8 $ 731.0 $ 168.8 $ 204.8 $ 373.6 Amortization — (5.3) (5.3) — (1.5) (1.5) Ending balance $ 439.2 $ 286.5 $ 725.7 $ 168.8 $ 203.3 $ 372.1 Nine Months Ended September 30, 2024 2023 Millions Goodwill Other Intangible Assets Total Goodwill and Other Intangible Assets Goodwill Other Intangible Assets Total Goodwill and Other Intangible Assets Beginning balance $ 168.8 $ 201.8 $ 370.6 $ 176.5 $ 215.9 $ 392.4 Acquisition of businesses (1) 270.4 101.0 371.4 — — — Acquisitions of intangible assets (2) — .3 .3 — — — Dispositions (2) — — — (6.7) (6.9) (13.6) Measurement period adjustments (3) — — — (1.0) — (1.0) Amortization — (16.6) (16.6) — (5.7) (5.7) Ending balance $ 439.2 $ 286.5 $ 725.7 $ 168.8 $ 203.3 $ 372.1 (1) Amounts relate to the fair values of goodwill and other intangible assets recognized in connection with the acquisition of Bamboo, which had not yet been finalized as of September 30, 2024. (2) Relates to acquisitions and dispositions within Other Operations. (3) Measurement period adjustments relate to updated information about acquisition date fair values of assets acquired and liabilities assumed. During the nine months ended September 30, 2023, adjustments relate to an acquisition within Other Operations.

Loss and Loss Adjustment Expe_2

Loss and Loss Adjustment Expense Reserves (Tables)	9 Months Ended
	Sep. 30, 2024
Insurance [Abstract]
Schedule of liability for unpaid claims and claims adjustment expense	The following table summarizes the loss and loss adjustment expense (“LAE”) reserve activity of the Ark/WM Outrigger segment for the three and nine months ended September 30, 2024 and 2023: Three Months Ended September 30, Nine Months Ended September 30, Millions 2024 2023 2024 2023 Gross beginning balance $ 1,890.1 $ 1,421.0 $ 1,605.1 $ 1,296.5 Less: beginning reinsurance recoverable on unpaid losses (1) (458.5) (362.7) (340.8) (505.0) Net loss and loss adjustment expense reserves 1,431.6 1,058.3 1,264.3 791.5 Loss and loss adjustment expenses incurred relating to: Current year losses 313.1 266.4 676.0 562.1 Prior year losses (24.8) (.6) (32.0) 19.0 Net incurred loss and loss adjustment expenses 288.3 265.8 644.0 581.1 Loss and loss adjustment expenses paid relating to: Current year losses (19.1) (8.8) (27.4) (18.4) Prior year losses (86.3) (90.4) (261.8) (281.1) Net paid loss and loss adjustment expenses (105.4) (99.2) (289.2) (299.5) Change in TPC Providers’ participation (2) — — — 145.4 Foreign currency translation and other adjustments to loss 11.3 (5.5) 6.7 .9 Net ending balance 1,625.8 1,219.4 1,625.8 1,219.4 Plus: ending reinsurance recoverable on unpaid losses 507.2 352.4 507.2 352.4 Gross ending balance $ 2,133.0 $ 1,571.8 $ 2,133.0 $ 1,571.8 (1) The beginning reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $145.4 as of December 31, 2022. (2) Amount represents the impact to net loss and loss adjustment expense reserves due to a change in the TPC Providers’ participation related to the annual RITC process.

Third-Party Reinsurance (Tables

Third-Party Reinsurance (Tables)	9 Months Ended
	Sep. 30, 2024
Reinsurance Disclosures [Abstract]
Effects of reinsurance	The following table summarizes the effects of reinsurance on written and earned premiums and on loss and LAE for the Ark/WM Outrigger segment for the three and nine months ended September 30, 2024 and 2023: Millions Three Months Ended September 30, Nine Months Ended September 30, 2024 2023 2024 2023 Written premiums: Direct $ 239.0 $ 189.4 $ 863.4 $ 727.1 Assumed 134.6 61.8 1,079.3 939.6 Gross written premiums 373.6 251.2 1,942.7 1,666.7 Ceded (1) (34.2) (20.2) (502.5) (360.3) Net written premiums $ 339.4 $ 231.0 $ 1,440.2 $ 1,306.4 Earned premiums: Direct $ 317.6 $ 273.1 $ 731.5 $ 624.1 Assumed 432.6 370.0 805.3 684.6 Gross earned premiums 750.2 643.1 1,536.8 1,308.7 Ceded (2) (198.0) (144.2) (363.5) (261.4) Net earned premiums $ 552.2 $ 498.9 $ 1,173.3 $ 1,047.3 Loss and loss adjustment expenses: Gross $ 350.7 $ 269.3 $ 852.7 $ 627.5 Ceded (3) (62.4) (3.5) (208.7) (46.4) Net loss and loss adjustment expenses $ 288.3 $ 265.8 $ 644.0 $ 581.1 (1) The three months ended September 30, 2024 and 2023 exclude $8.8 and $6.0, and the nine months ended September 30, 2024 and 2023 exclude $82.0 and $108.4 ceded by Ark to WM Outrigger Re, which eliminate in White Mountains’s consolidated financial statements. (2) The three months ended September 30, 2024 and 2023 exclude $45.0 and $60.6, and the nine months ended September 30, 2024 and 2023 exclude $62.8 and $75.4, ceded by Ark to WM Outrigger Re, which eliminate in White Mountains’s consolidated financial statements. (3) The three months ended September 30, 2024 and 2023 exclude $13.2 and $7.3, and the nine months ended September 30, 2024 and 2023 exclude $13.6 and $7.9, 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 September 30, 2024 and December 31, 2023: Millions September 30, 2024 December 31, 2023 Reinsurance recoverables on unpaid losses (1) $ 507.2 $ 340.8 Reinsurance recoverables on paid losses 27.9 27.4 Ceded unearned premiums (2) 212.9 73.8 Reinsurance recoverables $ 748.0 $ 442.0 (1) The reinsurance recoverables on unpaid losses exclude $28.0 and $15.6 ceded by Ark to WM Outrigger Re as of September 30, 2024 and December 31, 2023, which eliminate in White Mountains’s consolidated financial statements. (2) The ceded unearned premiums exclude $25.0 and $5.7 ceded by Ark to WM Outrigger Re as of September 30, 2024 and December 31, 2023, which eliminate in White Mountains’s consolidated financial statements. The following tables present schedules of written premiums and earned premiums included in the HG Global segment for the three and nine months ended September 30, 2024 and 2023: Three Months Ended September 30, 2024 Millions HG Global Written premiums: Direct $ — Assumed 14.0 Gross written premiums (1) $ 14.0 Earned premiums: Direct $ — Assumed 7.5 Gross earned premiums (1) $ 7.5 (1) There are no ceded premium amounts in the periods presented, such that gross written premiums and gross earned premiums are equivalent to net written premiums and net earned premiums, respectively. Three Months Ended September 30, 2023 Millions HG Global BAM Eliminations Total Written premiums: Direct $ — $ 16.5 $ — $ 16.5 Assumed 14.2 — (14.2) — Gross written premiums (1) $ 14.2 $ 16.5 $ (14.2) $ 16.5 Earned premiums: Direct $ — $ 7.4 $ — $ 7.4 Assumed 6.6 .5 (6.6) .5 Gross earned premiums (1) $ 6.6 $ 7.9 $ (6.6) $ 7.9 (1) There are no ceded premium amounts in the periods presented, such that gross written premiums and gross earned premiums are equivalent to net written premiums and net earned premiums, respectively. Nine Months Ended September 30, 2024 Millions HG Global BAM Eliminations Total Written premiums: Direct $ — $ 24.1 $ — $ 24.1 Assumed 34.5 — (20.5) 14.0 Gross written premiums (1) $ 34.5 $ 24.1 $ (20.5) $ 38.1 Earned premiums: Direct $ — $ 15.8 $ — $ 15.8 Assumed 21.5 1.0 (14.0) 8.5 Gross earned premiums (1) $ 21.5 $ 16.8 $ (14.0) $ 24.3 (1) There are no ceded premium amounts in the periods presented, such that gross written premiums and gross earned premiums are equivalent to net written premiums and net earned premiums, respectively. Nine Months Ended September 30, 2023 Millions HG Global BAM Eliminations Total Written premiums: Direct $ — $ 37.3 $ — $ 37.3 Assumed 31.9 — (31.9) — Gross written premiums (1) $ 31.9 $ 37.3 $ (31.9) $ 37.3 Earned premiums: Direct $ — $ 21.4 $ — $ 21.4 Assumed 19.4 1.9 (19.4) 1.9 Gross earned premiums (1) $ 19.4 $ 23.3 $ (19.4) $ 23.3 (1) There are no ceded premium amounts in the periods presented, such that gross written premiums and gross earned premiums are equivalent to net written premiums and net earned premiums, respectively.
Gross and net reinsurance recoverables by reinsurers ratings	The following table presents the Ark/WM Outrigger segment’s gross and net reinsurance recoverables by the reinsurer’s A.M. Best Company, Inc (“A.M. Best”) ratings as of September 30, 2024: $ in Millions As of September 30, 2024 A.M. Best Rating (1) Gross Collateral Net % of Total A+ or better $ 358.1 $ — $ 358.1 78.9 % A- to A 84.2 — 84.2 18.5 B++ or lower and not rated 92.8 (2) 81.1 (2) 11.7 2.6 Total $ 535.1 $ 81.1 $ 454.0 100.0 % (1) A.M. Best financial strength ratings as detailed above are: “A+ or better” (Superior) “A- to A” (Excellent), “B++” (Good). (2)

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2024
Debt Disclosure [Abstract]
Schedule of debt outstanding	The following table presents White Mountains’s debt outstanding as of September 30, 2024 and December 31, 2023: $ in Millions September 30, 2024 Effective (1) December 31, 2023 Effective (1) Ark 2007 Subordinated Notes, carrying value $ — $ 30.0 Ark 2021 Subordinated Notes Tranche 1 43.6 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 (3.8) (4.2) Ark 2021 Subordinated Notes, carrying value 156.8 155.5 Total Ark Subordinated Notes, carrying value 156.8 12.1% 185.5 11.0% HG Global Senior Notes (2) 150.0 11.8% 150.0 11.9% Unamortized discount and issuance cost (2.7) (3.1) HG Global Senior Notes, carrying value 147.3 146.9 Kudu Credit Facility (2) 225.3 10.5% 210.3 10.1% Unamortized issuance cost (6.8) (6.5) Kudu Credit Facility, carrying value 218.5 203.8 Other Operations debt 22.6 10.1% 28.9 10.2% Unamortized issuance cost (.3) (.5) Other Operations debt, carrying value 22.3 28.4 Total debt $ 544.9 $ 564.6 (1) Effective rate includes the effect of the amortization of debt issuance costs and, where applicable, the original issue discount. (2) Effective rate excludes the effect of the interest rate caps. The following table presents the change in debt under the Kudu Credit Facility for the three and nine months ended September 30, 2024 and 2023: Three Months Ended September 30, Nine Months Ended September 30, Millions 2024 2023 2024 2023 Kudu Credit Facility Beginning balance $ 210.3 $ 210.3 $ 210.3 $ 215.2 Term loans Borrowings 15.0 — 15.0 12.0 Repayments — — — (16.9) Ending balance $ 225.3 $ 210.3 $ 225.3 $ 210.3

Municipal Bond Guarantee Insu_2

Municipal Bond Guarantee Insurance (Tables)	9 Months Ended
	Sep. 30, 2024
Guarantees [Abstract]
Schedule of municipal bond guarantee insured obligations	The following table presents a schedule of the HG Global segment’s insured obligations as of September 30, 2024 and December 31, 2023: September 30, 2024 December 31, 2023 Contracts outstanding 15,524 14,485 Remaining weighted average contract period (in years) (1) 12.3 12.2 (2) Outstanding first loss exposure from policies assumed (in millions) (1) $ 17,667.2 $ 16,341.0 (3) Gross unearned insurance premiums (in millions) $ 287.0 $ 273.9 (4) (1) Under the FLRT, HG Re provides first loss 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. Accordingly, HG Re has no exposure to future interest outstanding and HG Re’s remaining weighted average contract period is calculated using its outstanding first loss exposure from policies assumed. (2) As of December 31, 2023, which was prior to the deconsolidation of BAM, HG Global/BAM’s remaining weighted average contract period was 11.1 years, calculated using HG Global/BAM’s total contractual debt service outstanding, including principal and interest. (3) As of December 31, 2023, which was prior to the deconsolidation of BAM, HG Global/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 the deconsolidation of BAM, HG Global/BAM’s gross unearned insurance premiums was $325.8.
Financial guarantee insurance contracts, premium received over contract period	The following table presents a schedule of HG Global’s future premium revenues as of September 30, 2024: Millions September 30, 2024 October 1, 2024 - December 31, 2024 $ 6.6 January 1, 2025 - March 31, 2025 6.4 April 1, 2025 - June 30, 2025 6.4 July 1, 2025 - September 30, 2025 6.2 October 1, 2025 - December 31, 2025 6.1 Total 2025 25.1 2026 23.7 2027 22.3 2028 20.8 2029 19.3 2030 and thereafter 169.2 Total gross unearned insurance premiums $ 287.0
Effects of reinsurance	The following table summarizes the effects of reinsurance on written and earned premiums and on loss and LAE for the Ark/WM Outrigger segment for the three and nine months ended September 30, 2024 and 2023: Millions Three Months Ended September 30, Nine Months Ended September 30, 2024 2023 2024 2023 Written premiums: Direct $ 239.0 $ 189.4 $ 863.4 $ 727.1 Assumed 134.6 61.8 1,079.3 939.6 Gross written premiums 373.6 251.2 1,942.7 1,666.7 Ceded (1) (34.2) (20.2) (502.5) (360.3) Net written premiums $ 339.4 $ 231.0 $ 1,440.2 $ 1,306.4 Earned premiums: Direct $ 317.6 $ 273.1 $ 731.5 $ 624.1 Assumed 432.6 370.0 805.3 684.6 Gross earned premiums 750.2 643.1 1,536.8 1,308.7 Ceded (2) (198.0) (144.2) (363.5) (261.4) Net earned premiums $ 552.2 $ 498.9 $ 1,173.3 $ 1,047.3 Loss and loss adjustment expenses: Gross $ 350.7 $ 269.3 $ 852.7 $ 627.5 Ceded (3) (62.4) (3.5) (208.7) (46.4) Net loss and loss adjustment expenses $ 288.3 $ 265.8 $ 644.0 $ 581.1 (1) The three months ended September 30, 2024 and 2023 exclude $8.8 and $6.0, and the nine months ended September 30, 2024 and 2023 exclude $82.0 and $108.4 ceded by Ark to WM Outrigger Re, which eliminate in White Mountains’s consolidated financial statements. (2) The three months ended September 30, 2024 and 2023 exclude $45.0 and $60.6, and the nine months ended September 30, 2024 and 2023 exclude $62.8 and $75.4, ceded by Ark to WM Outrigger Re, which eliminate in White Mountains’s consolidated financial statements. (3) The three months ended September 30, 2024 and 2023 exclude $13.2 and $7.3, and the nine months ended September 30, 2024 and 2023 exclude $13.6 and $7.9, 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 September 30, 2024 and December 31, 2023: Millions September 30, 2024 December 31, 2023 Reinsurance recoverables on unpaid losses (1) $ 507.2 $ 340.8 Reinsurance recoverables on paid losses 27.9 27.4 Ceded unearned premiums (2) 212.9 73.8 Reinsurance recoverables $ 748.0 $ 442.0 (1) The reinsurance recoverables on unpaid losses exclude $28.0 and $15.6 ceded by Ark to WM Outrigger Re as of September 30, 2024 and December 31, 2023, which eliminate in White Mountains’s consolidated financial statements. (2) The ceded unearned premiums exclude $25.0 and $5.7 ceded by Ark to WM Outrigger Re as of September 30, 2024 and December 31, 2023, which eliminate in White Mountains’s consolidated financial statements. The following tables present schedules of written premiums and earned premiums included in the HG Global segment for the three and nine months ended September 30, 2024 and 2023: Three Months Ended September 30, 2024 Millions HG Global Written premiums: Direct $ — Assumed 14.0 Gross written premiums (1) $ 14.0 Earned premiums: Direct $ — Assumed 7.5 Gross earned premiums (1) $ 7.5 (1) There are no ceded premium amounts in the periods presented, such that gross written premiums and gross earned premiums are equivalent to net written premiums and net earned premiums, respectively. Three Months Ended September 30, 2023 Millions HG Global BAM Eliminations Total Written premiums: Direct $ — $ 16.5 $ — $ 16.5 Assumed 14.2 — (14.2) — Gross written premiums (1) $ 14.2 $ 16.5 $ (14.2) $ 16.5 Earned premiums: Direct $ — $ 7.4 $ — $ 7.4 Assumed 6.6 .5 (6.6) .5 Gross earned premiums (1) $ 6.6 $ 7.9 $ (6.6) $ 7.9 (1) There are no ceded premium amounts in the periods presented, such that gross written premiums and gross earned premiums are equivalent to net written premiums and net earned premiums, respectively. Nine Months Ended September 30, 2024 Millions HG Global BAM Eliminations Total Written premiums: Direct $ — $ 24.1 $ — $ 24.1 Assumed 34.5 — (20.5) 14.0 Gross written premiums (1) $ 34.5 $ 24.1 $ (20.5) $ 38.1 Earned premiums: Direct $ — $ 15.8 $ — $ 15.8 Assumed 21.5 1.0 (14.0) 8.5 Gross earned premiums (1) $ 21.5 $ 16.8 $ (14.0) $ 24.3 (1) There are no ceded premium amounts in the periods presented, such that gross written premiums and gross earned premiums are equivalent to net written premiums and net earned premiums, respectively. Nine Months Ended September 30, 2023 Millions HG Global BAM Eliminations Total Written premiums: Direct $ — $ 37.3 $ — $ 37.3 Assumed 31.9 — (31.9) — Gross written premiums (1) $ 31.9 $ 37.3 $ (31.9) $ 37.3 Earned premiums: Direct $ — $ 21.4 $ — $ 21.4 Assumed 19.4 1.9 (19.4) 1.9 Gross earned premiums (1) $ 19.4 $ 23.3 $ (19.4) $ 23.3 (1) There are no ceded premium amounts in the periods presented, such that gross written premiums and gross earned premiums are equivalent to net written premiums and net earned premiums, respectively.

Earnings Per Share (Tables)

Earnings Per Share (Tables)	9 Months Ended
	Sep. 30, 2024
Earnings Per Share [Abstract]
Computation of earnings per share	The following table presents the Company’s computation of earnings per share from continuing operations for the three and nine months ended September 30, 2024 and 2023. Three Months Ended September 30, Nine Months Ended September 30, 2024 2023 2024 2023 Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’s $ 179.0 $ 23.6 $ 360.8 $ 222.7 Allocation of (earnings) losses to participating restricted common shares (1) (2.5) (.3) (4.6) (3.1) Basic and diluted earnings (losses) per share numerators $ 176.5 $ 23.3 $ 356.2 $ 219.6 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 2,568.1 2,560.5 2,564.8 2,564.2 Average unvested restricted common shares (2) (35.4) (37.6) (32.9) (35.5) Basic earnings (losses) per share denominator 2,532.7 2,522.9 2,531.9 2,528.7 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 2,568.1 2,560.5 2,564.8 2,564.2 Average unvested restricted common shares (2) (35.4) (37.6) (32.9) (35.5) Diluted earnings (losses) per share denominator 2,532.7 2,522.9 2,531.9 2,528.7 Basic and diluted earnings per share (in dollars): Distributed earnings - dividends declared and paid $ — $ — $ 1.00 $ 1.00 Undistributed earnings (losses) 69.68 9.19 139.66 85.82 Basic and diluted earnings (losses) per share $ 69.68 $ 9.19 $ 140.66 $ 86.82 (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (2) Restricted shares outstanding vest upon a stated date. See Note 12 — “Employee Share-Based Incentive Compensation Plans.”
Schedule of undistributed earnings	The following table presents the undistributed net earnings (losses) for the three and nine months ended September 30, 2024 and 2023. Three Months Ended September 30, Nine Months Ended September 30, Millions 2024 2023 2024 2023 Undistributed net earnings: Net income (loss) attributable to White Mountains’s common $ 176.5 $ 23.3 $ 356.2 $ 219.6 Dividends declared, net of restricted common share amounts (1) — — (2.5) (2.5) Total undistributed net earnings (losses), net of restricted $ 176.5 $ 23.3 $ 353.7 $ 217.1 (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.

Employee Share-Based Incentiv_2

Employee Share-Based Incentive Compensation Plans (Tables)	9 Months Ended
	Sep. 30, 2024
Share-Based Payment Arrangement [Abstract]
Summary of performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan	The following table presents the performance share activity for the three and nine months ended September 30, 2024 and 2023 for performance shares granted under the WTM Incentive Plan: Three Months Ended September 30, Nine Months Ended September 30, 2024 2023 2024 2023 $ in Millions Target Performance Accrued Target Performance Accrued Target Performance Accrued Target Performance Accrued Beginning of period 34,992 $ 52.0 37,031 $ 44.6 37,031 $ 69.4 39,449 $ 67.5 Shares paid (1) (2) — — — — (13,475) (44.9) (13,350) (40.8) New grants — — — — 11,405 — 10,895 — Forfeitures and cancellations (3) (133) — — — (102) .3 37 .2 Expense recognized — 3.7 — 10.5 — 30.9 — 28.2 End of period 34,859 $ 55.7 37,031 $ 55.1 34,859 $ 55.7 37,031 $ 55.1 (1) WTM performance share payments for the 2021-2023 performance cycle were made in March 2024 at 188% of target. (2) WTM performance share payments for the 2020-2022 performance cycle were made in March 2023 at 200% of target. (3) Amounts include changes in assumed forfeitures, as required under GAAP.
Share-based compensation, performance shares award outstanding activity	The following table presents performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan as of September 30, 2024 for each performance cycle: September 30, 2024 $ in Millions Target Performance Accrued Performance cycle: 2022 – 2024 13,150 $ 40.7 2023 – 2025 10,835 11.8 2024 – 2026 11,405 4.0 Sub-total 35,390 56.5 Assumed forfeitures (531) (.8) Total 34,859 $ 55.7
Unrecognized compensation cost for restricted share awards	The following table presents the unrecognized compensation cost associated with the outstanding restricted share awards under the WTM Incentive Plan for the three and nine months ended September 30, 2024 and 2023: Three Months Ended September 30, Nine Months Ended September 30, 2024 2023 2024 2023 $ in Millions Restricted Unamortized Restricted Unamortized Restricted Unamortized Issue Date Fair Value Restricted Unamortized Issue Date Fair Value Non-vested, Beginning of period 35,525 $ 28.8 37,595 $ 24.3 37,595 $ 16.2 38,350 $ 15.5 Issued — — — — 11,405 20.0 10,895 16.0 Vested — — — — (13,475) — (11,650) — Forfeited (135) — — — (135) — — — Expense recognized — (4.5) — (4.1) — (11.9) — (11.3) End of period 35,390 $ 24.3 37,595 $ 20.2 35,390 $ 24.3 37,595 $ 20.2

Noncontrolling Interests (Table

Noncontrolling Interests (Tables)	9 Months Ended
	Sep. 30, 2024
Noncontrolling Interest [Abstract]
Non-controlling interests	The following table presents the balance of noncontrolling interests included in White Mountains’s total equity and the related percentage of each consolidated entity’s total equity owned by noncontrolling shareholders as of September 30, 2024 and December 31, 2023: September 30, 2024 December 31, 2023 $ in Millions Noncontrolling Percentage (1) Noncontrolling Equity Noncontrolling Percentage (1) Noncontrolling Equity Noncontrolling interests, excluding BAM Ark 28.0 % $ 397.9 (2) 28.0 % $ 336.9 (2) HG Global 3.1 % (9.3) 3.1 % .8 Kudu 9.9 % 133.7 10.5 % 113.8 Bamboo 27.2 % 117.6 — % — Other various 9.3 various 9.4 Total, excluding BAM 649.2 460.9 BAM (3) — % — 100.0 % (139.8) Total noncontrolling interests $ 649.2 $ 321.1 (1) The noncontrolling percentage represents the basic ownership interests held by noncontrolling shareholders with the exception of HG Global, for which the noncontrolling percentage represents the preferred share ownership held by noncontrolling shareholders. (2) As of September 30, 2024 and December 31, 2023, Ark’s noncontrolling equity includes $40.3 and $21.0 related to management’s equity incentives. (3) 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)	9 Months Ended
	Sep. 30, 2024
Segment Reporting [Abstract]
Schedule of segments	The following tables present the financial information for White Mountains’s segments for the three and nine months ended September 30, 2024 and 2023: Ark/WM Outrigger Other Operations Total Millions Ark WM Outrigger Re HG Global Kudu Bamboo Three Months Ended September 30, 2024 Earned insurance premiums (1) $ 507.2 $ 45.0 $ 7.5 $ — $ 10.6 $ 11.2 $ 581.5 Net investment income (2) 21.3 3.0 6.0 17.2 .6 9.6 57.7 Net realized and unrealized investment gains (losses) (2) 53.2 — 22.5 29.5 .7 29.7 135.6 Net realized and unrealized investment gains — — — — — 88.2 88.2 Interest income from BAM Surplus Notes — — 7.9 — — — 7.9 Change in fair value of BAM Surplus Notes — — 15.8 — — — 15.8 Unrealized loss on deconsolidation of BAM — — (114.5) — — — (114.5) Commission and fee revenues — — — — 42.7 4.1 46.8 Other revenues 3.7 — — .5 1.1 14.8 20.1 Total revenues 585.4 48.0 (54.8) 47.2 55.7 157.6 839.1 Loss and loss adjustment expenses 275.1 13.2 — — 4.4 4.2 296.9 Acquisition expenses 83.3 13.0 1.9 — 3.7 4.5 106.4 Cost of sales — — — — — 7.6 7.6 Broker commission expenses — — — — 15.9 — 15.9 General and administrative expenses 68.9 — .3 3.6 16.2 32.9 121.9 Change in fair value of contingent consideration 34.2 — — — — — 34.2 Interest expense 4.9 — 5.8 5.7 — .3 16.7 Total expenses 466.4 26.2 8.0 9.3 40.2 49.5 599.6 Pre-tax income (loss) $ 119.0 $ 21.8 $ (62.8) $ 37.9 $ 15.5 $ 108.1 $ 239.5 (1) Ark’s earned insurance premiums based on the location of Ark’s underwriting offices in the United Kingdom and Bermuda are $326.4 and $180.8. (2) Bamboo’s net investment income and net realized and unrealized investment gains (losses) are included in other revenues in the consolidated statement of operations. Ark/WM Outrigger HG Global Other Operations Millions Ark WM Outrigger Re HG Global BAM (1) Kudu Total Three Months Ended September 30, 2023 Earned insurance premiums (2) $ 438.3 $ 60.6 $ 6.6 $ 1.3 $ — $ — $ 506.8 Net investment income 13.9 3.0 4.3 3.8 15.1 8.0 48.1 Net realized and unrealized investment gains (losses) (6.6) — (13.6) (10.5) 11.2 8.0 (11.5) Net realized and unrealized investment gains — — — — — (46.8) (46.8) Interest income (expense) from — — 6.6 (6.6) — — — Commission and fee revenues — — — — — 3.5 3.5 Other revenues 3.6 — — .7 — 15.2 19.5 Total revenues 449.2 63.6 3.9 (11.3) 26.3 (12.1) 519.6 Loss and loss adjustment expenses 258.5 7.3 — — — — 265.8 Acquisition expenses 71.0 19.3 2.0 .1 — — 92.4 Cost of sales — — — — — 8.0 8.0 General and administrative expenses 35.3 .1 .5 17.0 4.5 42.2 99.6 Change in fair value of contingent 17.0 — — — — — 17.0 Interest expense 5.5 — 3.8 — 5.5 .9 15.7 Total expenses 387.3 26.7 6.3 17.1 10.0 51.1 498.5 Pre-tax income (loss) $ 61.9 $ 36.9 $ (2.4) $ (28.4) $ 16.3 $ (63.2) $ 21.1 (1) BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the BAM Surplus Notes and is not reduced by accruals of interest expense on the BAM Surplus Notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the NYDFS. (2) Ark’s earned insurance premiums based on the location of Ark’s underwriting offices in the United Kingdom and Bermuda are $273.6 and $164.7. Ark/WM Outrigger HG Global Other Operations Millions Ark WM Outrigger Re HG Global BAM (1) (2) Kudu Bamboo Total Nine Months Ended September 30, 2024 Earned insurance premiums (3) $ 1,110.5 $ 62.8 $ 21.5 $ 2.8 $ — $ 27.0 $ 19.8 $ 1,244.4 Net investment income (4) 57.6 8.9 17.3 8.8 50.1 1.5 27.9 172.1 Net realized and unrealized investment gains (losses) (4) 84.1 — 13.2 (5.1) 77.5 .6 60.4 230.7 Net realized and unrealized investment gains — — — — — — 159.7 159.7 Interest income (expense) from — — 21.1 (13.2) — — — 7.9 Change in fair value of BAM Surplus Notes — — 15.8 — — — — 15.8 Unrealized loss on deconsolidation of BAM — — (114.5) — — — — (114.5) Commission and fee revenues — — — — — 97.3 11.1 108.4 Other revenues 9.6 — — 1.1 .5 2.4 43.7 57.3 Total revenues 1,261.8 71.7 (25.6) (5.6) 128.1 128.8 322.6 1,881.8 Loss and loss adjustment expenses 630.4 13.6 — — — 14.5 8.1 666.6 Acquisition expenses 212.9 17.9 5.9 .4 — 9.7 7.1 253.9 Cost of sales — — — — — — 22.2 22.2 Broker commission expenses — — — — — 37.9 — 37.9 General and administrative expenses 153.9 .1 1.3 33.5 10.5 43.9 126.4 369.6 Change in fair value of contingent consideration 47.5 — — — — — — 47.5 Interest expense 15.0 — 13.4 — 16.7 — 1.6 46.7 Total expenses 1,059.7 31.6 20.6 33.9 27.2 106.0 165.4 1,444.4 Pre-tax income (loss) $ 202.1 $ 40.1 $ (46.2) $ (39.5) $ 100.9 $ 22.8 $ 157.2 $ 437.4 (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) Ark’s earned insurance premiums based on the location of Ark’s underwriting offices in the United Kingdom and Bermuda are $697.5 and $413.0. (4) Bamboo’s net investment income and net realized and unrealized investment gains (losses) are included in other revenues in the consolidated statement of operations. Ark/WM Outrigger HG Global Other Operations Millions Ark WM Outrigger Re HG Global BAM (1) Kudu Total Nine Months Ended September 30, 2023 Earned insurance premiums (2) $ 971.9 $ 75.4 $ 19.4 $ 3.9 $ — $ — $ 1,070.6 Net investment income 33.5 7.7 12.4 10.5 44.0 22.0 130.1 Net realized and unrealized 35.9 — (11.4) (5.6) 45.4 125.8 190.1 Net realized and unrealized investment gains — — — — — (38.9) (38.9) Interest income (expense) from — — 19.7 (19.7) — — — Commission and fee revenues — — — — — 10.0 10.0 Other revenues (1.1) — — 2.0 — 67.1 68.0 Total revenues 1,040.2 83.1 40.1 (8.9) 89.4 186.0 1,429.9 Loss and loss adjustment expenses 573.2 7.9 — — — — 581.1 Acquisition expenses 189.3 22.2 5.6 .7 — — 217.8 Cost of sales — — — — — 33.5 33.5 General and administrative expenses 105.3 .2 1.9 47.8 12.3 130.7 298.2 Change in fair value of contingent 16.8 — — — — — 16.8 Interest expense 15.7 — 10.8 — 15.5 2.9 44.9 Total expenses 900.3 30.3 18.3 48.5 27.8 167.1 1,192.3 Pre-tax income (loss) $ 139.9 $ 52.8 $ 21.8 $ (57.4) $ 61.6 $ 18.9 $ 237.6 (1) BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the BAM Surplus Notes and is not reduced by accruals of interest expense on the BAM Surplus Notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the NYDFS. (2) Ark’s earned insurance premiums based on the location of Ark’s underwriting offices in the United Kingdom and Bermuda are $588.3 and $383.6.

Equity Method Eligible Invest_2

Equity Method Eligible Investments (Tables)	9 Months Ended
	Sep. 30, 2024
Equity Method Investments and Joint Ventures [Abstract]
Equity method investments	The following table presents the ownership interests and carrying values of White Mountains’s equity method eligible investments as of September 30, 2024 and December 31, 2023: September 30, 2024 December 31, 2023 $ in Millions Ownership Interest Carrying Value Ownership Interest Carrying Value Kudu’s Participation Contracts (1) 4.1% - 30.0% $ 930.6 4.1% - 30.0% $ 890.5 Investment in MediaAlpha 26.8% 323.4 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% 243.9 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.

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2024
Fair Value Disclosures [Abstract]
Summary of the fair value and carrying value of financial instruments	The following table presents the fair value and carrying value of these financial instruments as of September 30, 2024 and December 31, 2023: September 30, 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 $ 173.7 $ 156.8 $ 171.8 $ 155.5 HG Global Senior Notes $ 158.1 $ 147.3 $ 158.7 $ 146.9 Kudu Credit Facility $ 232.6 $ 218.5 $ 225.6 $ 203.8 Other Operations debt $ 23.4 $ 22.3 $ 30.0 $ 28.4

Basis of Presentation and Sig_3

Basis of Presentation and Significant Accounting Policies (Details) $ in Millions	3 Months Ended	9 Months Ended
	Jun. 30, 2024 USD ($)	Sep. 30, 2024 USD ($) segment	Apr. 30, 2024 USD ($)	Dec. 31, 2023	Oct. 25, 2023 USD ($)
Basis of Presentation
Number of reportable segments | segment		4
Percent of shares outstanding		12.30%
Subsidiary - HG Global
Basis of Presentation
Purchase of surplus notes		$ 503
Subsidiary - HG Global | Maximum
Basis of Presentation
Percentage of par value of policy reinsured		15%
Subsidiary - WTM Partners
Basis of Presentation
Expected capital contributions					$ 500
Subsidiary - Ark Insurance Holdings Limited
Basis of Presentation
Ownership percentage		72%		72%
Ownership percentage, after equity incentives		61.90%		61.90%
Subsidiary - WM Outrigger Re | Preferred Stock
Basis of Presentation
Ownership percentage		100%		100%
Subsidiary - HG Global | Preferred Stock
Basis of Presentation
Ownership percentage		96.90%		96.90%
Subsidiary - HG Global | Common Stock
Basis of Presentation
Ownership percentage		88.40%		88.40%
Subsidiary - Kudu Investment Management, LLC
Basis of Presentation
Ownership percentage		90.10%
Ownership percentage of basic units outstanding				89.60%
Ownership percentage, fully diluted		76.80%
Ownership percentage, fully converted				76.30%
Subsidiary - Bamboo Ide8 Insurance Services LLC
Basis of Presentation
Ownership percentage		72.80%
Ownership percentage, fully diluted		63.70%
Bamboo CRV
Basis of Presentation
Expected capital contributions		$ 30	$ 30
Preference share purchased	$ 12.1

Significant Transactions - Bamb

Significant Transactions - Bamboo (Details) - USD ($) $ in Millions		3 Months Ended
	Jan. 02, 2024	Dec. 31, 2023	Mar. 31, 2024
Bamboo Ide8 Insurance Services LLC
Other Significant Transactions [Line Items]
Assets and goodwill acquired	$ 479.5
Liabilities acquired	91.7
Noncontrolling interest acquired	111.1
Total goodwill and other intangible assets	371.4
Transaction costs	4		$ 0.3
Bamboo Ide8 Insurance Services LLC
Other Significant Transactions [Line Items]
Contributions to subsidiaries	296.7
Contribution of primary capital	36
Cash contributions for transaction	$ 20
Basic ownership	72.80%
Ownership percentage, after equity incentives	63.70%
Bamboo Ide8 Insurance Services LLC | Bamboo Management
Other Significant Transactions [Line Items]
Basic ownership	16.10%
Ownership percentage, after equity incentives	26.60%
Outrigger Re
Other Significant Transactions [Line Items]
Contributions to subsidiaries		$ 205

Significant Transactions - Sche

Significant Transactions - Schedule of Assets Acquired and Liabilities Assumed, Bamboo (Details) - USD ($) $ in Millions	Jan. 02, 2024	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Other Significant Transactions [Line Items]
Goodwill		$ 439.2	$ 439.2	$ 168.8	$ 168.8	$ 168.8	$ 176.5
Bamboo Ide8 Insurance Services LLC
Other Significant Transactions [Line Items]
Cash contributions for transaction	$ 20
Bamboo Ide8 Insurance Services LLC
Other Significant Transactions [Line Items]
Fixed maturity investments, at fair value	8.2
Short-term investments, at fair value	9.3
Restricted cash	37
Cash (restricted $37.0)	44.9
Premiums and commissions receivable	38
Other assets	7.7
Total tangible assets	108.1
Loss and loss adjustment expense reserves	(9.3)
Unearned insurance premiums	(20.1)
Premiums and commissions payable	(49.2)
Other liabilities	(13.1)
Total tangible liabilities	(91.7)
Net tangible assets acquired	16.4
Goodwill	270.4
Other intangible assets	101
Total goodwill and other intangible assets	371.4
Net assets acquired	$ 387.8

Significant Transactions - BAM

Significant Transactions - BAM (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Jul. 01, 2024	Dec. 31, 2023
Other Significant Transactions [Line Items]
Unrealized loss on deconsolidation of BAM	$ 114.5		$ 114.5	$ 0
Operating Segments | Financial Guarantee (HG Global)
Other Significant Transactions [Line Items]
Notes receivable, fair value disclosure	411.1		411.1		$ 387.4	$ 0
Unrealized loss on deconsolidation of BAM	$ 114.5	$ 0	$ 114.5	$ 0

Significant Transactions - Outr

Significant Transactions - Outrigger Re (Details) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2024	Mar. 31, 2024	Dec. 31, 2023
Outrigger Re
Other Significant Transactions [Line Items]
Contributions to subsidiaries			$ 205
Outrigger Re
Other Significant Transactions [Line Items]
Ceded unearned premiums			250
Rollover amount			130
Short-term investments, at fair value	$ 199.9		$ 265.3
Return of capital	$ 64.9	$ 68.1

Investment Securities - Schedul

Investment Securities - Schedule of Pre-Tax Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Net Investment Income [Line Items]
Total investment income	$ 58.4	$ 48.8	$ 174.3	$ 131.8
Third-party investment expenses	(0.7)	(0.7)	(2.2)	(1.7)
Net investment income	57.7	48.1	172.1	130.1
Fixed maturity investments
Net Investment Income [Line Items]
Total investment income	23.3	16.6	67.2	46.6
Short-term investments
Net Investment Income [Line Items]
Total investment income	13.2	13.8	45.3	34.8
Common equity securities
Net Investment Income [Line Items]
Total investment income	0.7	1	1.8	3.6
Other long-term investments
Net Investment Income [Line Items]
Total investment income	$ 21.2	$ 17.4	$ 60	$ 46.8

Investment Securities - Net Rea

Investment Securities - Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Gain (Loss) on Securities [Line Items]
Net realized and unrealized investment gains (losses) (1)	$ 223.8	$ (58.3)	$ 390.4	$ 151.2
MediaAlpha
Gain (Loss) on Securities [Line Items]
Net realized and unrealized investment gains (losses) (1)	88.2	(46.8)	159.7	(38.9)
Net realized and unrealized investment (losses) gains
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	4.3	1.3	119.2	60.7
Net unrealized investment gains (losses)	219.5	(59.6)	271.2	90.5
Fixed maturity investments
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	(0.5)	(0.3)	(9.2)	(1.7)
Net unrealized investment gains (losses)	56.9	(27.9)	49.7	(9.7)
Short-term investments
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	0	(0.2)	(0.3)	(0.3)
Net unrealized investment gains (losses)	0.5	(0.6)	(0.5)	0.7
Common equity securities
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	0	1.2	3.5	13.8
Net unrealized investment gains (losses)	24.7	(18.5)	63.6	29.8
Net realized and unrealized investment gains (losses) (1)	24.7	(17.3)	67.1	43.6
Equity Method Investments | MediaAlpha
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	0	0	91.2	0
Net unrealized investment gains (losses)	88.2	(46.8)	68.5	(38.9)
Net realized and unrealized investment gains (losses) (1)	88.2	(46.8)	159.7	(38.9)
Other long-term investments
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	4.8	0.6	34	48.9
Net unrealized investment gains (losses)	49.2	34.2	89.9	108.6
Fixed maturity and short-term investments
Gain (Loss) on Securities [Line Items]
Net realized and unrealized investment gains (losses) (1)	56.9	(29)	39.7	(11)
Less: net realized and unrealized gains (losses) on investment securities sold during the period	1.2	2.5	0.8	4.7
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	55.7	(31.5)	38.9	(15.7)
Common equity securities and investment in MediaAlpha
Gain (Loss) on Securities [Line Items]
Net realized and unrealized investment gains (losses) (1)	112.9	(64.1)	226.8	4.7
Less: net realized and unrealized gains (losses) on investment securities sold during the period	0	(0.1)	35.4	3.7
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	112.9	(64)	191.4	1
Total Investments After TPC Providers - Investments
Gain (Loss) on Securities [Line Items]
Gain (loss) on investments, exchange rate effect	$ 18.9	$ (12.8)	$ 7.1	$ (1.9)

Investment Securities - Investm

Investment Securities - Investment Gains (Losses) for Level 3 (Details) - Other long-term investments - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Gain (Loss) on Securities [Line Items]
Net unrealized investment gains (losses)	$ 49.2	$ 34.2	$ 89.9	$ 108.6
Level 3
Gain (Loss) on Securities [Line Items]
Net unrealized investment gains (losses)	$ 31.3	$ 24.6	$ 82.6	$ 76

Investment Securities - Fixed M

Investment Securities - Fixed Maturity Investments (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2024	Dec. 31, 2023
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	$ 2,434.1	$ 2,198.4
Gross Unrealized Gains	27.4	9.2
Gross Unrealized Losses	(41.5)	(97.1)
Net Foreign Currency Gains (Losses)	2.3	(1.2)
Carrying Value	2,422.3	2,109.3
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	446	209
Gross Unrealized Gains	4.8	0.3
Gross Unrealized Losses	(0.6)	(4.4)
Net Foreign Currency Gains (Losses)	0	0
Carrying Value	450.2	204.9
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	1,318.3	1,085.9
Gross Unrealized Gains	16.1	5.6
Gross Unrealized Losses	(24.4)	(45.4)
Net Foreign Currency Gains (Losses)	0.9	(1.1)
Carrying Value	1,310.9	1,045
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	3.2	275.1
Gross Unrealized Gains	0.1	0.9
Gross Unrealized Losses	0	(15.1)
Net Foreign Currency Gains (Losses)	0	0
Carrying Value	3.3	260.9
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	379	417.2
Gross Unrealized Gains	5.2	2
Gross Unrealized Losses	(16.1)	(29.8)
Net Foreign Currency Gains (Losses)	0	0
Carrying Value	368.1	389.4
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	287.6	211.2
Gross Unrealized Gains	1.2	0.4
Gross Unrealized Losses	(0.4)	(2.4)
Net Foreign Currency Gains (Losses)	1.4	(0.1)
Carrying Value	$ 289.8	$ 209.1

Investment Securities - Cost an

Investment Securities - Cost and Amortized Cost Maturity Schedule (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Fixed Maturities, Excluding Asset-Backed Securities And Collateralized Loan Obligations
Cost or Amortized Cost
Due in one year or less	$ 237
Due after one year through five years	1,277.8
Due after five years through ten years	228.4
Due after ten years	24.2
Carrying Value
Due in one year or less	235.1
Due after one year through five years	1,280.7
Due after five years through ten years	223.3
Due after ten years	25.3
Mortgage and asset-backed securities and collateralized loan obligations
Cost or Amortized Cost
Mortgage and asset-backed securities and collateralized loan obligations	666.7
Carrying Value
Mortgage and asset-backed securities and collateralized loan obligations	657.9
Fixed maturity investments
Cost or Amortized Cost
Total fixed maturity investments	2,434.1	$ 2,198.4
Carrying Value
Total fixed maturity investments	$ 2,422.3	$ 2,109.3

Investment Securities - Common

Investment Securities - Common Equity Securities and Other Long Term Investments (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2024	Dec. 31, 2023
Common equity securities
Cost or Amortized Cost	$ 537	$ 483.5
Gross Unrealized Gains	120.2	62.3
Gross Unrealized Losses	0	(1.2)
Net Foreign Currency Gains (Losses)	(1.8)	(6.2)
Carrying Value	655.4	538.4
Other long-term investments
Cost or Amortized Cost	1,638.6	1,655.7
Gross Unrealized Gains	547.9	446.3
Other Long-Term Investments, Unrealized Loss	(108.3)	(94.5)
Net Foreign Currency Gains (Losses)	(7.1)	(9.3)
Carrying Value	2,071.1	1,998.2
MediaAlpha
Investment in MediaAlpha
Cost or Amortized Cost	59.2	59.2
Gross Unrealized Gains	264.2	195.7
Gross Unrealized Losses	0	0
Net Foreign Currency Gains (Losses)	0	0
Carrying Value	$ 323.4	$ 254.9

Investment Securities - Narrati

Investment Securities - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2024 USD ($) $ / shares shares	Jun. 30, 2023 $ / shares shares	Sep. 30, 2024 USD ($) fund $ / shares shares	Dec. 31, 2023 USD ($)
Concentration Risk [Line Items]
Other long-term investments			$ 2,071.1	$ 1,998.2
Investments held in trusts			$ 641.4	585.6
MediaAlpha | IPO
Concentration Risk [Line Items]
Increase in book value per share (in dollars per share) | $ / shares			$ 7
Fair value measured on a recurring basis | MediaAlpha
Concentration Risk [Line Items]
Other long-term investments			$ 323.4	254.9
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments			172.7	158
Lloyds Bank
Concentration Risk [Line Items]
Fair value investments			356.8	344.7
Outrigger Re
Concentration Risk [Line Items]
Short-term investments			199.9	$ 265.3
Bamboo Captive
Concentration Risk [Line Items]
Short-term investments			32.9
Bamboo CRV
Concentration Risk [Line Items]
Pledged collateral			$ 12.1
MediaAlpha
Concentration Risk [Line Items]
Shares purchased (in shares) | shares	7.6	5.9
Purchase price (in dollars per share) | $ / shares	$ 19	$ 10	$ 18.11
Purchase price net of underwriting fees (in dollars per share) | $ / shares	$ 18.24
Investment owned (in shares) | shares			17.9
Ownership interest (as a percent)			26.80%	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]
Sale of stock, number of shares issued in transaction (in shares) | shares	5
Proceeds from sale of business	$ 91.2
Largest Investment | Private Equity and Hedge Fund Concentration Risk | Investments
Concentration Risk [Line Items]
Other long-term investments			$ 56.6	$ 57.3
HG Global Senior Notes | Senior Notes
Concentration Risk [Line Items]
Interest reserve account			29.3	30.4
Kudu Credit Facility | Secured Debt
Concentration Risk [Line Items]
Restricted cash			14.8	14.9
Asset Pledged as Collateral
Concentration Risk [Line Items]
Short-term investments			208	198.9
Asset Pledged as Collateral | Additional Investments, Insurance Regulators and Reinsurance Counterparties
Concentration Risk [Line Items]
Fair value investments			215.8	244.5
Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value			289.8	209.1
Collateralized loan obligations | Fair value measured on a recurring basis
Concentration Risk [Line Items]
Fixed maturity investments, at fair value			289.8	209.1
Private equity funds
Concentration Risk [Line Items]
Other long-term investments			$ 262
Number of investments | fund			17
Hedge funds
Concentration Risk [Line Items]
Number of investments | fund			2
Bank loan fund
Concentration Risk [Line Items]
Other long-term investments			$ 258.9	194.4
Foreign regulatory deposits
Concentration Risk [Line Items]
Other long-term investments			172.7	158
ILS funds
Concentration Risk [Line Items]
Other long-term investments			$ 71.2	$ 160.5
ILS funds | Minimum
Concentration Risk [Line Items]
Notice period			30 days
ILS funds | Maximum
Concentration Risk [Line Items]
Notice period			90 days

Investment Securities - Fair Va

Investment Securities - Fair Value Measurement by Level (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	$ 655.4	$ 538.4
Other long-term investments	2,071.1	1,998.2
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	450.2	204.9
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	1,310.9	1,045
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	3.3	260.9
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	368.1	389.4
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	289.8	209.1
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	2,422.3	2,109.3
Level 2 | Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	368.1	389.4
Level 2 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	289.8	209.1
Level 3 | Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Level 3 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	6,362.5	6,388.7
Fair value measured on a recurring basis | MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	323.4	254.9
Fair value measured on a recurring basis | U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	450.2	204.9
Fair value measured on a recurring basis | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	1,310.9	1,045
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	399	309.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	253.5	214.6
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	149.2	121.6
Fair value measured on a recurring basis | Debt securities issued by corporations | Healthcare
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	148	121.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	109.8	108.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	73	70.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	60.9	46.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	62.9	28.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	54.6	23.4
Fair value measured on a recurring basis | Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	3.3	260.9
Fair value measured on a recurring basis | Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	368.1	389.4
Fair value measured on a recurring basis | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	289.8	209.1
Fair value measured on a recurring basis | Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	2,422.3	2,109.3
Fair value measured on a recurring basis | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	890.3	1,487.9
Fair value measured on a recurring basis | Exchange-traded funds
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	219.8	137.8
Fair value measured on a recurring basis | Common stock, other
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	435.6	400.6
Fair value measured on a recurring basis | Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	655.4	538.4
Fair value measured on a recurring basis | Other Long-Term Investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	2,071.1	1,998.2
Fair value measured on a recurring basis | Fair Value, Inputs, Level 1, Level 2, and Level 3 | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,205.3	1,164.4
Fair value measured on a recurring basis | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,873.4	2,055.2
Fair value measured on a recurring basis | Level 1 | MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	323.4	254.9
Fair value measured on a recurring basis | Level 1 | U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	449.4	204.9
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Financials
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Consumer
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Healthcare
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Technology
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Utilities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Communications
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Energy
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Materials
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	449.4	204.9
Fair value measured on a recurring basis | Level 1 | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	880.8	1,457.6
Fair value measured on a recurring basis | Level 1 | Exchange-traded funds
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	219.8	137.8
Fair value measured on a recurring basis | Level 1 | Common stock, other
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	0	0
Fair value measured on a recurring basis | Level 1 | Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	219.8	137.8
Fair value measured on a recurring basis | Level 1 | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Fair value measured on a recurring basis | Level 1 | Other Long-Term Investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Fair value measured on a recurring basis | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	2,443.5	2,361.5
Fair value measured on a recurring basis | Level 2 | MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Fair value measured on a recurring basis | Level 2 | U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.8	0
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	1,310.9	1,045
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Financials
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	399	309.8
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Consumer
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	253.5	214.6
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	149.2	121.6
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Healthcare
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	148	121.3
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Technology
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	109.8	108.9
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Utilities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	73	70.2
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Communications
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	60.9	46.3
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Energy
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	62.9	28.9
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Materials
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	54.6	23.4
Fair value measured on a recurring basis | Level 2 | Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	3.3	260.9
Fair value measured on a recurring basis | Level 2 | Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	368.1	389.4
Fair value measured on a recurring basis | Level 2 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	289.8	209.1
Fair value measured on a recurring basis | Level 2 | Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	1,972.9	1,904.4
Fair value measured on a recurring basis | Level 2 | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	9.5	30.3
Fair value measured on a recurring basis | Level 2 | Exchange-traded funds
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	0	0
Fair value measured on a recurring basis | Level 2 | Common stock, other
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	435.6	400.6
Fair value measured on a recurring basis | Level 2 | Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	435.6	400.6
Fair value measured on a recurring basis | Level 2 | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	25.5	26.2
Fair value measured on a recurring basis | Level 2 | Other Long-Term Investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	25.5	26.2
Fair value measured on a recurring basis | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,179.8	1,138.2
Fair value measured on a recurring basis | Level 3 | MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Fair value measured on a recurring basis | Level 3 | U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Financials
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Consumer
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Healthcare
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Technology
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Utilities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Communications
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Energy
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Materials
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Exchange-traded funds
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Common stock, other
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	0	0
Fair value measured on a recurring basis | Level 3 | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,179.8	1,138.2
Fair value measured on a recurring basis | Level 3 | Other Long-Term Investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,179.8	1,138.2
Fair value measured on a recurring basis | NAV | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	$ 865.8	$ 833.8

Investment Securities - Debt Se

Investment Securities - Debt Securities Issued by Corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	$ 1,310.9	$ 1,045
AAA
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	15	11.5
AA
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	82.3	83.8
A
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	577.6	552.4
BBB
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	625.5	390.9
BB
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	3.2	0
Other
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	$ 7.3	$ 6.4

Investment Securities - Mortgag

Investment Securities - Mortgage and Asset-Backed Securities and Collateralized Loan Obligations (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Total mortgage and asset-backed securities and collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	$ 657.9	$ 598.5
Total mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	368.1	389.4
Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	363.4	366.4
Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	4.7	23
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	289.8	209.1
Level 2 | Total mortgage and asset-backed securities and collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	657.9	598.5
Level 2 | Total mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	368.1	389.4
Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	363.4	366.4
Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	4.7	23
Level 2 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	289.8	209.1
Level 3 | Total mortgage and asset-backed securities and collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Level 3 | Total mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Level 3 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Level 3 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Total non-agency | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.5	0.2
Total non-agency | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.5	0.2
Total non-agency | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Total agency | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	362.9	366.2
Total agency | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	362.9	366.2
Total agency | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
FNMA | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	194.8	198.8
FNMA | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	194.8	198.8
FNMA | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
FHLMC | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	142.1	137.2
FHLMC | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	142.1	137.2
FHLMC | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
GNMA | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	26	30.2
GNMA | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	26	30.2
GNMA | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Commercial | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.4	0
Commercial | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.4	0
Commercial | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Residential | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.1	0.2
Residential | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.1	0.2
Residential | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Vehicle receivables | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	1.7	17.2
Vehicle receivables | Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	1.7	17.2
Vehicle receivables | Level 3 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Credit card receivables | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	1.3	3.3
Credit card receivables | Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	1.3	3.3
Credit card receivables | Level 3 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0	0
Other | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	1.7	2.5
Other | Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	1.7	2.5
Other | Level 3 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	$ 0	$ 0

Investment Securities - Other L

Investment Securities - Other Long-Term Investments (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	$ 2,071.1	$ 1,998.2
PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	150	150
Elementum
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	35	35
Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	56.6	48.1
Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	349	312.9
Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	258.9	194.4
Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	172.7	158
ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	71.2	160.5
Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	15.8	15.8
Other
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	31.3	33
Kudu’s Participation Contracts | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	930.6	890.5
Investments, Excluding MediaAlpha | Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,172.2	1,123.6
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	544.8	440.9
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Elementum
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	81.9	55.5
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	258.9	194.4
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	172.7	158
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Other
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	31.3	33
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Kudu’s Participation Contracts | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Investments, Excluding MediaAlpha | Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	936.8	896.3
Operating Segments | Asset Management (Kudu) | PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Asset Management (Kudu) | Elementum
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Asset Management (Kudu) | Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Asset Management (Kudu) | Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Asset Management (Kudu) | Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Asset Management (Kudu) | Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Asset Management (Kudu) | ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Asset Management (Kudu) | Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	6.2	5.8
Operating Segments | Asset Management (Kudu) | Other
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Operating Segments | Asset Management (Kudu) | Kudu’s Participation Contracts | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	930.6	890.5
Operating Segments | Asset Management (Kudu) | Investments, Excluding MediaAlpha | Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	930.6	890.5
Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	589.5	661
Other Operations | PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	150	150
Other Operations | Elementum
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	35	35
Other Operations | Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	56.6	48.1
Other Operations | Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	267.1	257.4
Other Operations | Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Other Operations | Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Other Operations | ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	71.2	160.5
Other Operations | Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	9.6	10
Other Operations | Other
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Other Operations | Kudu’s Participation Contracts | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0	0
Other Operations | Investments, Excluding MediaAlpha | Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	$ 241.6	$ 233.1

Investment Securities - Private

Investment Securities - Private Equity Funds and Hedge Funds (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Total private equity funds and hedge funds included in other long-term investments
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 349	$ 312.9
Unfunded Commitments	85.3	57.7
Private equity funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	262	248.6
Unfunded Commitments	85.3	57.7
Private equity funds | Aerospace/Defense/Government
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	169	157.2
Unfunded Commitments	46.8	17.4
Private equity funds | Financial services
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	89.3	87.5
Unfunded Commitments	36.1	37.8
Private equity funds | Real estate
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	3.7	3.9
Unfunded Commitments	2.4	2.5
Hedge funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	87	64.3
Unfunded Commitments	0	0
Hedge funds | Long/short equity financials and business services
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	56.5	54.4
Unfunded Commitments	0	0
Hedge funds | Long/short all cap global
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	30.5	0
Unfunded Commitments	0	0
Hedge funds | European small/mid cap
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	0	9.9
Unfunded Commitments	$ 0	$ 0

Investment Securities - Inves_2

Investment Securities - Investments In Private Equity Funds Subject to Lock-Up Periods (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Debt and Equity Securities, FV-NI [Line Items]
1 – 3 years	$ 18.5
3 – 5 years	55.9
5 – 10 years	184.5
>10 years	3.1
Total	2,071.1	$ 1,998.2
Private equity funds
Debt and Equity Securities, FV-NI [Line Items]
Total	$ 262

Investment Securities - Rollfor

Investment Securities - Rollforward of Fair Value Measurements by Level (Details) - Other long-term investments - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2024	Sep. 30, 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	79.9	74
Purchases and contributions	3.1	200.1
Sales and distributions	(41.4)	(110.4)
Transfers in	0	0
Transfers out	0	0
Ending balance	$ 1,179.8	$ 1,075.3

Investment Securities - Signifi

Investment Securities - Significant Unobservable Inputs (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2024	Dec. 31, 2023
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	$ 2,071.1	$ 1,998.2
Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
New and existing contracts deployed	0.2	163.8
Level 3 | Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	18.7	19
Level 3 | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	15.4	15.5
Level 3 | Discounted cash flow | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	930.6	890.5
Level 3 | Discounted cash flow | PassportCard/DavidShield
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	150	150
Level 3 | Discounted cash flow | Elementum
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	35	35
Level 3 | Discounted cash flow | Preferred Securities
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	$ 30.1	28.2
Level 3 | Probability weighted expected return method | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value		$ 69.1
Level 3 | Discount Rate | Discounted cash flow | PassportCard/DavidShield
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	24%	24%
Level 3 | Discount Rate | Discounted cash flow | Elementum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	21%	21%
Level 3 | Discount Rate | Discounted cash flow | Preferred Securities
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	7%	8%
Level 3 | Discount Rate | Discounted cash flow | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	11%	11%
Level 3 | Discount Rate | Discounted cash flow | Minimum | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	16%	18%
Level 3 | Discount Rate | Discounted cash flow | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	12%	12%
Level 3 | Discount Rate | Discounted cash flow | Maximum | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	24%	25%
Level 3 | Terminal Cash Flow Exit Multiple | Discounted cash flow | Minimum | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	700%	700%
Level 3 | Terminal Cash Flow Exit Multiple | Discounted cash flow | Maximum | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	2,200%	2,200%
Level 3 | Terminal Revenue Growth Rate | Discounted cash flow | PassportCard/DavidShield
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	4%	4%
Level 3 | Terminal Revenue Growth Rate | Discounted cash flow | Elementum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	4%	4%
Level 3 | Measurement Input, Pre-Tax Discount Rate | Discounted cash flow | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	19%	20%
Level 3 | Measurement, Input, Pre-Tax Terminal Cash Flow Exit Multiple | Discounted cash flow | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	1,400%	1,300%

Goodwill and Other Intangible_3

Goodwill and Other Intangible Assets - Schedule of Acquisition Date Fair Values, Accumulated Amortization and Net Carrying Values for Other Intangibles Assets and Goodwill, By Company (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 439.2	$ 439.2	$ 168.8	$ 168.8	$ 168.8	$ 176.5
Other intangible assets, finite-lived, accumulated amortization	33.6		17
Total other intangible assets	320.1		218.8
Total other intangible assets	286.5	291.8	201.8	203.3	204.8	215.9
Total goodwill and other intangible assets	759.3		387.6
Goodwill and other intangible assets	725.7	$ 731	370.6	$ 372.1	$ 373.6	$ 392.4
Other
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, finite-lived, accumulated amortization	33.6		17
Total other intangible assets	286.5		201.8
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets	292.5		292.5
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	116.8		116.8
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Underwriting Capacity
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, indefinite-lived	175.7		175.7
Operating Segments | Asset Management (Kudu)
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	7.6		7.6
Goodwill and other intangible assets	$ 8		8.3
Operating Segments | Asset Management (Kudu) | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	7 years
Other intangible assets, finite-lived, acquisition date fair value	$ 2.2		2.2
Other intangible assets, finite-lived, accumulated amortization	1.8		1.5
Other intangible assets, finite-lived, net carrying value	0.4		0.7
Operating Segments | P&C Insurance Distribution (Bamboo)
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	270.4		0
Other intangible assets, finite-lived, acquisition date fair value	101		0
Other intangible assets, finite-lived, accumulated amortization	12.4		0
Other intangible assets, finite-lived, net carrying value	88.6		0
Goodwill and other intangible assets	$ 359		0
Operating Segments | P&C Insurance Distribution (Bamboo) | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	10 years
Other intangible assets, finite-lived, acquisition date fair value	$ 23.5		0
Other intangible assets, finite-lived, accumulated amortization	1.8		0
Other intangible assets, finite-lived, net carrying value	$ 21.7		0
Operating Segments | P&C Insurance Distribution (Bamboo) | Agency relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	6 years
Other intangible assets, finite-lived, acquisition date fair value	$ 72.4		0
Other intangible assets, finite-lived, accumulated amortization	9.1		0
Other intangible assets, finite-lived, net carrying value	$ 63.3		0
Operating Segments | P&C Insurance Distribution (Bamboo) | Developed technology
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	3 years
Other intangible assets, finite-lived, acquisition date fair value	$ 4.7		0
Other intangible assets, finite-lived, accumulated amortization	1.1		0
Other intangible assets, finite-lived, net carrying value	$ 3.6		0
Operating Segments | P&C Insurance Distribution (Bamboo) | Other
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	3 months 18 days
Other intangible assets, finite-lived, acquisition date fair value	$ 0.4		0
Other intangible assets, finite-lived, accumulated amortization	0.4		0
Other intangible assets, finite-lived, net carrying value	0		0
Other Operations
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	44.4		44.4
Other intangible assets, finite-lived, acquisition date fair value	41.2		40.9
Other intangible assets, finite-lived, accumulated amortization	19.4		15.5
Other intangible assets, finite-lived, net carrying value	21.8		25.4
Goodwill and other intangible assets	$ 66.2		69.8
Other Operations | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	13 years 3 months 18 days
Other intangible assets, finite-lived, acquisition date fair value	$ 13.3		13.3
Other intangible assets, finite-lived, accumulated amortization	5.3		4.1
Other intangible assets, finite-lived, net carrying value	$ 8		9.2
Other Operations | Customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
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, finite-lived, accumulated amortization	12.7		10.3
Other intangible assets, finite-lived, net carrying value	$ 12.1		14.5
Other Operations | Other
Acquired Finite-Lived Intangible Assets [Line Items]
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, finite-lived, accumulated amortization	1.4		1.1
Other intangible assets, finite-lived, net carrying value	$ 1.7		$ 1.7

Goodwill and Other Intangible_4

Goodwill and Other Intangible Assets - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Impairments to goodwill and other intangibles	$ 0	$ 0	$ 0	$ 0

Goodwill and Other Intangible_5

Goodwill and Other Intangible Assets - Schedule of Goodwill and Other Intangible Assets Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Goodwill
Goodwill, beginning balance	$ 439.2	$ 168.8	$ 168.8	$ 176.5
Acquisition of businesses			270.4	0
Dispositions			0	(6.7)
Measurement period adjustments			0	(1)
Goodwill, ending balance	439.2	168.8	439.2	168.8
Other Intangible Assets
Other intangible assets (excluding goodwill), beginning balance	291.8	204.8	201.8	215.9
Acquisition of businesses			101	0
Acquisition of intangible assets			0.3	0
Dispositions			0	(6.9)
Amortization	(5.3)	(1.5)	(16.6)	(5.7)
Other intangible assets (excluding goodwill), ending balance	286.5	203.3	286.5	203.3
Total Goodwill and Other Intangible Assets
Total Goodwill and Other Intangible Assets, beginning balance	731	373.6	370.6	392.4
Acquisition of businesses			371.4	0
Acquisition of intangible assets			0.3	0
Dispositions			0	(13.6)
Measurement period adjustments			0	(1)
Amortization	(5.3)	(1.5)	(16.6)	(5.7)
Total Goodwill and Other Intangible Assets, ending balance	$ 725.7	$ 372.1	$ 725.7	$ 372.1

Loss and Loss Adjustment Expe_3

Loss and Loss Adjustment Expense Reserves - Reserve Activity (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2021	Dec. 31, 2020
Loss and loss adjustment expenses incurred relating to:
Net incurred loss and loss adjustment expenses	$ 296.9	$ 265.8	$ 666.6	$ 581.1
Syndicate - Ark Insurance Holdings Limited
Loss and loss adjustment expenses paid relating to:
Percent of underwriting capital provided by third parties					0%	42.80%
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance	1,890.1	1,421	1,605.1	1,296.5
Less: beginning reinsurance recoverable on unpaid losses	(458.5)	(362.7)	(340.8)	(505)
Net loss and loss adjustment expense reserves	1,431.6	1,058.3	1,264.3	791.5
Loss and loss adjustment expenses incurred relating to:
Current year losses	313.1	266.4	676	562.1
Prior year losses	(24.8)	(0.6)	(32)	19
Net incurred loss and loss adjustment expenses	288.3	265.8	644	581.1
Loss and loss adjustment expenses paid relating to:
Current year losses	(19.1)	(8.8)	(27.4)	(18.4)
Prior year losses	(86.3)	(90.4)	(261.8)	(281.1)
Net paid loss and loss adjustment expenses	(105.4)	(99.2)	(289.2)	(299.5)
Change in TPC providers’ participation	0	0	0	145.4
Foreign currency translation and other adjustments to loss and loss adjustment expense reserves	11.3	(5.5)	6.7	0.9
Net ending balance	1,625.8	1,219.4	1,625.8	1,219.4
Plus: ending reinsurance recoverable on unpaid losses	(507.2)	(352.4)	(507.2)	(352.4)
Gross ending balance	2,133	1,571.8	2,133	1,571.8
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance			1,605.1
Loss and loss adjustment expenses incurred relating to:
Net incurred loss and loss adjustment expenses	288.3	265.8	644	581.1
Loss and loss adjustment expenses paid relating to:
Gross ending balance	2,133		2,133
P&C Insurance and Reinsurance (Ark/WM Outrigger) | TPC Providers
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Less: beginning reinsurance recoverable on unpaid losses				(145.4)
P&C Insurance Distribution (Bamboo) | Operating Segments
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance			0
Loss and loss adjustment expenses incurred relating to:
Net incurred loss and loss adjustment expenses	4.4	$ 0	14.5	$ 0
Loss and loss adjustment expenses paid relating to:
Gross ending balance	$ 17.7		$ 17.7

Loss and Loss Adjustment Expe_4

Loss and Loss Adjustment Expense Reserves - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss adjustment expense reserves, loss on delinquent payment	$ 0.3		$ 0.3
Net incurred loss and loss adjustment expenses	296.9	$ 265.8	666.6	$ 581.1
Other Operations
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	8.1		8.1			$ 0
Net incurred loss and loss adjustment expenses	4.2	0	8.1	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%
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year losses	(24.8)	(0.6)	(32)	19
Loss and loss adjustment expense reserves	2,133	1,571.8	2,133	1,571.8	$ 1,890.1	1,605.1	$ 1,421	$ 1,296.5
Net incurred loss and loss adjustment expenses	288.3	265.8	644	581.1
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	2,133		2,133			1,605.1
Net incurred loss and loss adjustment expenses	288.3	265.8	644	581.1
P&C Insurance Distribution (Bamboo) | Operating Segments
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	17.7		17.7			$ 0
Net incurred loss and loss adjustment expenses	$ 4.4	$ 0	$ 14.5	$ 0

Third-Party Reinsurance - Effec

Third-Party Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Written premiums:
Direct		$ 16.5	$ 24.1	$ 37.3
Assumed		0	14	0
Gross written premiums		16.5	38.1	37.3
Earned premiums:
Direct		7.4	15.8	21.4
Assumed		0.5	8.5	1.9
Gross earned premiums		7.9	24.3	23.3
Net earned premiums	$ 581.5	506.8	1,244.4	1,070.6
Loss and loss adjustment expenses:
Net incurred loss and loss adjustment expenses	296.9	265.8	666.6	581.1
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Written premiums:
Direct	239	189.4	863.4	727.1
Assumed	134.6	61.8	1,079.3	939.6
Gross written premiums	373.6	251.2	1,942.7	1,666.7
Ceded	(34.2)	(20.2)	(502.5)	(360.3)
Net written premiums	339.4	231	1,440.2	1,306.4
Earned premiums:
Direct	317.6	273.1	731.5	624.1
Assumed	432.6	370	805.3	684.6
Gross earned premiums	750.2	643.1	1,536.8	1,308.7
Ceded	(198)	(144.2)	(363.5)	(261.4)
Net earned premiums	552.2	498.9	1,173.3	1,047.3
Loss and loss adjustment expenses:
Gross	350.7	269.3	852.7	627.5
Ceded	(62.4)	(3.5)	(208.7)	(46.4)
Net incurred loss and loss adjustment expenses	288.3	265.8	644	581.1
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark
Written premiums:
Ceded	8.8	6	82	108.4
Earned premiums:
Ceded	45	60.6	62.8	75.4
Loss and loss adjustment expenses:
Ceded	$ 13.2	$ 7.3	$ 13.6	$ 7.9

Third-Party Reinsurance - Reins

Third-Party Reinsurance - Reinsurance Recoverables (Details) - P&C Insurance and Reinsurance (Ark/WM Outrigger) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Reinsurance recoverables on unpaid losses	$ 507.2	$ 340.8
Reinsurance recoverables on paid losses	27.9	27.4
Ceded unearned premiums	212.9	73.8
Reinsurance recoverables	748	442
Ark
Segment Reporting Information [Line Items]
Reinsurance recoverables on unpaid losses	(28)	(15.6)
Ceded unearned premiums	$ (25)	$ (5.7)

Third-Party Reinsurance - Remai

Third-Party Reinsurance - Remaining Gross and Net Reinsurance Recoverables (Details) - P&C Insurance and Reinsurance (Ark/WM Outrigger) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Reinsurance recoverables	$ 535.1
Collateral	748	$ 442
Net	454
Reinsurance recoverables on unpaid losses	507.2	340.8
Ark
Segment Reporting Information [Line Items]
Reinsurance recoverables on unpaid losses	$ (28)	$ (15.6)
Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting Information [Line Items]
% of Total	100%
Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 81.1
A+ or better
Segment Reporting Information [Line Items]
Reinsurance recoverables	358.1
Net	$ 358.1
A+ or better | Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting Information [Line Items]
% of Total	78.90%
A+ or better | Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 0
A- to A
Segment Reporting Information [Line Items]
Reinsurance recoverables	84.2
Net	$ 84.2
A- to A | Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting Information [Line Items]
% of Total	18.50%
A- to A | Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 0
B++ or lower and not rated
Segment Reporting Information [Line Items]
Reinsurance recoverables	92.8
Net	$ 11.7
B++ or lower and not rated | Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting Information [Line Items]
% of Total	2.60%
B++ or lower and not rated | Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 81.1

Third-Party Reinsurance - Narra

Third-Party Reinsurance - Narrative (Details) - Third Point Re - P&C Insurance and Reinsurance (Ark/WM Outrigger) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Annual crediting rate	3%
Carrying value of investment	$ 20.3	$ 20.4

Debt - Summary of Debt Outstand

Debt - Summary of Debt Outstanding (Details) € in Millions, $ in Millions	Sep. 30, 2024 USD ($)	Sep. 30, 2024 EUR (€)	Jun. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)	Sep. 30, 2023 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2022 USD ($)
Debt Instrument
Total debt	$ 544.9			$ 564.6
Ark 2021 Subordinated Notes
Debt Instrument
Total debt	156.8			155.5
HG Global Senior Notes
Debt Instrument
Total debt	147.3			146.9
Kudu Credit Facility
Debt Instrument
Total debt	218.5			203.8
Other Operations debt
Debt Instrument
Total debt	22.3			28.4
Subordinated Debt | Ark
Debt Instrument
Total debt	$ 156.8			$ 185.5
Effective Rate	12.10%	12.10%		11%
Subordinated Debt | Ark 2007 Notes | Ark Subordinated Debt
Debt Instrument
Total debt	$ 0			$ 30
Subordinated Debt | Ark 2021 Subordinated Notes
Debt Instrument
Unamortized issuance cost	(3.8)			(4.2)
Total debt	156.8			155.5
Subordinated Debt | Ark 2021 Subordinated Notes Tranche 1
Debt Instrument
Amount borrowed	43.6	€ 39.1		42.7
Subordinated Debt | Ark 2021 Subordinated Notes Tranche 2
Debt Instrument
Amount borrowed	47			47
Subordinated Debt | Ark 2021 Subordinated Notes Tranche 3
Debt Instrument
Amount borrowed	70			70
Senior Notes | HG Global Senior Notes
Debt Instrument
Amount borrowed	150			150
Unamortized discount and issuance cost	(2.7)			(3.1)
Total debt	$ 147.3			$ 146.9
Effective Rate	11.80%	11.80%		11.90%
Secured Debt | Kudu Credit Facility
Debt Instrument
Amount borrowed	$ 225.3			$ 210.3
Unamortized issuance cost	(6.8)			(6.5)
Total debt	$ 225.3		$ 210.3	$ 210.3	$ 210.3	$ 210.3	$ 215.2
Effective Rate	10.50%	10.50%		10.10%
Secured Debt | Other Operations debt
Debt Instrument
Amount borrowed	$ 22.6			$ 28.9
Unamortized issuance cost	$ (0.3)			$ (0.5)
Effective Rate	10.10%	10.10%		10.20%

Debt - Narrative (Details)

Debt - Narrative (Details) € in Millions			1 Months Ended					3 Months Ended				6 Months Ended		9 Months Ended
	Apr. 29, 2022 USD ($)	Sep. 08, 2021 USD ($)	Jul. 31, 2023	Sep. 30, 2021 USD ($)	Aug. 31, 2021	Jul. 31, 2021	Mar. 31, 2007 USD ($)	Sep. 30, 2024 USD ($) credit_facility	Mar. 31, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2021 USD ($)	Dec. 31, 2023 USD ($)	Jun. 30, 2023	Sep. 30, 2024 USD ($) credit_facility	Sep. 30, 2023 USD ($)	Sep. 30, 2024 EUR (€) credit_facility	Sep. 17, 2024 USD ($)	Aug. 22, 2024 USD ($)	Jul. 31, 2024	Jun. 28, 2024 USD ($)	Sep. 30, 2022 USD ($)	Jun. 16, 2022 USD ($)	Apr. 30, 2022 USD ($)	Dec. 31, 2021 USD ($) letterOfCredit	Aug. 11, 2021 USD ($)	Jul. 13, 2021 USD ($)	Jul. 13, 2021 EUR (€)	Mar. 23, 2021 USD ($)
Debt Instrument
Repayments of notes														$ 36,700,000	$ 24,300,000
Subsidiary - HG Global | Interest Rate Cap
Debt Instrument
Notional amount																		$ 150,000,000				$ 150,000,000
Maximum interest rate																		10.76%				9.76%
Kudu | Interest Rate Cap
Debt Instrument
Notional amount																	$ 150,000,000
Maximum 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 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								$ 43,600,000				$ 42,700,000		43,600,000		€ 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		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																				$ 350,000,000								$ 300,000,000
Basis spread on variable rate			0.26%										0.25%
Repayments of notes								0		$ 0				0	$ 16,900,000
Amount borrowed								225,300,000				210,300,000		225,300,000
Stated margin			4.30%										4.30%						4.45%
Restricted cash								$ 14,800,000				$ 14,900,000		$ 14,800,000
Debt covenant, maximum loan to value percent								23.70%						23.70%		23.70%
Percentage of borrowing base								35%						35%		35%
Kudu Credit Facility | Secured Debt | Debt Covenant, Period 1
Debt Instrument
Debt covenant, loan to value percent								50%						50%		50%
Kudu Credit Facility | Secured Debt | Debt Covenant, Period 2
Debt Instrument
Debt covenant, loan to value percent								40%						40%		40%
Kudu Credit Facility | Secured Debt | Debt Covenant, Period 3
Debt Instrument
Debt covenant, loan to value percent								25%						25%		25%
Kudu Credit Facility | Secured Debt | Debt Covenant, Period 4
Debt Instrument
Debt covenant, loan to value percent								15%						15%		15%
Kudu Credit Facility | Secured Debt | Debt Covenant, Period 5
Debt Instrument
Debt covenant, loan to value percent								0%						0%		0%
Kudu Credit Facility | Secured Debt | Secured Overnight Financing Rate (SOFR)
Debt Instrument
Basis spread on variable rate												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		$ 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								107,900,000						107,900,000
Pledged collateral								141,300,000						141,300,000
Lloyds LOC Facility | Line of Credit | Letter of Credit
Debt Instrument
Total commitment under revolving credit facility																					$ 100,000,000
Letters of credit outstanding								35,500,000						35,500,000
Pledged collateral								66,900,000						$ 66,900,000
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		$ 150,000,000
Stated margin	6%
Anniversary of funding date	5 years
Annual payments																							$ 15,000,000
Interest accrued multiplier																						8
Interest reserve account, maximum																						$ 29,300,000
Interest reserve account								29,300,000				30,400,000		29,300,000
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								$ 22,600,000				$ 28,900,000		$ 22,600,000
Secured credit facilities | credit_facility								4						4		4

Debt - Schedule of the Change i

Debt - Schedule of the Change in Debt (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Kudu Credit Facility
Beginning balance			$ 564.6
Borrowings			15.5	$ 12.2
Repayments			(36.7)	(24.3)
Ending balance	$ 544.9		544.9
Kudu Credit Facility
Kudu Credit Facility
Beginning balance			203.8
Ending balance	218.5		218.5
Secured Debt | Kudu Credit Facility
Kudu Credit Facility
Beginning balance	210.3	$ 210.3	210.3	215.2
Borrowings	15	0	15	12
Repayments	0	0	0	(16.9)
Ending balance	$ 225.3	$ 210.3	$ 225.3	$ 210.3

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Investments, Owned, Federal Income Tax Note [Line Items]
Net deferred tax benefit					$ 68
Effective tax rate	(4.80%)	34.60%	6.60%	8.20%
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark
Investments, Owned, Federal Income Tax Note [Line Items]
Net deferred tax benefit					51
Financial Guarantee (HG Global) | HG Global
Investments, Owned, Federal Income Tax Note [Line Items]
Net deferred tax benefit					$ 17
Deferred tax liabilities	$ 5		$ 5
Tax benefit	$ 5

Derivatives (Details)

Derivatives (Details) - Interest Rate Cap	3 Months Ended		9 Months Ended
	Sep. 30, 2024 USD ($) agreement	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($) agreement	Sep. 30, 2023 USD ($)	Jul. 25, 2028	Jul. 25, 2025	Sep. 17, 2024 USD ($)	Aug. 22, 2024 USD ($)	Dec. 31, 2023 USD ($)	Jun. 16, 2022 USD ($)
Derivative [Line Items]
Number of interest rate cap agreements | agreement	2		2
Subsidiary - HG Global
Derivative [Line Items]
Notional amount								$ 150,000,000		$ 150,000,000
Premiums paid								$ 1,300,000		$ 3,300,000
Proceeds from Periodic Settlement of Derivatives	$ 700,000	$ 600,000	$ 2,100,000	$ 1,200,000
Derivative, fair value, net	2,100,000		2,100,000						$ 2,900,000
Subsidiary - HG Global | Forecast
Derivative [Line Items]
Derivative, cap interest rate					4.50%	3.50%
Kudu
Derivative [Line Items]
Notional amount							$ 150,000,000
Premiums paid							$ 900,000
Derivative, cap interest rate							4.50%
Derivative, fair value, net	500,000		500,000
Interest Expense | Subsidiary - HG Global
Derivative [Line Items]
Gain (loss) on fair value of derivative instruments	2,000,000	$ 0	2,100,000	$ 700,000
Interest Expense | Kudu
Derivative [Line Items]
Gain (loss) on fair value of derivative instruments	$ 100,000		$ 100,000

Municipal Bond Guarantee Insu_3

Municipal Bond Guarantee Insurance - Narrative (Details)	1 Months Ended		3 Months Ended		9 Months Ended
	May 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Jul. 01, 2024 USD ($)
Guarantor Obligations [Line Items]
Member surplus contributions, insurance premium charges, percentage			55%		55%
Direct				$ 16,500,000	$ 38,100,000	$ 37,300,000
Gross earned premiums				7,900,000	24,300,000	23,300,000
Assets held-in-trust		$ 948,500,000	$ 976,400,000		976,400,000
Collateral trusts, investments and cash		588,600,000	644,700,000		644,700,000
Surplus notes		322,200,000	317,100,000		317,100,000
Interest receivable		37,700,000	14,600,000		14,600,000
Debt		564,600,000	544,900,000		544,900,000
Change in fair value of BAM Surplus Notes			$ 15,800,000		$ 15,800,000	0
BAM Surplus Notes | Surplus Notes
Guarantor Obligations [Line Items]
Basis spread on variable rate					3%
Stated margin			8.20%		8.20%		10%
Repayments of the BAM Surplus Notes subsequent to the deconsolidation of BAM	$ 8,000,000	27,400,000		0		0
Repayment of principal	5,100,000	17,800,000
Accrued interest payment held in trust	300,000	2,000,000
Accrued interest payment held outside of trust	$ 2,600,000	7,600,000
Debt		322,200,000	$ 317,100,000		$ 317,100,000
Interest receivable		174,500,000	$ 192,700,000		$ 192,700,000
BAM Surplus Notes | Surplus Notes | Discount Rate
Guarantor Obligations [Line Items]
Debt instrument, measurement input			0.0750		0.0750		0.0815
Financial Guarantee (HG Global)
Guarantor Obligations [Line Items]
Direct			$ 14,000,000	14,200,000	$ 34,500,000	31,900,000
Gross earned premiums			7,500,000	6,600,000	21,500,000	19,400,000
Financial Guarantee (HG Global) | Operating Segments
Guarantor Obligations [Line Items]
Debt		146,900,000	147,300,000		147,300,000
Notes receivable, fair value disclosure		0	411,100,000		411,100,000		$ 387,400,000
Change in fair value of BAM Surplus Notes			$ 15,800,000	$ 0	15,800,000	$ 0
Subsidiary - HG Global
Guarantor Obligations [Line Items]
Purchase of surplus notes					$ 503,000,000
Percentage of risk premium ceded			60%		60%
Renegotiation period					5 years
Direct			$ 14,000,000		$ 14,000,000
Gross earned premiums			7,500,000		$ 7,500,000
Percent over a target trust balance					102%
Subsidiary - HG Global | Regulation 114
Guarantor Obligations [Line Items]
Assets held-in-trust		341,600,000	363,200,000		$ 363,200,000
Supplement trust target			603,000,000		603,000,000
Subsidiary - HG Global | Supplemental Trust
Guarantor Obligations [Line Items]
Assets held-in-trust		$ 606,900,000	$ 613,200,000		$ 613,200,000
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					$ 125,000,000

Municipal Bond Guarantee Insu_4

Municipal Bond Guarantee Insurance - Schedule of Municipal Bond Guarantee Insured Obligations (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2024 USD ($) contract	Dec. 31, 2023 USD ($) contract
HG Global
Guarantor Obligations [Line Items]
Contracts outstanding | contract	15,524	14,485
Remaining weighted average contract period (in years)	12 years 3 months 18 days	12 years 2 months 12 days
Outstanding first loss exposure from policies assumed (in millions)	$ 17,667,200	$ 16,341,000
Total gross unearned insurance premiums	$ 287,000	$ 273,900
HG/BAM
Guarantor Obligations [Line Items]
Remaining weighted average contract period (in years)		11 years 1 month 6 days
Outstanding first loss exposure from policies assumed (in millions)		$ 109,673,800
Total gross unearned insurance premiums		325,800
Total debt service outstanding		165,686,000
Interest		$ 56,012,200

Municipal Bond Guarantee Insu_5

Municipal Bond Guarantee Insurance - Schedule of Municipal Guarantee Insurance Contracts, Premium Received Over Contract Period (Details) - Variable Interest Entity, Primary Beneficiary - HG Global $ in Millions	Sep. 30, 2024 USD ($)
Guarantor Obligations [Line Items]
October 1, 2024 - December 31, 2024	$ 6.6
January 1, 2025 - March 31, 2025	6.4
April 1, 2025 - June 30, 2025	6.4
July 1, 2025 - September 30, 2025	6.2
October 1, 2025 - December 31, 2025	6.1
Total 2025	25.1
2026	23.7
2027	22.3
2028	20.8
2029	19.3
2030 and thereafter	169.2
Total gross unearned insurance premiums	$ 287

Municipal Bond Guarantee Insu_6

Municipal Bond Guarantee Insurance - Schedule of Net Written Premiums (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Written premiums:
Direct		$ 16.5	$ 24.1	$ 37.3
Assumed		0	14	0
Gross written premiums		16.5	38.1	37.3
Earned premiums:
Direct		7.4	15.8	21.4
Assumed		0.5	8.5	1.9
Gross earned premiums		7.9	24.3	23.3
Financial Guarantee (HG Global)
Written premiums:
Direct	$ 0	0	0	0
Assumed	14	14.2	34.5	31.9
Gross written premiums	14	14.2	34.5	31.9
Earned premiums:
Direct	0	0	0	0
Assumed	7.5	6.6	21.5	19.4
Gross earned premiums	$ 7.5	6.6	21.5	19.4
BAM
Written premiums:
Direct		16.5	24.1	37.3
Assumed		0	0	0
Gross written premiums		16.5	24.1	37.3
Earned premiums:
Direct		7.4	15.8	21.4
Assumed		0.5	1	1.9
Gross earned premiums		7.9	16.8	23.3
Eliminations
Written premiums:
Direct		0	0	0
Assumed		(14.2)	(20.5)	(31.9)
Gross written premiums		(14.2)	(20.5)	(31.9)
Earned premiums:
Direct		0	0	0
Assumed		(6.6)	(14)	(19.4)
Gross earned premiums		$ (6.6)	$ (14)	$ (19.4)

Earnings Per Share - Basic and

Earnings Per Share - Basic and Diluted Earnings Per Share from Continuing Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 179	$ 23.6	$ 360.8	$ 222.7
Allocation of losses (earnings) to participating restricted common shares	(2.5)	(0.3)	(4.6)	(3.1)
Basic earnings (losses) per share numerators, Total	176.5	23.3	356.2	219.6
Diluted earnings (losses) per share numerators, Total	$ 176.5	$ 23.3	$ 356.2	$ 219.6
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	2,568,100	2,560,500	2,564,800	2,564,200
Average unvested restricted common shares (in shares)	(35,400)	(37,600)	(32,900)	(35,500)
Basic earnings (losses) per share denominator (in shares)	2,532,700	2,522,900	2,531,900	2,528,700
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)	2,568,100	2,560,500	2,564,800	2,564,200
Average unvested restricted common shares (in shares)	(35,400)	(37,600)	(32,900)	(35,500)
Diluted earnings (losses) per share denominator (in shares)	2,532,700	2,522,900	2,531,900	2,528,700
Basic and diluted earnings per share (in dollars):
Distributed earnings - dividends declared and paid (in usd per share)	$ 0	$ 0	$ 1	$ 1
Distributed earnings - dividends declared and paid (in usd per share)	0	0	1	1
Undistributed earnings (losses) (in usd per share)	69.68	9.19	139.66	85.82
Undistributed earnings (losses) (in usd per share)	69.68	9.19	139.66	85.82
Basic and diluted earnings (losses) per share (in usd per share)	69.68	9.19	140.66	86.82
Basic and diluted earnings (losses) per share (in usd per share)	$ 69.68	$ 9.19	$ 140.66	$ 86.82

Earnings Per Share - Undistribu

Earnings Per Share - Undistributed Net Earnings From Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 176.5	$ 23.3	$ 356.2	$ 219.6
Dividends declared, net of restricted common share amounts	0	0	(2.5)	(2.5)
Total undistributed net earnings (losses), net of restricted common share amounts	$ 176.5	$ 23.3	$ 353.7	$ 217.1

Employee Share-Based Incentiv_3

Employee Share-Based Incentive Compensation Plans - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended		36 Months Ended
	Sep. 30, 2024 USD ($) shares	Sep. 30, 2023 USD ($) shares	Sep. 30, 2024 USD ($) shares	Sep. 30, 2023 USD ($) shares	Dec. 31, 2023 USD ($) shares	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2022 USD ($)
Share-based compensation arrangement by share-based payment award
New grants (in shares)	0	0	11,405	10,895
Performance Share
Share-based compensation arrangement by share-based payment award
Vesting period			3 years
Performance cycle period			3 years
New grants (in shares)					100
Additional compensation cost that would be recognized if all outstanding performance shares vested | $	$ 28.1		$ 28.1
Performance Share | Minimum
Share-based compensation arrangement by share-based payment award
Share-based compensation arrangement by share-based payment award, multiplier			0
Performance Share | Maximum
Share-based compensation arrangement by share-based payment award
Share-based compensation arrangement by share-based payment award, multiplier			2
Restricted Stock
Share-based compensation arrangement by share-based payment award
Vesting period			34 months
Additional compensation cost that would be recognized if all outstanding performance shares vested | $	$ 24.3	$ 20.2	$ 24.3	$ 20.2	$ 16.2	$ 28.8	$ 24.3	$ 15.5
Issued (in shares)	0	0	11,405	10,895

Employee Share-Based Incentiv_4

Employee Share-Based Incentive Compensation Plans - Performance Shares (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended		9 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Target Performance Shares Outstanding
Beginning of period (in shares)			34,992	37,031	37,031	39,449
Shares paid (in shares)			0	0	(13,475)	(13,350)
New grants (in shares)			0	0	11,405	10,895
Forfeitures and cancellations (in shares)			(133)	0	(102)	37
End of period (in shares)			34,859	37,031	34,859	37,031
Accrued Expense
Beginning of period			$ 52	$ 44.6	$ 69.4	$ 67.5
Shares paid			0	0	(44.9)	(40.8)
New grants			0	0	0	0
Forfeitures and cancellations			0	0	0.3	0.2
Expense recognized			3.7	10.5	30.9	28.2
End of period			$ 55.7	$ 55.1	$ 55.7	$ 55.1
Performance Share
Accrued Expense
Percentage of target payout - minimum actual	188%	200%

Employee Share-Based Incentiv_5

Employee Share-Based Incentive Compensation Plans - Performance Shares Granted Under the WTM Incentive Plan (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	35,390
Assumed forfeitures (in shares)	(531)
Total target performance shares outstanding (in shares)	34,859	34,992	37,031	37,031	37,031	39,449
Accrued Expense	$ 56.5
Assumed forfeitures	(0.8)
Total	$ 55.7	$ 52	$ 69.4	$ 55.1	$ 44.6	$ 67.5
2022 – 2024
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,150
Accrued Expense	$ 40.7
2023 – 2025
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	10,835
Accrued Expense	$ 11.8
2024 – 2026
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	11,405
Accrued Expense	$ 4

Employee Share-Based Incentiv_6

Employee Share-Based Incentive Compensation Plans - Restricted Shares (Details) - Restricted Stock - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Restricted Shares
Beginning of period (in shares)	35,525	37,595	37,595	38,350
Issued (in shares)	0	0	11,405	10,895
Vested (in shares)	0	0	(13,475)	(11,650)
Forfeited (in shares)	(135)	0	(135)	0
End of period (in shares)	35,390	37,595	35,390	37,595
Unamortized Issue Date Fair Value
Beginning of period	$ 28.8	$ 24.3	$ 16.2	$ 15.5
Issued	0	0	20	16
Vested	0	0	0	0
Forfeited	0	0	0	0
Expense recognized	(4.5)	(4.1)	(11.9)	(11.3)
End of period	$ 24.3	$ 20.2	$ 24.3	$ 20.2

Noncontrolling Interests (Detai

Noncontrolling Interests (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Noncontrolling Interest [Line Items]
Noncontrolling Equity	$ 649.2	$ 321.1
Ark
Noncontrolling Interest [Line Items]
Noncontrolling Equity	397.9	336.9
Noncontrolling equity attributable to management's equity incentives	40.3	21
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling Equity	(9.3)	0.8
Kudu
Noncontrolling Interest [Line Items]
Noncontrolling Equity	133.7	113.8
Bamboo
Noncontrolling Interest [Line Items]
Noncontrolling Equity	117.6	0
Other
Noncontrolling Interest [Line Items]
Noncontrolling Equity	9.3	9.4
Total, excluding BAM
Noncontrolling Interest [Line Items]
Noncontrolling Equity	649.2	460.9
BAM
Noncontrolling Interest [Line Items]
Noncontrolling Equity	$ 0	$ (139.8)
Ark
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	28%	28%
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	3.10%	3.10%
Kudu
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	9.90%	10.50%
Bamboo
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	27.20%	0%
BAM
Noncontrolling Interest [Line Items]
Noncontrolling Percentage	0%	100%

Segment Information - Narrative

Segment Information - Narrative (Details)	9 Months Ended
	Sep. 30, 2024 segment
Segment Reporting [Abstract]
Number of reportable segments	4

Segment Information - Financial

Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Earned insurance premiums	$ 581.5	$ 506.8	$ 1,244.4	$ 1,070.6
Net investment income	57.7	48.1	172.1	130.1
Interest income (expense) from BAM Surplus Notes		0	7.9	0
Net realized and unrealized investment gains (losses)	223.8	(58.3)	390.4	151.2
Interest income from BAM Surplus Notes	7.9
Change in fair value of BAM Surplus Notes	15.8		15.8	0
Unrealized loss on deconsolidation of BAM	(114.5)		(114.5)	0
Commission and fee revenues	46.8	3.5	108.4	10
Other revenues	20.1	19.5	57.3	68
Total revenues	839.1	519.6	1,881.8	1,429.9
Loss and loss adjustment expenses	296.9	265.8	666.6	581.1
Acquisition expenses	106.4	92.4	253.9	217.8
Cost of sales	7.6	8	22.2	33.5
Broker commission expenses	15.9		37.9
General and administrative expenses	121.9	99.6	369.6	298.2
Change in fair value of contingent consideration	34.2	17	47.5	16.8
Interest expense	16.7	15.7	46.7	44.9
Total expenses	599.6	498.5	1,444.4	1,192.3
Pre-tax income (loss)	239.5	21.1	437.4	237.6
Other Operations
Segment Reporting Information [Line Items]
Earned insurance premiums	11.2	0	19.8	0
Net investment income	9.6	8	27.9	22
Interest income (expense) from BAM Surplus Notes		0	0	0
Interest income from BAM Surplus Notes	0
Change in fair value of BAM Surplus Notes	0		0
Unrealized loss on deconsolidation of BAM	0		0
Commission and fee revenues	4.1	3.5	11.1	10
Other revenues	14.8	15.2	43.7	67.1
Total revenues	157.6	(12.1)	322.6	186
Loss and loss adjustment expenses	4.2	0	8.1	0
Acquisition expenses	4.5	0	7.1	0
Cost of sales	7.6	8	22.2	33.5
Broker commission expenses	0		0
General and administrative expenses	32.9	42.2	126.4	130.7
Change in fair value of contingent consideration	0	0	0	0
Interest expense	0.3	0.9	1.6	2.9
Total expenses	49.5	51.1	165.4	167.1
Pre-tax income (loss)	108.1	(63.2)	157.2	18.9
Investments, Excluding MediaAlpha
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	135.6	(11.5)	230.7	190.1
Investments, Excluding MediaAlpha | Other Operations
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	29.7	8	60.4	125.8
MediaAlpha
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	88.2	(46.8)	159.7	(38.9)
MediaAlpha | Other Operations
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	88.2	(46.8)	159.7	(38.9)
Financial Guarantee (HG Global) | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	7.5	7.9	24.3	23.3
Net investment income	6	8.1	26.1	22.9
Net realized and unrealized investment gains (losses)	22.5	(24.1)	8.1	(17)
Interest income from BAM Surplus Notes	7.9	0	7.9	0
Change in fair value of BAM Surplus Notes	15.8	0	15.8	0
Unrealized loss on deconsolidation of BAM	(114.5)	0	(114.5)	0
Other revenues	0	0.7	1.1	2
Total revenues	(54.8)	(7.4)	(31.2)	31.2
Acquisition expenses	1.9	2.1	6.3	6.3
General and administrative expenses	0.3	17.5	34.8	49.7
Interest expense	5.8	3.8	13.4	10.8
Total expenses	8	23.4	54.5	66.8
Financial Guarantee (HG Global) | HG Global | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	7.5	6.6	21.5	19.4
Net investment income	6	4.3	17.3	12.4
Interest income (expense) from BAM Surplus Notes		6.6	21.1	19.7
Interest income from BAM Surplus Notes	7.9
Change in fair value of BAM Surplus Notes	15.8		15.8
Unrealized loss on deconsolidation of BAM	(114.5)		(114.5)
Commission and fee revenues	0	0	0	0
Other revenues	0	0	0	0
Total revenues	(54.8)	3.9	(25.6)	40.1
Loss and loss adjustment expenses	0	0	0	0
Acquisition expenses	1.9	2	5.9	5.6
Cost of sales	0	0	0	0
Broker commission expenses	0		0
General and administrative expenses	0.3	0.5	1.3	1.9
Change in fair value of contingent consideration	0	0	0	0
Interest expense	5.8	3.8	13.4	10.8
Total expenses	8	6.3	20.6	18.3
Pre-tax income (loss)	(62.8)	(2.4)	(46.2)	21.8
Financial Guarantee (HG Global) | HG Global | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	22.5	(13.6)	13.2	(11.4)
Financial Guarantee (HG Global) | HG Global | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0	0	0	0
Financial Guarantee (HG Global) | BAM | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums		1.3	2.8	3.9
Net investment income		3.8	8.8	10.5
Interest income (expense) from BAM Surplus Notes		(6.6)	(13.2)	(19.7)
Change in fair value of BAM Surplus Notes			0
Unrealized loss on deconsolidation of BAM			0
Commission and fee revenues		0	0	0
Other revenues		0.7	1.1	2
Total revenues		(11.3)	(5.6)	(8.9)
Loss and loss adjustment expenses		0	0	0
Acquisition expenses		0.1	0.4	0.7
Cost of sales		0	0	0
Broker commission expenses			0
General and administrative expenses		17	33.5	47.8
Change in fair value of contingent consideration		0	0	0
Interest expense		0	0	0
Total expenses		17.1	33.9	48.5
Pre-tax income (loss)		(28.4)	(39.5)	(57.4)
Financial Guarantee (HG Global) | BAM | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)		(10.5)	(5.1)	(5.6)
Financial Guarantee (HG Global) | BAM | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)		0	0	0
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment Reporting Information [Line Items]
Loss and loss adjustment expenses	288.3	265.8	644	581.1
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	552.2	498.9	1,173.3	1,047.3
Net investment income	24.3	16.9	66.5	41.2
Net realized and unrealized investment gains (losses)	53.2	(6.6)	84.1	35.9
Other revenues	3.7	3.6	9.6	(1.1)
Total revenues	633.4	512.8	1,333.5	1,123.3
Loss and loss adjustment expenses	288.3	265.8	644	581.1
Acquisition expenses	96.3	90.3	230.8	211.5
General and administrative expenses	68.9	35.4	154	105.5
Change in fair value of contingent consideration	34.2	17	47.5	16.8
Interest expense	4.9	5.5	15	15.7
Total expenses	492.6	414	1,091.3	930.6
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | UNITED KINGDOM
Segment Reporting Information [Line Items]
Earned insurance premiums	326.4	273.6	697.5	588.3
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | BERMUDA
Segment Reporting Information [Line Items]
Earned insurance premiums	180.8	164.7	413	383.6
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	507.2	438.3	1,110.5	971.9
Net investment income	21.3	13.9	57.6	33.5
Interest income (expense) from BAM Surplus Notes		0	0	0
Interest income from BAM Surplus Notes	0
Change in fair value of BAM Surplus Notes	0		0
Unrealized loss on deconsolidation of BAM	0		0
Commission and fee revenues	0	0	0	0
Other revenues	3.7	3.6	9.6	(1.1)
Total revenues	585.4	449.2	1,261.8	1,040.2
Loss and loss adjustment expenses	275.1	258.5	630.4	573.2
Acquisition expenses	83.3	71	212.9	189.3
Cost of sales	0	0	0	0
Broker commission expenses	0		0
General and administrative expenses	68.9	35.3	153.9	105.3
Change in fair value of contingent consideration	34.2	17	47.5	16.8
Interest expense	4.9	5.5	15	15.7
Total expenses	466.4	387.3	1,059.7	900.3
Pre-tax income (loss)	119	61.9	202.1	139.9
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	53.2	(6.6)	84.1	35.9
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0	0	0	0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	45	60.6	62.8	75.4
Net investment income	3	3	8.9	7.7
Interest income (expense) from BAM Surplus Notes		0	0	0
Interest income from BAM Surplus Notes	0
Change in fair value of BAM Surplus Notes	0		0
Unrealized loss on deconsolidation of BAM	0		0
Commission and fee revenues	0	0	0	0
Other revenues	0	0	0	0
Total revenues	48	63.6	71.7	83.1
Loss and loss adjustment expenses	13.2	7.3	13.6	7.9
Acquisition expenses	13	19.3	17.9	22.2
Cost of sales	0	0	0	0
Broker commission expenses	0		0
General and administrative expenses	0	0.1	0.1	0.2
Change in fair value of contingent consideration	0	0	0	0
Interest expense	0	0	0	0
Total expenses	26.2	26.7	31.6	30.3
Pre-tax income (loss)	21.8	36.9	40.1	52.8
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0	0	0	0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0	0	0	0
Asset Management (Kudu) | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	0	0	0	0
Net investment income	17.2	15.1	50.1	44
Interest income (expense) from BAM Surplus Notes		0	0	0
Net realized and unrealized investment gains (losses)	29.5	11.2	77.5	45.4
Interest income from BAM Surplus Notes	0
Change in fair value of BAM Surplus Notes	0		0
Unrealized loss on deconsolidation of BAM	0		0
Commission and fee revenues	0	0	0	0
Other revenues	0.5	0	0.5	0
Total revenues	47.2	26.3	128.1	89.4
Loss and loss adjustment expenses	0	0	0	0
Acquisition expenses	0	0	0	0
Cost of sales	0	0	0	0
Broker commission expenses	0		0
General and administrative expenses	3.6	4.5	10.5	12.3
Change in fair value of contingent consideration	0	0	0	0
Interest expense	5.7	5.5	16.7	15.5
Total expenses	9.3	10	27.2	27.8
Pre-tax income (loss)	37.9	16.3	100.9	61.6
Asset Management (Kudu) | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	29.5	11.2	77.5	45.4
Asset Management (Kudu) | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0	0	0	0
Bamboo | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	10.6	0	27	0
Net investment income	0.6		1.5
Interest income (expense) from BAM Surplus Notes			0
Interest income from BAM Surplus Notes	0
Change in fair value of BAM Surplus Notes	0		0
Unrealized loss on deconsolidation of BAM	0		0
Commission and fee revenues	42.7	0	97.3	0
Other revenues	2.4	0	4.5	0
Total revenues	55.7	0	128.8	0
Loss and loss adjustment expenses	4.4	0	14.5	0
Acquisition expenses	3.7	0	9.7	0
Cost of sales	0		0
Broker commission expenses	15.9	0	37.9	0
General and administrative expenses	16.2	0	43.9	0
Change in fair value of contingent consideration	0		0
Interest expense	0		0
Total expenses	40.2	$ 0	106	$ 0
Pre-tax income (loss)	15.5		22.8
Bamboo | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0.7		0.6
Bamboo | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0		0
Bamboo Ide8 Services, LLC - Segment, Net Of Reconciling Items | Operating Segments
Segment Reporting Information [Line Items]
Other revenues	$ 1.1		$ 2.4

Variable Interest Entities (Det

Variable Interest Entities (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2024	Jul. 01, 2024	Apr. 30, 2024	Dec. 31, 2023
Variable Interest Entity [Line Items]
Assets held-in-trust	$ 976.4			$ 948.5
BAM Surplus Notes | Surplus Notes
Variable Interest Entity [Line Items]
Stated margin	8.20%	10%
Bamboo CRV
Variable Interest Entity [Line Items]
Expected capital contributions	$ 30		$ 30
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%
Outrigger Re
Variable Interest Entity [Line Items]
Short-term investments, at fair value	$ 199.9			$ 265.3
PassportCard/DavidShield | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Ownership interest	53.80%
Partnership interest, carrying value	$ 159.2
Elementum | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Ownership interest	26.60%
Partnership interest, carrying value	$ 35
Limited Partnerships | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Partnership interest, carrying value	$ 262

Equity Method Eligible Invest_3

Equity Method Eligible Investments - Ownership Interests and Carrying Values (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Kudu's Participation Contracts
Investments in unconsolidated affiliates
Carrying Value	$ 930.6	$ 890.5
Kudu's Participation Contracts | Minimum
Investments in unconsolidated affiliates
Ownership Interest	4.10%	4.10%
Kudu's Participation Contracts | Maximum
Investments in unconsolidated affiliates
Ownership Interest	30%	30%
MediaAlpha
Investments in unconsolidated affiliates
Ownership Interest	26.80%	34.90%
Carrying Value	$ 323.4	$ 254.9
PassportCard/DavidShield
Investments in unconsolidated affiliates
Ownership Interest	53.80%	53.80%
Carrying Value	$ 150	$ 150
Elementum
Investments in unconsolidated affiliates
Ownership Interest	26.60%	26.60%
Carrying Value	$ 35	$ 35
Other equity method eligible investments, at fair value
Investments in unconsolidated affiliates
Carrying Value	243.9	291.7
Other equity method eligible investments, at fair value
Investments in unconsolidated affiliates
Carrying Value	$ 0	$ 24.8

Equity Method Eligible Invest_4

Equity Method Eligible Investments - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Equity Method Investments and Joint Ventures [Abstract]
Dividend and income distributions from equity method eligible investments	$ 19.7	$ 13.2	$ 57.9	$ 42.1

Fair Value of Financial Instr_3

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Balance Sheet Grouping, Financial Statement Captions
Carrying Value	$ 544.9	$ 564.6
Ark 2007 Subordinated Notes
Balance Sheet Grouping, Financial Statement Captions
Fair Value	0	30.5
Carrying Value	0	30
Ark 2021 Subordinated Notes
Balance Sheet Grouping, Financial Statement Captions
Fair Value	173.7	171.8
Carrying Value	156.8	155.5
HG Global Senior Notes
Balance Sheet Grouping, Financial Statement Captions
Fair Value	158.1	158.7
Carrying Value	147.3	146.9
Kudu Credit Facility
Balance Sheet Grouping, Financial Statement Captions
Fair Value	232.6	225.6
Carrying Value	218.5	203.8
Other Operations debt
Balance Sheet Grouping, Financial Statement Captions
Fair Value	23.4	30
Carrying Value	$ 22.3	$ 28.4