CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Assets
Common equity securities, at fair value	$ 483.0	$ 650.0
Other long-term investments	2,958.5	2,150.2
Total investments	8,323.9	6,417.6
Goodwill and other intangible assets	1,020.2	720.3
Total assets	12,306.5	9,925.6
Liabilities
Debt	837.0	562.5
Total liabilities	6,051.4	4,794.6
Redeemable noncontrolling interests	131.5	0.0
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 2,479,677 and 2,568,148 shares	2.5	2.6
Paid-in surplus	576.5	563.8
Retained earnings	4,845.6	3,919.0
Accumulated other comprehensive income (loss), after-tax:
Net unrealized gains (losses) from foreign currency translation	0.8	(1.7)
Total White Mountains’s common shareholders’ equity	5,425.4	4,483.7
Nonredeemable noncontrolling interests	698.2	647.3
Total equity	6,123.6	5,131.0
Total liabilities, redeemable noncontrolling interests and equity	12,306.5	9,925.6
Other Operations
Assets
Fixed maturity investments, at fair value	159.2	293.7
Common equity securities, at fair value	30.7	224.6
Investment in MediaAlpha, at fair value	231.2	201.6
Short-term investments, at fair value	807.4	260.1
Other long-term investments	977.4	588.4
Total investments	2,205.9	1,568.4
Cash	42.8	37.6
Goodwill and other intangible assets	142.3	64.8
Other assets	181.9	75.8
Total assets	2,577.9	2,338.8
Liabilities
Loss and loss adjustment expense reserves	13.6	12.1
Unearned insurance premiums	9.6	29.0
Debt	38.3	22.0
Accrued incentive compensation	102.9	79.3
Other liabilities	101.4	33.0
Total liabilities	269.4	349.0
Redeemable noncontrolling interests	0.0
Accumulated other comprehensive income (loss), after-tax:
Total White Mountains’s common shareholders’ equity	2,272.0	1,867.1
Nonredeemable noncontrolling interests	36.5	122.7
Other Operations | Bamboo Group
Assets
Total investments		58.0
Assets held for sale	0.0	585.7
Liabilities
Total liabilities held for sale	0.0	167.7
Other Operations | Other Disposal Group
Assets
Total investments	1.0	2.0
Assets held for sale	5.0	6.5
Liabilities
Total liabilities held for sale	3.6	5.9
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liabilities
Loss and loss adjustment expense reserves	2,481.0	2,127.5
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Assets
Fixed maturity investments, at fair value	1,917.9	1,565.1
Common equity securities, at fair value	452.3	425.4
Short-term investments, at fair value	866.6	601.4
Other long-term investments	689.7	547.8
Total investments	3,926.5	3,139.7
Cash	104.8	141.2
Reinsurance recoverables	836.1	589.0
Insurance premiums receivable	848.4	768.6
Deferred acquisition costs	211.1	165.2
Goodwill and other intangible assets	292.5	292.5
Other assets	134.7	202.8
Total assets	6,354.1	5,299.0
Liabilities
Loss and loss adjustment expense reserves	2,481.0	2,127.5
Unearned insurance premiums	1,026.1	853.3
Debt	159.7	154.5
Reinsurance payable	286.2	149.5
Contingent consideration	328.3	155.3
Other liabilities	247.2	224.7
Total liabilities	4,528.5	3,664.8
Redeemable noncontrolling interests	0.0
Accumulated other comprehensive income (loss), after-tax:
Total White Mountains’s common shareholders’ equity	1,360.4	1,223.8
Nonredeemable noncontrolling interests	465.2	410.4
Financial Guarantee (HG Global) | Operating Segments
Assets
Fixed maturity investments, at fair value	693.4	612.1
Short-term investments, at fair value	90.8	55.5
Total investments	784.2	667.6
Cash	0.1	11.5
BAM Surplus Notes, at fair value	339.0	381.7
Insurance premiums receivable	11.4	4.4
Deferred acquisition costs	96.9	86.6
Other assets	5.2	27.6
Total assets	1,236.8	1,179.4
Liabilities
Unearned insurance premiums	327.9	297.3
Debt	147.8	147.4
Other liabilities	23.8	19.4
Total liabilities	499.5	464.1
Redeemable noncontrolling interests	0.0
Accumulated other comprehensive income (loss), after-tax:
Total White Mountains’s common shareholders’ equity	755.7	728.7
Nonredeemable noncontrolling interests	(18.4)	(13.4)
Asset Management (Kudu) | Operating Segments
Assets
Short-term investments, at fair value	21.9	27.9
Other long-term investments	1,291.4	1,014.0
Total investments	1,313.3	1,041.9
Cash	34.5	0.6
Accrued investment income	25.3	18.0
Goodwill and other intangible assets	7.7	8.0
Other assets	21.5	39.9
Total assets	1,402.3	1,108.4
Liabilities
Debt	350.4	238.6
Other liabilities	96.5	78.1
Total liabilities	446.9	316.7
Redeemable noncontrolling interests	0.0
Accumulated other comprehensive income (loss), after-tax:
Total White Mountains’s common shareholders’ equity	814.2	664.1
Nonredeemable noncontrolling interests	141.2	127.6
Specialty Insurance Distribution (Distinguished) | Operating Segments
Assets
Short-term investments, at fair value	94.0	0.0
Total investments	94.0	0.0
Cash	2.7	0.0
Premiums, commissions and fees receivable	45.7	0.0
Goodwill and other intangible assets	577.7	0.0
Other assets	15.3	0.0
Total assets	735.4	0.0
Liabilities
Debt	140.8	0.0
Premiums and commissions payable	81.3	0.0
Other liabilities	85.0	0.0
Total liabilities	307.1	0.0
Redeemable noncontrolling interests	131.5
Accumulated other comprehensive income (loss), after-tax:
Total White Mountains’s common shareholders’ equity	223.1	0.0
Nonredeemable noncontrolling interests	$ 73.7	$ 0.0