CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Investments [Abstract]
Common equity securities, at fair value	$ 538.4	$ 668.4
Other long-term investments	1,998.2	1,488.0
Total investments	6,388.7	5,170.0
Goodwill and other intangible assets	370.6	392.4
Total assets	8,385.9	7,389.3
Liabilities
Debt	564.6	575.2
Total liabilities	3,824.3	3,454.3
White Mountains’s common shareholders’ equity
White Mountains’s common shares at $1 par value per share—authorized 50,000,000 shares; issued and outstanding 2,560,452 and 2,572,156 shares	2.6	2.6
Paid-in surplus	548.7	536.0
Retained earnings	3,690.8	3,211.8
Accumulated other comprehensive income (loss), after-tax:
Net unrealized gains (losses) from foreign currency translation and interest rate swap	(1.6)	(3.5)
Total White Mountains’s common shareholders’ equity	4,240.5	3,746.9
Noncontrolling interests	321.1	188.1
Total equity	4,561.6	3,935.0
Total liabilities and equity	8,385.9	7,389.3
Ark/WM Outrigger
Liabilities
Loss and loss adjustment expense reserves	1,605.1	1,296.5
Operating Segments | HG Global/BAM - Segment
Investments [Abstract]
Fixed maturity investments, at fair value	1,012.3	909.9
Short-term investments, at fair value	70.6	65.9
Total investments	1,082.9	975.8
Cash	6.7	18.2
Insurance premiums receivable	5.5	6.6
Deferred acquisition costs	40.1	36.0
Other assets	36.8	21.9
Total assets	1,172.0	1,058.5
Liabilities
Unearned insurance premiums	325.8	298.3
Debt	146.9	146.5
Accrued incentive compensation	27.2	28.0
Other liabilities	31.8	29.0
Total liabilities	531.7	501.8
Accumulated other comprehensive income (loss), after-tax:
Total White Mountains’s common shareholders’ equity	779.3	712.0
Noncontrolling interests	(139.0)	(155.3)
Operating Segments | Ark/WM Outrigger
Investments [Abstract]
Fixed maturity investments, at fair value	866.8	772.8
Common equity securities, at fair value	400.6	334.6
Short-term investments, at fair value	962.8	484.6
Other long-term investments	440.9	373.6
Total investments	2,671.1	1,965.6
Cash	90.5	101.5
Reinsurance recoverables	442.0	595.3
Insurance premiums receivable	612.2	544.1
Deferred acquisition costs	145.3	127.2
Goodwill and other intangible assets	292.5	292.5
Other assets	125.0	65.2
Total assets	4,378.6	3,691.4
Liabilities
Loss and loss adjustment expense reserves	1,605.1	1,296.5
Unearned insurance premiums	743.6	623.2
Debt	185.5	183.7
Reinsurance payable	81.1	251.1
Contingent consideration	94.0	45.3
Other liabilities	166.8	122.3
Total liabilities	2,876.1	2,522.1
Accumulated other comprehensive income (loss), after-tax:
Total White Mountains’s common shareholders’ equity	1,165.6	921.4
Noncontrolling interests	336.9	247.9
Operating Segments | Kudu
Investments [Abstract]
Short-term investments, at fair value	29.3	0.0
Other long-term investments	896.3	695.9
Total investments	925.6	695.9
Cash	1.4	101.4
Accrued investment income	17.6	12.4
Goodwill and other intangible assets	8.3	8.6
Other assets	6.5	7.6
Total assets	959.4	825.9
Liabilities
Debt	203.8	208.3
Other liabilities	71.6	65.0
Total liabilities	275.4	273.3
Accumulated other comprehensive income (loss), after-tax:
Total White Mountains’s common shareholders’ equity	570.2	477.5
Noncontrolling interests	113.8	75.1
Other Operations
Investments [Abstract]
Fixed maturity investments, at fair value	230.2	238.2
Common equity securities, at fair value	137.8	333.8
Investment in MediaAlpha, at fair value	254.9	168.6
Short-term investments, at fair value	425.2	373.6
Other long-term investments	661.0	418.5
Total investments	1,709.1	1,532.7
Cash	23.8	33.9
Goodwill and other intangible assets	69.8	91.3
Other assets	73.2	155.6
Total assets	1,875.9	1,813.5
Liabilities
Debt	28.4	36.7
Accrued incentive compensation	87.7	86.1
Other liabilities	25.0	34.3
Total liabilities	141.1	157.1
Accumulated other comprehensive income (loss), after-tax:
Total White Mountains’s common shareholders’ equity	1,725.4	1,636.0
Noncontrolling interests	$ 9.4	$ 20.4