CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Investments [Abstract]
Other long-term investments	$ 1,377.8	$ 786.8
Total investments	4,265.3	2,939.0
Total assets	7,000.7	4,831.4
Liabilities
Debt	693.0	376.4
Total liabilities	3,296.0	1,013.5
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 3,017,772 and 3,102,011 shares	3.0	3.1
Paid-in surplus	585.9	592.1
Retained earnings	2,957.5	3,311.2
Accumulated other comprehensive gain (loss), after-tax:
Net unrealized foreign currency translation and interest rate swap gains (losses)	1.7	(0.4)
Total White Mountains’s common shareholders’ equity	3,548.1	3,906.0
Non-controlling interests	156.6	(88.1)
Total equity	3,704.7	3,817.9
Total liabilities and equity	7,000.7	4,831.4
HG Global-BAM
Investments [Abstract]
Fixed maturity investments, at fair value	934.1	859.5
Short-term investments, at fair value	32.4	60.4
Total investments	966.5	919.9
Cash	19.8	42.8
Insurance premiums receivable	6.9	6.9
Deferred acquisition costs	33.1	27.8
Other assets	18.5	20.4
Total assets	1,044.8	1,017.8
Liabilities
Unearned insurance premiums	266.3	237.5
Accrued incentive compensation	24.7	25.7
Other liabilities	30.9	28.3
Total liabilities	321.9	291.5
Ark
Investments [Abstract]
Fixed maturity investments, at fair value	688.6	0.0
Short-term investments, at fair value	296.2	0.0
Common equity securities, at fair value	251.1	0.0
Other long-term investments	326.2	0.0
Total investments	1,562.1	0.0
Cash	67.8	0.0
Reinsurance recoverables	448.4	0.0
Cash pre-funded/placed in escrow for Ark Transaction		366.3
Insurance premiums receivable	416.0	0.0
Ceded unearned premiums	67.1	0.0
Deferred acquisition costs	108.2	0.0
Goodwill and other intangible assets	292.5	0.0
Other assets	64.9	0.0
Total assets	3,027.0	0.0
Liabilities
Loss and loss adjustment expense reserves	894.7	0.0
Unearned insurance premiums	495.9	0.0
Debt	185.9	0.0
Reinsurance payable	424.1	0.0
Contingent consideration	28.0	0.0
Other liabilities	93.8	0.0
Total liabilities	2,122.4	0.0
Accumulated other comprehensive gain (loss), after-tax:
Non-controlling interests	230.7	0.0
NSM
Investments [Abstract]
Cash	111.5	126.5
Premium and commission receivable	85.0	76.7
Goodwill and other intangible assets	725.4	736.8
Other assets	55.4	59.6
Total assets	977.3	999.6
Liabilities
Debt	272.1	272.6
Premiums payable	135.9	113.4
Contingent consideration	6.8	14.6
Other liabilities	80.5	91.2
Total liabilities	495.3	491.8
Kudu
Investments [Abstract]
Other long-term investments	669.5	400.6
Cash	21.4	7.8
Accrued investment income	16.9	9.8
Goodwill and other intangible assets	8.9	9.2
Other assets	10.4	2.8
Total assets	727.1	430.2
Liabilities
Debt	218.2	86.3
Other liabilities	42.8	10.0
Total liabilities	261.0	96.3
Other Operations
Investments [Abstract]
Fixed maturity investments, at fair value	286.2	347.7
Short-term investments, at fair value	137.3	82.4
Investment in MediaAlpha, at fair value	261.6	802.2
Other long-term investments	382.1	386.2
Total investments	1,067.2	1,618.5
Cash	38.8	34.1
Cash pre-funded/placed in escrow for Ark Transaction	0.0	646.3
Goodwill and other intangible assets	39.1	36.4
Other assets	63.3	46.2
Assets held for sale	16.1	2.3
Total assets	1,224.5	2,383.8
Liabilities
Debt	16.8	17.5
Accrued incentive compensation	48.5	70.1
Other liabilities	30.1	46.3
Total liabilities	$ 95.4	$ 133.9