CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Investments [Abstract]
Other long-term investments	$ 1,488.0	$ 1,377.8
Total investments	5,170.0	4,257.5
Total assets	7,389.3	7,000.7
Liabilities
Debt	575.2	420.9
Total liabilities	3,454.3	3,296.0
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 2,572,156 and 3,017,772 shares	2.6	3.0
Paid-in surplus	536.0	585.9
Retained earnings	3,211.8	2,957.5
Accumulated other comprehensive gain (loss), after-tax:
Net unrealized gains (losses) from foreign currency translation and interest rate swap	(3.5)	1.7
Total White Mountains’s common shareholders’ equity	3,746.9	3,548.1
Non-controlling interests	188.1	156.6
Total equity	3,935.0	3,704.7
Total liabilities and equity	7,389.3	7,000.7
HG Global-BAM
Investments [Abstract]
Fixed maturity investments, at fair value	909.9	934.1
Short-term investments, at fair value	65.9	32.4
Total investments	975.8	966.5
Cash	18.2	19.8
Insurance premiums receivable	6.6	6.9
Deferred acquisition costs	36.0	33.1
Other assets	21.9	18.5
Total assets	1,058.5	1,044.8
Liabilities
Unearned insurance premiums	298.3	266.3
Debt	146.5	0.0
Accrued incentive compensation	28.0	24.7
Other liabilities	29.0	30.9
Total liabilities	501.8	321.9
Ark
Investments [Abstract]
Fixed maturity investments, at fair value	772.8	688.6
Common equity securities, at fair value	334.6	251.1
Short-term investments, at fair value	280.9	296.2
Other long-term investments	373.6	326.2
Total investments	1,761.9	1,562.1
Cash	100.0	67.8
Reinsurance recoverables	536.1	448.4
Insurance premiums receivable	544.1	416.0
Ceded unearned premiums	59.2	67.1
Deferred acquisition costs	127.2	108.2
Goodwill and other intangible assets	292.5	292.5
Other assets	65.2	64.9
Total assets	3,486.2	3,027.0
Liabilities
Loss and loss adjustment expense reserves	1,296.5	894.7
Unearned insurance premiums	623.2	495.9
Debt	183.7	185.9
Reinsurance payable	251.1	424.1
Contingent consideration	45.3	28.0
Other liabilities	121.1	93.8
Total liabilities	2,520.9	2,122.4
Accumulated other comprehensive gain (loss), after-tax:
Non-controlling interests	247.9	230.7
Kudu
Investments [Abstract]
Other long-term investments	695.9	669.5
Cash	101.4	21.4
Accrued investment income	12.4	16.9
Goodwill and other intangible assets	8.6	8.9
Other assets	7.6	10.4
Total assets	825.9	727.1
Liabilities
Debt	208.3	218.2
Other liabilities	65.0	42.8
Total liabilities	273.3	261.0
Other Operations
Investments [Abstract]
Fixed maturity investments, at fair value	238.2	286.2
Common equity securities, at fair value	333.8	0.0
Short-term investments, at fair value	577.3	129.5
Investment in MediaAlpha, at fair value	168.6	261.6
Other long-term investments	418.5	382.1
Total investments	1,736.4	1,059.4
Cash	35.4	38.7
Goodwill and other intangible assets	91.3	39.1
Other assets	155.6	59.5
Assets held for sale - NSM Group	0.0	989.0
Assets held for sale - Other	0.0	16.1
Total assets	2,018.7	2,201.8
Liabilities
Debt	36.7	16.8
Accrued incentive compensation	86.1	48.5
Other liabilities	35.5	30.1
Liabilities held for sale - NSM Group	0.0	495.3
Total liabilities	$ 158.3	$ 590.7