CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Investments [Abstract]
Other long-term investments	$ 786.8
Total investments	2,939.1	$ 2,947.2
Goodwill and other intangible assets	782.4	656.7
Total assets	4,831.4	3,983.2
Liabilities
Debt	376.4	283.5
Total liabilities	1,013.5	838.5
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 3,102,011 and 3,185,353 shares	3.1	3.2
Paid-in surplus	592.1	593.1
Retained earnings	3,311.2	2,672.4
Accumulated other comprehensive loss, after-tax:
Net unrealized losses from foreign currency translation and interest rate swap	(0.4)	(7.2)
Total White Mountains’s common shareholders’ equity	3,906.0	3,261.5
Non-controlling interests	(88.1)	(116.8)
Total equity	3,817.9	3,144.7
Total liabilities and equity	4,831.4	3,983.2
HG Global-BAM
Investments [Abstract]
Fixed maturity investments, at fair value	859.5	799.3
Short-term investments, at fair value	60.4	46.3
Total investments	919.9	845.6
Cash	42.8	24.2
Insurance premiums receivable	6.9	6.7
Deferred acquisition costs	27.8	22.1
Accrued investment income	5.0	5.4
Accounts receivable on unsettled investment sales	0.0	3.9
Other assets	15.4	16.1
Total assets	1,017.8	924.0
Liabilities
Unearned insurance premiums	237.5	198.4
Accrued incentive compensation	25.7	21.7
Other liabilities	28.3	26.7
Total liabilities	291.5	246.8
NSM
Investments [Abstract]
Cash	126.5	89.7
Premium and commission receivable	76.7	70.8
Goodwill and other intangible assets	736.8	623.0
Other assets	59.6	41.7
Total assets	999.6	825.2
Liabilities
Debt	272.6	219.2
Premiums payable	113.4	102.3
Contingent consideration earnout liabilities	14.6	20.6
Other liabilities	91.2	59.0
Total liabilities	491.8	401.1
Kudu
Investments [Abstract]
Short-term investments, at fair value	0.1	0.1
Other long-term investments	400.6	266.5
Total investments	400.7	266.6
Cash	7.8	5.8
Accrued investment income	9.8	5.8
Goodwill and other intangible assets	9.2	9.6
Other assets	2.7	2.7
Total assets	430.2	290.5
Liabilities
Debt	86.3	53.6
Other liabilities	10.0	3.4
Total liabilities	96.3	57.0
Other Operations
Investments [Abstract]
Fixed maturity investments, at fair value	347.7	406.5
Short-term investments, at fair value	82.4	154.8
Investment in MediaAlpha, at fair value	802.2	180.0
Common equity securities, at fair value	0.0	683.9
Other long-term investments	386.2	409.8
Total investments	1,618.5	1,835.0
Cash	34.1	41.3
Cash pre-funded/placed in escrow for Ark Transaction	646.3	0.0
Accrued investment income	2.4	5.7
Accounts receivable on unsettled investment sales	3.4	5.1
Goodwill and other intangible assets	36.4	22.1
Other assets	40.4	31.3
Assets held for sale	2.3	3.0
Total assets	2,383.8	1,943.5
Liabilities
Debt	17.5	10.7
Accrued incentive compensation	70.1	55.1
Other liabilities	46.3	67.8
Total liabilities	$ 133.9	$ 133.6