Financial Statement Revisions - Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets	$ 3,659.2	$ 6,520.2
Investments	3,380.7	2,714.4
Goodwill	25.9	25.9	$ 18.6
Intangible assets	71.0	43.5
Total liabilities	298.4	2,804.2
Liabilities held for sale		2,569.3
White Mountains’s common shares	3.8	4.6
Paid-in surplus	666.8	806.1
Retained earnings	2,823.2	2,776.6
Accumulated other comprehensive income, net of tax		(4.6)
Total White Mountains’s common shareholders’ equity	3,492.5	3,582.7
Non-controlling interests	(131.7)	133.3
Total equity	3,360.8	3,716.0
Total liabilities and equity	3,659.2	6,520.2
Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets		6,544.7
Total liabilities		2,807.5
White Mountains’s common shares		4.6
Paid-in surplus		806.1
Retained earnings		2,797.2
Accumulated other comprehensive income, net of tax		(4.6)
Total White Mountains’s common shareholders’ equity		3,603.3
Non-controlling interests		133.9
Total equity		3,737.2
Total liabilities and equity		6,544.7
Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets		(24.5)
Total liabilities		(3.3)
White Mountains’s common shares		0.0
Paid-in surplus		0.0
Retained earnings		(20.6)
Accumulated other comprehensive income, net of tax		0.0
Total White Mountains’s common shareholders’ equity		(20.6)
Non-controlling interests		(0.6)
Total equity		(21.2)
Total liabilities and equity		(24.5)
HG Global BAM
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets	747.4	677.6
Investments	693.4	629.7
Total liabilities	167.0	108.9
Accrued incentive compensation	18.2	16.6
Other liabilities	11.4	9.4
HG Global BAM | Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets		677.6
Total liabilities		108.9
HG Global BAM | Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets		0.0
Total liabilities		0.0
MediaAlpha
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets	96.5	57.6
Total liabilities	59.8	28.3
Accrued incentive compensation	2.0	0.0
Other liabilities	2.4	2.6
MediaAlpha | Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets		57.6
Total liabilities		28.3
MediaAlpha | Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets		0.0
Total liabilities		0.0
Other
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets	2,815.3	5,785.0
Investments	2,687.3	2,084.7
Goodwill		7.6
Intangible assets		1.0
Other intangible assets		0.0
Other assets		85.3
Assets held for sale	3.3	3,606.4
Total liabilities	71.6	2,667.0
Accrued incentive compensation	60.6	79.1
Other liabilities	11.0	18.6
Liabilities held for sale	$ 0.0	2,569.3
Other | Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets		5,809.5
Investments		2,084.7
Goodwill		13.4
Intangible assets		4.7
Other intangible assets		14.8
Other assets		85.5
Assets held for sale		3,606.4
Total liabilities		2,670.3
Accrued incentive compensation		79.1
Other liabilities		21.9
Liabilities held for sale		2,569.3
Other | Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets		(24.5)
Investments		0.0
Goodwill		(5.8)
Intangible assets		(3.7)
Other intangible assets		(14.8)
Other assets		(0.2)
Assets held for sale		0.0
Total liabilities		(3.3)
Accrued incentive compensation		0.0
Other liabilities		(3.3)
Liabilities held for sale		$ 0.0