SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (including realized gains and losses)	$ 593.6	$ 501.3	$ 553.6	$ 529.6	$ 552.8	$ 551.0	$ 670.8	$ 792.1	$ 2,178.1	$ 2,566.7	$ 3,443.3
Expenses	535.6	500.1	517.5	563.3	457.2	475.8	664.9	815.7	2,116.5	2,413.6	2,646.1
Pre-tax loss	58.0	1.2	36.1	(33.7)	95.6	75.2	5.9	(23.6)	61.6	153.1	797.2
Income tax benefit (expense)	124.6	0.6	(10.0)	7.7	(6.6)	(11.7)	1.4		122.7	(16.9)	(214.2)
Net loss	182.6	1.8	26.1	(26.0)	89.0	63.5	7.3	(23.6)	184.3	136.2	583.0
Net income attributable to White Mountains' common shareholders	800.2	(15.8)	11.7	(28.2)	72.9	50.7	2.5	(39.6)	767.9	86.5	470.0
Comprehensive income attributable to White Mountains' common shareholders									686.2	214.4	750.8
Computation of net income (loss) available to common shareholders:
Net income available to common shareholders	800.2	(15.8)	11.7	(28.2)	72.9	50.7	2.5	(39.6)	767.9	86.5	470.0
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (including realized gains and losses)									6.4	1.1
Expenses									51.7	42.2	38.3
Pre-tax loss									(45.3)	(41.1)	(38.3)
Income tax benefit (expense)									6.5	(0.6)	(3.6)
Net loss									(38.8)	(41.7)	(41.9)
Equity in earnings (losses) from consolidated and unconsolidated affiliates									806.7	128.2	511.9
Net income attributable to White Mountains' common shareholders									767.9	86.5	470.0
Other comprehensive (loss) income items, after-tax									(81.7)	127.9	280.8
Comprehensive income attributable to White Mountains' common shareholders									686.2	214.4	750.8
Computation of net income (loss) available to common shareholders:
Net income available to common shareholders									$ 767.9	$ 86.5	$ 470.0