Segment Information (Details) (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	Dec. 31, 2008
Segment reporting information
Earned insurance and reinsurance premiums									$ 1,927.8	$ 2,251.8	$ 2,717.6
Net investment income									184.5	208.9	250.1
Net realized and unrealized investment gains									122.8	82.6	361.7
Other revenue									(57.0)	23.4	113.9
Total revenues	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
Losses and LAE									1,206.9	1,389.2	1,424.1
Insurance and reinsurance acquisition expenses									400.0	479.1	547.2
Other underwriting expenses									279.1	332.9	406.6
General and administrative expenses									175.3	155.1	197.4
Interest expense on debt									55.2	57.3	70.8
Total 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 income	58.0	1.2	36.1	(33.7)	95.6	75.2	5.9	(23.6)	61.6	153.1	797.2
Selected Balance Sheet Data
Total investments	8,268.0				7,943.5				8,268.0	7,943.5
Reinsurance recoverable on paid and unpaid losses	2,537.8				2,407.1				2,537.8	2,407.1
Assets held for sale	132.6				1,122.8				132.6	1,122.8
Total assets	14,064.0				14,534.1				14,064.0	14,534.1
Loss and LAE reserves	5,702.3				5,736.8				5,702.3	5,736.8	6,379.2	7,029.4
Liabilities held for sale	107.6				751.5				107.6	751.5
Total liabilities	9,396.1				10,273.3				9,396.1	10,273.3
Noncontrolling interest	580.2				607.8				580.2	607.8
Total White Mountains' common shareholders' equity	4,087.7				3,653.0				4,087.7	3,653.0
OneBeacon
Segment reporting information
Number of transactions completed, which transformed the reportable segment into a specialty insurance company									2
Earned insurance and reinsurance premiums									1,015.5	1,403.9	1,858.8
Net investment income									71.4	96.6	125.5
Net realized and unrealized investment gains									10.6	74.6	248.6
Other revenue									(10.7)	9.6	23.1
Total revenues									1,086.8	1,584.7	2,256.0
Losses and LAE									580.9	858.2	1,005.3
Insurance and reinsurance acquisition expenses									219.0	311.6	376.3
Other underwriting expenses									173.3	233.1	308.5
General and administrative expenses									9.8	13.0	18.5
Interest expense on debt									20.5	29.6	39.7
Total expenses									1,003.5	1,445.5	1,748.3
Pre-tax income									83.3	139.2	507.7
Selected Balance Sheet Data
Total investments	2,707.6				3,266.0				2,707.6	3,266.0
Reinsurance recoverable on paid and unpaid losses	2,184.1				1,937.7				2,184.1	1,937.7
Assets held for sale	132.6								132.6
Total assets	5,792.4				6,138.3				5,792.4	6,138.3
Loss and LAE reserves	3,358.6				3,295.5				3,358.6	3,295.5
Liabilities held for sale	107.6								107.6
Total liabilities	4,678.5				4,889.4				4,678.5	4,889.4
Noncontrolling interest	287.2				314.9				287.2	314.9
Total White Mountains' common shareholders' equity	826.7				934.0				826.7	934.0
Sirius
Segment reporting information
Earned insurance and reinsurance premiums									912.3	847.9	858.8
Net investment income									89.9	96.5	110.3
Net realized and unrealized investment gains									101.9	(9.8)	112.6
Other revenue									(44.6)	35.9	106.5
Total revenues									1,059.5	970.5	1,188.2
Losses and LAE									626.0	531.0	418.8
Insurance and reinsurance acquisition expenses									181.0	167.5	170.9
Other underwriting expenses									105.8	99.8	98.1
General and administrative expenses									34.1	31.6	74.4
Interest expense on debt									31.6	26.6	26.3
Total expenses									978.5	856.5	788.5
Pre-tax income									81.0	114.0	399.7
Selected Balance Sheet Data
Total investments	3,640.5				4,116.2				3,640.5	4,116.2
Reinsurance recoverable on paid and unpaid losses	353.7				469.4				353.7	469.4
Total assets	5,337.9				5,618.7				5,337.9	5,618.7
Loss and LAE reserves	2,343.7				2,441.3				2,343.7	2,441.3
Total liabilities	3,784.1				3,863.0				3,784.1	3,863.0
Noncontrolling interest	289.5				290.6				289.5	290.6
Total White Mountains' common shareholders' equity	1,264.3				1,465.1				1,264.3	1,465.1
Other Operations
Segment reporting information
Net investment income									23.2	15.8	14.3
Net realized and unrealized investment gains									10.3	17.8	0.5
Other revenue									(1.7)	(22.1)	(15.7)
Total revenues									31.8	11.5	(0.9)
General and administrative expenses									131.4	110.5	104.5
Interest expense on debt									3.1	1.1	4.8
Total expenses									134.5	111.6	109.3
Pre-tax income									(102.7)	(100.1)	(110.2)
Selected Balance Sheet Data
Total investments	1,919.9				561.3				1,919.9	561.3
Total assets	2,933.7				1,654.3				2,933.7	1,654.3
Total liabilities	933.5				769.4				933.5	769.4
Noncontrolling interest	3.5				2.3				3.5	2.3
Total White Mountains' common shareholders' equity	1,996.7				882.6				1,996.7	882.6
Discontinued Operations.
Selected Balance Sheet Data
Assets held for sale					1,122.8					1,122.8
Total assets					1,122.8					1,122.8
Liabilities held for sale					751.5					751.5
Total liabilities					751.5					751.5
Total White Mountains' common shareholders' equity					$ 371.3					$ 371.3