Segment Information (Assets) (Details) (USD $) In Millions, unless otherwise specified																12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2012 OneBeacon	Dec. 31, 2011 OneBeacon	Dec. 31, 2012 Sirius Group	Dec. 31, 2011 Sirius Group	Dec. 31, 2012 HG Global	Dec. 31, 2011 HG Global	Dec. 31, 2012 BAM		Dec. 31, 2011 BAM	Dec. 31, 2012 Other Operations	Dec. 31, 2011 Other Operations	Dec. 31, 2012 BAM
Segment reporting information
Surplus notes																$ 0
Total investments	7,278.1	8,268.0			2,291.5	2,707.6	3,534.3	3,640.5	101.5	0	472.4		0	878.4	1,919.9
Reinsurance recoverable on paid and unpaid losses	447.0	2,537.8			110.7	2,184.1	336.3	353.7	0	0	0		0	0	0
Assets held for sale	2,226.8	132.6			2,226.8	132.6	0	0	0	0	0		0	0	0
Assets	12,895.4	14,064.0			5,382.3	5,792.4	5,962.0	5,337.9	623.6	0	(28.6)	[1]	0	956.1	2,933.7
Loss and loss adjustment expense reserves	3,168.9	5,702.3	5,736.8	6,379.2	1,000.0	3,358.6	2,168.9	2,343.7	0	0	0		0	0	0
Liabilities held for sale	2,226.8	107.6			2,226.8	107.6	0	0	0	0	0		0	0	0
Liabilities	8,637.2	9,396.1			4,365.0	4,678.5	4,123.2	3,784.1	0.9	0	7.4		0	140.7	933.5
Non-controlling interest	526.4	580.2			254.2	287.2	279.1	289.5	16.5	0	(36.0)		0	12.6	3.5
Total White Mountains’ common shareholders’ equity	3,731.8	4,087.7			763.1	826.7	1,559.7	1,264.3	606.2	0	0		0	802.8	1,996.7
Interest income (expense), surplus notes																$ 0

[1] BAM total assets reflect the elimination of $503.0 in surplus notes issued to HG Global and its subsidiaries, and $18.4 in accrued interest related to those surplus notes.