Segment Information (Assets) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2010
Segment reporting information
Total investments	$ 7,192.6		$ 7,278.1
Reinsurance recoverables, gross	453.5		447.0
Assets held for sale	1,880.1		2,226.8
Assets	12,144.3		12,895.4
Loss and loss adjustment expense reserves	3,079.3		3,168.9		5,702.3	5,736.8
Liabilities held for sale	1,880.1		2,226.8
Liabilities	7,746.9		8,637.2
Total non-controlling interests	491.9		526.4
White Mountains’ common shareholders’ equity	3,905.5		3,731.8
OneBeacon
Segment reporting information
Total investments	2,364.9		2,291.5
Reinsurance recoverables, gross	89.9		110.7
Assets held for sale	1,880.1		2,226.8
Assets	5,191.3		5,382.3
Loss and loss adjustment expense reserves	1,054.3		1,000.0
Liabilities held for sale	1,880.1		2,226.8
Liabilities	4,083.9		4,365.0
Total non-controlling interests	276.8		254.2
White Mountains’ common shareholders’ equity	830.6		763.1
Sirius Group
Segment reporting information
Total investments	3,251.4		3,534.3
Reinsurance recoverables, gross	363.6	[1]	336.3
Assets held for sale	0		0
Assets	5,124.1		5,962.0
Loss and loss adjustment expense reserves	2,025.0		2,168.9
Liabilities held for sale	0		0
Liabilities	3,414.5		4,123.2
Total non-controlling interests	250.0		279.1
White Mountains’ common shareholders’ equity	1,459.6		1,559.7
HG Global
Segment reporting information
Total investments	107.9		101.5
Reinsurance recoverables, gross	0		0
Assets held for sale	0		0
Assets	675.0		623.6
Loss and loss adjustment expense reserves	0		0
Liabilities held for sale	0		0
Liabilities	68.9		0.9
Total non-controlling interests	16.6		16.6
White Mountains’ common shareholders’ equity	589.5		606.2
BAM
Segment reporting information
Surplus notes	503.0
Total investments	468.6		472.4
Reinsurance recoverables, gross	0		0
Assets held for sale	0		0
Assets	(77.9)	[2]	(28.6)	[2]
Loss and loss adjustment expense reserves	0		0
Liabilities held for sale	0		0
Liabilities	19.7		7.4
Total non-controlling interests	(97.6)		(36.0)
White Mountains’ common shareholders’ equity	0		0
Accrued interest on surplus notes	58.6		18.4
Other Operations
Segment reporting information
Total investments	999.8		878.4
Reinsurance recoverables, gross	0		0
Assets held for sale	0		0
Assets	1,231.8		956.1
Loss and loss adjustment expense reserves	0		0
Liabilities held for sale	0		0
Liabilities	159.9		140.7
Total non-controlling interests	46.1		12.5
White Mountains’ common shareholders’ equity	$ 1,025.8		$ 802.8

[1] Standard & Poor’s ratings as detailed above are: “AAA” (Extremely strong), “AA” (Very strong), “A” (Strong), and “BBB+” and “BBB” (Adequate).

[2] BAM total assets reflect the elimination of $503.0 in surplus notes issued to HG Global and its subsidiaries, and $58.6 and $18.4 in accrued interest related to those surplus notes as of December 31, 2013 and 2012.