Discontinued Operations (Earnings Per Share) (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Discontinued Operations
Net income (loss) from continuing operations attributable to White Mountains’ common shareholders	$ 125.9		$ 14.3		$ 254.6		$ (6.3)
Basic and diluted earnings (loss) per share numerators (in millions):
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	(15.8)		(12.0)		(24.5)		(7.8)
Income (Loss) Allocated to Participating Unvested Restricted Common Shares	(1.8)		(0.1)		(3.3)		0.1
Net income attributable to White Mountains' common shareholders, net of restricted common share amounts	19.1		(15.8)		139.2		(32.3)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	6,590,400		7,919,300		6,885,900		7,970,000
Average unvested restricted common shares	(96,500)	[1],[2]	(73,500)	[1],[2]	(89,500)	[1],[2]	(68,000)	[1],[2]
Basic earnings (loss) per share denominator	6,493,900		7,845,800		6,796,400		7,902,000
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	6,590,400		7,919,300		6,885,900		7,970,000
Average unvested restricted common shares	(96,500)	[1],[2]	(73,500)	[1],[2]	(89,500)	[1],[2]	(68,000)	[1],[2]
Weighted Average Number Diluted Shares Outstanding Adjustment	0	[2]	0	[2]	0	[2]	0	[2]
Diluted earnings (loss) per share denominator	6,493,900		7,845,800		6,796,400		7,902,000
Segment, Discontinued Operations [Member]
Discontinued Operations
Net income (loss) from continuing operations attributable to White Mountains’ common shareholders	(106.8)		(30.2)		(115.5)		(26.0)
Basic and diluted earnings (loss) per share numerators (in millions):
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	(15.8)		(12.0)		(24.5)		(7.8)
Income (Loss) Allocated to Participating Unvested Restricted Common Shares	1.5		0.3		1.5		0.2
Net income attributable to White Mountains' common shareholders, net of restricted common share amounts	$ (105.3)	[3]	$ (29.9)	[3]	$ (114.0)	[3]	$ (25.8)	[3]
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	6,590,400		7,919,300		6,885,900		7,970,000
Average unvested restricted common shares	(96,500)	[4]	(73,500)	[4]	(89,500)	[4]	(68,000)	[4]
Basic earnings (loss) per share denominator					6,796,400		7,902,000
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	6,590,400		7,919,300		6,885,900		7,970,000
Average unvested restricted common shares	(96,500)	[4]	(73,500)	[4]	(89,500)	[4]	(68,000)	[4]
Weighted Average Number Diluted Shares Outstanding Adjustment	0		0		0		0
Diluted earnings (loss) per share denominator	6,493,900		7,845,800		6,796,400		7,902,000
Basic and diluted earnings per share (in dollars per share):	$ (16.21)		$ (3.81)		$ (16.77)		$ (3.26)

[1] (2) Restricted shares outstanding vest either in equal annual installments or upon a stated date (see Note 12).

[2] (3) The diluted earnings (loss) per share denominator for the three and nine months ended September 30, 2012 and 2011 do not include common shares issuable upon exercise of the Non-Qualified Options as they are anti-dilutive to the calculation.

[3] (2) Net loss attributable to White Mountains’ common shareholders, net of restricted share amounts, is equal to undistributed loss for the three and nine months ended September 30, 2012 and 2011.

[4] (1) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities.