Earnings (Loss) 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
Basic and diluted earnings per share numerators (in millions):
Net income (loss) from continuing operations attributable to White Mountains’ common shareholders	$ 125.9		$ 14.3		$ 254.6		$ (6.3)
Allocation of (income) loss for unvested restricted common shares	(1.8)		(0.1)		(3.3)		0.1
Dividends declared on participating restricted common shares	0	[1]	0	[1]	0.1	[1]	0.1	[1]
Total allocation to restricted common shares	(1.8)		(0.1)		(3.4)		0
Net income (loss) attributable to White Mountains' common shareholders, net of restricted share amounts	124.1		14.2		251.2		(6.3)
Undistributed net earnings (in millions):
Net income (loss) attributable to White Mountains' common shareholders, net of restricted common share amounts	124.1		14.2		251.2		(6.3)
Dividends declared net of restricted common share amounts	0	[1]	0	[1]	6.5	[1]	7.9	[1]
Total undistributed net earnings (loss), net of restricted common share amounts	$ 124.1		$ 14.2		$ 244.7		$ (14.2)
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)	[2],[3]	(73,500)	[2],[3]	(89,500)	[2],[3]	(68,000)	[2],[3]
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)	[2],[3]	(73,500)	[2],[3]	(89,500)	[2],[3]	(68,000)	[2],[3]
Average outstanding dilutive options to acquire common shares	0	[3]	0	[3]	0	[3]	0	[3]
Diluted earnings (loss) per share denominator	6,493,900		7,845,800		6,796,400		7,902,000
Basic earnings per share (in dollars):
Net income (loss) attributable to White Mountains' common shareholders	$ 19.11		$ 1.81		$ 36.96		$ (0.80)
Undistributed earnings (loss) (in dollars per share)	$ 19.11		$ 1.81		$ 35.96		$ (1.80)
Earnings Per Share, Diluted [Abstract]
Net income (loss) attributable to White Mountains' common shareholders (in dollars per share)	$ 19.11		$ 1.81		$ 36.96		$ (0.80)
Dividends declared per White Mountains' common share (in dollars per share)	$ 0.00		$ 0.00		$ 1.00		$ 1.00
Undistributed earnings (loss) (in dollars per share)	$ 19.11		$ 1.81		$ 35.96		$ (1.80)

[1] (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.

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

[3] (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.