Earnings Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Earnings Per Share [Abstract]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	125,000		125,000		125,000		125,000
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’ common shareholders	$ 56.8		$ 125.9		$ 199.1		$ 254.6
Allocation of income for unvested restricted common shares	(0.9)		(1.8)		(2.9)		(3.3)
Dividends declared on participating restricted common shares	0	[1]	0	[1]	(0.1)	[1]	(0.1)	[1]
Total allocation to restricted common shares	(0.9)		(1.8)		(3.0)		(3.4)
Net income attributable to White Mountains’ common shareholders, net of restricted common share amounts	55.9		124.1		196.1		251.2
Undistributed net earnings (in millions):
Net income attributable to White Mountains’ common shareholders, net of restricted common share amounts	55.9		124.1		196.1		251.2
Dividends declared net of restricted common share amounts	0	[1]	0	[1]	(6.1)	[1]	(6.5)	[1]
Total undistributed net earnings, net of restricted common share amounts	$ 55.9		$ 124.1		$ 190.0		$ 244.7
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	6,176,600		6,590,400		6,208,400		6,885,900
Average unvested restricted common shares	(95,500)	[2]	(96,500)	[2]	(90,000)	[2]	(89,500)	[2]
Basic earnings per share denominator	6,081,100		6,493,900		6,118,400		6,796,400
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	6,176,600		6,590,400		6,208,400		6,885,900
Average unvested restricted common shares	(95,500)	[2]	(96,500)	[2]	(90,000)	[2]	(89,500)	[2]
Average outstanding dilutive options to acquire common shares	0	[3]	0	[3]	0	[3]	0	[3]
Diluted earnings (loss) per share denominator	6,081,100		6,493,900		6,118,400		6,796,400
Basic earnings per share (in dollars):
Net income (loss) attributable to White Mountains' common shareholders	$ 9.20		$ 19.11		$ 32.05		$ 36.96
Dividends declared	$ 0		$ 0		$ (1)		$ (1)
Undistributed earnings (loss) (in dollars per share)	$ 9.20		$ 19.11		$ 31.05		$ 35.96
Earnings Per Share, Diluted [Abstract]
Net income (loss) attributable to White Mountains' common shareholders (in dollars per share)	$ 9.20		$ 19.11		$ 32.05		$ 36.96
Dividends declared	$ 0		$ 0		$ (1)		$ (1)
Undistributed earnings (loss) (in dollars per share)	$ 9.20		$ 19.11		$ 31.05		$ 35.96

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

[2] Restricted shares outstanding vest either in equal annual installments or upon a stated date (see Note 13).

[3] The diluted earnings per share denominator for the three and nine months ended September 30, 2013 and 2012 do not include the impact of 125,000 common shares issuable upon exercise of the non-qualified options outstanding as they are anti-dilutive to the calculation.