Earnings Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’ common shareholders	$ 119.9		$ 103.1
Allocation of income for unvested restricted common shares	(1.5)		(1.1)
Dividends declared on participating restricted common shares	(0.1)	[1]	(0.1)	[1]
Total allocation to restricted common shares	(1.6)		(1.2)
Net income attributable to White Mountains’ common shareholders, net of restricted common share amounts	118.3		101.9
Undistributed net earnings (in millions):
Net income attributable to White Mountains’ common shareholders, net of restricted common share amounts	118.3		101.9
Dividends declared net of restricted common share amounts	(6.1)	[1]	(6.5)	[1]
Total undistributed net earnings, net of restricted common share amounts	$ 112.2		$ 95.4
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	6,273,500		7,431,700
Average unvested restricted common shares	(79,000)	[2]	(78,300)	[2]
Basic earnings per share denominator	6,194,500		7,353,400
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	6,273,500		7,431,700
Average unvested restricted common shares	(79,000)	[2]	(78,300)	[2]
Average outstanding dilutive options to acquire common shares	0	[3]	0	[3]
Diluted earnings (loss) per share denominator	6,194,500		7,353,400
Basic earnings per share (in dollars):
Net income (loss) attributable to White Mountains' common shareholders	$ 19.10		$ 13.86
Dividends declared	$ 1		$ 1
Undistributed earnings (loss) (in dollars per share)	$ 18.10		$ 12.86
Earnings Per Share, Diluted [Abstract]
Net income (loss) attributable to White Mountains' common shareholders (in dollars per share)	$ 19.10		$ 13.86
Dividends declared	$ 1		$ 1
Undistributed earnings (loss) (in dollars per share)	$ 18.10		$ 12.86

[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 months ended March 31, 2013 and 2012 do not include common shares issuable upon exercise of the Non-Qualified Options as they are anti-dilutive to the calculation.