Earnings Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Basic and diluted earnings per share numerators (in millions):
Net (loss) income attributable to White Mountains’ common shareholders									$ 322.4		$ 146.3		$ 116.6
Allocation of income for participating unvested restricted common shares									(4.3)		(1.2)		(1.3)
Dividends declared on participating restricted common shares (1)									(0.1)	[1]	(0.1)	[1]	(0.1)	[1]
Total allocation to restricted common shares									(4.4)		(1.3)		(1.4)
Net income attributable to White Mountains’ common shareholders, net of restricted share amounts									318.0		145.0		115.2
Undistributed net earnings:
Net income attributable to White Mountains’ common shareholders, net of restricted common share amounts									318.0		145.0		115.2
Dividends declared net of restricted common share amounts (1)									(6.5)	[1]	(7.9)	[1]	(8.7)	[1]
Total undistributed net earnings, net of restricted common share amounts									$ 311.5		$ 137.1		$ 106.5
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									6,799,800		7,881,000		8,548,400
Average unvested restricted common shares									(91,100)	[2]	(69,400)	[2]	(97,300)	[2]
Basic earnings per share denominator									6,708,700		7,811,600		8,451,100
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period									6,799,800		7,881,000		8,548,400
Average unvested restricted common shares									(91,100)	[2]	(69,400)	[2]	(97,300)	[2]
Average outstanding dilutive options to acquire common shares									0	[3]	0	[3]	500	[3]
Diluted earnings (loss) per share denominator									6,708,700		7,811,600		8,451,600
Basic earnings per share (in dollars):
Net income attributable to White Mountains’ common shareholders	$ 10.36	$ 19.11	$ 3.85	$ 13.85	$ 20.03	$ 1.80	$ 1.78	$ (4.33)	$ 47.41		$ 18.56		$ 13.63
Dividends declared and paid									$ (1.00)		$ (1.00)		$ (1.00)
Undistributed earnings									$ 46.41		$ 17.56		$ 12.63
Diluted earnings per share (in dollars)
Net income attributable to White Mountains’ common shareholders	$ 10.36	$ 19.11	$ 3.85	$ 13.85	$ 20.03	$ 1.80	$ 1.78	$ (4.33)	$ 47.41		$ 18.56		$ 13.63
Dividends declared and paid									$ (1.00)		$ (1.00)		$ (1.00)
Undistributed earnings									$ 46.41		$ 17.56		$ 12.63
Incremental common shares attributable to share-based payment arrangements													1,200
Average strike price of options (in dollars per share)													$ 189

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

[2] Restricted common shares outstanding vest either in equal annual installments or upon a stated date (see Note 11).

[3] The diluted earnings per share denominator for the year ended December 31, 2010 includes 1,200 common shares issuable upon exercise of incentive options at an average strike price of $189.31 per common share. The non-qualified options were not included in the diluted earnings per share denominator for any of the periods presented as their inclusion would be anti-dilutive.