Earnings Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’ common shareholders									$ 317.3		$ 322.4		$ 146.3
Basic and diluted earnings per share numerators (in millions):
Allocation of income for unvested restricted common shares									(4.7)		(4.3)		(1.2)
Dividends declared on participating restricted common shares(1)									(0.1)	[1]	(0.1)	[1]	(0.1)	[1]
Total allocation to restricted common shares									(4.8)		(4.4)		(1.3)
Net income attributable to White Mountains’ common shareholders, net of restricted share amounts									312.5		318.0		145.0
Undistributed net earnings (in millions):
Net income attributable to White Mountains’ common shareholders, net of restricted share amounts									312.5		318.0		145.0
Dividends declared net of restricted common share amounts(1)									(6.1)	[1]	(6.5)	[1]	(7.9)	[1]
Total undistributed net earnings, net of restricted common share amounts									$ 306.4		$ 311.5		$ 137.1
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									6,200,400		6,799,800		7,881,000
Average unvested restricted common shares									(91,400)	[2]	(91,100)	[2]	(69,400)	[2]
Basic earnings per share denominator									6,109,000		6,708,700		7,811,600
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period									6,200,400		6,799,800		7,881,000
Average unvested restricted common shares									(91,400)	[2]	(91,100)	[2]	(69,400)	[2]
Average outstanding dilutive options to acquire common shares									0	[3]	0	[3]	0	[3]
Diluted earnings (loss) per share denominator									6,109,000		6,708,700		7,811,600
Basic earnings per share (in dollars):
Net income attributable to White Mountains’ common shareholders	$ 19.14	$ 9.20	$ 3.62	$ 19.10	$ 10.36	$ 19.11	$ 3.85	$ 13.85	$ 51.15		$ 47.41		$ 18.56
Dividends declared and paid									$ (1.00)		$ (1.00)		$ (1.00)
Undistributed earnings									$ 50.15		$ 46.41		$ 17.56
Diluted earnings per share (in dollars)
Net income attributable to White Mountains’ common shareholders	$ 19.14	$ 9.20	$ 3.62	$ 19.10	$ 10.36	$ 19.11	$ 3.85	$ 13.85	$ 51.15		$ 47.41		$ 18.56
Dividends declared and paid									$ (1.00)		$ (1.00)		$ (1.00)
Undistributed earnings									$ 50.15		$ 46.41		$ 17.56
Incremental common shares attributable to share-based payment arrangements									125,000

[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 12).

[3] The diluted earnings per share denominator for the years ended December 31, 2013, 2012 and 2011 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.