Earnings Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Basic and diluted earnings per share numerators (in millions):
Net income from continuing operations attributable to White Mountains' common shareholders									$ 122.6	$ 93.1	$ 497.8
Allocation of income for participating unvested restricted common shares									(1.1)	(1.1)	(4.8)
Dividends paid on participating restricted common shares									(0.1)	(0.1)	(0.6)
Total allocation to restricted common shares									(1.2)	(1.2)	(5.4)
Net income attributable to White Mountains' common shareholders, net of restricted share amounts									121.4	91.9	492.4
Undistributed net earnings:
Net income attributable to White Mountains' common shareholders, net of restricted common share amounts									121.4	91.9	492.4
Dividends paid net of restricted common share amounts									(7.9)	(8.7)	(8.3)
Total undistributed net earnings, net of restricted common share amounts									113.5	83.2	484.1
Net income attributable to White Mountains' common shareholders, net of restricted share amounts									$ 122.6	$ 93.1	$ 497.8
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)									7,881,000	8,548,400	8,849,400
Average unvested restricted common shares (in shares)									(69,400)	(97,300)	(85,300)
Basic earnings per share denominator (in shares)									7,811,600	8,451,100	8,764,100
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)									7,881,000	8,548,400	8,849,400
Average unvested restricted common shares (in shares)									(69,400)	(97,300)	(85,300)
Average outstanding dilutive options to acquire common shares (in shares)										500	1,500
Diluted earnings per share denominator (in shares)									7,811,600	8,451,600	8,765,600
Basic earnings per share (in dollars):
Net income attributable to White Mountains' common shareholders	$ 16.54	$ 1.81	$ 1.66	$ (3.82)	$ 9.77	$ 5.58	$ 0.21	$ (4.14)	$ 15.56	$ 10.89	$ 56.25
Dividends declared and paid (in dollars per share)									$ (1.00)	$ (1.00)	$ (1.00)
Undistributed earnings (in dollars per share)									$ 14.56	$ 9.89	$ 55.25
Diluted earnings per share (in dollars)
Net income (loss) attributable to White Mountains' common shareholders (in dollars per share)	$ 16.54	$ 1.81	$ 1.66	$ (3.82)	$ 9.77	$ 5.58	$ 0.21	$ (4.14)	$ 15.56	$ 10.89	$ 56.24
Dividends declared and paid (in dollars per share)									$ (1.00)	$ (1.00)	$ (1.00)
Undistributed earnings (in dollars per share)									$ 14.56	$ 9.89	$ 55.25
Common shares issuable upon exercise of incentive options (in shares)										1,200	5,280
Average strike price of options (in dollars per share)					$ 189.31					$ 189.31	$ 183.03