Earnings (Loss) Per Share (Details) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains' common shareholders	$ (15.8)	$ 50.7	$ (32.3)	$ 13.6
Less: net (income) loss from discontinued operations	30.1	(3.6)	28.9	(1.2)
Net income (loss) attributable to White Mountains' common shareholders from continuing operations	14.3	47.1	(3.4)	12.4
Dividends declared on common shares			(8.0)	(8.8)
Undistributed earnings (loss)	$ 14.3	$ 47.1	$ (11.4)	$ 3.6
Basic earnings per share denominators (in thousands):
Average common shares outstanding during the period (in shares)	7,845	8,332	7,902	8,547
Average unvested restricted shares (in shares)	74	99	68	97
Basic earnings (loss) per share denominator (in shares)	7,919	8,431	7,970	8,644
Diluted earnings per share denominator (in thousands):
Average common shares outstanding during the period (in shares)	7,845	8,332	7,902	8,547
Average unvested restricted shares (in shares)	74	99	68	97
Average outstanding dilutive options to acquire common shares (in shares)				1
Diluted earnings (loss) per share denominator (in shares)	7,919	8,431	7,970	8,645
Basic earnings per share (in dollars):
Net income (loss) from continuing operations attributable to White Mountains' common shareholders (in dollars per share)	$ 1.81	$ 5.58	$ (0.43)	$ 1.44
Dividends declared and paid (in dollars per share)			$ (1.00)	$ (1.00)
Undistributed earnings (loss) per share (in dollars per share)	$ 1.81	$ 5.58	$ (1.43)	$ 0.44
Diluted earnings per share (in dollars)
Net income from continuing operations attributable to White Mountains' common shareholders (in dollars per share)	$ 1.81	$ 5.58	$ (0.43)	$ 1.44
Dividends declared and paid (in dollars per share)			$ (1.00)	$ (1.00)
Undistributed earnings (loss) per share (in dollars per share)	$ 1.81	$ 5.58	$ (1.43)	$ 0.44