Earnings (Loss) Per Share (Details) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Basic and diluted earnings per share numerators (in millions):
Net income from continuing operations attributable to White Mountains common shareholders	$ 12.0	$ 0.7	$ (19.4)	$ (36.3)
Dividends declared on common shares			(8.0)	(8.8)
Undistributed earnings (loss)	$ 12.0	$ 0.7	$ (27.4)	$ (45.1)
Basic earnings per share denominators (in thousands):
Average common shares outstanding during the period (in shares)	7,886	8,571	7,930	8,656
Average unvested restricted shares (in shares)	73	100	65	96
Basic earnings (loss) per share denominator (in shares)	7,959	8,671	7,995	8,752
Diluted earnings per share denominator (in thousands):
Average common shares outstanding during the period (in shares)	7,886	8,571	7,930	8,656
Average unvested restricted shares (in shares)	73	100	65	96
Diluted earnings (loss) per share denominator (in shares)	7,959	8,671	7,995	8,752
Basic earnings per share (in dollars):
Net income (loss) from continuing operations attributable to White Mountains' common shareholders (in dollars per share)	$ 1.51	$ 0.08	$ (2.42)	$ (4.15)
Dividends declared and paid (in dollars per share)			$ (1.00)	$ (1.00)
Undistributed earnings (loss) per share (in dollars per share)	$ 1.51	$ 0.08	$ (3.42)	$ (5.15)
Diluted earnings per share (in dollars)
Net income from continuing operations attributable to White Mountains' common shareholders (in dollars per share)	$ 1.51	$ 0.08	$ (2.42)	$ (4.15)
Dividends declared and paid (in dollars per share)			$ (1.00)	$ (1.00)
Undistributed earnings (loss) per share (in dollars per share)	$ 1.51	$ 0.08	$ (3.42)	$ (5.15)