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Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$ 562.1	$ 90.8	$ 604.7	$ 440.7
Total income (loss) from discontinued operations	(539.1)	(84.4)	(573.2)	(515.4)
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	23.0	6.4	31.5	(74.7)
Allocation of earnings to participating restricted common shares(1)	(0.3)	(0.1)	(0.4)	0.8
Basic and diluted earnings per share numerators	$ 22.7	$ 6.3	$ 31.1	$ (73.9)
Basic earnings per share denominators
Total average common shares outstanding during the period	4,297,200	4,867,400	4,477,000	5,166,600
Average unvested restricted common shares	(53,700)	(68,100)	(54,500)	(62,900)
Basic earnings per share denominator	4,243,500	4,799,300	4,422,500	5,103,700
Diluted earnings per share denominator
Total average common shares outstanding during the period	4,297,200	4,879,400	4,477,000	5,174,800
Average unvested restricted common shares	(53,700)	(68,100)	(54,500)	(62,900)
Diluted earnings per share denominator	4,243,500	4,811,300	4,422,500	5,111,900
Basic earnings per share (in dollars) - continuing operations:
Continuing operations (in usd per share)	$ 5.36	$ 1.31	$ 7.03	$ (14.47)
Dividends declared	(0.00)	(0.00)	(1.00)	(1.00)
Undistributed earnings (loss) (in dollars per share)	5.36	1.31	6.03	(15.47)
Earnings Per Share, Diluted [Abstract]
Continuing operations (in usd per share)	5.36	1.31	7.03	(14.47)
Dividends declared	(0.00)	(0.00)	(1.00)	(1.00)
Undistributed earnings (loss) (in dollars per share)	$ 5.36	$ 1.31	$ 6.03	$ (15.47)
Incremental common shares attributable to dilutive effect of share-based payment arrangements		11,943		8,208
Employee Stock Option
Earnings Per Share, Diluted [Abstract]
Incremental common shares attributable to dilutive effect of share-based payment arrangements		120,000		120,000

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