Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Basic and diluted earnings per share numerators (in millions):
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	$ 12.7	$ (42.5)	$ 15.7	$ (76.9)
Allocation of income for unvested restricted common shares	(0.2)	0.6	(0.1)	0.9
Dividends declared on participating restricted common shares	0.0	0.0	(0.1)	(0.1)
Total allocation to restricted common shares	(0.2)	0.6	(0.2)	0.8
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	12.5	(41.9)	15.5	(76.1)
Undistributed net earnings (in millions):
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	12.5	(41.9)	15.5	(76.1)
Dividends declared net of restricted common share amounts	0.0	0.0	(4.5)	(5.9)
Total undistributed net earnings, net of restricted common share amounts	$ 12.5	$ (41.9)	$ 11.0	$ (82.0)
Basic earnings per share denominators
Total average common shares outstanding during the period	4,572,100	5,096,100	4,568,400	5,317,800
Average unvested restricted common shares	(57,200)	(66,500)	(54,900)	(60,200)
Basic earnings per share denominator	4,514,900	5,029,600	4,513,500	5,257,600
Diluted earnings per share denominator
Total average common shares outstanding during the period	4,572,100	5,107,000	4,568,400	5,324,000
Average unvested restricted common shares	(57,200)	(66,500)	(54,900)	(60,200)
Diluted earnings per share denominator	4,514,900	5,040,500	4,513,500	5,263,800
Basic earnings per share (in dollars):
Continuing operations (in usd per share)	$ 2.78	$ (8.34)	$ 3.42	$ (14.47)
Dividends declared	(0.00)	(0.00)	(1.00)	(1.00)
Undistributed earnings (loss) (in dollars per share)	2.78	(8.34)	2.42	(15.47)
Earnings Per Share, Diluted [Abstract]
Continuing operations (in usd per share)	2.78	(8.32)	3.42	(14.46)
Dividends declared	(0.00)	(0.00)	(1.00)	(1.00)
Undistributed earnings (loss) (in dollars per share)	2.78	$ (8.32)	2.42	$ (15.46)
Incremental common shares attributable to dilutive effect of share-based payment arrangements		10,863		6,194
Earnings Per Share, Diluted (in usd per share)	$ 3.39	$ 66.79	$ 10.89	$ 66.50
Employee Stock Option
Earnings Per Share, Diluted [Abstract]
Incremental common shares attributable to dilutive effect of share-based payment arrangements		120,000		120,000