Discontinued Operations - EPS (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):
Allocation of income for participating unvested restricted common shares	$ (0.2)	$ 0.6	$ (0.1)	$ 0.9
Net income attributable to White Mountains’s common shareholders	$ 15.5	$ 341.1	$ 49.8	$ 354.1
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)	$ 3.39	$ 66.93	$ 10.89	$ 66.57
Diluted earnings per share (in dollars)	$ 3.39	$ 66.79	$ 10.89	$ 66.50
Incremental common shares attributable to dilutive effect of share-based payment arrangements		10,863		6,194
Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$ 2.8	$ 383.6	$ 34.1	$ 431.0
Allocation of income for participating unvested restricted common shares	0.0	(5.0)	(0.4)	(4.9)
Net income attributable to White Mountains’s common shareholders	$ 2.8	$ 378.6	$ 33.7	$ 426.1
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)	$ 0.61	$ 75.27	$ 7.47	$ 81.04
Diluted earnings per share (in dollars)	$ 0.61	$ 75.11	$ 7.47	$ 80.96
Employee Stock Option
Diluted earnings per share denominator
Incremental common shares attributable to dilutive effect of share-based payment arrangements		120,000		120,000