Discontinued Operations - EPS (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
Less: total income from continuing operations, net of tax	23.0	6.4	31.5	(74.7)
Net income (loss) from discontinued operations attributable to White Mountains’s common shareholders	(539.1)	(84.4)	(573.2)	(515.4)
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)	$ 130.81	$ 18.65	$ 135.06	$ 85.28
Diluted earnings per share (in dollars)	$ 130.81	$ 18.61	$ 135.06	$ 85.13
Incremental common shares attributable to dilutive effect of share-based payment arrangements		11,943		8,208
Discontinued Operations
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
Less: total income from continuing operations, net of tax	(23.0)	(6.4)	(31.5)	74.7
Net income (loss) from discontinued operations attributable to White Mountains’s common shareholders	539.1	84.4	573.2	515.4
Allocation of income for participating unvested restricted common shares	(6.7)	(1.2)	(7.0)	(6.3)
Basic and diluted earnings per share numerators	$ 532.4	$ 83.2	$ 566.2	$ 509.1
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)	$ 125.45	$ 17.34	$ 128.03	$ 99.75
Diluted earnings per share (in dollars)	$ 125.45	$ 17.30	$ 128.03	$ 99.60
Employee Stock Option
Diluted earnings per share denominator
Incremental common shares attributable to dilutive effect of share-based payment arrangements		120,000		120,000