Held for Sale and Discontinued Operations - EPS (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Basic and diluted earnings per share numerators (in millions):
Net (loss) income attributable to White Mountains’s common shareholders	$ (48.0)	$ 29.0
Less total loss from continuing operations, net of tax	(48.1)	(2.3)
Net income from discontinued operations attributable to White Mountains’s common shareholders	0.1	31.3
Basic and diluted earnings per share numerators	$ (47.7)	$ (2.3)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,746,100	4,564,600
Average unvested restricted common shares	(35,900)	(52,500)
Basic earnings per share denominator	3,710,200	4,512,100
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,746,100	4,564,600
Average unvested restricted common shares	(35,900)	(52,500)
Diluted earnings per share denominator	3,710,200	4,512,100
Basic earnings per share (in dollars)	$ (12.82)	$ 6.34
Diluted earnings per share (in dollars)	$ (12.82)	$ 6.34
Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Net (loss) income attributable to White Mountains’s common shareholders	$ (48.0)	$ 29.0
Less total loss from continuing operations, net of tax	48.1	2.3
Net income from discontinued operations attributable to White Mountains’s common shareholders	0.1	31.3
Allocation of income for participating unvested restricted common shares	0.0	(0.4)
Basic and diluted earnings per share numerators	$ 0.1	$ 30.9
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,746,100	4,564,600
Average unvested restricted common shares	(35,900)	(52,500)
Basic earnings per share denominator	3,710,200	4,512,100
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,746,100	4,564,600
Average unvested restricted common shares	(35,900)	(52,500)
Diluted earnings per share denominator	3,710,200	4,512,100
Basic earnings per share (in dollars)	$ 0.03	$ 6.86
Diluted earnings per share (in dollars)	$ 0.03	$ 6.86