Earnings Per Share (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
Total income (loss) from discontinued operations	0.1	31.3
Net loss from continuing operations attributable to White Mountains’s common shareholders	(48.1)	(2.3)
Allocation of earnings to participating restricted common shares	0.4	0.0
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) - continuing operations:
Distributed earnings - dividends declared and paid	$ (1.00)	$ (1.00)
Undistributed earnings (losses)	(13.85)	(1.52)
Basic and diluted earnings per share	$ (12.85)	$ (0.52)