Earnings Per Share - Schedule of Undistributed Net Earnings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 49.0	$ (120.1)	$ 158.2
Dividends declared net of restricted common share amounts	4.5	5.4	5.9
Total undistributed net earnings (losses), net of restricted common share amounts	$ 44.5	$ (125.5)	$ 152.3