Earnings Per Share - Undistributed Net Earnings (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 3.5	$ 10.7	$ (44.2)	$ 8.3
Dividends declared net of restricted common share amounts	0.0	0.0	(3.7)	(4.5)
Total undistributed net losses, net of restricted common share amounts	$ 3.5	$ 10.7	$ (47.9)	$ 3.8