Earnings Per Share - Schedule of Undistributed Net Earnings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 701.7	$ 408.4	$ (122.6)
Dividends declared, net of restricted common share amounts	(3.1)	(3.2)	(3.7)
Total undistributed net earnings (losses), net of restricted common share amounts	$ 698.6	$ 405.2	$ (126.3)