Earnings Per Share - Undistributed Net Earnings (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 47,100,000	$ 57,300,000	$ 347,600,000	$ 13,300,000
Dividends declared net of restricted common share amounts	0	0	(3,200,000)	(3,700,000)
Total undistributed net earnings, net of restricted common share amounts	$ 47,100,000	$ 57,300,000	$ 344,400,000	$ 9,600,000