Earnings Per Share - Undistributed Net Earnings (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
Net income (losses) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 20,200,000	$ 3,500,000	$ 300,600,000	$ (44,200,000)
Dividends declared net of restricted common share amounts	0	0	(3,200,000)	(3,700,000)
Total undistributed net earnings (losses), net of restricted common share amounts	$ 20,200,000	$ 3,500,000	$ 297,400,000	$ (47,900,000)