Earnings Per Share - Undistributed Net Earnings From Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ (53.9)	$ 19.3	$ 179.6	$ 196.4
Dividends declared, net of restricted common share amounts	0.0	0.0	(2.5)	(2.5)
Total undistributed net earnings (losses), net of restricted common share amounts	$ (53.9)	$ 19.3	$ 177.1	$ 193.9