Earnings Per Share - Undistributed Net Earnings From Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 176.5	$ 23.3	$ 356.2	$ 219.6
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	$ 176.5	$ 23.3	$ 353.7	$ 217.1