Earnings Per Share - Schedule of Undistributed Net Earnings (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Earnings Per Share [Abstract]
Net income (loss) available to White Mountains’s common shareholders, net of restricted common share amounts	$ 112.3	$ 176.5	$ 267.3	$ 356.2
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	$ 112.3	$ 176.5	$ 264.8	$ 353.7