Earnings Per Share - Schedule of Undistributed Net Earnings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 227.4	$ 502.0	$ (108.4)
Dividends declared, net of restricted common share amounts	(2.5)	(2.5)	(3.0)
Total undistributed net earnings (losses), net of restricted common share amounts	$ 224.9	$ 499.5	$ (111.4)