Earnings Per Share - Undistributed Net Earnings From Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 233.7	$ 177.3
Dividends declared, net of restricted common share amounts	(2.5)	(2.5)
Total undistributed net earnings (losses), net of restricted common share amounts	$ 231.2	$ 174.8