CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Statement of Comprehensive Income [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ (275.4)	$ 708.7	$ 414.5
Other comprehensive income (loss), net of tax	1.9	7.3	(1.4)
Comprehensive income (loss)	(273.5)	716.0	413.1
Comprehensive (income) loss attributable to non-controlling interests	0.2	(0.5)	0.0
Comprehensive income (loss) attributable to White Mountains’s common shareholders	$ (273.3)	$ 715.5	$ 413.1
Basic earnings (loss) per share
Continuing operations (usd per share)	$ (95.53)	$ 227.72	$ 130.02
Discontinued operations (usd per share)	6.07	(0.75)	0.25
Total consolidated operations (usd per share)	(89.46)	226.97	130.27
Diluted earnings (loss) per share
Continuing operations (usd per share)	(95.53)	227.72	130.02
Discontinued operations (usd per share)	6.07	(0.75)	0.25
Total consolidated operations (usd per share)	(89.46)	226.97	130.27
Dividends declared and paid per White Mountains’s common share (usd per share)	$ 1.00	$ 1.00	$ 1.00