CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Statement of Comprehensive Income [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ 33.4	$ (75.3)
Other comprehensive income (loss), net of tax	(2.3)	1.8
Comprehensive income (loss)	31.1	(73.5)
Other comprehensive (income) loss attributable to non-controlling interests	0.2	(0.1)
Comprehensive income (loss) attributable to White Mountains’s common shareholders	$ 31.3	$ (73.6)
Basic earnings (loss) per share
Continuing operations (in usd per share)	$ 11.10	$ (30.33)
Discontinued operations (in usd per share)	0	6.03
Total consolidated operations (in usd per share)	11.10	(24.30)
Diluted earnings (loss) per share
Continuing operations (in usd per share)	11.10	(30.33)
Discontinued operations (in usd per share)	0	6.03
Total consolidated operations (in usd per share)	11.10	(24.30)
Dividends declared and paid per White Mountains’s common share (in usd per share)	$ 1.00	$ 1.00