CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement of Comprehensive Income [Abstract]
Net (loss) income attributable to White Mountains’s common shareholders	$ (371.4)	$ 232.9	$ (308.2)	$ 219.5
Other comprehensive (loss) income, net of tax	(2.2)	3.9	0.6	1.0
Comprehensive (loss) income	(373.6)	236.8	(307.6)	220.5
Other comprehensive loss (income) attributable to non-controlling interests	0.2	(0.1)	0.0	(0.3)
Comprehensive (loss) income attributable to White Mountains’s common shareholders	$ (373.4)	$ 236.7	$ (307.6)	$ 220.2
Basic (loss) earnings per share
Continuing operations (in usd per share)	$ (120.18)	$ 75.32	$ (105.48)	$ 70.40
Discontinued operations (in usd per share)	0	(0.23)	6.03	(0.26)
Total consolidated operations (in usd per share)	(120.18)	75.09	(99.45)	70.14
Diluted (loss) earnings per share
Continuing operations (in usd per share)	(120.18)	75.32	(105.48)	70.40
Discontinued operations (in usd per share)	0	(0.23)	6.03	(0.26)
Total consolidated operations (in usd per share)	(120.18)	75.09	(99.45)	70.14
Dividends declared and paid per White Mountains’s common share (in usd per share)	$ 0	$ 0	$ 1.00	$ 1.00