CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Statement of Comprehensive Income [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ 888.2	$ (371.4)	$ 752.4	$ (308.2)
Other comprehensive income (loss), net of tax	(1.4)	0.0	(3.0)	(0.3)
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	0.7	(2.2)	(5.2)	0.9
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	2.9	0.0	2.9	0.0
Comprehensive income (loss)	890.4	(373.6)	747.1	(307.6)
Other comprehensive (income) loss attributable to non-controlling interests	(0.3)	0.2	0.4	0.0
Comprehensive income (loss) attributable to White Mountains’s common shareholders	$ 890.1	$ (373.4)	$ 747.5	$ (307.6)
Basic earnings (loss) per share
Continuing operations (in usd per share)	$ (1.66)	$ (121.90)	$ (50.73)	$ (98.16)
Discontinued operations (in usd per share)	308.59	1.72	304.97	(1.29)
Total consolidated operations (in usd per share)	306.93	(120.18)	254.24	(99.45)
Diluted earnings (loss) per share
Continuing operations (in usd per share)	(1.66)	(121.90)	(50.73)	(98.16)
Discontinued operations (in usd per share)	308.59	1.72	304.97	(1.29)
Total consolidated operations (in usd per share)	306.93	(120.18)	254.24	(99.45)
Dividends declared and paid per White Mountains’s common share (in usd per share)	$ 0	$ 0	$ 1.00	$ 1.00