CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Statement of Comprehensive Income [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ 179.5	$ 33.4
Other comprehensive income (loss), net of tax	1.2	(0.4)
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	0.0	(1.9)
Comprehensive income (loss)	180.7	31.1
Other comprehensive (income) loss attributable to noncontrolling interests	(0.4)	0.2
Comprehensive income (loss) attributable to White Mountains’s common shareholders	$ 180.3	$ 31.3
Basic earnings (loss) per share
Income (Loss) from Continuing Operations, Per Basic Share	$ 69.83	$ 9.90
Discontinued operations (in usd per share)	0	1.20
Total consolidated operations (in usd per share)	69.83	11.10
Diluted earnings (loss) per share
Continuing operations (in usd per share)	69.83	9.90
Discontinued operations (in usd per share)	0	1.20
Total consolidated operations (in usd per share)	69.83	11.10
Dividends declared and paid per White Mountains’s common share	$ 1.00	$ 1.00