CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Statement of Comprehensive Income [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ (54.6)	$ 19.6	$ 181.8	$ 199.1
Other comprehensive income (loss), net of tax	0.1	1.4	(0.2)	2.6
Comprehensive income (loss)	(54.5)	21.0	181.6	201.7
Other comprehensive (income) loss attributable to noncontrolling interests	(0.1)	(0.4)	0.0	(0.8)
Comprehensive income (loss) attributable to White Mountains’s common shareholders	$ (54.6)	$ 20.6	$ 181.6	$ 200.9
Basic earnings (loss) per share
Basic earnings (loss) per share continuing operations (in usd per share)	$ (21.24)	$ 7.65	$ 70.93	$ 77.57
Basic earnings (loss) per share discontinued operations (in usd per share)	0	0	0	0
Total consolidated operations (in usd per share)	(21.24)	7.65	70.93	77.57
Diluted earnings (loss) per share
Diluted earnings (loss) per share continuing operations (in usd per share)	(21.24)	7.65	70.93	77.57
Diluted earnings (loss) per share discontinued operations (in usd per share)	0	0	0	0
Total consolidated operations (in usd per share)	(21.24)	7.65	70.93	77.57
Dividends declared per White Mountains’s common share (in usd per share)	0	0	1.00	1.00
Dividends paid per White Mountains’s common share (in usd per share)	$ 0	$ 0	$ 1.00	$ 1.00