CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Statement of Comprehensive Income [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ 236.4	$ 179.5
Other comprehensive income, net of tax:
Other comprehensive income (loss), net of tax	(0.3)	1.2
Comprehensive income (loss) from discontinued operations attributable to White Mountains’s common shareholders	236.1	180.7
Other comprehensive (income) loss attributable to noncontrolling interests	0.1	(0.4)
Total comprehensive income (loss)	$ 236.2	$ 180.3
Basic earnings (loss) per share
Basic and diluted earnings (losses) per share (in usd per share)	$ 92.33	$ 69.83
Discontinued operations (in usd per share)	0	0
Total consolidated operations (in usd per share)	92.33	69.83
Diluted earnings (loss) per share
Continuing operations (in usd per share)	92.33	69.83
Discontinued operations (in usd per share)	0	0
Total consolidated operations (in usd per share)	92.33	69.83
Dividends declared per White Mountains’s common share (in usd per share)	1.00	1.00
Dividends paid per White Mountains’s common share (in usd per share)	$ 1.00	$ 1.00