Earnings Per Share - Schedule of Earnings Per Share from Continuing Operations (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 509.2	$ 792.8	$ (275.4)
Less: total income (loss) from discontinued operations, net of tax	0.0	903.2	(3.9)
Less: net (income) loss from discontinued operations attributable to noncontrolling interests	0.0	(0.7)	1.0
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	509.2	(109.7)	(272.5)
Allocation of losses (earnings) to participating restricted common shares	(7.2)	1.3	3.2
Basic earnings (losses) per share numerators	502.0	(108.4)	(269.3)
Diluted earnings (losses) per share numerators	$ 502.0	$ (108.4)	$ (269.3)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	2,563,300	2,862,400	3,079,000
Average unvested restricted common shares (in shares)	(36,000.0)	(36,200)	(36,500)
Basic earnings (losses) per share denominator (in shares)	2,527,300	2,826,200	3,042,500
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)	2,563,300	2,862,400	3,079,000
Average unvested restricted common shares (in shares)	(36,000.0)	(36,200)	(36,500)
Diluted earnings (loss) per share denominator (in shares)	2,527,300	2,826,200	3,042,500
Basic and diluted earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid (in usd per share)	$ 1.00	$ 1.00	$ 1.00
Distributed earnings - dividends declared and paid (in usd per share)	1.00	1.00	1.00
Undistributed earnings (losses) (in usd per share)	197.60	(39.34)	(89.52)
Undistributed earnings (losses) (in usd per share)	197.60	(39.34)	(89.52)
Continuing operations (usd per share)	198.60	(38.34)	(88.52)
Continuing operations (usd per share)	$ 198.60	$ (38.34)	$ (88.52)