Earnings Per Share - Basic and Diluted Earnings Per Share from Continuing Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 23.6	$ 888.2	$ 222.7	$ 752.4
Less: total income (loss) from discontinued operations, net of tax	0.0	893.1	0.0	903.2
Net (income) loss from discontinued operations attributable to noncontrolling interests	0.0	(0.1)	0.0	(0.7)
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	23.6	(4.8)	222.7	(150.1)
Allocation of losses (earnings) to participating restricted common shares	(0.3)	0.1	(3.1)	1.8
Basic earnings (losses) per share numerators	23.3	(4.7)	219.6	(148.3)
Diluted earnings (losses) per share numerators	$ 23.3	$ (4.7)	$ 219.6	$ (148.3)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	2,560,500	2,893,800	2,564,200	2,959,300
Average unvested restricted common shares (in shares)	(37,600)	(38,300)	(35,500)	(35,500)
Basic earnings (losses) per share denominator (in shares)	2,522,900	2,855,500	2,528,700	2,923,800
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)	2,560,500	2,893,800	2,564,200	2,959,300
Average unvested restricted common shares (in shares)	(37,600)	(38,300)	(35,500)	(35,500)
Diluted earnings (losses) per share denominator (in shares)	2,522,900	2,855,500	2,528,700	2,923,800
Basic and diluted earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid (in usd per share)	$ 0	$ 0	$ 1.00	$ 1.00
Distributed earnings - dividends declared and paid (in usd per share)	0	0	1.00	1.00
Undistributed earnings (losses) (in usd per share)	9.19	(1.66)	85.82	(51.73)
Undistributed earnings (losses) (in usd per share)	9.19	(1.66)	85.82	(51.73)
Basic and diluted earnings (losses) per share (in usd per share)	9.19	(1.66)	86.82	(50.73)
Basic and diluted earnings (losses) per share (in usd per share)	$ 9.19	$ (1.66)	$ 86.82	$ (50.73)