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, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 888.2	$ (371.4)	$ 752.4	$ (308.2)
Less: total income (loss) from discontinued operations, net of tax	893.1	5.3	903.2	(5.1)
Net (income) loss from discontinued operations attributable to non-controlling interests	(0.1)	0.0	(0.7)	1.1
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(4.8)	(376.7)	(150.1)	(304.2)
Allocation of losses (earnings) to participating restricted common shares	0.1	4.6	1.8	3.5
Basic and diluted earnings (losses) per share numerators	$ (4.7)	$ (372.1)	$ (148.3)	$ (300.7)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	2,893,800	3,090,300	2,959,300	3,099,400
Average unvested restricted common shares (in shares)	(38,300)	(37,800)	(35,500)	(36,000.0)
Basic earnings (losses) per share denominator (in shares)	2,855,500	3,052,500	2,923,800	3,063,400
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)	2,893,800	3,090,300	2,959,300	3,099,400
Average unvested restricted common shares (in shares)	(38,300)	(37,800)	(35,500)	(36,000.0)
Diluted earnings (losses) per share denominator (in shares)	2,855,500	3,052,500	2,923,800	3,063,400
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
Undistributed (losses) earnings (in usd per share)	(1.66)	(121.90)	(51.73)	(99.16)
Continuing operations (in usd per share)	(1.66)	(121.90)	(50.73)	(98.16)
Continuing operations (in usd per share)	$ (1.66)	$ (121.90)	$ (50.73)	$ (98.16)