Held for Sale and Discontinued Operations - Earnings Per Share for Discontinued 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)	$ 888.2	$ (371.4)	$ 752.4	$ (308.2)
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(4.8)	(376.7)	(150.1)	(304.2)
Total gain (loss) from discontinued operations attributable to White Mountains’s common shareholders	(893.1)	(5.3)	(903.2)	5.1
Allocation of 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
Discontinued operations (in usd per share)	$ 308.59	$ 1.72	$ 304.97	$ (1.29)
Discontinued operations (in usd per share)	$ 308.59	$ 1.72	$ 304.97	$ (1.29)
Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Net income (loss)	$ 888.2	$ (371.4)	$ 752.4	$ (308.2)
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(23.6)	(389.2)	(252.3)	(356.8)
Less: net (income) loss from continuing operations attributable to non-controlling interests	18.8	12.5	102.2	52.6
Total gain (loss) from discontinued operations attributable to White Mountains’s common shareholders	893.0	5.3	902.5	(4.0)
Allocation of earnings to participating restricted common shares	(11.8)	(0.1)	(10.8)	0.0
Basic and diluted earnings (losses) per share numerators	$ 881.2	$ 5.2	$ 891.7	$ (4.0)
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
Discontinued operations (in usd per share)	$ 308.59	$ 1.72	$ 304.97	$ (1.29)