Held for Sale and Discontinued Operations - Earnings Per Share for Discontinued Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Basic and diluted earnings per share numerators (in millions):
Net Income (Loss) Available to Common Stockholders, Basic, Total	$ 179.5	$ 33.4
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	179.5	29.8
Total gain (loss) from discontinued operations attributable to White Mountains’s common shareholders	0.0	(3.7)
Allocation of earnings to participating restricted common shares	(2.2)	(0.3)
Basic and diluted earnings (losses) per share numerators	$ 177.3	$ 29.5
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	2,569,900	3,006,200
Average unvested restricted common shares	(31,100)	(29,600)
Basic earnings (losses) per share denominator (in shares)	2,538,800	2,976,600
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	2,569,900	3,006,200
Average unvested restricted common shares	(31,100)	(29,600)
Diluted earnings (losses) per share denominator	2,538,800	2,976,600
Discontinued operations (in usd per share)	$ 0	$ 1.20
Discontinued operations (in usd per share)	$ 0	$ 1.20
Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Net Income (Loss) Available to Common Stockholders, Basic, Total		$ 33.4
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		(12.1)
Less: net (income) loss from continuing operations attributable to noncontrolling interests		41.9
Total gain (loss) from discontinued operations attributable to White Mountains’s common shareholders		3.6
Allocation of earnings to participating restricted common shares		0.0
Basic and diluted earnings (losses) per share numerators		$ 3.6
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period		3,006,200
Average unvested restricted common shares		(29,600)
Basic earnings (losses) per share denominator (in shares)		2,976,600
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period		3,006,200
Average unvested restricted common shares		(29,600)
Diluted earnings (losses) per share denominator		2,976,600
Discontinued operations (in usd per share)		$ 1.20