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, 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
Net income (loss) from continuing operations	$ 13.8	(23.6)	$ 218.2	(252.3)
Less: net (income) loss from continuing operations attributable to noncontrolling interests		18.8		102.2
Total gain (loss) from discontinued operations attributable to White Mountains’s common shareholders		893.0		902.5
Allocation of earnings to participating restricted common shares		(11.8)		(10.8)
Basic earnings per share numerators		881.2		891.7
Diluted earnings per share numerators		$ 881.2		$ 891.7
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):
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 earnings (losses) per share (in dollars) - discontinued operations (in dollars per share)	$ 0	$ 308.59	$ 0	$ 304.97
Diluted earnings (losses) per share (in dollars) - discontinued operations (in dollars per share)	$ 0	$ 308.59	$ 0	$ 304.97