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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Basic and diluted earnings per share numerators (in millions):
Net (loss) income attributable to White Mountains’s common shareholders	$ (371.4)	$ 232.9	$ (308.2)	$ 219.5
Less: total income (loss) from discontinued operations, net of tax	0.0	0.7	(18.7)	0.8
Less: total income (loss) from continuing operations, net of tax	(371.4)	233.6	(326.9)	220.3
Allocation of losses (earnings) to participating restricted common shares	4.5	(3.2)	3.8	(2.9)
Basic and diluted (losses) earnings per share numerators	$ (366.9)	$ 230.4	$ (323.1)	$ 217.4
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	3,090,300	3,101,800	3,099,400	3,129,000
Average unvested restricted common shares (in shares)	(37,800)	(43,100)	(36,000.0)	(40,000.0)
Basic earnings (losses) per share denominator (in shares)	3,052,500	3,058,700	3,063,400	3,089,000
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)	3,090,300	3,101,800	3,099,400	3,129,000
Average unvested restricted common shares (in shares)	(37,800)	(43,100)	(36,000.0)	(40,000.0)
Diluted earnings (losses) per share denominator (in shares)	3,052,500	3,058,700	3,063,400	3,089,000
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)	(120.18)	75.32	(106.48)	69.40
Basic and diluted (losses) earnings per share (in usd per share)	$ (120.18)	$ 75.32	$ (105.48)	$ 70.40