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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Basic and diluted earnings per share numerators (in millions):
Net (loss) income	$ (371.4)	$ 232.9	$ (308.2)	$ 219.5
Less: total income (loss) from continuing operations, net of tax	(371.4)	233.6	(326.9)	220.3
Net (loss) gain from discontinued operations attributable to White Mountains’s common shareholders	0.0	(0.7)	18.7	(0.8)
Allocation of 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 (losses) earnings per share (in usd per share)	$ (120.18)	$ 75.32	$ (105.48)	$ 70.40
Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Net (loss) income	$ (371.4)	$ 232.9	$ (308.2)	$ 219.5
Less: total income (loss) from continuing operations, net of tax	(371.4)	233.6	(326.9)	220.3
Net (loss) gain from discontinued operations attributable to White Mountains’s common shareholders	0.0	(0.7)	18.7	(0.8)
Allocation of earnings to participating restricted common shares	0.0	0.0	(0.2)	0.0
Basic and diluted (losses) earnings per share numerators	$ 0.0	$ (0.7)	$ 18.5	$ (0.8)
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 (losses) earnings per share (in usd per share)	$ 0	$ (0.23)	$ 6.03	$ (0.26)