Held for Sale and Discontinued Operations - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ (275.4)	$ 708.7	$ 414.5
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	(294.1)	711.0	413.7
Net income (loss) from discontinued operations attributable to White Mountains’s common shareholders	(18.7)	2.3	(0.8)
Allocation of (earnings) losses to participating restricted common shares	3.5	(9.3)	(5.3)
Basic and diluted earnings (losses) per share numerators	$ (290.6)	$ 701.7	$ 408.4
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	3,079,000	3,122,200	3,181,600
Average unvested restricted common shares (in shares)	(36,500)	(40,800)	(40,500)
Basic earnings (losses) per share denominator (in shares)	3,042,500	3,081,400	3,141,100
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)	3,079,000	3,122,200	3,181,600
Average unvested restricted common shares (in shares)	(36,500)	(40,800)	(40,500)
Diluted earnings (loss) per share denominator (in shares)	3,042,500	3,081,400	3,141,100
Basic (losses) earnings per share (in usd per share)	$ (89.46)	$ 226.97	$ 130.27
Diluted (losses) earnings per share (in usd per share)	$ (89.46)	$ 226.97	$ 130.27
Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ (275.4)	$ 708.7	$ 414.5
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	(294.1)	711.0	413.7
Net income (loss) from discontinued operations attributable to White Mountains’s common shareholders	18.7	(2.3)	0.8
Allocation of (earnings) losses to participating restricted common shares	(0.2)	0.0	0.0
Basic and diluted earnings (losses) per share numerators	$ 18.5	$ (2.3)	$ 0.8
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	3,079,000	3,122,200	3,181,600
Average unvested restricted common shares (in shares)	(36,500)	(40,800)	(40,500)
Basic earnings (losses) per share denominator (in shares)	3,042,500	3,081,400	3,141,100
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)	3,079,000	3,122,200	3,181,600
Average unvested restricted common shares (in shares)	(36,500)	(40,800)	(40,500)
Diluted earnings (loss) per share denominator (in shares)	3,042,500	3,081,400	3,141,100
Basic (losses) earnings per share (in usd per share)	$ 6.07	$ (0.75)	$ 0.25
Diluted (losses) earnings per share (in usd per share)	$ 6.07	$ (0.75)	$ 0.25