Held for Sale and Discontinued Operations - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 489.2	$ 232.9	$ 115.4	$ (128.8)	$ 60.9	$ 48.7	$ 20.5	$ 284.4	$ 708.7	$ 414.5	$ (141.2)
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders									711.0	413.7	(124.0)
Total income (loss) from discontinued operations									2.3	(0.8)	17.2
Allocation of (earnings) losses to participating restricted common shares									(9.3)	(5.3)	1.4
Basic and diluted earnings (losses) per share numerators									$ 701.7	$ 408.4	$ (122.6)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)									3,122,200	3,181,600	3,382,500
Average unvested restricted common shares (in shares)									(40,800)	(40,500)	(40,100)
Basic earnings (losses) per share denominator (in shares)									3,081,400	3,141,100	3,342,400
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)									3,122,200	3,181,600	3,382,500
Average unvested restricted common shares (in shares)									(40,800)	(40,500)	(40,100)
Diluted earnings (loss) per share denominator (in shares)									3,081,400	3,141,100	3,342,400
Basic (losses) earnings per share (in usd per share)	$ 157.70	$ 75.09	$ 37.14	$ (40.54)	$ 19.12	$ 15.29	$ 6.44	$ 89.64	$ 226.97	$ 130.27	$ (41.76)
Diluted (losses) earnings per share (in usd per share)	$ 157.70	$ 75.09	$ 37.14	$ (40.54)	$ 19.12	$ 15.29	$ 6.44	$ 89.64	$ 226.97	$ 130.27	$ (41.76)
Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders									$ 708.7	$ 414.5	$ (141.2)
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders									711.0	413.7	(124.0)
Total income (loss) from discontinued operations									(2.3)	0.8	(17.2)
Allocation of (earnings) losses to participating restricted common shares									0.0	0.0	0.2
Basic and diluted earnings (losses) per share numerators									$ (2.3)	$ 0.8	$ (17.0)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)									3,122,200	3,181,600	3,382,500
Average unvested restricted common shares (in shares)									(40,800)	(40,500)	(40,100)
Basic earnings (losses) per share denominator (in shares)									3,081,400	3,141,100	3,342,400
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)									3,122,200	3,181,600	3,382,500
Average unvested restricted common shares (in shares)									(40,800)	(40,500)	(40,100)
Diluted earnings (loss) per share denominator (in shares)									3,081,400	3,141,100	3,342,400
Basic (losses) earnings per share (in usd per share)									$ (0.75)	$ 0.25	$ (5.09)
Diluted (losses) earnings per share (in usd per share)									$ (0.75)	$ 0.25	$ (5.09)