Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 60.9	$ 48.7	$ 20.5	$ 284.4	$ (137.5)	$ 40.8	$ 3.5	$ (48.0)	$ 414.5	$ (141.2)	$ 627.2
Less: total income (loss) from discontinued operations, net of tax									0.8	(17.2)	577.5
Net income (loss) attributable to White Mountains’ common shareholders									413.7	(124.0)	49.7
Allocation of (earnings) losses to participating restricted common shares									(5.3)	1.4	(0.7)
Basic and diluted earnings per share numerators									$ 408.4	$ (122.6)	$ 49.0
Basic earnings per share denominators (in thousands):
Weighted Average Number of Shares Issued, Basic									3,181,600	3,382,500	4,293,800
Average unvested restricted common shares									(40,500)	(40,100)	(54,300)
Basic earnings (losses) per share denominator									3,141,100	3,342,400	4,239,500
Diluted earnings per share denominator (in thousands):
Average outstanding dilutive options to acquire common shares									3,181,600	3,382,500	4,293,800
Average unvested restricted common shares									(40,500)	(40,100)	(54,300)
Diluted earnings (loss) per share denominator									3,141,100	3,342,400	4,239,500
Diluted earnings per share (in dollars) - continuing operations:
Dividends declared and paid									$ 1.00	$ 1.00	$ 1.00
Undistributed earnings (losses)									129.02	(37.67)	10.56
Basic and diluted earnings (losses) per share									$ 130.02	$ (36.67)	$ 11.56