Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$ (137.5)	$ 40.8	$ 3.5	$ (48.0)	$ 22.5	$ 562.1	$ 13.6	$ 29.0	$ (141.2)	$ 627.2	$ 401.8
Less: total (loss) income from discontinued operations, net of tax									(17.2)	577.5	523.4
Net income (loss) attributable to White Mountains’ common shareholders									(124.0)	49.7	(121.6)
Allocation of earnings (losses) to participating restricted common shares									1.4	(0.7)	1.5
Basic and diluted earnings per share numerators									$ (122.6)	$ 49.0	$ (120.1)
Basic earnings per share denominators (in thousands):
Weighted Average Number of Shares Issued, Basic									3,382,500	4,293,800	5,014,900
Average unvested restricted common shares									(40,100)	(54,300)	(64,800.00)
Basic (losses) earnings per share denominator									3,342,400	4,239,500	4,950,100
Diluted earnings per share denominator (in thousands):
Average outstanding dilutive options to acquire common shares									3,382,500	4,293,800	5,018,100
Average unvested restricted common shares									(40,100)	(54,300)	(64,800.00)
Diluted earnings (loss) per share denominator									3,342,400.00	4,239,500	4,953,300.00
Diluted earnings per share (in dollars) - continuing operations:
Dividends declared and paid									$ 1.00	$ 1.00	$ 1.00
Undistributed earnings (loss)									(37.67)	10.56	(25.26)
Basic and diluted (losses) earnings per share									$ (36.67)	$ 11.56	$ (24.26)