Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income attributable to White Mountains’s common shareholders (3)	$ 22.5	$ 562.1	$ 13.6	$ 29.0	$ (38.9)	$ 90.8	$ 340.4	$ 9.5	$ 627.2	$ 401.8	$ 295.2
Less: total income from discontinued operations, net of tax									577.5	523.4	135.1
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders									49.7	(121.6)	160.1
Allocation of earnings to participating restricted common shares									0.7	(1.5)	1.9
Basic and diluted earnings per share numerators									49.0	(120.1)	158.2
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts									$ 49.0	$ (120.1)	$ 158.2
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									4,293,800	5,014,900	5,879,200
Average unvested restricted common shares									(54,300)	(64,800)	(68,000)
Basic earnings per share denominator									4,239,500	4,950,100	5,811,200
Diluted earnings per share denominator (in thousands):
Average outstanding dilutive shares outstanding									4,293,800	5,018,100	5,879,200
Average unvested restricted common shares									(54,300)	(64,800)	(68,000)
Diluted earnings (loss) per share denominator									4,239,500	4,953,300	5,811,200
Basic and diluted earnings per share (in dollars) - continuing operations:
Dividends declared and paid per White Mountains’s common share									$ 1	$ 1	$ 1
Undistributed earnings (loss)									10.56	(25.26)	26.22
Basic and diluted earnings per share (in usd per share)									$ 11.56	$ (24.26)	$ 27.22