Discontinued Operations (Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Basic and diluted earnings per share numerators (in millions):
Allocation of income for participating unvested restricted common shares									$ 0.0	$ (2.3)	$ (0.7)
Net income attributable to White Mountains’s common shareholders	$ (33.3)	$ 91.7	$ 341.1	$ 13.0	$ 267.9	$ (58.9)	$ 4.3	$ 84.3	$ 412.5	$ 297.6	$ 312.2
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									5,014,900	5,879,200	6,104,900
Average unvested restricted common shares									(64,800)	(68,000)	(78,900)
Basic earnings per share denominator									4,950,100	5,811,200	6,026,000
Diluted earnings per share denominator (in thousands):
Average outstanding dilutive shares outstanding									5,018,100	5,879,200	6,104,900
Average unvested restricted common shares									(64,800)	(68,000)	(78,900)
Diluted earnings per share denominator(4)									4,953,300	5,811,200	6,026,000
Basic earnings per share (in dollars):	$ (7.30)	$ 18.84	$ 66.93	$ 2.34	$ 47.28	$ (10.01)	$ 0.72	$ 14.09	$ 82.24	$ 50.60	$ 51.13
Earnings Per Share, Diluted	$ (7.30)	$ 18.80	$ 66.79	$ 2.34	$ 47.25	$ (10.01)	$ 0.72	$ 14.09	$ 82.19	$ 50.60	$ 51.13
Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total income from discontinued operations									$ 414.8	$ 96.9	$ 259.0
Basic and diluted earnings per share numerators (in millions):
Allocation of income for participating unvested restricted common shares									(5.4)	1.1	(3.3)
Net income attributable to White Mountains’s common shareholders									$ 409.4	$ 95.8	$ 255.7
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									5,014,900	5,879,200	6,104,900
Average unvested restricted common shares									(64,800)	(68,000)	(78,900)
Basic earnings per share denominator									4,950,100	5,811,200	6,026,000
Diluted earnings per share denominator (in thousands):
Average outstanding dilutive shares outstanding									5,018,100	5,879,200	6,104,900
Average unvested restricted common shares									(64,800)	(68,000)	(78,900)
Diluted earnings per share denominator(4)									4,953,300	5,811,200	6,026,000
Basic earnings per share (in dollars):									$ 82.71	$ 16.48	$ 42.43
Earnings Per Share, Diluted									$ 82.66	$ 16.48	$ 42.43