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
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income (loss) attributable to White Mountains’ common shareholders									$ (2.3)	$ 200.7	$ 53.2
Basic and diluted earnings per share numerators (in millions):
Allocation of income for unvested restricted common shares(1)									0.0	(2.3)	(0.7)
Dividends declared on participating restricted common shares(1)									0.0	(0.1)	(0.1)
Total allocation to restricted common shares									0.0	(2.4)	(0.8)
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts									(2.3)	198.3	52.4
Undistributed net earnings (in millions):
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts									(2.3)	198.3	52.4
Dividends declared net of restricted common share amounts(1)									(5.4)	(5.9)	(6.1)
Total undistributed net earnings, net of restricted common share amounts									$ (7.7)	$ 192.4	$ 46.3
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 (loss) per share denominator									4,953,300	5,811,200	6,026,000
Basic earnings per share (in dollars):
Net income attributable to White Mountains’s common shareholders	$ (5.81)	$ 6.12	$ (3.44)	$ 2.51	$ 43.82	$ (11.10)	$ 0.80	$ 1.76	$ (0.47)	$ 34.12	$ 8.70
Dividends declared and paid									(1.00)	(1.00)	(1.00)
Undistributed (loss) earnings									(1.47)	33.12	7.70
Diluted earnings per share (in dollars)
Net income attributable to White Mountains’s common shareholders	$ (5.81)	$ 6.11	$ (3.43)	$ 2.51	$ 43.79	$ (11.10)	$ 0.80	$ 1.76	(0.47)	34.12	8.70
Dividends declared and paid									(1.00)	(1.00)	(1.00)
Undistributed (loss) earnings									$ (1.47)	$ 33.12	$ 7.70