Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’ common shareholders	$ 29.8	$ (65.4)	$ 24.2	$ (47.6)
Allocation of income for unvested restricted common shares	(0.4)	0.0	(0.3)	0.0
Dividends declared on participating restricted common shares	0.0	0.0	(0.1)	(0.1)
Total allocation to restricted common shares	(0.4)	0.0	(0.4)	(0.1)
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	29.4	(65.4)	23.8	(47.7)
Undistributed net earnings (in millions):
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	29.4	(65.4)	23.8	(47.7)
Dividends declared net of restricted common share amounts	0.0	0.0	(5.4)	(5.9)
Total undistributed net earnings, net of restricted common share amounts	$ 29.4	$ (65.4)	$ 18.4	$ (53.6)
Basic earnings per share denominators
Total average common shares outstanding during the period	4,867,400	5,890,100	5,166,600	5,951,100
Average unvested restricted common shares	(68,100)	0	(62,900)	0
Basic earnings per share denominator	4,799,300	5,890,100	5,103,700	5,951,100
Diluted earnings per share denominator
Total average common shares outstanding during the period	4,867,400	5,890,100	5,166,600	5,951,100
Average unvested restricted common shares	(68,100)	0	(62,900)	0
Average outstanding dilutive options to acquire common shares	4,879,400	5,890,100	5,174,800	5,951,100
Diluted earnings (loss) per share denominator	4,811,300	5,890,100	5,111,900	5,951,100
Basic earnings per share (in dollars):
Continuing operations	$ 6.12	$ (11.10)	$ 4.67	$ (8.01)
Undistributed earnings (loss) (in dollars per share)	6.12	(11.10)	3.67	(9.01)
Earnings Per Share, Diluted [Abstract]
Continuing operations	6.11	(11.10)	4.66	(8.01)
Dividends declared	(0.00)	(0.00)	(1.00)	(1.00)
Undistributed earnings (loss) (in dollars per share)	$ 6.11	$ (11.10)	$ 3.66	$ (9.01)
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	11,943		8,208
Discontinued Operations [Member]
Basic and diluted earnings per share numerators (in millions):
Allocation of income for unvested restricted common shares	$ 0.9	$ 0.1	$ (5.1)	$ 0.9
Basic earnings per share denominators
Total average common shares outstanding during the period	4,879,400	5,890,100	5,174,800	5,951,100
Average unvested restricted common shares	(68,100)	(70,900)	(62,900)	(67,100)
Basic earnings per share denominator	4,799,300	5,819,200	5,103,700	5,884,000
Diluted earnings per share denominator
Total average common shares outstanding during the period	4,879,400	5,890,100	5,174,800	5,951,100
Average unvested restricted common shares	(68,100)	(70,900)	(62,900)	(67,100)
Diluted earnings (loss) per share denominator	4,811,300	5,819,200	5,111,900	5,884,000
Employee Stock Option [Member]
Earnings Per Share, Diluted [Abstract]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	120,000		120,000