Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2017	Sep. 30, 2016	Mar. 31, 2016
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’ common shareholders	$ 35.3		$ 11.9
Allocation of income for unvested restricted common shares	(0.4)		(0.1)
Dividends declared on participating restricted common shares	(0.1)		(0.1)
Total allocation to restricted common shares	(0.5)		(0.2)
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	34.8		11.7
Undistributed net earnings (in millions):
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	34.8		11.7
Dividends declared net of restricted common share amounts	(4.5)		(5.9)
Total undistributed net earnings, net of restricted common share amounts	$ 30.3		$ 5.8
Basic earnings per share denominators
Total average common shares outstanding during the period	4,564,600		5,539,600
Average unvested restricted common shares	(52,500)		(54,000)
Basic earnings per share denominator	4,512,100		5,485,600
Diluted earnings per share denominator
Total average common shares outstanding during the period	4,564,600		5,539,600
Average unvested restricted common shares	(52,500)		(54,000)
Average outstanding dilutive options to acquire common shares	4,564,600		5,540,500
Diluted earnings (loss) per share denominator	4,512,100		5,486,500
Basic earnings per share (in dollars):
Continuing operations	$ 7.72		$ 2.14
Dividends declared	(1.00)		(1.00)
Undistributed earnings (loss) (in dollars per share)	6.72		1.14
Earnings Per Share, Diluted [Abstract]
Continuing operations	7.72		2.14
Dividends declared	(1.00)		(1.00)
Undistributed earnings (loss) (in dollars per share)	6.72		$ 1.14
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements			882
Earnings Per Share, Diluted	$ 7.50		$ 2.34
Discontinued Operations [Member]
Basic and diluted earnings per share numerators (in millions):
Allocation of income for unvested restricted common shares	$ 0.0		$ 0.0
Basic earnings per share denominators
Total average common shares outstanding during the period	4,564,600		5,540,500
Average unvested restricted common shares	(52,500)		(54,000)
Basic earnings per share denominator	4,512,100		5,485,600
Diluted earnings per share denominator
Total average common shares outstanding during the period	4,564,600		5,540,500
Average unvested restricted common shares	(52,500)		(54,000)
Diluted earnings (loss) per share denominator	4,512,100		5,486,500
Earnings Per Share, Diluted [Abstract]
Earnings Per Share, Diluted	$ (0.22)		$ 0.20
Employee Stock Option [Member]
Earnings Per Share, Diluted [Abstract]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements		120,000
Employee Stock Option [Member]
Earnings Per Share, Diluted [Abstract]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements			125,000
Earnings Per Share, Diluted			$ 0.01