Derivatives (Pre-Tax Operating Results of WM Life Re) (Details) - USD ($) $ 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
Pre-tax operating results of WM Life Re
Total revenues		$ 87.5	$ 83.5	$ 88.8	$ (5.5)	$ 56.8	$ 48.4	$ 58.0	$ 373.8	$ 157.7	$ 440.0
General and administrative expenses									(218.7)	(186.0)	(200.7)
Pre-tax (loss) income from continuing operations	$ 5.2	$ 8.4	$ (2.2)	$ (3.6)	$ (74.5)	$ (13.8)	$ (25.7)	$ (33.3)	$ 7.8	(147.3)	129.2
White Mountains Life Re
Pre-tax operating results of WM Life Re
Fees, included in other revenue										1.2	9.3
Change in fair value of variable annuity liability, included in other revenue										(0.3)	(0.4)
Change in fair value of derivatives, included in other revenue										(2.0)	(8.8)
Foreign exchange, included in other revenue										1.3	(1.3)
Other investment loss										0.0	(0.4)
Total revenues										0.2	(1.6)
Death benefit claims paid, included in general and administrative expenses										(0.3)	(0.1)
General and administrative expenses										(2.6)	(4.0)
Pre-tax (loss) income from continuing operations										$ (2.7)	$ (5.7)