Derivatives (Pre-Tax Operating Results of WM Life Re) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pre-tax operating results of WM Life Re
Total revenues	$ 6.0	$ 198.7	$ 122.3	$ 42.1	$ 114.0	$ 87.5	$ 83.5	$ 88.8	$ 369.1	$ 373.8	$ 157.7
General and administrative expenses									(235.7)	(208.0)	(175.5)
Pre-tax income (loss)	$ (144.1)	$ 44.3	$ (12.4)	$ (66.0)	$ 5.2	$ 8.4	$ (2.2)	$ (3.6)	$ (178.2)	$ 7.8	(147.3)
White Mountains Life Re
Pre-tax operating results of WM Life Re
Fees, included in other revenue											1.2
Change in fair value of variable annuity liability, included in other revenue											(0.3)
Change in fair value of derivatives, included in other revenue											(2.0)
Foreign exchange, included in other revenue											1.3
Total revenues											0.2
Liability for Future Policy Benefits, Other Increase (Decrease)											(0.3)
General and administrative expenses											(2.6)
Pre-tax income (loss)											$ (2.7)