Derivatives Pre-Tax Operating Results of WM Life Re (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pre-tax operating results of WM Life Re
Total revenues	$ 85.4	$ 49.2	$ 178.9	$ 108.0
General and administrative expenses	(57.5)	(45.2)	(117.9)	(99.9)
Pre-tax (loss) income from continuing operations	$ (0.4)	(25.1)	$ 1.0	(54.8)
White Mountains Life Re
Pre-tax operating results of WM Life Re
Fees, included in other revenue		0.3		1.2
Change in fair value of variable annuity liability, included in other revenue		0.1		(0.3)
Change in fair value of derivatives, included in other revenue		(0.3)		(2.0)
Foreign exchange, included in other revenue		0.4		1.3
Total revenues		0.5		0.2
Death benefit claims paid, included in general and administrative expenses		(0.2)		(0.3)
General and administrative expenses		(1.2)		(1.9)
Pre-tax (loss) income from continuing operations		$ (0.9)		$ (2.0)