Derivatives Pre-Tax Operating Results of WM Life Re (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pre-tax operating results of WM Life Re
Total revenues	$ 334.7	$ 323.2	$ 1,130.1	$ 1,048.5
General and administrative expenses	(142.6)	(63.0)	(354.1)	(168.3)
Pre-tax (loss) income from continuing operations	(94.6)	(35.5)	(103.2)	36.7
White Mountains Life Re
Pre-tax operating results of WM Life Re
Fees, included in other revenue	2.1	4.5	7.7	15.0
Change in fair value of variable annuity liability, included in other revenue	(2.8)	19.9	(1.7)	40.5
Realized and unrealized gains (losses)	1.5	(23.8)	(6.3)	(56.5)
Foreign exchange, included in other revenue	0.4	(1.7)	(0.8)	(1.2)
Other investment income and (losses) gains	(0.2)	(0.9)	(0.3)	(0.4)
Total revenues	1.0	(2.0)	(1.4)	(2.6)
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	0.0	0.2	0.0	0.6
Death benefit claims paid, included in general and administrative expenses	0.0	0.0	0.0	(0.1)
General and administrative expenses	(0.8)	(0.9)	(3.1)	(3.3)
Pre-tax (loss) income from continuing operations	$ 0.2	$ (2.7)	$ (4.5)	$ (5.4)