Derivatives Pre-Tax Operating Results of WM Life Re (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Variable Annuity Reinsurance [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax	$ 0.0	$ 3.9	$ 0.0	$ 18.0
Pre-tax operating results of WM Life Re
Total revenues	365.0	296.1	1,092.5	1,019.1
General and administrative expenses	(70.2)	(97.3)	(230.4)	(231.6)
Pre-tax income (loss) from continuing operations	20.0	(86.9)	26.4	(89.0)
White Mountains Life Re
Pre-tax operating results of WM Life Re
Fees, included in other revenue	0.0	2.1	1.2	7.7
Change in fair value of variable annuity liability, included in other revenue	0.0	(2.8)	(0.3)	(1.7)
Realized and unrealized gains (losses)	0.0	1.5	(2.0)	(6.3)
Foreign exchange, included in other revenue	0.0	0.4	1.4	(0.8)
Other investment income and losses	0.0	(0.2)	0.0	(0.3)
Total revenues	0.0	1.0	0.3	(1.4)
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	0.0	0.0	(0.3)	0.0
Death benefit claims paid, included in general and administrative expenses	0.0	0.0	0.0	0.0
General and administrative expenses	(0.5)	(0.8)	(2.4)	(3.1)
Pre-tax income (loss) from continuing operations	$ (0.5)	$ 0.2	$ (2.4)	$ (4.5)