Derivatives (Pre-Tax Operating Results of WM Life Re) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Pre-tax operating results of WM Life Re
Total revenues	$ 652.8	$ 489.3	$ 1,231.1	$ 1,116.7
General and administrative expenses	(70.3)	(40.2)	(120.6)	(84.0)
Pre-tax income from continuing operations	109.0	3.6	221.1	155.4
White Mountains Life Re
Pre-tax operating results of WM Life Re
Fees, included in other revenue	5.1	6.2	10.5	13.0
Change in fair value of variable annuity liability, included in other revenue	22.0	55.9	20.7	240.2
Realized and unrealized gains (losses)	(26.2)	(63.5)	(32.7)	(245.3)
Foreign exchange, included in other revenue	0.2	0.1	0.5	(14.5)
Other investment income and gains (losses)	0.2	(1.5)	0.5	(5.5)
Total revenues	1.3	(2.8)	(0.5)	(12.1)
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	0.3	1.6	0.3	7.2
Death benefit claims paid, included in general and administrative expenses	(0.1)	(0.3)	(0.1)	(1.4)
General and administrative expenses	(1.0)	(1.1)	(2.4)	(2.8)
Pre-tax income from continuing operations	$ 0.5	$ (2.6)	$ (2.7)	$ (9.1)