Derivatives (Pre-Tax Operating Results of WM Life Re) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pre-tax operating results of WM Life Re
Total revenues	$ 627.4	$ 629.6
General and administrative expenses	(43.8)	(45.5)
Pre-tax income from continuing operations	151.8	140.8
White Mountains Life Re
Pre-tax operating results of WM Life Re
Fees, included in other revenues	6.8	8.0
Change in fair value of variable annuity liability, included in other revenues	184.3	211.2
Realized and unrealized (losses) gains	(181.8)	(206.5)
Foreign exchange, included in other revenues	(14.6)	(23.0)
Other investment income and gains (losses)	(4.0)	(2.1)
Total revenues	(9.3)	(12.4)
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	5.6	8.8
Death benefit claims paid, included in general and administrative expenses	(1.1)	(1.8)
General and administrative expenses	(1.7)	(1.3)
Pre-tax income from continuing operations	$ (6.5)	$ (6.7)