Derivatives (Realized and Unrealized Gains (Losses) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	$ (23.8)	$ (63.1)	$ (56.5)	$ (308.4)
Derivative, fair value, net	65.3		65.3		69.2
Fixed income/interest rate
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	(8.4)	(24.1)	(20.4)	(97.4)
Derivative, fair value, net	(2.1)		(2.1)		(9.7)
Foreign exchange
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	(0.4)	(28.5)	(13.3)	(144.5)
Derivative, fair value, net	52.3		52.3		58.0
Equity
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	(15.0)	(10.5)	(22.8)	(66.5)
Derivative, fair value, net	$ 15.1		$ 15.1		$ 20.9