Derivatives (Realized and Unrealized Gains (Losses) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 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)	$ (26.2)	$ (63.5)	$ (32.7)	$ (245.3)
Derivative, fair value, net	74.8		74.8		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)	(5.6)	(8.5)	(12.0)	(39.3)
Derivative, fair value, net	(3.3)		(3.3)		(9.7)
Foreign exchange
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	(6.4)	(22.0)	(12.9)	(75.6)
Derivative, fair value, net	58.2		58.2		58.0
Equity
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	(14.2)	(33.0)	(7.8)	(130.4)
Derivative, fair value, net	$ 19.9		$ 19.9		$ 20.9