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, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized (losses) gains	$ (63.5)	$ 95.5	$ (245.3)	$ (111.0)
Derivative, fair value, net	141.3		141.3		98.3
Fixed income/interest rate
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized (losses) gains	(8.5)	28.6	(39.3)	(50.5)
Derivative, fair value, net	11.4		11.4		27.1
Foreign exchange
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized (losses) gains	(22.0)	27.4	(75.6)	(27.5)
Derivative, fair value, net	70.6		70.6		52.8
Equity
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized (losses) gains	(33.0)	39.5	(130.4)	(33.0)
Derivative, fair value, net	$ 59.3		$ 59.3		$ 18.4