Derivatives (Realized and Unrealized Gains (Losses) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized (losses) gains	$ (181.8)	$ (206.5)
Derivative, fair value, net	75.8		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	(30.9)	(79.0)
Derivative, fair value, net	(16.2)		27.1
Foreign exchange
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized (losses) gains	(53.6)	(55.0)
Derivative, fair value, net	74.1		52.8
Equity
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized (losses) gains	(97.3)	(72.5)
Derivative, fair value, net	$ 17.9		$ 18.4