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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Variable Annuity Reinsurance [Line Items]
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	$ 1.3	$ 0.7	$ 8.5	$ 6.7
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized (losses) gains	(63.1)	(11.0)	(308.4)	(122.0)
Derivative, fair value, net	91.5		91.5		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	(24.1)	(22.6)	(97.4)	(73.1)
Derivative, fair value, net	102.4		102.4		27.1
Foreign exchange
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized (losses) gains	(28.5)	6.5	(144.5)	(21.0)
Derivative, fair value, net	(7.1)		(7.1)		52.8
Equity
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized (losses) gains	(10.5)	5.1	(66.5)	(27.9)
Derivative, fair value, net	$ (3.8)		$ (3.8)		$ 18.4