Derivatives (Realized and Unrealized Gains (Losses) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Variable Annuity Reinsurance [Line Items]
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	$ 0	$ 5.6
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	(6.6)	(181.8)
Derivative, fair value, net	89.6		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)	(6.4)	(30.9)
Derivative, fair value, net	(5.3)		(9.7)
Foreign exchange
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	(6.5)	(53.6)
Derivative, fair value, net	63.9		58.0
Equity
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	6.3	(97.3)
Derivative, fair value, net	$ 31.0		$ 20.9