Derivatives Additional Collateral (Details) (White Mountains Life Re [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Credit risk on derivative instruments
Derivative Collateral Right to Reclaim	$ 63.3	$ 73.2
Collateral Cash and Investments	477.5	485.3	419.7
Cash and Cash Equivalents [Member]
Credit risk on derivative instruments
Derivative Collateral Right to Reclaim	341.7	453.5
Short-term investments
Credit risk on derivative instruments
Derivative Collateral Right to Reclaim	14.6	0.6
Fixed maturity investments.
Credit risk on derivative instruments
Derivative Collateral Right to Reclaim	$ 121.2	$ 31.2