Derivatives Additional Collateral (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Credit risk on derivative instruments
Collateral cash and investments	$ 120.6	$ 393.6	$ 471.6
Cash
Credit risk on derivative instruments
Derivative collateral right to reclaim	61.2	249.8	360.0
Short-term investments
Credit risk on derivative instruments
Derivative collateral right to reclaim	35.1	5.1	20.6
Fixed maturity investments.
Credit risk on derivative instruments
Derivative collateral right to reclaim	$ 24.3	$ 138.7	$ 91.0