Derivatives Additional Collateral (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Credit risk on derivative instruments
Collateral cash and investments	$ 114.8	$ 393.6	$ 477.5
Cash
Credit risk on derivative instruments
Derivative collateral right to reclaim	74.4	249.8	341.7
Short-term investments
Credit risk on derivative instruments
Derivative collateral right to reclaim	15.8	5.1	14.6
Fixed maturity investments.
Credit risk on derivative instruments
Derivative collateral right to reclaim	$ 24.6	$ 138.7	$ 121.2