Derivatives Additional Collateral (Details) - White Mountains Life Re - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Credit risk on derivative instruments
Collateral cash and investments	$ 16.8	$ 33.2	$ 34.2
Cash
Credit risk on derivative instruments
Derivative collateral right to reclaim	16.8	23.7	23.8
Fixed maturity investments.
Credit risk on derivative instruments
Derivative collateral right to reclaim	$ 0.0	$ 9.5	$ 10.4