Derivatives Additional Collateral (Details) - White Mountains Life Re - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Credit risk on derivative instruments
Collateral cash and investments	$ 25.3	$ 33.2	$ 42.3
Cash
Credit risk on derivative instruments
Derivative collateral right to reclaim	20.9	23.7	27.8
Fixed maturity investments.
Credit risk on derivative instruments
Derivative collateral right to reclaim	$ 4.4	$ 9.5	$ 14.5