Derivatives (Additional Collateral) (Details) - White Mountains Life Re - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Credit risk on derivative instruments
Collateral Cash and Investments	$ 5.8	$ 33.2	$ 81.3
Cash
Credit risk on derivative instruments
Derivative Collateral Right to Reclaim	5.8	23.7
Fixed maturity investments
Credit risk on derivative instruments
Derivative Collateral Right to Reclaim	$ 0.0	$ 9.5