Derivatives (Additional Collateral) (Details) - White Mountains Life Re - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Credit risk on derivative instruments
Collateral Cash and Investments	$ 0.0	$ 5.8	$ 33.2
Cash
Credit risk on derivative instruments
Derivative Collateral Right to Reclaim	0.0	$ 5.8
Fixed maturity investments
Credit risk on derivative instruments
Derivative Collateral Right to Reclaim	$ 0.0