Investment Securities (Investments Held on Deposit or as Collateral) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Investment securities
Investments held in trusts	$ 104.8	$ 103.4
Assets held by insurance regulators	88.0	68.9
Surety Product Line [Member] | OneBeacon
Investment securities
Assets Held in Collateral	137.7	81.0
Fixed maturity investments | White Mountains Life Re
Investment securities
Derivative Collateral Right to Reclaim	$ 0.0	$ 9.5