Investment Securities (Investments Held on Deposit or as Collateral) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Investment securities
Investments held in trusts	$ 170.4	$ 169.9
Assets held by insurance regulators	290.7	319.3
Surety Product Line [Member] | OneBeacon
Investment securities
Assets Held in Collateral	63.3	5.9
Fixed maturity investments | White Mountains Life Re
Investment securities
Derivative Collateral Right to Reclaim	23.2	138.7
Short-term investments | White Mountains Life Re
Investment securities
Derivative Collateral Right to Reclaim	$ 2.0	$ 5.1