Summary of Significant Accounting Policies (Basis of Presentation) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Basis of Presentation
Assets held for sale	$ 58.1	$ 1,880.1
Percentage of par value of policy reinsured	15.00%
OneBeacon Ltd.
Basis of Presentation
Ownership interest (as a percent)	75.30%	75.20%
HG Global
Basis of Presentation
Surplus notes	503.0	503.0
Percentage of par value of policy reinsured	15.00%
Star & Shield Insurance Exchange [Member]
Basis of Presentation
Surplus notes	17.0
Redeemable preferred stocks \| HG Global
Basis of Presentation
Ownership interest (as a percent)	96.90%	97.30%
Common Stock \| HG Global
Basis of Presentation
Ownership interest (as a percent)	88.40%	88.70%
Wholly Owned Properties [Member] \| OneBeacon Ltd.
Basis of Presentation
Assets held for sale	$ 58.1

Represents the sum of the carrying value of all the Assets Held-for-Sale at the balance sheet date.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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Financial instruments issued by insurance entities (surplus notes) that are includable in surplus for statutory accounting purposes as prescribed or permitted by state laws and regulations that are liabilities of the issuer.

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Percentage of par value of policy reinsured.

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