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Summary of Significant Accounting Policies (Basis of Presentation) (Details) - USD ($) $ in Millions | Dec. 31, 2015 | Jun. 30, 2015 | Dec. 31, 2014 | Dec. 31, 2012
--- | --- | --- | --- | ---
Basis of Presentation |  |  |  |
Assets held for sale | $ 4,407.0 |  | $ 4,630.6 |
Percentage of par value of policy reinsured | 15.00% |  |  |
OneBeacon Ltd. |  |  |  |
Basis of Presentation |  |  |  |
Ownership interest (as a percent) | 75.50% |  | 75.30% |
HG Global |  |  |  |
Basis of Presentation |  |  |  |
Surplus notes |  |  |  | $ 503.0
Percentage of par value of policy reinsured | 15.00% |  |  |
Star & Shield Insurance Exchange [Member] |  |  |  |
Basis of Presentation |  |  |  |
Surplus notes | $ 21.0 |  | $ 17.0 |
Redeemable preferred stocks | HG Global |  |  |  |
Basis of Presentation |  |  |  |
Ownership interest (as a percent) | 96.90% |  |  |
Common Stock | HG Global |  |  |  |
Basis of Presentation |  |  |  |
Ownership interest (as a percent) | 88.40% |  | 88.40% |
Wholly Owned Properties [Member] | OneBeacon Ltd. |  |  |  |
Basis of Presentation |  |  |  |
Assets held for sale |  | $ 58.0 |  |

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