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Summary of Significant Accounting Policies (Basis of Presentation) (Details) - USD ($) $ in Millions | Dec. 31, 2016 | Dec. 31, 2015 | Jun. 30, 2015 | Dec. 31, 2012
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Basis of Presentation |  |  |  |
Debt issuance costs | $ 1.6 |  |  |
Assets held for sale | $ 10.1 | $ 4,790.4 |  |
Percentage of par value of policy reinsured | 15.00% |  |  |
OneBeacon Ltd. |  |  |  |
Basis of Presentation |  |  |  |
Ownership interest (as a percent) | 76.10% | 75.50% |  |
HG Global |  |  |  |
Basis of Presentation |  |  |  |
Surplus notes | $ 300.0 |  |  | $ 503.0
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% |  |  |
Wholly Owned Properties [Member] | OneBeacon Ltd. |  |  |  |
Basis of Presentation |  |  |  |
Assets held for sale |  |  | $ 58.0 |

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