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Summary of Significant Accounting Policies (Basis of Presentation) (Details) - USD ($) $ in Millions | Mar. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Jul. 31, 2012
--- | --- | --- | --- | ---
Basis of Presentation |  |  |  |
Debt Issuance Cost | $ 1.8 |  |  |
OneBeacon Ltd. |  |  |  |
Basis of Presentation |  |  |  |
Ownership interest (as a percent) | 75.70% | 76.10% |  |
HG Global |  |  |  |
Basis of Presentation |  |  |  |
Noncontrolling equity percentage | 3.10% | 3.10% |  |
Surplus Notes |  |  |  | $ 503.0
Percentage of par value of policy reinsured | 15.00% |  |  |
OneBeacon [Member] |  |  |  |
Basis of Presentation |  |  |  |
Noncontrolling equity percentage | 24.30% | 23.90% |  |
Debt Issuance Cost |  |  | $ 1.9 |
Preferred Stock | HG Global |  |  |  |
Basis of Presentation |  |  |  |
Noncontrolling equity percentage |  | 96.90% |  |
Ownership interest (as a percent) | 96.90% |  |  |
Common Stock | HG Global |  |  |  |
Basis of Presentation |  |  |  |
Noncontrolling equity percentage |  | 88.40% |  |
Ownership interest (as a percent) | 88.40% |  |  |

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