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Summary of Significant Accounting Policies (Basis of Presentation) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2016 | Mar. 31, 2015 | Dec. 31, 2015 | Jul. 31, 2012
Basis of Presentation |  |  |  |
Debt Issuance Cost | $ 3.8 |  |  |
Net investment income | 17.9 | $ 12.6 |  |
Income Tax Expense (Benefit) | (8.7) | 4.6 |  |
Equity in earnings of unconsolidated affiliates, net of tax | $ 0.0 | $ 7.3 |  |
OneBeacon Ltd. |  |  |  |
Basis of Presentation |  |  |  |
Ownership interest (as a percent) | 76.10% |  | 75.50% |
HG Global |  |  |  |
Basis of Presentation |  |  |  |
Surplus Notes |  |  |  | $ 503.0
Percentage of par value of policy reinsured | 15.00% |  |  |
Noncontrolling equity percentage | 3.10% |  | 3.10% |
Star & Shield Insurance Exchange |  |  |  |
Basis of Presentation |  |  |  |
Surplus Notes | $ 21.0 |  |  |
Noncontrolling equity percentage | 100.00% |  | 100.00% |
OneBeacon [Member] |  |  |  |
Basis of Presentation |  |  |  |
Debt Issuance Cost |  |  | $ 1.9 |
Noncontrolling equity percentage | 23.90% |  | 24.50% |
Preferred Stock | HG Global |  |  |  |
Basis of Presentation |  |  |  |
Ownership interest (as a percent) | 96.90% |  |  |
Noncontrolling equity percentage | 96.90% |  | 96.90% |
Common Stock | HG Global |  |  |  |
Basis of Presentation |  |  |  |
Ownership interest (as a percent) | 88.40% |  |  |
Noncontrolling equity percentage | 88.40% |  | 88.40% |

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