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Summary of Significant Accounting Policies (Basis of Presentation) (Details) £ in Millions, $ in Millions |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Aug. 04, 2016 GBP (£) | Aug. 04, 2016 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($) | Dec. 31, 2015 USD ($) | Jul. 31, 2012 USD ($)
Basis of Presentation |  |  |  |  |  |  |  |
Debt Issuance Cost |  |  |  |  | $ 1.9 |  |  |
Net investment income |  |  | $ 22.5 | $ 16.8 | 59.6 | $ 42.6 |  |
Income Tax Expense (Benefit) |  |  | (6.7) | (1.6) | (22.4) | 0.8 |  |
Equity in earnings of unconsolidated affiliates, net of tax |  |  | $ 0.0 | $ 3.9 | 0.0 | 18.0 |  |
Payments to Acquire Businesses and Interest in Affiliates |  |  |  |  | $ 13.7 | $ 2.4 |  |
OneBeacon Ltd. |  |  |  |  |  |  |  |
Basis of Presentation |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  | 76.10% |  | 76.10% |  | 75.50% |
HG Global |  |  |  |  |  |  |  |
Basis of Presentation |  |  |  |  |  |  |  |
Surplus Notes |  |  |  |  |  |  |  | $ 503.0
Percentage of par value of policy reinsured |  |  | 15.00% |  | 15.00% |  |  |
Noncontrolling equity percentage |  |  | 3.10% |  | 3.10% |  | 3.10% |
Buzzmove [Member] |  |  |  |  |  |  |  |
Basis of Presentation |  |  |  |  |  |  |  |
Ownership interest (as a percent) | 70.90% | 70.90% |  |  |  |  |  |
Noncontrolling equity percentage |  |  | 29.10% |  | 29.10% |  | 0.00% |
Star & Shield Insurance Exchange |  |  |  |  |  |  |  |
Basis of Presentation |  |  |  |  |  |  |  |
Noncontrolling equity percentage |  |  | 100.00% |  | 100.00% |  | 100.00% |
OneBeacon [Member] |  |  |  |  |  |  |  |
Basis of Presentation |  |  |  |  |  |  |  |
Debt Issuance Cost |  |  |  |  |  |  | $ 1.9 |
Noncontrolling equity percentage |  |  | 23.90% |  | 23.90% |  | 24.50% |
Preferred Stock | HG Global |  |  |  |  |  |  |  |
Basis of Presentation |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  | 96.90% |  | 96.90% |  |  |
Noncontrolling equity percentage |  |  | 96.90% |  | 96.90% |  | 96.90% |
Common Stock | HG Global |  |  |  |  |  |  |  |
Basis of Presentation |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  | 88.40% |  | 88.40% |  |  |
Noncontrolling equity percentage |  |  | 88.40% |  | 88.40% |  | 88.40% |
United Kingdom, Pounds | Buzzmove [Member] |  |  |  |  |  |  |  |
Basis of Presentation |  |  |  |  |  |  |  |
Payments to Acquire Businesses and Interest in Affiliates | £ 6.0 | $ 6.0 |  |  |  |  |  |
United States of America, Dollars | Buzzmove [Member] |  |  |  |  |  |  |  |
Basis of Presentation |  |  |  |  |  |  |  |
Payments to Acquire Businesses and Interest in Affiliates |  | $ 8.0 |  |  |  |  |  |

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