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Reserves for Unpaid Losses and Loss Adjustment Expenses Reserves for Unpaid Losses and Loss Adjustment Expenses (Reserve Development) (Details) - USD ($) $ in Millions |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Sep. 30, 2014 | Dec. 31, 2014 | Sep. 30, 2014 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |  |  |
Current Accident Year |  |  |  | $ 649.2 | $ 712.9 | $ 732.0
Prior Accident Year |  |  |  | 14.8 | (4.0) | 92.0
Total incurred losses and LAE |  |  |  | 664.0 | 708.9 | 824.0
OneBeacon |  |  |  |  |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |  |  |
Current Accident Year |  | $ 33.7 | $ 33.7 |  |  |
Prior Accident Year | $ 75.5 | 75.5 | 14.3 | 15.4 | $ (1.8) | 89.8
Total incurred losses and LAE |  | 109.2 | $ 109.2 |  |  | 89.8
Professional Insurance (1) | OneBeacon |  |  |  |  |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |  |  |
Current Accident Year |  | 22.9 |  |  |  |
Prior Accident Year |  | 46.4 |  |  |  |
Total incurred losses and LAE |  | 69.3 |  |  |  | 59.1
Specialty Property | OneBeacon |  |  |  |  |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |  |  |
Current Accident Year |  | (1.1) |  |  |  |
Prior Accident Year |  | 5.7 |  |  |  |
Total incurred losses and LAE |  | 4.6 |  |  |  | 1.1
Crop Business | OneBeacon |  |  |  |  |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |  |  |
Current Accident Year |  | 3.8 |  |  |  |
Prior Accident Year |  | 0.0 |  |  |  |
Total incurred losses and LAE |  | 3.8 |  |  |  | 0.0
Other | OneBeacon |  |  |  |  |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |  |  |
Current Accident Year |  | 2.8 |  |  |  |
Prior Accident Year |  | (0.4) |  |  |  |
Total incurred losses and LAE |  | 2.4 |  |  |  | 1.6
Specialty Products | OneBeacon |  |  |  |  |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |  |  |
Current Accident Year |  | 28.4 |  |  |  |
Prior Accident Year |  | 51.7 |  |  |  |
Total incurred losses and LAE |  | 80.1 |  |  |  | 61.8
Entertainment | OneBeacon |  |  |  |  |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |  |  |
Current Accident Year |  | 1.5 |  |  |  |
Prior Accident Year |  | 11.6 |  |  |  |
Total incurred losses and LAE |  | 13.1 |  |  |  | 13.5
Government Risks | OneBeacon |  |  |  |  |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |  |  |
Current Accident Year |  | 1.2 |  |  |  |
Prior Accident Year |  | 7.1 |  |  |  |
Total incurred losses and LAE |  | 8.3 |  |  |  | 8.5
Accident | OneBeacon |  |  |  |  |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |  |  |
Current Accident Year |  | 0.0 |  |  |  |
Prior Accident Year |  | 3.5 |  |  |  |
Total incurred losses and LAE |  | 3.5 |  |  |  | 6.0
Other | OneBeacon |  |  |  |  |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |  |  |
Current Accident Year |  | 2.6 |  |  |  |
Prior Accident Year |  | 1.6 |  |  |  |
Total incurred losses and LAE |  | 4.2 |  |  |  | 0.0
Specialty Industries | OneBeacon |  |  |  |  |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |  |  |
Current Accident Year |  | 5.3 |  |  |  |
Prior Accident Year |  | 23.8 |  |  |  |
Total incurred losses and LAE |  | $ 29.1 |  |  |  | $ 28.0
Healthcare Sector [Member] | OneBeacon |  |  |  |  |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |  |  |
Prior Accident Year |  |  |  | $ 40.7 |  |

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