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Segment Reporting - Summary of Gross and Net Written and Earned Premiums, Underwriting Results, Ratios and Reserves for Each of Company's Business Segments (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2016 | Dec. 31, 2015
Underwriting Revenues |  |  |  |  |  |
Gross written premiums | $ 852.5 | $ 763.5 | $ 2,672.6 | $ 2,540.9 |  |
Net written premiums | 607.4 | 638.4 | 1,872.3 | 2,162.9 |  |
Gross earned premiums | 913.3 | 809.8 | 2,414.3 | 2,346.7 |  |
Net earned premiums | 652.5 | 681.0 | 1,795.6 | 2,024.9 |  |
Underwriting Expenses |  |  |  |  |  |
Losses and loss adjustment expenses | 776.2 | 389.2 | 1,450.5 | 1,188.8 |  |
Amortization of deferred policy acquisition costs | 105.4 | 130.9 | 315.4 | 387.8 |  |
General and administrative expenses | 92.2 | 105.3 | 304.3 | 304.3 |  |
Underwriting loss | (321.3) | 55.6 | (274.6) | 144.0 |  |
Corporate expenses | (13.5) | (13.4) | (38.3) | (50.6) |  |
Non-operating expenses | (5.2) | (6.3) | (9.5) | (6.3) |  |
Net investment income | 46.4 | 46.4 | 141.5 | 143.9 |  |
Realized and unrealized investment gains | 29.9 | 26.7 | 130.1 | 137.4 |  |
Realized and unrealized investment losses | (12.4) | (5.2) | (24.4) | (34.1) |  |
Change in fair value of loan notes issued by variable interest entities | 9.8 | (9.8) | 3.6 | (13.7) |  |
Change in fair value of derivatives | 4.5 | 0.6 | 25.2 | (7.0) |  |
Interest expense on long term debt | (7.4) | (7.3) | (22.2) | (22.1) |  |
Net realized and unrealized foreign exchange gains | 8.4 | 10.8 | (21.1) | (10.2) |  |
Other income | (2.2) | 1.5 | 5.0 | 2.4 |  |
Net other income/(expense) | (2.2) | 2.4 |  |  |  |
Other expenses |  |  | (2.0) | (0.1) |  |
(Loss)/income from operations before income tax | (263.0) | 100.5 | (86.7) | 283.6 |  |
Net reserves for loss and loss adjustment expenses | $ 5,121.1 | $ 4,826.8 | $ 5,121.1 | $ 4,826.8 | $ 4,759.2 | $ 4,583.4
Ratios |  |  |  |  |  |
Loss ratio | 119.00% | 57.20% | 80.80% | 58.70% |  |
Policy acquisition expense ratio | 16.20% | 19.20% | 17.60% | 19.20% |  |
General and administrative expense ratio | 17.00% | 18.40% | 19.60% | 17.80% |  |
Expense ratio | 33.20% | 37.60% | 37.20% | 37.00% |  |
Combined ratio | 152.20% | 94.80% | 118.00% | 95.70% |  |
Reinsurance |  |  |  |  |  |
Underwriting Revenues |  |  |  |  |  |
Gross written premiums | $ 431.5 | $ 365.9 | $ 1,332.4 | $ 1,216.1 |  |
Net written premiums | 363.6 | 314.5 | 1,097.3 | 1,070.8 |  |
Gross earned premiums | 464.0 | 364.3 | 1,112.2 | 1,000.9 |  |
Net earned premiums | 382.0 | 316.3 | 932.2 | 896.0 |  |
Underwriting Expenses |  |  |  |  |  |
Losses and loss adjustment expenses | 502.2 | 178.7 | 797.9 | 494.3 |  |
Amortization of deferred policy acquisition costs | 61.5 | 53.0 | 174.4 | 163.1 |  |
General and administrative expenses | 32.8 | 47.4 | 117.4 | 130.6 |  |
Underwriting loss | (214.5) | 37.2 | (157.5) | 108.0 |  |
Net reserves for loss and loss adjustment expenses | $ 2,865.8 | $ 2,495.4 | $ 2,865.8 | $ 2,495.4 |  |
Ratios |  |  |  |  |  |
Loss ratio | 131.50% | 56.50% | 85.60% | 55.20% |  |
Policy acquisition expense ratio | 16.10% | 16.80% | 18.70% | 18.20% |  |
General and administrative expense ratio | 8.60% | 15.00% | 12.60% | 14.60% |  |
Expense ratio | 24.70% | 31.80% | 31.30% | 32.80% |  |
Combined ratio | 156.20% | 88.30% | 116.90% | 88.00% |  |
Insurance |  |  |  |  |  |
Underwriting Revenues |  |  |  |  |  |
Gross written premiums | $ 421.0 | $ 397.6 | $ 1,340.2 | $ 1,324.8 |  |
Net written premiums | 243.8 | 323.9 | 775.0 | 1,092.1 |  |
Gross earned premiums | 449.3 | 445.5 | 1,302.1 | 1,345.8 |  |
Net earned premiums | 270.5 | 364.7 | 863.4 | 1,128.9 |  |
Underwriting Expenses |  |  |  |  |  |
Losses and loss adjustment expenses | 274.0 | 210.5 | 652.6 | 694.5 |  |
Amortization of deferred policy acquisition costs | 43.9 | 77.9 | 141.0 | 224.7 |  |
General and administrative expenses | 59.4 | 57.9 | 186.9 | 173.7 |  |
Underwriting loss | (106.8) | 18.4 | (117.1) | 36.0 |  |
Net reserves for loss and loss adjustment expenses | $ 2,255.3 | $ 2,331.4 | $ 2,255.3 | $ 2,331.4 |  |
Ratios |  |  |  |  |  |
Loss ratio | 101.30% | 57.70% | 75.60% | 61.50% |  |
Policy acquisition expense ratio | 16.20% | 21.40% | 16.30% | 19.90% |  |
General and administrative expense ratio | 22.00% | 15.90% | 21.60% | 15.40% |  |
Expense ratio | 38.20% | 37.30% | 37.90% | 35.30% |  |
Combined ratio | 139.50% | 95.00% | 113.50% | 96.80% |  |

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