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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, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016
Underwriting Revenues |  |  |  |  |  |
Gross written premiums | $ 873.2 | $ 852.5 | $ 2,843.8 | $ 2,672.6 |  |
Net written premiums | 578.9 | 607.4 | 1,700.4 | 1,872.3 |  |
Gross earned premiums | 973.6 | 913.3 | 2,661.7 | 2,414.3 |  |
Net earned premiums | 623.2 | 652.5 | 1,676.2 | 1,795.6 |  |
Underwriting Expenses |  |  |  |  |  |
Losses and loss adjustment expenses | 431.1 | 776.2 | 1,051.7 | 1,450.5 |  |
Amortization of deferred policy acquisition costs | 101.0 | 105.4 | 277.7 | 315.4 |  |
General and administrative expenses | 93.4 | 92.2 | 273.7 | 304.3 |  |
Underwriting (loss)/income | (2.3) | (321.3) | 73.1 | (274.6) |  |
Corporate expenses | (16.0) | (13.5) | (45.7) | (38.3) |  |
Non-operating income (expense) | (51.0) | (5.2) | (72.2) | (9.5) |  |
Net investment income | 48.0 | 46.4 | 145.7 | 141.5 |  |
Realized and unrealized investment gains | 1.8 | 29.9 | 105.9 | 130.1 |  |
Realized and unrealized investment losses | (2.7) | (12.4) | (165.2) | (24.4) |  |
Realized loss on debt extinguishment |  |  | (8.6) |  |  |
Change in fair value of loan notes issued by variable interest entities | (1.7) | 9.8 | (4.1) | 3.6 |  |
Change in fair value of derivatives | 7.2 | 4.5 | (15.4) | 25.2 |  |
Interest expense on long term debt | (5.4) | (7.4) | (20.4) | (22.2) |  |
Net realized and unrealized foreign exchange losses | (9.5) | 8.4 | (9.0) | (21.1) |  |
Other income | 1.4 | (2.2) | 5.6 | 5.0 |  |
Net other expense |  | (2.2) |  |  |  |
Other expenses | (0.4) |  | (2.1) | (2.0) |  |
(Loss) from operations before income tax | (30.6) | (263.0) | (12.4) | (86.7) |  |
Net reserves for loss and loss adjustment expenses | $ 4,959.1 | $ 5,121.1 | $ 4,959.1 | $ 5,121.1 | $ 5,234.3 | $ 4,759.2
Ratios |  |  |  |  |  |
Loss ratio | 69.20% | 119.00% | 62.70% | 80.80% |  |
Policy acquisition expense ratio | 16.20% | 16.20% | 16.60% | 17.60% |  |
General and administrative expense ratio | 25.70% | 17.00% | 23.40% | 19.60% |  |
Expense ratio | 41.90% | 33.20% | 40.00% | 37.20% |  |
Combined ratio | 111.10% | 152.20% | 102.70% | 118.00% |  |
Non-operating expenses related to Effectiveness and Efficiency Program | $ 11.1 | $ 4.1 | $ 31.5 | $ 4.1 |  |
Non-operating expenses related to Merger Agreement | 38.6 |  | 38.6 |  |  |
Reinsurance |  |  |  |  |  |
Underwriting Revenues |  |  |  |  |  |
Gross written premiums | 396.4 | 431.5 | 1,345.9 | 1,332.4 |  |
Net written premiums | 356.4 | 363.6 | 1,048.3 | 1,097.3 |  |
Gross earned premiums | 475.3 | 464.0 | 1,216.5 | 1,112.2 |  |
Net earned premiums | 388.5 | 382.0 | 960.0 | 932.2 |  |
Underwriting Expenses |  |  |  |  |  |
Losses and loss adjustment expenses | 281.5 | 502.2 | 615.4 | 797.9 |  |
Amortization of deferred policy acquisition costs | 75.7 | 61.5 | 194.4 | 174.4 |  |
General and administrative expenses | 34.7 | 32.8 | 94.2 | 117.4 |  |
Underwriting (loss)/income | (3.4) | (214.5) | 56.0 | (157.5) |  |
Net reserves for loss and loss adjustment expenses | $ 2,829.1 | $ 2,865.8 | $ 2,829.1 | $ 2,865.8 |  |
Ratios |  |  |  |  |  |
Loss ratio | 72.50% | 131.50% | 64.10% | 85.60% |  |
Policy acquisition expense ratio | 19.50% | 16.10% | 20.30% | 18.70% |  |
General and administrative expense ratio | 8.90% | 8.60% | 9.80% | 12.60% |  |
Expense ratio | 28.40% | 24.70% | 30.10% | 31.30% |  |
Combined ratio | 100.90% | 156.20% | 94.20% | 116.90% |  |
Insurance |  |  |  |  |  |
Underwriting Revenues |  |  |  |  |  |
Gross written premiums | $ 476.8 | $ 421.0 | $ 1,497.9 | $ 1,340.2 |  |
Net written premiums | 222.5 | 243.8 | 652.1 | 775.0 |  |
Gross earned premiums | 498.3 | 449.3 | 1,445.2 | 1,302.1 |  |
Net earned premiums | 234.7 | 270.5 | 716.2 | 863.4 |  |
Underwriting Expenses |  |  |  |  |  |
Losses and loss adjustment expenses | 149.6 | 274.0 | 436.3 | 652.6 |  |
Amortization of deferred policy acquisition costs | 25.3 | 43.9 | 83.3 | 141.0 |  |
General and administrative expenses | 58.7 | 59.4 | 179.5 | 186.9 |  |
Underwriting (loss)/income | 1.1 | (106.8) | 17.1 | (117.1) |  |
Net reserves for loss and loss adjustment expenses | $ 2,130.0 | $ 2,255.3 | $ 2,130.0 | $ 2,255.3 |  |
Ratios |  |  |  |  |  |
Loss ratio | 63.70% | 101.30% | 60.90% | 75.60% |  |
Policy acquisition expense ratio | 10.80% | 16.20% | 11.60% | 16.30% |  |
General and administrative expense ratio | 25.00% | 22.00% | 25.10% | 21.60% |  |
Expense ratio | 35.80% | 38.20% | 36.70% | 37.90% |  |
Combined ratio | 99.50% | 139.50% | 97.60% | 113.50% |  |

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