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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 |  |
--- | --- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016
Underwriting Revenues |  |  |  |
Gross written premiums | $ 1,116.8 | $ 998.0 |  |
Net written premiums | 635.5 | 686.2 |  |
Gross earned premiums | 842.6 | 751.3 |  |
Net earned premiums | 533.5 | 581.1 |  |
Underwriting Expenses |  |  |  |
Losses and loss adjustment expenses | 310.2 | 328.2 |  |
Amortization of deferred policy acquisition costs | 90.8 | 113.7 |  |
General and administrative expenses | 95.2 | 105.7 |  |
Underwriting income | 37.3 | 33.5 |  |
Corporate expenses | (13.7) | (13.4) |  |
Non-operating expenses | (12.1) | (2.2) |  |
Net investment income | 47.3 | 47.7 |  |
Realized and unrealized investment gains | 100.6 | 51.2 |  |
Realized and unrealized investment losses | (138.3) | (5.0) |  |
Change in fair value of loan notes issued by variable interest entities | 1.0 | (2.9) |  |
Change in fair value of derivatives | 23.5 | 3.1 |  |
Interest expense on long term debt | (7.4) | (7.4) |  |
Net realized and unrealized foreign exchange (losses) | (4.7) | (8.9) |  |
Other income | 2.1 | 3.6 |  |
Other expenses | (1.2) | 0.0 |  |
Income from operations before income tax | 34.4 | 99.3 |  |
Net reserves for loss and loss adjustment expenses | $ 5,068.1 | $ 4,730.1 | $ 5,234.3 | $ 4,759.2
Ratios |  |  |  |
Loss ratio | 58.10% | 56.50% |  |
Policy acquisition expense ratio | 17.00% | 19.60% |  |
General and administrative expense ratio | 22.70% | 20.90% |  |
Expense ratio | 39.70% | 40.50% |  |
Combined ratio | 97.80% | 97.00% |  |
Non-operating expenses related to Effectiveness and Efficiency Program | $ 11.8 |  |  |
Reinsurance |  |  |  |
Underwriting Revenues |  |  |  |
Gross written premiums | 623.5 | $ 565.3 |  |
Net written premiums | 425.0 | 448.2 |  |
Gross earned premiums | 375.0 | 327.6 |  |
Net earned premiums | 282.5 | 277.5 |  |
Underwriting Expenses |  |  |  |
Losses and loss adjustment expenses | 166.9 | 143.1 |  |
Amortization of deferred policy acquisition costs | 55.9 | 59.5 |  |
General and administrative expenses | 31.6 | 43.9 |  |
Underwriting income | 28.1 | 31.0 |  |
Net reserves for loss and loss adjustment expenses | $ 2,823.6 | $ 2,445.4 |  |
Ratios |  |  |  |
Loss ratio | 59.10% | 51.60% |  |
Policy acquisition expense ratio | 19.80% | 21.40% |  |
General and administrative expense ratio | 11.20% | 15.80% |  |
Expense ratio | 31.00% | 37.20% |  |
Combined ratio | 90.10% | 88.80% |  |
Insurance |  |  |  |
Underwriting Revenues |  |  |  |
Gross written premiums | $ 493.3 | $ 432.7 |  |
Net written premiums | 210.5 | 238.0 |  |
Gross earned premiums | 467.6 | 423.7 |  |
Net earned premiums | 251.0 | 303.6 |  |
Underwriting Expenses |  |  |  |
Losses and loss adjustment expenses | 143.3 | 185.1 |  |
Amortization of deferred policy acquisition costs | 34.9 | 54.2 |  |
General and administrative expenses | 63.6 | 61.8 |  |
Underwriting income | 9.2 | 2.5 |  |
Net reserves for loss and loss adjustment expenses | $ 2,244.5 | $ 2,284.7 |  |
Ratios |  |  |  |
Loss ratio | 57.10% | 61.00% |  |
Policy acquisition expense ratio | 13.90% | 17.90% |  |
General and administrative expense ratio | 25.30% | 20.40% |  |
Expense ratio | 39.20% | 38.30% |  |
Combined ratio | 96.30% | 99.30% |  |

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