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RESERVE FOR LOSSES AND LOSS EXPENSES - Incurred losses narrative (Details) - USD ($) $ in Thousands |  | 3 Months Ended | 12 Months Ended |  |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Feb. 13, 2017 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 15, 2019 | Aug. 05, 2019 | Apr. 16, 2018 | Oct. 02, 2017 | Apr. 01, 2017 | Mar. 20, 2017 | Mar. 19, 2017 | Dec. 31, 2016
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net losses and loss expenses, net of reinstatement premiums |  |  |  |  |  |  |  |  |  | $ 3,123,698 | $ 3,389,949 | $ 3,487,826 |  |  |  |  |  |  |  |
Reinsurance recoverable on unpaid losses |  | $ 3,877,756 |  |  |  | $ 3,501,669 |  |  |  | 3,877,756 | 3,501,669 | 3,159,514 | $ 59,000 |  | $ 108,000 |  |  |  |  | $ 2,276,109
Reserve for losses and loss expenses |  | 12,752,081 |  |  |  | 12,280,769 |  |  |  | 12,752,081 | 12,280,769 | 12,997,553 |  |  |  |  |  |  |  | $ 9,697,827
Foreign exchange and other |  |  |  |  |  |  |  |  |  | (21,052) | 1,156,412 | (1,713,227) |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  | 78,900 | 199,662 | 200,054 |  |  |  |  |  |  |  |
Net premiums earned |  | $ 1,172,051 | $ 1,157,307 | $ 1,123,607 | $ 1,134,212 | $ 1,214,469 | $ 1,224,075 | $ 1,185,548 | $ 1,167,402 | 4,587,178 | 4,791,495 | 4,148,760 |  |  |  |  |  |  |  |
Discontinued Operations, Disposed of by Means Other than Sale |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Liability for unpaid claims and claims adjustment expense | $ (223,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reduction in reinsurance recoverables on unpaid and paid loses by | $ 223,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Novae |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interests acquired |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00% |  |  |  |
Reserve for losses and loss expenses |  |  |  |  |  |  |  |  |  |  |  | 2,126,000 |  |  |  |  |  |  |  |
Reinsurance recoverable on unpaid and paid losses |  |  |  |  |  |  |  |  |  |  |  | 788,000 |  |  |  |  |  |  |  |
Aviabel |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interests acquired |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00% |  |  |
Reinsurance recoverable on unpaid and paid losses |  |  |  |  |  |  |  |  |  |  |  | 5,000 |  |  |  |  |  |  |  |
Foreign exchange and other |  |  |  |  |  |  |  |  |  |  |  | 79,000 |  |  |  |  |  |  |  |
Catastrophe and Weather-related Events |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net losses and loss expenses, net of reinstatement premiums |  |  |  |  |  |  |  |  |  | 336,000 | 430,000 | 835,000 |  |  |  |  |  |  |  |
Short Tail Lines Insurance and Reinsurance Business |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  | (85,000) | 86,000 | 60,000 |  |  |  |  |  |  |  |
Syndicate 2007 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ceded reserves for losses and loss expenses |  |  |  |  |  |  |  |  |  |  | 819,000 |  |  |  |  |  |  |  |  |
Insurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  | 53,302 | 92,806 | 60,459 |  |  |  |  |  |  |  |
Insurance | Short Tail Lines Insurance and Reinsurance Business |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  | 11,000 |  |  |  |  |  |  |  |  |  |
Reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  | 25,598 | 106,856 | 139,595 |  |  |  |  |  |  |  |
Reinsurance | Short Tail Lines Insurance and Reinsurance Business |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  | (133,000) |  |  |  |  |  |  |  |  |  |
Professional lines | Insurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  | 12,000 | 32,000 | 26,000 |  |  |  |  |  |  |  |
Professional lines | Reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  |  | 21,000 | 44,000 |  |  |  |  |  |  |  |
Credit and surety | Reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  | 53,000 | 33,000 | 33,000 |  |  |  |  |  |  |  |
Liability Insurance Business | Insurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  | (25,000) | (22,000) | (8,000) |  |  |  |  |  |  |  |
Liability Insurance Business | Reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  | 31,000 | 23,000 | 43,000 |  |  |  |  |  |  |  |
Marine | Insurance | Short Tail Lines Insurance and Reinsurance Business |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  | 33,000 |  |  |  |  |  |  |  |  |  |
Credit and political risk | Reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  | 19,000 |  |  |  |  |  |  |  |  |  |
Motor Reserve Class | Reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  | 71,000 | 23,000 | 1,000 |  |  |  |  |  |  |  |
Discount rate |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.25% |  |  |  | 0.75% | (2.50%) |
Operating Segments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  | 4,587,178 | 4,791,495 | 4,148,760 |  |  |  |  |  |  |  |
Operating Segments | Insurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  | 2,190,084 | 2,362,606 | 1,816,438 |  |  |  |  |  |  |  |
Operating Segments | Reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  | 2,397,094 | 2,428,889 | 2,332,322 |  |  |  |  |  |  |  |
Operating Segments | Professional lines | Insurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  | 661,250 | 570,241 | 519,759 |  |  |  |  |  |  |  |
Operating Segments | Professional lines | Reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  | 206,328 | 220,687 | 226,622 |  |  |  |  |  |  |  |
Operating Segments | Marine | Insurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  | 281,764 | 300,944 | 181,533 |  |  |  |  |  |  |  |
Operating Segments | Credit and political risk | Insurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  | 91,698 | 102,825 | 56,432 |  |  |  |  |  |  |  |
Operating Segments | Accident and health | Insurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  | 144,499 | 207,777 | 199,121 |  |  |  |  |  |  |  |
Operating Segments | Accident and health | Reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  | $ 319,619 | $ 299,813 | $ 289,925 |  |  |  |  |  |  |  |

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