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RESERVE FOR LOSSES AND LOSS EXPENSES - Incurred losses narrative (Details) - USD ($) $ in Thousands |  | 3 Months Ended | 12 Months Ended |  |  |  |  |  |
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Feb. 13, 2017 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Apr. 16, 2018 | Oct. 02, 2017 | Apr. 01, 2017 | Mar. 20, 2017 | Mar. 19, 2017 | Dec. 31, 2015
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net losses and loss expenses, net of reinstatement premiums |  |  |  |  |  |  |  |  |  |  | $ 3,389,949 | $ 3,487,826 | $ 2,496,574 |  |  |  |  |  |
Reinsurance recoverable on unpaid losses |  | $ 3,501,669 |  |  |  | $ 3,159,514 | $ 3,159,514 |  |  |  | 3,501,669 | 3,159,514 | 2,276,109 | $ 108,000 |  |  |  |  | $ 2,031,309
Reserve for losses and loss expenses |  | 12,280,769 |  |  |  | 12,997,553 | 12,997,553 |  |  |  | 12,280,769 | 12,997,553 | 9,697,827 |  |  |  |  |  | $ 9,646,285
Foreign exchange and other |  |  |  |  |  |  |  |  |  |  | 1,156,412 | (1,713,227) | 205,606 |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  |  | 199,662 | 200,054 | 292,377 |  |  |  |  |  |
Net premiums earned |  | $ 1,214,469 | $ 1,224,075 | $ 1,185,548 | $ 1,167,402 | 1,211,495 |  | $ 1,017,131 | $ 981,431 | $ 938,703 | 4,791,495 | 4,148,760 | 3,705,625 |  |  |  |  |  |
Retroactive Reinsurance Recoverable |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Foreign exchange and other |  |  |  |  |  |  |  |  |  |  |  |  | 150,000 |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aviabel |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interests acquired |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00% |  |  |
Reinsurance recoverable on unpaid and paid losses |  |  |  |  |  | 5,000 | 5,000 |  |  |  |  | 5,000 |  |  |  |  |  |  |
Foreign exchange and other |  |  |  |  |  | 79,000 | 79,000 |  |  |  |  | 79,000 |  |  |  |  |  |  |
Novae |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interests acquired |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00% |  |  |  |
Reserve for losses and loss expenses |  |  |  |  |  | 2,126,000 | 2,126,000 |  |  |  |  | 2,126,000 |  |  |  |  |  |  |
Reinsurance recoverable on unpaid and paid losses |  |  |  |  |  | $ 788,000 | 788,000 |  |  |  |  | 788,000 |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  | $ 140,635 |  |  |  |  |  |  |  |  |  |  |  |
Catastrophe and Weather-related Events |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net losses and loss expenses, net of reinstatement premiums |  |  |  |  |  |  |  |  |  |  | 430,000 | 835,000 | 204,000 |  |  |  |  |  |
Syndicate 2007 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ceded reserves for losses and loss expenses |  |  |  |  |  |  |  |  |  |  | 819,000 |  |  |  |  |  |  |  |
Reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  |  | 106,856 | 139,595 | 243,399 |  |  |  |  |  |
Insurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  |  | 92,806 | 60,459 | 48,978 |  |  |  |  |  |
Short Tail Lines Insurance and Reinsurance Business |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  |  | 86,000 | 60,000 | 148,000 |  |  |  |  |  |
Professional lines | Reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  |  | 21,000 | 44,000 | 30,000 |  |  |  |  |  |
Professional lines | Insurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  |  | 32,000 | 26,000 | 14,000 |  |  |  |  |  |
Credit and surety | Reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  |  | 33,000 | 33,000 | 10,000 |  |  |  |  |  |
Motor Reserve Class | Reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  |  | 23,000 | 1,000 | 55,000 |  |  |  |  |  |
Discount rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (0.75%) | 2.50% |
Liability Insurance Business | Reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  |  | 23,000 | 43,000 | 44,000 |  |  |  |  |  |
Liability Insurance Business | Insurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable (adverse) prior year reserve development |  |  |  |  |  |  |  |  |  |  | (22,000) | (8,000) | (8,000) |  |  |  |  |  |
Operating Segments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  |  | 4,791,495 | 4,148,760 | 3,705,625 |  |  |  |  |  |
Operating Segments | Reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  |  | 2,428,889 | 2,332,322 | 2,171,343 |  |  |  |  |  |
Operating Segments | Insurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  |  | 2,362,606 | 1,816,438 | 1,534,282 |  |  |  |  |  |
Operating Segments | Professional lines | Reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  |  | 220,687 | 226,622 | 289,868 |  |  |  |  |  |
Operating Segments | Professional lines | Insurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  |  | 570,241 | 519,759 | 510,806 |  |  |  |  |  |
Operating Segments | Accident and health | Reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  |  | 299,813 | 289,925 | 243,039 |  |  |  |  |  |
Operating Segments | Accident and health | Insurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Information Related to Net Incurred Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  |  | $ 207,777 | $ 199,121 | $ 141,107 |  |  |  |  |  |

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