RESERVE FOR LOSSES AND LOSS EXPENSES - Reconciliation of development tables to consolidated balance sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Claims Development [Line Items]
Net outstanding liabilities	$ 8,676,295
Reinsurance recoverable on unpaid claims	3,501,669
Gross outstanding liabilities	12,177,964
Unallocated claims adjustment expenses	145,768
Foreign exchange and other	53,695
(Ceded)/assumed reserves related to retroactive transactions	(96,658)
Reserve for losses and loss expenses	12,280,769	$ 12,997,553	$ 9,697,827	$ 9,646,285
Insurance
Claims Development [Line Items]
Net outstanding liabilities	3,540,908
Reinsurance recoverable on unpaid claims	2,692,252
Gross outstanding liabilities	6,233,160
Insurance | Property and other
Claims Development [Line Items]
Net outstanding liabilities	771,659
Reinsurance recoverable on unpaid claims	482,207
Gross outstanding liabilities	1,253,866
Insurance | Marine
Claims Development [Line Items]
Net outstanding liabilities	340,668
Reinsurance recoverable on unpaid claims	187,164
Gross outstanding liabilities	527,832
Insurance | Aviation
Claims Development [Line Items]
Net outstanding liabilities	91,404
Reinsurance recoverable on unpaid claims	14,310
Gross outstanding liabilities	105,714
Insurance | Credit and political risk
Claims Development [Line Items]
Net outstanding liabilities	99,011
Reinsurance recoverable on unpaid claims	24,536
Gross outstanding liabilities	123,547
Insurance | Professional lines
Claims Development [Line Items]
Net outstanding liabilities	1,587,696
Reinsurance recoverable on unpaid claims	1,005,316
Gross outstanding liabilities	2,593,012
Insurance | Liability
Claims Development [Line Items]
Net outstanding liabilities	650,470
Reinsurance recoverable on unpaid claims	978,719
Gross outstanding liabilities	1,629,189
Reinsurance
Claims Development [Line Items]
Net outstanding liabilities	5,135,387
Reinsurance recoverable on unpaid claims	809,417
Gross outstanding liabilities	5,944,804
Reinsurance | Property and other
Claims Development [Line Items]
Net outstanding liabilities	1,393,776
Reinsurance recoverable on unpaid claims	435,671
Gross outstanding liabilities	1,829,447
Reinsurance | Credit and surety
Claims Development [Line Items]
Net outstanding liabilities	325,491
Reinsurance recoverable on unpaid claims	36,180
Gross outstanding liabilities	361,671
Reinsurance | Professional lines
Claims Development [Line Items]
Net outstanding liabilities	1,039,085
Reinsurance recoverable on unpaid claims	81,608
Gross outstanding liabilities	1,120,693
Reinsurance | Motor
Claims Development [Line Items]
Net outstanding liabilities	1,172,467
Reinsurance recoverable on unpaid claims	128,575
Gross outstanding liabilities	1,301,042
Reinsurance | Liability
Claims Development [Line Items]
Net outstanding liabilities	1,204,568
Reinsurance recoverable on unpaid claims	127,383
Gross outstanding liabilities	$ 1,331,951