RESERVE FOR LOSSES AND LOSS EXPENSES - Reconciliation of development tables to consolidated balance sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Claims Development [Line Items]
Net outstanding liabilities	$ 9,654,715
Reinsurance recoverable on unpaid claims	3,159,514
Gross outstanding liabilities	12,814,229
Unallocated claims adjustment expenses	170,168
Foreign exchange and other	(45,262)
Assumed reserves related to retroactive transactions	58,418
Reserve for losses and loss expenses	12,997,553	$ 9,697,827	$ 9,646,285	$ 9,596,797
Aviabel
Claims Development [Line Items]
Net outstanding liabilities	77,405
Reinsurance recoverable on unpaid claims	5,329
Gross outstanding liabilities	82,734
Novae
Claims Development [Line Items]
Net outstanding liabilities	1,300,361
Reinsurance recoverable on unpaid claims	771,258
Gross outstanding liabilities	2,071,619
Insurance
Claims Development [Line Items]
Net outstanding liabilities	3,331,536
Reinsurance recoverable on unpaid claims	2,047,292
Gross outstanding liabilities	5,378,828
Insurance | Property and Other
Claims Development [Line Items]
Net outstanding liabilities	880,954
Reinsurance recoverable on unpaid claims	181,101
Gross outstanding liabilities	1,062,055
Insurance | Marine
Claims Development [Line Items]
Net outstanding liabilities	213,390
Reinsurance recoverable on unpaid claims	106,466
Gross outstanding liabilities	319,856
Insurance | Aviation
Claims Development [Line Items]
Net outstanding liabilities	56,400
Reinsurance recoverable on unpaid claims	8,945
Gross outstanding liabilities	65,345
Insurance | Credit and Political Risk
Claims Development [Line Items]
Net outstanding liabilities	51,469
Reinsurance recoverable on unpaid claims	1,384
Gross outstanding liabilities	52,853
Insurance | Professional Lines
Claims Development [Line Items]
Net outstanding liabilities	1,559,564
Reinsurance recoverable on unpaid claims	853,130
Gross outstanding liabilities	2,412,694
Insurance | Liability
Claims Development [Line Items]
Net outstanding liabilities	569,759
Reinsurance recoverable on unpaid claims	896,266
Gross outstanding liabilities	1,466,025
Reinsurance
Claims Development [Line Items]
Net outstanding liabilities	4,945,413
Reinsurance recoverable on unpaid claims	335,635
Gross outstanding liabilities	5,281,048
Reinsurance | Property and Other
Claims Development [Line Items]
Net outstanding liabilities	1,220,069
Reinsurance recoverable on unpaid claims	214,394
Gross outstanding liabilities	1,434,463
Reinsurance | Credit and Surety
Claims Development [Line Items]
Net outstanding liabilities	375,762
Reinsurance recoverable on unpaid claims	13,061
Gross outstanding liabilities	388,823
Reinsurance | Professional Lines
Claims Development [Line Items]
Net outstanding liabilities	1,119,534
Reinsurance recoverable on unpaid claims	37,822
Gross outstanding liabilities	1,157,356
Reinsurance | Motor
Claims Development [Line Items]
Net outstanding liabilities	1,082,423
Reinsurance recoverable on unpaid claims	3,666
Gross outstanding liabilities	1,086,089
Reinsurance | Liability
Claims Development [Line Items]
Net outstanding liabilities	1,147,625
Reinsurance recoverable on unpaid claims	66,692
Gross outstanding liabilities	$ 1,214,317