RESERVE FOR LOSSES AND LOSS EXPENSES (DETAILS 4) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Additional Information Related to Net Incurred Losses [Line Items]
Current year net loss and loss expenses	$ 2,496,574	$ 2,419,247	$ 2,445,666
Net favorable (adverse) prior year reserve development	292,377	243,048	258,944
Foreign exchange and other	205,606	215,018	230,372
Short Tail Lines Insurance and Reinsurance Business [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development	148,000	152,000	207,000
Professional Lines Reinsurance Business [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development	30,000	38,000	33,000
Professional Lines Insurance Business [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development	14,000	(14,000)
Credit and Surety Business [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development	10,000	27,000
Credit and Political Risk Business [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development		(15,000)
Liability and Motor Reinsurance Business [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development	99,000	82,000	40,000
Liability Insurance Business [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development	(8,000)	(27,000)	(23,000)
Catastrophe and Weather-related Events [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Current year net loss and loss expenses	204,000	$ 100,000	$ 93,000
Retroactive Reinsurance Recoverable [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Foreign exchange and other	$ 150,000