RESERVE FOR LOSSES AND LOSS EXPENSES (Details 2) - USD ($) $ in Thousands		3 Months Ended
	Feb. 13, 2017	Mar. 31, 2017	Mar. 31, 2016	Mar. 20, 2017	Mar. 19, 2017	Dec. 31, 2016
Additional Information Related to Net Incurred Losses [Line Items]
Current year		$ 631,735	$ 569,356
Reduction of reserves for losses and loss expenses	$ 223,000
Reduction in reinsurance recoverables on unpaid and paid loses by	$ 223,000	(2,070,341)				$ (2,334,922)
Foreign exchange and other		(36,797)	(87,420)
Net favorable (adverse) prior year reserve development		24,793	70,394
Short Tail Lines Insurance And Reinsurance Business [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development		4,000	48,000
Professional Lines Insurance and Reinsurance Business [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development		23,000
Liability and Motor Reinsurance Business [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development		(22,000)	16,000
Ogden rate				(0.75%)	2.50%
Liability Insurance Business [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development		23,000	6,000
Catastrophe and Weather-related Events [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Current year		$ 35,000	$ 14,000