RESERVE FOR LOSSES AND LOSS EXPENSES (Details 2) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
	Feb. 13, 2017	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Mar. 20, 2017	Mar. 19, 2017
Additional Information Related to Net Incurred Losses [Line Items]
Current year				$ 1,308,000	$ 1,279,369
Foreign exchange and other				(242,973)	106,629
Net favorable (adverse) prior year reserve development		$ 70,933	$ 77,719	95,727	148,113
Short Tail Lines Insurance And Reinsurance Business [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development		31,000	27,000	35,000	75,000
Professional Lines Insurance And Reinsurance Business [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development		22,000	15,000	46,000	16,000
Liability Reinsurance Business [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development		$ 16,000	15,000	39,000	22,000
Liability Insurance Business [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development				(7,000)
Motor Reinsurance Business [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development			$ 17,000	(20,000)	33,000
Ogden rate						(0.75%)	2.50%
Catastrophe and Weather-related Events [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Current year				$ 85,000	$ 124,000
Discontinued Operations, Disposed of by Means Other than Sale [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Liability for unpaid claims and claims adjustment Eexpense	$ (223,000)
Reduction in reinsurance recoverables on unpaid and paid loses by	$ 223,000