RESERVE FOR LOSSES AND LOSS EXPENSES - Incurred losses narrative (Details) - USD ($) $ in Thousands			12 Months Ended
	Apr. 01, 2017	Feb. 13, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 02, 2017	Mar. 20, 2017	Mar. 19, 2017	Dec. 31, 2014
Additional Information Related to Net Incurred Losses [Line Items]
Current year net loss and loss expenses			$ 3,487,826	$ 2,496,574	$ 2,419,247
Reserve for losses and loss expenses			12,997,553	9,697,827	9,646,285				$ 9,596,797
Reinsurance recoverable on unpaid and paid losses			3,338,840	2,334,922
Foreign exchange and other			(1,713,227)	205,606	215,018
Net favorable (adverse) prior year reserve development			200,054	292,377	243,048
Retroactive Reinsurance Recoverable
Additional Information Related to Net Incurred Losses [Line Items]
Foreign exchange and other			150,000
Discontinued Operations, Disposed of by Means Other than Sale
Additional Information Related to Net Incurred Losses [Line Items]
Liability for unpaid claims and claims adjustment expense		$ (223,000)
Reduction in reinsurance recoverables on unpaid and paid loses by		$ 223,000
Aviabel
Additional Information Related to Net Incurred Losses [Line Items]
Interests acquired	100.00%
Reserves of businesses acquired	$ 79,000
Reinsurance recoverable on unpaid and paid losses	$ 5,000
Novae
Additional Information Related to Net Incurred Losses [Line Items]
Interests acquired						100.00%
Reserve for losses and loss expenses			2,126,000
Reinsurance recoverable on unpaid and paid losses			788,000
Reinsurance
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development			151,085	236,472	219,601
Insurance
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development			48,969	55,905	23,447
Short Tail Lines Insurance and Reinsurance Business
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development			60,000	148,000	152,000
Professional Lines | Reinsurance
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development			44,000	30,000	38,000
Professional Lines | Insurance
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development			26,000	14,000	(14,000)
Credit and Surety
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development			33,000	10,000	27,000
Credit and Political Risk
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development					(15,000)
Motor Reserve Class
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development			1,000	55,000	37,000
Short-Duration Contracts, Discounted Liabilities, Discount Rate							(0.75%)	2.50%
Liability Insurance Business | Reinsurance
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development			43,000	44,000	46,000
Liability Insurance Business | Insurance
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development			(8,000)	(8,000)	(27,000)
Catastrophe and Weather-related Events
Additional Information Related to Net Incurred Losses [Line Items]
Current year net loss and loss expenses			$ 835,000	$ 204,000	$ 100,000