RESERVE FOR LOSSES AND LOSS EXPENSES - Incurred losses narrative (Details) - USD ($) $ in Thousands		3 Months Ended									12 Months Ended
	Feb. 13, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Apr. 16, 2018	Oct. 02, 2017	Apr. 01, 2017	Mar. 20, 2017	Mar. 19, 2017	Dec. 31, 2015
Additional Information Related to Net Incurred Losses [Line Items]
Net losses and loss expenses, net of reinstatement premiums											$ 3,389,949	$ 3,487,826	$ 2,496,574
Reinsurance recoverable on unpaid losses		$ 3,501,669				$ 3,159,514	$ 3,159,514				3,501,669	3,159,514	2,276,109	$ 108,000					$ 2,031,309
Reserve for losses and loss expenses		12,280,769				12,997,553	12,997,553				12,280,769	12,997,553	9,697,827						$ 9,646,285
Foreign exchange and other											1,156,412	(1,713,227)	205,606
Net favorable (adverse) prior year reserve development											199,662	200,054	292,377
Net premiums earned		$ 1,214,469	$ 1,224,075	$ 1,185,548	$ 1,167,402	1,211,495		$ 1,017,131	$ 981,431	$ 938,703	4,791,495	4,148,760	3,705,625
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%
Reinsurance recoverable on unpaid and paid losses						5,000	5,000					5,000
Foreign exchange and other						79,000	79,000					79,000
Novae
Additional Information Related to Net Incurred Losses [Line Items]
Interests acquired															100.00%
Reserve for losses and loss expenses						2,126,000	2,126,000					2,126,000
Reinsurance recoverable on unpaid and paid losses						$ 788,000	788,000					788,000
Net premiums earned							$ 140,635
Catastrophe and Weather-related Events
Additional Information Related to Net Incurred Losses [Line Items]
Net losses and loss expenses, net of reinstatement premiums											430,000	835,000	204,000
Syndicate 2007
Additional Information Related to Net Incurred Losses [Line Items]
Ceded reserves for losses and loss expenses											819,000
Reinsurance
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development											106,856	139,595	243,399
Insurance
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development											92,806	60,459	48,978
Short Tail Lines Insurance and Reinsurance Business
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development											86,000	60,000	148,000
Professional lines | Reinsurance
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development											21,000	44,000	30,000
Professional lines | Insurance
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development											32,000	26,000	14,000
Credit and surety | Reinsurance
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development											33,000	33,000	10,000
Motor Reserve Class | Reinsurance
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development											23,000	1,000	55,000
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											23,000	43,000	44,000
Liability Insurance Business | Insurance
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable (adverse) prior year reserve development											(22,000)	(8,000)	(8,000)
Operating Segments
Additional Information Related to Net Incurred Losses [Line Items]
Net premiums earned											4,791,495	4,148,760	3,705,625
Operating Segments | Reinsurance
Additional Information Related to Net Incurred Losses [Line Items]
Net premiums earned											2,428,889	2,332,322	2,171,343
Operating Segments | Insurance
Additional Information Related to Net Incurred Losses [Line Items]
Net premiums earned											2,362,606	1,816,438	1,534,282
Operating Segments | Professional lines | Reinsurance
Additional Information Related to Net Incurred Losses [Line Items]
Net premiums earned											220,687	226,622	289,868
Operating Segments | Professional lines | Insurance
Additional Information Related to Net Incurred Losses [Line Items]
Net premiums earned											570,241	519,759	510,806
Operating Segments | Accident and health | Reinsurance
Additional Information Related to Net Incurred Losses [Line Items]
Net premiums earned											299,813	289,925	243,039
Operating Segments | Accident and health | Insurance
Additional Information Related to Net Incurred Losses [Line Items]
Net premiums earned											$ 207,777	$ 199,121	$ 141,107