RESERVE FOR LOSSES AND LOSS EXPENSES (Narrative) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
	Feb. 13, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Apr. 16, 2018	Dec. 31, 2017	Apr. 01, 2017	Mar. 20, 2017	Mar. 19, 2017	Dec. 31, 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Current year				$ 1,482,409	$ 1,308,000
Reinsurance recoverable on unpaid losses		$ 3,152,706	$ 2,110,869	3,152,706	2,110,869	$ 108,000	$ 3,159,514				$ 2,276,109
Reinsurance recoverable on unpaid and paid losses		3,289,236		3,289,236			$ 3,338,840
Net favorable (adverse) prior year reserve development		60,116	70,933	114,423	95,727
Short Tail Reserving Class Insurance And Reinsurance Business
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net favorable (adverse) prior year reserve development		43,000	31,000	81,000	35,000
Reinsurance Credit and Surety
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net favorable (adverse) prior year reserve development		9,000		14,000
Liability Reinsurance Business
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net favorable (adverse) prior year reserve development		6,000	16,000	8,000	40,000
Motor Reinsurance Business
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net favorable (adverse) prior year reserve development		5,000		9,000	(20,000)
Catastrophe and Weather-related Events
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Current year				73,000	85,000
Discontinued Operations, Disposed of by Means Other than Sale
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Liability for unpaid claims and claims adjustment expense	$ 223,000
Reduction in reinsurance recoverables on unpaid and paid loses by	$ 223,000
Motor Reserve Class
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Ogden rate									(0.75%)	2.50%
Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net favorable (adverse) prior year reserve development		35,822	51,146	67,355	68,075
Reinsurance | Professional Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net favorable (adverse) prior year reserve development			11,000	8,000	27,000
Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net favorable (adverse) prior year reserve development		$ 24,294	19,787	47,068	27,652
Insurance | Professional Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net favorable (adverse) prior year reserve development			11,000		19,000
Insurance | Liability Insurance Business
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net favorable (adverse) prior year reserve development				$ (7,000)	(7,000)
Novae Syndicate 2007
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Policyholder benefits and claims incurred, assumed and ceded					819,000
Compagnie Belge d'Assurances Aviation NV/SA
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Interests acquired								100.00%
Reserves of businesses acquired			79,000		79,000
Reinsurance recoverable on unpaid and paid losses			$ 5,000		$ 5,000