RESERVE FOR LOSSES AND LOSS EXPENSES (Narrative) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Apr. 16, 2018	Dec. 31, 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Current year				$ 1,374,784	$ 1,482,409
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	$ 3,152,706	$ 3,564,812	$ 3,152,706	3,564,812	3,152,706	$ 3,501,669	$ 108,000	$ 3,159,514
Net favorable (adverse) prior year reserve development		23,621	60,116	38,293	114,423
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		(19,000)	43,000	(52,000)	81,000
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		11,000	6,000	23,000	8,000
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	11,000	9,000
Catastrophe and Weather-related Events
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Current year				36,000	73,000
Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net favorable (adverse) prior year reserve development		2,295	35,822	10,053	67,355
Reinsurance | 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		(30,000)		(59,000)
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		17,000	9,000	27,000	14,000
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		6,000		8,000	8,000
Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net favorable (adverse) prior year reserve development		21,326	24,294	28,240	$ 47,068
Insurance | 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		6,000		(16,000)
Insurance | 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		7,000		9,000
Insurance | Marine | 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		5,000		22,000
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		$ 4,000		$ 10,000
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)
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