RESERVE FOR LOSSES AND LOSS EXPENSES (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net loss and loss expenses			$ 1,593,102	$ 1,374,784
Reinsurance recoverable on unpaid losses and loss expenses (Allowance for expected credit losses 2020: $20,987)	$ 4,160,521	$ 3,564,812	4,160,521	3,564,812	$ 3,877,756	$ 3,501,669
Net favorable (adverse) prior year reserve development	2,655	23,621	8,767	38,293
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	22,000	(19,000)	32,000	(52,000)
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	(21,000)	23,000
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	4,000		18,000	11,000
Liability Insurance Business [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net favorable (adverse) prior year reserve development		(17,000)	(21,000)
Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net favorable (adverse) prior year reserve development	420	21,326	4,251	28,240
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	25,000	6,000	37,000	(16,000)
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	(9,000)	(4,000)	(14,000)	(10,000)
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 | Aviation | 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
Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net favorable (adverse) prior year reserve development	2,235	2,295	4,516	10,053
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	(4,000)	(30,000)	(8,000)	(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	10,000	17,000	15,000	27,000
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	$ (5,000)	$ 6,000		8,000
Catastrophe and Weather-related Events
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net loss and loss expenses			$ 336,000	$ 36,000