RESERVE FOR LOSSES AND LOSS EXPENSES (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net loss and loss expenses			$ 2,473,363	$ 2,252,424
Net favorable (adverse) prior year reserve development	$ 584	$ 26,727	9,351	65,021
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	14,000	2,000	44,000	(50,000)
Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net favorable (adverse) prior year reserve development	270	14,609	4,521	42,849
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	7,000	11,000	43,000	(4,000)
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	4,000			24,000
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	(7,000)	4,000	(19,000)	14,000
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				10,000
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
Insurance | Marine And Aviation 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		3,000
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	(4,000)	(4,000)	(25,000)
Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net favorable (adverse) prior year reserve development	314	12,118	4,830	22,172
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	3,000	(12,000)	(5,000)	(71,000)
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	11,000	6,000	28,000	33,000
Reinsurance | 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		23,000	19,000	34,000
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	$ (14,000)	$ (7,000)	(13,000)
Reinsurance | 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			(23,000)	26,000
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			$ 576,000	$ 206,000