RESERVES FOR LOSSES AND LOSS SETTLEMENT EXPENSES (Narrative) (Details) $ in Thousands	Dec. 31, 2020 USD ($)
Workers' compensation
Liability for Claims and Claims Adjustment Expense [Line Items]
Unfavorable (favorable) reserve development	$ (25,428)
Commercial fire and allied
Liability for Claims and Claims Adjustment Expense [Line Items]
Unfavorable (favorable) reserve development	(10,655)
Fidelity and surety
Liability for Claims and Claims Adjustment Expense [Line Items]
Unfavorable (favorable) reserve development	(2,068)
Personal automobile
Liability for Claims and Claims Adjustment Expense [Line Items]
Unfavorable (favorable) reserve development	(1,851)
Commercial liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Unfavorable (favorable) reserve development	12,845
Reinsurance assumed
Liability for Claims and Claims Adjustment Expense [Line Items]
Unfavorable (favorable) reserve development	5,972
Commercial automobile
Liability for Claims and Claims Adjustment Expense [Line Items]
Unfavorable (favorable) reserve development	$ 4,024