RESERVES FOR LOSSES AND LOSS SETTLEMENT EXPENSES (Reconciliation of the Disclosure Claims Development to the Liability for Claims and Claim Adjustment Expenses) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	$ 1,347,699
Net outstanding liabilities for unpaid unallocated loss settlement expenses	97,379
Fair value adjustment (purchase accounting adjustment for Mercer acquisition)	1,210
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	1,446,288
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	133,001
Reinsurance fair value amortization (purchase accounting adjustment for Mercer acquisition)	(1,158)
Total reinsurance recoverable on unpaid losses and loss settlement expenses	131,843
Total gross liability for unpaid losses and loss settlement expenses	1,578,131	$ 1,421,754	$ 1,312,483	$ 1,224,183
Commercial other liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	585,811
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	29,497
Commercial fire and allied
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	129,290
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	42,481
Commercial automobile
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	434,293
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	3,334
Commercial workers' compensation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	116,050
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	47,130
Personal
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	39,210
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	8,867
All other lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	43,045
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	$ 1,692