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, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	$ 1,257,183
Net outstanding liabilities for unpaid unallocated loss settlement expenses	94,359
Fair value adjustment (purchase accounting adjustment for Mercer acquisition)	1,676
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	1,353,218
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	70,139
Reinsurance fair value amortization (purchase accounting adjustment for Mercer acquisition)	(1,603)
Total reinsurance recoverable on unpaid losses and loss settlement expenses	68,536
Total gross liability for unpaid losses and loss settlement expenses	1,421,754	$ 1,312,483	$ 1,224,183	$ 1,123,896
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	532,153
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	29,070
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	113,569
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	8,652
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	410,716
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	993
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	142,587
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	28,681
Personal
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	29,390
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	12
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	28,768
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	$ 2,731