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, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	$ 1,166,388
Net outstanding liabilities for unpaid unallocated loss settlement expenses	86,753
Fair value adjustment (purchase accounting adjustment for Mercer acquisition)	2,248
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	1,255,389
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	59,243
Reinsurance fair value amortization (purchase accounting adjustment for Mercer acquisition)	(2,149)
Total reinsurance recoverable on unpaid losses and loss settlement expenses	57,094
Total gross liability for unpaid losses and loss settlement expenses	1,312,483	$ 1,224,183	$ 1,123,896	$ 1,003,895
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	496,332
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	22,556
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	103,575
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	2,723
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	338,664
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	3,166
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	168,554
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	27,465
Personal
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	26,668
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	4
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	32,595
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	$ 3,329