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, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	$ 1,073,195
Net outstanding liabilities for unpaid unallocated loss settlement expenses	88,193
Fair value adjustment (purchase accounting adjustment for Mercer acquisition)	2,924
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	1,164,312
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	62,667
Reinsurance fair value amortization (purchase accounting adjustment for Mercer acquisition)	(2,796)
Total reinsurance recoverable on unpaid losses and loss settlement expenses	59,871
Total gross liability for unpaid losses and loss settlement expenses	1,224,183	$ 1,123,896	$ 1,003,895	$ 969,437
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	443,753
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	20,367
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	104,806
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	10,806
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	284,552
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	1,994
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	168,640
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	26,799
Personal
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	27,735
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	26
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	43,709
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	$ 2,675