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, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	$ 980,636
Net outstanding liabilities for unpaid unallocated loss settlement expenses	79,751
Fair value adjustment (purchase accounting adjustment for Mercer acquisition)	3,715
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	1,064,102
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	63,347
Reinsurance fair value amortization (purchase accounting adjustment for Mercer acquisition)	(3,553)
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses:	59,794
Total gross liability for unpaid losses and loss settlement expenses	1,123,896	$ 1,003,895	$ 969,437	$ 960,651
Commercial other liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	458,261
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	21,173
Commercial fire and allied
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	97,785
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	9,748
Commercial automobile
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	209,984
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	1,544
Workers' compensation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	160,303
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	29,686
Personal
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	27,793
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	41
All other lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	26,510
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	$ 1,155