SEGMENT INFORMATION (Net Premiums) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting, Revenue Reconciling Item [Line Items]
Total net premiums earned	$ 1,023,401	$ 930,890	$ 828,330
Property and Casualty Insurance
Segment Reporting, Revenue Reconciling Item [Line Items]
Reinsurance assumed	17,037	18,396	16,265
Other	2,835	3,179	3,735
Total net premiums earned	936,131	851,695	766,939
Property and Casualty Insurance | Other liability
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums earned	289,982	261,303	228,426
Property and Casualty Insurance | Fire and allied lines
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums earned	265,221	246,450	226,086
Property and Casualty Insurance | Automobile
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums earned	239,216	210,090	187,813
Property and Casualty Insurance | Workers' compensation
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums earned	103,605	95,672	88,522
Property and Casualty Insurance | Fidelity and surety
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums earned	22,507	21,362	19,212
Property and Casualty Insurance | Reinsurance assumed
Segment Reporting, Revenue Reconciling Item [Line Items]
Reinsurance assumed	12,765	13,639	13,145
Life Insurance
Segment Reporting, Revenue Reconciling Item [Line Items]
Other	58	388	261
Total net premiums earned	87,270	79,984	61,921
Life Insurance | Ordinary life (excluding universal life)
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums earned	63,668	53,114	35,557
Life Insurance | Universal life policy fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums earned	11,577	12,834	13,190
Life Insurance | Immediate annuities with life contingencies
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums earned	10,533	12,223	11,639
Life Insurance | Accident and health
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums earned	$ 1,434	$ 1,425	$ 1,274