SEGMENT INFORMATION - Net premiums earned by segment and lines of insurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue from External Customer [Line Items]
Premiums earned	$ 607,158	$ 592,408	$ 570,266
Property and casualty insurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	472,369	456,467	447,197
Property and casualty insurance [Member] | Commercial Lines [Member]
Revenue from External Customer [Line Items]
Premiums earned	434,331	417,474	403,779
Property and casualty insurance [Member] | Commercial Lines [Member] | Automobile Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	118,224	110,941	105,904
Property and casualty insurance [Member] | Commercial Lines [Member] | Property Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	108,162	105,012	104,303
Property and casualty insurance [Member] | Commercial Lines [Member] | Workers' Compensation [Member]
Revenue from External Customer [Line Items]
Premiums earned	100,552	96,517	92,828
Property and casualty insurance [Member] | Commercial Lines [Member] | Other Liability [Member]
Revenue from External Customer [Line Items]
Premiums earned	98,674	96,630	92,665
Property and casualty insurance [Member] | Commercial Lines [Member] | Other Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	8,719	8,374	8,079
Property and casualty insurance [Member] | Personal Lines [Member]
Revenue from External Customer [Line Items]
Premiums earned	38,038	38,993	43,418
Reinsurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	134,789	135,941	123,069
Reinsurance [Member] | Assumed Pro Rata [Member]
Revenue from External Customer [Line Items]
Premiums earned	44,636	56,317	47,421
Reinsurance [Member] | Assumed Excess of Loss [Member]
Revenue from External Customer [Line Items]
Premiums earned	$ 90,153	$ 79,624	$ 75,648