SEGMENT INFORMATION - Net premiums earned by segment and lines of insurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenue from External Customer [Line Items]
Premiums earned	$ 570,266	$ 540,722	$ 515,506
Property and casualty insurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	447,197	422,381	392,719
Property and casualty insurance [Member] | Commercial Lines [Member]
Revenue from External Customer [Line Items]
Premiums earned	403,779	375,943	342,318
Property and casualty insurance [Member] | Commercial Lines [Member] | Automobile Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	105,904	96,908	86,230
Property and casualty insurance [Member] | Commercial Lines [Member] | Property Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	104,303	97,155	87,446
Property and casualty insurance [Member] | Commercial Lines [Member] | Workers Compensation Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	92,828	88,356	83,172
Property and casualty insurance [Member] | Commercial Lines [Member] | Liability Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	92,665	86,108	77,983
Property and casualty insurance [Member] | Commercial Lines [Member] | Other Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	8,079	7,416	7,487
Property and casualty insurance [Member] | Personal Lines [Member]
Revenue from External Customer [Line Items]
Premiums earned	43,418	46,438	50,401
Property and casualty insurance [Member] | Personal Lines [Member] | Automobile Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	22,855	25,094	27,408
Property and casualty insurance [Member] | Personal Lines [Member] | Homeowners Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	20,563	21,344	22,993
Reinsurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	123,069	118,341	122,787
Reinsurance [Member] | Pro rata reinsurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	47,421	41,883	47,648
Reinsurance [Member] | Pro rata reinsurance [Member] | Multiline (primarily property) Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	7,089	8,552	7,489
Reinsurance [Member] | Pro rata reinsurance [Member] | Property Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	15,324	8,482	20,239
Reinsurance [Member] | Pro rata reinsurance [Member] | Liability Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	20,629	9,919	5,172
Reinsurance [Member] | Pro rata reinsurance [Member] | Marine Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	4,379	14,930	14,748
Reinsurance [Member] | Excess of loss reinsurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	75,648	76,458	75,139
Reinsurance [Member] | Excess of loss reinsurance [Member] | Property Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	63,416	64,956	64,069
Reinsurance [Member] | Excess of loss reinsurance [Member] | Liability Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	$ 12,232	$ 11,502	$ 11,070