SEGMENT INFORMATION - Net premiums earned by segment and lines of insurance (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenue from External Customer [Line Items]
Premiums earned	$ 540,722	$ 515,506	$ 458,846
Property and casualty insurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	422,381	392,719	357,139
Property and casualty insurance [Member] | Commercial Lines [Member]
Revenue from External Customer [Line Items]
Premiums earned	375,943	342,318	306,060
Property and casualty insurance [Member] | Commercial Lines [Member] | Automobile Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	96,908	86,230	76,362
Property and casualty insurance [Member] | Commercial Lines [Member] | Property Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	97,155	87,446	77,726
Property and casualty insurance [Member] | Commercial Lines [Member] | Workers Compensation Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	88,356	83,172	75,697
Property and casualty insurance [Member] | Commercial Lines [Member] | Liability Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	86,108	77,983	68,661
Property and casualty insurance [Member] | Commercial Lines [Member] | Other Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	7,416	7,487	7,614
Property and casualty insurance [Member] | Personal Lines [Member]
Revenue from External Customer [Line Items]
Premiums earned	46,438	50,401	51,079
Property and casualty insurance [Member] | Personal Lines [Member] | Automobile Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	25,094	27,408	28,437
Property and casualty insurance [Member] | Personal Lines [Member] | Property Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	20,562	22,285	22,020
Property and casualty insurance [Member] | Personal Lines [Member] | Liability Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	782	708	622
Reinsurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	118,341	122,787	101,707
Reinsurance [Member] | Pro rata reinsurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	41,883	47,648	30,461
Reinsurance [Member] | Pro rata reinsurance [Member] | Property And Liability Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	8,552	7,489	6,232
Reinsurance [Member] | Pro rata reinsurance [Member] | Property Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	4,793	15,775	13,509
Reinsurance [Member] | Pro rata reinsurance [Member] | Crop Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	3,636	4,455	3,841
Reinsurance [Member] | Pro rata reinsurance [Member] | Liability Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	9,919	5,172	1,171
Reinsurance [Member] | Pro rata reinsurance [Member] | Marine And Aviation Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	14,983	14,757	5,708
Reinsurance [Member] | Excess of loss reinsurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	76,458	75,139	71,246
Reinsurance [Member] | Excess of loss reinsurance [Member] | Property Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	64,956	64,069	59,537
Reinsurance [Member] | Excess of loss reinsurance [Member] | Liability Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	11,408	11,070	11,698
Reinsurance [Member] | Excess of loss reinsurance [Member] | Fidelity And Surety Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	$ 94	$ 0	$ 11