SEGMENT INFORMATION - Net premiums earned by segment and lines of insurance (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Revenue from External Customer [Line Items]
Premiums earned	$ 144,605	$ 133,952	$ 283,336	$ 267,032
Property and casualty insurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	111,254	103,517	219,459	204,764
Property and casualty insurance [Member] | Commercial Lines [Member]
Revenue from External Customer [Line Items]
Premiums earned	100,287	91,960	197,498	181,366
Property and casualty insurance [Member] | Commercial Lines [Member] | Automobile Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	26,222	23,855	51,618	46,657
Property and casualty insurance [Member] | Commercial Lines [Member] | Property Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	25,926	23,328	50,992	46,645
Property and casualty insurance [Member] | Commercial Lines [Member] | Workers Compensation Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	23,006	21,910	45,373	42,963
Property and casualty insurance [Member] | Commercial Lines [Member] | Liability Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	23,087	21,055	45,503	41,510
Property and casualty insurance [Member] | Commercial Lines [Member] | Other Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	2,046	1,812	4,012	3,591
Property and casualty insurance [Member] | Personal Lines [Member]
Revenue from External Customer [Line Items]
Premiums earned	10,967	11,557	21,961	23,398
Property and casualty insurance [Member] | Personal Lines [Member] | Automobile Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	5,779	6,304	11,596	12,715
Property and casualty insurance [Member] | Personal Lines [Member] | Homeowners Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	5,188	5,253	10,365	10,683
Reinsurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	33,351	30,435	63,877	62,268
Reinsurance [Member] | Pro rata reinsurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	14,812	11,888	27,117	25,370
Reinsurance [Member] | Pro rata reinsurance [Member] | Multiline Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	2,156	3,267	3,394	4,331
Reinsurance [Member] | Pro rata reinsurance [Member] | Property Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	3,859	1,063	7,715	7,307
Reinsurance [Member] | Pro rata reinsurance [Member] | Liability Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	5,367	2,721	9,168	5,513
Reinsurance [Member] | Pro rata reinsurance [Member] | Marine And Aviation Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	3,430	4,837	6,840	8,219
Reinsurance [Member] | Excess of loss reinsurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	18,539	18,547	36,760	36,898
Reinsurance [Member] | Excess of loss reinsurance [Member] | Property Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	15,714	15,800	30,176	31,259
Reinsurance [Member] | Excess of loss reinsurance [Member] | Liability Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	$ 2,825	$ 2,747	$ 6,584	$ 5,639