SEGMENT INFORMATION - Net premiums earned by segment and lines of insurance (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue from External Customer [Line Items]
Premiums earned	$ 157,946	$ 149,837	$ 313,732	$ 294,324
Property and casualty insurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	121,495	116,187	240,127	229,835
Property and casualty insurance [Member] | Commercial Lines [Member]
Revenue from External Customer [Line Items]
Premiums earned	111,904	106,877	221,190	211,351
Property and casualty insurance [Member] | Commercial Lines [Member] | Automobile Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	31,660	29,014	62,304	57,046
Property and casualty insurance [Member] | Commercial Lines [Member] | Property Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	27,196	26,069	53,788	51,571
Property and casualty insurance [Member] | Commercial Lines [Member] | Workers' Compensation [Member]
Revenue from External Customer [Line Items]
Premiums earned	25,229	25,343	50,131	50,046
Property and casualty insurance [Member] | Commercial Lines [Member] | Other Liability [Member]
Revenue from External Customer [Line Items]
Premiums earned	25,591	24,254	50,553	48,382
Property and casualty insurance [Member] | Commercial Lines [Member] | Other Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	2,228	2,197	4,414	4,306
Property and casualty insurance [Member] | Personal Lines [Member]
Revenue from External Customer [Line Items]
Premiums earned	9,591	9,310	18,937	18,484
Reinsurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	36,451	33,650	73,605	64,489
Reinsurance [Member] | Assumed Pro Rata [Member]
Revenue from External Customer [Line Items]
Premiums earned	10,070	12,016	23,143	22,451
Reinsurance [Member] | Assumed Excess of Loss [Member]
Revenue from External Customer [Line Items]
Premiums earned	$ 26,381	$ 21,634	$ 50,462	$ 42,038