SEGMENT INFORMATION - Net premiums earned by segment and lines of insurance (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenue from External Customer [Line Items]
Premiums earned	$ 149,837	$ 146,446	$ 294,324	$ 289,183
Property and casualty insurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	116,187	111,771	229,835	222,217
Property and casualty insurance [Member] | Commercial Lines [Member]
Revenue from External Customer [Line Items]
Premiums earned	106,877	102,183	211,351	202,846
Property and casualty insurance [Member] | Commercial Lines [Member] | Automobile Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	29,014	27,409	57,046	54,336
Property and casualty insurance [Member] | Commercial Lines [Member] | Property Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	26,069	25,073	51,571	49,821
Property and casualty insurance [Member] | Commercial Lines [Member] | Workers' Compensation [Member]
Revenue from External Customer [Line Items]
Premiums earned	25,343	23,489	50,046	46,736
Property and casualty insurance [Member] | Commercial Lines [Member] | Other Liability [Member]
Revenue from External Customer [Line Items]
Premiums earned	24,254	24,139	48,382	47,809
Property and casualty insurance [Member] | Commercial Lines [Member] | Other Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	2,197	2,073	4,306	4,144
Property and casualty insurance [Member] | Personal Lines [Member]
Revenue from External Customer [Line Items]
Premiums earned	9,310	9,588	18,484	19,371
Reinsurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	33,650	34,675	64,489	66,966
Reinsurance [Member] | Assumed Pro Rata [Member]
Revenue from External Customer [Line Items]
Premiums earned	12,016	15,468	22,451	29,109
Reinsurance [Member] | Assumed Excess of Loss [Member]
Revenue from External Customer [Line Items]
Premiums earned	$ 21,634	$ 19,207	$ 42,038	$ 37,857