SEGMENT INFORMATION - Net premiums earned by segment and lines of insurance (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue from External Customer [Line Items]
Premiums earned	$ 155,786	$ 144,487
Property and casualty insurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	118,632	113,648
Property and casualty insurance [Member] | Commercial Lines [Member]
Revenue from External Customer [Line Items]
Premiums earned	109,123	104,474
Property and casualty insurance [Member] | Commercial Lines [Member] | Automobile Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	30,644	28,032
Property and casualty insurance [Member] | Commercial Lines [Member] | Property Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	26,429	25,502
Property and casualty insurance [Member] | Commercial Lines [Member] | Workers' Compensation [Member]
Revenue from External Customer [Line Items]
Premiums earned	24,902	24,703
Property and casualty insurance [Member] | Commercial Lines [Member] | Other Liability [Member]
Revenue from External Customer [Line Items]
Premiums earned	24,962	24,128
Property and casualty insurance [Member] | Commercial Lines [Member] | Other Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	2,186	2,109
Property and casualty insurance [Member] | Personal Lines [Member]
Revenue from External Customer [Line Items]
Premiums earned	9,509	9,174
Reinsurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	37,154	30,839
Reinsurance [Member] | Assumed Pro Rata [Member]
Revenue from External Customer [Line Items]
Premiums earned	13,073	10,435
Reinsurance [Member] | Assumed Excess of Loss [Member]
Revenue from External Customer [Line Items]
Premiums earned	$ 24,081	$ 20,404