SEGMENT INFORMATION - Net premiums earned by segment and lines of insurance (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenue from External Customer [Line Items]
Premiums earned	$ 167,302	$ 155,786
Property and casualty insurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	124,772	118,632
Property and casualty insurance [Member] | Commercial Lines [Member]
Revenue from External Customer [Line Items]
Premiums earned	115,532	109,123
Property and casualty insurance [Member] | Commercial Lines [Member] | Automobile Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	32,907	30,644
Property and casualty insurance [Member] | Commercial Lines [Member] | Property Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	27,671	26,429
Property and casualty insurance [Member] | Commercial Lines [Member] | Workers' Compensation [Member]
Revenue from External Customer [Line Items]
Premiums earned	23,543	24,902
Property and casualty insurance [Member] | Commercial Lines [Member] | Other Liability [Member]
Revenue from External Customer [Line Items]
Premiums earned	28,905	24,962
Property and casualty insurance [Member] | Commercial Lines [Member] | Other Insurance L O B [Member]
Revenue from External Customer [Line Items]
Premiums earned	2,506	2,186
Property and casualty insurance [Member] | Personal Lines [Member]
Revenue from External Customer [Line Items]
Premiums earned	9,240	9,509
Reinsurance [Member]
Revenue from External Customer [Line Items]
Premiums earned	42,530	37,154
Reinsurance [Member] | Assumed Pro Rata [Member]
Revenue from External Customer [Line Items]
Premiums earned	13,006	13,073
Reinsurance [Member] | Assumed Excess of Loss [Member]
Revenue from External Customer [Line Items]
Premiums earned	$ 29,524	$ 24,081