OPERATING SEGMENT INFORMATION - Major Products (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenue by major product
Net premiums earned	$ 182,025	$ 183,595	$ 549,641	$ 540,739
Casualty segment
Revenue by major product
Net premiums earned	118,393	115,619	354,636	336,572
Casualty segment | Commercial and personal umbrella
Revenue by major product
Net premiums earned	28,848	27,844	86,286	82,510
Casualty segment | General liability
Revenue by major product
Net premiums earned	22,138	21,563	66,572	63,735
Casualty segment | Transportation
Revenue by major product
Net premiums earned	18,047	21,557	59,543	60,037
Casualty segment | Professional services
Revenue by major product
Net premiums earned	19,584	19,318	58,826	56,748
Casualty segment | Small commercial
Revenue by major product
Net premiums earned	12,419	11,706	36,471	33,973
Casualty segment | Executive products
Revenue by major product
Net premiums earned	4,439	4,716	13,289	14,126
Casualty segment | Medical professional liability
Revenue by major product
Net premiums earned	4,169	4,512	12,725	12,509
Casualty segment | Other casualty
Revenue by major product
Net premiums earned	8,749	4,403	20,924	12,934
Property segment
Revenue by major product
Net premiums earned	33,559	37,532	103,849	114,011
Property segment | Commercial
Revenue by major product
Net premiums earned	15,600	16,819	47,191	51,528
Property segment | Marine
Revenue by major product
Net premiums earned	13,112	11,964	37,005	36,070
Property segment | Specialty Personal
Revenue by major product
Net premiums earned	4,844	6,186	16,370	18,731
Property segment | Property reinsurance
Revenue by major product
Net premiums earned		2,544	3,200	7,925
Net premiums earned	(62)
Property segment | Crop reinsurance business
Revenue by major product
Net premiums earned		19
Net premiums earned	(11)		(6)	(243)
Property segment | Other property
Revenue by major product
Net premiums earned	76		89
Surety segment
Revenue by major product
Net premiums earned	30,073	30,444	91,156	90,156
Surety segment | Miscellaneous
Revenue by major product
Net premiums earned	11,780	11,714	35,491	34,304
Surety segment | Contract
Revenue by major product
Net premiums earned	7,130	7,221	21,361	22,030
Surety segment | Commercial surety product
Revenue by major product
Net premiums earned	6,861	6,906	20,942	20,332
Surety segment | Energy Surety
Revenue by major product
Net premiums earned	$ 4,302	$ 4,603	$ 13,362	$ 13,490