OPERATING SEGMENT INFORMATION - Major Products (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue by major product
Net premiums earned	$ 200,815	$ 182,025	$ 587,364	$ 549,641
Casualty segment
Revenue by major product
Net premiums earned	131,605	118,393	387,068	354,636
Casualty segment | Commercial and personal umbrella
Revenue by major product
Net premiums earned	31,244	28,848	91,845	86,286
Casualty segment | General liability
Revenue by major product
Net premiums earned	22,485	22,138	68,988	66,572
Casualty segment | Transportation
Revenue by major product
Net premiums earned	20,747	18,047	60,570	59,543
Casualty segment | Professional services
Revenue by major product
Net premiums earned	19,890	19,584	59,559	58,826
Casualty segment | Small commercial
Revenue by major product
Net premiums earned	12,883	12,419	38,674	36,471
Casualty segment | Executive products
Revenue by major product
Net premiums earned	5,352	4,439	15,548	13,289
Casualty segment | Medical professional liability
Revenue by major product
Net premiums earned	3,988	4,169	12,822	12,725
Casualty segment | Other casualty
Revenue by major product
Net premiums earned	15,016	8,749	39,062	20,924
Property segment
Revenue by major product
Net premiums earned	39,067	33,559	111,439	103,849
Property segment | Commercial
Revenue by major product
Net premiums earned	18,489	15,600	53,296	47,191
Property segment | Marine
Revenue by major product
Net premiums earned	16,068	13,112	44,866	37,005
Property segment | Specialty Personal
Revenue by major product
Net premiums earned	4,228	4,844	12,499	16,370
Property segment | Property reinsurance
Revenue by major product
Net premiums earned	13		39	3,200
Net premiums earned		(62)
Property segment | Other property
Revenue by major product
Net premiums earned	269	65	739	83
Surety segment
Revenue by major product
Net premiums earned	30,143	30,073	88,857	91,156
Surety segment | Miscellaneous
Revenue by major product
Net premiums earned	11,822	11,780	35,183	35,491
Surety segment | Contract
Revenue by major product
Net premiums earned	7,474	7,130	20,832	21,361
Surety segment | Commercial surety product
Revenue by major product
Net premiums earned	6,863	6,861	20,337	20,942
Surety segment | Energy Surety
Revenue by major product
Net premiums earned	$ 3,984	$ 4,302	$ 12,505	$ 13,362