OPERATING SEGMENT INFORMATION - Major Products (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenue by major product
Net premiums earned	$ 204,689	$ 190,027
Casualty segment
Revenue by major product
Net premiums earned	136,900	125,850
Casualty segment | Commercial excess and personal umbrella
Revenue by major product
Net premiums earned	32,281	29,952
Casualty segment | General liability
Revenue by major product
Net premiums earned	23,888	23,165
Casualty segment | Transportation
Revenue by major product
Net premiums earned	20,404	19,175
Casualty segment | Professional services
Revenue by major product
Net premiums earned	20,299	19,823
Casualty segment | Small commercial
Revenue by major product
Net premiums earned	13,188	12,890
Casualty segment | Executive products
Revenue by major product
Net premiums earned	6,069	5,052
Casualty segment | Medical professional liability
Revenue by major product
Net premiums earned	3,106	4,472
Casualty segment | Other casualty
Revenue by major product
Net premiums earned	17,665	11,321
Property segment
Revenue by major product
Net premiums earned	38,746	35,182
Property segment | Marine
Revenue by major product
Net premiums earned	17,021	13,857
Property segment | Commercial
Revenue by major product
Net premiums earned	16,875	16,951
Property segment | Specialty Personal
Revenue by major product
Net premiums earned	4,584	4,142
Property segment | Other property
Revenue by major product
Net premiums earned	266	232
Surety segment
Revenue by major product
Net premiums earned	29,043	28,995
Surety segment | Miscellaneous
Revenue by major product
Net premiums earned	11,602	11,642
Surety segment | Commercial surety product
Revenue by major product
Net premiums earned	6,734	6,713
Surety segment | Contract
Revenue by major product
Net premiums earned	6,629	6,346
Surety segment | Energy Surety
Revenue by major product
Net premiums earned	$ 4,078	$ 4,294