OPERATING SEGMENT INFORMATION - Major Products (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenue by major product
Net premiums earned	$ 215,582	$ 204,689
Casualty segment
Revenue by major product
Net premiums earned	143,420	136,900
Casualty segment | Commercial excess and personal umbrella
Revenue by major product
Net premiums earned	40,088	32,281
Casualty segment | General liability
Revenue by major product
Net premiums earned	23,998	23,888
Casualty segment | Transportation
Revenue by major product
Net premiums earned	21,185	20,404
Casualty segment | Professional services
Revenue by major product
Net premiums earned	20,695	20,299
Casualty segment | Small commercial
Revenue by major product
Net premiums earned	15,633	13,188
Casualty segment | Executive products
Revenue by major product
Net premiums earned	7,331	6,069
Casualty segment | Other casualty
Revenue by major product
Net premiums earned	14,490	20,771
Property segment
Revenue by major product
Net premiums earned	44,348	38,746
Property segment | Marine
Revenue by major product
Net premiums earned	19,577	17,021
Property segment | Commercial
Revenue by major product
Net premiums earned	19,155	16,875
Property segment | Specialty Personal
Revenue by major product
Net premiums earned	5,000	4,584
Property segment | Other property
Revenue by major product
Net premiums earned	616	266
Surety segment
Revenue by major product
Net premiums earned	27,814	29,043
Surety segment | Commercial surety product
Revenue by major product
Net premiums earned	10,938	10,812
Surety segment | Miscellaneous
Revenue by major product
Net premiums earned	10,516	11,602
Surety segment | Contract
Revenue by major product
Net premiums earned	$ 6,360	$ 6,629