OPERATING SEGMENT INFORMATION - Revenue by Product (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue by major product
Net premiums earned	$ 204,002	$ 200,815	$ 196,522	$ 190,027	$ 188,296	$ 182,025	$ 184,331	$ 183,285	$ 791,366	$ 737,937	$ 728,608
Casualty segment
Revenue by major product
Net premiums earned									523,472	478,603	454,843
Casualty segment | Commercial excess and personal umbrella
Revenue by major product
Net premiums earned									124,350	115,543	111,079
Casualty segment | General liability
Revenue by major product
Net premiums earned									93,928	90,283	86,853
Casualty segment | Transportation
Revenue by major product
Net premiums earned									81,053	78,061	81,402
Casualty segment | Professional services
Revenue by major product
Net premiums earned									79,951	78,508	75,872
Casualty segment | Small commercial
Revenue by major product
Net premiums earned									51,519	49,601	45,660
Casualty segment | Executive products
Revenue by major product
Net premiums earned									21,326	18,086	18,755
Casualty segment | Medical professional liability
Revenue by major product
Net premiums earned									16,042	17,072	17,449
Casualty segment | Other casualty
Revenue by major product
Net premiums earned									55,303	31,449	17,773
Property segment
Revenue by major product
Net premiums earned									149,261	138,346	152,167
Property segment | Commercial property
Revenue by major product
Net premiums earned									71,501	63,117	68,165
Property segment | Marine
Revenue by major product
Net premiums earned									59,795	50,931	48,301
Property segment | Specialty Personal
Revenue by major product
Net premiums earned									16,901	20,793	24,981
Property segment | Other property
Revenue by major product
Net premiums earned									1,064	3,505	10,720
Surety segment
Revenue by major product
Net premiums earned									118,633	120,988	121,598
Surety segment | Miscellaneous
Revenue by major product
Net premiums earned									46,968	47,237	46,235
Surety segment | Contract
Revenue by major product
Net premiums earned									28,196	28,573	28,240
Surety segment | Commercial
Revenue by major product
Net premiums earned									26,751	27,625	29,105
Surety segment | Energy Surety
Revenue by major product
Net premiums earned									$ 16,718	$ 17,553	$ 18,018