OPERATING SEGMENT INFORMATION - Revenue by Product (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue by major product
Net premiums earned	$ 215,626	$ 211,255	$ 207,541	$ 204,689	$ 204,002	$ 200,815	$ 196,522	$ 190,027	$ 839,111	$ 791,366	$ 737,937
Casualty segment
Revenue by major product
Net premiums earned									558,458	523,472	478,603
Casualty segment | Commercial excess and personal umbrella
Revenue by major product
Net premiums earned									140,483	124,350	115,543
Casualty segment | General liability
Revenue by major product
Net premiums earned									98,880	93,928	90,283
Casualty segment | Transportation
Revenue by major product
Net premiums earned									83,213	81,053	78,061
Casualty segment | Professional services
Revenue by major product
Net premiums earned									81,329	79,951	78,508
Casualty segment | Small commercial
Revenue by major product
Net premiums earned									55,701	51,519	49,601
Casualty segment | Executive products
Revenue by major product
Net premiums earned									27,088	21,326	18,086
Casualty segment | Other casualty
Revenue by major product
Net premiums earned									71,764	71,345	48,521
Property segment
Revenue by major product
Net premiums earned									164,022	149,261	138,346
Property segment | Marine
Revenue by major product
Net premiums earned									74,887	59,795	50,931
Property segment | Commercial property
Revenue by major product
Net premiums earned									68,310	71,501	63,117
Property segment | Specialty Personal
Revenue by major product
Net premiums earned									19,316	16,901	20,793
Property segment | Other property
Revenue by major product
Net premiums earned									1,509	1,064	3,505
Surety segment
Revenue by major product
Net premiums earned									116,631	118,633	120,988
Surety segment | Miscellaneous
Revenue by major product
Net premiums earned									44,721	46,968	47,237
Surety segment | Commercial
Revenue by major product
Net premiums earned									43,553	43,469	45,178
Surety segment | Contract
Revenue by major product
Net premiums earned									$ 28,357	$ 28,196	$ 28,573