OPERATING SEGMENT INFORMATION - Major Products (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenue by major product
Net premiums earned	$ 208,734	$ 207,541	$ 424,316	$ 412,230
Casualty segment
Revenue by major product
Net premiums earned	135,215	138,344	278,635	275,244
Casualty segment | Commercial excess and personal umbrella
Revenue by major product
Net premiums earned	42,321	34,340	82,409	66,621
Casualty segment | General liability
Revenue by major product
Net premiums earned	22,651	25,347	46,649	49,235
Casualty segment | Professional services
Revenue by major product
Net premiums earned	21,384	19,275	42,079	39,574
Casualty segment | Transportation
Revenue by major product
Net premiums earned	11,139	20,508	32,324	40,912
Casualty segment | Small commercial
Revenue by major product
Net premiums earned	15,791	13,315	31,424	26,503
Casualty segment | Executive products
Revenue by major product
Net premiums earned	6,975	6,381	14,306	12,450
Casualty segment | Other casualty
Revenue by major product
Net premiums earned	14,954	19,178	29,444	39,949
Property segment
Revenue by major product
Net premiums earned	45,387	39,972	89,735	78,718
Property segment | Marine
Revenue by major product
Net premiums earned	20,712	18,579	40,289	35,600
Property segment | Commercial
Revenue by major product
Net premiums earned	19,048	16,275	38,203	33,150
Property segment | Specialty Personal
Revenue by major product
Net premiums earned	4,936	4,792	9,936	9,376
Property segment | Other property
Revenue by major product
Net premiums earned	691	326	1,307	592
Surety segment
Revenue by major product
Net premiums earned	28,132	29,225	55,946	58,268
Surety segment | Commercial surety product
Revenue by major product
Net premiums earned	10,806	10,975	21,744	21,787
Surety segment | Miscellaneous
Revenue by major product
Net premiums earned	10,499	11,280	21,015	22,882
Surety segment | Contract
Revenue by major product
Net premiums earned	$ 6,827	$ 6,970	$ 13,187	$ 13,599