OPERATING SEGMENT INFORMATION - Major Products (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenue by major product
Net premiums earned	$ 207,541	$ 196,522	$ 412,230	$ 386,549
Casualty segment
Revenue by major product
Net premiums earned	138,344	129,613	275,244	255,463
Casualty segment | Commercial excess and personal umbrella
Revenue by major product
Net premiums earned	34,340	30,649	66,621	60,601
Casualty segment | General liability
Revenue by major product
Net premiums earned	25,347	23,338	49,235	46,503
Casualty segment | Transportation
Revenue by major product
Net premiums earned	20,508	20,648	40,912	39,823
Casualty segment | Professional services
Revenue by major product
Net premiums earned	19,275	19,846	39,574	39,669
Casualty segment | Small commercial
Revenue by major product
Net premiums earned	13,315	12,901	26,503	25,791
Casualty segment | Executive products
Revenue by major product
Net premiums earned	6,381	5,144	12,450	10,196
Casualty segment | Other casualty
Revenue by major product
Net premiums earned	19,178	17,087	39,949	32,880
Property segment
Revenue by major product
Net premiums earned	39,972	37,190	78,718	72,372
Property segment | Marine
Revenue by major product
Net premiums earned	18,579	14,941	35,600	28,798
Property segment | Commercial
Revenue by major product
Net premiums earned	16,275	17,856	33,150	34,807
Property segment | Specialty Personal
Revenue by major product
Net premiums earned	4,792	4,129	9,376	8,271
Property segment | Other property
Revenue by major product
Net premiums earned	326	264	592	496
Surety segment
Revenue by major product
Net premiums earned	29,225	29,719	58,268	58,714
Surety segment | Miscellaneous
Revenue by major product
Net premiums earned	11,280	11,719	22,882	23,361
Surety segment | Commercial surety product
Revenue by major product
Net premiums earned	7,077	6,761	13,811	13,474
Surety segment | Contract
Revenue by major product
Net premiums earned	6,970	7,012	13,599	13,358
Surety segment | Energy Surety
Revenue by major product
Net premiums earned	$ 3,898	$ 4,227	$ 7,976	$ 8,521