OPERATING SEGMENT INFORMATION - Major Products (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue by major product
Net premiums earned	$ 196,522	$ 184,331	$ 386,549	$ 367,616
Casualty segment
Revenue by major product
Net premiums earned	129,613	119,259	255,463	236,243
Casualty segment | Commercial and personal umbrella
Revenue by major product
Net premiums earned	30,649	28,861	60,601	57,438
Casualty segment | General liability
Revenue by major product
Net premiums earned	23,338	22,851	46,503	44,434
Casualty segment | Transportation
Revenue by major product
Net premiums earned	20,648	20,395	39,823	41,496
Casualty segment | Professional services
Revenue by major product
Net premiums earned	19,846	20,016	39,669	39,242
Casualty segment | Small commercial
Revenue by major product
Net premiums earned	12,901	11,765	25,791	24,052
Casualty segment | Executive products
Revenue by major product
Net premiums earned	5,144	4,438	10,196	8,850
Casualty segment | Medical professional liability
Revenue by major product
Net premiums earned	4,362	4,265	8,834	8,556
Casualty segment | Other casualty
Revenue by major product
Net premiums earned	12,725	6,668	24,046	12,175
Property segment
Revenue by major product
Net premiums earned	37,190	34,485	72,372	70,290
Property segment | Commercial
Revenue by major product
Net premiums earned	17,856	15,873	34,807	31,591
Property segment | Marine
Revenue by major product
Net premiums earned	14,941	11,607	28,798	23,893
Property segment | Specialty Personal
Revenue by major product
Net premiums earned	4,129	5,508	8,271	11,526
Property segment | Property reinsurance
Revenue by major product
Net premiums earned	14	1,479	26	3,262
Property segment | Other property
Revenue by major product
Net premiums earned	250	18	470	18
Surety segment
Revenue by major product
Net premiums earned	29,719	30,587	58,714	61,083
Surety segment | Miscellaneous
Revenue by major product
Net premiums earned	11,719	11,854	23,361	23,711
Surety segment | Commercial surety product
Revenue by major product
Net premiums earned	6,761	6,938	13,474	14,081
Surety segment | Contract
Revenue by major product
Net premiums earned	7,012	7,147	13,358	14,231
Surety segment | Energy Surety
Revenue by major product
Net premiums earned	$ 4,227	$ 4,648	$ 8,521	$ 9,060