OPERATING SEGMENT INFORMATION - Revenue by Product (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue by major product
Net premiums earned	$ 187,869	$ 183,595	$ 180,226	$ 176,918	$ 179,371	$ 179,448	$ 172,339	$ 169,003	$ 728,608	$ 700,161	$ 687,375
Casualty segment
Revenue by major product
Net premiums earned									454,843	412,248	382,105
Casualty segment | Commercial and personal umbrella
Revenue by major product
Net premiums earned									111,079	104,598	100,420
Casualty segment | General liability
Revenue by major product
Net premiums earned									83,958	81,213	80,820
Casualty segment | Transportation
Revenue by major product
Net premiums earned									81,402	65,564	58,911
Casualty segment | Professional services
Revenue by major product
Net premiums earned									75,872	71,034	58,327
Casualty segment | P&C package business
Revenue by major product
Net premiums earned									45,660	40,410	35,371
Casualty segment | Executive products
Revenue by major product
Net premiums earned									18,755	17,892	18,915
Casualty segment | Medical professional liability
Revenue by major product
Net premiums earned									17,449	12,292	15,943
Casualty segment | Other casualty
Revenue by major product
Net premiums earned									20,668	19,245	13,398
Property segment
Revenue by major product
Net premiums earned									152,167	170,924	197,776
Property segment | Commercial property
Revenue by major product
Net premiums earned									68,165	75,749	80,719
Property segment | Marine
Revenue by major product
Net premiums earned									48,301	47,016	49,235
Property segment | Specialty Personal
Revenue by major product
Net premiums earned									24,981	26,395	26,627
Property segment | Property reinsurance
Revenue by major product
Net premiums earned									10,953	12,330	12,756
Property segment | Crop reinsurance business
Revenue by major product
Net premiums earned										9,358	28,293
Net premiums earned									(233)
Property segment | Other property
Revenue by major product
Net premiums earned										76	146
Surety segment
Revenue by major product
Net premiums earned									121,598	116,989	107,494
Surety segment | Miscellaneous
Revenue by major product
Net premiums earned									46,235	42,372	39,026
Surety segment | Commercial
Revenue by major product
Net premiums earned									29,105	29,529	25,778
Surety segment | Contract
Revenue by major product
Net premiums earned									28,240	28,269	26,592
Surety segment | Energy Surety
Revenue by major product
Net premiums earned									$ 18,018	$ 16,819	$ 16,098