OPERATING SEGMENT INFORMATION - Major Products (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Revenue by major product
Net premiums earned	$ 183,285	$ 176,918
Casualty segment
Revenue by major product
Net premiums earned	116,984	108,593
Casualty segment | Commercial and personal umbrella
Revenue by major product
Net premiums earned	28,577	27,130
Casualty segment | General liability
Revenue by major product
Net premiums earned	21,583	20,724
Casualty segment | Transportation
Revenue by major product
Net premiums earned	21,101	18,274
Casualty segment | Professional services
Revenue by major product
Net premiums earned	19,226	18,538
Casualty segment | Small commercial
Revenue by major product
Net premiums earned	12,287	10,900
Casualty segment | Executive products
Revenue by major product
Net premiums earned	4,412	4,835
Casualty segment | Medical professional liability
Revenue by major product
Net premiums earned	4,291	3,815
Casualty segment | Other casualty
Revenue by major product
Net premiums earned	5,507	4,377
Property segment
Revenue by major product
Net premiums earned	35,805	38,685
Property segment | Commercial
Revenue by major product
Net premiums earned	15,718	17,596
Property segment | Marine
Revenue by major product
Net premiums earned	12,286	12,300
Property segment | Specialty Personal
Revenue by major product
Net premiums earned	6,018	6,308
Property segment | Property reinsurance
Revenue by major product
Net premiums earned	1,783	2,762
Property segment | Crop reinsurance business
Revenue by major product
Net premiums earned	5
Net premiums earned		(281)
Property segment | Other property
Revenue by major product
Net premiums earned	(5)
Surety segment
Revenue by major product
Net premiums earned	30,496	29,640
Surety segment | Miscellaneous
Revenue by major product
Net premiums earned	11,857	11,097
Surety segment | Commercial surety product
Revenue by major product
Net premiums earned	7,143	7,354
Surety segment | Contract
Revenue by major product
Net premiums earned	7,084	6,743
Surety segment | Energy Surety
Revenue by major product
Net premiums earned	$ 4,412	$ 4,446