OPERATING SEGMENT INFORMATION - Revenue by Product (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenue by major product
Net premiums earned	$ 179,371	$ 179,448	$ 172,339	$ 169,003	$ 179,892	$ 177,747	$ 168,604	$ 161,132	$ 700,161	$ 687,375	$ 630,802
Casualty segment
Revenue by major product
Net premiums earned									412,248	382,105	324,022
Casualty segment | Commercial and personal umbrella
Revenue by major product
Net premiums earned									104,598	100,420	85,532
Casualty segment | General liability
Revenue by major product
Net premiums earned									81,213	80,820	81,427
Casualty segment | Professional services
Revenue by major product
Net premiums earned									71,034	58,327	42,063
Casualty segment | Commercial transportation
Revenue by major product
Net premiums earned									65,564	58,911	50,287
Casualty segment | P&C package business
Revenue by major product
Net premiums earned									40,410	35,371	30,603
Casualty segment | Executive products
Revenue by major product
Net premiums earned									17,892	18,915	19,123
Casualty segment | Medical professional liability
Revenue by major product
Net premiums earned									12,292	15,943	8,626
Casualty segment | Other casualty
Revenue by major product
Net premiums earned									19,245	13,398	6,361
Property segment
Revenue by major product
Net premiums earned									170,924	197,776	200,141
Property segment | Commercial property
Revenue by major product
Net premiums earned									75,749	80,719	76,939
Property segment | Marine
Revenue by major product
Net premiums earned									47,016	49,235	57,122
Property segment | Specialty Personal
Revenue by major product
Net premiums earned									26,395	26,627	16,308
Property segment | Property reinsurance
Revenue by major product
Net premiums earned									12,330	12,756	15,770
Property segment | Crop reinsurance
Revenue by major product
Net premiums earned									9,358	28,293	31,421
Property segment | Other property
Revenue by major product
Net premiums earned									76	146	2,581
Surety segment
Revenue by major product
Net premiums earned									116,989	107,494	106,639
Surety segment | Miscellaneous
Revenue by major product
Net premiums earned									42,372	39,026	38,131
Surety segment | Commercial
Revenue by major product
Net premiums earned									29,529	25,778	23,133
Surety segment | Contract
Revenue by major product
Net premiums earned									28,269	26,592	27,176
Surety segment | Energy Surety
Revenue by major product
Net premiums earned									$ 16,819	$ 16,098	$ 18,199