Quarterly Information (unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 127,283	$ 121,978	$ 125,542	$ 115,756	$ 126,431	$ 122,931	$ 125,665	$ 115,253	$ 490,559	$ 490,280	$ 501,867
Gross profit	21,882	20,004	20,344	17,927	22,412	20,369	20,076	19,135	80,157	81,992	89,575
Net income	$ 6,732	$ 4,802	$ 4,913	$ 3,800	$ 6,252	$ 3,716	$ 4,714	$ 4,107	$ 20,247	$ 18,789	$ 27,098
Earnings per share:
Basic (in dollars per share)	$ 0.40	$ 0.29	$ 0.29	$ 0.23	$ 0.37	$ 0.22	$ 0.27	$ 0.24	$ 1.21	$ 1.10	$ 1.45
Diluted (in dollars per share)	$ 0.40	$ 0.29	$ 0.29	$ 0.23	$ 0.37	$ 0.22	$ 0.27	$ 0.24	$ 1.21	$ 1.09	$ 1.43