Quarterly Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 131,503	$ 124,097	$ 131,161	$ 122,447	$ 127,283	$ 121,978	$ 125,542	$ 115,756	$ 509,208	$ 490,559	$ 490,280
Gross profit	21,558	18,646	22,435	19,388	21,882	20,004	20,344	17,927	82,027	80,157	81,992
Net income	$ (339)	$ 3,281	$ 5,863	$ 4,086	$ 6,732	$ 4,802	$ 4,913	$ 3,800	$ 12,891	$ 20,247	$ 18,789
Earnings per share:
Basic (in dollars per share)	$ (0.02)	$ 0.20	$ 0.35	$ 0.24	$ 0.40	$ 0.29	$ 0.29	$ 0.23	$ 0.77	$ 1.21	$ 1.10
Diluted (in dollars per share)	$ (0.02)	$ 0.19	$ 0.35	$ 0.24	$ 0.40	$ 0.29	$ 0.29	$ 0.23	$ 0.76	$ 1.21	$ 1.09
Revenue and gross profit reduction related to contract		$ 2,600
Revenue from External Customer [Line Items]
Loss on contract termination	$ 1,800
Gross Profit [Member]
Revenue from External Customer [Line Items]
Loss on contract termination	500
Selling, General and Administrative Expenses [Member]
Revenue from External Customer [Line Items]
Bad debt reserve	$ 1,300