Quarterly Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Gross profit	$ 18,292	$ 19,199	$ 22,573	$ 17,679	$ 21,558	$ 18,646	$ 22,435	$ 19,388	$ 77,743	$ 82,027	$ 80,157
Net income	$ 385	$ 3,244	$ 3,575	$ 2,632	$ (339)	$ 3,281	$ 5,863	$ 4,086	$ 9,836	$ 12,891	$ 20,247
Earnings per share:
Basic earnings per share (in dollars per share)	$ 0.02	$ 0.20	$ 0.22	$ 0.16	$ (0.02)	$ 0.20	$ 0.35	$ 0.24	$ 0.59	$ 0.77	$ 1.21
Diluted (in dollars per share)	$ 0.02	$ 0.20	$ 0.22	$ 0.16	$ (0.02)	$ 0.19	$ 0.35	$ 0.24	$ 0.59	$ 0.76	$ 1.21
Revenue and gross profit reduction related to contract						$ 2,600
Revenue from External Customer [Line Items]
Revenue	$ 132,871	$ 123,566	$ 133,691	$ 125,032	$ 131,503	$ 124,097	$ 131,161	$ 122,447	$ 515,160	$ 509,208	$ 490,559
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