Selected Quarterly Financial Data (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
Selected Quarterly Financial Data (Unaudited)
Total revenue	$ 10,123	$ 9,008	$ 8,925	$ 7,880	$ 7,064	$ 7,164	$ 6,659	$ 6,207	$ 35,936	$ 27,094	$ 26,546
Gross profit	6,221	5,255	5,792	4,217	3,232	4,305	3,500	3,662	21,485	14,699	13,978
Net loss	$ (4,381)	$ (5,446)	$ (5,187)	$ (6,086)	$ (5,308)	$ (1,444)	$ (5,416)	$ (4,540)	$ (21,100)	$ (16,708)	$ (18,183)
Net loss per common share, basic and diluted	$ (0.35)	$ (0.43)	$ (0.42)	$ (0.49)	$ (0.48)	$ (0.54)	$ (2.12)	$ (1.78)	$ (1.69)	$ (3.53)	$ (7.12)