Selected Quarterly Financial Data (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
Selected Quarterly Financial Data
Total revenue	$ 19,378	$ 15,795	$ 14,941	$ 20,645	$ 16,022	$ 11,603	$ 12,359	$ 12,535	$ 70,759	$ 52,519	$ 49,274
Gross profit	5,272	5,630	4,414	4,165	4,735	4,203	4,884	6,183	19,481	20,005	29,574
Loss from operations	(10,612)	(10,568)	(15,802)	(10,790)	(13,963)	(14,731)	(17,267)	(13,556)	(47,772)	(59,517)	(48,981)
Net loss	$ (11,563)	$ (10,993)	$ (16,521)	$ (11,423)	$ (14,536)	$ (15,408)	$ (17,989)	$ (13,917)	$ (50,500)	$ (61,850)	$ (50,601)
Per share data:
Net loss per common share - basic and diluted	$ (0.21)	$ (0.20)	$ (0.30)	$ (0.21)	$ (0.26)	$ (0.28)	$ (0.37)	$ (0.30)	$ (0.91)	$ (1.21)	$ (1.15)