Supplemental Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 3,470	$ 2,271	$ 1,806	$ 1,750	$ 1,822	$ 1,355	$ 1,692	$ 801	$ 9,297	$ 5,670	$ 4,177
Gross profit	1,513	1,154	899	834	524	675	975	309	4,400	2,483	3,175
Loss from operations	(18,269)	(17,653)	(18,087)	(18,822)	(19,759)	(19,369)	(20,415)	(20,826)	(72,831)	(80,369)	(64,184)
Net loss	$ (21,335)	$ (20,434)	$ (20,815)	$ (21,721)	$ (22,191)	$ (22,098)	$ (23,225)	$ (20,812)	$ (84,305)	$ (88,326)	$ (64,028)
Basic and diluted net loss per share (dollars per share)	$ (0.40)	$ (0.37)	$ (0.38)	$ (0.40)	$ (0.41)	$ (0.41)	$ (0.43)	$ (0.37)