Supplemental 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
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 2,120	$ 828	$ 699	$ 530	$ 39	$ 24	$ 20	$ 163	$ 4,177	$ 246	$ 147
Gross profit	1,470	637	564	504	39	24	20	163	3,175	246	147
Loss from operations	(16,136)	(17,315)	(16,423)	(14,310)	(16,024)	(17,416)	(17,889)	(15,172)	(64,184)	(66,501)	(45,549)
Net loss	$ (16,296)	$ (17,075)	$ (16,457)	$ (14,200)	$ (16,135)	$ (17,299)	$ (17,866)	$ (15,074)	$ (64,028)	$ (66,374)	$ (45,498)
Basic and diluted net loss per share (dollars per share)	$ (0.29)	$ (0.31)	$ (0.31)	$ (0.27)	$ (0.31)	$ (0.34)	$ (0.35)	$ (0.29)