Selected Quarterly Financial Data (Unaudited) - Schedule of Quarterly Financial Data (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
Statements of Operations Data:
Revenue from Contract with Customer, Excluding Assessed Tax	$ 2,256	$ 829	$ 650	$ 492	$ 504	$ 498	$ 648	$ 340	$ 4,227	$ 1,990	$ 1,923
Loss from operations	(21,401)	(23,144)	(21,599)	(19,658)	(17,283)	(14,816)	(13,564)	(13,455)	(85,802)	(59,118)	(61,923)
Net loss	$ (26,017)	$ (18,778)	$ (24,453)	$ (17,124)	$ (17,399)	$ (15,010)	$ (13,804)	$ (13,765)	$ (86,372)	$ (59,978)	$ (63,386)
Basic net loss per common share	$ (0.53)	$ (0.40)	$ (0.57)	$ (0.41)	$ (0.42)	$ (0.38)	$ (0.37)	$ (0.40)
Diluted net loss per common share	$ (0.53)	$ (0.45)	$ (0.57)	$ (0.45)	$ (0.42)	$ (0.38)	$ (0.37)	$ (0.40)