Selected Quarterly Financial 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
Selected Quarterly Financial Data (Unaudited)
Total revenue	$ 31	$ 0	$ 0	$ 0	$ 232	$ 3,000	$ 2,500	$ 0	$ 31	$ 5,732	$ 0
Revenue, Product and Service [Extensible List]	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember
Loss from operations	$ (6,470)	$ (8,755)	$ (6,141)	$ (5,214)	$ (7,951)	$ (4,258)	$ (8,595)	$ (9,693)	$ (26,580)	$ (30,498)	$ (70,387)
Net loss	$ (6,445)	$ (8,566)	$ (4,691)	$ (5,169)	$ (7,976)	$ (4,483)	$ (9,304)	$ (9,021)	$ (24,871)	$ (30,784)	$ (71,865)
Net loss per share of common stock, basic and diluted (in dollars per share)	$ (0.07)	$ (0.09)	$ (0.05)	$ (0.06)	$ (0.10)	$ (0.06)	$ (0.13)	$ (0.14)	$ (0.27)	$ (0.42)	$ (1.21)
Weighted average common shares outstanding, basic and diluted (in shares)	92,777,480	92,569,993	92,414,644	88,897,292	82,323,393	77,445,675	69,664,994	64,562,236	91,677,963	73,558,548	59,436,649