Selected Quarterly Financial Data (Unaudited) - Schedule of Selected Quarterly Results from Operations (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 Information [Abstract]
Grant revenues	$ 0	$ 0	$ 2,378	$ 1,510	$ 1,483	$ 1,261	$ 1,479	$ 982	$ 3,888	$ 5,205	$ 4,628
Type of revenue [extensible list]	us-gaap:GrantMember	us-gaap:GrantMember	us-gaap:GrantMember	us-gaap:GrantMember	us-gaap:GrantMember	us-gaap:GrantMember	us-gaap:GrantMember	us-gaap:GrantMember	us-gaap:GrantMember	us-gaap:GrantMember	us-gaap:GrantMember
Loss from operations	$ (15,305)	$ (12,243)	$ (9,670)	$ (8,245)	$ (6,627)	$ (7,998)	$ (6,720)	$ (6,331)	$ (45,463)	$ (27,676)	$ (21,906)
Net loss	$ (14,898)	$ (11,917)	$ (9,414)	$ (8,119)	$ (6,483)	$ (7,874)	$ (6,632)	$ (6,247)	$ (44,348)	$ (27,236)	$ (21,698)
Basic and diluted net loss per common share	$ (0.62)	$ (0.54)	$ (0.46)	$ (0.49)	$ (0.39)	$ (0.48)	$ (0.47)	$ (0.47)	$ (2.13)	$ (1.80)	$ (2.22)