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, 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 Information [Abstract]
Grant revenues							$ 2,378	$ 1,510	$ 0	$ 3,888	$ 5,205
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
Loss from operations	$ (21,909)	$ (22,146)	$ (18,622)	$ (17,657)	$ (15,305)	$ (12,243)	$ (9,670)	$ (8,245)	$ (80,334)	$ (45,463)	$ (27,676)
Net loss	$ (21,495)	$ (21,573)	$ (18,019)	$ (17,167)	$ (14,898)	$ (11,917)	$ (9,414)	$ (8,119)	$ (78,254)	$ (44,348)	$ (27,236)
Basic and diluted net loss per common share	$ (0.65)	$ (0.66)	$ (0.55)	$ (0.59)	$ (0.62)	$ (0.54)	$ (0.46)	$ (0.49)	$ (2.45)	$ (2.13)	$ (1.80)

The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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