Selected 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
Quarterly Financial Information Disclosure [Abstract]
Grant and collaboration revenue	$ 6,654	$ 0	$ 13	$ 10	$ 903	$ 0	$ 0	$ 0	$ 6,677	$ 903	$ 207
Operating expenses	19,224	11,794	16,248	11,866	15,402	15,941	18,769	15,813	59,132	65,925	63,991
Net loss	(14,888)	(11,994)	(16,394)	(12,074)	(14,651)	(16,001)	(18,796)	(15,888)	(55,350)	(65,336)	(65,321)
Net (loss) income attributable to common stockholders - basic & diluted	$ (14,888)	$ (11,994)	$ (16,394)	$ (12,074)	$ (14,651)	$ (16,001)	$ (18,796)	$ (15,888)	$ (55,350)	$ (65,336)	$ (65,321)
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (0.28)	$ (0.26)	$ (0.37)	$ (0.31)	$ (0.65)	$ (0.71)	$ (0.84)	$ (0.71)	$ (1.22)	$ (2.92)	$ (3.20)

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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