Description of the Business and Liquidity (Details) $ in Thousands, € in Millions				1 Months Ended			3 Months Ended				6 Months Ended
	Jul. 30, 2021 EUR (€)	May 11, 2021 USD ($) tranche	Sep. 23, 2020 shares	Dec. 31, 2020 USD ($) shares	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($) shares	Jun. 30, 2021 USD ($) shares	Mar. 31, 2021 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2021 USD ($) shares	Jun. 30, 2020 USD ($)
Liquidity
Net loss							$ 23,823	$ 27,141	$ 15,675	$ 18,672	$ 50,964	$ 34,347
Accumulated deficit				$ 311,929			$ 362,893				$ 362,893
Reverse stock split ratio			0.25
Common stock, shares authorized | shares			150,000,000	150,000,000			150,000,000				150,000,000
Oaktree Capital Management LP Credit Agreement
Liquidity
Term of loan facility		5 years
Maximum borrowing capacity available under the credit agreement		$ 125,000
Number of tranches | tranche		5
BARDA Contract
Liquidity
Maximum amount to be awarded under the contract					$ 51,000
Contract base period					2 years
Amount of funding to be provided during contract base period					$ 21,000
Amount of additional funding to be provided following successful completion of clinical trial and preclinical studies in base period					30,000
Cost sharing, amount					33,000
Cost sharing, BARDA amount					$ 51,000
Contract period-of-performance					5 years
Collaboration Agreement with Orion
Liquidity
Upfront fee received | €	€ 25
R&D reimbursement and cash milestone payments to be received | €	€ 97
Number of days from receipt of the final report, in which the collaboration agreement may be terminated	90 days
Percentage of upfront fee which must be refunded in the event of termination	75.00%
Common Stock | Public Offering
Liquidity
Issuance of common stock (in shares) | shares				5,000,000		4,600,000
Proceeds from equity offerings, net of offering costs				$ 64,900		$ 42,900