U.S. Department of Defense (DoD) expense reimbursement contract (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2021	May 31, 2021	Dec. 31, 2020	Jul. 31, 2020
Unusual or Infrequent Item, or Both [Line Items]
Prepaid expenses and other current assets	$ 2,986,000		$ 2,986,000				$ 3,128,000
Other Transaction Authority for Prototype Agreement
Unusual or Infrequent Item, or Both [Line Items]
Grants Receivable						$ 17,600,000		$ 13,300,000
Contra-research and development expense	4,100,000	$ 0	8,100,000	$ 0
Prepaid expenses and other current assets	1,700,000		1,700,000
Deferred research obligation liability	$ 500,000		$ 500,000
Grant expected to be received					$ 17,600,000