| Accounts Payable and Accrued Expenses | 3 Months Ended | |||||||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Mar. 31, 2021 | ||||||||||||||||||||||||||||||||||||||||||||||
| Payables and Accruals [Abstract] | ||||||||||||||||||||||||||||||||||||||||||||||
| Accounts Payable and Accrued Expenses | 4. Accounts Payable and Accrued Expenses 
 Accounts payable and accrued expenses consisted of the following (in thousands) as of: 
 
 
 Accounts payable and accrued other expenses contain unpaid general and administrative expenses and costs related to research and development that have been billed and estimated unbilled, respectively, as of period-end. | |||||||||||||||||||||||||||||||||||||||||||||