Prepaid Expenses and Other Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid insurance	$ 1,661	$ 1,021
Prepaid software licenses and maintenance	207	563
Foreign R&D credit refund	1,345	692
Prepaid research and development expenses	9,136	5,963
Interest receivable	288	478
Zentera receivable	2,940	0
Other prepaid expenses	888	441
Total prepaid expenses and other assets	16,465	9,158
Less long-term portion	6,042	2,976
Total prepaid expenses and other assets, current	$ 10,423	$ 6,182