Summary of Significant Accounting Policies - Schedule of Compensating Balance Arrangements (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Cash and Cash Equivalents [Line Items]
Total restricted cash	$ 1,327	$ 1,730	$ 1,905	$ 1,902
Restricted Cash	1,327	1,994
Restricted cash, current portion	0	264	$ 0	$ 0
Letter of Credit | Cambridge Lease Agreement
Cash and Cash Equivalents [Line Items]
Restricted Cash	0	264
Letter of Credit | Waltham Lease
Cash and Cash Equivalents [Line Items]
Restricted Cash	250	250
Letter of Credit | Vienna Austria Lease
Cash and Cash Equivalents [Line Items]
Restricted Cash	222	336
Letter of Credit | Allston Lease Agreement
Cash and Cash Equivalents [Line Items]
Restricted Cash	$ 855	$ 1,144