Summary of Significant Accounting Policies - Schedule of Compensating Balance Arrangements (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Cash and Cash Equivalents [Line Items]
Total restricted cash (non-current)	$ 1,905	$ 1,902	$ 755	$ 364
Corporate Credit Card Collateral
Cash and Cash Equivalents [Line Items]
Total restricted cash (non-current)	150	150
Letter of Credit | Cambridge Lease Agreement
Cash and Cash Equivalents [Line Items]
Total restricted cash (non-current)	264	264
Letter of Credit | Waltham Lease
Cash and Cash Equivalents [Line Items]
Total restricted cash (non-current)	250	250
Letter of Credit | Vienna Austria Lease
Cash and Cash Equivalents [Line Items]
Total restricted cash (non-current)	97	94
Letter of Credit | Allston Lease Agreement
Cash and Cash Equivalents [Line Items]
Total restricted cash (non-current)	$ 1,144	$ 1,144