Restricted cash - Schedule of Collateralized Bank Account of Financial Institution (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Total restricted cash	$ 56,023	$ 54,495
60 Binney Street lease
Debt and Equity Securities, FV-NI [Line Items]
Total restricted cash	13,763	13,763
50 Binney Street sublease
Debt and Equity Securities, FV-NI [Line Items]
Total restricted cash	40,072	40,072
Other
Debt and Equity Securities, FV-NI [Line Items]
Total restricted cash	$ 2,188	$ 660