Restricted cash - Schedule of Collateralized Bank Account of Financial Institution (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Total restricted cash	$ 54,495	$ 14,520
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	0
Other leases
Debt and Equity Securities, FV-NI [Line Items]
Total restricted cash	$ 660	$ 757