Subsequent events - Additional Information (Detail) - Subsequent Event [Member] - 50 Binney Street Sublease [Member] $ in Millions	Apr. 23, 2019 USD ($) ft² $ / ft²
Subsequent Event [Line Items]
Sublease rent description	Under the terms of the 50 Binney Street Sublease, the Company will lease 267,278 square feet of office space for $99.95 per square foot, or $26.7 million per year in base rent subject to certain operating expenses, taxes and annual rent increases. The Company anticipates the lease will commence on July 1, 2021 and end on December 31, 2030, unless the Company earlier occupies the premises or other conditions specified in the 50 Binney Street Sublease occur. The sublessor has the right to postpone the commencement date until January 1, 2022 by providing not less than nine months’ prior written notice to the Company.
Area of office space for sublease | ft²	267,278
Rate of area for sublease per square feet | $ / ft²	99.95
Sublease annual base rent	$ 26.7
Sublease rent payments	Payments will commence at the earlier of (i) the date which is 90 days following the commencement date and (ii) the date the Company takes occupancy of all or any portion of the premises
Cash-Collateralized Letter of Credit [Member]
Subsequent Event [Line Items]
Collateralized letter of credit	$ 40.1