Commitments and contingencies - License agreements and Lease commitments (Details)	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jul. 01, 2019 USD ($)	Oct. 02, 2018 USD ($) ft²
License agreements
Drawing capacity					$ 1,000,000
Letter of credit will be reduced at 41 months after the commencement date	$ 750,000				750,000
Letter of credit will be reduced at 65 months after the commencement date	250,000				$ 250,000
Minimum future rent payments under the lease agreement
2020	2,081,000
2021	3,322,000
2022	3,396,000
2023	3,480,000
2024 and Forward	25,760,000
Total	38,039,000
Rent expense	3,300,000	$ 1,600,000	$ 1,100,000
Other licenses
License agreements
Royalty expense	800,000	400,000	200,000
Annual minimum royalty to be paid under the license agreement	50,000
Tufts | License agreements
License agreements
Royalty expense	1,000,000	$ 700,000	$ 500,000
Lease for facilities in Billerica, Massachusetts | Aushon
Lease commitments
Consideration for the early termination				$ 75,000
Company headquarters
License agreements
Term of operating lease					137 months
Square footage of office and laboratory space | ft²					92,000
Other non-current liabilities [Member]
Lease commitments
Deferred rent expense	$ 3,000,000