Commitments and contingencies - License agreements and Lease commitments (Details)		12 Months Ended
	Oct. 02, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
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
2019		1,172,000
2020		2,013,000
2021		3,290,000
2022		3,372,000
2023 and Forward		29,207,000
Total		39,054,000
Rent expense		1,600,000	$ 1,100,000	$ 1,100,000
Other non-current liabilities
License agreements
Deferred rent expense		300,000
Other accrued expenses | Maximum
License agreements
Deferred rent expense		100,000
Company headquarters
License agreements
Term of operating lease	137 months
Square footage of office and laboratory space	92,000
License agreements | Tufts
License agreements
Royalty expense		700,000	500,000	300,000
Other licenses
License agreements
Royalty expense		400,000	$ 200,000	$ 200,000
Annual minimum royalty to be paid under the license agreement		$ 50,000