Commitments and contingencies - License agreements and Lease commitments (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Oct. 02, 2018
License agreements
Drawing capacity					$ 1,000,000
Letter of credit will be reduced at 41 months after the commencement date					750,000
Letter of credit will be reduced at 65 months after the commencement date					$ 250,000
Minimum future rent payments under the lease agreement
2019	$ 473,000		$ 473,000
2020	2,312,000		2,312,000
2021	3,575,000		3,575,000
2022	3,372,000		3,372,000
2023 and Forward	29,208,000		29,208,000
Total	38,940,000		38,940,000
Rent expense	700,000	$ 400,000	2,600,000	$ 1,000,000
Tufts
License agreements
Royalty expense	300,000	200,000	700,000	500,000
Other licenses | Maximum
License agreements
Royalty expense	100,000	$ 100,000	100,000	$ 100,000
Company headquarters
License agreements
Term of operating lease					137 months
Other non-current liabilities
Lease commitments
Deferred rent expense	$ 2,600,000		$ 2,600,000