Commitments and contingencies - License agreements and Lease commitments (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
License agreements
Cost of revenue	$ 11,081,000	$ 10,283,000	$ 7,917,000	$ 8,914,000	$ 9,052,000	$ 7,911,000	$ 6,605,000	$ 6,330,000	$ 38,195,000	$ 29,898,000	$ 19,684,000
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
Rent expense										3,300,000	1,600,000
Other licenses
License agreements
Royalty expense									0	800,000	400,000
Annual minimum royalty to be paid under the license agreement	$ 50,000								50,000
Tufts | License agreements
License agreements
Royalty expense									1,100,000	$ 1,000,000.0	$ 700,000
Collaboration and license revenue
License agreements
Cost of revenue		$ 1,000,000							1,000,000
Collaboration and license revenue | Tufts
License agreements
Cost of revenue									$ 1,000,000.0