Lease obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Lease expense	$ 6,200	$ 5,700	$ 4,000
Restricted cash included in deposits and other noncurrent assets	$ 1,779	$ 735	$ 735
Weighted average remaining lease term	6 years 9 months 18 days
Weighted average incremental borrowing rate	8.00%
Total minimum lease payments
2021	$ 7,822
2022	8,469
2023	8,723
2024	8,985
2025	9,644
Thereafter	19,945
Total future minimum lease payments	63,588
Less: imputed interest	(15,981)
Total lease liability	$ 47,607
Operating Lease, Liability, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent us-gaap:OtherLiabilitiesNoncurrent	us-gaap:OtherLiabilitiesCurrent us-gaap:OtherLiabilitiesNoncurrent
Current lease liabilities	$ 4,198	$ 3,193
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	Other Liabilities, Current	Other Liabilities, Current
Non-current lease liabilities	$ 43,409	$ 30,975
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Other Liabilities, Noncurrent	Other Liabilities, Noncurrent
75 and 64 Sidney Street Leases
Leasehold improvements	$ 5,300
75 Hayden Avenue
Leasehold improvements	$ 5,600