Lease obligation (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)
Rent expense	$ 1,800	$ 1,400
Restricted cash included in deposits and other noncurrent assets	$ 1,779	735
Weighted average remaining lease term	6 years 8 months 12 days
Weighted average incremental borrowing rate	8.5
Total minimum lease payments
2020 (remainder of year)	$ 4,470
2021	6,138
2022	6,323
2023	6,512
2024	6,707
2025+	14,190
Total future minimum lease payments	44,340
Less: imputed interest	(10,946)
Total lease liability	33,394
Current lease liabilities	$ 3,306	3,193
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherCurrentLiabilitiesMember
Non-current lease liabilities	$ 30,088	$ 30,975
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherNoncurrentLiabilitiesMember
75 Sidney Street
Leasehold improvements	$ 1,300
Entitled leasehold improvements	100
64 Sidney Street
Leasehold improvements	$ 3,500
75 Hayden Avenue
Lease term	10 years
Entitled leasehold improvements	$ 6,100
Total minimum lease payments
Total lease liability	25,600
Deposits and other non-current assets
Restricted cash included in deposits and other noncurrent assets	$ 1,700