Lease obligation (Details) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Apr. 30, 2014 item	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Lease, Practical Expedients, Package [true false]				true
Lessee, Operating Lease, Existence of Option to Extend [true false]				true
Rent expense		$ 1,100	$ 1,000	$ 4,100	$ 2,800
Restricted cash included in deposits and other noncurrent assets		$ 735		$ 735		$ 735
Weighted average remaining lease term		7 years 2 months 12 days		7 years 2 months 12 days
Weighted average incremental borrowing rate		8.5		8.5
Total minimum lease payments
2019 (remainder of year)		$ 1,377		$ 1,377
2020		5,960		5,960
2021		6,138		6,138
2022		6,323		6,323
2023		6,512		6,512
2024+		20,897		20,897
Total		47,207		47,207
Less: imputed interest		(12,393)		(12,393)
Total lease liability		34,814		34,814
Current lease liabilities		$ 3,016		$ 3,016
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]		Other Liabilities, Current		Other Liabilities, Current
Non-current lease liabilities		$ 31,798		$ 31,798
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]		Other non-current liabilities		Other non-current liabilities
75 Sidney Street
Number of renewal options | item	2
Term of each renewal option	5 years
Leasehold improvements		$ 1,300		$ 1,300
Entitled leasehold improvements		300		300
64 Sidney Street
Leasehold improvements		$ 3,500		$ 3,500