Commitments and contingencies - Operating Leases - (Details) $ in Thousands	1 Months Ended
	Mar. 31, 2014 item	Mar. 31, 2018 USD ($)	Feb. 28, 2018 USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018 (remainder of year)		$ 3,160
2019		4,325
2020		4,731
2021		4,863
2022		5,001
2023 +		10,427
Total minimum lease payments		32,507
75 Sidney Street
Number of renewal options | item	2
Leasehold improvement receivable			$ 300
Term of each renewal option	5 years
Leasehold incentive received from landlord		1,300
64 Sidney Street
Leasehold incentive received from landlord		$ 3,500