Commitments and contingencies - Operating Leases - (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Mar. 31, 2014 item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Number of renewal options | item	2
Term of each renewal option	5 years
Rent expense		$ 900	$ 2,000	$ 700
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2017		3,211
2018		3,290
2019		3,382
2020		3,762
2021		3,868
2022 +		12,273
Total minimum lease payments		29,786
75 Sidney Street
Leasehold incentive received from landlord		1,300
64 Sidney Street
Leasehold incentive received from landlord		$ 3,500