Summary of significant accounting policies - Other Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Other Assets
Other assets	$ 1,804	$ 371
Restricted cash included in other assets	$ 371	$ 371	$ 371	$ 371
Restricted cash, Balance Sheet Location	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent
Held by service provider	$ 1,433