Summary of significant accounting policies - Other Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Other Assets
Other assets	$ 1,453	$ 1,453
Security deposit	321	321
Retainage deposit	1,132	1,132
Restricted cash included in other assets, noncurrent	$ 321	$ 321	$ 371	$ 371
Restricted cash, Balance Sheet Location	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent