Significant accounting policies - Cash and Cash equivalents and Restricted cash and deposits (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Cash and Cash equivalents
Cash	$ 1,821	$ 1,500
Money Market funds invested in U.S. Treasury obligations	42,608	78,182
Total cash and cash equivalents	44,429	79,682
Restricted cash and deposits
Restricted cash and deposits	1,400	$ 400
Restricted Cash	1,000
Other current and noncurrent assets
Restricted cash and deposits
Restricted cash and deposits	$ 400