Stockholders' and Mezzanine equity - Series A Perpetual Convertible Preferred Stock (Details)	Jul. 13, 2017	Apr. 17, 2017	Jan. 19, 2017	Aug. 25, 2016
Series A preferred stock
Series A Perpetual Convertible Preferred Stock
Dividend rate (as a percent)	8.00%	8.00%	8.00%	8.00%