Stockholders' and Mezzanine equity - Mezzanine Equity - General Information (Details) $ / shares in Units, $ in Thousands					9 Months Ended
	Jul. 13, 2017 $ / shares	Apr. 17, 2017 $ / shares	Jan. 19, 2017 $ / shares	Aug. 25, 2016 USD ($) $ / shares shares	Sep. 30, 2017 item $ / shares	Sep. 30, 2016 USD ($)	Dec. 31, 2016 $ / shares
Series A Perpetual Convertible Preferred Stock
Net proceeds from offering | $						$ 88,236
Series A preferred stock, par value (in dollars per share)					$ 0.001		$ 0.001
Series A preferred stock
Series A Perpetual Convertible Preferred Stock
Dividend rate (as a percent)	8.00%	8.00%	8.00%	8.00%
Shares issued (in shares) | shares				1,840,000
Net proceeds from offering | $				$ 88,300
Liquidation preference (in dollars per share)	$ 50.00	$ 50.00	$ 50.00	$ 50.00
Series A preferred stock, par value (in dollars per share)				$ 0.001
Conversion ratio				17.0683
Equivalent conversion price (in dollars per share)				$ 2.93
Shares issuable upon conversion | shares				31,400,000
Conversion threshold (as a percent)					175.00%
Minimum number of trading days | item					20
Consecutive trading days					30 days