Stockholders' and Mezzanine equity - Mezzanine Equity - General Information (Details) $ / shares in Units, $ in Millions				6 Months Ended
	Apr. 17, 2017 $ / shares	Jan. 19, 2017 $ / shares	Aug. 25, 2016 USD ($) $ / shares shares	Jun. 30, 2017 item $ / shares	Dec. 31, 2016 $ / shares
Series A Perpetual Convertible Preferred Stock
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%
Shares issued (in shares) | shares			1,840,000
Net proceeds from offering | $			$ 88.3
Liquidation preference (in dollars per share)	$ 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