Stockholders' and Mezzanine equity - Mezzanine Equity - General Information (Details) $ / shares in Units, $ in Millions			3 Months Ended
	Jan. 19, 2017 $ / shares	Aug. 25, 2016 USD ($) $ / shares shares	Mar. 31, 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%
Shares issued (in shares) | shares		1,840,000
Net proceeds from offering | $		$ 88.3
Liquidation preference (in dollars per share)	$ 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