Fresh start accounting (Enterprise Value to Fair Value of the Successor's Common Stock) (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Mar. 21, 2017	Dec. 31, 2016
Fresh Start Accounting [Abstract]
Enterprise value				$ 1,200,000
Plus: cash and cash equivalents	$ 22,595	$ 37,446		45,123
Less: fair value of outstanding debt				(296,061)
Less: fair value of warrants (consideration for previously accrued consulting fees)				(118)
Fair value of Successor common stock on the Effective Date				$ 948,944
Total shares issued under the Reorganization Plan	47,942,230	46,390,513	46,827,762	44,982,142	1,392,706
Per share value (in dollars per share)				$ 21.10