Fresh start accounting (Additional information) (Details) $ in Thousands	Mar. 21, 2017 USD ($)
Fresh-Start Adjustment [Line Items]
Enterprise value	$ 1,200,000
Fair value of proved reserves	604,065
Fair value of undeveloped leasehold acreage	$ 585,574
Measurement Input, Discount Rate
Fresh-Start Adjustment [Line Items]
Oil and gas, measurement input	0.085
High
Fresh-Start Adjustment [Line Items]
Fresh start voting share percentage of existing shareholders in emerging entity	50.00%
Plan of Reorganization | Midpoint
Fresh-Start Adjustment [Line Items]
Reorganization plan, enterprise value of the post emergence	$ 1,200,000
Plan of Reorganization | Low
Fresh-Start Adjustment [Line Items]
Reorganization plan, enterprise value of the post emergence	1,050,000
Plan of Reorganization | High
Fresh-Start Adjustment [Line Items]
Reorganization plan, enterprise value of the post emergence	$ 1,350,000