Quarterly report pursuant to Section 13 or 15(d)

Derivative Instruments and Hedging Activities (Oil contracts (Midland basis differential)) (Details)

v3.8.0.1
Derivative Instruments and Hedging Activities (Oil contracts (Midland basis differential)) (Details) - Not Designated as Hedging Instrument - Oil - Swap contracts - Forecast
 3 Months Ended 12 Months Ended
Dec. 31, 2017
$ / MMBTU
MBbls
Dec. 31, 2018
$ / MMBTU
MBbls
Derivative [Line Items]    
Total volume (MBbls) | MBbls 184,000 1,460,000
Average swap price (in dollars per share) | $ / MMBTU 45.74 50.93
Oil contracts (Midland basis differential)    
Derivative [Line Items]    
Total volume (MBbls) | MBbls 552,000 4,563,000
Average swap price (in dollars per share) | $ / MMBTU 0.52 0.98
Swap Contracts (Midland Basis Differential), Subsequent Contracts    
Derivative [Line Items]    
Total volume (MBbls) | MBbls 0 546,000
Average swap price (in dollars per share) | $ / MMBTU 0.00 0.23
Swap Contracts (WTI), Subsequent Contracts    
Derivative [Line Items]    
Total volume (MBbls) | MBbls 0 365,000
Average swap price (in dollars per share) | $ / MMBTU 0.00 53.40