Quarterly report pursuant to Section 13 or 15(d)

Derivative Instruments and Hedging Activities (Schedule of Outstanding Oil and Natural Gas Derivative Contracts) (Details)

v3.8.0.1
Derivative Instruments and Hedging Activities (Schedule of Outstanding Oil and Natural Gas Derivative Contracts) (Details) - Not Designated as Hedging Instrument - Forecast
 3 Months Ended 12 Months Ended
Dec. 31, 2017
MMBTU
$ / MMBTU
MBbls
Dec. 31, 2018
MMBTU
$ / MMBTU
MBbls
Oil | Swap contracts    
Derivative [Line Items]    
Total volume (MBbls) | MBbls 184,000 1,460,000
Average swap price (in dollars per share) 45.74 50.93
Oil | Swap contracts combined with short puts (enhanced swaps) | Swap contracts    
Derivative [Line Items]    
Total volume (MBbls) | MBbls 184,000 0
Average swap price (in dollars per share) 44.50 0
Oil | Swap contracts combined with short puts (enhanced swaps) | Swap contracts | Put option | Short    
Derivative [Line Items]    
Strike price (in dollars per share) 30.00 0
Oil | Deferred premium put spread option    
Derivative [Line Items]    
Total volume (MBbls) | MBbls 253,000 0
Average swap price (in dollars per share) 2.45 0.00
Oil | Deferred premium put spread option | Put option | Short    
Derivative [Line Items]    
Strike price (in dollars per share) 40.00 0
Oil | Deferred premium put spread option | Put option | Long    
Derivative [Line Items]    
Strike price (in dollars per share) 50.00 0
Oil | Collar contracts (two-way collars)    
Derivative [Line Items]    
Total volume (MBbls) | MBbls 340,000 0
Oil | Collar contracts (two-way collars) | Put option | Long    
Derivative [Line Items]    
Strike price (in dollars per share) 47.50 0
Oil | Collar contracts (two-way collars) | Call option | Short    
Derivative [Line Items]    
Strike price (in dollars per share) 58.19 0
Oil | Call option contracts | Call option    
Derivative [Line Items]    
Total volume (MBbls) | MBbls 169,000 0
Strike price (in dollars per share) 1.82 0
Oil | Call option contracts | Call option | Short    
Derivative [Line Items]    
Strike price (in dollars per share) 50.00 0
Oil | Call option contracts | Call option | Long    
Derivative [Line Items]    
Strike price (in dollars per share) 50.00 0
Oil | Collar contracts combined with short puts (three-way collars)    
Derivative [Line Items]    
Total volume (MBbls) | MBbls 0 3,468,000
Oil | Collar contracts combined with short puts (three-way collars) | Put option | Short    
Derivative [Line Items]    
Strike price (in dollars per share) 0 39.21
Oil | Collar contracts combined with short puts (three-way collars) | Put option | Long    
Derivative [Line Items]    
Strike price (in dollars per share) 0 48.95
Oil | Collar contracts combined with short puts (three-way collars) | Call option | Short    
Derivative [Line Items]    
Strike price (in dollars per share) 0 60.86
Natural gas | Swap contracts    
Derivative [Line Items]    
Strike price (in dollars per share) 3.39 0
Total volume (BBtu) | MMBTU 124,000 0
Natural gas | Collar contracts combined with short puts (Henry Hub, three-way collars)    
Derivative [Line Items]    
Total volume (BBtu) | MMBTU 368,000 0
Natural gas | Collar contracts combined with short puts (Henry Hub, three-way collars) | Put option | Short    
Derivative [Line Items]    
Strike price (in dollars per share) 2.50 0
Natural gas | Collar contracts combined with short puts (Henry Hub, three-way collars) | Put option | Long    
Derivative [Line Items]    
Strike price (in dollars per share) 3.00 0
Natural gas | Collar contracts combined with short puts (Henry Hub, three-way collars) | Call option | Short    
Derivative [Line Items]    
Strike price (in dollars per share) 3.71 0
Natural gas | Collar contracts (Henry Hub, two-way collars)    
Derivative [Line Items]    
Total volume (BBtu) | MMBTU 856,000 720,000
Natural gas | Collar contracts (Henry Hub, two-way collars) | Put option | Long    
Derivative [Line Items]    
Strike price (in dollars per share) 3.23 3,400.00
Natural gas | Collar contracts (Henry Hub, two-way collars) | Call option | Short    
Derivative [Line Items]    
Strike price (in dollars per share) 3.77 3,840.00