Derivative Instruments and Hedging Activities (Schedule of Outstanding Oil and Natural Gas Derivative Contracts) (Details) - Forecast - Not Designated as Hedging Instrument	3 Months Ended	12 Months Ended
	Dec. 31, 2018 MMBTU $ / MMBTU bbl	Dec. 31, 2020 MMBTU $ / MMBTU bbl	Dec. 31, 2019 MMBTU $ / MMBTU bbl
Oil | Put option
Derivative [Line Items]
Total volume (Bbls) | bbl	276,000	0	1,825,000
Strike price (in dollars per share)	65	0	65
Oil | Swap contracts
Derivative [Line Items]
Total volume (Bbls) | bbl	552,000	0	0
Strike price (in dollars per share)	52.07	0	0
Oil | Collar contracts (two-way collars)
Derivative [Line Items]
Total volume (Bbls) | bbl	92,000	0	1,095,000
Oil | Collar contracts (two-way collars) | Short
Derivative [Line Items]
Strike price (in dollars per share)	60.50	0	80
Oil | Collar contracts (two-way collars) | Long
Derivative [Line Items]
Strike price (in dollars per share)	50	0	65
Oil | Collar contracts combined with short puts (three-way collars)
Derivative [Line Items]
Total volume (Bbls) | bbl	874,000	0	3,469,000
Oil | Collar contracts combined with short puts (three-way collars) | Call option | Short
Derivative [Line Items]
Strike price (in dollars per share)	60.86	0	63.71
Oil | Collar contracts combined with short puts (three-way collars) | Put option | Short
Derivative [Line Items]
Strike price (in dollars per share)	39.21	0	43.95
Oil | Collar contracts combined with short puts (three-way collars) | Put option | Long
Derivative [Line Items]
Strike price (in dollars per share)	48.95	0	53.95
Oil | Oil contracts (Midland basis differential)
Derivative [Line Items]
Total volume (Bbls) | bbl	1,518,000	4,024,000	4,746,500
Weighted average swap price (in dollars per share)	(5.30)	(1.51)	(4.72)
Natural gas | Collar contracts (two-way collars)
Derivative [Line Items]
Total volume (MMBtu) | MMBTU	552,000	0	2,372,500
Natural gas | Collar contracts (two-way collars) | Short
Derivative [Line Items]
Strike price (in dollars per share)	3.19	0	2.95
Natural gas | Collar contracts (two-way collars) | Long
Derivative [Line Items]
Strike price (in dollars per share)	2.75	0	2.65
Natural gas | Swap contracts (Henry Hub) | Swap contracts
Derivative [Line Items]
Strike price (in dollars per share)	2.91	0	0
Total volume (MMBtu) | MMBTU	1,380,000	0	0
Natural gas | Waha basis differential | Swap contracts
Derivative [Line Items]
Total volume (MMBtu) | MMBTU	552,000	2,196,000	5,840,000
Weighted average swap price (in dollars per share)	(1.14)	(1.14)	(1.21)