Derivative Instruments and Hedging Activities (Schedule of Outstanding Oil and Natural Gas Derivative Contracts) (Details) - Forecast - Not Designated as Hedging Instrument	6 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	552,000,000	0	1,825,000,000
Average swap price (in dollars per share)	65.00	0.00	65.00
Oil | Swap contracts
Derivative [Line Items]
Total volume (Bbls) | bbl	1,104,000,000	0	0
Average swap price (in dollars per share)	52.07	0.00	0.00
Oil | Collar contracts (two-way collars)
Derivative [Line Items]
Total volume (Bbls) | bbl	184,000,000	0	0
Oil | Collar contracts (two-way collars) | Short
Derivative [Line Items]
Strike price (in dollars per share)	60.50	0	0
Oil | Collar contracts (two-way collars) | Long
Derivative [Line Items]
Strike price (in dollars per share)	50	0	0
Oil | Collar contracts combined with short puts (three-way collars)
Derivative [Line Items]
Total volume (Bbls) | bbl	1,748,000,000	0	3,469,000,000
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	2,208,000,000	3,660,000,000	4,380,000,000
Oil | Oil contracts (Midland basis differential) | Short
Derivative [Line Items]
Average swap price (in dollars per share)	4.26	1.47	4.77
Natural gas | Swap contracts
Derivative [Line Items]
Average swap price (in dollars per share)	2.91	0	0
Total volume (MMBtu) | MMBTU	2,760,000,000	0	0
Natural gas | Collar contracts (two-way collars)
Derivative [Line Items]
Total volume (MMBtu) | MMBTU	1,104,000,000	0	2,372,500,000
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 | Waha basis differential | Swap contracts
Derivative [Line Items]
Total volume (MMBtu) | MMBTU	1,104,000,000	2,196,000,000	2,190,000,000
Natural gas | Waha basis differential | Swap contracts | Short
Derivative [Line Items]
Average swap price (in dollars per share)	1.14	1.14	1.14