Derivatives and Risk Management (Balance Sheet Classification of Derivative Financial Instruments) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Derivatives, Fair Value [Line Items]
Derivative assets	$ 296	$ 387
Derivative liabilities	506	428
Interest rate swaps | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	1	0
Natural Gas | Purchased fixed price swaps | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	1	1
Natural Gas | Fixed price swaps | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	25	37
Natural Gas | Fixed price swaps | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	0	7
Natural Gas | Fixed price swaps | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	12	7
Natural Gas | Fixed price swaps | Long-term derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	1	3
Natural Gas | Two-way costless collars | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	39	54
Natural Gas | Two-way costless collars | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	2	20
Natural Gas | Two-way costless collars | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	39	43
Natural Gas | Two-way costless collars | Long-term derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	4	21
Natural Gas | Three-way costless collars | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	39	57
Natural Gas | Three-way costless collars | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	82	87
Natural Gas | Three-way costless collars | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	73	82
Natural Gas | Three-way costless collars | Long-term derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	88	102
Natural Gas | Basis swaps | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	46	60
Natural Gas | Basis swaps | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	39	15
Natural Gas | Basis swaps | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	9	3
Natural Gas | Basis swaps | Long-term derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	8	7
Natural Gas | Call options | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	1	4
Natural Gas | Call options | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	13	12
Natural Gas | Call options | Long-term derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	21	28
Natural Gas | Put options | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	1	1
Natural Gas | Swaptions | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	1	2
Oil | Fixed price swaps | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	0	13
Oil | Fixed price swaps | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	0	2
Oil | Fixed price swaps | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	32	12
Oil | Fixed price swaps | Long-term derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	7	2
Oil | Two-way costless collars | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	2	1
Oil | Three-way costless collars | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	6	15
Oil | Three-way costless collars | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	14	15
Oil | Three-way costless collars | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	17	15
Oil | Three-way costless collars | Long-term derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	21	15
Oil | Call options | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	1	0
Propane | Fixed price swaps | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	1	0
Propane | Fixed price swaps | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	76	36
Propane | Fixed price swaps | Long-term derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	8	2
Propane | Three-way costless collars | Long-term derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	1	0
Ethane | Fixed price swaps | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	11	10
Ethane | Fixed price swaps | Long-term derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	1	0
Normal Butane | Fixed price swaps | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	21	8
Normal Butane | Fixed price swaps | Long-term derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	3	1
Natural Gasoline | Fixed price swaps | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	30	13
Natural Gasoline | Fixed price swaps | Long-term derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	$ 5	$ 2