Derivatives and Risk Management (Balance Sheet Classification of Derivative Financial Instruments) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative assets	$ 335	$ 191
Derivative liabilities	166	139
Not Designated as Hedging Instrument | Storage - fixed price swap | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	1	0
Not Designated as Hedging Instrument | Interest rate swaps | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	0	1
Natural Gas | Fixed price swaps
Derivatives, Fair Value [Line Items]
Premium paid	9
Natural Gas | Not Designated as Hedging Instrument | Fixed price swaps
Derivatives, Fair Value [Line Items]
Premium paid	9
Natural Gas | Not Designated as Hedging Instrument | Fixed price swaps | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	68	32
Premium paid	9
Natural Gas | Not Designated as Hedging Instrument | Fixed price swaps | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	5	6
Natural Gas | Not Designated as Hedging Instrument | Fixed price swaps | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	0	9
Natural Gas | Not Designated as Hedging Instrument | Fixed price swaps | Other long-term liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	0	1
Natural Gas | Not Designated as Hedging Instrument | Two-way costless collars | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	2	11
Natural Gas | Not Designated as Hedging Instrument | Two-way costless collars | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	0	7
Natural Gas | Not Designated as Hedging Instrument | Three-way costless collars | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	84	41
Natural Gas | Not Designated as Hedging Instrument | Three-way costless collars | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	63	34
Natural Gas | Not Designated as Hedging Instrument | Three-way costless collars | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	53	33
Natural Gas | Not Designated as Hedging Instrument | Three-way costless collars | Other long-term liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	60	35
Natural Gas | Not Designated as Hedging Instrument | Basis swaps | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	17	8
Natural Gas | Not Designated as Hedging Instrument | Basis swaps | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	7	3
Natural Gas | Not Designated as Hedging Instrument | Basis swaps | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	24	18
Natural Gas | Not Designated as Hedging Instrument | Basis swaps | Other long-term liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	5	4
Natural Gas | Not Designated as Hedging Instrument | Sold call options | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	4	3
Natural Gas | Not Designated as Hedging Instrument | Sold call options | Other long-term liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	10	19
Natural Gas | Not Designated as Hedging Instrument | Purchased call options | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	1	0
Natural Gas | Not Designated as Hedging Instrument | Purchased call options | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	2	6
Oil | Not Designated as Hedging Instrument | Purchased fixed price swaps | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	1	6
Oil | Not Designated as Hedging Instrument | Fixed price swaps | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	15	13
Oil | Not Designated as Hedging Instrument | Fixed price swaps | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	4	6
Oil | Not Designated as Hedging Instrument | Two-way costless collars | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	6	6
Oil | Not Designated as Hedging Instrument | Two-way costless collars | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	1	5
Oil | Not Designated as Hedging Instrument | Two-way costless collars | Other long-term liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	0	1
Oil | Not Designated as Hedging Instrument | Three-way costless collars | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	5	0
Oil | Not Designated as Hedging Instrument | Three-way costless collars | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	8	0
Oil | Not Designated as Hedging Instrument | Three-way costless collars | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	3	0
Oil | Not Designated as Hedging Instrument | Three-way costless collars | Other long-term liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	6	0
Propane | Not Designated as Hedging Instrument | Fixed price swaps | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	25	11
Propane | Not Designated as Hedging Instrument | Fixed price swaps | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	5	0
Propane | Not Designated as Hedging Instrument | Two-way costless collars | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	3	0
Ethane | Not Designated as Hedging Instrument | Fixed price swaps | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	12	7
Ethane | Not Designated as Hedging Instrument | Fixed price swaps | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	1	1
Ethane | Not Designated as Hedging Instrument | Fixed price swaps | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	$ 0	$ 3