Derivatives and Risk Management (Balance Sheet Classification of Derivative Financial Instruments) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative assets	$ 261	$ 191
Derivative liabilities	112	139
Not Designated as Hedging Instrument | Storage - fixed price swap | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	1	0
Not Designated as Hedging Instrument | Interest rate swaps | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	0	1
Not Designated as Hedging Instrument | Interest rate swaps | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	1	0
Natural Gas | Purchased call options
Derivatives, Fair Value [Line Items]
Premium paid	1
Natural Gas | Not Designated as Hedging Instrument | Fixed price swaps | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	79	32
Natural Gas | Not Designated as Hedging Instrument | Fixed price swaps | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	4	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	13	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	65	41
Natural Gas | Not Designated as Hedging Instrument | Three-way costless collars | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	31	34
Natural Gas | Not Designated as Hedging Instrument | Three-way costless collars | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	37	33
Natural Gas | Not Designated as Hedging Instrument | Three-way costless collars | Other long-term liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	28	35
Natural Gas | Not Designated as Hedging Instrument | Basis swaps | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	5	8
Natural Gas | Not Designated as Hedging Instrument | Basis swaps | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	2	3
Natural Gas | Not Designated as Hedging Instrument | Basis swaps | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	19	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	5	3
Natural Gas | Not Designated as Hedging Instrument | Sold call options | Other long-term liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	11	19
Natural Gas | Not Designated as Hedging Instrument | Purchased call options | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	1	0
Premium paid	1
Natural Gas | Not Designated as Hedging Instrument | Purchased call options | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	4	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	7	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	2	5
Oil | Not Designated as Hedging Instrument | Two-way costless collars | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	1	0
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	2	0
Oil | Not Designated as Hedging Instrument | Three-way costless collars | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	2	0
Oil | Not Designated as Hedging Instrument | Three-way costless collars | Derivative liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	2	0
Oil | Not Designated as Hedging Instrument | Three-way costless collars | Other long-term liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	1	0
Propane | Not Designated as Hedging Instrument | Fixed price swaps | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	18	11
Propane | Not Designated as Hedging Instrument | Fixed price swaps | Other long-term assets
Derivatives, Fair Value [Line Items]
Derivative assets	2	0
Propane | Not Designated as Hedging Instrument | Two-way costless collars | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	2	0
Ethane | Not Designated as Hedging Instrument | Fixed price swaps | Derivative assets
Derivatives, Fair Value [Line Items]
Derivative assets	11	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