Derivatives and Risk Management (Balance Sheet Classification of Derivative Financial Instruments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	$ 187	$ 191
Derivative liabilities	110	139
Not Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets		1
Not Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	1
Natural Gas [Member] | Purchased Call Options [Member]
Derivatives, Fair Value [Line Items]
Premium paid	1
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	38	32
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Other Long-Term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	18	6
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities		9
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	1	1
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Two-Way Costless Collars [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	5	11
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Two-Way Costless Collars [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	1	7
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Three-Way Costless Collars [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	82	41
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Three-Way Costless Collars [Member] | Other Long-Term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	39	34
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Three-Way Costless Collars [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	36	33
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Three-Way Costless Collars [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	30	35
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	2	8
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Other Long-Term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets		3
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	23	18
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities		4
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Purchased Call Options [Member]
Derivatives, Fair Value [Line Items]
Premium paid	1
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Purchased Call Options [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	3
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Purchased Call Options [Member] | Other Long-Term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets		6
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Sold Call Options [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	3	3
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Sold Call Options [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	$ 15	19
Oil [Member] | Not Designated as Hedging Instrument [Member] | Purchased Fixed Price Swap [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities		6
Oil [Member] | Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets		13
Oil [Member] | Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Other Long-Term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets		6
Oil [Member] | Not Designated as Hedging Instrument [Member] | Two-Way Costless Collars [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets		6
Oil [Member] | Not Designated as Hedging Instrument [Member] | Two-Way Costless Collars [Member] | Other Long-Term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets		5
Oil [Member] | Not Designated as Hedging Instrument [Member] | Two-Way Costless Collars [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities		1
Propane [Member] | Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets		11
Ethane [Member] | Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets		7
Ethane [Member] | Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Other Long-Term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets		1
Ethane [Member] | Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities		$ 3