Derivatives and Risk Management (Balance Sheet Classification of Derivative Financial Instruments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Mar. 31, 2018
Purchased Call Options - 2017 [Member]
Derivatives, Fair Value [Line Items]
Premium paid	$ 1
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	186	$ 189
Derivative liabilities	110	111
Not Designated as Hedging Instrument [Member] | Two-Way Costless Collars [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	5	4
Not Designated as Hedging Instrument [Member] | Two-Way Costless Collars [Member] | Other Long-Term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets		12
Not Designated as Hedging Instrument [Member] | Two-Way Costless Collars [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	1	3
Not Designated as Hedging Instrument [Member] | Two-Way Costless Collars [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities		6
Not Designated as Hedging Instrument [Member] | Three-Way Costless Collars [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	82	63
Not Designated as Hedging Instrument [Member] | Three-Way Costless Collars [Member] | Other Long-Term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	39	28
Not Designated as Hedging Instrument [Member] | Three-Way Costless Collars [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	36	25
Not Designated as Hedging Instrument [Member] | Three-Way Costless Collars [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	30	16
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	2	6
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	23	7
Not Designated as Hedging Instrument [Member] | Purchased Call Options [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	2
Not Designated as Hedging Instrument [Member] | Purchased Call Options [Member] | Other Long-Term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets		18
Not Designated as Hedging Instrument [Member] | Sold Call Options [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	3	3
Not Designated as Hedging Instrument [Member] | Sold Call Options [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	15	49
Not Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member] | Other Long-Term 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
Not Designated as Hedging Instrument [Member] | Purchased Call Options - 2017 [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	34
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	20
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities		2
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
Propane [Member] | Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets		$ 3