Derivatives and Risk Management (Balance Sheet Classification of Derivative Financial Instruments) (Details) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Derivative assets	$ 141	$ 187
Derivative liabilities	147	110
Interest Rate Swaps [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	1
Interest Rate Swaps [Member] | Other Long-Term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	1
Interest Rate Swaps [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities		1
Natural Gas [Member] | Sold Fixed Price Swaps [Member]
Derivatives, Fair Value [Line Items]
Premium paid	22
Natural Gas [Member] | Sold Fixed Price Swaps [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	46	38
Natural Gas [Member] | Sold Fixed Price Swaps [Member] | Other Long-Term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	6	18
Natural Gas [Member] | Sold Fixed Price Swaps [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	6
Natural Gas [Member] | Sold Fixed Price Swaps [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities		1
Natural Gas [Member] | Two-Way Costless Collars [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	12	5
Natural Gas [Member] | Two-Way Costless Collars [Member] | Other Long-Term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	2
Natural Gas [Member] | Two-Way Costless Collars [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	5	1
Natural Gas [Member] | Two-Way Costless Collars [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	1
Natural Gas [Member] | Three-Way Costless Collars [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	40	82
Natural Gas [Member] | Three-Way Costless Collars [Member] | Other Long-Term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	24	39
Natural Gas [Member] | Three-Way Costless Collars [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	28	36
Natural Gas [Member] | Three-Way Costless Collars [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	17	30
Natural Gas [Member] | Sold Basis Swaps [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	5	2
Natural Gas [Member] | Sold Basis Swaps [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	15	23
Natural Gas [Member] | Purchased Call Options [Member]
Derivatives, Fair Value [Line Items]
Premium paid		1
Natural Gas [Member] | Purchased Call Options [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets		3
Natural Gas [Member] | Purchased Call Options [Member] | Other Long-Term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	4
Natural Gas [Member] | Sold Call Options [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	2	3
Natural Gas [Member] | Sold Call Options [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	14	$ 15
Propane [Member] | Sold Fixed Price Swaps [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	19
Propane [Member] | Sold Fixed Price Swaps [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	2
Ethane [Member] | Sold Fixed Price Swaps [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	31
Ethane [Member] | Sold Fixed Price Swaps [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	2
Oil [Member] | Sold Call Options [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	$ 5