Derivatives and Risk Management (Schedule of Derivative Assets and Liabilities Held for Sale) (Details) - Not Designated as Hedging Instrument [Member] $ in Millions	Sep. 30, 2018 USD ($)
Derivatives, Fair Value [Line Items]
Derivative assets held for sale	$ 34
Derivative liabilities held for sale	49
Natural Gas [Member] | Sold Fixed Price Swaps [Member] | Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets held for sale	23
Natural Gas [Member] | Sold Fixed Price Swaps [Member] | Other Long-Term Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets held for sale	11
Natural Gas [Member] | Sold Fixed Price Swaps [Member] | Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities held for sale	10
Natural Gas [Member] | Sold Fixed Price Swaps [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities held for sale	$ 39