Derivative Instruments and Hedging Activities (Fair Values) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Fair Values by Balance Sheet Location [Abstract]
Asset value	$ 802	$ 1,318
Liability value	684	1,217
Not Designated as Hedging Instrument
Fair Values by Balance Sheet Location [Abstract]
Asset value	802	1,318
Liability value	684	1,217
Not Designated as Hedging Instrument | Current Assets | Energy Commodities, net
Fair Values by Balance Sheet Location [Abstract]
Asset value	576	1,079
Not Designated as Hedging Instrument | Noncurrent Assets | Energy Commodities, net
Fair Values by Balance Sheet Location [Abstract]
Asset value	226	239
Not Designated as Hedging Instrument | Current Liabilities | Energy Commodities, net
Fair Values by Balance Sheet Location [Abstract]
Liability value	506	1,024
Not Designated as Hedging Instrument | Noncurrent Liabilities | Energy Commodities, net
Fair Values by Balance Sheet Location [Abstract]
Liability value	$ 178	$ 193