Financial and Other Derivative Instruments (Fair Value of Derivative Instruments) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Total fair value of derivatives	$ 1,178	$ 833
Total fair value of derivatives	(1,110)	(936)
Natural Gas Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Total fair value of derivatives	787	396
Total fair value of derivatives	(718)	(503)
Electricity Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Total fair value of derivatives	342	400
Total fair value of derivatives	(342)	(398)
Other Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Total fair value of derivatives	45	37
Total fair value of derivatives	(45)	(34)
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Total fair value of derivatives	1,178	833
Total fair value of derivatives	(1,110)	(936)
Not Designated as Hedging Instrument [Member] | Foreign Currency Exchange Contract [Member]
Derivatives, Fair Value [Line Items]
Total fair value of derivatives	4	0
Total fair value of derivatives	(5)	(1)
Not Designated as Hedging Instrument [Member] | Natural Gas Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Total fair value of derivatives	787	396
Total fair value of derivatives	(718)	(503)
Not Designated as Hedging Instrument [Member] | Electricity Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Total fair value of derivatives	342	400
Total fair value of derivatives	(342)	(398)
Not Designated as Hedging Instrument [Member] | Other Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Total fair value of derivatives	45	37
Total fair value of derivatives	(45)	(34)
Current Derivative Asset [Member]
Derivatives, Fair Value [Line Items]
Total fair value of derivatives	1,083	691
Noncurrent Derivative Asset [Member]
Derivatives, Fair Value [Line Items]
Total fair value of derivatives	95	142
Current Derivative Liability [Member]
Derivatives, Fair Value [Line Items]
Total fair value of derivatives	(1,041)	(773)
Noncurrent Derivative Liability [Member]
Derivatives, Fair Value [Line Items]
Total fair value of derivatives	$ (69)	$ (163)