Financial and Other Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivatives designated as hedging instruments:
Derivative Asset Fair Value Gross Asset	$ 870	$ 1,016
Derivative Liability Fair Value Gross Liability	(944)	(1,020)
Derivative Assets, Current	56	108
Derivative Assets, Noncurrent	17	39
Derivative Liabilities, Current	81	125
Derivative Liabilities, Noncurrent	14	26
Estimate of Fair Value, Fair Value Disclosure [Member]
Derivatives designated as hedging instruments:
Derivative Assets, Current	775	862
Derivative Assets, Noncurrent	95	154
Derivative Liabilities, Current	(856)	(879)
Derivative Liabilities, Noncurrent	(88)	(141)
Derivative [Member]
Derivatives designated as hedging instruments:
Derivative Assets, Current	56	108
Derivative Assets, Noncurrent	17	39
Derivative Liabilities, Current	(81)	(125)
Derivative Liabilities, Noncurrent	(14)	(26)
Designated as Hedging Instrument [Member] | Interest rate contracts [Member]
Derivatives designated as hedging instruments:
Derivative Asset Fair Value Gross Asset	0	0
Derivative Liability Fair Value Gross Liability	(1)	(1)
Not Designated as Hedging Instrument [Member]
Derivatives designated as hedging instruments:
Derivative Asset Fair Value Gross Asset	870	1,016
Derivative Liability Fair Value Gross Liability	(943)	(1,019)
Not Designated as Hedging Instrument [Member] | Natural Gas [Member]
Derivatives designated as hedging instruments:
Derivative Asset Fair Value Gross Asset	539	645
Derivative Liability Fair Value Gross Liability	(617)	(661)
Not Designated as Hedging Instrument [Member] | Electricity [Member]
Derivatives designated as hedging instruments:
Derivative Asset Fair Value Gross Asset	308	360
Derivative Liability Fair Value Gross Liability	(307)	(351)
Not Designated as Hedging Instrument [Member] | Other [Member]
Derivatives designated as hedging instruments:
Derivative Asset Fair Value Gross Asset	23	11
Derivative Liability Fair Value Gross Liability	$ (19)	$ (7)