Financial and Other Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivatives designated as hedging instruments:
Derivative Asset Fair Value Gross Asset	$ 1,016	$ 2,805
Derivative Liability Fair Value Gross Liability	(1,020)	(2,811)
Derivative Assets, Current	108	222
Derivative Assets, Noncurrent	39	74
Derivative Liabilities, Current	125	158
Derivative Liabilities, Noncurrent	26	89
Estimate of Fair Value, Fair Value Disclosure [Member]
Derivatives designated as hedging instruments:
Derivative Assets, Current	862	2,272
Derivative Assets, Noncurrent	154	533
Derivative Liabilities, Current	(879)	(2,282)
Derivative Liabilities, Noncurrent	(141)	(529)
Derivative Asset [Member]
Derivatives designated as hedging instruments:
Derivative Assets, Current	862	2,272
Derivative Assets, Noncurrent	154	533
Derivative Liabilities [Member]
Derivatives designated as hedging instruments:
Derivative Liabilities, Current	(879)	(2,282)
Derivative Liabilities, Noncurrent	(141)	(529)
Counterparty Netting [Member]
Derivatives designated as hedging instruments:
Derivative Assets, Current	(754)	(2,050)
Derivative Assets, Noncurrent	(115)	(440)
Derivative Liabilities, Current	754	2,050
Derivative Liabilities, Noncurrent	115	440
Collateral Adjustment [Member]
Derivatives designated as hedging instruments:
Derivative Assets, Current	0	0
Derivative Assets, Noncurrent	0	(19)
Derivative Liabilities, Current	0	74
Derivative Liabilities, Noncurrent	0	0
Derivative [Member]
Derivatives designated as hedging instruments:
Derivative Assets, Current	108	222
Derivative Assets, Noncurrent	39	74
Derivative Liabilities, Current	(125)	(158)
Derivative Liabilities, Noncurrent	(26)	(89)
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	1,016	2,805
Derivative Liability Fair Value Gross Liability	(1,019)	(2,810)
Not Designated as Hedging Instrument [Member] | Foreign currency exchange contracts [Member]
Derivatives designated as hedging instruments:
Derivative Asset Fair Value Gross Asset	0	3
Derivative Liability Fair Value Gross Liability	0	(5)
Not Designated as Hedging Instrument [Member] | Natural Gas [Member]
Derivatives designated as hedging instruments:
Derivative Asset Fair Value Gross Asset	645	2,024
Derivative Liability Fair Value Gross Liability	(661)	(2,080)
Not Designated as Hedging Instrument [Member] | Electricity [Member]
Derivatives designated as hedging instruments:
Derivative Asset Fair Value Gross Asset	360	747
Derivative Liability Fair Value Gross Liability	(351)	(705)
Not Designated as Hedging Instrument [Member] | Other [Member]
Derivatives designated as hedging instruments:
Derivative Asset Fair Value Gross Asset	11	31
Derivative Liability Fair Value Gross Liability	$ (7)	$ (20)