Derivative Instruments (Details 2) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Derivative assets, current	$ 514	$ 339
Derivative Assets, Noncurrent	(149)	(98)
Total derivative assets	663	437
Current derivative liabilities	595	357
Derivative Liabilities, Noncurrent	289	293
Total derivative liabilities	884	650
Net derivative assets (liabilities)	(221)	(213)
Fair Value of Derivative Assets	663	437
Fair Value of Derivative Liabilities	884	650
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Fair Value of Derivative Assets	5	4
Fair Value of Derivative Liabilities	174	184
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Fair Value of Derivative Assets	658	433
Fair Value of Derivative Liabilities	710	466
Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	0	0
Long-term derivative assets	5	4
Total derivative assets	5	4
Current derivative liabilities	44	40
Derivative Liabilities, Noncurrent	144	160
Total derivative liabilities	188	200
Net derivative assets (liabilities)	(183)	(196)
Interest Rate Swap [Member] \| Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Fair Value of Derivative Assets	5	4
Fair Value of Derivative Liabilities	174	184
Interest Rate Swap [Member] \| Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Fair Value of Derivative Assets	0	0
Fair Value of Derivative Liabilities	14	16
Commodity Option [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	514	339
Long-term derivative assets	144	94
Total derivative assets	658	433
Current derivative liabilities	551	317
Derivative Liabilities, Noncurrent	145	133
Total derivative liabilities	696	450
Net derivative assets (liabilities)	(38)	(17)
Commodity Option [Member] \| Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Fair Value of Derivative Assets	658	433
Fair Value of Derivative Liabilities	$ 696	$ 450

Fair value of the gross assets less the gross liabilities of a derivative asset or group of derivative assets.

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