Derivative Instruments (Details 2) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Derivative assets, current	$ 601	$ 339
Derivative Assets, Noncurrent	136	98
Total derivative assets	737	437
Current derivative liabilities	630	357
Derivative Liabilities, Noncurrent	291	293
Total derivative liabilities	921	650
Net derivative assets (liabilities)	(184)	(213)
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Total derivative assets	8	4
Total derivative liabilities	134	184
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Total derivative assets	729	433
Total derivative liabilities	787	466
Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative assets, current	0	0
Derivative Assets, Noncurrent	8	4
Total derivative assets	8	4
Current derivative liabilities	47	40
Derivative Liabilities, Noncurrent	98	160
Total derivative liabilities	145	200
Net derivative assets (liabilities)	(137)	(196)
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Total derivative assets	8	4
Total derivative liabilities	134	184
Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Total derivative assets	0	0
Total derivative liabilities	11	16
Commodity Option [Member]
Derivatives, Fair Value [Line Items]
Derivative assets, current	601	339
Derivative Assets, Noncurrent	128	94
Total derivative assets	729	433
Current derivative liabilities	583	317
Derivative Liabilities, Noncurrent	193	133
Total derivative liabilities	776	450
Net derivative assets (liabilities)	(47)	(17)
Commodity Option [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Total derivative assets	729	433
Total derivative liabilities	$ 776	$ 450