Derivative Instruments (Details 2) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Derivative Assets, Current	$ 1,049		$ 1,051
Derivative Assets, Noncurrent	158		113
Total derivative assets	1,207		1,164
Current derivative liabilities	1,243		1,144
Derivative Liabilities, Noncurrent	276		279
Total derivative liabilities	(1,519)		(1,423)
Net derivative assets (liabilities)	(312)		(259)
Fair Value of Derivative Assets	1,207		1,164
Fair Value of Derivative Liabilities	1,519		1,423
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Fair Value of Derivative Assets	34	[1]	61	[1]
Fair Value of Derivative Liabilities	180	[1]	167	[1]
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Fair Value of Derivative Assets	1,173		1,103
Fair Value of Derivative Liabilities	1,339		1,256
Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	0		0
Long-term derivative assets	6		10
Total derivative assets	6		10
Current derivative liabilities	31		166
Derivative Liabilities, Noncurrent	166		154
Total derivative liabilities	(197)		(320)
Net derivative assets (liabilities)	(191)		(310)
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Fair Value of Derivative Assets	6	[1]	10	[1]
Fair Value of Derivative Liabilities	176	[1]	149	[1]
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	21		171
Commodity Option [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	1,049		1,051
Long-term derivative assets	152		103
Total derivative assets	1,201		1,154
Current derivative liabilities	1,212		978
Derivative Liabilities, Noncurrent	110		125
Total derivative liabilities	(1,322)		(1,103)
Net derivative assets (liabilities)	(121)		51
Commodity Option [Member] | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Fair Value of Derivative Assets	28	[1]	51	[1]
Fair Value of Derivative Liabilities	4	[1]	18	[1]
Commodity Option [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Fair Value of Derivative Assets	1,173		1,103
Fair Value of Derivative Liabilities	$ 1,318		$ 1,085

[1] Includes accumulated fair value of derivative instruments as of the date hedge accounting was discontinued, net of amortized fair value for settlement periods which have transpired.