Fair Value Measurements (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Fair Value - Other Financial Instruments
Debt fair value	$ 8,200,000,000	$ 6,400,000,000
Debt carrying value	7,600,000,000	5,900,000,000
Recurring | Level 1
Assets:
Commodity derivatives	684,000,000	310,000,000
Liabilities:
Commodity derivatives	627,000,000	311,000,000
Recurring | Level 2
Assets:
Commodity derivatives	745,000,000	640,000,000
Liabilities:
Commodity derivatives	893,000,000	652,000,000
Recurring | Netting and Collateral
Assets:
Commodity derivatives	(1,197,000,000)	(758,000,000)
Liabilities:
Commodity derivatives	(1,272,000,000)	(782,000,000)
Recurring | Total Fair Value
Assets:
Commodity derivatives	232,000,000	192,000,000
Liabilities:
Commodity derivatives	$ 248,000,000	$ 181,000,000