Fair Value Measurements (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Level 1
Fair Value - Other Financial Instruments
Debt estimated fair value	$ 7,700,000,000	$ 8,200,000,000
Debt carrying value	7,600,000,000	7,600,000,000
Recurring | Level 1
Assets:
Commodity derivatives	551,000,000	107,000,000
Liabilities:
Commodity derivatives	524,000,000	99,000,000
Recurring | Level 2
Assets:
Commodity derivatives	314,000,000	312,000,000
Liabilities:
Commodity derivatives	344,000,000	398,000,000
Recurring | Netting and Collateral
Assets:
Commodity derivatives	(750,000,000)	(301,000,000)
Liabilities:
Commodity derivatives	(784,000,000)	(371,000,000)
Recurring | Total Fair Value
Assets:
Commodity derivatives	115,000,000	118,000,000
Liabilities:
Commodity derivatives	$ 84,000,000	$ 126,000,000