Supplemental Disclosures About Natural Gas, Oil and NGL Producing Activities (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 Mcfe	Dec. 31, 2011 Mcfe	Dec. 31, 2010 Mcfe
Reserve Quantities [Line Items]
Capitalized Costs, Unproved Properties	$ 976	$ 727	$ 711
Term Which Unproved Properties Will Be Transerred Into Amortization Base	5 years
Proved Undeveloped Reserves Drilling and Completion Costs	1,035
Oil And Gas Benefit From Drilling Carries	784	2,570	1,151
Reserves, Energy, Duration	42,000,000	30,000,000	89,000,000
Proved Reserves Volume Percentage	89.00%	77.00%	78.00%
Proved Undeveloped Reserves Term	5 years
Proved Undeveloped Reserves Converted to Proved Developed Reserves	961,000,000,000
Costs Incurred, Acquisition of Oil and Gas Properties with Proved Reserves	332	48	243
Proceeds From Sale Of Proved Reserves	2,381	2,612	2,876
Oil Price Before Price Differentials	94.84	95.97	79.42
Natural Gas Price After Price Differentials	1.75	3.19	3.52
Oil Price After Price Differentials	91.78	88.50	75.17
NGL Price After Price Differentials	30.81	40.38	32.06
Annual Discount Factor Oil And Gas Properties	0.00%
Revisions From Lower Natural Gas Prices [Member]
Reserve Quantities [Line Items]
Reserves, Energy, Duration	1,349,000,000,000
Revisions From Positive Performance [Member]
Reserve Quantities [Line Items]
Reserves, Energy, Duration		273,000,000	183,000,000
Revisions From Negative Performance [Member]
Reserve Quantities [Line Items]
Reserves, Energy, Duration	5,414,000,000,000	337,000,000	6,000,000
Revisions Because of Higher Oil Prices [Member]
Reserve Quantities [Line Items]
Reserves, Energy, Duration		14,000,000
Revisions From Higher Natural Gas Prices [Member]
Reserve Quantities [Line Items]
Reserves, Energy, Duration			189,000,000
Reserves Revisions of Previous Estimates
Reserve Quantities [Line Items]
Reserves, Energy, Duration	6,763,000,000,000	50,000,000
Reserves Sale of Reserves-In-Place
Reserve Quantities [Line Items]
Reserves, Energy, Duration	1,347,000,000,000	2,776,000,000,000	1,493,000,000,000
Undeveloped Reserves [Member]
Reserve Quantities [Line Items]
Oil And Gas Benefit From Drilling Carries	$ 86