Basis of Presentation and Summary of Significant Accounting Policies - Additional information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Mcfe	Sep. 30, 2011	Dec. 31, 2011
Disclosure Basis Of Presentation And Summary Of Significant Accounting Policies Additional Information [Abstract]
Value of Properties Other than that Attributable to Proved Reserves	$ 1,684		$ 1,684
Unamortized Costs Capitalized Less Related Deferred Income Taxes Exceed Ceiling Limitations, Expense	3,315	0	3,315	0
Effects of Cash Flow Hedges Considered in Calculation Ceiling Limitation, Amount	$ 279		$ 279
Percent of estimated proved reserves volumes that are natural gas	70.00%		70.00%		83.00%
Natural gas, percent of sales volume of natural gas			80.00%		84.00%
Proved Developed And Undeveloped Reserves Revisions Of Previous Estimates Increase Decrease Energy			5,500,000,000
Increase (Decrease) In Price Used To Estimate Reserves			31.00%
Natural gas prices used in estimating proved reserves (usd per mcf)			2.83		4.12