Derivative Instruments - Additional Information (Detail) (USD $)					12 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Dec. 31, 2009	Mar. 31, 2009	Dec. 31, 2009 Interest Rate Contract [Member]	Sep. 30, 2011 Marketing and Trading Derivative [Member] Natural Gas [Member] Bcf	Dec. 31, 2010 Marketing and Trading Derivative [Member] Natural Gas [Member] Bcf	Sep. 30, 2011 Securing most derivative counterparties [Member] Five Billion Dollar Facility [Member]	Sep. 30, 2011 Five Billion Dollar Facility [Member]	Sep. 30, 2011 Reference Period Start End Dates June 2014 to June 2024 and June 2014 to June 2044 [Member]	Sep. 30, 2011 Eligible for setoff against gross derivative asset in the event of default [Member]	Dec. 31, 2010 Eligible for setoff against gross derivative asset in the event of default [Member]
Derivative Instrument Detail [Abstract]
Accumulated other comprehensive loss, before tax	$ 113,000,000	$ 125,000,000
Accumulated other comprehensive loss, after tax	72,000,000	79,000,000
Derivative [Line Items]
Fixed-price physical transactions						28	32
Derivative transactions						15	28
Net positions						13	4
Realized gain on derivative instruments					552,000,000
Notional principal amount of interest-rate swap										1,850,000,000
Gross derivative liability	1,287,000,000	724,000,000									340,000,000	394,000,000
Line of credit, maximum borrowing capacity								5,000,000,000	5,000,000,000
Weighted-average interest rate for interest-rate swap			4.80%	3.25%
Aggregate fair value of all derivative instruments with credit-risk-related contingent features for which a net liability position existed, net of collateral	$ 10,000,000	$ 10,000,000