Derivative Instruments - Additional Information (Detail) (USD $)													3 Months Ended	10 Months Ended
	Dec. 31, 2011	Oct. 31, 2011	Dec. 31, 2010	Jun. 30, 2009	Mar. 31, 2009	Dec. 31, 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]	Sep. 30, 2011 Reference Period Start End Dates June 2014 to June 2024 and June 2014 to June 2044 [Member]	Dec. 31, 2011 Five Billion Dollar Facility [Member]	Dec. 31, 2011 Securing most derivative counterparties [Member] Five Billion Dollar Facility [Member]	Dec. 31, 2011 Marketing and Trading Derivative [Member] Natural Gas [Member] Bcf	Dec. 31, 2010 Marketing and Trading Derivative [Member] Natural Gas [Member] Bcf	Jun. 30, 2009 Interest Rate Contract [Member]	Oct. 31, 2011 Interest Rate Contract [Member]
Derivative Instrument Detail [Abstract]
Accumulated other comprehensive loss, before tax	$ 109,000,000		$ 125,000,000
Accumulated other comprehensive loss, after tax	70,000,000		79,000,000
Derivative [Line Items]
Fixed-price physical transactions											22	32
Derivative transactions											21	28
Net positions											1	4
Realized (gains) losses on derivatives, net													(552,000,000)	57,000,000
Notional principal amount of interest-rate swap		150,000,000						1,850,000,000
Gross derivative liability	1,617,000,000		724,000,000			749,000,000	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	$ 2,000,000		$ 10,000,000