Derivative Instruments - Additional Information (Detail) (USD $)						10 Months Ended		9 Months Ended		9 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Five Billion Dollar Facility [Member]	Dec. 31, 2012 Permitted to offset gross derivative asset with financial institutions [Member]	Dec. 31, 2011 Permitted to offset gross derivative asset with financial institutions [Member]	Oct. 31, 2012 Interest-Rate Swap Agreement [Member]	Oct. 31, 2011 Interest-Rate Swap Agreement [Member]	Sep. 30, 2012 Interest Rate Swap 3 and 4 [Member]	Dec. 31, 2012 Interest Rate Swap 3 and 4 [Member]	Sep. 30, 2011 Interest Rate Swap 1 and 2 [Member]	Dec. 31, 2011 Interest Rate Swap 1 and 2 [Member]	Dec. 31, 2012 Marketing and Trading Derivative [Member] Natural Gas [Member] Mcf	Dec. 31, 2011 Marketing and Trading Derivative [Member] Natural Gas [Member] Mcf	Dec. 31, 2012 Securing most derivative counterparties [Member] Five Billion Dollar Facility [Member]
Derivative Instrument Detail [Abstract]
Accumulated other comprehensive loss, before tax balance related to interest-rate derivatives that were previously subject to hedge accounting	$ 96,000,000	$ 109,000,000
Accumulated other comprehensive loss, after tax balance related to interest-rate derivatives that were previously subject to hedge accounting	61,000,000	70,000,000
Derivative [Line Items]
Fixed-price physical transactions												10,000,000	22,000,000
Derivative transactions												10,000,000	21,000,000
Swap maturity date								Oct. 01, 2012		Oct. 01, 2011
Aggregate notional principal amount of interest-rate swap						200,000,000	150,000,000		800,000,000		1,850,000,000
Realized (gains) losses on derivatives, net						64,000,000	57,000,000
Gross derivative liability	1,419,000,000	1,617,000,000		339,000,000	749,000,000
Five billion dollar facility			5,000,000,000											5,000,000,000
Aggregate fair value of derivative instruments with credit-risk-related contingent features for which a net liability position existed (net of collateral)	$ 94,000,000	$ 2,000,000