Derivative Instruments - Additional Information (Detail) (USD $)	Sep. 30, 2012	Dec. 31, 2011
Derivative Instrument Detail [Abstract]
Accumulated other comprehensive loss, before tax balance related to interest-rate derivatives previously subject to hedge accounting	$ 100,000,000	$ 109,000,000
Accumulated other comprehensive loss, after tax balance related to interest-rate derivatives previously subject to hedge accounting	64,000,000	70,000,000
Derivative [Line Items]
Gross derivative liability	1,655,000,000	1,617,000,000
Aggregate fair value of all derivative instruments with credit-risk-related contingent features for which a net liability position existed, net of collateral	158,000,000	2,000,000
Permitted to offset gross derivative asset with financial institutions [Member]
Derivative [Line Items]
Gross derivative liability	422,000,000	749,000,000
Reference Period Start End Dates September 2016 to September 2026 and September 2016 to September 2046 [Member]
Derivative [Line Items]
Notional principal amount of interest-rate swap	$ 800,000,000
Marketing and Trading Derivative [Member] | Natural Gas [Member]
Derivative [Line Items]
Fixed-price physical transactions	12,000,000	22,000,000
Derivative transactions	11,000,000	21,000,000
Net position	1,000,000	1,000,000