Derivative Instruments - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Derivative [Line Items]
Cash received (paid) in settlement of derivative instruments	$ 85,000,000	$ 685,000,000	$ 147,000,000
Revolving Credit Facility [Member]
Derivative [Line Items]
Five billion dollar facility	5,000,000,000
Securing most derivative counterparties [Member] | Revolving Credit Facility [Member]
Derivative [Line Items]
Five billion dollar facility	5,000,000,000
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Gross derivative liability	986,000,000	1,419,000,000
Aggregate fair value of derivative instruments with credit-risk-related contingent features for which a net liability position existed, net of collateral	42,000,000	94,000,000
Not Designated as Hedging Instrument [Member] | Permitted to Offset Gross Derivative Asset with Financial Institutions [Member]
Derivative [Line Items]
Gross derivative liability	76,000,000	339,000,000
Not Designated as Hedging Instrument [Member] | Interest Rate Swap 3 and 4 [Member]
Derivative [Line Items]
Swap maturity date		Oct. 01, 2012
Aggregate notional principal amount of interest-rate swap		800,000,000
Not Designated as Hedging Instrument [Member] | Interest Rate Swap 1 and 2 [Member]
Derivative [Line Items]
Swap maturity date			Oct. 01, 2011
Aggregate notional principal amount of interest-rate swap			1,850,000,000
Not Designated as Hedging Instrument [Member] | Interest-Rate Swap Agreement [Member]
Derivative [Line Items]
Aggregate notional principal amount of interest-rate swap		200,000,000	150,000,000
Cash received (paid) in settlement of derivative instruments		(64,000,000)	(57,000,000)
Gross derivative liability	$ 654,000,000	$ 1,194,000,000
Not Designated as Hedging Instrument [Member] | Commodity [Member] | Natural Gas [Member]
Derivative [Line Items]
Financial derivative transactions	16,000,000	22,000,000