Derivative Instruments - Additional Information (Detail) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2013 Mcf	Dec. 31, 2012 Mcf
Derivative [Line Items]
Gross derivative liability	$ 1,011,000,000	$ 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	52,000,000	94,000,000
Five Billion Dollar Facility [Member]
Derivative [Line Items]
Five billion dollar facility	5,000,000,000
Permitted to Offset Gross Derivative Asset with Financial Institutions [Member]
Derivative [Line Items]
Gross derivative liability	305,000,000	339,000,000
Securing most derivative counterparties [Member] | Five Billion Dollar Facility [Member]
Derivative [Line Items]
Five billion dollar facility	$ 5,000,000,000
Commodity [Member] | Natural Gas [Member]
Derivative [Line Items]
Fixed-price physical transactions	10,000,000	10,000,000
Fixed-to-floating derivative transaction	10,000,000	10,000,000