Derivative Instruments - Additional Information (Detail) (USD $)	6 Months Ended								6 Months Ended			12 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 Revolving Credit Facility [Member] $5.0 billion Facility [Member]	Jun. 30, 2014 Securing most derivative counterparties [Member] Revolving Credit Facility [Member] $5.0 billion Facility [Member]	Jun. 30, 2014 Not Designated as Hedging Instrument [Member]	Dec. 31, 2013 Not Designated as Hedging Instrument [Member]	Jun. 30, 2014 Not Designated as Hedging Instrument [Member] Permitted to Offset Gross Derivative Asset with Financial Institutions [Member]	Dec. 31, 2013 Not Designated as Hedging Instrument [Member] Permitted to Offset Gross Derivative Asset with Financial Institutions [Member]	Jun. 30, 2014 Not Designated as Hedging Instrument [Member] Modified interest-rate swap [Member]	Jun. 30, 2014 Not Designated as Hedging Instrument [Member] Settled interest-rate swap [Member]	Jun. 30, 2014 Commodity [Member] Natural Gas [Member] Not Designated as Hedging Instrument [Member] Mcf	Dec. 31, 2013 Commodity [Member] Natural Gas [Member] Not Designated as Hedging Instrument [Member] Mcf
Derivative [Line Items]
Financial derivative transactions											4,000,000	16,000,000
Interest-rate swap reference-period start date									Jun. 01, 2014
Aggregate notional principal amount of interest-rate swap									$ 1,100,000,000	$ 750,000,000
Cash paid in settlement of interest-rate swap agreements	222,000,000	0								222,000,000
Gross derivative liability					1,075,000,000	986,000,000	53,000,000	76,000,000
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					$ 92,000,000	$ 42,000,000