Description of Derivative Instruments (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	$ 50	$ 104
Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	41	89
Other Assets
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	9	15
Other liabilities and deferred credits: Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	(2)
Hedge Derivatives, Net | Cash Flow Hedging
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	14	25
Hedge Derivatives, Net | Cash Flow Hedging | Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	9	19
Hedge Derivatives, Net | Cash Flow Hedging | Other Assets
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	5	6
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	36	79
Fuel contracts, liabilities	(2)
Not Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	4	9
Not Designated as Hedging Instrument | Other liabilities and deferred credits: Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	(2)
Not Designated as Hedging Instrument | Cash Flow Hedging | Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	$ 32	$ 70