Description of Derivative Instruments (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	$ 118	$ 104
Fuel contracts, liabilities	2
Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	73	89
Other Assets
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	45	15
Current liabilities: Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	1
Other liabilities and deferred credits: Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	1
Hedge Derivatives, Net | Cash Flow Hedging
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	46	25
Hedge Derivatives, Net | Cash Flow Hedging | Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	17	19
Hedge Derivatives, Net | Cash Flow Hedging | Other Assets
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	29	6
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	72	79
Fuel contracts, liabilities	2
Not Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	16	9
Not Designated as Hedging Instrument | Current liabilities: Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	1
Not Designated as Hedging Instrument | Other liabilities and deferred credits: Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	1
Not Designated as Hedging Instrument | Cash Flow Hedging | Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	$ 56	$ 70