Description of Derivative Instruments (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	$ 104	$ 51
Fuel contracts, liabilities		5
Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	89	51
Other Assets
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	15
Current liabilities: Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities		4
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	25	7
Hedge Derivatives, Net | Cash Flow Hedging | Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	19	7
Hedge Derivatives, Net | Cash Flow Hedging | Other Assets
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	6
Hedge Derivatives, Net | Cash Flow Hedging | Current liabilities: Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities		2
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	79	44
Fuel contracts, liabilities		3
Not Designated as Hedging Instrument | Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	70	44
Not Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	9
Not Designated as Hedging Instrument | Current liabilities: Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities		2
Not Designated as Hedging Instrument | Other liabilities and deferred credits: Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities		$ 1