Description of Derivative Instruments (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	$ 73	$ 51
Fuel contracts, liabilities		5
Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	70	51
Other Liabilities Current
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities		4
Other Assets Noncurrent
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	3
Other Liabilities Noncurrent
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities		1
Designated as Hedging Instrument | Cash Flow Hedging | Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	19	7
Designated as Hedging Instrument | Cash Flow Hedging | Other Liabilities Current
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities		2
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	54	44
Fuel contracts, liabilities		3
Not Designated as Hedging Instrument | Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	51	44
Not Designated as Hedging Instrument | Other Liabilities Current
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities		2
Not Designated as Hedging Instrument | Other Assets Noncurrent
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	3
Not Designated as Hedging Instrument | Other Liabilities Noncurrent
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities		$ 1