Description of Derivative Instruments (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	$ 33	$ 5
Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	10	51
Other Liabilities Current
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	26	4
Other Liabilities Noncurrent
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	7	1
Designated as Hedging Instrument | Cash Flow Hedging
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	7	2
Designated as Hedging Instrument | Cash Flow Hedging | Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	6	7
Designated as Hedging Instrument | Cash Flow Hedging | Other Liabilities Current
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	4	2
Designated as Hedging Instrument | Cash Flow Hedging | Other Liabilities Noncurrent
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	3
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	26	3
Not Designated as Hedging Instrument | Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	4	44
Not Designated as Hedging Instrument | Other Liabilities Current
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	22	2
Not Designated as Hedging Instrument | Other Liabilities Noncurrent
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	$ 4	$ 1