Description of Derivative Instruments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	$ 54	$ 51
Fuel contracts, liabilities	2	5
Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	52	51
Other Liabilities Current
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities		4
Other Liabilities Noncurrent
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	2	1
Other Assets
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	2
Designated as Hedging Instrument | Cash Flow Hedging
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	1	2
Designated as Hedging Instrument | Cash Flow Hedging | Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	23	7
Designated as Hedging Instrument | Cash Flow Hedging | Other Liabilities Current
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities		2
Designated as Hedging Instrument | Cash Flow Hedging | Other Liabilities Noncurrent
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	1
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	31	44
Fuel contracts, liabilities	1	3
Not Designated as Hedging Instrument | Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	29	44
Not Designated as Hedging Instrument | Other Liabilities Current
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities		2
Not Designated as Hedging Instrument | Other Liabilities Noncurrent
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	1	1
Not Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	$ 2