Description of Derivative Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	$ 124	$ 723
Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets		6
Fuel Derivative Instruments
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	124	694
Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities		29
Designated as Hedging Instrument | Cash Flow Hedging
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	119	477
Designated as Hedging Instrument | Cash Flow Hedging | Fuel Derivative Instruments
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	119	450
Designated as Hedging Instrument | Cash Flow Hedging | Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities		27
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	5	246
Not Designated as Hedging Instrument | Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets		6
Not Designated as Hedging Instrument | Fuel Derivative Instruments
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	$ 5	244
Not Designated as Hedging Instrument | Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities		$ 2