Description of Derivative Instruments (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	$ 40	$ 6
Fuel contracts, liabilities	401	723
Fuel Derivative Instruments
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	401	694
Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	32	6
Other Assets
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	8
Other liabilities and deferred credits: Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities		29
Designated as Hedging Instrument | Cash Flow Hedging
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	23
Fuel contracts, liabilities	261	477
Designated as Hedging Instrument | Cash Flow Hedging | Fuel Derivative Instruments
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	261	450
Designated as Hedging Instrument | Cash Flow Hedging | Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	23
Designated as Hedging Instrument | Cash Flow Hedging | Other liabilities and deferred credits: Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities		27
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	17	6
Fuel contracts, liabilities	140	246
Not Designated as Hedging Instrument | Fuel Derivative Instruments
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	140	244
Not Designated as Hedging Instrument | Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	9	6
Not Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	$ 8
Not Designated as Hedging Instrument | Other liabilities and deferred credits: Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities		$ 2