Description of Derivative Instruments (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	$ 6	$ 104
Fuel contracts, liabilities	723
Fuel Derivative Instruments
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	694
Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	6	89
Other Assets
Derivatives, Fair Value [Line Items]
Fuel contracts, assets		15
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		25
Fuel contracts, liabilities	477
Designated as Hedging Instrument | Cash Flow Hedging | Fuel Derivative Instruments
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	450
Designated as Hedging Instrument | Cash Flow Hedging | Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets		19
Designated as Hedging Instrument | Cash Flow Hedging | Other Assets
Derivatives, Fair Value [Line Items]
Fuel contracts, assets		6
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	6	79
Fuel contracts, liabilities	246
Not Designated as Hedging Instrument | Fuel Derivative Instruments
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	244
Not Designated as Hedging Instrument | Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	6	70
Not Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Fuel contracts, assets		9
Not Designated as Hedging Instrument | Other liabilities and deferred credits: Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	$ 2