Description of Derivative Instruments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	$ 1	$ 104
Fuel contracts, liabilities	115
Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets		89
Other Assets
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	1	15
Current liabilities: Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	93
Other liabilities and deferred credits: Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	22
Designated as Hedging Instrument | Cash Flow Hedging
Derivatives, Fair Value [Line Items]
Fuel contracts, assets		25
Fuel contracts, liabilities	82
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 | Current liabilities: Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	66
Designated as Hedging Instrument | Cash Flow Hedging | Other liabilities and deferred credits: Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	16
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	1	79
Fuel contracts, liabilities	33
Not Designated as Hedging Instrument | Receivables
Derivatives, Fair Value [Line Items]
Fuel contracts, assets		70
Not Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Fuel contracts, assets	1	9
Not Designated as Hedging Instrument | Current liabilities: Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	27
Not Designated as Hedging Instrument | Other liabilities and deferred credits: Other
Derivatives, Fair Value [Line Items]
Fuel contracts, liabilities	$ 6