Fuel - Effects of Derivative Instruments Gains and Losses (Details) (Cash Flow Hedging, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized in Other Comprehensive Income (OCI) on Derivatives (Effective Portion)	$ 0		$ 9
Amount of Gain or (Loss) Recognized from AOCL into Income (Effective Portion)	0		21
Amount of Gain or (Loss) Recognized in Income on Derivatives (Ineffective Portion and Amount Excluded from Effectiveness Testing)	0	[1]	(2)	[1]
Fuel Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized in Other Comprehensive Income (OCI) on Derivatives (Effective Portion)	0		9
Fuel Contracts | Fuel Expense
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized from AOCL into Income (Effective Portion)	0		21
Amount of Gain or (Loss) Recognized in Income on Derivatives (Ineffective Portion and Amount Excluded from Effectiveness Testing)	$ 0	[1]	$ (2)	[1]

[1] No portion of the loss was excluded from the assessment of hedge effectiveness for the periods then ended.