Fuel - Effects of Derivative Instruments Gains and Losses (Details) (Cash Flow Hedging, USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized in Other Comprehensive Income (OCI) on Derivatives (Effective Portion)	$ 0		$ (2)		$ 0		$ 7
Amount of Gain or (Loss) Recognized from AOCL into Income (Effective Portion)	0		4		0		25
Amount of Gain or (Loss) Recognized in Income on Derivatives (Ineffective Portion and Amount Excluded from Effectiveness Testing)	0	[1]	(1)	[1]	0	[1]	(3)	[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		(2)		0		7
Fuel Contracts | Fuel Expense
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized from AOCL into Income (Effective Portion)	0		4		0		25
Amount of Gain or (Loss) Recognized in Income on Derivatives (Ineffective Portion and Amount Excluded from Effectiveness Testing)	$ 0	[1]	$ (1)	[1]	$ 0	[1]	$ (3)	[1]

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