Financial Derivative Instruments - Offsetting of Derivative Liabilities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities	$ 101	$ 327
Gross asset amounts offset in the Balance Sheet	(101)	(327)
Liability derivative contracts, net	0	0
Fuel derivatives | Other Assets
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities	38	367
Gross asset amounts offset in the Balance Sheet	(38)	(367)
Liability derivative contracts, net	0	0
Fuel derivatives | Accrued Liabilities
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities	178	60
Gross asset amounts offset in the Balance Sheet	(142)	(10)
Liability derivative contracts, net	36	50
Fuel derivatives | Other Noncurrent Liabilities
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities	189	0
Gross asset amounts offset in the Balance Sheet	(159)	0
Liability derivative contracts, net	30	0
Interest rate derivatives | Other Noncurrent Liabilities
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities	90	126
Gross asset amounts offset in the Balance Sheet	(41)	(89)
Liability derivative contracts, net	$ 49	$ 37