Financial Derivative Instruments - Offsetting of Derivative Assets (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets	$ 303		$ 375
Gross liability amounts offset in the Balance Sheet	(253)		(327)
Asset derivative contracts, net	50	[1]	48	[1]
Fuel derivatives | Other Assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets	178		588
Gross liability amounts offset in the Balance Sheet	(120)		(367)
Asset derivative contracts, net	58		221
Fuel derivatives | Accrued Liabilities
Offsetting Assets [Line Items]
Gross amounts of recognized assets	2		10
Gross liability amounts offset in the Balance Sheet	(2)		(10)
Asset derivative contracts, net	0		0
Fuel derivatives | Other Noncurrent Liabilities
Offsetting Assets [Line Items]
Gross amounts of recognized assets	19		0
Gross liability amounts offset in the Balance Sheet	(19)		0
Asset derivative contracts, net	0		0
Interest rate derivatives | Other Assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets	22		31
Gross liability amounts offset in the Balance Sheet	0		0
Asset derivative contracts, net	$ 22		$ 31

[1] Amounts included in Prepaid expenses and other current assets.