Financial Derivative Instruments - Offsetting of Derivative Liabilities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities	$ 169		$ 182
Gross asset amounts offset in the Balance Sheet	(169)		(182)
Liability derivative contracts, net	0		0
Fuel derivatives | Other Assets
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities	69		100
Gross asset amounts offset in the Balance Sheet	(69)		(100)
Liability derivative contracts, net	0	[1]	0	[1]
Fuel derivatives | Accrued Liabilities
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities	0		21
Gross asset amounts offset in the Balance Sheet	0		(9)
Liability derivative contracts, net	0	[1]	12	[1]
Interest rate derivatives | Other Noncurrent Liabilities
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities	73		77
Gross asset amounts offset in the Balance Sheet	(32)		(32)
Liability derivative contracts, net	$ 41	[1]	$ 45	[1]

[1] The net amounts of derivative assets and liabilities are reconciled to the individual line item amounts presented in the unaudited Condensed Consolidated Balance Sheet in Note 5.