Financial Derivative Instruments - Offsetting of Derivative Assets (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Offsetting Assets [Line Items]
Net amounts of assets presented in the Balance Sheet	$ 13		$ 145
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets	0		249
Gross amounts offset in the Balance Sheet	0		(182)
Net amounts of assets presented in the Balance Sheet	0		67
Fuel derivatives | Other Assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets	0		225
Gross amounts offset in the Balance Sheet	0		(100)
Net amounts of assets presented in the Balance Sheet	0	[1]	125	[1]
Fuel derivatives | Accrued Liabilities
Offsetting Assets [Line Items]
Gross amounts of recognized assets	1,258		9
Gross amounts offset in the Balance Sheet	(1,258)		(9)
Net amounts of assets presented in the Balance Sheet	0	[1]	0	[1]
Fuel derivatives | Other Noncurrent Liabilities
Offsetting Assets [Line Items]
Gross amounts of recognized assets	355		0
Gross amounts offset in the Balance Sheet	(355)		0
Net amounts of assets presented in the Balance Sheet	0	[1]	0	[1]
Interest rate derivatives | Other Assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets	13		20
Gross amounts offset in the Balance Sheet	0		0
Net amounts of assets presented in the Balance Sheet	$ 13	[1]	$ 20	[1]

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