Financial Derivative Instruments - Offsetting of Derivative Assets (Details) - USD ($) $ in Millions		Jun. 30, 2015	Dec. 31, 2014
Offsetting Assets [Line Items]
Asset derivative contracts, net		$ 1	$ 13
Fuel derivatives | Accrued Liabilities
Offsetting Assets [Line Items]
Gross amounts of recognized assets		639	1,258
Gross liability amounts offset in the Balance Sheet		(639)	(1,258)
Asset derivative contracts, net	[1]	0	0
Fuel derivatives | Other Noncurrent Liabilities
Offsetting Assets [Line Items]
Gross amounts of recognized assets		647	355
Gross liability amounts offset in the Balance Sheet		(647)	(355)
Asset derivative contracts, net	[1]	0	0
Interest rate derivatives | Other Assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets		1	13
Gross liability amounts offset in the Balance Sheet		0	0
Asset derivative contracts, net	[1]	$ 1	$ 13

[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.