Financial Derivative Instruments - Offsetting of Derivative Assets (Details) - USD ($) $ in Millions		Jun. 30, 2016	Dec. 31, 2015
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets		$ 19	$ 41
Gross liability amounts offset in the Balance Sheet		(19)	(26)
Asset derivative contracts, net	[1]	0	15
Fuel derivatives | Other Assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets		11	7
Gross liability amounts offset in the Balance Sheet		(11)	0
Asset derivative contracts, net	[1]	0	7
Fuel derivatives | Accrued Liabilities
Offsetting Assets [Line Items]
Gross amounts of recognized assets		957	1,737
Gross liability amounts offset in the Balance Sheet		(957)	(1,737)
Asset derivative contracts, net	[1]	0	0
Fuel derivatives | Other Noncurrent Liabilities
Offsetting Assets [Line Items]
Gross amounts of recognized assets		521	985
Gross liability amounts offset in the Balance Sheet		(494)	(985)
Asset derivative contracts, net	[1]	27	0
Interest rate derivatives | Other Assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets		21	2
Gross liability amounts offset in the Balance Sheet		0	0
Asset derivative contracts, net	[1]	$ 21	$ 2

[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.