Financial Derivative Instruments - Offsetting of Derivative Assets (Details) - USD ($) $ in Millions		Mar. 31, 2016	Dec. 31, 2015
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets		$ 30	$ 41
Gross liability amounts offset in the Balance Sheet		(28)	(26)
Asset derivative contracts, net	[1]	2	15
Fuel derivatives | Other Assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets		45	7
Gross liability amounts offset in the Balance Sheet		(30)	0
Asset derivative contracts, net	[1]	15	7
Fuel derivatives | Accrued Liabilities
Offsetting Assets [Line Items]
Gross amounts of recognized assets		1,580	1,737
Gross liability amounts offset in the Balance Sheet		(1,580)	(1,737)
Asset derivative contracts, net	[1]	0	0
Fuel derivatives | Other Noncurrent Liabilities
Offsetting Assets [Line Items]
Gross amounts of recognized assets		846	985
Gross liability amounts offset in the Balance Sheet		(846)	(985)
Asset derivative contracts, net	[1]	0	0
Interest rate derivatives | Other Assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets		16	2
Gross liability amounts offset in the Balance Sheet		0	0
Asset derivative contracts, net	[1]	$ 16	$ 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.