Financial Derivative Instruments - Offsetting of Derivative Assets (Details) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Offsetting Assets [Line Items]
Asset derivative contracts, net		$ 175	$ 136
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets		168	147
Gross liability amounts offset in the Balance Sheet		(83)	(50)
Asset derivative contracts, net		85	97
Fuel derivatives | Other Assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets		195	136
Gross liability amounts offset in the Balance Sheet		(20)	0
Asset derivative contracts, net	[1]	$ 175	$ 136

[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 9.