Financial Derivative Instruments - Offsetting of Derivative Assets (Details) - USD ($) $ in Millions		Mar. 31, 2019	Dec. 31, 2018
Offsetting Assets [Line Items]
Asset derivative contracts, net		$ 150	$ 95
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets		104	43
Gross liability amounts offset in the Balance Sheet		(15)	0
Asset derivative contracts, net		89	43
Fuel derivatives | Other Assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets		150	95
Gross liability amounts offset in the Balance Sheet		0	0
Asset derivative contracts, net	[1]	$ 150	$ 95

[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.