Financial Derivative Instruments - Offsetting of Derivative Assets (Details) - USD ($) $ in Millions		Mar. 31, 2021	Dec. 31, 2020
Offsetting Assets [Line Items]
Asset derivative contracts, net		$ 131	$ 90
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets		67	13
Gross liability amounts offset in the Balance Sheet		(15)	(3)
Asset derivative contracts, net		52	10
Fuel derivatives | Other Assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets		182	121
Gross liability amounts offset in the Balance Sheet		(57)	(31)
Asset derivative contracts, net	[1]	125	90
Interest rate derivatives | Other Assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets		6	0
Gross liability amounts offset in the Balance Sheet		0	0
Asset derivative contracts, net	[1]	$ 6	$ 0

[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.