Financial Derivative Instruments - Offsetting of Derivative Assets (Details) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019
Offsetting Assets
Asset derivative contracts, net		$ 90	$ 49
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Assets
Gross amounts of recognized assets		13	48
Gross amounts offset in the Balance Sheet		(3)	(10)
Asset derivative contracts, net		10	38
Fuel derivatives | Other Assets
Offsetting Assets
Gross amounts of recognized assets		121	62
Gross amounts offset in the Balance Sheet		(31)	(15)
Asset derivative contracts, net	[1]	90	47
Interest rate derivatives | Other Assets
Offsetting Assets
Gross amounts of recognized assets		0	2
Gross amounts offset in the Balance Sheet		0	0
Asset derivative contracts, net	[1]	$ 0	$ 2

[1] The net amounts of derivative assets and liabilities are reconciled to the individual line item amounts presented in the Consolidated Balance Sheet in Note 16.