Financial Derivative Instruments - Offsetting of Derivative Liabilities (Details) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Liabilities
Gross amounts of recognized liabilities		$ 3	$ 10
Gross amounts offset in the Balance Sheet		(3)	(10)
Liabilities derivative contracts, net		0	0
Fuel derivatives | Other Assets
Offsetting Liabilities
Gross amounts of recognized liabilities		31	15
Gross amounts offset in the Balance Sheet		(31)	(15)
Liabilities derivative contracts, net	[1]	0	0
Interest rate derivatives | Accrued Liabilities
Offsetting Liabilities
Gross amounts of recognized liabilities		0	5
Gross amounts offset in the Balance Sheet		0	0
Liabilities derivative contracts, net		0	5
Interest rate derivatives | Other Noncurrent Liabilities
Offsetting Liabilities
Gross amounts of recognized liabilities		6	1
Gross amounts offset in the Balance Sheet		0	0
Liabilities derivative contracts, net		$ 6	$ 1

[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.