Financial Derivative Instruments - Offsetting Derivative Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities	$ 3	$ 6
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities	48	3
Gross amounts offset in the Balance Sheet	48	3
Net amounts of liabilities presented in the Balance Sheet	0	0
Fuel derivatives | Other assets
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities	72	31
Gross amounts offset in the Balance Sheet	72	31
Net amounts of liabilities presented in the Balance Sheet	0	0
Interest rate derivatives | Other noncurrent liabilities
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities	3	6
Gross amounts offset in the Balance Sheet	0	0
Net amounts of liabilities presented in the Balance Sheet	$ 3	$ 6