Operating Leases - Aircraft Rent Expense and Maintenance Obligations (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) aircraft	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Lessee, Lease, Description [Line Items]
Aircraft rent expense	$ 554	$ 556	$ 530
Supplemental rent expense for maintenance-related reserves deemed non-recoverable	(2)	(1)	(3)
Supplemental rent expense related to probable lease return condition obligations	20	90	$ 61
Leased aircraft return cost liability	26	102
Aircraft maintenance deposits expected to be recoverable	96	117
Aircraft maintenance deposits expected to be recoverable, eligible maintenance performed	12	12
Aircraft maintenance deposits	84	$ 105
Maintenance reserve payments, due in year one	3
Maintenance reserve payments, due in year two	3
Maintenance reserve payments, due in year three	3
Maintenance reserve payments, due in year four	3
Maintenance reserve payments, due in year five	4
Maintenance reserve payments, due after year five	1
Rent benefit related to previously accrued lease return costs	$ 53
Aircraft and spare engines
Lessee, Lease, Description [Line Items]
Number of lease terms extended | aircraft	6
Aircraft Engine
Lessee, Lease, Description [Line Items]
Number of lease terms extended | aircraft	5