Operating Leases - Aircraft Rent Expense and Maintenance Obligations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Leases [Abstract]
Aircraft rent expense (benefit)	$ 159	$ 131
Supplemental rent expense for maintenance-related reserves deemed non-recoverable	1	1
Supplemental rent expense related to probable lease return condition obligations	13	$ 2
Leased aircraft return cost liability	33		$ 26
Aircraft maintenance deposits expected to be recoverable	98		96
Aircraft maintenance deposits expected to be recoverable, eligible maintenance performed	11		12
Aircraft maintenance deposits	87		$ 84
Maintenance reserve payments, due remainder of fiscal year	2
Maintenance reserve payments, due in year one	3
Maintenance reserve payments, due in year two	3
Maintenance reserve payments, due in year three	4
Maintenance reserve payments, due in year four	4
Maintenance reserve payments, due after year four	$ 1