Operating Leases - Aircraft Rent Expense and Maintenance Obligations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022
Leases [Abstract]
Aircraft rent expense (benefit)	$ 148	$ 133	$ 279	$ 261
Supplemental rent expense for maintenance-related reserves deemed non-recoverable	(2)	(1)	(2)	(1)
Supplemental rent expense related to probable lease return condition obligations	22	$ 18	24	$ 33
Leased aircraft return cost liability	58		58		$ 102
Aircraft maintenance deposits expected to be recoverable	109		109		117
Aircraft maintenance deposits expected to be recoverable, eligible maintenance performed	24		24		12
Aircraft maintenance deposits	85		85		$ 105
Maintenance reserve payments, due remainder of fiscal year	2		2
Maintenance reserve payments, due in year one	3		3
Maintenance reserve payments, due in year two	3		3
Maintenance reserve payments, due in year three	3		3
Maintenance reserve payments, due in year four	4		4
Maintenance reserve payments, due after year four	$ 5		$ 5