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Operating Leases - Aircraft Rent Expense and Maintenance Obligations (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2024
Jun. 30, 2023
Jun. 30, 2024
Jun. 30, 2023
Dec. 31, 2023
Leases [Abstract]          
Aircraft rent expense (benefit) $ (147) $ (148) $ (306) $ (279)  
Supplemental rent expense for maintenance-related reserves deemed non-recoverable (7) (2) (7) (2)  
Supplemental rent expense related to probable lease return condition obligations 1 $ 22 14 $ 24  
Leased aircraft return cost liability 31   31   $ 26
Disbursement previously paid $ 104   $ 104