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Financial Derivative Instruments - (Gain) Loss by Hedging Relationship (Details) - Fuel derivative contracts - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2025
Jun. 30, 2024
Jun. 30, 2025
Jun. 30, 2024
Derivative Instruments, Gain (Loss) [Line Items]        
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Other Nonoperating Income (Expense) Other Nonoperating Income (Expense) Other Nonoperating Income (Expense) Other Nonoperating Income (Expense)
Designated as Hedging Instrument        
Derivative Instruments, Gain (Loss) [Line Items]        
(Gain) loss recognized in AOCI on derivatives, net of tax $ 40 $ 23 $ 69 $ 22
Not Designated as Hedging Instrument        
Derivative Instruments, Gain (Loss) [Line Items]        
(Gain) loss recognized in income on derivatives $ 0 $ 2 $ 0 $ 3