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FAIR VALUE MEASUREMENTS - Measured at Fair Value on a Recurring Basis (Details) - Recurring - USD ($)
$ in Millions
Jun. 30, 2024
Dec. 31, 2023
Fair Value    
Cash equivalents $ 2,840 $ 1,545
Restricted cash equivalents 397 653
Long-term investments and related 2,445 2,867
Fuel hedge contracts (23) 5
U.S. Government securities    
Fair Value    
Short-term investments 100 859
Corporate obligations    
Fair Value    
Short-term investments 10 218
Other fixed income securities    
Fair Value    
Short-term investments 14 50
Level 1    
Fair Value    
Cash equivalents 2,840 1,545
Restricted cash equivalents 397 653
Long-term investments and related 2,188 2,614
Fuel hedge contracts 0 0
Level 1 | U.S. Government securities    
Fair Value    
Short-term investments 0 204
Level 1 | Corporate obligations    
Fair Value    
Short-term investments 0 0
Level 1 | Other fixed income securities    
Fair Value    
Short-term investments 0 0
Level 2    
Fair Value    
Cash equivalents 0 0
Restricted cash equivalents 0 0
Long-term investments and related 143 134
Fuel hedge contracts (23) 5
Level 2 | U.S. Government securities    
Fair Value    
Short-term investments 100 655
Level 2 | Corporate obligations    
Fair Value    
Short-term investments 10 218
Level 2 | Other fixed income securities    
Fair Value    
Short-term investments 14 50
Level 3    
Fair Value    
Cash equivalents 0 0
Restricted cash equivalents 0 0
Long-term investments and related 114 119
Fuel hedge contracts 0 0
Level 3 | U.S. Government securities    
Fair Value    
Short-term investments 0 0
Level 3 | Corporate obligations    
Fair Value    
Short-term investments 0 0
Level 3 | Other fixed income securities    
Fair Value    
Short-term investments $ 0 $ 0