v3.25.1
FAIR VALUE MEASUREMENTS, INVESTMENTS AND NOTES RECEIVABLE - Narrative (Details)
$ in Millions
3 Months Ended
Mar. 31, 2025
USD ($)
aircraft
Apr. 04, 2025
Fair Value [Line Items]    
Equity method investments $ 249  
Carrying amount of loan receivable $ 79  
Republic Airways    
Fair Value [Line Items]    
Number of aircraft operated | aircraft 66  
Champlain Enterprises LLC    
Fair Value [Line Items]    
Number of aircraft operated | aircraft 59  
Republic Airways    
Fair Value [Line Items]    
Ownership stake 19.00%  
CommuteAir LLC    
Fair Value [Line Items]    
Ownership stake 40.00%  
United Airlines Ventures Sustainable Flight Fund    
Fair Value [Line Items]    
Ownership stake 33.00%  
Other Equity Investments    
Fair Value [Line Items]    
Investment carrying value $ 262  
New Surviving Entity | Maximum | Scenario, Plan | Subsequent Event    
Fair Value [Line Items]    
Equity interests exchanged for commitment to facilitate merger transaction   6.00%
Mesa    
Fair Value [Line Items]    
Variable interest 10.00%  
Corporate debt    
Fair Value [Line Items]    
Available-for-sale securities remaining maturities 2 years