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COMMITMENTS, CONTINGENCIES, AND GUARANTEES - Narrative (Details) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2025 USD ($) employee aircraft	Jun. 30, 2025 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2025 USD ($) employee aircraft	Sep. 30, 2024 USD ($)
Commitments and Contingencies [Line Items]
Special charges related to ratification payment	$ 0	$ 561	$ 0	$ 561	$ 0
United Airlines, Inc.
Commitments and Contingencies [Line Items]
Number of employees \| employee	111,900			111,900
Percentage of employees represented by various U.S. labor organizations				82.00%
Tax-Exempt Special Facilities Revenue Bonds
Commitments and Contingencies [Line Items]
Aggregate principal amount of guarantee	$ 2,900			$ 2,900
Floating Rate Debt
Commitments and Contingencies [Line Items]
Carrying amount	8,600			$ 8,600
Remaining term of debt				12 years
Loans and Leases from Non-U.S. Entities
Commitments and Contingencies [Line Items]
Remaining term of debt				12 years
Aggregate balance	$ 5,500			$ 5,500
Purchase commitments \| Airbus A319
Commitments and Contingencies [Line Items]
Number of aircrafts agreed to purchase between 2026 and 2029 \| aircraft	25			25