GENERAL AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - PROPERTY, EQUIPMENT AND DEPRECIATION (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Special items - net non-operating	$ (16)	$ 18	$ 0
Capitalized Computer Software, Net	155	66
Capitalized Computer Software, Amortization	47	25	$ 17
Aircraft Maintenance Deposits, Current	$ 42	$ 70
Buildings
Property, Plant and Equipment [Line Items]
Estimated Salvage Value	10.00%
Buildings | Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	30 years
Buildings | Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	40 years
Minor building and land improvements
Property, Plant and Equipment [Line Items]
Estimated Useful Life	10 years
Estimated Salvage Value	0.00%
Capitalized leases and leasehold improvements
Property, Plant and Equipment [Line Items]
Estimated Salvage Value	0.00%
Computer hardware and software
Property, Plant and Equipment [Line Items]
Estimated Salvage Value	0.00%
Computer hardware and software | Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	3 years
Computer hardware and software | Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	5 years
Other furniture and equipment
Property, Plant and Equipment [Line Items]
Estimated Salvage Value	0.00%
Other furniture and equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	3 years
Other furniture and equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	10 years
Boeing 717-200 aircraft | Aircraft
Property, Plant and Equipment [Line Items]
Estimated Salvage Value	10.00%
Boeing 717-200 aircraft | Aircraft | Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	3 years
Boeing 717-200 aircraft | Aircraft | Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	4 years
Boeing 737 aircraft | Aircraft
Property, Plant and Equipment [Line Items]
Estimated Salvage Value	10.00%
Boeing 737 aircraft | Aircraft | Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	20 years
Boeing 737 aircraft | Aircraft | Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	25 years
Boeing 787-9 aircraft | Aircraft
Property, Plant and Equipment [Line Items]
Estimated Useful Life	25 years
Estimated Salvage Value	10.00%
Airbus A321neo aircraft | Aircraft
Property, Plant and Equipment [Line Items]
Estimated Salvage Value	10.00%
Airbus A321neo aircraft | Aircraft | Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	18 years
Airbus A321neo aircraft | Aircraft | Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	20 years
Airbus A330 aircraft | Aircraft
Property, Plant and Equipment [Line Items]
Estimated Salvage Value	10.00%
Airbus A330 aircraft | Aircraft | Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	11 years
Airbus A330 aircraft | Aircraft | Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	15 years
Embraer E175 aircraft | Aircraft
Property, Plant and Equipment [Line Items]
Estimated Salvage Value	10.00%
Embraer E175 aircraft | Aircraft | Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	20 years