GENERAL AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - PROPERTY, EQUIPMENT AND DEPRECIATION (Details) $ in Millions	12 Months Ended
	Dec. 31, 2025 USD ($) aircraft	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Property, Plant and Equipment [Line Items]
Less accumulated depreciation and amortization	$ (4,945.0)	$ (4,548.0)
Total Property and Equipment, net	11,857.0	10,781.0
Capitalized computer software	118.0	155.0
Amortization expenses related to computer software	79.0	47.0	$ 25.0
Aircraft and other flight equipment
Property, Plant and Equipment [Line Items]
Property and Equipment, gross	13,647.0	12,273.0
Other property and equipment
Property, Plant and Equipment [Line Items]
Property and Equipment, gross	2,424.0	2,173.0
Deposits for future flight equipment
Property, Plant and Equipment [Line Items]
Property and Equipment, gross	$ 731.0	$ 883.0
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 and other flight equipment
Property, Plant and Equipment [Line Items]
Estimated Salvage Value	10.00%
Boeing 717-200 aircraft | Aircraft and other flight equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	3 years
Boeing 717-200 aircraft | Aircraft and other flight equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	4 years
Boeing 737 aircraft | Aircraft and other flight equipment
Property, Plant and Equipment [Line Items]
Estimated Salvage Value	10.00%
Boeing 737 aircraft | Aircraft and other flight equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	20 years
Boeing 737 aircraft | Aircraft and other flight equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	25 years
Boeing 787-9 aircraft | Aircraft and other flight equipment
Property, Plant and Equipment [Line Items]
Estimated Useful Life	25 years
Estimated Salvage Value	10.00%
Airbus A321neo aircraft | Aircraft and other flight equipment
Property, Plant and Equipment [Line Items]
Estimated Salvage Value	10.00%
Airbus A321neo aircraft | Aircraft and other flight equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	18 years
Airbus A321neo aircraft | Aircraft and other flight equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	20 years
Airbus A330 aircraft | Aircraft and other flight equipment
Property, Plant and Equipment [Line Items]
Estimated Salvage Value	10.00%
Airbus A330 aircraft | Aircraft and other flight equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	11 years
Airbus A330 aircraft | Aircraft and other flight equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	15 years
Embraer E175 aircraft | Aircraft and other flight equipment
Property, Plant and Equipment [Line Items]
Estimated Salvage Value	10.00%
Embraer E175 aircraft | Aircraft and other flight equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	20 years
B-737-900 | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Property, Plant and Equipment [Line Items]
Number of aircraft sold | aircraft	12
B-737-900 | Aircraft and other flight equipment | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Property, Plant and Equipment [Line Items]
Gain on sale	$ 57.0
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other