DEBT - Narrative (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2026	Sep. 30, 2026	Jun. 30, 2026	Mar. 31, 2026
Debt Instrument [Line Items]
Weighted-average interest rate (in percent)	5.40%	5.40%
Debt payments		$ 398,000,000
Prepayments of debt		113,000,000
Fixed-rate unsecured notes payable due through 2031
Debt Instrument [Line Items]
Debt instrument, face amount	$ 500,000,000	$ 500,000,000
Interest rate (in percent)	6.50%	6.50%
Loyalty financing, variable-rate term loan facility due through 2033
Debt Instrument [Line Items]
Increase in senior secured term loan facility	$ 500,000,000
Other Financing
Debt Instrument [Line Items]
Debt instrument, face amount	183,000,000	$ 183,000,000
Other Financing, Secured By Aircraft
Debt Instrument [Line Items]
Debt instrument, face amount	111,000,000	111,000,000
Other Financing, Aircraft Delivery Finance Agreement
Debt Instrument [Line Items]
Debt instrument, face amount	72,000,000	72,000,000
Alaskan And Hawaiian Credit Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, borrowing capacity			$ 1,100,000,000	$ 850,000,000
Outstanding borrowing	0	0
Alaskan Credit Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, borrowing capacity	106,000,000	106,000,000
Interest rate swap agreements | Cash Flow Hedging | Derivative Instruments Designated as Hedges
Debt Instrument [Line Items]
Notional amount	$ 596,000,000	$ 596,000,000