REVENUE FROM CONTRACTS WITH CUSTOMERS Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) aircraft	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Deferred Revenue Arrangement [Line Items]
Percent of Mileage Breakage	17.40%	17.40%
Contract cost recognized	$ 334	$ 311	$ 90
Receivable	$ 383	296
Number of Partner AIrlines | aircraft	30
Credit Card Receivables	$ 243	201
Prepaid Expenses and Other Current Assets [Member]
Deferred Revenue Arrangement [Line Items]
Capitalized Contract Cost, Net	35	33
Passenger Revenue [Member]
Deferred Revenue Arrangement [Line Items]
Contract with Customer, Liability, Revenue Recognized	823	700
Mileage Plan Revenue [Member]
Deferred Revenue Arrangement [Line Items]
Receivable	$ 102	$ 83