REVENUE FROM CONTRACTS WITH CUSTOMERS Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) agreement	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Deferred Revenue Arrangement [Line Items]
Percent of Mileage Breakage	17.40%	17.40%
Contract cost recognized	$ 43	$ 208	$ 217
Receivable	$ 480	323
Number of Partner AIrlines | agreement	17
Deferred Revenue, Refund Payments	$ 600
COVID-19 Credits Issued, Balance	569
COVID-19 Related Credits Issued	1,000
Credit Card Receivables	83	212
Prepaid Expenses and Other Current Assets [Member]
Deferred Revenue Arrangement [Line Items]
Capitalized Contract Cost, Net	24	27
Passenger Revenue [Member]
Deferred Revenue Arrangement [Line Items]
Contract with Customer, Liability, Revenue Recognized	502	577
Mileage Plan Revenue [Member]
Deferred Revenue Arrangement [Line Items]
Receivable	$ 48	$ 105