REVENUE FROM CONTRACTS WITH CUSTOMERS - Contract Liabilities Activity (Details) - Mileage Plan Revenue [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Change in Contract with Customer, Liability [Roll Forward]
Total Deferred Revenue balance at January 1	$ 2,358	$ 2,277
Increase in liability for mileage credits issued	259	136
Total Deferred Revenue balance at Sept 30	2,432	2,325
Passenger revenue [Member]
Change in Contract with Customer, Liability [Roll Forward]
Revenue recognized	(176)	(84)
Mileage plan other revenue [Member]
Change in Contract with Customer, Liability [Roll Forward]
Revenue recognized	$ (9)	$ (4)