REVENUE FROM CONTRACTS WITH CUSTOMERS - Contract Liabilities Activity (Details) - Mileage Plan Revenue [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Change in Contract with Customer, Liability [Roll Forward]
Total Deferred Revenue balance at January 1	$ 2,277	$ 1,990
Increase in liability for mileage credits issued	136	207
Total Deferred Revenue balance at Sept 30	2,325	2,023
Passenger revenue [Member]
Change in Contract with Customer, Liability [Roll Forward]
Revenue recognized	(84)	(152)
Mileage plan other revenue [Member]
Change in Contract with Customer, Liability [Roll Forward]
Revenue recognized	$ (4)	$ (22)