REVENUE FROM CONTRACTS WITH CUSTOMERS - Contract Liabilities Activity (Details) - Mileage Plan Revenue [Member] - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Change in Contract with Customer, Liability [Roll Forward]
Total Deferred Revenue balance at January 1	$ 1,990	$ 1,874
Increase in liability for mileage credits issued	486	692
Total Deferred Revenue balance at Sept 30	2,183	1,971
Passenger revenue [Member]
Change in Contract with Customer, Liability [Roll Forward]
Revenue recognized	(272)	(511)
Mileage plan other revenue [Member]
Change in Contract with Customer, Liability [Roll Forward]
Revenue recognized	$ (21)	$ (84)