REVENUE FROM CONTRACTS WITH CUSTOMERS - Contract Liabilities Activity (Details) - Mileage Plan Revenue [Member] - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Change in Contract with Customer, Liability [Roll Forward]
Total Deferred Revenue balance at January 1	$ 1,831	$ 1,874	$ 1,725
Increase in liability for mileage credits issued		692	631
Total Deferred Revenue balance at September 30		1,971
Passenger revenue [Member]
Change in Contract with Customer, Liability [Roll Forward]
Revenue recognized		(511)	(459)
Mileage plan other revenue [Member]
Change in Contract with Customer, Liability [Roll Forward]
Revenue recognized		$ (84)	$ (66)