REVENUE FROM CONTRACTS WITH CUSTOMERS - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Passenger Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 2,111	$ 1,997	$ 3,827	$ 3,681
Passenger Revenue [Member] | Passenger ticket revenue, including ticket breakage and net of taxes and fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,792	1,693	3,231	3,121
Passenger Revenue [Member] | Passenger ancillary revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	147	137	271	254
Passenger Revenue [Member] | Mileage Plan passenger revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	172	167	325	306
Passenger Revenue [Member] | Passenger [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	2,111	1,997	3,827	3,681
Mileage Plan Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	290	275	553	521
Mileage Plan Revenue [Member] | Passenger [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	172	167	325	306
Mileage Plan Revenue [Member] | Mileage plan other revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	118	108	228	215
Cargo and Other Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	59	51	109	92
Cargo and Other Revenue [Member] | Cargo revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	38	34	68	60
Cargo and Other Revenue [Member] | Other revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 21	$ 17	$ 41	$ 32