REVENUE FROM CONTRACTS WITH CUSTOMERS - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Passenger Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 1,481	$ 1,716
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	1,213	1,439
Passenger Revenue [Member] | Passenger ancillary revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	116	124
Passenger Revenue [Member] | Mileage Plan passenger revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	152	153
Passenger Revenue [Member] | Passenger [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,481	1,716
Mileage Plan Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	261	263
Mileage Plan Revenue [Member] | Passenger [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	152	153
Mileage Plan Revenue [Member] | Mileage plan other revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	109	110
Cargo and Other Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	46	50
Cargo and Other Revenue [Member] | Cargo revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	24	30
Cargo and Other Revenue [Member] | Other revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 22	$ 20