REVENUE FROM CONTRACTS WITH CUSTOMERS - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Passenger Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 572	$ 2,211	$ 2,362	$ 6,038
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	459	1,868	1,894	5,099
Passenger Revenue [Member] | Passenger ancillary revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	49	157	196	428
Passenger Revenue [Member] | Mileage Plan passenger revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	64	186	272	511
Passenger Revenue [Member] | Passenger [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	572	2,211	2,362	6,038
Mileage Plan Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	148	304	538	857
Mileage Plan Revenue [Member] | Passenger [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	64	186	272	511
Mileage Plan Revenue [Member] | Mileage plan other revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	84	118	266	346
Cargo and Other Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	45	60	130	169
Cargo and Other Revenue [Member] | Cargo revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	31	36	83	104
Cargo and Other Revenue [Member] | Other revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 14	$ 24	$ 47	$ 65