REVENUE FROM CONTRACTS WITH CUSTOMERS - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Mileage plan other revenue	$ 107	$ 100
Passenger Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,685	1,602
Passenger Revenue [Member] | Passenger ticket revenue, including ticket breakage and net of taxes and fees [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,428	1,358
Passenger Revenue [Member] | Passenger ancillary revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue	118	116
Passenger Revenue [Member] | Mileage Plan passenger revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue	139	128
Mileage Plan Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue	246	228
Mileage Plan Revenue [Member] | Passenger revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue	139	128
Mileage Plan Revenue [Member] | Mileage plan other revenue [Member]
Disaggregation of Revenue [Line Items]
Mileage plan other revenue	107	100
Cargo and Other Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue	40	38
Cargo and Other Revenue [Member] | Cargo revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue	26	24
Cargo and Other Revenue [Member] | Other revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue	$ 14	$ 14