REVENUE FROM CONTRACTS WITH CUSTOMERS Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) agreement	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Deferred Revenue Arrangement [Line Items]
Percent of Mileage Breakage	17.40%	17.40%
Contract cost recognized	$ 208	$ 217	$ 238
Receivable	$ 323	366
Number of Partner AIrlines | agreement	17
Prepaid Expenses and Other Current Assets [Member]
Deferred Revenue Arrangement [Line Items]
Capitalized Contract Cost, Net	$ 27	23
Passenger Revenue [Member]
Deferred Revenue Arrangement [Line Items]
Contract with Customer, Liability, Revenue Recognized	577	583
Mileage Plan Revenue [Member]
Deferred Revenue Arrangement [Line Items]
Receivable	$ 105	$ 119