REVENUE FROM CONTRACTS WITH CUSTOMERS Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) airline	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Deferred Revenue Arrangement [Line Items]
Percent of Mileage Breakage	17.40%	17.40%
Contract cost recognized	$ 217	$ 238	$ 173
Receivable	$ 366	341
Number of Partner AIrlines | airline	17
Prepaid Expenses and Other Current Assets [Member]
Deferred Revenue Arrangement [Line Items]
Capitalized Contract Cost, Net	$ 23	24
Passenger Revenue [Member]
Deferred Revenue Arrangement [Line Items]
Contract with Customer, Liability, Revenue Recognized	583	551
Mileage Plan Revenue [Member]
Deferred Revenue Arrangement [Line Items]
Receivable	$ 119	$ 101