REVENUE FROM CONTRACTS WITH CUSTOMERS - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) airline	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Deferred Revenue Arrangement [Line Items]
Number of airline partners | airline	16
Breakage rate	17.40%	17.40%
Air traffic liability	$ 1,122		$ 806
Receivable	350		341
Contract cost recognized	51	$ 56
Prepaid Expenses and Other Current Assets [Member]
Deferred Revenue Arrangement [Line Items]
Contract costs	29		24
Mileage Plan Revenue [Member]
Deferred Revenue Arrangement [Line Items]
Receivable	101		$ 101
Mileage Plan Revenue [Member] | Mileage plan other revenue [Member]
Deferred Revenue Arrangement [Line Items]
Contract liability revenue recognized	(19)	(23)
Passenger Revenue [Member]
Deferred Revenue Arrangement [Line Items]
Contract liability revenue recognized	$ 434	$ 431