REVENUE FROM CONTRACTS WITH CUSTOMERS - Narrative (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) airline	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Deferred Revenue Arrangement [Line Items]
Number of airline partners | airline			16
Breakage rate	17.40%	17.40%	17.40%	17.40%
Air traffic liability	$ 1,112		$ 1,112		$ 806
Receivable	411		411		341
Contract cost recognized	58	$ 66	109	$ 122
Prepaid Expenses and Other Current Assets [Member]
Deferred Revenue Arrangement [Line Items]
Contract costs	31		31		24
Mileage Plan Revenue [Member]
Deferred Revenue Arrangement [Line Items]
Receivable	103		103		$ 101
Mileage Plan Revenue [Member] | Mileage plan other revenue [Member]
Deferred Revenue Arrangement [Line Items]
Contract liability revenue recognized			(44)	(44)
Passenger Revenue [Member]
Deferred Revenue Arrangement [Line Items]
Contract liability revenue recognized	$ 79	$ 89	$ 513	$ 520