REVENUE RECOGNITION - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Mar. 31, 2018	Sep. 30, 2019	Dec. 31, 2018
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Deferred revenue recognized			$ 133
Deferred revenue refund payments			15
Payments received from travelers			217
Deferred revenue in deferred merchant bookings [Member] | Deferred Merchant Bookings
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Customer Contract Liability	$ 218		218	$ 149
Customer Incentive Programs [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Reduction in Loyalty and Other Incentive Program Liability	(37)
Loyalty Programs [Member] | Customer Incentive Programs [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Reduction in Loyalty and Other Incentive Program Liability		$ (27)
Loyalty Programs [Member] | Customer Incentive Programs [Member] | Accrued Liabilities [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Customer Contract Liability	79		79	73
Other customer incentive programs [Member] | Customer Incentive Programs [Member] | Accrued Liabilities [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Customer Contract Liability	$ 25		$ 25	$ 61