REVENUE - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Reduction in revenue for refunds paid or estimated to be payable	$ 63
Loyalty Programs [Member] | Customer Incentive Programs [Member] | Accrued Liabilities [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Contract liabilities, current	76		$ 80
Other customer incentive programs [Member] | Customer Incentive Programs [Member] | Accrued Liabilities [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Contract liabilities, current	$ 22		$ 22
Online accommodation reservation services [Member] | Revenue Benchmark [Member] | Product Concentration Risk [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Concentration risk percentage	85.00%	84.00%
Other sources of online travel reservation services or advertising and other revenues [Member] | Revenue Benchmark [Member] | Product Concentration Risk [Member] | Maximum
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Concentration risk percentage	10.00%	10.00%