REVENUE - Deferred Revenue (Details) $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($)
Contract With Customer, Change In Contract Liability, Current [Roll Forward]
Revenues recognized from the beginning deferred revenue balance	$ (153)
Cancellations and amendments	(65)
Payments received from travelers, net of amounts estimated to be payable to travel service providers, and other	73
Deferred revenue in deferred merchant bookings | Deferred merchant bookings
Contract With Customer, Change In Contract Liability, Current [Roll Forward]
Balance, December 31, 2019	220
Balance, September 30, 2020	$ 75