BUSINESS SEGMENTS - Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 5,875	$ 5,663	$ 5,795	$ 5,561	$ 5,456	$ 3,942	$ 3,902	$ 3,772	$ 22,894	$ 17,072	$ 14,486
Total segment | International Operations
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									4,840	3,181	2,761
Total segment | North American Full-Service
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									14,300	10,376	8,825
Total segment | North American Limited-Service
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									4,002	3,561	3,193
Total segment | Asia Pacific
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									1,344	761	516
Total segment | Other International
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									2,658	1,875	1,684
Unallocated corporate
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									$ 590	$ 499	$ 268