BUSINESS SEGMENTS - Segment Revenues (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue		$ 5,371	$ 5,284	$ 5,305	$ 5,012	$ 5,289	$ 5,051	$ 5,409	$ 5,009	$ 20,972	$ 20,758	$ 20,452
Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue										20,377	19,673	19,470
Total segment | International Operations
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue										4,400	4,246	3,830
Total segment | North American Full-Service
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue										13,455	13,072	12,909
Total segment | North American Limited-Service
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue										3,378	3,217	3,219
Total segment | Asia Pacific
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue										1,189	1,118	1,054
Total segment | Other International
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue										2,355	2,266	2,288
Unallocated corporate
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue										595	1,085	982
Net fee revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues										3,823	3,638	3,295
Contract investment amortization	[1]									(62)	(58)	(50)
Total revenue										3,761	3,580	3,245
Net fee revenues | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues										3,297	3,155	2,951
Contract investment amortization										(62)	(58)	(50)
Total revenue										3,235	3,097	2,901
Net fee revenues | Total segment | North American Full-Service
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues										1,299	1,255	1,202
Contract investment amortization										(35)	(33)	(25)
Total revenue										1,264	1,222	1,177
Net fee revenues | Total segment | North American Limited-Service
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues										966	903	842
Contract investment amortization										(13)	(12)	(11)
Total revenue										953	891	831
Net fee revenues | Total segment | Asia Pacific
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues										477	479	431
Contract investment amortization										(2)	(2)	(1)
Total revenue										475	477	430
Net fee revenues | Total segment | Other International
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues										555	518	476
Contract investment amortization										(12)	(11)	(13)
Total revenue										543	507	463
Owned, leased, and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue	[1]									1,612	1,635	1,752
Owned, leased, and other | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue										1,556	1,571	1,705
Owned, leased, and other | Total segment | North American Full-Service
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue										581	593	697
Owned, leased, and other | Total segment | North American Limited-Service
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue										134	128	132
Owned, leased, and other | Total segment | Asia Pacific
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue										178	182	191
Owned, leased, and other | Total segment | Other International
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue										663	668	685
Reimbursements
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue	[1]									15,599	15,543	15,455
Reimbursements | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue										15,586	15,005	14,864
Reimbursements | Total segment | North American Full-Service
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue										11,610	11,257	11,035
Reimbursements | Total segment | North American Limited-Service
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue										2,291	2,198	2,256
Reimbursements | Total segment | Asia Pacific
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue										536	459	433
Reimbursements | Total segment | Other International
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue										$ 1,149	$ 1,091	$ 1,140

[1] See Note 16 for disclosure of related party amounts.