BUSINESS SEGMENTS - Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue	$ 5,289	$ 5,051	$ 5,409	$ 5,009	$ 5,251	$ 5,078	$ 5,211	$ 4,912	$ 20,758		$ 20,452		$ 15,407
Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue									19,673		19,470		14,612
Total segment | International Operations
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue									4,246		3,830		3,187
Total segment | North American Full-Service
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue									13,072		12,909		9,424
Total segment | North American Limited-Service
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue									3,217		3,219		2,894
Total segment | Asia Pacific
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue									1,118		1,054		631
Total segment | Other International
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue									2,266		2,288		1,663
Unallocated corporate
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue									1,085		982		795
Net fee revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues		932	951	845					3,638		3,295		2,388
Contract investment amortization		(13)	(13)	(18)					(58)	[1]	(50)	[1]	(40)	[1]
Total revenue		919	938	827					3,580		3,245		2,348
Net fee revenues | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues									3,155		2,951		2,145
Contract investment amortization									(58)		(50)		(40)
Total revenue									3,097		2,901		2,105
Net fee revenues | Total segment | North American Full-Service
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues									1,255		1,202		856
Contract investment amortization									(33)		(25)		(21)
Total revenue									1,222		1,177		835
Net fee revenues | Total segment | North American Limited-Service
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues									903		842		746
Contract investment amortization									(12)		(11)		(9)
Total revenue									891		831		737
Net fee revenues | Total segment | Asia Pacific
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues									479		431		231
Contract investment amortization									(2)		(1)		(1)
Total revenue									477		430		230
Net fee revenues | Total segment | Other International
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues									518		476		312
Contract investment amortization									(11)		(13)		(9)
Total revenue									507		463		303
Owned, leased, and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue		397	423	406					1,635	[1]	1,752	[1]	1,125	[1]
Owned, leased, and other | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue									1,571		1,705		1,074
Owned, leased, and other | Total segment | North American Full-Service
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue									593		697		390
Owned, leased, and other | Total segment | North American Limited-Service
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue									128		132		119
Owned, leased, and other | Total segment | Asia Pacific
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue									182		191		127
Owned, leased, and other | Total segment | Other International
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue									668		685		438
Reimbursements
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue		$ 3,735	$ 4,048	$ 3,776					15,543	[1]	15,455	[1]	11,934	[1]
Reimbursements | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue									15,005		14,864		11,433
Reimbursements | Total segment | North American Full-Service
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue									11,257		11,035		8,199
Reimbursements | Total segment | North American Limited-Service
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue									2,198		2,256		2,038
Reimbursements | Total segment | Asia Pacific
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue									459		433		274
Reimbursements | Total segment | Other International
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue									$ 1,091		$ 1,140		$ 922

[1] See Footnote 18. Related Party Transactions for disclosure of related party amounts.