SUPPLEMENTARY DATA QUARTERLY FINANCIAL DATA - UNAUDITED - Schedule of Adjustments (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								9 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	Sep. 30, 2018	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
REVENUES
Total revenue	$ 5,289	$ 5,051	$ 5,409	$ 5,009	$ 5,251	$ 5,078	$ 5,211	$ 4,912		$ 20,758		$ 20,452		$ 15,407
OPERATING COSTS AND EXPENSES
Depreciation, amortization, and other		52	58	54						226	[1]	229	[1]	119	[1]
General, administrative, and other		221	217	247						927	[1]	921	[1]	743	[1]
Merger-related costs and charges		12	18	34						155		159		386
Costs and Expenses, Total		4,455	4,591	4,479						18,392		17,948		13,983
OPERATING INCOME	422	596	818	530	424	790	744	546		2,366		2,504		1,424
Gains and other income, net		18	114	59						194	[1]	688	[1]	5	[1]
Interest expense		(86)	(85)	(75)						(340)		(288)		(234)
Interest income		5	6	5						22	[1]	38	[1]	35	[1]
Equity in earnings		61	21	13						103	[1]	40	[1]	9	[1]
INCOME BEFORE INCOME TAXES		594	874	532						2,345		2,982		1,239
Provision for income taxes		(91)	(207)	(112)						(438)		(1,523)		(431)
NET INCOME	$ 317	$ 503	$ 667	$ 420	$ 114	$ 485	$ 489	$ 371		$ 1,907		$ 1,459		$ 808
EARNINGS PER SHARE
Earnings per share - basic (in USD per share)	$ 0.93	$ 1.45	$ 1.89	$ 1.17	$ 0.31	$ 1.30	$ 1.29	$ 0.96		$ 5.45		$ 3.89		$ 2.78
Earnings per share - diluted (in USD per share)	$ 0.92	$ 1.43	$ 1.87	$ 1.16	$ 0.31	$ 1.29	$ 1.28	$ 0.95		$ 5.38		$ 3.84		$ 2.73
As Previously Reported
REVENUES
Total revenue		$ 5,049	$ 5,346	$ 5,006								$ 22,894		$ 17,072
OPERATING COSTS AND EXPENSES
Depreciation, amortization, and other		52	58	54								290		168
General, administrative, and other		221	217	247								894		704
Merger-related costs and charges		12	18	34								159		386
Costs and Expenses, Total		4,479	4,606	4,506								20,535		15,704
OPERATING INCOME		570	740	500								2,359		1,368
Gains and other income, net		18	114	59								688		5
Interest expense		(86)	(85)	(75)								(288)		(234)
Interest income		5	6	5								38		35
Equity in earnings		61	21	13								39		10
INCOME BEFORE INCOME TAXES		568	796	502								2,836		1,184
Provision for income taxes		(85)	(186)	(104)								(1,464)		(404)
NET INCOME		$ 483	$ 610	$ 398								$ 1,372		$ 780
EARNINGS PER SHARE
Earnings per share - basic (in USD per share)		$ 1.39	$ 1.73	$ 1.11								$ 3.66		$ 2.68
Earnings per share - diluted (in USD per share)		$ 1.38	$ 1.71	$ 1.09								$ 3.61		$ 2.64
Loyalty Program Error | Effect of Prior Period Adjustment
REVENUES
Total revenue		$ 2	$ 63	$ 3
OPERATING COSTS AND EXPENSES
Depreciation, amortization, and other		0	0	0
General, administrative, and other		0	0	0
Merger-related costs and charges		0	0	0
Costs and Expenses, Total		(24)	(15)	(27)
OPERATING INCOME		26	78	30
Gains and other income, net		0	0	0
Interest expense		0	0	0
Interest income		0	0	0
Equity in earnings		0	0	0
INCOME BEFORE INCOME TAXES		26	78	30
Provision for income taxes		(6)	(21)	(8)
NET INCOME		$ 20	$ 57	$ 22					$ 99
EARNINGS PER SHARE
Earnings per share - basic (in USD per share)		$ 0.06	$ 0.16	$ 0.06
Earnings per share - diluted (in USD per share)		$ 0.05	$ 0.16	$ 0.07
Base management fees
REVENUES
Gross fee revenues		$ 279	$ 300	$ 273						$ 1,140	[1]	$ 1,102	[1]	$ 806	[1]
Base management fees | As Previously Reported
REVENUES
Gross fee revenues		279	300	273								1,102		806
Base management fees | Loyalty Program Error | Effect of Prior Period Adjustment
REVENUES
Gross fee revenues		0	0	0
Franchise fees
REVENUES
Gross fee revenues		502	475	417						1,849		1,586		1,157
Franchise fees | As Previously Reported
REVENUES
Gross fee revenues		502	475	417								1,618		1,169
Franchise fees | Loyalty Program Error | Effect of Prior Period Adjustment
REVENUES
Gross fee revenues		0	0	0
Incentive management fees
REVENUES
Gross fee revenues		151	176	155						649	[1]	607	[1]	425	[1]
Incentive management fees | As Previously Reported
REVENUES
Gross fee revenues		151	176	155								607		425
Incentive management fees | Loyalty Program Error | Effect of Prior Period Adjustment
REVENUES
Gross fee revenues		0	0	0
Net fee revenues
REVENUES
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 | As Previously Reported
REVENUES
Gross fee revenues		932	951	845								3,327		2,400
Contract investment amortization		(13)	(13)	(18)								0		0
Total revenue		919	938	827								3,327		2,400
Net fee revenues | Loyalty Program Error | Effect of Prior Period Adjustment
REVENUES
Gross fee revenues		0	0	0
Contract investment amortization		0	0	0
Total revenue		0	0	0
Owned, leased, and other
REVENUES
Total revenue		397	423	406						1,635	[1]	1,752	[1]	1,125	[1]
OPERATING COSTS AND EXPENSES
Cost of revenues		315	334	336						1,306		1,411		901
Owned, leased, and other | As Previously Reported
REVENUES
Total revenue		397	423	406								1,802		1,126
OPERATING COSTS AND EXPENSES
Cost of revenues		315	334	336								1,427		900
Owned, leased, and other | Loyalty Program Error | Effect of Prior Period Adjustment
REVENUES
Total revenue		0	0	0
OPERATING COSTS AND EXPENSES
Cost of revenues		0	0	0
Reimbursements
REVENUES
Total revenue		3,735	4,048	3,776						15,543	[1]	15,455	[1]	11,934	[1]
OPERATING COSTS AND EXPENSES
Cost of revenues		3,855	3,964	3,808						$ 15,778	[1]	15,228	[1]	11,834	[1]
Reimbursements | As Previously Reported
REVENUES
Total revenue		3,733	3,985	3,773								17,765		13,546
OPERATING COSTS AND EXPENSES
Cost of revenues		3,879	3,979	3,835								$ 17,765		$ 13,546
Reimbursements | Loyalty Program Error | Effect of Prior Period Adjustment
REVENUES
Total revenue		2	63	3
OPERATING COSTS AND EXPENSES
Cost of revenues		$ (24)	$ (15)	$ (27)

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