SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Effect of Adoption of ASU 2014-09 on Statement of Operations (Details) - USD ($) $ / shares in Units, $ 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
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 - 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
Base management fees
REVENUES
Gross fee revenues		$ 279	$ 300	$ 273					$ 1,140	[1]	$ 1,102	[1]	$ 806	[1]
Franchise fees
REVENUES
Gross fee revenues		502	475	417					1,849		1,586		1,157
Incentive management fees
REVENUES
Gross fee revenues		151	176	155					649	[1]	607	[1]	425	[1]
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
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
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]
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 - 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
As Previously Reported | Base management fees
REVENUES
Gross fee revenues		$ 279	$ 300	$ 273							$ 1,102		$ 806
As Previously Reported | Franchise fees
REVENUES
Gross fee revenues		502	475	417							1,618		1,169
As Previously Reported | Incentive management fees
REVENUES
Gross fee revenues		151	176	155							607		425
As Previously Reported | Net fee revenues
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
As Previously Reported | Owned, leased, and other
REVENUES
Total revenue		397	423	406							1,802		1,126
OPERATING COSTS AND EXPENSES
Cost of revenues		315	334	336							1,427		900
As Previously Reported | Reimbursements
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
Accounting Standards Update 2014-09 | Adoption of ASU
REVENUES
Total revenue											(2,442)		(1,665)
OPERATING COSTS AND EXPENSES
Depreciation, amortization, and other											(61)		(49)
General, administrative, and other											27		39
Merger-related costs and charges											0		0
Costs and Expenses, Total											(2,587)		(1,721)
OPERATING INCOME											145		56
Gains and other income, net											0		0
Interest expense											0		0
Interest income											0		0
Equity in earnings											1		(1)
INCOME BEFORE INCOME TAXES											146		55
Provision for income taxes											(59)		(27)
NET INCOME											$ 87		$ 28
Earnings per share - basic (in USD per share)											$ 0.23		$ 0.10
Earnings per share - diluted (in USD per share)											$ 0.23		$ 0.09
Accounting Standards Update 2014-09 | Adoption of ASU | Base management fees
REVENUES
Gross fee revenues											$ 0		$ 0
Accounting Standards Update 2014-09 | Adoption of ASU | Franchise fees
REVENUES
Gross fee revenues											(32)		(12)
Accounting Standards Update 2014-09 | Adoption of ASU | Incentive management fees
REVENUES
Gross fee revenues											0		0
Accounting Standards Update 2014-09 | Adoption of ASU | Net fee revenues
REVENUES
Gross fee revenues											(32)		(12)
Contract investment amortization											(50)		(40)
Total revenue											(82)		(52)
Accounting Standards Update 2014-09 | Adoption of ASU | Owned, leased, and other
REVENUES
Total revenue											(50)		(1)
OPERATING COSTS AND EXPENSES
Cost of revenues											(16)		1
Accounting Standards Update 2014-09 | Adoption of ASU | Reimbursements
REVENUES
Total revenue											(2,310)		(1,612)
OPERATING COSTS AND EXPENSES
Cost of revenues											$ (2,537)		$ (1,712)

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