Basis of Presentation - Effect of Adoption of ASU 2014-09 on Statement of Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
REVENUES
Revenues	$ 5,049	$ 5,078	$ 15,401	$ 15,201
OPERATING COSTS AND EXPENSES
Depreciation, amortization, and other	52	54	164	176
General, administrative, and other	221	205	685	651
Merger-related costs and charges	12	28	64	100
Costs and expenses	4,479	4,288	13,591	13,121
OPERATING INCOME	570	790	1,810	2,080
Gains and other income, net	18	6	191	31
Interest expense	(86)	(73)	(246)	(216)
Interest income	5	9	16	24
Equity in earnings	61	6	95	29
INCOME BEFORE INCOME TAXES	568	738	1,866	1,948
Income taxes	(85)	(253)	(375)	(603)
NET INCOME	$ 483	$ 485	$ 1,491	$ 1,345
EARNINGS PER SHARE
Earnings per share - basic (in USD per share)	$ 1.39	$ 1.30	$ 4.23	$ 3.55
Earnings per share - diluted (in USD per share)	$ 1.38	$ 1.29	$ 4.18	$ 3.51
Base management fees
REVENUES
Gross revenues	$ 279	$ 269	$ 852	$ 818
Franchise fees
REVENUES
Gross revenues	502	419	1,394	1,182
Incentive management fees
REVENUES
Gross revenues	151	138	482	433
Fee revenues
REVENUES
Gross revenues	932	826	2,728	2,433
Contract investment amortization	(13)	(11)	(44)	(34)
Revenues	919	815	2,684	2,399
Owned, leased, and other
REVENUES
Revenues	397	433	1,226	1,309
OPERATING COSTS AND EXPENSES
Operating costs	315	351	985	1,057
Reimbursements
REVENUES
Revenues	3,733	3,830	11,491	11,493
OPERATING COSTS AND EXPENSES
Operating costs	$ 3,879	3,650	$ 11,693	11,137
As Previously Reported
REVENUES
Revenues		5,663		17,019
OPERATING COSTS AND EXPENSES
Depreciation, amortization, and other		68		218
General, administrative, and other		199		635
Merger-related costs and charges		28		100
Costs and expenses		5,031		15,230
OPERATING INCOME		632		1,789
Gains and other income, net		6		31
Interest expense		(73)		(216)
Interest income		9		24
Equity in earnings		6		29
INCOME BEFORE INCOME TAXES		580		1,657
Income taxes		(188)		(486)
NET INCOME		$ 392		$ 1,171
EARNINGS PER SHARE
Earnings per share - basic (in USD per share)		$ 1.05		$ 3.09
Earnings per share - diluted (in USD per share)		$ 1.04		$ 3.06
As Previously Reported | Base management fees
REVENUES
Gross revenues		$ 269		$ 818
As Previously Reported | Franchise fees
REVENUES
Gross revenues		426		1,207
As Previously Reported | Incentive management fees
REVENUES
Gross revenues		136		437
As Previously Reported | Fee revenues
REVENUES
Gross revenues		831		2,462
Contract investment amortization		0		0
Revenues		831		2,462
As Previously Reported | Owned, leased, and other
REVENUES
Revenues		452		1,349
OPERATING COSTS AND EXPENSES
Operating costs		356		1,069
As Previously Reported | Reimbursements
REVENUES
Revenues		4,380		13,208
OPERATING COSTS AND EXPENSES
Operating costs		4,380		13,208
Adjustment | Adoption of ASU 2014-09
REVENUES
Revenues		(585)		(1,818)
OPERATING COSTS AND EXPENSES
Depreciation, amortization, and other		(14)		(42)
General, administrative, and other		6		16
Merger-related costs and charges		0		0
Costs and expenses		(743)		(2,109)
OPERATING INCOME		158		291
Gains and other income, net		0		0
Interest expense		0		0
Interest income		0		0
Equity in earnings		0		0
INCOME BEFORE INCOME TAXES		158		291
Income taxes		(65)		(117)
NET INCOME		$ 93		$ 174
EARNINGS PER SHARE
Earnings per share - basic (in USD per share)		$ 0.25		$ 0.46
Earnings per share - diluted (in USD per share)		$ 0.25		$ 0.45
Adjustment | Base management fees | Adoption of ASU 2014-09
REVENUES
Gross revenues		$ 0		$ 0
Adjustment | Franchise fees | Adoption of ASU 2014-09
REVENUES
Gross revenues		(7)		(25)
Adjustment | Incentive management fees | Adoption of ASU 2014-09
REVENUES
Gross revenues		2		(4)
Adjustment | Fee revenues | Adoption of ASU 2014-09
REVENUES
Gross revenues		(5)		(29)
Contract investment amortization		(11)		(34)
Revenues		(16)		(63)
Adjustment | Owned, leased, and other | Adoption of ASU 2014-09
REVENUES
Revenues		(19)		(40)
OPERATING COSTS AND EXPENSES
Operating costs		(5)		(12)
Adjustment | Reimbursements | Adoption of ASU 2014-09
REVENUES
Revenues		(550)		(1,715)
OPERATING COSTS AND EXPENSES
Operating costs		$ (730)		$ (2,071)