Basis of Presentation - Effect of Adoption of ASU 2014-09 on Statement of Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
REVENUES
Revenues	$ 5,346	$ 5,211	$ 10,352	$ 10,123
OPERATING COSTS AND EXPENSES
Depreciation, amortization, and other	58	71	112	122
General, administrative, and other	217	234	464	446
Merger-related costs and charges	18	21	52	72
Costs and expenses	4,606	4,467	9,112	8,833
OPERATING INCOME	740	744	1,240	1,290
Gains and other income, net	114	25	173	25
Interest expense	(85)	(73)	(160)	(143)
Interest income	6	8	11	15
Equity in earnings	21	12	34	23
INCOME BEFORE INCOME TAXES	796	716	1,298	1,210
Income taxes	(186)	(227)	(290)	(350)
NET INCOME	$ 610	$ 489	$ 1,008	$ 860
EARNINGS PER SHARE
Earnings per share - basic (in USD per share)	$ 1.73	$ 1.29	$ 2.83	$ 2.25
Earnings per share - diluted (in USD per share)	$ 1.71	$ 1.28	$ 2.80	$ 2.23
Base management fees
REVENUES
Gross revenues	$ 300	$ 285	$ 573	$ 549
Franchise fees
REVENUES
Gross revenues	475	408	892	763
Incentive management fees
REVENUES
Gross revenues	176	155	331	295
Fee revenues
REVENUES
Gross revenues	951	848	1,796	1,607
Contract investment amortization	(13)	(12)	(31)	(23)
Revenues	938	836	1,765	1,584
Owned, leased, and other
REVENUES
Revenues	423	448	829	876
OPERATING COSTS AND EXPENSES
Operating costs	334	350	670	706
Reimbursements
REVENUES
Revenues	3,985	3,927	7,758	7,663
OPERATING COSTS AND EXPENSES
Operating costs	$ 3,979	3,791	$ 7,814	7,487
As Previously Reported
REVENUES
Revenues		5,795		11,356
OPERATING COSTS AND EXPENSES
Depreciation, amortization, and other		85		8,828
General, administrative, and other		226		150
Merger-related costs and charges		21		436
Costs and expenses		5,175		10,199
OPERATING INCOME		620		1,157
Gains and other income, net		25		25
Interest expense		(73)		(143)
Interest income		8		15
Equity in earnings		12		23
INCOME BEFORE INCOME TAXES		592		1,077
Income taxes		(178)		(298)
NET INCOME		$ 414		$ 779
EARNINGS PER SHARE
Earnings per share - basic (in USD per share)		$ 1.09		$ 2.04
Earnings per share - diluted (in USD per share)		$ 1.08		$ 2.02
As Previously Reported | Base management fees
REVENUES
Gross revenues		$ 285		$ 549
As Previously Reported | Franchise fees
REVENUES
Gross revenues		416		781
As Previously Reported | Incentive management fees
REVENUES
Gross revenues		148		301
As Previously Reported | Fee revenues
REVENUES
Gross revenues		849		1,631
Contract investment amortization		0		0
Revenues		849		1,631
As Previously Reported | Owned, leased, and other
REVENUES
Revenues		458		897
OPERATING COSTS AND EXPENSES
Operating costs		355		713
As Previously Reported | Reimbursements
REVENUES
Revenues		4,488		8,828
OPERATING COSTS AND EXPENSES
Operating costs		4,488		72
Adjustment | Adoption of ASU 2014-09
REVENUES
Revenues		(584)		(1,233)
OPERATING COSTS AND EXPENSES
Depreciation, amortization, and other		(14)		(1,341)
General, administrative, and other		8		(28)
Merger-related costs and charges		0		10
Costs and expenses		(708)		(1,366)
OPERATING INCOME		124		133
Gains and other income, net		0		0
Interest expense		0		0
Interest income		0		0
Equity in earnings		0		0
INCOME BEFORE INCOME TAXES		124		133
Income taxes		(49)		(52)
NET INCOME		$ 75		$ 81
EARNINGS PER SHARE
Earnings per share - basic (in USD per share)		$ 0.20		$ 0.21
Earnings per share - diluted (in USD per share)		$ 0.20		$ 0.21
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		(8)		(18)
Adjustment | Incentive management fees | Adoption of ASU 2014-09
REVENUES
Gross revenues		7		(6)
Adjustment | Fee revenues | Adoption of ASU 2014-09
REVENUES
Gross revenues		(1)		(24)
Contract investment amortization		(12)		(23)
Revenues		(13)		(47)
Adjustment | Owned, leased, and other | Adoption of ASU 2014-09
REVENUES
Revenues		(10)		(21)
OPERATING COSTS AND EXPENSES
Operating costs		(5)		(7)
Adjustment | Reimbursements | Adoption of ASU 2014-09
REVENUES
Revenues		(561)		(1,165)
OPERATING COSTS AND EXPENSES
Operating costs		$ (697)		$ 0