SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Effect of Adoption of ASUs on Statement of Cash Flows (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
OPERATING ACTIVITIES
Net income	$ 317		$ 503	$ 667	$ 420		$ 114		$ 485	$ 489	$ 371	$ 1,907		$ 1,459		$ 808
Adjustments to reconcile to cash provided by operating activities:
Depreciation, amortization, and other												284		279		159
Share-based compensation												184		181		212
Income taxes												(239)		887		103
Contract acquisition costs												(152)		(185)		(76)
Merger-related charges												16		(124)		209
Working capital changes												(76)		(30)		(106)
(Gain) loss on asset dispositions												(194)		(687)		1
Other												107		149		88
Net cash provided by operating activities												2,357		2,227		1,619
INVESTING ACTIVITIES
Acquisition of a business, net of cash acquired												0		0		(2,392)
Capital expenditures												(556)		(240)		(199)
Dispositions												479		1,418		211
Loan advances												(13)		(93)		(32)
Loan collections												48		187		67
Contract acquisition costs														0		0
Other												(10)		(61)		(1)
Net cash (used in) provided by investing activities												(52)		1,211		(2,346)
FINANCING ACTIVITIES
Commercial paper/Credit Facility, net												(129)		60		1,373
Issuance of long-term debt												1,646		0		1,482
Repayment of long-term debt												(397)		(310)		(326)
Issuance of Class A Common Stock												4		6		34
Dividends paid												(543)		(482)		(374)
Purchase of treasury stock												(2,850)		(3,013)		(568)
Share-based compensation withholding taxes												(105)		(157)		(100)
Other												0		0		(24)
Net cash (used in) provided by financing activities												(2,374)		(3,896)		1,497
DECREASE IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH												(69)		(458)		770
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH, beginning of period					429	[1]					887	429	[1]	887		117
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH, end of period	$ 360	[1]					429	[1]				360	[1]	429	[1]	887
As Previously Reported
OPERATING ACTIVITIES
Net income			$ 483	$ 610	398									1,372		780
Adjustments to reconcile to cash provided by operating activities:
Depreciation, amortization, and other														290		168
Share-based compensation														181		212
Income taxes														828		76
Contract acquisition costs														0		0
Merger-related charges														(124)		113
Working capital changes														81		(77)
(Gain) loss on asset dispositions														(687)		1
Other														117		66
Net cash provided by operating activities														2,436		1,682
INVESTING ACTIVITIES
Acquisition of a business, net of cash acquired														0		(2,412)
Capital expenditures														(240)		(199)
Dispositions														1,418		218
Loan advances														(93)		(32)
Loan collections														187		67
Contract acquisition costs														(189)		(80)
Other														(63)		29
Net cash (used in) provided by investing activities														1,020		(2,409)
FINANCING ACTIVITIES
Commercial paper/Credit Facility, net														25		1,365
Issuance of long-term debt														0		1,482
Repayment of long-term debt														(310)		(326)
Issuance of Class A Common Stock														6		34
Dividends paid														(482)		(374)
Purchase of treasury stock														(3,013)		(568)
Share-based compensation withholding taxes														(157)		(100)
Other														0		(24)
Net cash (used in) provided by financing activities														(3,931)		1,489
DECREASE IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH														(475)		762
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH, beginning of period					383						858	383		858		96
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH, end of period							383							383		858
Accounting Standards Update 2014-09 | Adoption of ASU
OPERATING ACTIVITIES
Net income														87		28
Adjustments to reconcile to cash provided by operating activities:
Depreciation, amortization, and other														(11)		(9)
Share-based compensation														0		0
Income taxes														59		27
Contract acquisition costs														(185)		(76)
Merger-related charges														0		96
Working capital changes														(128)		(24)
(Gain) loss on asset dispositions														0		0
Other														67		30
Net cash provided by operating activities														(191)		(50)
INVESTING ACTIVITIES
Acquisition of a business, net of cash acquired														0		0
Capital expenditures														0		0
Dispositions														0		0
Loan advances														0		0
Loan collections														0		0
Contract acquisition costs														189		80
Other														2		(30)
Net cash (used in) provided by investing activities														191		50
FINANCING ACTIVITIES
Commercial paper/Credit Facility, net														0		0
Issuance of long-term debt														0		0
Repayment of long-term debt														0		0
Issuance of Class A Common Stock														0		0
Dividends paid														0		0
Purchase of treasury stock														0		0
Share-based compensation withholding taxes														0		0
Other														0		0
Net cash (used in) provided by financing activities														0		0
DECREASE IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH														0		0
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH, beginning of period					0						0	0		0		0
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH, end of period							0							0		0
Accounting Standards Updated 2016-18 and 2016-15 | Adoption of ASU
OPERATING ACTIVITIES
Net income														0		0
Adjustments to reconcile to cash provided by operating activities:
Depreciation, amortization, and other														0		0
Share-based compensation														0		0
Income taxes														0		0
Contract acquisition costs														0		0
Merger-related charges														0		0
Working capital changes														17		(5)
(Gain) loss on asset dispositions														0		0
Other														(35)		(8)
Net cash provided by operating activities														(18)		(13)
INVESTING ACTIVITIES
Acquisition of a business, net of cash acquired														0		20
Capital expenditures														0		0
Dispositions														0		(7)
Loan advances														0		0
Loan collections														0		0
Contract acquisition costs														0		0
Other														0		0
Net cash (used in) provided by investing activities														0		13
FINANCING ACTIVITIES
Commercial paper/Credit Facility, net														35		8
Issuance of long-term debt														0		0
Repayment of long-term debt														0		0
Issuance of Class A Common Stock														0		0
Dividends paid														0		0
Purchase of treasury stock														0		0
Share-based compensation withholding taxes														0		0
Other														0		0
Net cash (used in) provided by financing activities														35		8
DECREASE IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH														17		8
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH, beginning of period					$ 46						$ 29	46		29		21
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH, end of period							$ 46							46		29
Guest loyalty program
Adjustments to reconcile to cash provided by operating activities:
Liability for guest loyalty program												$ 520		298		221
Guest loyalty program | As Previously Reported
Adjustments to reconcile to cash provided by operating activities:
Liability for guest loyalty program														378		343
Guest loyalty program | Accounting Standards Update 2014-09 | Adoption of ASU
Adjustments to reconcile to cash provided by operating activities:
Liability for guest loyalty program														(80)		(122)
Guest loyalty program | Accounting Standards Updated 2016-18 and 2016-15 | Adoption of ASU
Adjustments to reconcile to cash provided by operating activities:
Liability for guest loyalty program														$ 0		$ 0

[1] The 2018 amounts include beginning restricted cash of $46 million at December 31, 2017, and ending restricted cash of $44 million at December 31, 2018, which we present in the “Prepaid expenses and other” and “Other noncurrent assets” captions of our Balance Sheets.