Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2021	Feb. 04, 2022	Jun. 30, 2021
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Trading Symbol	MSCI
Entity Registrant Name	MSCI INC.
Entity Central Index Key	0001408198
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		81,268,195
Entity Current Reporting Status	Yes
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity File Number	001-33812
Entity Tax Identification Number	13-4038723
Entity Address, Address Line One	7 World Trade Center
Entity Address, Address Line Two	250 Greenwich Street, 49th Floor
Entity Address, City or Town	New York
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10007
City Area Code	212
Local Phone Number	804-3900
Entity Interactive Data Current	Yes
Entity Incorporation, State or Country Code	DE
Title of 12(b) Security	Common stock, par value $0.01 per share
Security Exchange Name	NYSE
Document Annual Report	true
Document Transition Report	false
Entity Public Float			$ 43,427,568,686
Documents Incorporated by Reference	Portions of the registrant’s proxy statement for its annual meeting of stockholders, to be held on April 26, 2022, are incorporated herein by reference into Part III of this Form 10-K.
Auditor Firm ID	238
Auditor Name	PricewaterhouseCoopers LLP
Auditor Location	New York, New York