| Document and Entity Information | Mar. 12, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001408198 |
| Document Type | 8-K |
| Document Period End Date | Mar. 12, 2021 |
| Entity Registrant Name | MSCI Inc. |
| Entity Incorporation State Country Code | DE |
| 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 |
| Entity Address, Address Line Three | 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, par value $0.01 per share |
| Trading Symbol | MSCI |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
