| Document and Entity Information | Dec. 10, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K/A |
| Amendment Flag | true |
| Amendment Description | This Form 8-K/A is being filed as an amendment (“Amendment No. 1”) to the Current Report on Form 8-K filed by MSCI Inc. (the “Company”) under Item 5.03 on December 11, 2020 to amend Item 5.03 and refile the Company’s Amended and Restated Bylaws (as amended and restated, the “Bylaws”) filed as Exhibit 3.1 to correct a typographical error in Exhibit 3.1 to the Original Form 8-K. The corrected Bylaws are filed as Exhibit 3.1 hereto and supersede and replace in their entirety the Bylaws filed as Exhibit 3.1 to the Original Form 8-K. |
| Document Period End Date | Dec. 10, 2020 |
| Entity Registrant Name | MSCI Inc. |
| Entity Central Index Key | 0001408198 |
| Entity Emerging Growth Company | false |
| Entity File Number | 001-33812 |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 13-4038723 |
| Entity Address, Address Line One | 7 World Trade Center |
| Entity Address, Address Line Two | 250 Greenwich St., 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 |
| Current Fiscal Year End Date | --12-31 |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | MSCI |
| Security Exchange Name | NYSE |
