v2.4.0.6
Document And Entity Information (USD $)
12 Months Ended
Dec. 31, 2011
Jun. 30, 2011
Feb. 23, 2012
Common Class A [Member]
Feb. 23, 2012
Common Class B [Member]
Document Type 10-K      
Amendment Flag false      
Document Period End Date Dec. 31, 2011      
Document Fiscal Year Focus 2011      
Document Fiscal Period Focus FY      
Entity Registrant Name MSCI Inc.      
Entity Central Index Key 0001408198      
Current Fiscal Year End Date --12-31      
Entity Filer Category Large Accelerated Filer      
Entity Common Stock, Shares Outstanding     121,457,571 0
Entity Public Float   $ 4,461,163,352    
Entity Voluntary Filers No      
Entity Current Reporting Status Yes      
Entity Well-known Seasoned Issuer Yes