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Cover Page - USD ($)
$ in Billions
 12 Months Ended
Dec. 31, 2019
Jan. 31, 2020
Jun. 30, 2019
Cover page.      
Document Type 10-K    
Document Annual Report true    
Document Transition Report false    
Entity File Number 1-14443    
Entity Registrant Name GARTNER, INC.    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 04-3099750    
Entity Address, Address Line One P.O. Box 10212    
Entity Address, Address Line Two 56 Top Gallant Road    
Entity Address, City or Town Stamford,    
Entity Address, State or Province CT    
Entity Address, Postal Zip Code 06902-7700    
City Area Code 203    
Local Phone Number 316-1111    
Title of 12(b) Security Common Stock, $0.0005 par value per share    
Trading Symbol IT    
Security Exchange Name NYSE    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Central Index Key 0000749251    
Current Fiscal Year End Date --12-31    
Entity Filer Category Large Accelerated Filer    
Entity Emerging Growth Company false    
Entity Small Business false    
Entity Shell Company false    
Document Period End Date Dec. 31, 2019    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Amendment Flag false    
Entity Public Float     $ 14.1
Entity Common Stock, Shares Outstanding   89,101,606  
Documents Incorporated by Reference
The definitive Proxy Statement for the Annual Meeting of Stockholders to be held on June 8, 2020 is incorporated by reference into Part III to the extent described therein.