Document and Entity Information - shares
9 Months Ended
Sep. 30, 2019
Nov. 04, 2019
Cover [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Sep. 30, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q3  
Document Quarterly Report true  
Document Transition Report false  
Title of 12(b) Security Common Stock, $.01 Par Value  
Trading Symbol FORR  
Security Exchange Name NASDAQ  
Entity Registrant Name FORRESTER RESEARCH, INC.  
Entity Central Index Key 0001023313  
Current Fiscal Year End Date --12-31  
Entity Filer Category Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Common Stock, Shares Outstanding   18,622,000
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Shell Company false  
Entity Incorporation, State or Country Code DE  
Entity File Number 000-21433  
Entity Tax Identification Number 04-2797789  
Entity Address, Address Line One 60 Acorn Park Drive  
Entity Address, City or Town CAMBRIDGE  
Entity Address, State or Province MA  
Entity Address, Postal Zip Code 02140  
City Area Code 617  
Local Phone Number 613-6000