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Document And Entity Information - shares
 6 Months Ended
Jun. 30, 2019
Jul. 31, 2019
Document And Entity Information [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Jun. 30, 2019  
Document Fiscal Period Focus Q2  
Document Fiscal Year Focus 2019  
Trading Symbol FISI  
Entity Registrant Name FINANCIAL INSTITUTIONS INC  
Entity Central Index Key 0000862831  
Current Fiscal Year End Date --12-31  
Entity Current Reporting Status Yes  
Entity Filer Category Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity File Number 000-26481  
Entity Tax Identification Number 160816610  
Entity Address, Address Line One 220 LIBERTY STREET  
Entity Address, City or Town WARSAW  
Entity Address, State or Province NEW YORK  
Entity Address, Postal Zip Code 14569  
City Area Code 585  
Local Phone Number 786-1100  
Entity Common Stock, Shares Outstanding   15,994,697