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Document and Entity Information
Dec. 08, 2023
Cover [Abstract]  
Document Type 8-K
Amendment Flag false
Document Period End Date Dec. 08, 2023
Entity File Number 000-26481
Entity Registrant Name FINANCIAL INSTITUTIONS INC
Entity Central Index Key 0000862831
Entity Incorporation, State or Country Code NY
Entity Tax Identification Number 16-0816610
Entity Address, Address Line One 220 Liberty Street
Entity Address, City or Town Warsaw
Entity Address, State or Province NY
Entity Address, Postal Zip Code 14569
City Area Code 585
Local Phone Number 786-1100
Title of 12(b) Security Common stock, par value $0.01 per share
Trading Symbol FISI
Security Exchange Name NASDAQ
Entity Emerging Growth Company false
Written Communications false
Soliciting Material false
Pre-commencement Tender Offer false
Pre-commencement Issuer Tender Offer false