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Document And Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2024
Feb. 28, 2025
Jun. 30, 2024
Cover [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2024    
Document Fiscal Period Focus FY    
Document Fiscal Year Focus 2024    
Trading Symbol FISI    
Entity Registrant Name Financial Institutions, Inc.    
Entity Central Index Key 0000862831    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer No    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
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 16-0816610    
Entity Incorporation, State or Country Code NY    
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    
Entity Common Stock, Shares Outstanding   20,077,893  
Entity Interactive Data Current Yes    
Document Annual Report true    
Document Transition Report false    
Title of 12(b) Security Common stock, par value $0.01 per share    
Security Exchange Name NASDAQ    
ICFR Auditor Attestation Flag true    
Entity Public Float     $ 290,557,000
Documents Incorporated by Reference

Portions of the registrant’s proxy statement for the 2025 Annual Meeting of Shareholders are incorporated by reference in Part III of this Annual Report on Form 10-K.

    
Auditor Name RSM US LLP    
Auditor Location Chicago, Illinois    
Auditor Firm ID 49    
Document Financial Statement Error Correction Flag false