    <SEC-DOCUMENT>0000950170-24-007391-index.html : 20240125
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    <ACCEPTANCE-DATETIME>20240125160623
    ACCESSION NUMBER:		0000950170-24-007391
    CONFORMED SUBMISSION TYPE:	8-K
    PUBLIC DOCUMENT COUNT:		12
    CONFORMED PERIOD OF REPORT:	20240125
    ITEM INFORMATION:		Results of Operations and Financial Condition
    ITEM INFORMATION:		Regulation FD Disclosure
    ITEM INFORMATION:		Financial Statements and Exhibits
    FILED AS OF DATE:		20240125
    DATE AS OF CHANGE:		20240125

    FILER:

    	COMPANY DATA:
    		COMPANY CONFORMED NAME:			FINANCIAL INSTITUTIONS INC
    		CENTRAL INDEX KEY:			0000862831
    		STANDARD INDUSTRIAL CLASSIFICATION:	NATIONAL COMMERCIAL BANKS [6021]
    		ORGANIZATION NAME:           	02 Finance
    		IRS NUMBER:				160816610
    		STATE OF INCORPORATION:			NY
    		FISCAL YEAR END:			1231

    	FILING VALUES:
    		FORM TYPE:		8-K
    		SEC ACT:		1934 Act
    		SEC FILE NUMBER:	000-26481
    		FILM NUMBER:		24562200

    	BUSINESS ADDRESS:
    		STREET 1:		220 LIBERTY STREET
    		CITY:			WARSAW
    		STATE:			NY
    		ZIP:			14569
    		BUSINESS PHONE:		5857861100

    	MAIL ADDRESS:
    		STREET 1:		220 LIBERTY STREET
    		CITY:			WARSAW
    		STATE:			NY
    		ZIP:			14569
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    <TYPE>8-K
    <SEQUENCE>1
    <FILENAME>fisi-20240125.htm
    <DESCRIPTION>8-K
    <TEXT>
    Document 1 - file: fisi-20240125.htm

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    <DOCUMENT>
    <TYPE>EX-99.1
    <SEQUENCE>2
    <FILENAME>fisi-ex99_1.htm
    <DESCRIPTION>EX-99.1
    <TEXT>
    Document 2 - file: fisi-ex99_1.htm

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    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>3
    <FILENAME>img145321324_0.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
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    <DOCUMENT>
    <TYPE>EX-101.SCH
    <SEQUENCE>4
    <FILENAME>fisi-20240125.xsd
    <DESCRIPTION>XBRL TAXONOMY EXTENSION SCHEMA WITH EMBEDDED LINKBASES DOCUMENT
    <TEXT>
    Document 4 - file: fisi-20240125.xsd

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    <SEQUENCE>5
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    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
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    <DOCUMENT>
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    <FILENAME>fisi-20240125_htm.xml
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    <TEXT>
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    <DOCUMENT>
    <TYPE>EXCEL
    <SEQUENCE>7
    <FILENAME>Financial_Report.xlsx
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 7 - file: Financial_Report.xlsx

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    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>8
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    <TEXT>
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    <SEQUENCE>9
    <FILENAME>report.css
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    <TEXT>
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    <TYPE>XML
    <SEQUENCE>10
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    <TEXT>
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    <DOCUMENT>
    <TYPE>JSON
    <SEQUENCE>12
    <FILENAME>MetaLinks.json
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
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    <DOCUMENT>
    <TYPE>ZIP
    <SEQUENCE>13
    <FILENAME>0000950170-24-007391-xbrl.zip
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 13 - file: 0000950170-24-007391-xbrl.zip

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