    <SEC-DOCUMENT>0000950170-25-036259-index.html : 20250310
    <SEC-HEADER>0000950170-25-036259.hdr.sgml : 20250310
    <ACCEPTANCE-DATETIME>20250310160609
    ACCESSION NUMBER:		0000950170-25-036259
    CONFORMED SUBMISSION TYPE:	8-K/A
    PUBLIC DOCUMENT COUNT:		12
    CONFORMED PERIOD OF REPORT:	20241231
    ITEM INFORMATION:		Results of Operations and Financial Condition
    ITEM INFORMATION:		Regulation FD Disclosure
    FILED AS OF DATE:		20250310
    DATE AS OF CHANGE:		20250310

    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/A
    		SEC ACT:		1934 Act
    		SEC FILE NUMBER:	000-26481
    		FILM NUMBER:		25723686

    	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
    </SEC-HEADER>
    <DOCUMENT>
    <TYPE>8-K/A
    <SEQUENCE>1
    <FILENAME>fisi-20241231.htm
    <DESCRIPTION>8-K/A
    <TEXT>
    Document 1 - file: fisi-20241231.htm

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>2
    <FILENAME>img39886387_0.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 2 - file: img39886387_0.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>EX-101.SCH
    <SEQUENCE>3
    <FILENAME>fisi-20241231.xsd
    <DESCRIPTION>XBRL TAXONOMY EXTENSION SCHEMA WITH EMBEDDED LINKBASES DOCUMENT
    <TEXT>
    Document 3 - file: fisi-20241231.xsd

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>5
    <FILENAME>R1.htm
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 5 - file: R1.htm

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>EXCEL
    <SEQUENCE>6
    <FILENAME>Financial_Report.xlsx
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 6 - file: Financial_Report.xlsx

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>7
    <FILENAME>Show.js
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 7 - file: Show.js

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>8
    <FILENAME>report.css
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 8 - file: report.css

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>10
    <FILENAME>FilingSummary.xml
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 10 - file: FilingSummary.html

    Document 10 - RAW XML: FilingSummary.xml

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>JSON
    <SEQUENCE>12
    <FILENAME>MetaLinks.json
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 12 - file: MetaLinks.json

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>ZIP
    <SEQUENCE>13
    <FILENAME>0000950170-25-036259-xbrl.zip
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 13 - file: 0000950170-25-036259-xbrl.zip

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>14
    <FILENAME>fisi-20241231_htm.xml
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 14 - file: fisi-20241231_htm.html

    Document 14 - RAW XML: fisi-20241231_htm.xml

    </DOCUMENT>
    </SEC-DOCUMENT>