Document And Entity Information - shares	9 Months Ended
	Sep. 30, 2019	Oct. 31, 2019
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2019
Document Fiscal Period Focus	Q3
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	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
Entity Common Stock, Shares Outstanding		15,998,339
Entity Interactive Data Current	Yes
Document Quarterly Report	true
Document Transition Report	false
Title of 12(b) Security	Common stock, par value $0.01 per share
Security Exchange Name	NASDAQ
Entity Incorporation, State or Country Code	NY