| Document And Entity Information | Mar. 21, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Mar. 21, 2021 |
| Entity Registrant Name | Financial Institutions, Inc. |
| Entity Central Index Key | 0000862831 |
| Entity Emerging Growth Company | false |
| Entity File Number | 0-26481 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock, par value $0.01 per share |
| Trading Symbol | FISI |
| Security Exchange Name | NASDAQ |
