| Document and Entity Information | Dec. 08, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Dec. 8, 2020 |
| Entity Registrant Name | BYLINE BANCORP, INC. |
| Entity Central Index Key | 0001702750 |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Entity File Number | 001-38139 |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 36-3012593 |
| Entity Address, Address Line One | 180 North LaSalle Street |
| Entity Address, Address Line Two | Suite 300 |
| Entity Address, City or Town | Chicago |
| Entity Address, State or Province | IL |
| Entity Address, Postal Zip Code | 60601 |
| City Area Code | 773 |
| Local Phone Number | 244-7000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of each class | Common Stock |
| Trading Symbol | BY |
| Name of each exchange on which registered | NYSE |
