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Document And Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2019
Mar. 09, 2020
Jun. 28, 2019
Cover [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2019    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Entity Registrant Name Byline Bancorp, Inc.    
Entity Central Index Key 0001702750    
Current Fiscal Year End Date --12-31    
Entity Filer Category Accelerated Filer    
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    
Entity Well-known Seasoned Issuer No    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Interactive Data Current Yes    
Entity Public Float     $ 484,533,744
Entity Common Stock, Shares Outstanding   38,443,504  
Entity Shell Company false    
Entity Small Business false    
Entity Emerging Growth Company true    
Entity Ex Transition Period false    
Document Annual Report true    
Document Transition Report false    
Documents Incorporated by Reference

Portions of the Registrant’s Definitive Proxy Statement relating to its 2020 Annual Meeting of Stockholders, scheduled to be held on June 9, 2020, are incorporated by reference into Part III of this Report.

    
Title of each class Common Stock    
Trading Symbol BY    
Name of each exchange on which registered NYSE