Employee Benefit Plans	12 Months Ended
Dec. 31, 2017
Defined Contribution Pension And Other Postretirement Plans Disclosure [Abstract]
Employee Benefit Plans	Note 16— Employee Benefit Plans The Company’s defined contribution 401(k) savings plan (the “Plan”) covers substantially all employees that have completed certain service requirements. The Board of Directors determines the amount of any discretionary profit sharing contribution made to the plan. There were no profit sharing contributions to the Plan for the years ended December 31, 2017 and 2016. The net assets of the plan are not included in the Consolidated Statements of Financial Condition. As of February 1, 2016, a 401(k) employer match contribution was adopted by the Company, pursuant to which the Company will make a contribution equal to 50% of the first 4% contributed by employees. Total expense for the employer contributions made to the Plan was $655,000 during the year ended December 31, 2016. Effective January 1, 2017, the employer match was increased to equal 100% of the first 3% and 50% for the next 2% contributed by employees. Total expense for the employer contributions made to the Plan was $1.5 million during the year ended December 31, 2017. The Company’s board of directors adopted the Byline Bancorp, Inc. Employee Stock Purchase Plan (the “ESPP”) in connection with the IPO. The ESPP will allow employees to purchase shares of common stock at a discount from the market price through automatic payroll deductions. A total of 200,000 shares of common stock are reserved and available for sale under the ESPP, subject to adjustment in accordance with the terms of the ESPP. The Company intends for the ESPP to be qualified under Section 423 of the Code. The plan offering was in the first quarter of 2018, and there was no compensation expense for the years ended December 31, 2017, 2016, or 2015.

Note 16— Employee Benefit Plans

The Company’s defined contribution 401(k) savings plan (the “Plan”) covers substantially all employees that have completed certain service requirements. The Board of Directors determines the amount of any discretionary profit sharing contribution made to the plan. There were no profit sharing contributions to the Plan for the years ended December 31, 2017 and 2016. The net assets of the plan are not included in the Consolidated Statements of Financial Condition.

As of February 1, 2016, a 401(k) employer match contribution was adopted by the Company, pursuant to which the Company will make a contribution equal to 50% of the first 4% contributed by employees. Total expense for the employer contributions made to the Plan was $655,000 during the year ended December 31, 2016. Effective January 1, 2017, the employer match was increased to equal 100% of the first 3% and 50% for the next 2% contributed by employees. Total expense for the employer contributions made to the Plan was $1.5 million during the year ended December 31, 2017.

The Company’s board of directors adopted the Byline Bancorp, Inc. Employee Stock Purchase Plan (the “ESPP”) in connection with the IPO. The ESPP will allow employees to purchase shares of common stock at a discount from the market price through automatic payroll deductions. A total of 200,000 shares of common stock are reserved and available for sale under the ESPP, subject to adjustment in accordance with the terms of the ESPP. The Company intends for the ESPP to be qualified under Section 423 of the Code. The plan offering was in the first quarter of 2018, and there was no compensation expense for the years ended December 31, 2017, 2016, or 2015.