Basis of Presentation (Policies)	6 Months Ended
Jun. 30, 2026
Accounting Policies [Abstract]
Basis of Presentation	These unaudited interim condensed consolidated financial statements include the accounts of Byline Bancorp, Inc., a Delaware corporation (the "Company," "Byline," "we," "us," "our"), a bank holding company whose principal activity is the ownership and management of its Illinois state chartered subsidiary bank, Byline Bank (the "Bank"), based in Chicago, Illinois. These unaudited interim condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America ("GAAP") for interim financial information and in accordance with the instructions to Form 10-Q and Article 10 of Regulation S-X as promulgated by the Securities and Exchange Commission ("SEC"). In preparing these financial statements, the Company has evaluated events and transactions subsequent to June 30, 2026 for potential recognition or disclosure. In the opinion of management, all adjustments (consisting of normal recurring accruals) considered necessary for a fair presentation of the financial position and results of operations for the periods presented have been included. Certain information in footnote disclosures normally included in financial statements prepared in accordance with GAAP has been condensed or omitted pursuant to the rules and regulations of the SEC and the accounting standards for interim financial statements. These financial statements should be read in conjunction with the consolidated financial statements and notes thereto included in the Company’s Consolidated Financial Statements for the years ended December 31, 2025 and 2024. The Company has one reportable segment. The Company’s chief operating decision makers evaluate the operations of the Company using consolidated information for purposes of allocating resources and assessing performance. Refer to Note 21—Segment Information for additional disclosures. In accordance with the Financial Accounting Standards Board ("FASB") Accounting Standards Codification ("ASC") Topic 855, "Subsequent Events," the Company’s management has evaluated subsequent events for potential recognition or disclosure through the date of the issuance of these condensed consolidated financial statements. No subsequent events were identified that would have required a change to the condensed consolidated financial statements or disclosure in the notes to the condensed consolidated financial statements. Certain prior period amounts have been reclassified to conform to current period presentation. These reclassifications did not result in any changes to previously reported net income or stockholders’ equity.
Accounting Pronouncements Recently Adopted or Issued	The following reflect recent accounting pronouncements that have been adopted or are pending adoption by the Company. Adopted Accounting Pronouncements Financial Instruments – Credit Losses (Topic 326) – Purchased Loans – In November 2025, the FASB issued ASU 2025-08 to change the accounting for certain acquired purchased seasoned loans ("PSL") by applying the gross‑up method, which records an allowance for expected credit losses at acquisition as an adjustment to amortized cost basis rather than a Day 1 provision through earnings. The guidance is intended to simplify post‑acquisition accounting, reduce inconsistency between PCD and non-PCD loans, and eliminate Day 1 credit loss expense for in‑scope PSLs. The amendments are effective for public business entities for annual periods beginning after December 15, 2026, with early adoption permitted. As of January 1, 2026, the Company adopted this guidance on a prospective basis. Early adoption will eliminate Day 1 credit loss expense on future eligible purchased loans and provide consistency in accounting with PCD loans. Derivatives and Hedging (Topic 815) – Hedge Accounting Improvements – In November 2025, the FASB issued ASU 2025-09 to better align hedge accounting with entities’ risk management activities. Key amendments include expanding the ability to group forecasted transactions with similar (rather than identical) risk exposure, establishing a model for hedging interest payments on choose‑your‑rate debt, expanding hedge accounting for certain forecasted nonfinancial transactions, and updating guidance on net written options and foreign‑currency‑denominated debt. The amendments are effective for public business entities for annual periods beginning after December 15, 2026, with early adoption permitted. As of January 1, 2026, the Company adopted this guidance to provide additional flexibility in forecasting hedge transactions, which included making certain adjustments to our existing designation documentation for active hedging relationships to take advantage of specific provisions in the new guidance. Issued Accounting Pronouncements Pending Adoption Income Statement (Topic 220) – In November 2024, the FASB issued ASU 2024-03, Income Statement (Topic 220) – Reporting Comprehensive Income – Expense Disaggregation Disclosures; to address requests from investors for more detailed information about certain expense types. The standard requires that public business entities disclose disaggregated information about specific relevant natural expense categories underlying certain income statement expense line items. The ASU requires entities to disaggregate any relevant expense caption presented on the face of the income statement within continuing operations into the following required natural expense categories, as applicable: (1) purchases of inventory, (2) employee compensation, (3) depreciation, (4) intangible asset amortization, and (5) depreciation, depletion, and amortization recognized as part of oil- and gas-producing activities or other depletion expenses. The amendments are effective for public business entities for annual periods beginning after December 15, 2026, and interim periods within fiscal years beginning after December 15, 2027. In January 2025, the FASB issued ASU 2025-01 to amend the effective date of ASU 2024-03 to clarify that all public business entities are required to adopt the guidance in annual reporting periods beginning after December 15, 2026, and interim periods within annual reporting periods beginning after December 15, 2027. Entities are required to adopt prospectively. The Company is evaluating the accounting and disclosure requirements of this update and does not expect them to have a material effect on the consolidated financial statements.

Basis of Presentation (Policies)

6 Months Ended

Jun. 30, 2026

These unaudited interim condensed consolidated financial statements include the accounts of Byline Bancorp, Inc., a Delaware corporation (the "Company," "Byline," "we," "us," "our"), a bank holding company whose principal activity is the ownership and management of its Illinois state chartered subsidiary bank, Byline Bank (the "Bank"), based in Chicago, Illinois.

These unaudited interim condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America ("GAAP") for interim financial information and in accordance with the instructions to Form 10-Q and Article 10 of Regulation S-X as promulgated by the Securities and Exchange Commission ("SEC"). In preparing these financial statements, the Company has evaluated events and transactions subsequent to June 30, 2026 for potential recognition or disclosure. In the opinion of management, all adjustments (consisting of normal recurring accruals) considered necessary for a fair presentation of the financial position and results of operations for the periods presented have been included. Certain information in footnote disclosures normally included in financial statements prepared in accordance with GAAP has been condensed or omitted pursuant to the rules and regulations of the SEC and the accounting standards for interim financial statements. These financial statements should be read in conjunction with the consolidated financial statements and notes thereto included in the Company’s Consolidated Financial Statements for the years ended December 31, 2025 and 2024.

The Company has one reportable segment. The Company’s chief operating decision makers evaluate the operations of the Company using consolidated information for purposes of allocating resources and assessing performance. Refer to Note 21—Segment Information for additional disclosures.

In accordance with the Financial Accounting Standards Board ("FASB") Accounting Standards Codification ("ASC") Topic 855, "Subsequent Events," the Company’s management has evaluated subsequent events for potential recognition or disclosure through the date of the issuance of these condensed consolidated financial statements. No subsequent events were identified that would have required a change to the condensed consolidated financial statements or disclosure in the notes to the condensed consolidated financial statements.

Certain prior period amounts have been reclassified to conform to current period presentation. These reclassifications did not result in any changes to previously reported net income or stockholders’ equity.

Accounting Pronouncements Recently Adopted or Issued

The following reflect recent accounting pronouncements that have been adopted or are pending adoption by the Company.

Adopted Accounting Pronouncements

Financial Instruments – Credit Losses (Topic 326) – Purchased Loans – In November 2025, the FASB issued ASU 2025-08 to change the accounting for certain acquired purchased seasoned loans ("PSL") by applying the gross‑up method, which records an allowance for expected credit losses at acquisition as an adjustment to amortized cost basis rather than a Day 1 provision through earnings. The guidance is intended to simplify post‑acquisition accounting, reduce inconsistency between PCD and non-PCD loans, and eliminate Day 1 credit loss expense for in‑scope PSLs. The amendments are effective for public business entities for annual periods beginning after December 15, 2026, with early adoption permitted. As of January 1, 2026, the Company adopted this guidance on a prospective basis. Early adoption will eliminate Day 1 credit loss expense on future eligible purchased loans and provide consistency in accounting with PCD loans.

Derivatives and Hedging (Topic 815) – Hedge Accounting Improvements – In November 2025, the FASB issued ASU 2025-09 to better align hedge accounting with entities’ risk management activities. Key amendments include expanding the ability to group forecasted transactions with similar (rather than identical) risk exposure, establishing a model for hedging interest payments on choose‑your‑rate debt, expanding hedge accounting for certain forecasted nonfinancial transactions, and updating guidance on net written options and foreign‑currency‑denominated debt. The amendments are effective for public business entities for annual periods beginning after December 15, 2026, with early adoption permitted. As of January 1, 2026, the Company adopted this guidance to provide additional flexibility in forecasting hedge transactions, which included making certain adjustments to our existing designation documentation for active hedging relationships to take advantage of specific provisions in the new guidance.

Issued Accounting Pronouncements Pending Adoption

Income Statement (Topic 220) – In November 2024, the FASB issued ASU 2024-03, Income Statement (Topic 220) – Reporting Comprehensive Income – Expense Disaggregation Disclosures; to address requests from investors for more detailed information about certain expense types. The standard requires that public business entities disclose disaggregated information about specific relevant natural expense categories underlying certain income statement expense line items. The ASU requires entities to disaggregate any relevant expense caption presented on the face of the income statement within continuing operations into the following required natural expense categories, as applicable: (1) purchases of inventory, (2) employee compensation, (3) depreciation, (4) intangible asset amortization, and (5) depreciation, depletion, and amortization recognized as part of oil- and gas-producing activities or other depletion expenses. The amendments are effective for public business entities for annual periods beginning after December 15, 2026, and interim periods within fiscal years beginning after December 15, 2027. In January 2025, the FASB issued ASU 2025-01 to amend the effective date of ASU 2024-03 to clarify that all public business entities are required to adopt the guidance in annual reporting periods beginning after December 15, 2026, and interim periods within annual reporting periods beginning after December 15, 2027. Entities are required to adopt prospectively. The Company is evaluating the accounting and disclosure requirements of this update and does not expect them to have a material effect on the consolidated financial statements.