Basis of Presentation (Policies)	9 Months Ended
Sep. 30, 2020
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 September 30, 2020 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, 2019, 2018, and 2017. 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. The Company has one reportable segment. The Company’s chief operating decision maker evaluates the operations of the Company using consolidated information for purposes of allocating resources and assessing performance. Therefore, segments disclosures are not required. 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. As the Company qualifies as an emerging growth company and has elected the extended transition period for complying with new or revised accounting pronouncements, it is not subject to new or revised accounting standards applicable to public companies during the extended transition period. The accounting pronouncements pending adoption below reflect effective dates for the Company as an emerging growth company with the extended transition period. Adopted Accounting Pronouncement Nonrefundable Fees and Other Costs (Subtopic 310‑20)—In March 2017, FASB issued ASU No. 2017‑08, Receivables—Nonrefundable Fees and Other Costs. The amendments in the ASU shorten the amortization period for certain callable debt securities held at a premium at the earliest call date. Under current GAAP, the Company amortizes the premium as an adjustment of yield over the contractual life of the instrument. As a result, upon exercise of a call on a callable debt security held at a premium, the unamortized premium is charged to earnings. The ASU shortens the amortization period for certain callable debt securities held at a premium and requires the premium to be amortized to the earliest call date. However, the amendments do not require an accounting change for securities held at a discount; the discount continues to be amortized to maturity. The amendments are effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. The Company is required to apply the amendments on a modified retrospective basis through a cumulative-effect adjustment directly to retained earnings as of the beginning of the period of adoption. Given our emerging growth status, the Company adopted these amendments on January 1, 2020 in conjunction with ASU No. 2017-08, which did not have a material impact on the Company’s Consolidated Financial Statements. Issued Accounting Pronouncements Pending Adoption Leases (Topic 842)—In February 2016, FASB issued ASU No. 2016‑02, Leases. The amendments in this ASU require lessees to recognize the following for all leases (with the exception of short-term) at the commencement date: a lease liability, which is a lessee’s obligation to make lease payments arising from a lease, measured on a discounted basis; and a right-of-use asset, which is an asset that represents the lessee’s right to use, or control the use of, a specified asset for the lease term. The amendments in this ASU leave lessor accounting largely unchanged, although certain targeted improvements were made to align lessor accounting with the lessee accounting model. This ASU simplifies the accounting for sale and leaseback transactions primarily because lessees must recognize lease assets and lease liabilities. The amendments in this ASU are effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. Early application is permitted upon issuance. Lessees (for capital and operating leases) and lessors (for sales-type, direct financing, and operating leases) must apply a modified retrospective transition approach for leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements. The modified retrospective approach would not require any transition accounting for leases that expired before the earliest comparative period presented. Lessees and lessors may not apply a full retrospective transition approach. The Company is evaluating the new guidance and its impact on the Company’s Consolidated Statements of Operations and Consolidated Statements of Financial Condition. In November 2019, FASB issued ASU No. 2019-10, Effective Dates, which delays the effective date of this ASU for entities not classified as Public Business Entities (PBEs). Our status as an emerging growth company makes us eligible for this deferral. Assuming the Company remains an emerging growth company, the new authoritative guidance is effective for fiscal years beginning after December 15, 2020, and interim periods with fiscal years beginning after December 15, 2021. The Company expects an increase in assets and liabilities as a result of recognizing additional right-of-use assets and liabilities under lease contracts in which the Company is lessee. While the Company has not quantified the impact of this ASU on its direct financing lease portfolio, it does not expect a material change in its accounting for the initial direct costs related to these leases. Financial Instruments—Credit Losses (Topic 326)—In June 2016, FASB issued ASU No. 2016‑13, Measurement of Credit Losses on Financial Instruments. Current GAAP requires an “incurred loss” methodology for recognizing credit losses that delays recognition until it is probable a loss has been incurred. The main objective of this ASU is to provide financial statement users with more decision-useful information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. The amendments in this ASU replace the incurred loss impairment methodology in current GAAP with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. The amendments in this ASU require a financial asset (or group of financial assets) measured at amortized cost basis to be presented at the net amount expected to be collected. The allowance for credit losses is a valuation account that is deducted from the amortized cost basis of the financial asset(s) to present the net carrying value at the amount expected to be collected on the financial asset. The measurement of expected credit losses will be based on relevant information about past events, including historical experience, current conditions, and reasonable and supportable forecasts that affect the collectability of the reported amount. The amendments in this ASU broaden the information that an entity must consider in developing its expected credit loss estimate for assets measured either collectively or individually. The use of forecasted information incorporates more timely information in the estimate of expected credit loss, which will be more useful to users of the financial statements. In November 2019, FASB issued ASU No. 2019-10, Effective Dates, which delays the effective date of the ASU for entities not classified as PBEs. Assuming the Company remains an emerging growth company, the new authoritative guidance is effective for fiscal years beginning after December 15, 2022, including interim periods within those fiscal years. The Company is in the process of implementation and determining the impact that this ASU will have on the Company’s Consolidated Financial Statements. Income Taxes (Topic 740)—In December 2019, FASB issued ASU No. 2019-12, Simplifying the Accounting for Income Taxes. The amendments in the ASU simplify the accounting for income taxes by removing the following: the exception to the incremental approach for intraperiod tax allocation when there is a loss from continuing operations and income or a gain from other items; the exception to the requirement to or not to recognize a deferred tax liability for a foreign entity when it becomes an equity method investment or it becomes a subsidiary, respectively; and the exception to the general methodology for calculating income taxes in an interim period when a year-to-date loss exceeds the anticipated loss for the year. The amendments in the ASU changes current authoritative guidance by requiring the recognition of franchise tax that is partially based on income as an income-based tax and account for any incremental amount incurred as a non-income-based tax; requiring an evaluation when a step up in the tax basis of goodwill should be considered part the of business combination; specifying that it is not required to allocate the consolidated amount of current and deferred tax expense to a legal entity that is not subject to tax in its separate financial statements; and requiring that an entity reflect the effect of an enacted change in tax laws or rates in the annual effective tax rate computation in the interim period that includes the enactment date. The amendments are effective for annual periods beginning after December 15, 2021, and interim periods within annual periods beginning after December 15, 2022. Early adoption is permitted. Assuming the Company remains an emerging growth company, the new authoritative guidance will be effective for reporting periods after January 1, 2022. The Company is currently evaluating the provisions of ASU No. 2019-12 to determine the potential impact the new standard will have on the Company’s Consolidated Financial Statements. Reference Rate Reform (Topic 848)—In March 2020, FASB issued ASU No. 2020-04, Facilitation of the Effects of Reference Rate Reform on Financial Reporting. The amendments in the ASU provide optional guidance for a limited period of time to ease the potential burden in accounting for (or recognizing the effects of) reference rate reform on financial reporting. The amendments in the ASU provide optional expedients and exceptions for applying generally accepted accounting principles to contract modifications and hedging relationships, subject to meeting certain criteria, that reference LIBOR or another reference rate expected to be discontinued. The ASU is intended to help stakeholders during the global market-wide reference rate transition period. The amendments in the ASU will be in effect for all entities as of March 12, 2020 through December 31, 2022. The Company is currently evaluating the provisions of ASU No. 2020-04 to determine the potential impact the new standard will have on the Company’s Consolidated Financial Statements.

Basis of Presentation (Policies)

9 Months Ended

Sep. 30, 2020

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 September 30, 2020 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, 2019, 2018, and 2017.

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.

The Company has one reportable segment. The Company’s chief operating decision maker evaluates the operations of the Company using consolidated information for purposes of allocating resources and assessing performance. Therefore, segments disclosures are not required.

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. As the Company qualifies as an emerging growth company and has elected the extended transition period for complying with new or revised accounting pronouncements, it is not subject to new or revised accounting standards applicable to public companies during the extended transition period. The accounting pronouncements pending adoption below reflect effective dates for the Company as an emerging growth company with the extended transition period.

Adopted Accounting Pronouncement

Nonrefundable Fees and Other Costs (Subtopic 310‑20)—In March 2017, FASB issued ASU No. 2017‑08, Receivables—Nonrefundable Fees and Other Costs. The amendments in the ASU shorten the amortization period for certain callable debt securities held at a premium at the earliest call date. Under current GAAP, the Company amortizes the premium as an adjustment of yield over the contractual life of the instrument. As a result, upon exercise of a call on a callable debt security held at a premium, the unamortized premium is charged to earnings. The ASU shortens the amortization period for certain callable debt securities held at a premium and requires the premium to be amortized to the earliest call date. However, the amendments do not require an accounting change for securities held at a discount; the discount continues to be amortized to maturity. The amendments are effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. The Company is required to apply the amendments on a modified retrospective basis through a cumulative-effect adjustment directly to retained earnings as of the beginning of the period of adoption. Given our emerging growth status, the Company adopted these amendments on January 1, 2020 in conjunction with ASU No. 2017-08, which did not have a material impact on the Company’s Consolidated Financial Statements.

Issued Accounting Pronouncements Pending Adoption

Leases (Topic 842)—In February 2016, FASB issued ASU No. 2016‑02, Leases. The amendments in this ASU require lessees to recognize the following for all leases (with the exception of short-term) at the commencement date: a lease liability, which is a lessee’s obligation to make lease payments arising from a lease, measured on a discounted basis; and a right-of-use asset, which is an asset that represents the lessee’s right to use, or control the use of, a specified asset for the lease term. The amendments in this ASU leave lessor accounting largely unchanged, although certain targeted improvements were made to align lessor accounting with the lessee accounting model. This ASU simplifies the accounting for sale and leaseback transactions primarily because lessees must recognize lease assets and lease liabilities. The amendments in this ASU are effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. Early application is permitted upon issuance. Lessees (for capital and operating leases) and lessors (for sales-type, direct financing, and operating leases) must apply a modified retrospective transition approach for leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements. The modified retrospective approach would not require any transition accounting for leases that expired before the earliest comparative period presented. Lessees and lessors may not apply a full retrospective transition approach. The Company is evaluating the new guidance and its impact on the Company’s Consolidated Statements of Operations and Consolidated Statements of Financial Condition. In November 2019, FASB issued ASU No. 2019-10, Effective Dates, which delays the effective date of this ASU for entities not classified as Public Business Entities (PBEs). Our status as an emerging growth company makes us eligible for this deferral. Assuming the Company remains an emerging growth company, the new authoritative guidance is effective for fiscal years beginning after December 15, 2020, and interim periods with fiscal years beginning after December 15, 2021. The Company expects an increase in assets and liabilities as a result of recognizing additional right-of-use assets and liabilities under lease contracts in which the Company is lessee. While the Company has not quantified the impact of this ASU on its direct financing lease portfolio, it does not expect a material change in its accounting for the initial direct costs related to these leases.

Financial Instruments—Credit Losses (Topic 326)—In June 2016, FASB issued ASU No. 2016‑13, Measurement of Credit Losses on Financial Instruments. Current GAAP requires an “incurred loss” methodology for recognizing credit losses that delays recognition until it is probable a loss has been incurred. The main objective of this ASU is to provide financial statement users with more decision-useful information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. The amendments in this ASU replace the incurred loss impairment methodology in current GAAP with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. The amendments in this ASU require a financial asset (or group of financial assets) measured at amortized cost basis to be presented at the net amount expected to be collected. The allowance for credit losses is a valuation account that is deducted from the amortized cost basis of the financial asset(s) to present the net carrying value at the amount expected to be collected on the financial asset. The measurement of expected credit losses will be based on relevant information about past events, including historical experience, current conditions, and reasonable and supportable forecasts that affect the collectability of the reported amount. The amendments in this ASU broaden the information that an entity must consider in developing its expected credit loss estimate for assets measured either collectively or individually. The use of forecasted information incorporates more timely information in the estimate of expected credit loss, which will be more useful to users of the financial statements. In November 2019, FASB issued ASU No. 2019-10, Effective Dates, which delays the effective date of the ASU for entities not classified as PBEs. Assuming the Company remains an emerging growth company, the new authoritative guidance is effective for fiscal years beginning after December 15, 2022, including interim periods within those fiscal years. The Company is in the process of implementation and determining the impact that this ASU will have on the Company’s Consolidated Financial Statements.

Income Taxes (Topic 740)—In December 2019, FASB issued ASU No. 2019-12, Simplifying the Accounting for Income Taxes. The amendments in the ASU simplify the accounting for income taxes by removing the following: the exception to the incremental approach for intraperiod tax allocation when there is a loss from continuing operations and income or a gain from other items; the exception to the requirement to or not to recognize a deferred tax liability for a foreign entity when it becomes an equity method investment or it becomes a subsidiary, respectively; and the exception to the general methodology for calculating income taxes in an interim period when a year-to-date loss exceeds the anticipated loss for the year. The amendments in the ASU changes current authoritative guidance by requiring the recognition of franchise tax that is partially based on income as an income-based tax and account for any incremental amount incurred as a non-income-based tax; requiring an evaluation when a step up in the tax basis of goodwill should be considered part the of business combination; specifying that it is not required to allocate the consolidated amount of current and deferred tax expense to a legal entity that is not subject to tax in its separate financial statements; and requiring that an entity reflect the effect of an enacted change in tax laws or rates in the annual effective tax rate computation in the interim period that includes the

enactment date. The amendments are effective for annual periods beginning after December 15, 2021, and interim periods within annual periods beginning after December 15, 2022. Early adoption is permitted. Assuming the Company remains an emerging growth company, the new authoritative guidance will be effective for reporting periods after January 1, 2022. The Company is currently evaluating the provisions of ASU No. 2019-12 to determine the potential impact the new standard will have on the Company’s Consolidated Financial Statements.

Reference Rate Reform (Topic 848)—In March 2020, FASB issued ASU No. 2020-04, Facilitation of the Effects of Reference Rate Reform on Financial Reporting. The amendments in the ASU provide optional guidance for a limited period of time to ease the potential burden in accounting for (or recognizing the effects of) reference rate reform on financial reporting. The amendments in the ASU provide optional expedients and exceptions for applying generally accepted accounting principles to contract modifications and hedging relationships, subject to meeting certain criteria, that reference LIBOR or another reference rate expected to be discontinued. The ASU is intended to help stakeholders during the global market-wide reference rate transition period. The amendments in the ASU will be in effect for all entities as of March 12, 2020 through December 31, 2022. The Company is currently evaluating the provisions of ASU No. 2020-04 to determine the potential impact the new standard will have on the Company’s Consolidated Financial Statements.