Basis of Presentation (Policies)	9 Months Ended
Sep. 30, 2024
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation The interim Consolidated Financial Statements include the accounts of S&T Bancorp, Inc., or S&T, and its wholly owned subsidiaries. All significant intercompany transactions have been eliminated in consolidation. Investments of 20 percent to 50 percent of the outstanding common stock of investees are accounted for using the equity method of accounting.
Basis of Presentation	Basis of Presentation The accompanying unaudited interim Consolidated Financial Statements of S&T have been prepared in accordance with generally accepted accounting principles, or GAAP, in the United States for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements and should be read in conjunction with the audited Consolidated Financial Statements included in our Annual Report on Form 10-K for the year ended December 31, 2023, filed with the Securities and Exchange Commission, or SEC, on February 27, 2024 (2023 Form 10-K). In the opinion of management, the accompanying interim financial information reflects all adjustments, consisting of normal recurring adjustments, necessary to present fairly our financial position and the results of operations for each of the interim periods presented. Results of operations for interim periods are not necessarily indicative of the results of operations that may be expected for a full year or any future period.
Reclassification	Reclassification Amounts in prior period financial statements and footnotes are reclassified whenever necessary to conform to the current period presentation. Reclassifications had no effect on our consolidated financial statements.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities at the date of the financial statements. Actual results could differ from those estimates.
Recently Adopted Accounting Standards and Recently Issued Accounting Standards Not Yet Adopted	Recently Adopted Accounting Standards Investments Equity Method and Joint Ventures (Topic 323) Accounting for Investments in Tax Credit Structures Using the Proportional Amortization Method In March 2023, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, 2023-02, Investments Equity Method and Joint Ventures (Topic 323): Accounting for Investments in Tax Credit Structures Using the Proportional Amortization Method, or PAM, to allow reporting entities to consistently account for equity investments made primarily for the purpose of receiving income tax credits and other income tax benefits. If certain conditions are met, a reporting entity may elect to account for its tax equity investments using the PAM regardless of the program from which it receives income tax credits, instead of only using it for low-income-housing tax credit, or LIHTC, structures. This amendment also eliminates the ability to account for LIHTC investments using the cost method. The amendments in this update were effective for fiscal years beginning after December 15, 2023, and interim periods within those fiscal years. We adopted this ASU, as of January 1, 2024, using a modified retrospective transition approach, which resulted in a $1.0 million cumulative effect adjustment being recorded to retained earnings related to the transition of the cost method to the PAM on LIHTC partnerships. We also elected to apply PAM to our qualifying historic tax credit, or HTC, equity investments. Results for reporting periods beginning after January 1, 2024 are presented using the PAM, while prior period amounts continue to be reported in accordance with previously applicable GAAP. Under the previously applicable accounting guidance, tax credit investments were accounted for using the cost method. The investment was amortized on a straight-line basis over a maximum of 10 years, which represents the period over which the tax credits will be utilized. The amortization expense was recognized in other noninterest expense and the tax credits offset income tax expense. Under the PAM, the equity investment is amortized in proportion to the income tax credits and other income tax benefits received. The amortization expense and the income tax credits are required to be presented on a net basis in income tax expense on the Condensed Consolidated Statements of Comprehensive Income. Refer to Note 7 Tax Credit Equity Investments for additional disclosures. Recently Issued Accounting Standards Not Yet Adopted Segment Reporting (Topic 280) Improvements to Reportable Segment Disclosures In November 2023, the FASB issued ASU 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures to improve disclosure requirements, primarily through enhanced disclosures about significant segment expenses. This update does not change how a public entity identifies its operating segments; however, it does require that an entity that has a single reportable segment provide all the disclosures required by the amendments in this update. The amendments in this update are effective for fiscal years beginning after December 15, 2023, and interim periods within fiscal years beginning after December 15, 2024. A public entity should apply the amendments in this update retrospectively to all prior periods presented in the consolidated financial statements. Early adoption is permitted. We currently have one reportable operating segment, Community Banking. This ASU will not impact our consolidated financial statements and will have minimal impact to our disclosures, requiring identification of the chief operating decision maker and the information used to make operating decisions and to allocate resources. Income Taxes (Topic 740) Improvements to Income Tax Disclosures In December 2023, the FASB issued ASU 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures to enhance the transparency and decision usefulness of the disclosures. The amendments in this update address investor requests for more transparency about income tax information through improvements to disclosures primarily related to the rate reconciliation and income taxes paid information. The amendments in this update are effective for fiscal years beginning after December 15, 2024. Early adoption is permitted for annual consolidated financial statements that have not yet been issued. This ASU is not expected to have a significant impact on disclosures, and will not impact our consolidated financial statements.
Fair Value Measurements	We use fair value measurements when recording and disclosing certain financial assets and liabilities. Debt securities, equity securities, securities held in a deferred compensation plan and derivative financial instruments are recorded at fair value on a recurring basis. Additionally, from time to time, we may be required to record other financial instruments at fair value on a nonrecurring basis, such as loans held for sale, individually assessed loans, other real estate owned, or OREO, and other repossessed assets, mortgage servicing rights, or MSRs, and certain other assets. Fair value is the price that would be received to sell an asset or paid to transfer a liability in the principal or most advantageous market in an orderly transaction between market participants at the measurement date. An orderly transaction is a transaction that assumes exposure to the market for a period prior to the measurement date to allow for marketing activities that are usual and customary for transactions involving such assets or liabilities; it is not a forced transaction. In determining fair value, we use various valuation approaches, including market, income and cost approaches. The fair value standard establishes a hierarchy for inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that observable inputs be used when available. Observable inputs are inputs that market participants would use in pricing an asset or liability, which are developed based on market data that we have obtained from independent sources. Unobservable inputs reflect our estimates of assumptions that market participants would use in pricing an asset or liability, which are developed based on the best information available in the circumstances. The fair value hierarchy gives the highest priority to unadjusted quoted market prices in active markets for identical assets or liabilities (Level 1 measurement) and the lowest priority to unobservable inputs (Level 3 measurement). The fair value hierarchy is broken down into three levels based on the reliability of inputs as follows. Level 1: valuation is based upon unadjusted quoted market prices for identical instruments traded in active markets. Level 2: valuation is based upon quoted market prices for similar instruments traded in active markets, quoted market prices for identical or similar instruments traded in markets that are not active and model-based valuation techniques for which all significant assumptions are observable in the market or can be corroborated by market data. Level 3: valuation is derived from other valuation methodologies, including discounted cash flow models and similar techniques that use significant assumptions not observable in the market. These unobservable assumptions reflect estimates of assumptions that market participants would use in determining fair value. A financial instrument’s level within the fair value hierarchy is based on the lowest level of input that is significant to the fair value measurement. There have been no changes in our valuation methodologies during the three and nine months ended September 30, 2024. Refer to Note 1 Summary of Significant Accounting Policies of the Notes to Consolidated Financial Statements in our 2023 Form 10-K for more information on the valuation methodologies that we use for financial instruments recorded at fair value on a recurring or nonrecurring basis.
Allowance for Credit Losses	Allowance for Credit Losses We maintain an Allowance for Credit Losses, or ACL, at a level determined to be adequate to absorb estimated expected credit losses within the loan portfolio over the contractual life of an instrument that considers our historical loss experience, current conditions and forecasts of future economic conditions as of the balance sheet date. We develop and document a systematic ACL methodology based on the following portfolio segments: 1) CRE, 2) C&I, 3) Commercial Construction, 4) Business Banking, 5) Consumer Real Estate and 6) Other Consumer. The following are key risks within each portfolio segment: CRE—Loans secured by commercial purpose real estate, including both owner-occupied properties and investment properties for various purposes such as hotels, retail, multifamily and health care. Operations of the individual projects and global cash flows of the debtors are the primary sources of repayment for these loans. The condition of the local economy is an important indicator of risk, but there are also more specific risks depending on the collateral type and the business prospects of the lessee, if the project is not owner-occupied. C&I—Loans made to operating companies or manufacturers for the purpose of production, operating capacity, accounts receivable, inventory or equipment financing. Cash flow from the operations of the company is the primary source of repayment for these loans. The condition of the local economy is an important indicator of risk, but there are also more specific risks depending on the industry of the company. Collateral for these types of loans often does not have sufficient value in a distressed or liquidation scenario to satisfy the outstanding debt. Commercial Construction—Loans made to finance construction of buildings or other structures, as well as to finance the acquisition and development of raw land for various purposes. While these loans are generally confined to the construction/development period, if there are problems, the project may not be completed, and as such, may not provide sufficient cash flow on its own to service the debt or have sufficient value in a liquidation to cover the outstanding principal. The condition of the local economy is an important indicator of risk, but there are also more specific risks depending on the type of project and the experience and resources of the developer. Business Banking—Commercial purpose loans made to small businesses that are standard, non-complex products evaluated through a streamlined credit approval process that has been designed to maximize efficiency while maintaining high credit quality standards that meet small business market customers’ needs. The business banking portfolio is monitored by utilizing a standard and closely managed process focusing on behavioral and performance criteria. The condition of the local economy is an important indicator of risk, but there are also more specific risks depending on the collateral type and business. Consumer Real Estate—Loans secured by first and second liens such as 1-4 family residential mortgages, home equity loans and home equity lines of credit. The primary source of repayment for these loans is the income and assets of the borrower. The condition of the local economy, in particular the unemployment rate, is an important indicator of risk for this segment. The state of the local housing market can also have a significant impact on this segment because low demand and/or declining home values can limit the ability of borrowers to sell a property and satisfy the debt. Other Consumer—Loans made to individuals that may be secured by assets other than 1-4 family residences, as well as unsecured loans. This segment includes auto loans, unsecured loans and lines of credit. The primary source of repayment for these loans is the income and assets of the borrower. The condition of the local economy, in particular the unemployment rate, is an important indicator of risk for this segment. The value of the collateral, if there is any, is less likely to be a source of repayment due to less certain collateral values. Management monitors various credit quality indicators for the commercial, business banking and consumer loan portfolios, including changes in risk ratings, nonperforming status and delinquency on a monthly basis. We monitor the commercial loan portfolio through an internal risk rating system. Loan risk ratings are assigned based upon the creditworthiness of the borrower and are reviewed on an ongoing basis according to our internal policies. Loans within the pass rating generally have a lower risk of loss than loans risk rated as special mention or substandard. Our risk ratings are consistent with regulatory guidance and are as follows: Pass—The loan is currently performing and is of high quality. Special Mention—A special mention loan has potential weaknesses that warrant management’s close attention. If left uncorrected, these potential weaknesses may result in deterioration of the repayment prospects or in the strength of our credit position at some future date. Substandard—A substandard loan is not adequately protected by the net worth and/or paying capacity of the borrower or by the collateral pledged, if any. Substandard loans have a well-defined weakness or weaknesses that jeopardize the liquidation of the debt. These loans are characterized by the distinct possibility that we will sustain some loss if the deficiencies are not corrected. Doubtful—Loans classified doubtful have all the weaknesses inherent in those classified substandard with the added characteristic that the weaknesses make collection or liquidation in full, on the basis of currently known facts, conditions and values, highly questionable and improbable.

Basis of Presentation (Policies)

9 Months Ended

Sep. 30, 2024

The interim Consolidated Financial Statements include the accounts of S&T Bancorp, Inc., or S&T, and its wholly owned subsidiaries. All significant intercompany transactions have been eliminated in consolidation. Investments of 20 percent to 50 percent of the outstanding common stock of investees are accounted for using the equity method of accounting.

Basis of Presentation

The accompanying unaudited interim Consolidated Financial Statements of S&T have been prepared in accordance with generally accepted accounting principles, or GAAP, in the United States for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements and should be read in conjunction with the audited Consolidated Financial Statements included in our Annual Report on Form 10-K for the year ended December 31, 2023, filed with the Securities and Exchange Commission, or SEC, on February 27, 2024 (2023 Form 10-K). In the opinion of management, the accompanying interim financial information reflects all adjustments, consisting of normal recurring adjustments, necessary to present fairly our financial position and the results of operations for each of the interim periods presented. Results of operations for interim periods are not necessarily indicative of the results of operations that may be expected for a full year or any future period.

Reclassification

Amounts in prior period financial statements and footnotes are reclassified whenever necessary to conform to the current period presentation. Reclassifications had no effect on our consolidated financial statements.

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities at the date of the financial statements. Actual results could differ from those estimates.

Recently Adopted Accounting Standards and Recently Issued Accounting Standards Not Yet Adopted

Recently Adopted Accounting Standards

Investments Equity Method and Joint Ventures (Topic 323) Accounting for Investments in Tax Credit Structures Using the Proportional Amortization Method

In March 2023, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, 2023-02, Investments Equity Method and Joint Ventures (Topic 323): Accounting for Investments in Tax Credit Structures Using the Proportional Amortization Method, or PAM, to allow reporting entities to consistently account for equity investments made primarily for the purpose of receiving income tax credits and other income tax benefits. If certain conditions are met, a reporting entity may elect to account for its tax equity investments using the PAM regardless of the program from which it receives income tax credits, instead of only using it for low-income-housing tax credit, or LIHTC, structures. This amendment also eliminates the ability to account for LIHTC investments using the cost method. The amendments in this update were effective for fiscal years beginning after December 15, 2023, and interim periods within those fiscal years. We adopted this ASU, as of January 1, 2024, using a modified retrospective transition approach, which resulted in a $1.0 million cumulative effect adjustment being recorded to retained earnings related to the transition of the cost method to the PAM on LIHTC partnerships. We also elected to apply PAM to our qualifying historic tax credit, or HTC, equity investments. Results for reporting periods beginning after January 1, 2024 are presented using the PAM, while prior period amounts continue to be reported in accordance with previously applicable GAAP. Under the previously applicable accounting guidance, tax credit investments were accounted for using the cost method. The investment was amortized on a straight-line basis over a maximum of 10 years, which represents the period over which the tax credits will be utilized. The amortization expense was recognized in other noninterest expense and the tax credits offset income tax expense. Under the PAM, the equity investment is amortized in proportion to the income tax credits and other income tax benefits received. The amortization expense and the income tax credits are required to be presented on a net basis in income tax expense on the Condensed Consolidated Statements of Comprehensive Income. Refer to Note 7 Tax Credit Equity Investments for additional disclosures.

Recently Issued Accounting Standards Not Yet Adopted

Segment Reporting (Topic 280) Improvements to Reportable Segment Disclosures

In November 2023, the FASB issued ASU 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures to improve disclosure requirements, primarily through enhanced disclosures about significant segment expenses. This update does not change how a public entity identifies its operating segments; however, it does require that an entity that has a single reportable segment provide all the disclosures required by the amendments in this update. The amendments in this update are effective for fiscal years beginning after December 15, 2023, and interim periods within fiscal years beginning after December 15, 2024. A public entity should apply the amendments in this update retrospectively to all prior periods presented in the consolidated financial statements. Early adoption is permitted. We currently have one reportable operating segment, Community Banking. This ASU will not impact our consolidated financial statements and will have minimal impact to our disclosures, requiring identification of the chief operating decision maker and the information used to make operating decisions and to allocate resources.

Income Taxes (Topic 740) Improvements to Income Tax Disclosures

In December 2023, the FASB issued ASU 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures to enhance the transparency and decision usefulness of the disclosures. The amendments in this update address investor requests for more transparency about income tax information through improvements to disclosures primarily related to the rate reconciliation and income taxes paid information. The amendments in this update are effective for fiscal years beginning after December 15, 2024. Early adoption is permitted for annual consolidated financial statements that have not yet been issued. This ASU is not expected to have a significant impact on disclosures, and will not impact our consolidated financial statements.

Fair Value Measurements

We use fair value measurements when recording and disclosing certain financial assets and liabilities. Debt securities, equity securities, securities held in a deferred compensation plan and derivative financial instruments are recorded at fair value on a recurring basis. Additionally, from time to time, we may be required to record other financial instruments at fair value on a nonrecurring basis, such as loans held for sale, individually assessed loans, other real estate owned, or OREO, and other repossessed assets, mortgage servicing rights, or MSRs, and certain other assets.

Fair value is the price that would be received to sell an asset or paid to transfer a liability in the principal or most advantageous market in an orderly transaction between market participants at the measurement date. An orderly transaction is a transaction that assumes exposure to the market for a period prior to the measurement date to allow for marketing activities that are usual and customary for transactions involving such assets or liabilities; it is not a forced transaction. In determining fair value, we use various valuation approaches, including market, income and cost approaches. The fair value standard establishes a hierarchy for inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that observable inputs be used when available. Observable inputs are inputs that market participants would use in pricing an asset or liability, which are developed based on market data that we have obtained from independent sources. Unobservable inputs reflect our estimates of assumptions that market participants would use in pricing an asset or liability, which are developed based on the best information available in the circumstances.

The fair value hierarchy gives the highest priority to unadjusted quoted market prices in active markets for identical assets or liabilities (Level 1 measurement) and the lowest priority to unobservable inputs (Level 3 measurement). The fair value hierarchy is broken down into three levels based on the reliability of inputs as follows.

Level 1: valuation is based upon unadjusted quoted market prices for identical instruments traded in active markets.

Level 2: valuation is based upon quoted market prices for similar instruments traded in active markets, quoted market prices for identical or similar instruments traded in markets that are not active and model-based valuation techniques for which all significant assumptions are observable in the market or can be corroborated by market data.

Level 3: valuation is derived from other valuation methodologies, including discounted cash flow models and similar techniques that use significant assumptions not observable in the market. These unobservable assumptions reflect estimates of assumptions that market participants would use in determining fair value.

A financial instrument’s level within the fair value hierarchy is based on the lowest level of input that is significant to the fair value measurement.

There have been no changes in our valuation methodologies during the three and nine months ended September 30, 2024. Refer to Note 1 Summary of Significant Accounting Policies of the Notes to Consolidated Financial Statements in our 2023 Form 10-K for more information on the valuation methodologies that we use for financial instruments recorded at fair value on a recurring or nonrecurring basis.

Allowance for Credit Losses

We maintain an Allowance for Credit Losses, or ACL, at a level determined to be adequate to absorb estimated expected credit losses within the loan portfolio over the contractual life of an instrument that considers our historical loss experience, current conditions and forecasts of future economic conditions as of the balance sheet date. We develop and document a systematic ACL methodology based on the following portfolio segments: 1) CRE, 2) C&I, 3) Commercial Construction, 4) Business Banking, 5) Consumer Real Estate and 6) Other Consumer.

The following are key risks within each portfolio segment:

CRE—Loans secured by commercial purpose real estate, including both owner-occupied properties and investment properties for various purposes such as hotels, retail, multifamily and health care. Operations of the individual projects and global cash flows of the debtors are the primary sources of repayment for these loans. The condition of the local economy is an important indicator of risk, but there are also more specific risks depending on the collateral type and the business prospects of the lessee, if the project is not owner-occupied.

C&I—Loans made to operating companies or manufacturers for the purpose of production, operating capacity, accounts receivable, inventory or equipment financing. Cash flow from the operations of the company is the primary source of repayment for these loans. The condition of the local economy is an important indicator of risk, but there are also more specific risks depending on the industry of the company. Collateral for these types of loans often does not have sufficient value in a distressed or liquidation scenario to satisfy the outstanding debt.

Commercial Construction—Loans made to finance construction of buildings or other structures, as well as to finance the acquisition and development of raw land for various purposes. While these loans are generally confined to the construction/development period, if there are problems, the project may not be completed, and as such, may not provide sufficient cash flow on its own to service the debt or have sufficient value in a liquidation to cover the outstanding principal. The condition of the local economy is an important indicator of risk, but there are also more specific risks depending on the type of project and the experience and resources of the developer.

Business Banking—Commercial purpose loans made to small businesses that are standard, non-complex products evaluated through a streamlined credit approval process that has been designed to maximize efficiency while maintaining high credit quality standards that meet small business market customers’ needs. The business banking portfolio is monitored by utilizing a standard and closely managed process focusing on behavioral and performance criteria. The condition of the local economy is an important indicator of risk, but there are also more specific risks depending on the collateral type and business.

Consumer Real Estate—Loans secured by first and second liens such as 1-4 family residential mortgages, home equity loans and home equity lines of credit. The primary source of repayment for these loans is the income and assets of the borrower. The condition of the local economy, in particular the unemployment rate, is an important indicator of risk for this segment. The state of the local housing market can also have a significant impact on this segment because low demand and/or declining home values can limit the ability of borrowers to sell a property and satisfy the debt.

Other Consumer—Loans made to individuals that may be secured by assets other than 1-4 family residences, as well as unsecured loans. This segment includes auto loans, unsecured loans and lines of credit. The primary source of repayment for these loans is the income and assets of the borrower. The condition of the local economy, in particular the unemployment rate, is an important indicator of risk for this segment. The value of the collateral, if there is any, is less likely to be a source of repayment due to less certain collateral values.

Management monitors various credit quality indicators for the commercial, business banking and consumer loan portfolios, including changes in risk ratings, nonperforming status and delinquency on a monthly basis.

We monitor the commercial loan portfolio through an internal risk rating system. Loan risk ratings are assigned based upon the creditworthiness of the borrower and are reviewed on an ongoing basis according to our internal policies. Loans within the pass rating generally have a lower risk of loss than loans risk rated as special mention or substandard.

Our risk ratings are consistent with regulatory guidance and are as follows:

Pass—The loan is currently performing and is of high quality.

Special Mention—A special mention loan has potential weaknesses that warrant management’s close attention. If left uncorrected, these potential weaknesses may result in deterioration of the repayment prospects or in the strength of our credit position at some future date.

Substandard—A substandard loan is not adequately protected by the net worth and/or paying capacity of the borrower or by the collateral pledged, if any. Substandard loans have a well-defined weakness or weaknesses that jeopardize the liquidation of the debt. These loans are characterized by the distinct possibility that we will sustain some loss if the deficiencies are not corrected.

Doubtful—Loans classified doubtful have all the weaknesses inherent in those classified substandard with the added characteristic that the weaknesses make collection or liquidation in full, on the basis of currently known facts, conditions and values, highly questionable and improbable.