Basis of Presentation	9 Months Ended
Sep. 30, 2019
Accounting Policies [Abstract]
BASIS OF PRESENTATION	BASIS OF PRESENTATION 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 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, 2018, filed with the Securities and Exchange Commission, or SEC, on February 21, 2019. 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. On June 5, 2019, S&T and DNB Financial Corporation, or DNB, based in Downingtown, Pennsylvania with 14 branches in Chester, Delaware and Philadelphia counties and approximately $1.2 billion in assets as of June 30, 2019, entered into a definitive Agreement and Plan of Merger pursuant to which DNB will merge with and into S&T, with S&T continuing as the surviving entity. Under the terms of the Agreement and Plan of Merger, DNB shareholders will have the right to receive 1.22 shares of common stock, par value $2.50 per share, of S&T for each share of common stock, par value $1.00 per share, of DNB. Immediately following the merger of DNB and S&T, DNB’s wholly owned bank subsidiary, DNB First, National Association, will merge with and into S&T’s wholly owned bank subsidiary, S&T Bank, with S&T Bank as the surviving entity. The Agreement and Plan of Merger was unanimously approved by the Board of Directors of each of S&T and DNB. DNB shareholders approved the merger at a Special Meeting of Shareholders on September 25, 2019. All required bank regulatory approvals have been received for the merger. The transaction is expected to be effective on or about November 30, 2019 and remains subject to the satisfaction or waiver of other customary closing conditions. On January 1, 2018, we sold a 70 percent majority interest in the assets of our wholly-owned subsidiary S&T Evergreen Insurance, LLC. We transferred our remaining 30 percent ownership interest in the net assets of S&T Evergreen Insurance, LLC to a new entity for a 30 percent ownership interest in a new insurance entity (see Note 15: Sale of a Majority Interest of Insurance Business). We use the equity method of accounting to recognize our partial ownership interest in the new entity. 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 results of operations or financial condition. 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 Updates, or ASU or Update Leases - Section A-Amendments to the FASB Accounting Standards Codification, Section B-Conforming Amendments Related to Leases and Section C-Background Information and Basis for Conclusions In February 2016, the Financial Accounting Standards Board, or FASB, established ASC Topic 842, by issuing ASU No. 2016-02, which requires lessees to recognize leases on the balance sheet and disclose key information about leasing arrangements. Topic 842 was subsequently amended by ASU No. 2018-01, Land Easement Practical Expedient for Transition to Topic 842; ASU No. 2018-10, Codification Improvements to Topic 842, Leases; and ASU No. 2018-11, Targeted Improvements. The new standard establishes a right-of-use, or ROU, model that requires a lessee to recognize ROU assets and lease liabilities on the balance sheet. Leases will be classified as finance or operating leases, with classification affecting the pattern and classification of expense recognition in the statement of operations. We adopted the new standard on January 1, 2019 (see Note 7: Right-of-use Assets and Lease Liabilities). The new standard provides several optional practical expedients to elect in transition to the new lease guidance. We have elected the "package of practical expedients," which permit us not to reassess under the new standard our prior conclusions about lease identification, lease classification and initial direct costs. We elected the "use-of-hindsight" practical expedient which allows us to use hindsight in judgments that impact the lease term. We have also elected an accounting policy not to restate comparative periods upon adoption. The most significant effects of adopting the new standard relate to the recognition of ROU assets and lease liabilities on our balance sheet for our real estate leases and providing significant new disclosures about our leasing activities. The carrying value of our ROU assets will be tested annually for impairment or more frequently if events or changes in circumstances indicate that an impairment might exist. Upon adoption, we recognized additional finance lease liabilities of approximately $1.2 million and operating lease liabilities, net of deferred rent, of approximately $33.7 million based on the present value of the remaining minimum rental payments under current leasing standards for existing leases. We also recognized corresponding finance ROU assets of $1.2 million and operating ROU assets of approximately $33.4 million. The adoption had no material impact on the Consolidated Statements of Comprehensive Income. The new standard also provides practical expedients for our ongoing lease accounting. We elected the short-term lease recognition exemption for all leases with terms of 12 months or less. This means that we will not recognize ROU assets or lease liabilities for existing short-term leases of those assets in transition. Beginning in 2019, we made changes to our disclosed lease recognition policies and practices, as well as to other related financial statement disclosures due to the adoption of this standard (See Note 7: Right-of-use Assets and Lease Liabilities). Leases - Land Easement Practical Expedient for Transition to Topic 842 In January 2018, the FASB issued ASU No. 2018-01, Leases - Land Easement Practical Expedient for Transition to Topic 842. The amendments in this ASU permit an entity to elect an optional transition practical expedient to not evaluate under Topic 842 land easements that existed or expired before the entity's adoption of Topic 842 and that were not previously accounted for as leases under Topic 840. We have one land easement lease that we previously accounted for under Topic 840; as such, this lease has been recognized as an operating lease under Topic 842. We adopted the amendments in this ASU in conjunction with the adoption of the new lease standard, ASU 2016-02. Accounting Standards Issued But Not Yet Adopted Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract In August 2018, the FASB issued ASU No. 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. The amendments in this ASU apply to an entity that is a customer in a hosting arrangement that is a service contract. These amendments relate to accounting for implementation costs (e.g., implementation, setup and other upfront costs). These amendments require an entity in a hosting arrangement that is a service contract to follow the guidance in Subtopic 350-40 to determine which costs to capitalize and which costs to expense. These amendments require the entity to expense the capitalized implementation costs of a hosting arrangement that is a service contract over the term of the hosting arrangement. This ASU is effective for annual and interim periods beginning after December 15, 2019. Early adoption of the amendments is permitted, including adoption in any interim period. The amendments in this ASU will not materially impact our Consolidated Balance Sheets or Consolidated Statements of Comprehensive Income. Compensation—Retirement Benefits—Defined Benefit Plans—General (Subtopic 715-20): Changes to the Disclosure Requirements for Defined Benefit Plans In August 2018, the FASB issued ASU No. 2018-14, Compensation—Retirement Benefits—Defined Benefit Plans—General (Subtopic 715-20): Changes to the Disclosure Requirements for Defined Benefit Plans. The amendments in this ASU apply to all employers that sponsor defined benefit pension or other postretirement plans. These amendments remove certain disclosures from Topic 715-20 and require additional disclosures. The amendments in this ASU will require S&T to update our employee benefits disclosures beginning with our Form 10-Q for the period ended March 31, 2021. The amendments in this ASU will have no impact on our Consolidated Balance Sheets or Consolidated Statements of Comprehensive Income. Fair Value Measurement - Changes to the Disclosure Requirements for Fair Value Measurement In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement - Changes to the Disclosure Requirements for Fair Value Measurement. The amendments in this ASU remove certain disclosures from Topic 820, modify disclosures and/or require additional disclosures. The amendments in this Update will require us to change our Fair Value disclosures beginning with our Form 10-Q for the period ended March 31, 2020. The amendments in this ASU will have no impact on our Consolidated Balance Sheets or Consolidated Statements of Comprehensive Income. Intangibles - Goodwill and Other - Simplifying the Test for Goodwill Impairment In January 2017, the FASB issued ASU No. 2017-04, Intangibles - Goodwill and Other - Simplifying the Test for Goodwill Impairment (Topic 350). The main objective of this ASU is to simplify the current requirements for testing goodwill for impairment by eliminating step two from the goodwill impairment test. The amendments are expected to reduce the complexity and costs associated with performing the goodwill impairment test, which could result in recording impairment charges sooner than under the current guidance. This Update is effective for any interim and annual impairment tests in reporting periods in fiscal years beginning after December 15, 2019. Early adoption is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. The amendments in this ASU are not expected to have any impact on our Consolidated Balance Sheets or Consolidated Statements of Comprehensive Income. Financial Instruments - Credit Losses In June 2016, the FASB issued ASU No. 2016-13, Measurement of Credit Losses on Financial Instruments. 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 of this Update 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 form credit loss estimates. The collective changes to the recognition and measurement accounting standards for financial instruments and their anticipated impact on the allowance for credit losses modeling have been universally referred to as CECL, or current expected credit loss model. Credit losses related to available-for-sale debt securities (regardless of whether the impairment is considered to be other-than-temporary) will be measured in a manner similar to the present, except that such losses will be recorded as allowances rather than as reductions in the amortized cost of the related securities. This Update is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2019. Early adoption is permitted for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. We did not adopt this ASU at its early adoption date. Our CECL Committee governs the implementation of these amendments and the Committee consists of key stakeholders from Credit Administration, Finance, Accounting, Risk Management and Audit and Advisory Services. We engaged a third-party consultant to assist us in developing our CECL methodology and we have completed a preliminary CECL model. The Model includes our loan segmentation and the quantitative and qualitative components of the calculation which incorporates a forecasting component of certain economic variables. We are currently working to finalize our documentation around the CECL methodology, designing and updating internal controls and financial statement disclosures. We will run parallel credit loss models for both the third and fourth quarters of 2019 prior to adoption of the CECL standard. The CECL model will be reviewed and validated by an independent consultant during the fourth quarter of 2019 and recommendations will be reviewed and implemented prior to adoption on January 1, 2020. While we anticipate an increase in the allowance for credit losses upon adoption of CECL, we are still in the process of validating the results of our model in order to determine the financial impact. The impact of adopting CECL depends on various factors including, the mix of the loan portfolio, forecasted macroeconomic variables at the time of adoption and the assessment of loans acquired from DNB. We will continue to evaluate the provisions of this ASU to determine the potential impact on our Consolidated Balance Sheets and Consolidated Statements of Comprehensive Income and expect to disclose such impact in our 2019 Form 10-K.

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.

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, 2018, filed with the Securities and Exchange Commission, or SEC, on February 21, 2019. 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.

On June 5, 2019, S&T and DNB Financial Corporation, or DNB, based in Downingtown, Pennsylvania with 14 branches in Chester, Delaware and Philadelphia counties and approximately $1.2 billion in assets as of June 30, 2019, entered into a definitive Agreement and Plan of Merger pursuant to which DNB will merge with and into S&T, with S&T continuing as the surviving entity. Under the terms of the Agreement and Plan of Merger, DNB shareholders will have the right to receive 1.22 shares of common stock, par value $2.50 per share, of S&T for each share of common stock, par value $1.00 per share, of DNB. Immediately following the merger of DNB and S&T, DNB’s wholly owned bank subsidiary, DNB First, National Association, will merge with and into S&T’s wholly owned bank subsidiary, S&T Bank, with S&T Bank as the surviving entity. The Agreement and Plan of Merger was unanimously approved by the Board of Directors of each of S&T and DNB. DNB shareholders approved the merger at a Special Meeting of Shareholders on September 25, 2019. All required bank regulatory approvals have been received for the merger. The transaction is expected to be effective on or about November 30, 2019 and remains subject to the satisfaction or waiver of other customary closing conditions.

On January 1, 2018, we sold a 70 percent majority interest in the assets of our wholly-owned subsidiary S&T Evergreen Insurance, LLC. We transferred our remaining 30 percent ownership interest in the net assets of S&T Evergreen Insurance, LLC to a new entity for a 30 percent ownership interest in a new insurance entity (see Note 15: Sale of a Majority Interest of Insurance Business). We use the equity method of accounting to recognize our partial ownership interest in the new entity.

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.

In February 2016, the Financial Accounting Standards Board, or FASB, established ASC Topic 842, by issuing ASU No. 2016-02, which requires lessees to recognize leases on the balance sheet and disclose key information about leasing arrangements. Topic 842 was subsequently amended by ASU No. 2018-01, Land Easement Practical Expedient for Transition to Topic 842; ASU No. 2018-10, Codification Improvements to Topic 842, Leases; and ASU No. 2018-11, Targeted Improvements. The new standard establishes a right-of-use, or ROU, model that requires a lessee to recognize ROU assets and lease liabilities on the balance sheet. Leases will be classified as finance or operating leases, with classification affecting the

The new standard provides several optional practical expedients to elect in transition to the new lease guidance. We have elected the "package of practical expedients," which permit us not to reassess under the new standard our prior conclusions about lease identification, lease classification and initial direct costs. We elected the "use-of-hindsight" practical expedient which allows us to use hindsight in judgments that impact the lease term. We have also elected an accounting policy not to restate comparative periods upon adoption.

The most significant effects of adopting the new standard relate to the recognition of ROU assets and lease liabilities on our balance sheet for our real estate leases and providing significant new disclosures about our leasing activities. The carrying value of our ROU assets will be tested annually for impairment or more frequently if events or changes in circumstances indicate that an impairment might exist.

Upon adoption, we recognized additional finance lease liabilities of approximately $1.2 million and operating lease liabilities, net of deferred rent, of approximately $33.7 million based on the present value of the remaining minimum rental payments under current leasing standards for existing leases. We also recognized corresponding finance ROU assets of $1.2 million and operating ROU assets of approximately $33.4 million. The adoption had no material impact on the Consolidated Statements of Comprehensive Income.

The new standard also provides practical expedients for our ongoing lease accounting. We elected the short-term lease recognition exemption for all leases with terms of 12 months or less. This means that we will not recognize ROU assets or lease liabilities for existing short-term leases of those assets in transition. Beginning in 2019, we made changes to our disclosed lease recognition policies and practices, as well as to other related financial statement disclosures due to the adoption of this standard (See Note 7: Right-of-use Assets and Lease Liabilities).

In January 2018, the FASB issued ASU No. 2018-01, Leases - Land Easement Practical Expedient for Transition to Topic 842. The amendments in this ASU permit an entity to elect an optional transition practical expedient to not evaluate under Topic 842 land easements that existed or expired before the entity's adoption of Topic 842 and that were not previously accounted for as leases under Topic 840. We have one land easement lease that we previously accounted for under Topic 840; as such, this lease has been recognized as an operating lease under Topic 842. We adopted the amendments in this ASU in conjunction with the adoption of the new lease standard, ASU 2016-02.

In August 2018, the FASB issued ASU No. 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. The amendments in this ASU apply to an entity that is a customer in a hosting arrangement that is a service contract. These amendments relate to accounting for implementation costs (e.g., implementation, setup and other upfront costs). These amendments require an entity in a hosting arrangement that is a service contract to follow the guidance in Subtopic 350-40 to determine which costs to capitalize and which costs to expense. These amendments require the entity to expense the capitalized implementation costs of a hosting arrangement that is a service contract over the term of the hosting arrangement. This ASU is effective for annual and interim periods beginning after December 15, 2019. Early adoption of the amendments is permitted, including adoption in any interim period. The amendments in this ASU will not materially impact our Consolidated Balance Sheets or Consolidated Statements of Comprehensive Income.

In August 2018, the FASB issued ASU No. 2018-14, Compensation—Retirement Benefits—Defined Benefit Plans—General (Subtopic 715-20): Changes to the Disclosure Requirements for Defined Benefit Plans. The amendments in this ASU apply to all employers that sponsor defined benefit pension or other postretirement plans. These amendments remove certain disclosures from Topic 715-20 and require additional disclosures. The amendments in this ASU will require S&T to update our employee benefits disclosures beginning with our Form 10-Q for the period ended March 31, 2021. The amendments in this ASU will have no impact on our Consolidated Balance Sheets or Consolidated Statements of Comprehensive Income.

In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement - Changes to the Disclosure Requirements for Fair Value Measurement. The amendments in this ASU remove certain disclosures from Topic 820, modify disclosures and/or require additional disclosures. The amendments in this Update will require us to change our Fair Value disclosures beginning with our Form 10-Q for the period ended March 31, 2020. The amendments in this ASU will have no impact on our Consolidated Balance Sheets or Consolidated Statements of Comprehensive Income.

In January 2017, the FASB issued ASU No. 2017-04, Intangibles - Goodwill and Other - Simplifying the Test for Goodwill Impairment (Topic 350). The main objective of this ASU is to simplify the current requirements for testing goodwill for impairment by eliminating step two from the goodwill impairment test. The amendments are expected to reduce the complexity and costs associated with performing the goodwill impairment test, which could result in recording impairment charges sooner than under the current guidance. This Update is effective for any interim and annual impairment tests in reporting periods in fiscal years beginning after December 15, 2019. Early adoption is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. The amendments in this ASU are not expected to have any impact on our Consolidated Balance Sheets or Consolidated Statements of Comprehensive Income.

In June 2016, the FASB issued ASU No. 2016-13, Measurement of Credit Losses on Financial Instruments. 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 of this Update 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 form credit loss estimates. The collective changes to the recognition and measurement accounting standards for financial instruments and their anticipated impact on the allowance for credit losses modeling have been universally referred to as CECL, or current expected credit loss model. Credit losses related to available-for-sale debt securities (regardless of whether the impairment is considered to be other-than-temporary) will be measured in a manner similar to the present, except that such losses will be recorded as allowances rather than as reductions in the amortized cost of the related securities. This Update is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2019. Early adoption is permitted for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. We did not adopt this ASU at its early adoption date.

Our CECL Committee governs the implementation of these amendments and the Committee consists of key stakeholders from Credit Administration, Finance, Accounting, Risk Management and Audit and Advisory Services. We engaged a third-party consultant to assist us in developing our CECL methodology and we have completed a preliminary CECL model. The Model includes our loan segmentation and the quantitative and qualitative components of the calculation which incorporates a forecasting component of certain economic variables. We are currently working to finalize our documentation around the CECL methodology, designing and updating internal controls and financial statement disclosures. We will run parallel credit loss models for both the third and fourth quarters of 2019 prior to adoption of the CECL standard. The CECL model will be reviewed and validated by an independent consultant during the fourth quarter of 2019 and recommendations will be reviewed and implemented prior to adoption on January 1, 2020. While we anticipate an increase in the allowance for credit losses upon adoption of CECL, we are still in the process of validating the results of our model in order to determine the financial impact. The impact of adopting CECL depends on various factors including, the mix of the loan portfolio, forecasted macroeconomic variables at the time of adoption and the assessment of loans acquired from DNB. We will continue to evaluate the provisions