Basis of Presentation	6 Months Ended
Jun. 30, 2015
Accounting Policies [Abstract]
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 our annual report on Form 10-K for the year ended December 31, 2014, filed with the Securities and Exchange Commission, or SEC, on February 20, 2015. In the opinion of management, the accompanying interim financial information reflects all adjustments, including 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 Certain amounts in the prior periods’ financial statements and footnotes have been reclassified to conform to the current period’s presentation. The reclassifications had no significant 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 Repurchase-To-Maturity Transactions, Repurchase Financings, and Disclosures In June 2014, the Financial Accounting Standards Board, or FASB, issued ASU No. 2014-11, Repurchase-to-Maturity Transactions, Repurchase Financings, and Disclosures which introduces two accounting changes to the Transfers and Servicing guidance (Topic 860). Repurchase-to-maturity transactions will be accounted for as secured borrowing transactions on the balance sheet and for repurchase financing arrangements, an entity will account separately for a transfer of a financial asset executed contemporaneously with a repurchase agreement with the same counterparty. This will also generally result in secured borrowing accounting for the repurchase agreement. With respect to disclosures, a transferor is required to disclose information about transactions accounted for as a sale in which the transferor retains substantially all of the exposure to the economic return on the transferred financial assets through an agreement with the transferee. Additionally, new disclosures are required for repurchase agreements, securities lending transactions, and repurchase-to-maturity transactions that are accounted for as secured borrowings. The new disclosure for transactions accounted for as secured borrowings is required for interim periods beginning after March 15, 2015. These new disclosures are included in Note 9. Borrowings. The adoption of this ASU had no impact on our results of operations or financial position. Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity In April 2014, the FASB issued ASU No. 2014-08, Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity, which changes the criteria for determining which disposals can be presented as discontinued operations and modifies related disclosure requirements. The guidance applies to all entities that dispose of components. It will significantly change current practices for assessing discontinued operations and affect an entity’s income and earnings per share from continuing operations. An entity is required to reclassify assets and liabilities of a discontinued operation that are classified as held for sale or disposed of in the current period for all comparative periods presented. The ASU requires that an entity present in the statement of cash flows or disclose in a note either total operating and investing cash flows for discontinued operations, or depreciation, amortization, capital expenditures and significant operating and investing noncash items related to discontinued operations. Additional disclosures are required when an entity retains significant continuing involvement with a discontinued operation after its disposal, including the amount of cash flows to and from a discontinued operation. The new standard applies prospectively after the effective date of December 15, 2014, and early adoption was permitted. The adoption of this ASU had no impact on our results of operations or financial position. Reclassification of Residential Real Estate Collateralized Consumer Mortgage Loans upon Foreclosure In January 2014, the FASB issued ASU No. 2014-04, Reclassification of Residential Real Estate Collateralized Consumer Mortgage Loans upon Foreclosure. The ASU clarifies that an in substance repossession or foreclosure has occurred and a creditor is considered to have received physical possession of residential real estate property collateralizing a consumer mortgage loan, upon either the creditor obtaining legal title to the residential real estate property upon completion of a foreclosure or the borrower conveying all interest in the residential real estate property to the creditor to satisfy that loan through completion of a deed in lieu of foreclosure. Interim and annual disclosure is required of both the amount of foreclosed residential real estate property held by the creditor and the recorded investment in consumer mortgage loans collateralized by residential real estate property that are in the process of foreclosure. The new standard is effective using either the modified retrospective transition method or a prospective transition method for fiscal years and interim periods within those years, beginning after December 15, 2014, and early adoption was permitted. The adoption of this ASU had no impact on our results of operations or financial position. Accounting for Investments in Qualified Affordable Housing Projects In January 2014, the FASB issued ASU No. 2014-01, Accounting for Investments in Qualified Affordable Housing Projects. The ASU permits reporting entities to make an accounting policy election to account for investments in qualified affordable housing projects using the proportional amortization method if certain conditions are met. The proportional amortization method permits the amortization of the initial cost of the investment in proportion to the tax credits and other tax benefits received, and recognizes the net investment performance in the income statement as a component of income tax expense (benefit). The new standard is effective retrospectively for fiscal years and interim periods within those years, beginning after December 15, 2014, and early adoption was permitted. This ASU did not have a material impact on our results of operations or financial position. We did not adopt the proportional amortization method. Refer to Note 14 for additional disclosure. Recently Issued Accounting Standards Updates not yet Adopted Intangibles – Goodwill and Other – Internal-Use Software: Customer's Accounting for Fees Paid in a Cloud Computing Arrangement In April 2015, the FASB issued ASU No. 2015-05, Intangibles – Goodwill and Other – Internal-Use Software (Subtopic 350-40): Customer's Accounting for Fees Paid in a Cloud Computing Arrangement. The main provisions of ASU 2015-05 provide a basis for evaluating whether a cloud computing arrangement includes a software license. If a cloud computing arrangement includes a software license, then the customer should account for the software license element of the arrangement consistent with the acquisition of other software licenses. If a cloud computing arrangement does not include a software license, then the arrangement should be accounted for as a service contract. The standard is effective for annual periods and interim periods beginning after December 15, 2015. We do not expect that this ASU will have a material impact on our results of operations or financial position. Interest – Imputation of Interest: Simplifying the Presentation of Debt Issuance Costs In April 2015, the FASB issued ASU No. 2015-03, Interest – Imputation of Interest (Subtopic 835-30): Simplifying the Presentation of Debt Issuance Costs. The amendments in this ASU require that debt issuance costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from the carrying amount of that debt liability, consistent with debt discounts. The standard is required to be adopted by public business entities in annual periods beginning on or after December 15, 2016. We do not expect that this ASU will have a material impact on our results of operations or financial position. Consolidation: Amendments to the Consolidation Analysis In April 2015, the FASB issued ASU No. 2015-02, Consolidation (Topic 810): Amendments to the Consolidation Analysis. The amendments in this ASU affect reporting entities that are required to evaluate whether they should consolidate certain legal entities. All legal entities are subject to reevaluation under the revised consolidation model. Specifically, the amendments: 1) Modify the evaluation of whether limited partnerships and similar legal entities are variable interest entities (VIEs) or voting interest entities, 2) Eliminate the presumption that a general partner should consolidate a limited partnership, 3) Affect the consolidation analysis of reporting entities that are involved with VIEs, particularly those that have fee arrangements and related party relationships, 4) Provide a scope exception from consolidation guidance for reporting entities with interests in legal entities that are required to comply with or operate in accordance with requirements that are similar to those in Rule 2A-7 of the Investment Company Act of 1940 for registered money market funds. The amendments in this ASU are effective for public business entities for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2015. We are currently evaluating the impact that these amendments may have on our consolidated financial statements. We do not expect that this ASU will have a material impact on our results of operations or financial position. Income Statement – Extraordinary and Unusual Items: Simplifying Income Statement Presentation by Eliminating the Concept of Extraordinary In January 2015, the FASB issued ASU No. 2015-01, Income Statement – Extraordinary and Unusual Items (Subtopic 225-20): Simplifying Income Statement Presentation by Eliminating the Concept of Extraordinary. The amendments in this ASU eliminate from GAAP the concept of extraordinary items and eliminate the requirements for reporting entities to consider whether an underlying event or transaction is extraordinary. The presentation and disclosure guidance for items that are unusual in nature or occur infrequently will be retained and will be expanded to include items that are both unusual in nature and infrequently occurring. The standard is required to be adopted by public business entities in annual periods beginning on or after December 15, 2015. We do not expect that this ASU will have a material impact on our results of operations or financial position.

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 our annual report on Form 10-K for the year ended December 31, 2014, filed with the Securities and Exchange Commission, or SEC, on February 20, 2015. In the opinion of management, the accompanying interim financial information reflects all adjustments, including 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

Certain amounts in the prior periods’ financial statements and footnotes have been reclassified to conform to the current period’s presentation. The reclassifications had no significant 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

Repurchase-To-Maturity Transactions, Repurchase Financings, and Disclosures

In June 2014, the Financial Accounting Standards Board, or FASB, issued ASU No. 2014-11, Repurchase-to-Maturity Transactions, Repurchase Financings, and Disclosures which introduces two accounting changes to the Transfers and Servicing guidance (Topic 860). Repurchase-to-maturity transactions will be accounted for as secured borrowing transactions on the balance sheet and for repurchase financing arrangements, an entity will account separately for a transfer of a financial asset executed contemporaneously with a repurchase agreement with the same counterparty. This will also generally result in secured borrowing accounting for the repurchase agreement. With respect to disclosures, a transferor is required to disclose information about transactions accounted for as a sale in which the transferor retains substantially all of the exposure to the economic return on the transferred financial assets through an agreement with the transferee. Additionally, new disclosures are required for repurchase agreements, securities lending transactions, and repurchase-to-maturity transactions that are accounted for as secured borrowings. The new disclosure for transactions accounted for as secured borrowings is required for interim periods beginning after March 15, 2015. These new disclosures are included in Note 9. Borrowings. The adoption of this ASU had no impact on our results of operations or financial position.

Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity

In April 2014, the FASB issued ASU No. 2014-08, Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity, which changes the criteria for determining which disposals can be presented as discontinued operations and modifies related disclosure requirements. The guidance applies to all entities that dispose of components. It will significantly change current practices for assessing discontinued operations and affect an entity’s income and earnings per share from continuing operations. An entity is required to reclassify assets and liabilities of a discontinued operation that are classified as held for sale or disposed of in the current period for all comparative periods presented. The ASU requires that an entity present in the statement of cash flows or disclose in a note either total operating and investing cash flows for discontinued operations, or depreciation, amortization, capital expenditures and significant operating and investing noncash items related to discontinued operations. Additional disclosures are required when an entity retains significant continuing involvement with a discontinued operation after its disposal, including the amount of cash flows to and from a discontinued operation. The new standard applies prospectively after the effective date of December 15, 2014, and early adoption was permitted. The adoption of this ASU had no impact on our results of operations or financial position.

Reclassification of Residential Real Estate Collateralized Consumer Mortgage Loans upon Foreclosure

In January 2014, the FASB issued ASU No. 2014-04, Reclassification of Residential Real Estate Collateralized Consumer Mortgage Loans upon Foreclosure. The ASU clarifies that an in substance repossession or foreclosure has occurred and a creditor is considered to have received physical possession of residential real estate property collateralizing a consumer mortgage loan, upon either the creditor obtaining legal title to the residential real estate property upon completion of a foreclosure or the borrower conveying all interest in the residential real estate property to the creditor to satisfy that loan through completion of a deed in lieu of foreclosure. Interim and annual disclosure is required of both the amount of foreclosed residential real estate property held by the creditor and the recorded investment in consumer mortgage loans collateralized by residential real estate property that are in the process of foreclosure. The new standard is effective using either the modified retrospective transition method or a prospective transition method for fiscal years and interim periods within those years, beginning after December 15, 2014, and early adoption was permitted. The adoption of this ASU had no impact on our results of operations or financial position.

Accounting for Investments in Qualified Affordable Housing Projects

In January 2014, the FASB issued ASU No. 2014-01, Accounting for Investments in Qualified Affordable Housing Projects. The ASU permits reporting entities to make an accounting policy election to account for investments in qualified affordable housing projects using the proportional amortization method if certain conditions are met. The proportional amortization method permits the amortization of the initial cost of the investment in proportion to the tax credits and other tax benefits received, and recognizes the net investment performance in the income statement as a component of income tax expense (benefit). The new standard is effective retrospectively for fiscal years and interim periods within those years, beginning after December 15, 2014, and early adoption was permitted. This ASU did not have a material impact on our results of operations or financial position. We did not adopt the proportional amortization method. Refer to Note 14 for additional disclosure.

Recently Issued Accounting Standards Updates not yet Adopted

Intangibles – Goodwill and Other – Internal-Use Software: Customer's Accounting for Fees Paid in a Cloud Computing Arrangement

In April 2015, the FASB issued ASU No. 2015-05, Intangibles – Goodwill and Other – Internal-Use Software (Subtopic 350-40): Customer's Accounting for Fees Paid in a Cloud Computing Arrangement. The main provisions of ASU 2015-05 provide a basis for evaluating whether a cloud computing arrangement includes a software license. If a cloud computing arrangement includes a software license, then the customer should account for the software license element of the arrangement consistent with the acquisition of other software licenses. If a cloud computing arrangement does not include a software license, then the arrangement should be accounted for as a service contract. The standard is effective for annual periods and interim periods beginning after December 15, 2015. We do not expect that this ASU will have a material impact on our results of operations or financial position.

Interest – Imputation of Interest: Simplifying the Presentation of Debt Issuance Costs

In April 2015, the FASB issued ASU No. 2015-03, Interest – Imputation of Interest (Subtopic 835-30): Simplifying the Presentation of Debt Issuance Costs. The amendments in this ASU require that debt issuance costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from the carrying amount of that debt liability, consistent with debt discounts. The standard is required to be adopted by public business entities in annual periods beginning on or after December 15, 2016. We do not expect that this ASU will have a material impact on our results of operations or financial position.

Consolidation: Amendments to the Consolidation Analysis

In April 2015, the FASB issued ASU No. 2015-02, Consolidation (Topic 810): Amendments to the Consolidation Analysis. The amendments in this ASU affect reporting entities that are required to evaluate whether they should consolidate certain legal entities. All legal entities are subject to reevaluation under the revised consolidation model. Specifically, the amendments: 1) Modify the evaluation of whether limited partnerships and similar legal entities are variable interest entities (VIEs) or voting interest entities, 2) Eliminate the presumption that a general partner should consolidate a limited partnership, 3) Affect the consolidation analysis of reporting entities that are involved with VIEs, particularly those that have fee arrangements and related party relationships, 4) Provide a scope exception from consolidation guidance for reporting entities with interests in legal entities that are required to comply with or operate in accordance with requirements that are similar to those in Rule 2A-7 of the Investment Company Act of 1940 for registered money market funds. The amendments in this ASU are effective for public business entities for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2015. We are currently evaluating the impact that these amendments may have on our consolidated financial statements. We do not expect that this ASU will have a material impact on our results of operations or financial position.

Income Statement – Extraordinary and Unusual Items: Simplifying Income Statement Presentation by Eliminating the Concept of Extraordinary

In January 2015, the FASB issued ASU No. 2015-01, Income Statement – Extraordinary and Unusual Items (Subtopic 225-20): Simplifying Income Statement Presentation by Eliminating the Concept of Extraordinary. The amendments in this ASU eliminate from GAAP the concept of extraordinary items and eliminate the requirements for reporting entities to consider whether an underlying event or transaction is extraordinary. The presentation and disclosure guidance for items that are unusual in nature or occur infrequently will be retained and will be expanded to include items that are both unusual in nature and infrequently occurring. The standard is required to be adopted by public business entities in annual periods beginning on or after December 15, 2015. We do not expect that this ASU will have a material impact on our results of operations or financial position.