Basis of Presentation (Policies)	3 Months Ended
Basis of Presentation (Policies)	Mar. 31, 2014
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 our annual report on Form 10-K for the year ended December 31, 2013, filed with the Securities and Exchange Commission, or SEC, on February 21, 2014. 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	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	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.
Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Tax Credit Carry Forward Exists	Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Tax Credit Carry forward Exists In July 2013, the Financial Accounting Standards Board (FASB) issued ASU No. 2013-11, Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Tax Credit Carry forward Exists. The ASU requires that entities should present an unrecognized tax benefit as a reduction of the deferred tax asset for a net operating loss, or NOL, or similar tax loss or tax credit carry forward rather than as a liability when the uncertain tax position would reduce the NOL or other carry forward under the tax law. The new standard is effective for fiscal years, and interim periods within those years, beginning after December 15, 2013, and should be applied prospectively to all unrecognized tax benefits that exist at the effective date. Retrospective application is permitted. The adoption of this ASU had no impact on our results of operations or financial position.
Obligations Resulting from Joint and Several Liability Arrangements for Which the Total Amount of the Obligation is Fixed at the Reporting Date	Obligations Resulting from Joint and Several Liability Arrangements for Which the Total Amount of the Obligation is Fixed at the Reporting Date In February 2013, the FASB issued ASU No. 2013-04, Obligations Resulting from Joint and Several Liability Arrangements for Which the Total Amount of the Obligation is Fixed at the Reporting Date. The ASU requires the measurement of obligations resulting from joint and several liability arrangements for which the total amount of the obligation is fixed at the reporting date as the sum of the amount the reporting entity agreed to pay on the basis of its arrangement with its co-obligors as well as any additional amount that the entity expects to pay on behalf of its co-obligors. The new standard is effective retrospectively for fiscal years and interim periods within those years, beginning after December 15, 2013, and early adoption is permitted. The adoption of this ASU had no impact on our results of operations or financial position.
New Accounting Pronouncements	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 is permitted. We do not expect that this ASU will have a material 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 is permitted. We do not expect that this ASU will have a material 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 is permitted. We do not expect that this ASU will have a material impact on our results of operations or financial position.

Disclosure of accounting policy regarding (1) the principles it follows in consolidating or combining the separate financial statements, including the principles followed in determining the inclusion or exclusion of subsidiaries or other entities in the consolidated or combined financial statements and (2) its treatment of interests (for example, common stock, a partnership interest or other means of exerting influence) in other entities, for example consolidation or use of the equity or cost methods of accounting. The accounting policy may also address the accounting treatment for intercompany accounts and transactions, noncontrolling interest, and the income statement treatment in consolidation for issuances of stock by a subsidiary.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02, 03

-Article 3A

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6367646&loc=d3e18780-107790

Reference 3: http://www.xbrl.org/2003/role/presentationRef

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-URI http://asc.fasb.org/subtopic&trid=2197480

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 40

-Section 45

-URI http://asc.fasb.org/section&trid=2197723

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 323

-SubTopic 10

-URI http://asc.fasb.org/subtopic&trid=2196966

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-URI http://asc.fasb.org/subtopic&trid=2197087

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 810

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.3A-02)

-URI http://asc.fasb.org/extlink&oid=27015204&loc=d3e355033-122828

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-Publisher FASB

-Name Accounting Standards Codification

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 810

-SubTopic 10

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=18733093&loc=d3e5614-111684

Reference 10: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph k

-Article 1

+ Details

- Definition

Disclosure of accounting policy for income taxes, which may include its accounting policies for recognizing and measuring deferred tax assets and liabilities and related valuation allowances, recognizing investment tax credits, operating loss carryforwards, tax credit carryforwards, and other carryforwards, methodologies for determining its effective income tax rate and the characterization of interest and penalties in the financial statements.

+ References+ Details