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Qualified Affordable Housing Projects (Details) (USD $) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014
Qualified Affordable Housing Projects [Abstract] |  |  |
Investment in qualified affordable housing projects | $ 17,700,000 stba_AmortizationMethodQualifiedAffordableHousingProjectInvestments |  | $ 18,600,000 stba_AmortizationMethodQualifiedAffordableHousingProjectInvestments
Open commitments | 0 stba_QualifiedAffordableHousingProjectInvestmentsCommitment |  | 0 stba_QualifiedAffordableHousingProjectInvestmentsCommitment
Amortization expense included in noninterest expense | 900,000 stba_AmortizationMethodQualifiedAffordableHousingProjectInvestmentsAmortization | 1,000,000 stba_AmortizationMethodQualifiedAffordableHousingProjectInvestmentsAmortization |
Tax credits | $ 1,000,000 stba_AffordableHousingTaxCreditsandOtherBenefitsAmount | $ 1,100,000 stba_AffordableHousingTaxCreditsandOtherBenefitsAmount |

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- Definition Amount of affordable housing tax credits and other tax benefits recognized. + References No definition available. + Details
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- Definition Amount of investment in qualified affordable housing projects. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
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- Definition Amount of amortization expense of investment in qualified affordable housing projects. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
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- Definition Amount of commitment or contingent commitments to qualified affordable housing project investments. + References No definition available. + Details
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Namespace Prefix: | stba_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Details
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