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Derivative Instruments and Hedging Activities - Amount of Gain or Loss Recognized in Income on Derivatives (Detail) (Not Designated as Hedging Instruments [Member], USD $) In Thousands, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total Derivatives Gain (Loss) | $ 132 us-gaap_DerivativeGainLossOnDerivativeNet | $ (29)us-gaap_DerivativeGainLossOnDerivativeNet
Interest Rate Swap Contracts-Commercial Loans [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total Derivatives Gain (Loss) | 13 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateSwapsCommercialLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | (8)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateSwapsCommercialLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Interest Rate Lock Commitments - Mortgage Loans [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total Derivatives Gain (Loss) | 136 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateLockCommitmentsMortgageLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 8 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateLockCommitmentsMortgageLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Forward Sale Contracts-Mortgage Loans [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total Derivatives Gain (Loss) | $ (17)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = stba_ForwardSaleContractsMortgageLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | $ (29)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = stba_ForwardSaleContractsMortgageLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember

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- Definition Amount of increase (decrease) in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5618551-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (a),(c),(d),(e) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624171-113959 + Details
Name: | us-gaap_DerivativeGainLossOnDerivativeNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
Name: | us-gaap_DerivativeInstrumentsGainLossLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration