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Derivative Instruments and Hedging Activities - Value of Derivative Assets and Derivative Liabilities (Detail) (Not Designated as Hedging Instruments [Member], USD $) In Thousands, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Derivatives not Designated as Hedging Instruments |  |
Fair value, Derivatives (included in Other Assets) | $ 14,020 us-gaap_DerivativeAssets | $ 12,981 us-gaap_DerivativeAssets
Fair value, Derivatives (included in Other Liabilities) | 13,979 us-gaap_DerivativeLiabilities | 12,953 us-gaap_DerivativeLiabilities
Other Assets [Member] | Interest Rate Swap Contracts-Commercial Loans [Member] |  |
Derivatives not Designated as Hedging Instruments |  |
Fair value, Derivatives (included in Other Assets) | 14,020 us-gaap_DerivativeAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateSwapsCommercialLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 12,981 us-gaap_DerivativeAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateSwapsCommercialLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Notional amount, Derivatives (included in Other Assets) | 228,800 us-gaap_DerivativeAssetNotionalAmount / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateSwapsCommercialLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 245,152 us-gaap_DerivativeAssetNotionalAmount / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateSwapsCommercialLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Collateral posted | 0 us-gaap_CollateralAlreadyPostedAggregateFairValue / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateSwapsCommercialLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 0 us-gaap_CollateralAlreadyPostedAggregateFairValue / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateSwapsCommercialLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Other Assets [Member] | Interest Rate Lock Commitments - Mortgage Loans [Member] |  |
Derivatives not Designated as Hedging Instruments |  |
Fair value, Derivatives (included in Other Assets) | 371 us-gaap_DerivativeAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateLockCommitmentsMortgageLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 235 us-gaap_DerivativeAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateLockCommitmentsMortgageLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Notional amount, Derivatives (included in Other Assets) | 11,923 us-gaap_DerivativeAssetNotionalAmount / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateLockCommitmentsMortgageLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 8,822 us-gaap_DerivativeAssetNotionalAmount / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateLockCommitmentsMortgageLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Other Assets [Member] | Forward Sale Contracts-Mortgage Loans [Member] |  |
Derivatives not Designated as Hedging Instruments |  |
Fair value, Derivatives (included in Other Assets) | 0 us-gaap_DerivativeAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = stba_ForwardSaleContractsMortgageLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 0 us-gaap_DerivativeAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = stba_ForwardSaleContractsMortgageLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Notional amount, Derivatives (included in Other Assets) | 0 us-gaap_DerivativeAssetNotionalAmount / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = stba_ForwardSaleContractsMortgageLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 0 us-gaap_DerivativeAssetNotionalAmount / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = stba_ForwardSaleContractsMortgageLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Other Liabilities [Member] | Interest Rate Swap Contracts-Commercial Loans [Member] |  |
Derivatives not Designated as Hedging Instruments |  |
Collateral posted | 12,498 us-gaap_CollateralAlreadyPostedAggregateFairValue / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateSwapsCommercialLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 12,059 us-gaap_CollateralAlreadyPostedAggregateFairValue / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateSwapsCommercialLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Fair value, Derivatives (included in Other Liabilities) | 13,979 us-gaap_DerivativeLiabilities / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateSwapsCommercialLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 12,953 us-gaap_DerivativeLiabilities / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateSwapsCommercialLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Notional amount, Derivatives (included in Other Liabilities) | 228,800 us-gaap_DerivativeLiabilityNotionalAmount / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateSwapsCommercialLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 245,152 us-gaap_DerivativeLiabilityNotionalAmount / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateSwapsCommercialLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Other Liabilities [Member] | Interest Rate Lock Commitments - Mortgage Loans [Member] |  |
Derivatives not Designated as Hedging Instruments |  |
Fair value, Derivatives (included in Other Liabilities) | 0 us-gaap_DerivativeLiabilities / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateLockCommitmentsMortgageLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 0 us-gaap_DerivativeLiabilities / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateLockCommitmentsMortgageLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Notional amount, Derivatives (included in Other Liabilities) | 0 us-gaap_DerivativeLiabilityNotionalAmount / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateLockCommitmentsMortgageLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 0 us-gaap_DerivativeLiabilityNotionalAmount / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = stba_InterestRateLockCommitmentsMortgageLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Other Liabilities [Member] | Forward Sale Contracts-Mortgage Loans [Member] |  |
Derivatives not Designated as Hedging Instruments |  |
Fair value, Derivatives (included in Other Liabilities) | 74 us-gaap_DerivativeLiabilities / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = stba_ForwardSaleContractsMortgageLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 57 us-gaap_DerivativeLiabilities / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = stba_ForwardSaleContractsMortgageLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Notional amount, Derivatives (included in Other Liabilities) | $ 10,680 us-gaap_DerivativeLiabilityNotionalAmount / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = stba_ForwardSaleContractsMortgageLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | $ 7,789 us-gaap_DerivativeLiabilityNotionalAmount / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = stba_ForwardSaleContractsMortgageLoansMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember

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- Definition The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624258-113959 + Details
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- Definition Nominal or face amount used to calculate payments on the derivative asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5580258-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5579245-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Notional Amount -URI http://asc.fasb.org/extlink&oid=6519104 + Details
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- Definition Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13495-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226008-175313 + Details
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- Definition Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13495-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226008-175313 + Details
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- Definition Nominal or face amount used to calculate payments on the derivative liability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5580258-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5579245-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Notional Amount -URI http://asc.fasb.org/extlink&oid=6519104 + Details
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