XML 76 R64.htm IDEA: XBRL DOCUMENT v2.4.1.9
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
Derivatives not Designated as Hedging Instruments    
Fair value, Derivatives (included in Other Assets) $ 14,020us-gaap_DerivativeAssets $ 12,981us-gaap_DerivativeAssets
Fair value, Derivatives (included in Other Liabilities) 13,979us-gaap_DerivativeLiabilities 12,953us-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) 14,020us-gaap_DerivativeAssets
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= stba_InterestRateSwapsCommercialLoansMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 12,981us-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) 228,800us-gaap_DerivativeAssetNotionalAmount
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= stba_InterestRateSwapsCommercialLoansMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 245,152us-gaap_DerivativeAssetNotionalAmount
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= stba_InterestRateSwapsCommercialLoansMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Collateral posted 0us-gaap_CollateralAlreadyPostedAggregateFairValue
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= stba_InterestRateSwapsCommercialLoansMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 0us-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) 371us-gaap_DerivativeAssets
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= stba_InterestRateLockCommitmentsMortgageLoansMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 235us-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) 11,923us-gaap_DerivativeAssetNotionalAmount
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= stba_InterestRateLockCommitmentsMortgageLoansMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 8,822us-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) 0us-gaap_DerivativeAssets
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= stba_ForwardSaleContractsMortgageLoansMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 0us-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) 0us-gaap_DerivativeAssetNotionalAmount
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= stba_ForwardSaleContractsMortgageLoansMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 0us-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 12,498us-gaap_CollateralAlreadyPostedAggregateFairValue
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember
/ us-gaap_DerivativeInstrumentRiskAxis
= stba_InterestRateSwapsCommercialLoansMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 12,059us-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) 13,979us-gaap_DerivativeLiabilities
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember
/ us-gaap_DerivativeInstrumentRiskAxis
= stba_InterestRateSwapsCommercialLoansMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 12,953us-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) 228,800us-gaap_DerivativeLiabilityNotionalAmount
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember
/ us-gaap_DerivativeInstrumentRiskAxis
= stba_InterestRateSwapsCommercialLoansMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 245,152us-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) 0us-gaap_DerivativeLiabilities
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember
/ us-gaap_DerivativeInstrumentRiskAxis
= stba_InterestRateLockCommitmentsMortgageLoansMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 0us-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) 0us-gaap_DerivativeLiabilityNotionalAmount
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember
/ us-gaap_DerivativeInstrumentRiskAxis
= stba_InterestRateLockCommitmentsMortgageLoansMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 0us-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) 74us-gaap_DerivativeLiabilities
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember
/ us-gaap_DerivativeInstrumentRiskAxis
= stba_ForwardSaleContractsMortgageLoansMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 57us-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) $ 10,680us-gaap_DerivativeLiabilityNotionalAmount
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember
/ us-gaap_DerivativeInstrumentRiskAxis
= stba_ForwardSaleContractsMortgageLoansMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 $ 7,789us-gaap_DerivativeLiabilityNotionalAmount
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember
/ us-gaap_DerivativeInstrumentRiskAxis
= stba_ForwardSaleContractsMortgageLoansMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember