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Loans and Loans Held for Sale - Restructured Loans for Periods Stated (Detail) (USD $) In Thousands, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 SecurityLoan | Mar. 31, 2014 SecurityLoan
Financing Receivable, Modifications [Line Items] |  |  |  |
Number of Loans | 20 us-gaap_FinancingReceivableModificationsNumberOfContracts2 |  | 12 us-gaap_FinancingReceivableModificationsNumberOfContracts2 |
Pre-Modification Outstanding Recorded Investment | $ 4,612 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 | [1] | $ 1,808 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 | [1]
Post-Modification Outstanding Recorded Investment | 4,585 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 | [1] | 1,791 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 | [1]
Total Difference in Recorded Investment | (27)stba_FinancingReceivableModificationsDifferenceInRecordedInvestments |  | (17)stba_FinancingReceivableModificationsDifferenceInRecordedInvestments |
Principal Deferral [Member] |  |  |  |
Financing Receivable, Modifications [Line Items] |  |  |  |
Number of Loans | 9 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember |  | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember |
Pre-Modification Outstanding Recorded Investment | 3,616 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember | [1] | 1,019 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember | [1]
Post-Modification Outstanding Recorded Investment | 3,615 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember | [1] | 1,019 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember | [1]
Total Difference in Recorded Investment | (1)stba_FinancingReceivableModificationsDifferenceInRecordedInvestments / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember |  | 0 stba_FinancingReceivableModificationsDifferenceInRecordedInvestments / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember |
Chapter 7 Bankruptcy [Member] |  |  |  |
Financing Receivable, Modifications [Line Items] |  |  |  |
Number of Loans | 9 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [2] | 11 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [2]
Pre-Modification Outstanding Recorded Investment | 145 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [1],[2] | 789 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [1],[2]
Post-Modification Outstanding Recorded Investment | 134 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [1],[2] | 772 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [1],[2]
Total Difference in Recorded Investment | (11)stba_FinancingReceivableModificationsDifferenceInRecordedInvestments / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [2] | (17)stba_FinancingReceivableModificationsDifferenceInRecordedInvestments / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [2]
Maturity Date Extension [Member] |  |  |  |
Financing Receivable, Modifications [Line Items] |  |  |  |
Number of Loans | 2 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_MaturityDateExtensionMember |  |  |
Pre-Modification Outstanding Recorded Investment | 851 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_MaturityDateExtensionMember | [1] |  |
Post-Modification Outstanding Recorded Investment | 836 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_MaturityDateExtensionMember | [1] |  |
Total Difference in Recorded Investment | (15)stba_FinancingReceivableModificationsDifferenceInRecordedInvestments / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_MaturityDateExtensionMember |  |  |
Commercial Real Estate [Member] | Principal Deferral [Member] |  |  |  |
Financing Receivable, Modifications [Line Items] |  |  |  |
Number of Loans | 2 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember |  |  |
Pre-Modification Outstanding Recorded Investment | 2,851 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember | [1] |  |
Post-Modification Outstanding Recorded Investment | 2,851 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember | [1] |  |
Total Difference in Recorded Investment | 0 stba_FinancingReceivableModificationsDifferenceInRecordedInvestments / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember |  |  |
Commercial and Industrial [Member] | Principal Deferral [Member] |  |  |  |
Financing Receivable, Modifications [Line Items] |  |  |  |
Number of Loans | 6 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember |  |  |
Pre-Modification Outstanding Recorded Investment | 661 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember | [1] |  |
Post-Modification Outstanding Recorded Investment | 661 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember | [1] |  |
Total Difference in Recorded Investment | 0 stba_FinancingReceivableModificationsDifferenceInRecordedInvestments / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember |  |  |
Commercial and Industrial [Member] | Chapter 7 Bankruptcy [Member] |  |  |  |
Financing Receivable, Modifications [Line Items] |  |  |  |
Number of Loans | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [2] | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [2]
Pre-Modification Outstanding Recorded Investment | 3 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [1],[2] | 287 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [1],[2]
Post-Modification Outstanding Recorded Investment | 1 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [1],[2] | 286 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [1],[2]
Total Difference in Recorded Investment | (2)stba_FinancingReceivableModificationsDifferenceInRecordedInvestments / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [2] | (1)stba_FinancingReceivableModificationsDifferenceInRecordedInvestments / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [2]
Commercial and Industrial [Member] | Maturity Date Extension [Member] |  |  |  |
Financing Receivable, Modifications [Line Items] |  |  |  |
Number of Loans | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_MaturityDateExtensionMember |  |  |
Pre-Modification Outstanding Recorded Investment | 780 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_MaturityDateExtensionMember | [1] |  |
Post-Modification Outstanding Recorded Investment | 765 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_MaturityDateExtensionMember | [1] |  |
Total Difference in Recorded Investment | (15)stba_FinancingReceivableModificationsDifferenceInRecordedInvestments / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_MaturityDateExtensionMember |  |  |
Commercial Construction [Member] | Principal Deferral [Member] |  |  |  |
Financing Receivable, Modifications [Line Items] |  |  |  |
Number of Loans | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember |  | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember |
Pre-Modification Outstanding Recorded Investment | 104 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember | [1] | 1,019 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember | [1]
Post-Modification Outstanding Recorded Investment | 103 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember | [1] | 1,019 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember | [1]
Total Difference in Recorded Investment | (1)stba_FinancingReceivableModificationsDifferenceInRecordedInvestments / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember |  | 0 stba_FinancingReceivableModificationsDifferenceInRecordedInvestments / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_PrincipalDeferralMember |
Residential Mortgage [Member] | Interest Rate Reduction [Member] |  |  |  |
Financing Receivable, Modifications [Line Items] |  |  |  |
Number of Loans | 4 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_InterestRateReductionMember | [2] |  |
Pre-Modification Outstanding Recorded Investment | 277 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_InterestRateReductionMember | [1],[2] |  |
Post-Modification Outstanding Recorded Investment | 276 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_InterestRateReductionMember | [1],[2] |  |
Total Difference in Recorded Investment | (1)stba_FinancingReceivableModificationsDifferenceInRecordedInvestments / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_InterestRateReductionMember | [2] |  |
Home Equity [Member] | Chapter 7 Bankruptcy [Member] |  |  |  |
Financing Receivable, Modifications [Line Items] |  |  |  |
Number of Loans | 8 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [2] | 6 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [2]
Pre-Modification Outstanding Recorded Investment | 142 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [1],[2] | 225 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [1],[2]
Post-Modification Outstanding Recorded Investment | 133 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [1] | 210 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [1],[2]
Total Difference in Recorded Investment | (9)stba_FinancingReceivableModificationsDifferenceInRecordedInvestments / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [2] | (15)stba_FinancingReceivableModificationsDifferenceInRecordedInvestments / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_BankruptcyMember | [2]
Home Equity [Member] | Maturity Date Extension [Member] |  |  |  |
Financing Receivable, Modifications [Line Items] |  |  |  |
Number of Loans | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_MaturityDateExtensionMember |  |  |
Pre-Modification Outstanding Recorded Investment | 71 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_MaturityDateExtensionMember | [1] |  |
Post-Modification Outstanding Recorded Investment | 71 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_MaturityDateExtensionMember | [1] |  |
Total Difference in Recorded Investment | $ 0 stba_FinancingReceivableModificationsDifferenceInRecordedInvestments / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = stba_MaturityDateExtensionMember |  |  |
[1] | Excludes loans that were fully paid off or fully charged-off by period end. The pre-modification balance represents the balance outstanding prior to modification. The post-modification balance represents the outstanding balance at period end.
[2] | Chapter 7 bankruptcy loans where the debt has been legally discharged through the bankruptcy court and not reaffirmed.

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- Definition Post-modification amount of investment in financing receivables modified by troubled debt restructurings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 33 -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524 + Details
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