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Loans and Loans Held for Sale - Additional Information (Detail) (USD $) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unearned income on loans | $ 2,500,000 stba_UnearnedIncomeOnLoan | $ 2,100,000 stba_UnearnedIncomeOnLoan
Percentage of commercial loans in total portfolio loans | 78.00%stba_PercentageOfCommercialLoansInPortfolioLoans | 75.00%stba_PercentageOfCommercialLoansInPortfolioLoans
Commercial Loans | 3,649,632,000 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial | 2,892,522,000 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
Maximum concentration of commercial real estate and commercial construction portfolio in loans (in excess of) | 9.00%stba_MaximumConcentrationOfCommercialRealEstateAndCommercialConstructionPortfolioInLoans | 9.00%stba_MaximumConcentrationOfCommercialRealEstateAndCommercialConstructionPortfolioInLoans
Minimum period of loan payment defaults following restructure for TDRs to be in default | 90 days |
Commercial Real Estate and Commercial Construction [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Commercial Loans | 2,400,000,000 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialRealEstateAndCommercialConstructionMember |
Combined percentage of commercial real estate and commercial construction in total commercial loans | 67.00%stba_CombinedPercentageOfCommercialRealEstateAndCommercialConstructionInCommercialLoans / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialRealEstateAndCommercialConstructionMember | 66.00%stba_CombinedPercentageOfCommercialRealEstateAndCommercialConstructionInCommercialLoans / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialRealEstateAndCommercialConstructionMember
Combined percentage of commercial real estate and commercial construction in total portfolio loans | 52.00%stba_CombinedPercentageOfCommercialRealEstateAndCommercialConstructionInPortfolioLoans / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialRealEstateAndCommercialConstructionMember | 49.00%stba_CombinedPercentageOfCommercialRealEstateAndCommercialConstructionInPortfolioLoans / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialRealEstateAndCommercialConstructionMember
Out Of State Exposure Of Combined Portfolio | 6.10%stba_OutOfStateExposureOfCombinedPortfolio / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialRealEstateAndCommercialConstructionMember | 8.00%stba_OutOfStateExposureOfCombinedPortfolio / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialRealEstateAndCommercialConstructionMember
Percentage of total loans out-of-state excluding contiguous states | 3.20%stba_NetLoansOutOfStatePercentageExcludingContiguousStates / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialRealEstateAndCommercialConstructionMember | 3.90%stba_NetLoansOutOfStatePercentageExcludingContiguousStates / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialRealEstateAndCommercialConstructionMember
Commercial and Industrial [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Commercial Loans | 1,211,053,000 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember | 994,138,000 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember
Number Of Loans Modified Not Considered To Be Troubled Debt Restructuring | 2 stba_NumberOfLoansModifiedNotConsideredToBeTroubledDebtRestructuring / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember |
Loans modified not considered to be troubled debt restructuring | 200,000 stba_LoansModifiedNotConsideredToBeTroubledDebtRestructuring / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember |
Commercial Real Estate [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Commercial Loans | 2,152,413,000 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember | 1,682,236,000 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember
Number Of Loans Modified Not Considered To Be Troubled Debt Restructuring | 2 stba_NumberOfLoansModifiedNotConsideredToBeTroubledDebtRestructuring / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember |
Loans modified not considered to be troubled debt restructuring | 200,000 stba_LoansModifiedNotConsideredToBeTroubledDebtRestructuring / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember |
Integrity Bancshares, Inc. [Member] | Commercial Real Estate and Commercial Construction [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Commercial Loans | $ 527,900,000 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial / us-gaap_BusinessAcquisitionAxis = stba_IntegrityBancsharesInc.Member / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialRealEstateAndCommercialConstructionMember |

X
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- Definition Percentage of commercial loans in portfolio loans. + References No definition available. + Details
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- Definition Reflects the aggregate gross carrying amount of all commercial loans and leases held in portfolio, prior to deducting deferred income and the allowance for losses on commercial loans and leases. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5074-111524 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5066-111524 + Details
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