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Allowance for Loan Losses - Summary of Allowance for Loan Losses and Recorded Investments (Detail) (USD $)
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]        
Allowance for Loan Losses, Individually Evaluated for Impairment $ 88,000us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 $ 54,000us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1    
Allowance for Loan Losses, Collectively Evaluated for Impairment 48,018,000us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment 47,857,000us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment    
Total Allowance for Loan Losses 48,106,000us-gaap_LoansAndLeasesReceivableAllowance 47,911,000us-gaap_LoansAndLeasesReceivableAllowance 46,616,000us-gaap_LoansAndLeasesReceivableAllowance 46,255,000us-gaap_LoansAndLeasesReceivableAllowance
Portfolio Loans, Individually Evaluated for Impairment 48,489,000us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment 43,977,000us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment    
Portfolio Loans, Collectively Evaluated for Impairment 4,635,209,000us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment 3,824,769,000us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment    
Total Portfolio Loans 4,683,698,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome [1] 3,868,746,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome [1]    
Allowance for loan losses on acquired loans 0stba_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedAllowanceforLoanLosses      
Commercial Real Estate [Member]        
Financing Receivable, Allowance for Credit Losses [Line Items]        
Allowance for Loan Losses, Individually Evaluated for Impairment 39,000us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
0us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
   
Allowance for Loan Losses, Collectively Evaluated for Impairment 19,032,000us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
20,164,000us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
   
Total Allowance for Loan Losses 19,071,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
20,164,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
19,880,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
18,921,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
Portfolio Loans, Individually Evaluated for Impairment 23,326,000us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
19,890,000us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
   
Portfolio Loans, Collectively Evaluated for Impairment 2,129,087,000us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
1,662,346,000us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
   
Total Portfolio Loans 2,152,413,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
[1] 1,682,236,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
[1]    
Commercial and Industrial [Member]        
Financing Receivable, Allowance for Credit Losses [Line Items]        
Allowance for Loan Losses, Individually Evaluated for Impairment 0us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
0us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
   
Allowance for Loan Losses, Collectively Evaluated for Impairment 13,711,000us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
13,668,000us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
   
Total Allowance for Loan Losses 13,711,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
13,668,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
13,979,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
14,433,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
Portfolio Loans, Individually Evaluated for Impairment 10,338,000us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
9,218,000us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
   
Portfolio Loans, Collectively Evaluated for Impairment 1,200,715,000us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
984,920,000us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
   
Total Portfolio Loans 1,211,053,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
[1] 994,138,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
[1]    
Commercial Construction [Member]        
Financing Receivable, Allowance for Credit Losses [Line Items]        
Allowance for Loan Losses, Individually Evaluated for Impairment 0us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
0us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
   
Allowance for Loan Losses, Collectively Evaluated for Impairment 6,869,000us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
6,093,000us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
   
Total Allowance for Loan Losses 6,869,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
6,093,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
5,183,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
5,374,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
Portfolio Loans, Individually Evaluated for Impairment 7,696,000us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
7,605,000us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
   
Portfolio Loans, Collectively Evaluated for Impairment 278,470,000us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
208,543,000us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
   
Total Portfolio Loans 286,166,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
[1] 216,148,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
[1]    
Consumer Real Estate [Member]        
Financing Receivable, Allowance for Credit Losses [Line Items]        
Allowance for Loan Losses, Individually Evaluated for Impairment 41,000us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_ConsumerRealEstateMember
43,000us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_ConsumerRealEstateMember
   
Allowance for Loan Losses, Collectively Evaluated for Impairment 6,682,000us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_ConsumerRealEstateMember
6,290,000us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_ConsumerRealEstateMember
   
Total Allowance for Loan Losses 6,723,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_ConsumerRealEstateMember
6,333,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_ConsumerRealEstateMember
6,408,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_ConsumerRealEstateMember
6,362,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_ConsumerRealEstateMember
Portfolio Loans, Individually Evaluated for Impairment 7,080,000us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_ConsumerRealEstateMember
7,202,000us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_ConsumerRealEstateMember
   
Portfolio Loans, Collectively Evaluated for Impairment 961,232,000us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_ConsumerRealEstateMember
903,455,000us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_ConsumerRealEstateMember
   
Total Portfolio Loans 968,312,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_ConsumerRealEstateMember
[1] 910,657,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_ConsumerRealEstateMember
[1]    
Other Consumer [Member]        
Financing Receivable, Allowance for Credit Losses [Line Items]        
Allowance for Loan Losses, Individually Evaluated for Impairment 8,000us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_OtherConsumerMember
11,000us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_OtherConsumerMember
   
Allowance for Loan Losses, Collectively Evaluated for Impairment 1,724,000us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_OtherConsumerMember
1,642,000us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_OtherConsumerMember
   
Total Allowance for Loan Losses 1,732,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_OtherConsumerMember
1,653,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_OtherConsumerMember
1,166,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_OtherConsumerMember
1,165,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_OtherConsumerMember
Portfolio Loans, Individually Evaluated for Impairment 49,000us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_OtherConsumerMember
62,000us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_OtherConsumerMember
   
Portfolio Loans, Collectively Evaluated for Impairment 65,705,000us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_OtherConsumerMember
65,505,000us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_OtherConsumerMember
   
Total Portfolio Loans 65,754,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_OtherConsumerMember
[1] 65,567,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_OtherConsumerMember
[1]    
Fair Value Measurements [Member]        
Financing Receivable, Allowance for Credit Losses [Line Items]        
Allowance for loan losses on acquired loans $ 0stba_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedAllowanceforLoanLosses
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
     
[1] (1) Includes acquired loans.