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Allowance for Loan Losses - Recorded Investment in Commercial Loan Classes by Internally Assigned Risk Ratings (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in commercial loan $ 3,649,632us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial $ 2,892,522us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
Total percentage of recorded investment in commercial loan 100.00%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage 100.00%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
Pass [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in commercial loan 3,458,560us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember		 2,780,315us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Total percentage of recorded investment in commercial loan 94.80%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember		 96.10%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Special Mention [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in commercial loan 101,333us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 65,968us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Total percentage of recorded investment in commercial loan 2.80%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 2.30%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Substandard [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in commercial loan 89,739us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 46,239us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Total percentage of recorded investment in commercial loan 2.40%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 1.60%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Commercial Real Estate [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in commercial loan 2,152,413us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember		 1,682,236us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
Total percentage of recorded investment in commercial loan 100.00%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember		 100.00%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
Commercial Real Estate [Member] | Pass [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in commercial loan 2,073,492us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember		 1,635,132us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Total percentage of recorded investment in commercial loan 96.30%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember		 97.20%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Commercial Real Estate [Member] | Special Mention [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in commercial loan 25,204us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 23,597us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Total percentage of recorded investment in commercial loan 1.20%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 1.40%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Commercial Real Estate [Member] | Substandard [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in commercial loan 53,717us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 23,507us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Total percentage of recorded investment in commercial loan 2.50%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 1.40%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Commercial and Industrial [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in commercial loan 1,211,053us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember		 994,138us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
Total percentage of recorded investment in commercial loan 100.00%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember		 100.00%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
Commercial and Industrial [Member] | Pass [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in commercial loan 1,137,101us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember		 948,663us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Total percentage of recorded investment in commercial loan 93.90%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember		 95.40%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Commercial and Industrial [Member] | Special Mention [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in commercial loan 55,529us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 30,357us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Total percentage of recorded investment in commercial loan 4.60%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 3.10%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Commercial and Industrial [Member] | Substandard [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in commercial loan 18,423us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 15,118us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Total percentage of recorded investment in commercial loan 1.50%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 1.50%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Commercial Construction [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in commercial loan 286,166us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember		 216,148us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
Total percentage of recorded investment in commercial loan 100.00%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember		 100.00%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
Commercial Construction [Member] | Pass [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in commercial loan 247,967us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember		 196,520us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Total percentage of recorded investment in commercial loan 86.70%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember		 90.90%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Commercial Construction [Member] | Special Mention [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in commercial loan 20,600us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 12,014us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Total percentage of recorded investment in commercial loan 7.20%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 5.60%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Commercial Construction [Member] | Substandard [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in commercial loan $ 17,599us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 $ 7,614us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Total percentage of recorded investment in commercial loan 6.10%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 3.50%stba_LoansAndLeasesReceivableGrossCarryingAmountCommercialPercentage
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stba_CommercialConstructionMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember