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Allowance for Loan Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Detail) (USD $) In Thousands, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Loans, Current | $ 4,653,941 us-gaap_FinancingReceivableRecordedInvestmentCurrent | $ 3,850,597 us-gaap_FinancingReceivableRecordedInvestmentCurrent
Loans, 30 to 59 Days Past Due | 11,393 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue | 6,448 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
Loans, 60 to 89 Days Past Due | 1,327 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue | 2,214 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
Loans, 90 Days Past Due | 5,029 us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue | 0 us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
Total nonaccrual loans | 18,134 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus | 12,457 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
Loans, Past Due, Total | 35,883 us-gaap_FinancingReceivableRecordedInvestmentPastDue | 21,119 us-gaap_FinancingReceivableRecordedInvestmentPastDue
Total Loans | 4,689,824 us-gaap_LoansReceivableNet | 3,871,716 us-gaap_LoansReceivableNet
Commercial Real Estate [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Loans, Current | 2,138,815 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember | 1,674,930 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember
Loans, 30 to 59 Days Past Due | 2,955 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember | 2,548 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember
Loans, 60 to 89 Days Past Due | 522 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember | 323 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember
Loans, 90 Days Past Due | 1,735 us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember |
Total nonaccrual loans | 8,386 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember | 4,435 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember
Loans, Past Due, Total | 13,598 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember | 7,306 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember
Total Loans | 2,152,413 us-gaap_LoansReceivableNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember | 1,682,236 us-gaap_LoansReceivableNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember
Commercial and Industrial [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Loans, Current | 1,202,897 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember | 991,136 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember
Loans, 30 to 59 Days Past Due | 4,089 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember | 1,227 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember
Loans, 60 to 89 Days Past Due | 517 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember | 153 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember
Loans, 90 Days Past Due | 0 us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember | 0 us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember
Total nonaccrual loans | 3,550 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember | 1,622 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember
Loans, Past Due, Total | 8,156 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember | 3,002 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember
Total Loans | 1,211,053 us-gaap_LoansReceivableNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember | 994,138 us-gaap_LoansReceivableNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember
Commercial Construction [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Loans, Current | 281,084 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember | 214,174 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember
Loans, 30 to 59 Days Past Due | 969 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember |
Loans, 90 Days Past Due | 2,140 us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember |
Total nonaccrual loans | 1,973 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember | 1,974 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember
Loans, Past Due, Total | 5,082 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember | 1,974 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember
Total Loans | 286,166 us-gaap_LoansReceivableNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember | 216,148 us-gaap_LoansReceivableNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember
Residential Mortgage [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Loans, Current | 516,515 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember | 485,465 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember
Loans, 30 to 59 Days Past Due | 1,548 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember | 565 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember
Loans, 60 to 89 Days Past Due | 63 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember | 1,220 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember
Loans, 90 Days Past Due | 1,154 us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember |
Total nonaccrual loans | 2,226 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember | 2,336 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember
Loans, Past Due, Total | 4,991 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember | 4,121 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember
Total Loans | 521,506 us-gaap_LoansReceivableNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember | 489,586 us-gaap_LoansReceivableNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember
Home Equity [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Loans, Current | 438,629 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember | 414,303 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember
Loans, 30 to 59 Days Past Due | 1,583 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember | 1,756 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember
Loans, 60 to 89 Days Past Due | 198 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember | 445 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember
Total nonaccrual loans | 1,986 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember | 2,059 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember
Loans, Past Due, Total | 3,767 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember | 4,260 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember
Total Loans | 442,396 us-gaap_LoansReceivableNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember | 418,563 us-gaap_LoansReceivableNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember
Installment and Other Consumer [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Loans, Current | 65,465 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_InstallmentAndOtherConsumerMember | 65,111 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_InstallmentAndOtherConsumerMember
Loans, 30 to 59 Days Past Due | 249 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_InstallmentAndOtherConsumerMember | 352 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_InstallmentAndOtherConsumerMember
Loans, 60 to 89 Days Past Due | 27 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_InstallmentAndOtherConsumerMember | 73 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_InstallmentAndOtherConsumerMember
Total nonaccrual loans | 13 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_InstallmentAndOtherConsumerMember | 31 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_InstallmentAndOtherConsumerMember
Loans, Past Due, Total | 289 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_InstallmentAndOtherConsumerMember | 456 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_InstallmentAndOtherConsumerMember
Total Loans | 65,754 us-gaap_LoansReceivableNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_InstallmentAndOtherConsumerMember | 65,567 us-gaap_LoansReceivableNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_InstallmentAndOtherConsumerMember
Consumer Construction [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Loans, Current | 4,410 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_ConsumerConstructionMember | 2,508 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_ConsumerConstructionMember
Total Loans | 4,410 us-gaap_LoansReceivableNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_ConsumerConstructionMember | 2,508 us-gaap_LoansReceivableNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_ConsumerConstructionMember
Loans Held for Sale [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Loans, Current | 6,126 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_LoansHeldforSaleMember | 2,970 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_LoansHeldforSaleMember
Total Loans | $ 6,126 us-gaap_LoansReceivableNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_LoansHeldforSaleMember | $ 2,970 us-gaap_LoansReceivableNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_LoansHeldforSaleMember

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- Definition Financing receivables that are less than 60 days past due but more than 29 days past due. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953401-111524 + Details
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- Definition Financing receivables that are less than 90 days past due but more than 59 days past due. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953401-111524 + Details
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- Definition Financing receivables that are current. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953401-111524 + Details
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- Definition Financing receivables that are equal to or greater than 90 days past due. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953401-111524 + Details
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- Definition Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes loans held for sale. Excludes loans and leases covered under loss sharing agreements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Subparagraph a -Article 9 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(a)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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