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Loans and Loans Held for Sale - Composition of Loans (Detail) (USD $) In Thousands, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Composition of the loans |  |  |  |
Commercial Loans | $ 3,649,632 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial |  | $ 2,892,522 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial |
Consumer Loans | 1,034,066 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountConsumer |  | 976,224 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountConsumer |
Total Portfolio Loans | 4,683,698 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome | [1] | 3,868,746 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome | [1]
Loans held for sale | 6,126 us-gaap_LoansReceivableHeldForSaleNet |  | 2,970 us-gaap_LoansReceivableHeldForSaleNet |
Total Loans | 4,689,824 us-gaap_LoansReceivableNet |  | 3,871,716 us-gaap_LoansReceivableNet |
Commercial Real Estate [Member] |  |  |  |
Composition of the loans |  |  |  |
Commercial Loans | 2,152,413 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember |  | 1,682,236 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember |
Total Portfolio Loans | 2,152,413 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember | [1] | 1,682,236 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember | [1]
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] |  |  |  |
Composition of the loans |  |  |  |
Commercial Loans | 1,211,053 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember |  | 994,138 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember |
Total Portfolio Loans | 1,211,053 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember | [1] | 994,138 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialAndIndustrialMember | [1]
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] |  |  |  |
Composition of the loans |  |  |  |
Commercial Loans | 286,166 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember |  | 216,148 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCommercial / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember |
Total Portfolio Loans | 286,166 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember | [1] | 216,148 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_CommercialConstructionMember | [1]
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] |  |  |  |
Composition of the loans |  |  |  |
Consumer Loans | 521,506 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountConsumer / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember |  | 489,586 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountConsumer / 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] |  |  |  |
Composition of the loans |  |  |  |
Consumer Loans | 442,396 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountConsumer / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember |  | 418,563 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountConsumer / 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] |  |  |  |
Composition of the loans |  |  |  |
Consumer Loans | 65,754 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountConsumer / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_InstallmentAndOtherConsumerMember |  | 65,567 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountConsumer / 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] |  |  |  |
Composition of the loans |  |  |  |
Consumer Loans | 4,410 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountConsumer / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = stba_ConsumerConstructionMember |  | 2,508 us-gaap_LoansAndLeasesReceivableGrossCarryingAmountConsumer / 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 |
[1] | (1) Includes acquired loans.

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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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- Definition Reflects the aggregate gross carrying amount of all consumer loans and leases held in portfolio, prior to deducting deferred income and the allowance for losses on consumer 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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- Definition Amount before allowance and after 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. Excludes loans and leases covered under loss sharing agreements. + 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 + Details
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- Definition Amount classified as loans attributable to disposal group held for sale or disposed of. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities. + References Reference 1: 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 2: 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 3: 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 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 35 -Paragraph 49 -URI http://asc.fasb.org/extlink&oid=35742846&loc=d3e3084-111518 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 948 -SubTopic 310 -Section 30 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6490209&loc=d3e48214-111002 + 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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