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Borrowings - Summary of Information Pertaining to Borrowings (Detail) (USD $) In Thousands, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Long Term And Short Term Debt [Line Items] |  |
Short-term borrowings | $ 246,294 us-gaap_ShortTermBorrowings | $ 320,605 us-gaap_ShortTermBorrowings
Long-term borrowings | 69,457 us-gaap_LongTermDebt | 65,061 us-gaap_LongTermDebt
Total Borrowings | 315,751 us-gaap_DebtLongtermAndShorttermCombinedAmount | 385,666 us-gaap_DebtLongtermAndShorttermCombinedAmount
Short-term borrowings, Weighted Average Rate | 0.27%us-gaap_ShortTermDebtWeightedAverageInterestRate | 0.27%us-gaap_ShortTermDebtWeightedAverageInterestRate
Long-term borrowings, Weighted Average Rate | 2.94%us-gaap_LongtermDebtWeightedAverageInterestRate | 2.79%us-gaap_LongtermDebtWeightedAverageInterestRate
Total Borrowings, Weighted Average Rate | 0.86%us-gaap_DebtWeightedAverageInterestRate | 0.70%us-gaap_DebtWeightedAverageInterestRate
Securities Sold under Repurchase Agreements [Member] |  |
Long Term And Short Term Debt [Line Items] |  |
Short-term borrowings | 46,721 us-gaap_ShortTermBorrowings / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 30,605 us-gaap_ShortTermBorrowings / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
Short-term borrowings, Weighted Average Rate | 0.01%us-gaap_ShortTermDebtWeightedAverageInterestRate / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 0.01%us-gaap_ShortTermDebtWeightedAverageInterestRate / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
Short-Term Borrowings [Member] |  |
Long Term And Short Term Debt [Line Items] |  |
Short-term borrowings | 199,573 us-gaap_ShortTermBorrowings / us-gaap_DebtInstrumentAxis = stba_OtherShortTermBorrowingsMember | 290,000 us-gaap_ShortTermBorrowings / us-gaap_DebtInstrumentAxis = stba_OtherShortTermBorrowingsMember
Short-term borrowings, Weighted Average Rate | 0.33%us-gaap_ShortTermDebtWeightedAverageInterestRate / us-gaap_DebtInstrumentAxis = stba_OtherShortTermBorrowingsMember | 0.31%us-gaap_ShortTermDebtWeightedAverageInterestRate / us-gaap_DebtInstrumentAxis = stba_OtherShortTermBorrowingsMember
Other Long-Term Borrowings [Member] |  |
Long Term And Short Term Debt [Line Items] |  |
Long-term borrowings | 18,838 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = stba_OtherLongTermDebtMember | 19,442 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = stba_OtherLongTermDebtMember
Long-term borrowings, Weighted Average Rate | 2.92%us-gaap_LongtermDebtWeightedAverageInterestRate / us-gaap_DebtInstrumentAxis = stba_OtherLongTermDebtMember | 3.00%us-gaap_LongtermDebtWeightedAverageInterestRate / us-gaap_DebtInstrumentAxis = stba_OtherLongTermDebtMember
Junior Subordinated Debt Securities [Member] |  |
Long Term And Short Term Debt [Line Items] |  |
Long-term borrowings | $ 50,619 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_JuniorSubordinatedDebtMember | $ 45,619 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_JuniorSubordinatedDebtMember
Long-term borrowings, Weighted Average Rate | 2.95%us-gaap_LongtermDebtWeightedAverageInterestRate / us-gaap_DebtInstrumentAxis = us-gaap_JuniorSubordinatedDebtMember | 2.70%us-gaap_LongtermDebtWeightedAverageInterestRate / us-gaap_DebtInstrumentAxis = us-gaap_JuniorSubordinatedDebtMember

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- Definition Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 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 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20, 22 -Article 5 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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