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Securities Available-for-Sale - Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 640,036 us-gaap_AvailableForSaleSecuritiesAmortizedCost | $ 629,463 us-gaap_AvailableForSaleSecuritiesAmortizedCost
Fair Value | 655,829 us-gaap_AvailableForSaleSecurities | 640,273 us-gaap_AvailableForSaleSecurities
Obligations of the U.S. Treasury and U.S. government corporations and agencies, and obligations of states and political subdivisions [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Due in one year or less, Amortized Cost | 29,757 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = stba_UsGovernmentCorporationsAndAgenciesSecuritiesAndUsStatesAndPoliticalSubdivisionsMember |
Due after one year through five years, Amortized Cost | 206,322 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = stba_UsGovernmentCorporationsAndAgenciesSecuritiesAndUsStatesAndPoliticalSubdivisionsMember |
Due after five years through ten years, Amortized Cost | 80,865 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = stba_UsGovernmentCorporationsAndAgenciesSecuritiesAndUsStatesAndPoliticalSubdivisionsMember |
Due after ten years, Amortized Cost | 100,390 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = stba_UsGovernmentCorporationsAndAgenciesSecuritiesAndUsStatesAndPoliticalSubdivisionsMember |
Available-for-sale Securities, Debt Maturities, Amortized Cost | 417,334 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = stba_UsGovernmentCorporationsAndAgenciesSecuritiesAndUsStatesAndPoliticalSubdivisionsMember |
Due in one year or less, Fair Value | 30,068 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = stba_UsGovernmentCorporationsAndAgenciesSecuritiesAndUsStatesAndPoliticalSubdivisionsMember |
Due after one year through five years, Fair Value | 208,787 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = stba_UsGovernmentCorporationsAndAgenciesSecuritiesAndUsStatesAndPoliticalSubdivisionsMember |
Due after five years through ten years, Fair Value | 83,329 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = stba_UsGovernmentCorporationsAndAgenciesSecuritiesAndUsStatesAndPoliticalSubdivisionsMember |
Due after ten years, Fair Value | 104,905 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = stba_UsGovernmentCorporationsAndAgenciesSecuritiesAndUsStatesAndPoliticalSubdivisionsMember |
Fair Value, Debt securities | 427,089 us-gaap_AvailableForSaleSecuritiesDebtSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = stba_UsGovernmentCorporationsAndAgenciesSecuritiesAndUsStatesAndPoliticalSubdivisionsMember |
Collateralized Mortgage Obligations of U.S. Government Corporations and Agencies [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 129,010 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 116,897 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Fair Value | 131,404 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 118,006 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Residential Mortgage-Backed Securities of U.S. Government Corporations and Agencies [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 46,412 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | 45,274 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Fair Value | 48,165 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | 46,668 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Commercial Mortgage-Backed Securities of U.S. Government Corporations and Agencies [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 39,701 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 39,834 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Fair Value | 40,080 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 39,673 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Debt Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 632,457 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_DebtSecuritiesMember | 621,884 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_DebtSecuritiesMember
Fair Value | 646,738 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_DebtSecuritiesMember | 631,214 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_DebtSecuritiesMember
Marketable Equity Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 7,579 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 7,579 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Fair Value | $ 9,091 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | $ 9,059 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember

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- Definition This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 + Details
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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562 + Details
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- Definition Amount after adjustments of available-for-sale debt securities at cost, maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27198-111563 + Details
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- Definition Amount after adjustments of available-for-sale debt securities at cost, maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27198-111563 + Details
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- Definition Amount after adjustments of available-for-sale debt securities at cost, maturing after the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27198-111563 + Details
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- Definition Amount after adjustments of available-for-sale debt securities at cost, maturing in the next fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27198-111563 + Details
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