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Securities Available-for-sale - Unrealized Gains (Losses) (Details) (USD $) In Thousands, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Investments, Debt and Equity Securities [Abstract] |  |
Total unrealized gains/(losses) on securities available for sale, Gross Unrealized Gains | $ 16,250 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax | $ 12,647 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Total unrealized gains/(losses) on securities available for sale, Gross Unrealized Losses | (457)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax | (1,837)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Total unrealized gains/(losses) on securities available for sale, Net Unrealized Gains/(Losses) | 15,793 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax | 10,810 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
Income Tax expense/(benefit), Gross Unrealized Gains | 5,688 stba_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesGrossUnrealizedGainAdjustmentTaxExpenseBenefit | 4,426 stba_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesGrossUnrealizedGainAdjustmentTaxExpenseBenefit
Income Tax expense/(benefit), Gross Unrealized Losses | (160)stba_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesGrossUnrealizedLossAdjustmentTaxExpenseBenefit | (643)stba_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesGrossUnrealizedLossAdjustmentTaxExpenseBenefit
Income Tax expense/(benefit), Net Unrealized Gains/ (Losses) | 5,528 stba_AccumulatedOtherComprehensiveIncomeLossAvailableforsaleSecuritiesAdjustmentTaxExpenseBenefit | 3,783 stba_AccumulatedOtherComprehensiveIncomeLossAvailableforsaleSecuritiesAdjustmentTaxExpenseBenefit
Net unrealized gains/(losses), net of tax included in accumulated other comprehensive income/(loss), Gross Unrealized Gains | 10,562 stba_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesGrossUnrealizedGainAdjustmentNetOfTax | 8,221 stba_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesGrossUnrealizedGainAdjustmentNetOfTax
Net unrealized gains/(losses), net of tax included in accumulated other comprehensive income/(loss), Gross Unrealized Losses | (297)stba_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesGrossUnrealizedLossesAdjustmentNetOfTax | (1,194)stba_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesGrossUnrealizedLossesAdjustmentNetOfTax
Net unrealized gains/(losses), net of tax included in accumulated other comprehensive income/(loss), Net Unrealized Gains/ (Losses) | $ 10,265 us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax | $ 7,027 us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax

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- Definition Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Tax Expense (Benefit) + References No definition available. + Details
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- Definition Accumulated Other Comprehensive Income Loss Available For Sale Securities Gross Unrealized Gain Adjustment Net Of Tax + References No definition available. + Details
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- Definition Accumulated Other Comprehensive Income Loss Available For Sale Securities Gross Unrealized Gain Adjustment, Tax Expense (Benefit) + References No definition available. + Details
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- Definition Accumulated Other Comprehensive Income Loss Available For Sale Securities Gross Unrealized Loss Adjustment, Tax Expense (Benefit) + References No definition available. + Details
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- Definition Accumulated appreciation or loss, net of tax, in value of the total of available-for-sale securities at the end of an accounting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e637-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e681-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669686-108580 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669646-108580 + Details
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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 + Details
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