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Securities Available-for-Sale - Additional Information (Detail) (USD $) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Dec. 31, 2014 security
Schedule of Available-for-sale Securities [Line Items] |  |
Number of debt securities on which unrealized losses were primarily attributable to changes in interest | 17 stba_NumberOfDebtSecuritiesOnWhichUnrealizedLossesWerePrimarilyAttributableToChangesInInterest | 20 stba_NumberOfDebtSecuritiesOnWhichUnrealizedLossesWerePrimarilyAttributableToChangesInInterest
Carrying values of securities | $ 334,600,000 us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesForFederalHomeLoanBank | $ 289,100,000 us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesForFederalHomeLoanBank
Marketable Equity Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Unrealized losses reclassified out of accumulated other comprehensive income into earnings | $ 0 stba_AvailableforsaleSecuritiesUnrealizedLossesReclassifiedIntoEarningsToRecordOtherThanTemporaryImpairmentLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |

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- Definition Available-for-sale securities, unrealized losses reclassified into earnings to record other than temporary impairment losses. + References No definition available. + Details
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- Definition Number of debt securities on which unrealized losses were primarily attributable to changes in interest. + References No definition available. + Details
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- Definition The carrying amount as of the date of the latest financial statement presented of securities which are owned but transferred to serve as collateral for FHLB debt obligations, and for which the transferee does not have the right by contract or custom to sell or re-pledge them to an unrelated party. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 30 -Section 50 -Paragraph 1A -Subparagraph (b)(1)(i) -URI http://asc.fasb.org/extlink&oid=34720971&loc=SL6224234-111729 + Details
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