XML 33 R47.htm IDEA: XBRL DOCUMENT v2.4.1.9
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 17stba_NumberOfDebtSecuritiesOnWhichUnrealizedLossesWerePrimarilyAttributableToChangesInInterest 20stba_NumberOfDebtSecuritiesOnWhichUnrealizedLossesWerePrimarilyAttributableToChangesInInterest
Carrying values of securities $ 334,600,000us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesForFederalHomeLoanBank $ 289,100,000us-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 $ 0stba_AvailableforsaleSecuritiesUnrealizedLossesReclassifiedIntoEarningsToRecordOtherThanTemporaryImpairmentLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember