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Sale of Merchant Card Servicing Business - Additional Information (Detail) (USD $)
 12 Months Ended 3 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Mar. 31, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Proceeds from sale of business   $ 4,750,000us-gaap_ProceedsFromDivestitureOfBusinesses    
Gain on sale of business 0us-gaap_GainLossOnSaleOfBusiness 3,093,000us-gaap_GainLossOnSaleOfBusiness 0us-gaap_GainLossOnSaleOfBusiness  
Merchant Card Servicing Business [Member]        
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Proceeds from sale of business       4,800,000us-gaap_ProceedsFromDivestitureOfBusinesses
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= stba_MerchantCardServicingBusinessMember
Termination fee       1,700,000stba_ContractTerminationCharge
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= stba_MerchantCardServicingBusinessMember
Gain on sale of business       $ 3,100,000us-gaap_GainLossOnSaleOfBusiness
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= stba_MerchantCardServicingBusinessMember
Initial term of agreement 10 years