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Segments - Total Assets by Reportable Operating Segment (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Segment Reporting, Asset Reconciling Item [Line Items]    
Total Assets $ 4,964,686us-gaap_Assets $ 4,533,190us-gaap_Assets
Community Banking [Member]    
Segment Reporting, Asset Reconciling Item [Line Items]    
Total Assets 4,954,728us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= stba_CommunityBankingMember		 4,524,939us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= stba_CommunityBankingMember
Insurance [Member]    
Segment Reporting, Asset Reconciling Item [Line Items]    
Total Assets 7,468us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= stba_InsuranceMember		 6,926us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= stba_InsuranceMember
Wealth Management [Member]    
Segment Reporting, Asset Reconciling Item [Line Items]    
Total Assets $ 2,490us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= stba_WealthManagementMember		 $ 1,325us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= stba_WealthManagementMember