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Segments - Total Assets by Reportable Operating Segment (Detail) (USD $) In Thousands, unless otherwise specified | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 segment | Dec. 31, 2014
Segment Reporting, Asset Reconciling Item [Line Items] |  |
Number of Reportable Segments | 3 us-gaap_NumberOfReportableSegments |
Total Assets | $ 5,971,293 us-gaap_Assets | $ 4,964,686 us-gaap_Assets
Community Banking [Member] |  |
Segment Reporting, Asset Reconciling Item [Line Items] |  |
Total Assets | 5,959,651 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = stba_CommunityBankingMember | 4,954,728 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = stba_CommunityBankingMember
Insurance [Member] |  |
Segment Reporting, Asset Reconciling Item [Line Items] |  |
Total Assets | 8,098 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = stba_InsuranceMember | 7,468 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = stba_InsuranceMember
Wealth Management [Member] |  |
Segment Reporting, Asset Reconciling Item [Line Items] |  |
Total Assets | $ 3,544 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = stba_WealthManagementMember | $ 2,490 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = stba_WealthManagementMember

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- Definition Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.18) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 12 -Article 7 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 18 -Article 5 + Details
Name: | us-gaap_Assets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements. + References No definition available. + Details
Name: | us-gaap_NumberOfReportableSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_SegmentReportingAssetReconcilingItemLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=stba_CommunityBankingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=stba_InsuranceMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=stba_WealthManagementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |