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Commitments and Contingencies - Additional Information (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract] |  |  |
Allowance for unfunded commitments | $ 2,600,000 | $ 2,500,000 |
Data processing and communication charges | 10,400,000 | 11,700,000 | $ 9,800,000
Data processing, contract termination and conversion cost expenses | $ 0 | $ 1,300,000 | $ 0

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- Definition Allowance for unfunded commitments. + References No definition available. + Details
Name: | stba_AllowanceForUnfundedCommitments
Namespace Prefix: | stba_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Data processing contract termination and conversion cost expenses. + References No definition available. + Details
Name: | stba_DataProcessingContractTerminationAndConversionCostExpenses
Namespace Prefix: | stba_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The amount of expense in the period for communications and data processing expense. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.4) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
Name: | us-gaap_CommunicationsAndInformationTechnology
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration