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v3.20.2

Derivative Instruments and Hedging Activities - Additional Information (Details) | 9 Months Ended
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Sep. 30, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract] |
Period for commitments (in days) | 60 days

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- Definition Period for commitments. + References No definition available. + Details
Name: | stba_PeriodForCommitments
Namespace Prefix: | stba_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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