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Derivative Instruments and Hedging Activities - Additional Information (Detail) | 6 Months Ended
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Jun. 30, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract] |
Period for commitments | 60 days

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- Definition Period for commitments. + References No definition available. + Details
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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