XML 70 R60.htm IDEA: XBRL DOCUMENT
v3.5.0.2

Derivative Instruments and Hedging Activities - Additional Information (Detail) | 9 Months Ended
--- | ---
Sep. 30, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract] |
Period for commitments | 60 days

X
- 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

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