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Allowance for Loan Losses - Additional Information (Details) | 3 Months Ended
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Mar. 31, 2018 USD ($)
Receivables [Abstract] |
Loans considered nonperforming (in days) | 90 days
Evaluation for impairment of substandard and nonaccrual commercial loans | $ 500,000.0

X
- Definition Minimum amount over which evaluation for impairment of substandard and nonaccrual commercial loans are made. + References No definition available. + Details
Name: | stba_MinimumAmountOverWhichEvaluationForImpairmentOfSubstandardAndNonaccrualCommercialLoansAreMade
Namespace Prefix: | stba_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Period interest and principal of loan due treated as loans considered nonperforming. + References No definition available. + Details
Name: | stba_PeriodInterestAndPrincipalOfLoanDueTreatedAsLoansConsideredNonperforming
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_ReceivablesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration