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Allowance for Loan Losses - Additional Information (Detail) - Jun. 30, 2015 - USD ($) | Total
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Financing Receivable, Impaired [Line Items] |
Evaluation for impairment of substandard and nonaccrual commercial loans | $ 500,000.0
Minimum [Member] |
Financing Receivable, Impaired [Line Items] |
Loans considered nonperforming (in days) | 90 days

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

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- Definition Period interest and principal of loan due treated as loans considered nonperforming. + 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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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_FinancingReceivableImpairedLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |