XML 87 R58.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Allowance for Loan Losses - Additional Information (Detail) (USD $) | 3 Months Ended
--- | ---
Mar. 31, 2015
Financing Receivable, Impaired [Line Items] |
Evaluation for impairment of substandard and nonaccrual commercial loans | 500,000.0 stba_MinimumAmountOverWhichEvaluationForImpairmentOfSubstandardAndNonaccrualCommercialLoansAreMade
Minimum [Member] |
Financing Receivable, Impaired [Line Items] |
Loans considered nonperforming (in days) | 90 days

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

X
- Details
Name: | us-gaap_FinancingReceivableImpairedLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |