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Dividend and Loan Restrictions - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2015
Equity [Abstract] |
Percentage of collateralized loans | 10.00%

X
- Definition Percentage of loan collateral balance. + References No definition available. + Details
Name: | stba_PercentageOfLoanCollateralBalance
Namespace Prefix: | stba_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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