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Dividend and Loan Restrictions - Additional Information (Details)	12 Months Ended
	Dec. 31, 2016
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