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
Minimum [Member]
Financing Receivable, Impaired [Line Items]
Loans considered nonperforming (in days)	90 days

Minimum amount over which evaluation for impairment of substandard and nonaccrual commercial loans are made.

No definition available.

Name:	stba_MinimumAmountOverWhichEvaluationForImpairmentOfSubstandardAndNonaccrualCommercialLoansAreMade
Namespace Prefix:	stba_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant