Loans and Allowance for Credit Losses - Amortized Cost of Loans to Borrowers, Modified (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified in period	$ 32,949	$ 25,883
% of Portfolio Segment	0.41%	0.33%
Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified in period	$ 19,499	$ 12,934
Payment Delays (Other Than Insignificant)
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified in period	0	12,264
Term Extension and Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified in period	0	685
Term Extension and Payment Delays
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified in period	13,450	0
Commercial loans \| Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified in period		$ 3,689
% of Portfolio Segment		0.14%
Commercial loans \| Commercial real estate \| Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified in period		$ 3,004
Commercial loans \| Commercial real estate \| Payment Delays (Other Than Insignificant)
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified in period		0
Commercial loans \| Commercial real estate \| Term Extension and Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified in period		685
Commercial loans \| Commercial real estate \| Term Extension and Payment Delays
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified in period		0
Commercial loans \| Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified in period	$ 32,372	$ 21,701
% of Portfolio Segment	2.43%	1.61%
Commercial loans \| Commercial and industrial \| Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified in period	$ 18,922	$ 9,437
Commercial loans \| Commercial and industrial \| Payment Delays (Other Than Insignificant)
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified in period	0	12,264
Commercial loans \| Commercial and industrial \| Term Extension and Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified in period	0	0
Commercial loans \| Commercial and industrial \| Term Extension and Payment Delays
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified in period	13,450	0
Consumer loans \| Consumer real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified in period	$ 577	$ 493
% of Portfolio Segment	0.03%	0.03%
Consumer loans \| Consumer real estate \| Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified in period	$ 577	$ 493
Consumer loans \| Consumer real estate \| Payment Delays (Other Than Insignificant)
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified in period	0	0
Consumer loans \| Consumer real estate \| Term Extension and Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified in period	0	0
Consumer loans \| Consumer real estate \| Term Extension and Payment Delays
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified in period	$ 0	$ 0

Amortized cost excluding accrued interest, of financing receivable modified during period for debtor experiencing financial difficulty.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestModifiedPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Percentage, excluding accrued interest, of financing receivable modified during period for debtor experiencing financial difficulty to total financing receivables.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationToTotalFinancingReceivablesExcludingAccruedInterestPercent
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- 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

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481933/310-10-55-12A

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-34

Reference 3: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 33
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-33

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

Name:	us-gaap_FinancingReceivableModificationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_PaymentDeferralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityAndInterestRateReductionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=stba_ExtendedMaturityAndPaymentDeferralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=stba_CommercialAndIndustrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=stba_ConsumerRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: