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

Combined percentage of commercial real estate and commercial construction in commercial loans.

No definition available.

Name:	stba_CombinedPercentageOfCommercialRealEstateAndCommercialConstructionInCommercialLoans
Namespace Prefix:	stba_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

Combined percentage of commercial real estate and commercial construction in portfolio loans.

No definition available.

Name:	stba_CombinedPercentageOfCommercialRealEstateAndCommercialConstructionInPortfolioLoans
Namespace Prefix:	stba_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

Commitment To Lend Additional Funds

No definition available.

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

Net loans out of state percentage excluding contiguous states.

No definition available.

Name:	stba_NetLoansOutOfStatePercentageExcludingContiguousStates
Namespace Prefix:	stba_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

Number of troubled debt restructuring loans returned back to accruing status.

No definition available.

Name:	stba_NumberOfTroubledDebtRestructuringLoansReturnedBackToAccruingStatus
Namespace Prefix:	stba_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Commitment To Lend Additional Funds

No definition available.

Name:	stba_NumberofCommitmentToLendAdditionalFunds
Namespace Prefix:	stba_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Out of state exposure of combined portfolio.

No definition available.

Name:	stba_OutOfStateExposureOfCombinedPortfolio
Namespace Prefix:	stba_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

Percentage of commercial loans in portfolio loans.

No definition available.

Name:	stba_PercentageOfCommercialLoansInPortfolioLoans
Namespace Prefix:	stba_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

Threshold Period of Satisfactory Payments to Remove from Nonaccrual Status

No definition available.

Name:	stba_ThresholdPeriodofSatisfactoryPaymentstoRemovefromNonaccrualStatus
Namespace Prefix:	stba_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

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.

No definition available.

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

Amount excluding accrued interest, after allowance for credit loss, of financing receivable.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestAfterAllowanceForCreditLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Amount excluding accrued interest, before allowance for credit loss, of financing receivable.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919230-210447

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 3B
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL120267963-210447

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Amount of unamortized loan commitment, origination, and other fees (costs) on financing receivable. Excludes financing receivable covered under loss sharing agreement.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6378536&loc=d3e10092-111533

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5074-111524

Reference 3: http://www.xbrl.org/2003/role/recommendedDisclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 20
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84159169&loc=d3e10152-111534

Name:	us-gaap_FinancingReceivableUnamortizedLoanFeeCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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

Name:	us-gaap_LoanRestructuringModificationNameAxis=stba_PaycheckProtectionProgramLoanCARESActMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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

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

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

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

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

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

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