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

No definition available.

Name:	stba_CombinedPercentageOfCommercialRealEstateAndCommercialConstructionInCommercialLoans
Namespace Prefix:	stba_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

Name:	stba_CombinedPercentageOfCommercialRealEstateAndCommercialConstructionInPortfolioLoans
Namespace Prefix:	stba_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Commitment to lend additional funds on TDRs

+ References

No definition available.

+ Details

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

- Definition

Financing Receivable, Modifications, Post-Modification Loans Removed From Nonaccrual Status

+ References

No definition available.

+ Details

Name:	stba_FinancingReceivableModificationsPostModificationLoansRemovedFromNonaccrualStatus
Namespace Prefix:	stba_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Loans modified not considered to be troubled debt restructuring.

+ References

No definition available.

+ Details

Name:	stba_LoansModifiedNotConsideredToBeTroubledDebtRestructuring
Namespace Prefix:	stba_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Maximum concentration of commercial real estate and commercial construction portfolio in loans.

+ References

No definition available.

+ Details

Name:	stba_MaximumConcentrationOfCommercialRealEstateAndCommercialConstructionPortfolioInLoans
Namespace Prefix:	stba_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Net loans out of state percentage excluding contiguous states.

+ References

No definition available.

+ Details

Name:	stba_NetLoansOutOfStatePercentageExcludingContiguousStates
Namespace Prefix:	stba_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Number Of Loans Modified Not Considered To Be Troubled Debt Restructuring

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Out of state exposure of combined portfolio.

+ References

No definition available.

+ Details

Name:	stba_OutOfStateExposureOfCombinedPortfolio
Namespace Prefix:	stba_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of commercial loans in portfolio loans.

+ References

No definition available.

+ Details

Name:	stba_PercentageOfCommercialLoansInPortfolioLoans
Namespace Prefix:	stba_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Period of loans in default treated as troubled debt restructuring.

+ References

No definition available.

+ Details

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

- Definition

Threshold Period of Satisfactory Payments to Remove from Nonaccrual Status

+ References

No definition available.

+ Details

Name:	stba_ThresholdPeriodofSatisfactoryPaymentstoRemovefromNonaccrualStatus
Namespace Prefix:	stba_
Data Type:	xbrli:durationItemType
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

No definition available.

+ Details

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

- Definition

Total remaining unamortized or unaccreted amounts of premiums, discounts, and intangible assets, in years subsequent to the year of acquisition, when the total amounts of remaining discounts, premiums, and intangible assets exceed 30 percent of stockholders' equity.

+ References

No definition available.

+ Details

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

- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13531-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6404-108592

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13537-108611

+ Details

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The increase (decrease) during the reporting period in the value of loans or securitized loans that are held with the intention to sell in the near future.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 21
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3461-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 230
-Section 55
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82926135&loc=d3e60097-112785

+ Details

Name:	us-gaap_IncreaseDecreaseInLoansHeldForSale
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of deferred interest and fee income, unamortized costs incurred to originate loans and leases, unamortized loan commitments and loan syndication fees, and premiums over or discounts from face amounts of acquired loans. Excludes amounts for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=84165051&loc=d3e45377-112738

+ Details

Name:	us-gaap_LoansAndLeasesReceivableDeferredIncome
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

+ Details

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

- Definition

Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes loans held for sale. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(a))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Details

Name:	us-gaap_DisposalGroupClassificationAxis=us-gaap_DisposalGroupHeldforsaleNotDiscontinuedOperationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=stba_StateCollegeRetailBranchMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=stba_IntegrityBancsharesInc.Member
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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

Loans and Loans Held for Sale - Additional Information (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) contract tdr loan	Sep. 30, 2016 USD ($) contract tdr loan	Sep. 30, 2017 USD ($) contract tdr loan	Sep. 30, 2016 USD ($) contract tdr loan	Dec. 31, 2016 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	$ 3,600,000		$ 3,600,000		$ 5,200,000
Loans, net of discount related to purchase accounting adjustments	4,600,000		4,600,000		7,100,000
Loans receivable, net	$ 5,868,694,000		5,868,694,000		$ 5,615,212,000
Maximum concentration of commercial real estate and commercial construction portfolio in loans (in excess of)					7.00%
Increase in loans held for sale during the period			$ 44,100,000
Threshold period of satisfactory performance for troubled debt restructuring to be restored to accruing status			6 months
Number of troubled debt restructuring loans returned back to accruing status \| tdr	0	0	1	0
Amount of troubled debt restructuring loans returned to accruing status			$ 2,000,000
Number of loans modified not considered to be troubled debt restructuring \| contract			13
Loans modified not considered to be troubled debt restructuring			$ 11,800,000
Commitment to lend additional funds on troubled debt restructuring	$ 0		$ 0
Minimum period of loan payment defaults following restructure for TDRs to be in default			90 days
Number of troubled debt restructuring loans that defaulted \| loan	0	0	0	0
Disposal Group, Held-for-sale, Not Discontinued Operations \| State College Retail Branch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase in loans held for sale during the period			$ 43,400,000
Integrity Bancshares, Inc.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	$ 415,000,000		$ 415,000,000		$ 543,000,000
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of commercial loans in total portfolio loans	78.00%		78.00%		78.00%
Commercial loans	$ 4,561,391,000		$ 4,561,391,000		$ 4,355,395,000
Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	$ 2,681,693,000		$ 2,681,693,000		2,498,476,000
Number of loans modified not considered to be troubled debt restructuring \| contract	1	1	4	2
Loans modified not considered to be troubled debt restructuring	$ 1,000,000	$ 1,900,000	$ 1,500,000	$ 2,500,000
Commercial Real Estate \| Integrity Bancshares, Inc.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	219,000,000		219,000,000		273,000,000
Commercial Real Estate \| Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loans	2,681,693,000		2,681,693,000		2,498,476,000
Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	1,446,811,000		$ 1,446,811,000		1,401,035,000
Number of loans modified not considered to be troubled debt restructuring \| contract		12	9	15
Loans modified not considered to be troubled debt restructuring		$ 16,900,000	$ 10,300,000	$ 25,600,000
Commercial and Industrial \| Integrity Bancshares, Inc.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	103,000,000		103,000,000		141,000,000
Commercial and Industrial \| Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loans	1,446,811,000		1,446,811,000		1,401,035,000
Commercial Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	432,887,000		432,887,000		455,884,000
Commercial Construction \| Integrity Bancshares, Inc.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	15,000,000		15,000,000		33,000,000
Commercial Construction \| Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loans	432,887,000		432,887,000		455,884,000
Residential Mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	697,367,000		697,367,000		701,982,000
Residential Mortgage \| Integrity Bancshares, Inc.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	60,000,000		60,000,000		74,000,000
Home Equity, Installment and Other Consumer Construction \| Integrity Bancshares, Inc.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net	18,000,000		18,000,000		22,000,000
Commercial Real Estate and Commercial Construction \| Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loans	$ 3,100,000,000		$ 3,100,000,000		$ 3,000,000,000
Combined percentage of commercial real estate and commercial construction in total commercial loans	68.00%		68.00%		68.00%
Combined percentage of commercial real estate and commercial construction in total portfolio loans	54.00%		54.00%		53.00%
Concentration risk percentage of commercial real estate and commercial construction			0.00%		0.00%
Maximum concentration of commercial real estate and commercial construction portfolio in loans (in excess of)	14.00%		14.00%		14.00%
Out of market exposure of combined portfolio (percent)	5.10%		5.10%		5.20%
Percentage of total loans out-of-state excluding contiguous states	2.70%		2.70%		2.70%