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.

+ References

No definition available.

+ Details

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

- 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

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

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=64851502&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=73719730&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=64851502&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=73719730&loc=d3e13537-108611

+ Details

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

- Definition

The net gain (loss) resulting from a sale of loans, including adjustments to record loans classified as held-for-sale at the lower-of-cost-or-market and fair value adjustments to loan held for investment purposes.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.3(a))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.13(h))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585

+ Details

Name:	us-gaap_GainLossOnSalesOfLoansNet
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=65890831&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=66008989&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=25496975&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=65884687&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 for bad debt, of increase (decrease) in loan receivables held-for-sale that are not part of a disposal group.

+ References

+ Details

Name:	us-gaap_LoansReceivableHeldForSaleReconciliationToCashFlowPeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_EquitySecuritiesByIndustryAxis=us-gaap_CommercialAndIndustrialSectorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerBorrowerMember
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_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
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 (Detail) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016 USD ($) contract	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) contract	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	$ 4,600		$ 4,600		$ 3,200
Loans, net of discount related to purchase accounting adjustments	8,200		8,200		$ 10,900
Maximum concentration of commercial real estate and commercial construction portfolio in loans (in excess of)					7.00%
Decrease in loans held for sale during the period			23,600
Gain on sale of credit card portfolio	0	$ 0	2,066	$ 0
Commitment to lend additional funds on troubled debt restructuring	$ 300		$ 300
Minimum period of loan payment defaults following restructure for TDRs to be in default			90 days
Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans modified not considered to be troubled debt restructuring \| contract	12		15
Loans modified not considered to be troubled debt restructuring	$ 16,900		$ 25,600
Installment and Other Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Decrease in loans held for sale during the period			25,000
Gain on sale of credit card portfolio			$ 2,100
Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans modified not considered to be troubled debt restructuring \| contract	1		2
Loans modified not considered to be troubled debt restructuring	$ 1,900		$ 2,500
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of commercial loans in total portfolio loans	77.00%		77.00%		76.00%
Commercial Loans	$ 4,173,585		$ 4,173,585		$ 3,836,877
Maximum concentration of commercial real estate and commercial construction portfolio in loans (in excess of)	7.00%		7.00%		7.00%
Commercial \| Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	$ 1,344,297		$ 1,344,297		$ 1,256,830
Commercial \| Commercial Real Estate and Commercial Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	$ 2,800,000		$ 2,800,000		$ 2,600,000
Combined percentage of commercial real estate and commercial construction in total commercial loans	68.00%		68.00%		67.00%
Combined percentage of commercial real estate and commercial construction in total portfolio loans	52.00%		52.00%		51.00%
Concentration risk percentage of commercial real estate and commercial construction			0.00%		0.00%
Out of market exposure of combined portfolio (percent)	5.40%		5.40%		5.80%
Percentage of total loans out-of-state excluding contiguous states	2.80%		2.80%		3.00%
Commercial \| Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	$ 2,427,164		$ 2,427,164		$ 2,166,603