Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
ASSETS
Debt securities, available-for-sale	$ 770,393	$ 779,133
Marketable equity securities	3,300	5,150
U.S. Treasury securities
ASSETS
Debt securities, available-for-sale	10,282	10,040
Obligations of U.S. government corporations and agencies
ASSETS
Debt securities, available-for-sale	82,904	157,697
Collateralized mortgage obligations of U.S. government corporations and agencies
ASSETS
Debt securities, available-for-sale	209,296	189,348
Residential mortgage-backed securities of U.S. government corporations and agencies
ASSETS
Debt securities, available-for-sale	67,778	22,418
Commercial mortgage-backed securities of U.S. government corporations and agencies
ASSETS
Debt securities, available-for-sale	273,681	275,870
Obligations of states and political subdivisions
ASSETS
Debt securities, available-for-sale	124,427	116,133
Fair Value Measurements, Recurring
ASSETS
Debt securities, available-for-sale	770,393	779,133
Marketable equity securities	3,300	5,150
Total Securities	773,693	784,283
Securities held in a deferred compensation plan	6,794	5,987
Total Assets	861,706	816,239
LIABILITIES
Total Liabilities	79,418	25,615
Fair Value Measurements, Recurring \| U.S. Treasury securities
ASSETS
Debt securities, available-for-sale	10,282	10,040
Fair Value Measurements, Recurring \| Obligations of U.S. government corporations and agencies
ASSETS
Debt securities, available-for-sale	82,904	157,697
Fair Value Measurements, Recurring \| Collateralized mortgage obligations of U.S. government corporations and agencies
ASSETS
Debt securities, available-for-sale	209,296	189,348
Fair Value Measurements, Recurring \| Residential mortgage-backed securities of U.S. government corporations and agencies
ASSETS
Debt securities, available-for-sale	67,778	22,418
Fair Value Measurements, Recurring \| Commercial mortgage-backed securities of U.S. government corporations and agencies
ASSETS
Debt securities, available-for-sale	273,681	275,870
Fair Value Measurements, Recurring \| Corporate obligations
ASSETS
Debt securities, available-for-sale	2,025	7,627
Fair Value Measurements, Recurring \| Obligations of states and political subdivisions
ASSETS
Debt securities, available-for-sale	124,427	116,133
Fair Value Measurements, Recurring \| Interest rate swaps
ASSETS
Derivative financial assets	78,319	25,647
LIABILITIES
Derivative financial liabilities	79,033	25,615
Fair Value Measurements, Recurring \| Interest rate lock commitments
ASSETS
Derivative financial assets	2,900	321
Fair Value Measurements, Recurring \| Forward sale contracts
ASSETS
Derivative financial assets		1
LIABILITIES
Derivative financial liabilities	385
Fair Value Measurements, Recurring \| Level 1
ASSETS
Debt securities, available-for-sale	0	0
Marketable equity securities	3,228	5,078
Total Securities	3,228	5,078
Securities held in a deferred compensation plan	6,794	5,987
Total Assets	10,022	11,065
LIABILITIES
Total Liabilities	0	0
Fair Value Measurements, Recurring \| Level 1 \| U.S. Treasury securities
ASSETS
Debt securities, available-for-sale	0	0
Fair Value Measurements, Recurring \| Level 1 \| Obligations of U.S. government corporations and agencies
ASSETS
Debt securities, available-for-sale	0	0
Fair Value Measurements, Recurring \| Level 1 \| Collateralized mortgage obligations of U.S. government corporations and agencies
ASSETS
Debt securities, available-for-sale	0	0
Fair Value Measurements, Recurring \| Level 1 \| Residential mortgage-backed securities of U.S. government corporations and agencies
ASSETS
Debt securities, available-for-sale	0	0
Fair Value Measurements, Recurring \| Level 1 \| Commercial mortgage-backed securities of U.S. government corporations and agencies
ASSETS
Debt securities, available-for-sale	0	0
Fair Value Measurements, Recurring \| Level 1 \| Corporate obligations
ASSETS
Debt securities, available-for-sale	0	0
Fair Value Measurements, Recurring \| Level 1 \| Obligations of states and political subdivisions
ASSETS
Debt securities, available-for-sale	0	0
Fair Value Measurements, Recurring \| Level 1 \| Interest rate swaps
ASSETS
Derivative financial assets	0	0
LIABILITIES
Derivative financial liabilities	0	0
Fair Value Measurements, Recurring \| Level 1 \| Interest rate lock commitments
ASSETS
Derivative financial assets	0	0
Fair Value Measurements, Recurring \| Level 1 \| Forward sale contracts
ASSETS
Derivative financial assets		0
LIABILITIES
Derivative financial liabilities	0
Fair Value Measurements, Recurring \| Level 2
ASSETS
Debt securities, available-for-sale	770,393	779,133
Marketable equity securities	72	72
Total Securities	770,465	779,205
Securities held in a deferred compensation plan	0	0
Total Assets	848,784	805,174
LIABILITIES
Total Liabilities	79,418	25,615
Fair Value Measurements, Recurring \| Level 2 \| U.S. Treasury securities
ASSETS
Debt securities, available-for-sale	10,282	10,040
Fair Value Measurements, Recurring \| Level 2 \| Obligations of U.S. government corporations and agencies
ASSETS
Debt securities, available-for-sale	82,904	157,697
Fair Value Measurements, Recurring \| Level 2 \| Collateralized mortgage obligations of U.S. government corporations and agencies
ASSETS
Debt securities, available-for-sale	209,296	189,348
Fair Value Measurements, Recurring \| Level 2 \| Residential mortgage-backed securities of U.S. government corporations and agencies
ASSETS
Debt securities, available-for-sale	67,778	22,418
Fair Value Measurements, Recurring \| Level 2 \| Commercial mortgage-backed securities of U.S. government corporations and agencies
ASSETS
Debt securities, available-for-sale	273,681	275,870
Fair Value Measurements, Recurring \| Level 2 \| Corporate obligations
ASSETS
Debt securities, available-for-sale	2,025	7,627
Fair Value Measurements, Recurring \| Level 2 \| Obligations of states and political subdivisions
ASSETS
Debt securities, available-for-sale	124,427	116,133
Fair Value Measurements, Recurring \| Level 2 \| Interest rate swaps
ASSETS
Derivative financial assets	78,319	25,647
LIABILITIES
Derivative financial liabilities	79,033	25,615
Fair Value Measurements, Recurring \| Level 2 \| Interest rate lock commitments
ASSETS
Derivative financial assets	0	321
Fair Value Measurements, Recurring \| Level 2 \| Forward sale contracts
ASSETS
Derivative financial assets		1
LIABILITIES
Derivative financial liabilities	385
Fair Value Measurements, Recurring \| Level 3
ASSETS
Debt securities, available-for-sale	0	0
Marketable equity securities	0	0
Total Securities	0	0
Securities held in a deferred compensation plan	0	0
Total Assets	2,900	0
LIABILITIES
Total Liabilities	0	0
Fair Value Measurements, Recurring \| Level 3 \| U.S. Treasury securities
ASSETS
Debt securities, available-for-sale	0	0
Fair Value Measurements, Recurring \| Level 3 \| Obligations of U.S. government corporations and agencies
ASSETS
Debt securities, available-for-sale	0	0
Fair Value Measurements, Recurring \| Level 3 \| Collateralized mortgage obligations of U.S. government corporations and agencies
ASSETS
Debt securities, available-for-sale	0	0
Fair Value Measurements, Recurring \| Level 3 \| Residential mortgage-backed securities of U.S. government corporations and agencies
ASSETS
Debt securities, available-for-sale	0	0
Fair Value Measurements, Recurring \| Level 3 \| Commercial mortgage-backed securities of U.S. government corporations and agencies
ASSETS
Debt securities, available-for-sale	0	0
Fair Value Measurements, Recurring \| Level 3 \| Corporate obligations
ASSETS
Debt securities, available-for-sale	0	0
Fair Value Measurements, Recurring \| Level 3 \| Obligations of states and political subdivisions
ASSETS
Debt securities, available-for-sale	0	0
Fair Value Measurements, Recurring \| Level 3 \| Interest rate swaps
ASSETS
Derivative financial assets	0	0
LIABILITIES
Derivative financial liabilities	0	0
Fair Value Measurements, Recurring \| Level 3 \| Interest rate lock commitments
ASSETS
Derivative financial assets	2,900	0
Fair Value Measurements, Recurring \| Level 3 \| Forward sale contracts
ASSETS
Derivative financial assets		$ 0
LIABILITIES
Derivative financial liabilities	$ 0

Assets Held-in-Trust, Fair Value Disclosure

No definition available.

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

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

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

- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

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

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

+ Details

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

- Definition

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

+ Details

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

- References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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

- Definition

Fair value of financial and nonfinancial obligations.

+ References

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CollateralizedMortgageObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateBondSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_InterestRateLockCommitmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ForwardContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: