Debt instrument call date description.

No definition available.

Name:	stba_DebtInstrumentCallDateDescription
Namespace Prefix:	stba_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Junior Subordinated Debentures [Line Items]

+ References

No definition available.

+ Details

Name:	stba_JuniorSubordinatedDebenturesLineItems
Namespace Prefix:	stba_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Regulatory capital.

+ References

No definition available.

+ Details

Name:	stba_RegulatoryCapital
Namespace Prefix:	stba_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Carrying value as of the balance sheet date of long-term debt (with maturities initially due after one year or beyond the operating cycle if longer) identified as Junior Subordinated Notes, which have a lower priority than senior instruments, excluding current portion.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Weighted average interest rate of long-term debt outstanding calculated at point in time.

+ References

No definition available.

+ Details

Name:	us-gaap_LongtermDebtWeightedAverageInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_JuniorSubordinatedDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=stba_TrustPreferredSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=stba_A2001TrustPreferredSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=stba_A2005TrustPreferredSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=stba_A2015JuniorSubordinatedDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_PrimeRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=stba_A2006JuniorSubordinatedDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=stba_A2008TrustPreferredSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Long-Term Borrowings and Subordinated Debt - Key Terms of Junior Subordinated Debt Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Junior Subordinated Debentures [Line Items]
Junior subordinated debt securities	$ 64,277	$ 45,619	$ 45,619
Interest Rate at December 31, 2019	2.60%	2.84%	1.88%
Junior Subordinated Debt
Junior Subordinated Debentures [Line Items]
Junior subordinated debt securities	$ 34,753	$ 25,000	$ 25,000
Trust Preferred Securities
Junior Subordinated Debentures [Line Items]
Junior subordinated debt securities	29,524	$ 20,619	$ 20,619
2001 Trust Preferred Securities \| Trust Preferred Securities
Junior Subordinated Debentures [Line Items]
Junior subordinated debt securities	$ 5,155
Stated Maturity Date	Jul. 25, 2031
Optional redemption date at par	Any time after 7/25/2011
Regulatory Capital	Tier 1
Interest Rate at December 31, 2019	6.00%
2001 Trust Preferred Securities \| Trust Preferred Securities \| LIBOR
Junior Subordinated Debentures [Line Items]
Interest Rate	3.75%
2005 Trust Preferred Securities \| Trust Preferred Securities
Junior Subordinated Debentures [Line Items]
Junior subordinated debt securities	$ 4,124
Stated Maturity Date	May 23, 2035
Optional redemption date at par	Any time after 5/23/2010
Regulatory Capital	Tier 1
Interest Rate at December 31, 2019	3.68%
2005 Trust Preferred Securities \| Trust Preferred Securities \| LIBOR
Junior Subordinated Debentures [Line Items]
Interest Rate	1.77%
2015 Junior Subordinated Debt \| Junior Subordinated Debt
Junior Subordinated Debentures [Line Items]
Junior subordinated debt securities	$ 9,750
Stated Maturity Date	Mar. 06, 2025
Optional redemption date at par	Quarterly after 4/1/2020
Regulatory Capital	Tier 2
Interest Rate at December 31, 2019	4.25%
2015 Junior Subordinated Debt \| Junior Subordinated Debt \| Prime Rate
Junior Subordinated Debentures [Line Items]
Interest Rate	1.00%
2006 Junior Subordinated Debt \| Junior Subordinated Debt
Junior Subordinated Debentures [Line Items]
Junior subordinated debt securities	$ 25,000
Stated Maturity Date	Dec. 15, 2036
Optional redemption date at par	Any time after 9/15/2011
Regulatory Capital	Tier 2
Interest Rate at December 31, 2019	3.49%
2006 Junior Subordinated Debt \| Junior Subordinated Debt \| LIBOR
Junior Subordinated Debentures [Line Items]
Interest Rate	1.60%
2008 Trust Preferred Securities \| Trust Preferred Securities
Junior Subordinated Debentures [Line Items]
Junior subordinated debt securities	$ 20,619
Stated Maturity Date	Mar. 15, 2038
Optional redemption date at par	Any time after 3/15/2013
Regulatory Capital	Tier 1
Interest Rate at December 31, 2019	5.39%
2008 Trust Preferred Securities \| Trust Preferred Securities \| LIBOR
Junior Subordinated Debentures [Line Items]
Interest Rate	3.50%