Debt instrument call date description.

No definition available.

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

- Details

Name:	stba_JuniorSubordinatedDebenturesLineItems
Namespace Prefix:	stba_
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Balance Type:	na
Period Type:	duration

- Definition

Long-term Debt, Trust Preferred Securities

+ References

No definition available.

+ Details

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Namespace Prefix:	stba_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-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

+ 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://www.xbrl.org/2003/role/presentationRef

-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

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

+ 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.

+ References

No definition available.

+ Details

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

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
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Long-Term Borrowings and Subordinated Debt - Schedule of Junior Subordinated Debt Securities (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Junior Subordinated Debentures [Line Items]
Junior Subordinated Debt	$ 45,619us-gaap_JuniorSubordinatedLongTermNotes	$ 45,619us-gaap_JuniorSubordinatedLongTermNotes	$ 90,619us-gaap_JuniorSubordinatedLongTermNotes
Interest Rate at December 31, 2014	3.00%	3.01%	3.17%
2006 Junior Subordinated Debt [Member]
Junior Subordinated Debentures [Line Items]
Junior Subordinated Debt	25,000	25,000	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, 2014	1.84%
2008 Junior Subordinated Debt-Trust Preferred Securities [Member]
Junior Subordinated Debentures [Line Items]
Junior Subordinated Debt	20,619	20,619	20,619
Trust Preferred 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, 2014	3.74%
2008 Junior Subordinated Debt [Member]
Junior Subordinated Debentures [Line Items]
Junior Subordinated Debt	20,000
Stated Maturity Date	Jun. 15, 2018
Optional redemption date at par	Any time after 6/15/2013
Regulatory Capital	Tier 2
Interest Rate at December 31, 2014	0.00%
2008 Junior Subordinated Debt [Member]
Junior Subordinated Debentures [Line Items]
Junior Subordinated Debt	$ 25,000
Stated Maturity Date	May 30, 2018
Optional redemption date at par	Any time after 5/30/2013
Regulatory Capital	Tier 2
Interest Rate at December 31, 2014	0.00%
3-Month LIBOR [Member] \| 2006 Junior Subordinated Debt [Member]
Junior Subordinated Debentures [Line Items]
Basis spread on base interest rate	1.60%
3-Month LIBOR [Member] \| 2008 Junior Subordinated Debt-Trust Preferred Securities [Member]
Junior Subordinated Debentures [Line Items]
Basis spread on base interest rate	3.50%
3-Month LIBOR [Member] \| 2008 Junior Subordinated Debt [Member]
Junior Subordinated Debentures [Line Items]
Basis spread on base interest rate	3.50%
3-Month LIBOR [Member] \| 2008 Junior Subordinated Debt [Member]
Junior Subordinated Debentures [Line Items]
Basis spread on base interest rate	2.50%