Loans and Loans Held for Sale - Additional Information (Detail) (USD $)	3 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unearned income on loans	$ 2,500,000stba_UnearnedIncomeOnLoan	$ 2,100,000stba_UnearnedIncomeOnLoan
Percentage of commercial loans in total portfolio loans	78.00%	75.00%
Commercial Loans	3,649,632,000	2,892,522,000
Maximum concentration of commercial real estate and commercial construction portfolio in loans (in excess of)	9.00%	9.00%
Minimum period of loan payment defaults following restructure for TDRs to be in default	90 days
Commercial Real Estate and Commercial Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	2,400,000,000
Combined percentage of commercial real estate and commercial construction in total commercial loans	67.00%	66.00%
Combined percentage of commercial real estate and commercial construction in total portfolio loans	52.00%	49.00%
Out Of State Exposure Of Combined Portfolio	6.10%	8.00%
Percentage of total loans out-of-state excluding contiguous states	3.20%	3.90%
Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	1,211,053,000	994,138,000
Number Of Loans Modified Not Considered To Be Troubled Debt Restructuring	2
Loans modified not considered to be troubled debt restructuring	200,000
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	2,152,413,000	1,682,236,000
Number Of Loans Modified Not Considered To Be Troubled Debt Restructuring	2
Loans modified not considered to be troubled debt restructuring	200,000
Integrity Bancshares, Inc. [Member] \| Commercial Real Estate and Commercial Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	$ 527,900,000

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

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

Loans modified not considered to be troubled debt restructuring.

+ References

No definition available.

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- Definition

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

+ References

No definition available.

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- Definition

Net loans out of state percentage excluding contiguous states.

+ References

No definition available.

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- Definition

Number Of Loans Modified Not Considered To Be Troubled Debt Restructuring

+ References

No definition available.

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- Definition

Out of state exposure of combined portfolio.

+ References

No definition available.

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- Definition

Percentage of commercial loans in portfolio loans.

+ References

No definition available.

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- Definition

Period of loans in default treated as troubled debt restructuring.

+ References

No definition available.

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- Definition

Unearned income on loan.

+ References

No definition available.

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- Definition

Reflects the aggregate gross carrying amount of all commercial loans and leases held in portfolio, prior to deducting deferred income and the allowance for losses on commercial loans and leases.

+ References

Reference 1: 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=35738203&loc=d3e5074-111524

Reference 2: 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=28367877&loc=d3e4428-111522

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5066-111524

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