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Business Combinations - Pro Forma Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Business Acquisition [Line Items] |  |
Net interest income | $ 40,259 us-gaap_InterestIncomeExpenseNet | $ 35,591 us-gaap_InterestIncomeExpenseNet
Net income | 12,835 us-gaap_NetIncomeLoss | 14,033 us-gaap_NetIncomeLoss
Integrity Bancshares, Inc. [Member] |  |
Business Acquisition [Line Items] |  |
Net interest income | 2,200 us-gaap_InterestIncomeExpenseNet / us-gaap_BusinessAcquisitionAxis = stba_IntegrityBancsharesInc.Member |
Net income | 300 us-gaap_NetIncomeLoss / us-gaap_BusinessAcquisitionAxis = stba_IntegrityBancsharesInc.Member |
Business Acquisition, Pro Forma Information [Abstract] |  |
Total revenue | 58,208 us-gaap_BusinessAcquisitionsProFormaRevenue / us-gaap_BusinessAcquisitionAxis = stba_IntegrityBancsharesInc.Member | 53,996 us-gaap_BusinessAcquisitionsProFormaRevenue / us-gaap_BusinessAcquisitionAxis = stba_IntegrityBancsharesInc.Member
Net income (1) | $ 14,502 us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss / us-gaap_BusinessAcquisitionAxis = stba_IntegrityBancsharesInc.Member | $ 16,052 us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss / us-gaap_BusinessAcquisitionAxis = stba_IntegrityBancsharesInc.Member
Earnings per common share: (1) |  |
Basic (in USD per share) | $ 0.42 us-gaap_BusinessAcquisitionProFormaEarningsPerShareBasic / us-gaap_BusinessAcquisitionAxis = stba_IntegrityBancsharesInc.Member | $ 0.46 us-gaap_BusinessAcquisitionProFormaEarningsPerShareBasic / us-gaap_BusinessAcquisitionAxis = stba_IntegrityBancsharesInc.Member
Diluted (in USD per share) | $ 0.42 us-gaap_BusinessAcquisitionProFormaEarningsPerShareDiluted / us-gaap_BusinessAcquisitionAxis = stba_IntegrityBancsharesInc.Member | $ 0.46 us-gaap_BusinessAcquisitionProFormaEarningsPerShareDiluted / us-gaap_BusinessAcquisitionAxis = stba_IntegrityBancsharesInc.Member

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- Definition The pro forma basic net income per share for a period as if the business combination or combinations had been completed at the beginning of a period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(2)-(3) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463 + Details
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