Mortgage Servicing Rights - Additional Information (Detail) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Mortgage Loan Activity [Line Items]
Sale of 1-4 family mortgage loans to Federal National Mortgage Association (FNMA)	$ 42,361,000	$ 87,932,000	$ 86,886,000
Total servicing portfolio	325,800,000stba_PortfolioLoansFairValueDisclosure	327,400,000stba_PortfolioLoansFairValueDisclosure	329,200,000stba_PortfolioLoansFairValueDisclosure
Federal National Mortgage Association or (FNMA) [Member]
Mortgage Loan Activity [Line Items]
Sale of 1-4 family mortgage loans to Federal National Mortgage Association (FNMA)	$ 40,100,000	$ 62,900,000	$ 82,900,000

The cash inflow from sales of loans that are secured with real estate mortgages and are held with the intention to resell in the near future.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 21

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3461-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 25

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3536-108585

Name:	us-gaap_ProceedsFromSaleOfMortgageLoansHeldForSale
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration