XML 131 R115.htm IDEA: XBRL DOCUMENT
v3.3.1.900

Commitments and Contingencies - Additional Information (Detail) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Commitments and Contingencies Disclosure [Abstract] |  |  |
Allowance for unfunded commitments | $ 2,500,000 | $ 2,300,000 |
Data processing and communication charges | 11,700,000 | 9,800,000 | $ 9,500,000
Data Processing, Contract Termination and Conversion Cost Expenses | $ 1,300,000 | $ 0 | $ 800,000

X
- Definition Allowance for unfunded commitments. + References No definition available. + Details
Name: | stba_AllowanceForUnfundedCommitments
Namespace Prefix: | stba_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Data processing contract termination and conversion cost expenses. + References No definition available. + Details
Name: | stba_DataProcessingContractTerminationAndConversionCostExpenses
Namespace Prefix: | stba_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The amount of expense in the period for communications and data processing expense. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.4) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 + Details
Name: | us-gaap_CommunicationsAndInformationTechnology
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration