XML 48 R72.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Employee Benefits - Additional Information (Detail) | 0 Months Ended | 3 Months Ended
--- | --- | ---
Jan. 01, 2015 | Mar. 31, 2015
Compensation and Retirement Disclosure [Abstract] |  |
Number of consecutive years of employee's compensation |  | 5 years
Number of total years of employee's compensation |  | 10 years
Expected long-term rate of return on plan assets |  | 8.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
Lump sum death benefit to unmarried beneficiary as percent of accrued benefit payable (percent) | 80.00%stba_DefinedBenefitPlanLumpSumDeathBenefitforBeneficiariesofUnmarriedParticipantsPercent |

X
- Definition Defined Benefit Plan, Lump Sum Death Benefit for Beneficiaries of Unmarried Participants, Percent + References No definition available. + Details
Name: | stba_DefinedBenefitPlanLumpSumDeathBenefitforBeneficiariesofUnmarriedParticipantsPercent
Namespace Prefix: | stba_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number Of Consecutive Periods Of Highest Compensation Used To Determine Defined Benefit Pension Plan Benefits + References No definition available. + Details
Name: | stba_NumberOfConsecutivePeriodsOfHighestCompensationUsedToDetermineDefinedBenefitPensionPlanBenefits
Namespace Prefix: | stba_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number Of Periods Used To Determine Defined Benefit Pension Plan Benefits + References No definition available. + Details
Name: | stba_NumberOfPeriodsUsedToDetermineDefinedBenefitPensionPlanBenefits
Namespace Prefix: | stba_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_CompensationAndRetirementDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Glossary Expected Long-Term Rate of Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512013 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration