Employee Benefits - Components of Net Periodic Pension Cost and Other Changes in Plan Assets and Benefit Obligation Recognized in Other Comprehensive Income (Loss) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Components of Net Periodic Pension Cost
Service cost-benefits earned during the period	$ 2,369us-gaap_DefinedBenefitPlanServiceCost	$ 2,767us-gaap_DefinedBenefitPlanServiceCost	$ 2,788us-gaap_DefinedBenefitPlanServiceCost
Interest cost on projected benefit obligation	4,470us-gaap_DefinedBenefitPlanInterestCost	3,985us-gaap_DefinedBenefitPlanInterestCost	4,358us-gaap_DefinedBenefitPlanInterestCost
Expected return on plan assets	(6,907)	(6,207)	(5,564)
Amortization of prior service cost (credit)	(137)	(138)	(137)
Recognized net actuarial loss	941	2,425	2,474
Net Periodic Pension Expense	736	2,832	3,919
Other Changes in Plan Assets and Benefit Obligation Recognized in Other Comprehensive Income (Loss)
Net actuarial loss (gain)	13,294	(15,499)	2,477
Recognized net actuarial loss	(941)	(2,425)	(2,474)
Recognized prior service credit	137	138	137
Total (Before Tax Effects)	12,490	(17,786)	140
Total Recognized in Net Benefit Cost and Other Comprehensive Income (Loss) (Before Tax Effects)	$ 13,226	$ (14,954)	$ 4,059

Employee Benefits - Components of Net Periodic Pension Cost and Other Changes in Plan Assets and Benefit Obligation Recognized in Other Comprehensive Income (Loss) (Detail) (USD $)
In Thousands, unless otherwise specified

12 Months Ended

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Components of Net Periodic Pension Cost

Service cost-benefits earned during the period

$ 2,369us-gaap_DefinedBenefitPlanServiceCost

$ 2,767us-gaap_DefinedBenefitPlanServiceCost

$ 2,788us-gaap_DefinedBenefitPlanServiceCost

Interest cost on projected benefit obligation

4,470us-gaap_DefinedBenefitPlanInterestCost

3,985us-gaap_DefinedBenefitPlanInterestCost

4,358us-gaap_DefinedBenefitPlanInterestCost

Expected return on plan assets

(6,907)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets

(6,207)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets

(5,564)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets

Amortization of prior service cost (credit)

(137)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit

(138)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit

(137)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit

Recognized net actuarial loss

941us-gaap_DefinedBenefitPlanActuarialGainLoss

2,425us-gaap_DefinedBenefitPlanActuarialGainLoss

2,474us-gaap_DefinedBenefitPlanActuarialGainLoss

Net Periodic Pension Expense

736us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost

2,832us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost

3,919us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost

Other Changes in Plan Assets and Benefit Obligation Recognized in Other Comprehensive Income (Loss)

Net actuarial loss (gain)

13,294us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax

(15,499)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax

2,477us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax

Recognized net actuarial loss

(941)stba_DefinedBenefitPlanActuarialGainLossOnOtherChangeInPlanAssetsAndBenefitObligation

(2,425)stba_DefinedBenefitPlanActuarialGainLossOnOtherChangeInPlanAssetsAndBenefitObligation

(2,474)stba_DefinedBenefitPlanActuarialGainLossOnOtherChangeInPlanAssetsAndBenefitObligation

Recognized prior service credit

137us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax

138us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax

137us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax

Total (Before Tax Effects)

12,490stba_OtherComprehensiveIncomeLossDefinedBenefitPlansBeforeTax

(17,786)stba_OtherComprehensiveIncomeLossDefinedBenefitPlansBeforeTax

140stba_OtherComprehensiveIncomeLossDefinedBenefitPlansBeforeTax

Total Recognized in Net Benefit Cost and Other Comprehensive Income (Loss) (Before Tax Effects)

$ 13,226us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax

$ (14,954)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax

$ 4,059us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax

- Definition

Defined benefit plan actuarial gain loss on other change in plan assets and benefit obligation.

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Other comprehensive income loss defined benefit plans before tax.

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Amount of gain (loss) related to change in benefit obligation resulting from changes in actuarial assumptions, for example, but not limited to, interest, mortality, employee turnover, salary, and temporary deviation from the substantive plan.

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- Definition

The amount of the prior service cost or credit recognized in net periodic benefit cost relating to benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation.

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- Definition

An amount calculated as a basis for determining the extent of delayed recognition of the effects of changes in the fair value of assets. The expected return on plan assets is determined based on the expected long-term rate of return on plan assets and the market-related value of plan assets.

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- Definition

The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.

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- Definition

The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments.

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- Definition

The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan.

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Amount before tax of reclassification adjustment from accumulated other comprehensive (income) loss for prior service cost (credit) related to pension and other postretirement benefit plans.

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Amount before tax of (increase) decrease in the value of the projected benefit obligation related to a change in an actuarial assumption and increase (decrease) in the value of the plan assets resulting from experience different from that assumed, that has not been recognized in net periodic benefit (cost) credit.

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Amount before tax of reclassification adjustment from accumulated other comprehensive income (loss) for actuarial gain (loss) related to pension and other postretirement benefit plans.

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