Segments - Financial Information of Segments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Segment Reporting, Asset Reconciling Item [Line Items]
Interest income	$ 43,916	$ 38,665
Interest expense	3,657us-gaap_InterestExpense	3,074us-gaap_InterestExpense
NET INTEREST INCOME	40,259us-gaap_InterestIncomeExpenseNet	35,591us-gaap_InterestIncomeExpenseNet
Provision for loan losses	1,207us-gaap_ProvisionForLoanAndLeaseLosses	289us-gaap_ProvisionForLoanAndLeaseLosses
Noninterest income	12,084us-gaap_NoninterestIncome	11,416us-gaap_NoninterestIncome
Noninterest expense	32,243stba_NoninterestExpenseBySegment	27,777stba_NoninterestExpenseBySegment
Depreciation expense	1,030us-gaap_Depreciation	821us-gaap_Depreciation
Amortization of intangible assets	348us-gaap_AmortizationOfIntangibleAssets	316us-gaap_AmortizationOfIntangibleAssets
Provision for income taxes	4,680us-gaap_IncomeTaxExpenseBenefit	3,771us-gaap_IncomeTaxExpenseBenefit
Net Income	12,835us-gaap_NetIncomeLoss	14,033us-gaap_NetIncomeLoss
Operating Segments [Member] \| Community Banking [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Interest income	43,887	38,630
Interest expense	3,889	3,482
NET INTEREST INCOME	39,998	35,148
Provision for loan losses	1,207	289
Noninterest income	7,537	6,854
Noninterest expense	28,687	23,973
Depreciation expense	1,011	803
Amortization of intangible assets	326	292
Provision for income taxes	4,256	3,365
Net Income	12,048	13,280
Operating Segments [Member] \| Insurance [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Interest income	0	0
Interest expense	0	0
NET INTEREST INCOME	0	0
Provision for loan losses	0	0
Noninterest income	1,553	1,521
Noninterest expense	1,137	1,155
Depreciation expense	12	11
Amortization of intangible assets	13	13
Provision for income taxes	137	120
Net Income	254	222
Operating Segments [Member] \| Wealth Management [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Interest income	140	174
Interest expense	0	0
NET INTEREST INCOME	140	174
Provision for loan losses	0	0
Noninterest income	2,916	2,935
Noninterest expense	2,220	2,274
Depreciation expense	7	7
Amortization of intangible assets	9	11
Provision for income taxes	287	286
Net Income	533	531
Eliminations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Interest income	(111)	(139)
Interest expense	(232)	(408)
NET INTEREST INCOME	121	269
Provision for loan losses	0	0
Noninterest income	78	106
Noninterest expense	199	375
Depreciation expense	0	0
Amortization of intangible assets	0	0
Provision for income taxes	0	0
Net Income	$ 0	$ 0

Noninterest expense by segment.

No definition available.

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- Definition

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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- Definition

The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

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- Definition

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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-Glossary Income Tax Expense (or Benefit)

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- Definition

Represents the total of interest and dividend income, including any amortization and accretion (as applicable) of discounts and premiums, earned from (1) loans and leases whether held-for-sale or held-in-portfolio; (2) investment securities; (3) federal funds sold; (4) securities purchased under agreements to resell; (5) investments in banker's acceptances, commercial paper, or certificates of deposit; (6) dividend income; or (7) other investments not otherwise specified herein.

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- Definition

Amount of the cost of borrowed funds accounted for as interest expense.

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The net amount of operating interest income (expense).

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- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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The total amount of noninterest income which may be derived from: (1) fees and commissions; (2) premiums earned; (3) insurance policy charges; (4) the sale or disposal of assets; and (5) other sources not otherwise specified.

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- Definition

Amount of expense related to estimated loss from loan and lease transactions.

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