Regulatory Matters - Summary of Risk-Based Capital Amounts and Ratios for S&T and S&T Bank (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Minimum requirement for most highly-rated financial institutions	3.00%
S&T [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 537,935	$ 494,986
Total Capital (to Risk-Weighted Assets), Actual Ratio	14.27%	14.36%
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	301,548	275,684
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Ratio	8.00%	8.00%
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Amount	376,936	344,606
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	10.00%	10.00%
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	465,114	426,234
Tier 1 Capital (to Risk-Weighted Assets), Actual Ratio	12.34%	12.37%
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	150,774	137,842
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Ratio	4.00%	4.00%
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Amount	226,161	206,763
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	6.00%	6.00%
Leverage Ratio, Actual Amount	465,114	426,234
Leverage Ratio, Actual Ratio	9.80%	9.75%
Leverage Ratio, Minimum Regulatory Capital Requirements Amount	189,895	174,824
Leverage Ratio, Minimum Regulatory Capital Requirements Ratio	4.00%	4.00%
Leverage Ratio, To be Well Capitalized Amount	237,369	218,530
Leverage Ratio, To be Well Capitalized Ratio	5.00%	5.00%
S&T Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to Risk-Weighted Assets), Actual Amount	475,538	457,540
Total Capital (to Risk-Weighted Assets), Actual Ratio	12.68%	13.35%
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	300,095	274,257
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Ratio	8.00%	8.00%
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Amount	375,119	342,821
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	10.00%	10.00%
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	403,593	389,584
Tier 1 Capital (to Risk-Weighted Assets), Actual Ratio	10.76%	11.36%
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	150,048	137,128
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Ratio	4.00%	4.00%
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Amount	225,071	205,693
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	6.00%	6.00%
Leverage Ratio, Actual Amount	403,593	389,584
Leverage Ratio, Actual Ratio	8.53%	8.95%
Leverage Ratio, Minimum Regulatory Capital Requirements Amount	189,182	174,081
Leverage Ratio, Minimum Regulatory Capital Requirements Ratio	4.00%	4.00%
Leverage Ratio, To be Well Capitalized Amount	$ 236,477	$ 217,601
Leverage Ratio, To be Well Capitalized Ratio	5.00%	5.00%

Minimum required leverage capital to assets percentage.

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The amount of total capital as defined in the regulations.

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The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action.

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The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

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Total Capital divided by risk weighted assets as defined in the regulations.

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Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital.

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Tier 1 Risk Based Capital as defined in the regulations.

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Tier 1 capital divided by risk weighted assets as defined by regulations.

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