Regulatory Matters - Summary of Risk-Based Capital Amounts and Ratios (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
S&T
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Leverage Ratio, Actual Amount	$ 628,876	$ 582,155
Leverage Ratio, Actual Ratio	9.17%	8.98%
Leverage Ratio, Minimum Regulatory Capital Requirements Amount	$ 274,254	$ 259,170
Leverage Ratio, Minimum Regulatory Capital Requirements Ratio	4.00%	4.00%
Leverage Ratio, To be Well Capitalized Amount	$ 342,818	$ 323,963
Leverage Ratio, To be Well Capitalized Ratio	5.00%	5.00%
Tier One Common Equity Capital	$ 608,876	$ 562,155
Tier One Common Equity Capital to Risk Weighted Assets	10.71%	10.04%
Tier One Common Equity Capital Required for Capital Adequacy	$ 255,778	$ 252,079
Tier One Common Equity Capital Required for Capital Adequacy to Risk Weighted Assets	4.50%	4.50%
Tier One Common Equity Capital Required to be Well Capitalized	$ 369,457	$ 364,114
Tier One Common Equity Capital Required to be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	$ 628,876	$ 582,155
Tier 1 Capital (to Risk-Weighted Assets), Actual Ratio	11.06%	10.39%
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 341,037	$ 336,105
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Ratio	6.00%	6.00%
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Amount	$ 454,717	$ 448,140
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	8.00%	8.00%
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 713,056	$ 664,184
Total Capital (to Risk-Weighted Assets), Actual Ratio	12.55%	11.86%
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 454,717	$ 448,140
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	$ 568,396	$ 560,175
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	10.00%	10.00%
S&T Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Leverage Ratio, Actual Amount	$ 582,929	$ 542,048
Leverage Ratio, Actual Ratio	8.52%	8.39%
Leverage Ratio, Minimum Regulatory Capital Requirements Amount	$ 273,538	$ 258,460
Leverage Ratio, Minimum Regulatory Capital Requirements Ratio	4.00%	4.00%
Leverage Ratio, To be Well Capitalized Amount	$ 341,922	$ 323,075
Leverage Ratio, To be Well Capitalized Ratio	5.00%	5.00%
Tier One Common Equity Capital	$ 582,929	$ 542,048
Tier One Common Equity Capital to Risk Weighted Assets	10.29%	9.71%
Tier One Common Equity Capital Required for Capital Adequacy	$ 255,024	$ 251,213
Tier One Common Equity Capital Required for Capital Adequacy to Risk Weighted Assets	4.50%	4.50%
Tier One Common Equity Capital Required to be Well Capitalized	$ 368,368	$ 362,864
Tier One Common Equity Capital Required to be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	$ 582,929	$ 542,048
Tier 1 Capital (to Risk-Weighted Assets), Actual Ratio	10.29%	9.71%
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 340,032	$ 334,951
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Ratio	6.00%	6.00%
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Amount	$ 453,375	$ 446,601
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	8.00%	8.00%
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 666,560	$ 622,469
Total Capital (to Risk-Weighted Assets), Actual Ratio	11.76%	11.15%
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 453,375	$ 446,602
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	$ 566,719	$ 558,252
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	10.00%	10.00%

Tier One Common Equity Capital

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Tier One Common Equity Capital Required for Capital Adequacy to Risk Weighted Assets

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Tier One Common Equity Capital Required to be Well Capitalized

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Tier One Common Equity Capital Required to be Well Capitalized to Risk Weighted Assets

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Tier One Common Equity Capital to Risk Weighted Assets

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The amount of total capital as defined in the regulations.

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The minimum amount of Total Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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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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The minimum amount of Tier 1 Leverage Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum amount of Tier 1 Leverage Capital for capital adequacy purposes divided by average assets as defined in the regulations.

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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized divided by average assets as defined in the regulatory framework for prompt corrective action.

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Tier 1 Leverage Capital divided by average assets as defined in the regulations.

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Tier 1 Risk Based Capital as defined in the regulations.

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The minimum amount of Tier 1 Risk Based Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum Tier One Capital Ratio (Tier one 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 Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action.

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

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