Regulatory Matters - Summary of Risk-Based Capital Amounts and Ratios (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
S&T
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Leverage Ratio, Actual Amount	$ 777,913	$ 713,056
Leverage Ratio, Actual Ratio	13.21%	12.55%
Leverage Ratio, Minimum Regulatory Capital Requirements Amount	$ 470,992	$ 454,717
Leverage Ratio, Minimum Regulatory Capital Requirements Ratio	8.00%	8.00%
Leverage Ratio, To be Well Capitalized Amount	$ 588,741	$ 568,396
Leverage Ratio, To be Well Capitalized Ratio	10.00%	10.00%
Tier One Common Equity Capital	$ 689,778	$ 628,876
Tier One Common Equity Capital to Risk Weighted Assets	10.05%	9.17%
Tier One Common Equity Capital Required for Capital Adequacy	$ 274,497	$ 274,254
Tier One Common Equity Capital Required for Capital Adequacy to Risk Weighted Assets	4.00%	4.00%
Tier One Common Equity Capital Required to be Well Capitalized	$ 343,121	$ 342,818
Tier One Common Equity Capital Required to be Well Capitalized to Risk Weighted Assets	5.00%	5.00%
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	$ 689,778	$ 628,876
Tier 1 Capital (to Risk-Weighted Assets), Actual Ratio	11.72%	11.06%
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 353,244	$ 341,037
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	$ 470,992	$ 454,717
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	8.00%	8.00%
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 669,778	$ 608,876
Total Capital (to Risk-Weighted Assets), Actual Ratio	11.38%	10.71%
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 264,933	$ 255,778
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Ratio	4.50%	4.50%
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Amount	$ 382,681	$ 369,457
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	6.50%	6.50%
S&T Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Leverage Ratio, Actual Amount	$ 747,438	$ 666,560
Leverage Ratio, Actual Ratio	12.73%	11.76%
Leverage Ratio, Minimum Regulatory Capital Requirements Amount	$ 469,560	$ 453,375
Leverage Ratio, Minimum Regulatory Capital Requirements Ratio	8.00%	8.00%
Leverage Ratio, To be Well Capitalized Amount	$ 586,950	$ 566,719
Leverage Ratio, To be Well Capitalized Ratio	10.00%	10.00%
Tier One Common Equity Capital	$ 659,304	$ 582,929
Tier One Common Equity Capital to Risk Weighted Assets	9.63%	8.52%
Tier One Common Equity Capital Required for Capital Adequacy	$ 273,820	$ 273,538
Tier One Common Equity Capital Required for Capital Adequacy to Risk Weighted Assets	4.00%	4.00%
Tier One Common Equity Capital Required to be Well Capitalized	$ 342,275	$ 341,922
Tier One Common Equity Capital Required to be Well Capitalized to Risk Weighted Assets	5.00%	5.00%
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	$ 659,304	$ 582,929
Tier 1 Capital (to Risk-Weighted Assets), Actual Ratio	11.23%	10.29%
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 352,170	$ 340,032
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	$ 469,560	$ 453,375
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	8.00%	8.00%
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 659,304	$ 582,929
Total Capital (to Risk-Weighted Assets), Actual Ratio	11.23%	10.29%
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 264,127	$ 255,024
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Ratio	4.50%	4.50%
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Amount	$ 381,517	$ 368,368
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	6.50%	6.50%

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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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

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