Regulatory Matters - Summary of Risk-Based Capital Amounts and Ratios for S&T and S&T Bank (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
S&T
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Leverage Ratio, Actual Amount	$ 535,234	$ 465,114
Leverage Ratio, Actual Ratio	8.96%	9.80%
Leverage Ratio, Minimum Regulatory Capital Requirements Amount	$ 238,841	$ 189,895
Leverage Ratio, Minimum Regulatory Capital Requirements Ratio	4.00%	4.00%
Leverage Ratio, To be Well Capitalized Amount	$ 298,551	$ 237,369
Leverage Ratio, To be Well Capitalized Ratio	5.00%	5.00%
Tier One Common Equity Capital	$ 515,234	$ 445,114
Tier One Common Equity Capital to Risk Weighted Assets	9.77%	11.81%
Tier One Common Equity Capital Required for Capital Adequacy	$ 237,315	$ 169,621
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	$ 342,788	$ 245,008
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	$ 535,234	$ 465,114
Tier 1 Capital (to Risk-Weighted Assets), Actual Ratio	10.15%	12.34%
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 316,419	$ 150,774
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Ratio	6.00%	4.00%
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Amount	$ 421,892	$ 226,161
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	8.00%	6.00%
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 611,859	$ 537,935
Total Capital (to Risk-Weighted Assets), Actual Ratio	11.60%	14.27%
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 421,892	$ 301,548
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	$ 527,366	$ 376,936
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	$ 502,114	$ 403,593
Leverage Ratio, Actual Ratio	8.43%	8.53%
Leverage Ratio, Minimum Regulatory Capital Requirements Amount	$ 238,121	$ 189,182
Leverage Ratio, Minimum Regulatory Capital Requirements Ratio	4.00%	4.00%
Leverage Ratio, To be Well Capitalized Amount	$ 297,651	$ 236,477
Leverage Ratio, To be Well Capitalized Ratio	5.00%	5.00%
Tier One Common Equity Capital	$ 502,114	$ 403,593
Tier One Common Equity Capital to Risk Weighted Assets	9.55%	10.76%
Tier One Common Equity Capital Required for Capital Adequacy	$ 236,482	$ 168,804
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	$ 341,584	$ 243,827
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	$ 502,114	$ 403,593
Tier 1 Capital (to Risk-Weighted Assets), Actual Ratio	9.55%	10.76%
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 315,309	$ 150,048
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Ratio	6.00%	4.00%
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Amount	$ 420,412	$ 225,071
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	8.00%	6.00%
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 577,824	$ 475,538
Total Capital (to Risk-Weighted Assets), Actual Ratio	11.00%	12.68%
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 420,412	$ 300,095
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	$ 525,515	$ 375,119
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	10.00%	10.00%