Regulatory Matters - Summary of Risk-Based Capital Amounts and Ratios (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
S&T
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Leverage Ratio, Actual Amount	$ 854,146	$ 689,778
Leverage Ratio, Actual Ratio	10.29%	10.05%
Leverage Ratio, Minimum Regulatory Capital Requirements Amount	$ 331,925	$ 274,497
Leverage Ratio, Minimum Regulatory Capital Requirements Ratio	4.00%	4.00%
Leverage Ratio, To be Well Capitalized Amount	$ 414,907	$ 343,121
Leverage Ratio, To be Well Capitalized Ratio	5.00%	5.00%
Tier One Common Equity Capital	$ 825,146	$ 669,778
Tier One Common Equity Capital to Risk Weighted Assets	11.43%	11.38%
Tier One Common Equity Capital Required for Capital Adequacy	$ 324,745	$ 264,933
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	$ 469,077	$ 382,681
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	$ 854,146	$ 689,778
Tier 1 Capital (to Risk-Weighted Assets), Actual Ratio	11.84%	11.72%
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 432,994	$ 353,244
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	$ 577,325	$ 470,992
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	8.00%	8.00%
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 954,094	$ 777,913
Total Capital (to Risk-Weighted Assets), Actual Ratio	13.22%	13.21%
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 577,325	$ 470,992
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	$ 721,656	$ 588,741
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	$ 832,113	$ 659,304
Leverage Ratio, Actual Ratio	10.04%	9.63%
Leverage Ratio, Minimum Regulatory Capital Requirements Amount	$ 331,355	$ 273,820
Leverage Ratio, Minimum Regulatory Capital Requirements Ratio	4.00%	4.00%
Leverage Ratio, To be Well Capitalized Amount	$ 414,194	$ 342,275
Leverage Ratio, To be Well Capitalized Ratio	5.00%	5.00%
Tier One Common Equity Capital	$ 832,113	$ 659,304
Tier One Common Equity Capital to Risk Weighted Assets	11.56%	11.23%
Tier One Common Equity Capital Required for Capital Adequacy	$ 324,048	$ 264,127
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	$ 468,069	$ 381,517
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	$ 832,113	$ 659,304
Tier 1 Capital (to Risk-Weighted Assets), Actual Ratio	11.56%	11.23%
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 432,064	$ 352,170
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	$ 576,085	$ 469,560
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	8.00%	8.00%
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 922,310	$ 747,438
Total Capital (to Risk-Weighted Assets), Actual Ratio	12.81%	12.73%
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 576,085	$ 469,560
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	$ 720,106	$ 586,950
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	10.00%	10.00%