Regulatory Matters - Summary of Risk-Based Capital Amounts and Ratios for S&T and S&T Bank (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
S&T
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Leverage Ratio, Actual Amount	$ 582,155	$ 535,234
Leverage Ratio, Actual Ratio	8.98%	8.96%
Leverage Ratio, Minimum Regulatory Capital Requirements Amount	$ 259,170	$ 238,841
Leverage Ratio, Minimum Regulatory Capital Requirements Ratio	4.00%	4.00%
Leverage Ratio, To be Well Capitalized Amount	$ 323,963	$ 298,551
Leverage Ratio, To be Well Capitalized Ratio	5.00%	5.00%
Tier One Common Equity Capital	$ 562,155	$ 515,234
Tier One Common Equity Capital to Risk Weighted Assets	10.04%	9.77%
Tier One Common Equity Capital Required for Capital Adequacy	$ 252,079	$ 237,315
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	$ 364,114	$ 342,788
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	$ 582,155	$ 535,234
Tier 1 Capital (to Risk-Weighted Assets), Actual Ratio	10.39%	10.15%
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 336,105	$ 316,419
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	$ 448,140	$ 421,892
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	8.00%	8.00%
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 664,184	$ 611,859
Total Capital (to Risk-Weighted Assets), Actual Ratio	11.86%	11.60%
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 448,140	$ 421,892
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	$ 560,175	$ 527,366
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	$ 542,048	$ 502,114
Leverage Ratio, Actual Ratio	8.39%	8.43%
Leverage Ratio, Minimum Regulatory Capital Requirements Amount	$ 258,460	$ 238,121
Leverage Ratio, Minimum Regulatory Capital Requirements Ratio	4.00%	4.00%
Leverage Ratio, To be Well Capitalized Amount	$ 323,075	$ 297,651
Leverage Ratio, To be Well Capitalized Ratio	5.00%	5.00%
Tier One Common Equity Capital	$ 542,048	$ 502,114
Tier One Common Equity Capital to Risk Weighted Assets	9.71%	9.55%
Tier One Common Equity Capital Required for Capital Adequacy	$ 251,213	$ 236,482
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	$ 362,864	$ 341,584
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	$ 542,048	$ 502,114
Tier 1 Capital (to Risk-Weighted Assets), Actual Ratio	9.71%	9.55%
Tier 1 Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 334,951	$ 315,309
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	$ 446,601	$ 420,412
Tier 1 Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	8.00%	8.00%
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 622,469	$ 577,824
Total Capital (to Risk-Weighted Assets), Actual Ratio	11.15%	11.00%
Total Capital (to Risk-Weighted Assets), Minimum Regulatory Capital Requirements Amount	$ 446,602	$ 420,412
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	$ 558,252	$ 525,515
Total Capital (to Risk-Weighted Assets), To be Well Capitalized Ratio	10.00%	10.00%