Regulatory Matters - Summary of Risk-Based Capital Amounts and Ratios (Details) $ in Thousands	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
S&T
Leverage Ratio, Amount
Actual	$ 889,785	$ 852,515
Minimum Regulatory Capital Requirements	365,535	350,311
To be Well Capitalized Under Prompt Corrective Action Provisions	$ 456,918	$ 437,889
Leverage Ratio
Actual	0.0974	0.0943
Minimum Regulatory Capital Requirements	0.0400	0.0400
To be Well Capitalized Under Prompt Corrective Action Provisions	0.0500	0.0500
Common Equity Tier 1 (to Risk-Weighted Assets), Amount
Actual	$ 860,785	$ 796,515
Minimum Regulatory Capital Requirements	322,109	316,338
To be Well Capitalized Under Prompt Corrective Action Provisions	$ 465,268	$ 456,933
Common Equity Tier 1 (to Risk-Weighted Assets), Ratio
Actual	0.1203	0.1133
Minimum Regulatory Capital Requirements	0.0450	0.0450
To be Well Capitalized Under Prompt Corrective Action Provisions	0.0650	0.0650
Tier 1 Capital (to Risk-Weighted Assets), Amount
Actual	$ 889,785	$ 825,515
Minimum Regulatory Capital Requirements	429,479	421,784
To be Well Capitalized Under Prompt Corrective Action Provisions	$ 572,638	$ 562,379
Tier 1 Capital (to Risk-Weighted Assets), Ratio
Actual	0.1243	0.1174
Minimum Regulatory Capital Requirements	0.0600	0.0600
To be Well Capitalized Under Prompt Corrective Action Provisions	0.0800	0.0800
Total Capital (to Risk-Weighted Assets), Amount
Actual	$ 987,420	$ 944,686
Minimum Regulatory Capital Requirements	572,638	562,379
To be Well Capitalized Under Prompt Corrective Action Provisions	$ 715,798	$ 702,974
Total Capital (to Risk-Weighted Assets), Ratio
Actual	0.1379	0.1344
Minimum Regulatory Capital Requirements	0.0800	0.0800
To be Well Capitalized Under Prompt Corrective Action Provisions	0.1000	0.1000
S&T Bank
Leverage Ratio, Amount
Actual	$ 864,127	$ 810,636
Minimum Regulatory Capital Requirements	365,544	349,739
To be Well Capitalized Under Prompt Corrective Action Provisions	$ 456,930	$ 437,174
Leverage Ratio
Actual	0.0946	0.0927
Minimum Regulatory Capital Requirements	0.0400	0.0400
To be Well Capitalized Under Prompt Corrective Action Provisions	0.0500	0.0500
Common Equity Tier 1 (to Risk-Weighted Assets), Amount
Actual	$ 864,127	$ 810,636
Minimum Regulatory Capital Requirements	321,711	315,792
To be Well Capitalized Under Prompt Corrective Action Provisions	$ 464,694	$ 456,144
Common Equity Tier 1 (to Risk-Weighted Assets), Ratio
Actual	0.1209	0.1155
Minimum Regulatory Capital Requirements	0.0450	0.0450
To be Well Capitalized Under Prompt Corrective Action Provisions	0.0650	0.0650
Tier 1 Capital (to Risk-Weighted Assets), Amount
Actual	$ 864,127	$ 810,636
Minimum Regulatory Capital Requirements	428,948	421,056
To be Well Capitalized Under Prompt Corrective Action Provisions	$ 571,931	$ 561,408
Tier 1 Capital (to Risk-Weighted Assets), Ratio
Actual	0.1209	0.1155
Minimum Regulatory Capital Requirements	0.0600	0.0600
To be Well Capitalized Under Prompt Corrective Action Provisions	0.0800	0.0800
Total Capital (to Risk-Weighted Assets), Amount
Actual	$ 961,762	$ 922,007
Minimum Regulatory Capital Requirements	571,931	561,408
To be Well Capitalized Under Prompt Corrective Action Provisions	$ 714,913	$ 701,760
Total Capital (to Risk-Weighted Assets), Ratio
Actual	0.1345	0.1314
Minimum Regulatory Capital Requirements	0.0800	0.0800
To be Well Capitalized Under Prompt Corrective Action Provisions	0.1000	0.1000