Regulatory Matters - Summary of Risk-Based Capital Amounts and Ratios (Details) $ in Thousands	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
S&T
Leverage Ratio, Amount
Actual	$ 825,515	$ 854,146
Minimum Regulatory Capital Requirements	350,311	331,925
To be Well Capitalized Under Prompt Corrective Action Provisions	$ 437,889	$ 414,907
Leverage Ratio
Actual	0.0943	0.1029
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	$ 796,515	$ 825,146
Minimum Regulatory Capital Requirements	316,338	324,745
To be Well Capitalized Under Prompt Corrective Action Provisions	$ 456,933	$ 469,077
Common Equity Tier 1 (to Risk-Weighted Assets), Ratio
Actual	0.1133	0.1143
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	$ 825,515	$ 854,146
Minimum Regulatory Capital Requirements	421,784	432,994
To be Well Capitalized Under Prompt Corrective Action Provisions	$ 562,379	$ 577,325
Tier 1 Capital (to Risk-Weighted Assets), Ratio
Actual	0.1174	0.1184
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	$ 944,686	$ 954,094
Minimum Regulatory Capital Requirements	562,379	577,325
To be Well Capitalized Under Prompt Corrective Action Provisions	$ 702,974	$ 721,656
Total Capital (to Risk-Weighted Assets), Ratio
Actual	0.1344	0.1322
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	$ 810,636	$ 832,113
Minimum Regulatory Capital Requirements	349,739	331,355
To be Well Capitalized Under Prompt Corrective Action Provisions	$ 437,174	$ 414,194
Leverage Ratio
Actual	0.0927	0.1004
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	$ 810,636	$ 832,113
Minimum Regulatory Capital Requirements	315,792	324,048
To be Well Capitalized Under Prompt Corrective Action Provisions	$ 456,144	$ 468,069
Common Equity Tier 1 (to Risk-Weighted Assets), Ratio
Actual	0.1155	0.1156
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	$ 810,636	$ 832,113
Minimum Regulatory Capital Requirements	421,056	432,064
To be Well Capitalized Under Prompt Corrective Action Provisions	$ 561,408	$ 576,085
Tier 1 Capital (to Risk-Weighted Assets), Ratio
Actual	0.1155	0.1156
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	$ 922,007	$ 922,310
Minimum Regulatory Capital Requirements	561,408	576,085
To be Well Capitalized Under Prompt Corrective Action Provisions	$ 701,760	$ 720,106
Total Capital (to Risk-Weighted Assets), Ratio
Actual	0.1314	0.1281
Minimum Regulatory Capital Requirements	0.0800	0.0800
To be Well Capitalized Under Prompt Corrective Action Provisions	0.1000	0.1000