Derivative Instruments and Hedging Activities - Schedule of Gross Amounts of Derivative Assets and Derivative Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Derivative Assets (Included in Other Assets)
Derivative Asset, Statement of Financial Position [Extensible Enumeration]	Other assets	Other assets
Notional Amount	$ 759,245	$ 850,104
Derivative financial assets	$ 35,758	$ 60,890
Derivative Liabilities (Included in Other Liabilities)
Derivative Liability, Statement of Financial Position [Extensible Enumeration]	Other liabilities	Other liabilities
Notional Amount	$ 1,158,665	$ 1,350,104
Derivative financial liabilities	39,241	70,860
Interest rate swap contracts—commercial loans
Derivative Assets (Included in Other Assets)
Derivative financial assets	35,739	60,890
Derivative Liabilities (Included in Other Liabilities)
Derivative financial liabilities	39,241	70,860
Designated as Hedging Instruments
Derivative Assets (Included in Other Assets)
Notional Amount	0	0
Derivative financial assets	0	0
Derivative Liabilities (Included in Other Liabilities)
Notional Amount	400,000	500,000
Derivative financial liabilities	3,199	9,589
Designated as Hedging Instruments \| Cash Flow Hedge
Derivative Assets (Included in Other Assets)
Notional Amount	0	0
Derivative financial assets	0	0
Derivative Liabilities (Included in Other Liabilities)
Notional Amount	400,000	500,000
Derivative financial liabilities	3,199	9,589
Not Designated as Hedging Instruments
Derivative Assets (Included in Other Assets)
Notional Amount	759,245	850,104
Derivative financial assets	35,758	60,890
Derivative Liabilities (Included in Other Liabilities)
Notional Amount	758,665	850,104
Derivative financial liabilities	36,042	61,271
Not Designated as Hedging Instruments \| Interest rate swap contracts—commercial loans
Derivative Assets (Included in Other Assets)
Notional Amount	758,665	850,104
Derivative financial assets	35,739	60,890
Derivative Liabilities (Included in Other Liabilities)
Notional Amount	758,665	850,104
Derivative financial liabilities	36,042	61,271
Not Designated as Hedging Instruments \| Interest rate lock commitments - mortgage loans
Derivative Assets (Included in Other Assets)
Notional Amount	580	0
Derivative financial assets	19	0
Derivative Liabilities (Included in Other Liabilities)
Notional Amount	0	0
Derivative financial liabilities	$ 0	$ 0

S&T BANCORP INC 10-Q