Loans and Allowance for Credit Losses - Summary of Aging Analysis of Modifications (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Sep. 30, 2024
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months	$ 39,563	$ 34,758
Current \| Accrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months	21,261	27,176
30-59 Days Past Due \| Accrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months	18,056	7,475
60-89 Days Past Due \| Accrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months	0	0
90+ Days Past Due \| Accrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months	246	107
Commercial loans \| Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months	15,044	12,482
Commercial loans \| Commercial real estate \| Current \| Accrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months	470	12,482
Commercial loans \| Commercial real estate \| 30-59 Days Past Due \| Accrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months	14,574	0
Commercial loans \| Commercial real estate \| 60-89 Days Past Due \| Accrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months	0	0
Commercial loans \| Commercial real estate \| 90+ Days Past Due \| Accrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months	0	0
Commercial loans \| Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months	24,089	21,839
Commercial loans \| Commercial and industrial \| Current \| Accrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months	20,607	14,364
Commercial loans \| Commercial and industrial \| 30-59 Days Past Due \| Accrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months	3,482	7,475
Commercial loans \| Commercial and industrial \| 60-89 Days Past Due \| Accrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months	0	0
Commercial loans \| Commercial and industrial \| 90+ Days Past Due \| Accrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months	0	0
Business banking
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months		107
Business banking \| Current \| Accrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months		107
Business banking \| 30-59 Days Past Due \| Accrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months		0
Business banking \| 60-89 Days Past Due \| Accrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months		0
Business banking \| 90+ Days Past Due \| Accrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months		0
Consumer loans \| Consumer real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months	430	330
Consumer loans \| Consumer real estate \| Current \| Accrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months	184	223
Consumer loans \| Consumer real estate \| 30-59 Days Past Due \| Accrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months	0	0
Consumer loans \| Consumer real estate \| 60-89 Days Past Due \| Accrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months	0	0
Consumer loans \| Consumer real estate \| 90+ Days Past Due \| Accrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, after 12 months	$ 246	$ 107

S&T BANCORP INC 10-Q