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Subordinated Debentures Subordinated Debentures, Additional Information (Details) $ in Millions	Jun. 30, 2025 USD ($)
First Commonwealth Capital Trust II [Member]
Debt Instrument [Line Items]
Debt Issuance Costs, Gross	$ 0.5
First Commonwealth Bank 2033 [Member]
Debt Instrument [Line Items]
Subordinated Debt Par Amount	50.0
Debt Issuance Costs, Gross	1.1
First Commonwealth Capital Trust III [Member]
Debt Instrument [Line Items]
Debt Issuance Costs, Gross	0.6
First Commonwealth Financial Corporation
Debt Instrument [Line Items]
Subordinated Debt Par Amount	6.0
Debt Issuance Costs, Gross	$ 0.6

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