Subordinated Debentures Subordinated Debentures, Additional Information (Details) $ in Millions	Sep. 30, 2023 USD ($)
First Commonwealth Capital Trust II [Member]
Debt Instrument [Line Items]
Debt Issuance Costs, Gross	$ 0.5
First Commonwealth Bank 2028 [Member]
Debt Instrument [Line Items]
Subordinated Debt Par Amount	50.0
Debt Issuance Costs, Gross	0.9
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
Business Combination, Subordinated Debentures, Discount (Premium)	$ 0.6