Subordinated Debentures Subordinated Debentures, Additional Information (Details) $ in Millions	Mar. 31, 2024 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
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	0.4
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