Subordinated Debentures Subordinated Debentures, Additional Information (Details) $ in Millions	Mar. 31, 2022 USD ($)
First Commonwealth Capital Trust II [Member]
Debt Instrument [Line Items]
Debt Issuance Costs, Gross	$ 0.6
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	$ 0.5