Subordinated Debentures Subordinated Debentures, Additional Information (Details) $ in Thousands	Mar. 31, 2020 USD ($)
First Commonwealth Capital Trust II [Member]
Debt Instrument [Line Items]
Debt Issuance Costs, Gross	$ 600
First Commonwealth Bank 2028 [Member]
Debt Instrument [Line Items]
Subordinated Debt Par Amount	50,000
Debt Issuance Costs, Gross	900
First Commonwealth Bank 2033 [Member]
Debt Instrument [Line Items]
Subordinated Debt Par Amount	50,000
Debt Issuance Costs, Gross	1,100
First Commonwealth Capital Trust III [Member]
Debt Instrument [Line Items]
Debt Issuance Costs, Gross	$ 500