Subordinated Debentures Subordinated Debentures, Additional Information (Details) $ in Thousands	Sep. 30, 2018 USD ($)
First Commonwealth Capital Trust II [Member]
Debt Instrument [Line Items]
Debt Issuance Costs, Gross	$ 630
First Commonwealth Bank 2028 [Member]
Debt Instrument [Line Items]
Debt Issuance Costs, Gross	923
First Commonwealth Bank 2033 [Member]
Debt Instrument [Line Items]
Debt Issuance Costs, Gross	1,051
First Commonwealth Capital Trust III [Member]
Debt Instrument [Line Items]
Debt Issuance Costs, Gross	$ 471