Subordinated Debentures Schedule of Subordinated Debentures Outstanding (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Subordinated debentures	$ 170,734	$ 170,612
First Commonwealth Bank 2028 [Member]
Debt Instrument [Line Items]
Subordinated debentures	49,383	49,314
First Commonwealth Bank 2033 [Member]
Debt Instrument [Line Items]
Subordinated debentures	49,184	49,131
First Commonwealth Capital Trust II [Member]
Debt Instrument [Line Items]
Subordinated debentures	30,929	30,929
First Commonwealth Capital Trust III [Member]
Debt Instrument [Line Items]
Subordinated debentures	$ 41,238	$ 41,238