Subordinated Debentures - Subordinated Debentures Outstanding (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2021	Dec. 31, 2021	Dec. 31, 2020	May 21, 2018
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Amount	$ 170,775	$ 170,775	$ 170,775	$ 170,612
First Commonwealth Bank due 2028 [Member]
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Due	Jun. 01, 2028
Subordinated Debentures outstanding, Amount	$ 49,407	49,407	49,407	49,314
First Commonwealth Bank due 2033 [Member]
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Due	Jun. 01, 2033
Subordinated Debentures outstanding, Amount	$ 49,201	49,201	49,201	49,131
First Commonwealth Capital Trust II [Member]
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Due	Jan. 23, 2034
Subordinated Debentures outstanding, Amount	$ 30,929	30,929	30,929	30,929
First Commonwealth Capital Trust III [Member]
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Due	Apr. 06, 2034
Subordinated Debentures outstanding, Amount	$ 41,238	$ 41,238	$ 41,238	$ 41,238
First Commonwealth Bank 2028 [Member]
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Amount					$ 50,000
First Commonwealth Bank 2033 [Member]
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Amount					$ 50,000
Subordinated Debt [Member] | LIBOR [Member] | First Commonwealth Capital Trust II [Member]
Subordinated Borrowing [Line Items]
Variable interest rate		2.85%	2.85%	2.85%
Subordinated Debt [Member] | LIBOR [Member] | First Commonwealth Capital Trust III [Member]
Subordinated Borrowing [Line Items]
Variable interest rate		2.85%	2.85%	2.85%
Subordinated Debt [Member] | LIBOR [Member] | First Commonwealth Bank 2028 [Member]
Subordinated Borrowing [Line Items]
Variable interest rate		1.845%	1.845%	1.845%
Debt Instrument, Interest Rate During Period		4.875%		4.875%
Subordinated Debt [Member] | LIBOR [Member] | First Commonwealth Bank 2033 [Member]
Subordinated Borrowing [Line Items]
Variable interest rate		2.37%	2.37%	2.37%
Debt Instrument, Interest Rate During Period		5.50%		5.50%