Subordinated Debentures - Subordinated Debentures Outstanding (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	May 21, 2018
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Amount	$ 170,612	$ 170,450
First Commonwealth Bank due 2028 [Member]
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Due	Jun. 01, 2028
Subordinated Debentures outstanding, Amount	$ 49,314	49,222
First Commonwealth Bank due 2033 [Member]
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Due	Jun. 01, 2033
Subordinated Debentures outstanding, Amount	$ 49,131	49,061
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
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
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%
Subordinated Debt [Member] | LIBOR [Member] | First Commonwealth Capital Trust III [Member]
Subordinated Borrowing [Line Items]
Variable interest rate	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%
Debt Instrument, Interest Rate During Period	4.88%	4.88%
Subordinated Debt [Member] | LIBOR [Member] | First Commonwealth Bank 2033 [Member]
Subordinated Borrowing [Line Items]
Variable interest rate	2.37%	2.37%
Debt Instrument, Interest Rate During Period	5.50%	5.50%