Subordinated Debentures - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) trust Rate	Dec. 31, 2019 USD ($) Rate	May 21, 2018 USD ($)
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Amount	$ 170,612	$ 170,450
Number of trusts | trust	2
First Commonwealth Bank 2028 [Member]
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Amount			$ 50,000
Deferred finance costs	$ 900
First Commonwealth Bank 2033 [Member]
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Amount			$ 50,000
Deferred finance costs	1,100
First Commonwealth Capital Trust III [Member]
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Amount	$ 41,238	41,238
Redemption price as percentage of principal amount	100.00%
Deferred finance costs	$ 630
First Commonwealth Capital Trust II [Member]
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Amount	$ 30,929	$ 30,929
Redemption price as percentage of principal amount	100.00%
Deferred finance costs	$ 471
LIBOR [Member] | Subordinated Debt [Member] | First Commonwealth Bank 2028 [Member]
Subordinated Borrowing [Line Items]
Debt Instrument, Interest Rate During Period | Rate	4.88%	4.88%
Variable interest rate	1.845%	1.845%
LIBOR [Member] | Subordinated Debt [Member] | First Commonwealth Bank 2033 [Member]
Subordinated Borrowing [Line Items]
Debt Instrument, Interest Rate During Period | Rate	5.50%	5.50%
Variable interest rate	2.37%	2.37%
LIBOR [Member] | Subordinated Debt [Member] | First Commonwealth Capital Trust III [Member]
Subordinated Borrowing [Line Items]
Variable interest rate	2.85%	2.85%
LIBOR [Member] | Subordinated Debt [Member] | First Commonwealth Capital Trust II [Member]
Subordinated Borrowing [Line Items]
Variable interest rate	2.85%	2.85%