Subordinated Debentures - Subordinated Debentures Outstanding (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	May 21, 2018
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Amount	$ 128,466	$ 128,305
First Commonwealth Bank due 2033 [Member]
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Amount	49,480	49,411
First Commonwealth Capital Trust II [Member]
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Amount	30,929	30,929
First Commonwealth Capital Trust III [Member]
Subordinated Borrowing [Line Items]
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
First Commonwealth Financial Corporation
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Amount	$ 6,819	$ 6,727	$ 6,000
Subordinated Debt [Member] | First Commonwealth Capital Trust II [Member]
Subordinated Borrowing [Line Items]
Debt Instrument, Interest Rate During Period	1.515%
Subordinated Debt [Member] | First Commonwealth Capital Trust III [Member]
Subordinated Borrowing [Line Items]
Debt Instrument, Interest Rate During Period	1.525%
Subordinated Debt [Member] | First Commonwealth Bank 2033 [Member]
Subordinated Borrowing [Line Items]
Debt Instrument, Interest Rate During Period	5.50%
Subordinated Debt [Member] | First Commonwealth Financial Corporation
Subordinated Borrowing [Line Items]
Debt Instrument, Interest Rate During Period	4.50%