Subordinated Debentures - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2025 USD ($) trust Rate	Dec. 31, 2024 USD ($)	May 21, 2018 USD ($)
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Amount	$ 128,466	$ 128,305
Number of trusts | trust	2
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
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	$ 600
First Commonwealth Capital Trust II [Member]
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Amount	30,929	30,929
Deferred finance costs	500
First Commonwealth Financial Corporation
Subordinated Borrowing [Line Items]
Subordinated Debentures outstanding, Amount	$ 6,819	$ 6,727	6,000
Subordinated Debt, Fair Value Premium			$ 600
Subordinated Debt [Member] | First Commonwealth Bank 2033 [Member]
Subordinated Borrowing [Line Items]
Debt Instrument, Interest Rate During Period | Rate	5.50%
Subordinated Debt [Member] | First Commonwealth Capital Trust III [Member]
Subordinated Borrowing [Line Items]
Debt Instrument, Interest Rate During Period | Rate	1.525%
Subordinated Debt [Member] | First Commonwealth Capital Trust II [Member]
Subordinated Borrowing [Line Items]
Debt Instrument, Interest Rate During Period | Rate	1.515%
Subordinated Debt [Member] | First Commonwealth Financial Corporation
Subordinated Borrowing [Line Items]
Debt Instrument, Interest Rate During Period | Rate	4.50%
Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate | Subordinated Debt [Member] | First Commonwealth Bank 2028 [Member]
Subordinated Borrowing [Line Items]
Variable interest rate | Rate	1.845%
Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate | Subordinated Debt [Member] | First Commonwealth Bank 2033 [Member]
Subordinated Borrowing [Line Items]
Variable interest rate | Rate	2.37%
Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate | Subordinated Debt [Member] | First Commonwealth Capital Trust III [Member]
Subordinated Borrowing [Line Items]
Variable interest rate | Rate	2.85%
Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate | Subordinated Debt [Member] | First Commonwealth Capital Trust II [Member]
Subordinated Borrowing [Line Items]
Variable interest rate | Rate	2.85%
Prime Rate | Subordinated Debt [Member] | First Commonwealth Financial Corporation
Subordinated Borrowing [Line Items]
Variable interest rate | Rate	1.00%