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Subordinated Debentures - Additional Information (Detail)
$ in Thousands
 12 Months Ended
Dec. 31, 2023
USD ($)
trust
Rate
Dec. 31, 2022
USD ($)
May 21, 2018
USD ($)
Subordinated Borrowing [Line Items]      
Subordinated Debentures outstanding, Amount $ 177,741 $ 170,937  
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 $ 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 $ 6,641 $ 0 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%