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Subordinated Debentures - Additional Information (Detail)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
USD ($)
trust
Rate
Dec. 31, 2017
USD ($)
May 21, 2018
USD ($)
Subordinated Borrowing [Line Items]      
Subordinated Debentures outstanding, Amount $ 170,288 $ 72,167  
Number of trusts | trust 2    
First Commonwealth Bank 2028 [Member]      
Subordinated Borrowing [Line Items]      
Subordinated Debentures outstanding, Amount     $ 50,000
Deferred finance costs $ 923    
First Commonwealth Bank 2033 [Member]      
Subordinated Borrowing [Line Items]      
Subordinated Debentures outstanding, Amount     $ 50,000
Deferred finance costs 1,051    
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 4.90%    
Variable interest rate 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%    
Variable interest rate 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%