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Subordinated Debentures - Subordinated Debentures Outstanding (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
May 21, 2018
Subordinated Borrowing [Line Items]      
Subordinated Debentures outstanding, Amount $ 170,288 $ 72,167  
First Commonwealth Bank due 2028 [Member]      
Subordinated Borrowing [Line Items]      
Subordinated Debentures outstanding, Due Jan. 01, 2028    
Subordinated Debentures outstanding, Amount $ 49,131    
First Commonwealth Bank due 2033 [Member]      
Subordinated Borrowing [Line Items]      
Subordinated Debentures outstanding, Due Jan. 01, 2033    
Subordinated Debentures outstanding, Amount $ 48,990    
First Commonwealth Capital Trust II [Member]      
Subordinated Borrowing [Line Items]      
Subordinated Debentures outstanding, Due Jan. 01, 2034    
Subordinated Debentures outstanding, Amount $ 30,929 30,929  
First Commonwealth Capital Trust III [Member]      
Subordinated Borrowing [Line Items]      
Subordinated Debentures outstanding, Due Jan. 01, 2034    
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
Subordinated Debt [Member] | LIBOR [Member] | First Commonwealth Capital Trust II [Member]      
Subordinated Borrowing [Line Items]      
Variable interest rate 2.85% 2.85%  
Subordinated Debt [Member] | LIBOR [Member] | First Commonwealth Capital Trust III [Member]      
Subordinated Borrowing [Line Items]      
Variable interest rate 2.85% 2.85%  
Subordinated Debt [Member] | LIBOR [Member] | First Commonwealth Bank 2028 [Member]      
Subordinated Borrowing [Line Items]      
Variable interest rate 1.845%    
Debt Instrument, Interest Rate During Period 4.90%    
Subordinated Debt [Member] | LIBOR [Member] | First Commonwealth Bank 2033 [Member]      
Subordinated Borrowing [Line Items]      
Variable interest rate 2.37%    
Debt Instrument, Interest Rate During Period 5.50%