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Condensed Financial Information of First Commonwealth Financial Corporation (Parent Company Only) - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Condensed Financial Information Disclosure [Abstract]      
Cash Dividends Declared per Common Share (in dollars per share) $ 0.35 $ 0.32 $ 0.28
Line of credit $ 15,000,000    
Borrowings $ 0 $ 0 $ 0