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Derivative Financial Instruments - Additional Information (Detail)
Sep. 30, 2018
USD ($)
Agreement
Dec. 31, 2017
USD ($)
Derivative Instruments, Gain (Loss) [Line Items]    
Derivative assets and liabilities offset in balance sheet $ 0  
Total notional amount of the Company's swaps $ 204,100,000 $ 198,500,000
Interest-rate Swap Agreements with Customers [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Number of interest-rate swap agreements | Agreement 76