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Derivative Hedging Instruments - Summary of Interest Rate Swap Transactions (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Net Exposure, Notional Amount $ 0 $ 0
Derivative Financial Instruments, Assets [Member]    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Net Exposure, Notional Amount $ 66,227,000 $ 52,975,000
Weighted Average Rate Received/(Paid) 5.08% 5.07%
Net Exposure, Impact of a 1 basis point change in interest rates $ 36,000 $ 30,000
Repricing Frequency Monthly  
Derivative Financial Instruments, Liabilities [Member]    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Net Exposure, Notional Amount $ (66,227,000) $ (52,975,000)
Weighted Average Rate Received/(Paid) 5.08% 5.07%
Net Exposure, Impact of a 1 basis point change in interest rates $ (36,000) $ (30,000)
Repricing Frequency Monthly