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Secondary Market Mortgage Activities - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Debt Disclosure [Abstract]      
Total loans serviced to others $ 159,208,000 $ 157,073,000  
Valuation allowance $ 0 $ 0 $ 0