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Commitments, Off Balance Sheets Risk and Contingencies - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]    
Commitments to make loans $ 802,900,000 $ 808,300,000
Commitments under outstanding standby letters of credit 3,400,000 $ 1,000,000
Anticipated losses from unused commitments $ 0