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Summary of Significant Accounting Policies - Additional Information (Detail)
 1 Months Ended 12 Months Ended
Dec. 31, 2017
USD ($)
Sep. 20, 2017
Mar. 31, 2017
USD ($)
Dec. 31, 2017
USD ($)
Jan. 19, 2018
shares
Dec. 31, 2016
USD ($)
Summary Of Significant Accounting Policies [Line Items]            
Period for federal funds purchased and sold       1 day    
Average aggregate reserve $ 11,400,000     $ 11,400,000   $ 5,000,000
Loan non-accrual classification period       90 days    
Days to charged consumer loans to the value of the collateral       90 days    
Maximum time for re-evaluation (in months)       12 months    
Re-evaluation period for real estate       2 years    
Other real estate owned 674,000     $ 674,000   $ 774,000
Percentage of likelihood of realization of recognized tax benefit, minimum       50.00%    
Share stock split ratio   2        
Amount of reclassification effect       $ 0    
Accelerated amortization expense adjustment     $ 30,000      
Adjustment of deferred tax assets       $ (8,000)    
Subsequent Event [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Stock Repurchase Program, Number of Shares Authorized to be Repurchased | shares         400,000  
Maximum [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Maturity period of cash equivalents       3 months    
Adjustment of deferred tax assets $ 8,000