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Nature of Banking Activities and Significant Accounting policies - Additional Information (Detail)
 12 Months Ended
Dec. 31, 2017
USD ($)
Entity
Dec. 31, 2016
USD ($)
Nature Of Banking Activities And Significant Accounting Policies [Line Items]    
Number of accounts included by parent company | Entity 6  
Gain (loss) on rate lock commitments $ 0  
Accrual status of loan 90 days  
Troubled debt restructurings (TDRs), amount $ 333,000 $ 460,000
Bank-owned life insurance, death benefits received 720,000 527,000
Tax Cuts and Jobs Act of 2017, Income tax expense $ 752,000  
Largest amount of tax benefit realized by taxing authority 50.00%  
Liability for unrecognized tax benefits $ 0 0
Advertising cost 379,000 402,000
Reclassification of tax effects stranded in accumulated other comprehensive loss by tax rate change 0  
Life Insurance Product Line    
Nature Of Banking Activities And Significant Accounting Policies [Line Items]    
Bank-owned life insurance, death benefits received $ 312,000 $ 102,000
Minimum    
Nature Of Banking Activities And Significant Accounting Policies [Line Items]    
Period of time between issuance of a loan commitment and closing and sale of the loan 30 days  
Management's policy on evaluation of impairment $ 250,000  
Company not considered value of impairment on individual consumer, residential and small commercial loans $ 250,000  
Minimum | Software    
Nature Of Banking Activities And Significant Accounting Policies [Line Items]    
Estimated useful lives 3 years  
Minimum | Core Deposits    
Nature Of Banking Activities And Significant Accounting Policies [Line Items]    
Intangible assets, estimated useful lives 6 years  
Minimum | Premises and Equipment    
Nature Of Banking Activities And Significant Accounting Policies [Line Items]    
Estimated useful lives 3 years  
Maximum    
Nature Of Banking Activities And Significant Accounting Policies [Line Items]    
Period of time between issuance of a loan commitment and closing and sale of the loan 60 days  
Maximum | Software    
Nature Of Banking Activities And Significant Accounting Policies [Line Items]    
Estimated useful lives 7 years  
Maximum | Core Deposits    
Nature Of Banking Activities And Significant Accounting Policies [Line Items]    
Intangible assets, estimated useful lives 9 years  
Maximum | Premises and Equipment    
Nature Of Banking Activities And Significant Accounting Policies [Line Items]    
Estimated useful lives 40 years  
Retained Earnings    
Nature Of Banking Activities And Significant Accounting Policies [Line Items]    
Reclassification of tax effects stranded in accumulated other comprehensive loss by tax rate change $ 158,000  
Accumulated Other Comprehensive Loss    
Nature Of Banking Activities And Significant Accounting Policies [Line Items]    
Reclassification of tax effects stranded in accumulated other comprehensive loss by tax rate change (158,000)  
ASU 2018-02 | Retained Earnings    
Nature Of Banking Activities And Significant Accounting Policies [Line Items]    
Reclassification of tax effects stranded in accumulated other comprehensive loss by tax rate change 158,000  
ASU 2018-02 | Accumulated Other Comprehensive Loss    
Nature Of Banking Activities And Significant Accounting Policies [Line Items]    
Reclassification of tax effects stranded in accumulated other comprehensive loss by tax rate change $ (158,000)