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Summary of Significant Accounting Policies (Detail Textuals) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Summary Of Significant Accounting Policies [Line Items]      
Mortgage loans held for sale $ 383,000 $ 162,000  
Depreciation methods Straight line and accelerated methods    
Sustained period of repayment performance for non-accrual TDRs 6 months    
Period of employees highest compensation before retirement 10 years    
Term of weighted average maturity of interest bearing deposits 4 years 2 months 12 days    
Certificates of deposit in other banks $ 3,460,000 $ 1,000,000  
Tax at statutory federal income tax rate (in percent) 34.00% 34.00% 34.00%
Corporate income tax rate effective in 2018 21.00%    
Amounts reclassified from accumulated other comprehensive loss per ASU 2018-02 $ 0    
Retained Earnings      
Summary Of Significant Accounting Policies [Line Items]      
Amounts reclassified from accumulated other comprehensive loss per ASU 2018-02 1,218,000    
Accumulated Other Comprehensive Loss      
Summary Of Significant Accounting Policies [Line Items]      
Amounts reclassified from accumulated other comprehensive loss per ASU 2018-02 $ (1,218,000)    
Minimum      
Summary Of Significant Accounting Policies [Line Items]      
Future effective tax rate after impact of tax exempt income 18.00%    
Maximum      
Summary Of Significant Accounting Policies [Line Items]      
Future effective tax rate after impact of tax exempt income 19.00%    
Buildings and improvements      
Summary Of Significant Accounting Policies [Line Items]      
Estimated useful lives 5 to 40 years    
Furniture and equipment      
Summary Of Significant Accounting Policies [Line Items]      
Estimated useful lives 3 to 15 years    
Federal Home Loan Bank of Des Moines      
Summary Of Significant Accounting Policies [Line Items]      
Investment in capital stock of the Federal Home Loan Bank of total assets 0.12%    
Investment in capital stock of Federal Home Loan Bank required percentage of advances 4.00%