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Summary of Significant Accounting Policies - Additional Information (Detail)
 12 Months Ended
Dec. 31, 2020
USD ($)
Segment
$ / shares
Dec. 31, 2019
Dec. 31, 2018
Significant Accounting Policies [Line Items]      
Allowance for loan losses related to commercial, commercial real estate and farm real estate loans $ 750,000    
Number of days reaching which loans are considered for nonaccrual status 90 days    
Federal Home Loan Bank par value | $ / shares $ 100    
Reserve amount $ 0    
Number of operating segments | Segment 1    
ASU 2018-13 [Member]      
Significant Accounting Policies [Line Items]      
Change in Accounting Principle, Accounting Standards Update, Adopted [true false] true    
Change in Accounting Principle, Accounting Standards Update, Adoption Date Jan. 01, 2020    
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false] true    
ASU 2018-15 [Member]      
Significant Accounting Policies [Line Items]      
Change in Accounting Principle, Accounting Standards Update, Adopted [true false] true    
Change in Accounting Principle, Accounting Standards Update, Adoption Date Jan. 01, 2020    
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false] true    
ASU 2018-17 [Member]      
Significant Accounting Policies [Line Items]      
Change in Accounting Principle, Accounting Standards Update, Adopted [true false] true    
Change in Accounting Principle, Accounting Standards Update, Adoption Date Jan. 01, 2020    
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false] true    
ASU 2018-18 [Member]      
Significant Accounting Policies [Line Items]      
Change in Accounting Principle, Accounting Standards Update, Adopted [true false] true    
Change in Accounting Principle, Accounting Standards Update, Adoption Date Jan. 01, 2020    
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false] true    
ASU 2016-13 [Member]      
Significant Accounting Policies [Line Items]      
Change in Accounting Principle, Accounting Standards Update, Early Adoption [true false] true    
ASU 2019-05 [Member]      
Significant Accounting Policies [Line Items]      
Change in Accounting Principle, Accounting Standards Update, Early Adoption [true false] true    
Cash and Cash Equivalents [Member] | Interest Rate Swap [Member]      
Significant Accounting Policies [Line Items]      
Cash pledged as collateral $ 11,300,000    
Unsecured Debt [Member]      
Significant Accounting Policies [Line Items]      
Retail loans past due charge off period 90 days    
Secured Debt [Member]      
Significant Accounting Policies [Line Items]      
Residential real estate loans past due assessment of value period 180 days    
Other Debt [Member]      
Significant Accounting Policies [Line Items]      
Loans past due charged down to the net realizable value period 90 days    
Maximum [Member]      
Significant Accounting Policies [Line Items]      
Percentage of insurance commission revenue of total revenue 1.00% 1.00% 1.00%
Original maturities for cash and cash equivalents 90 days    
Estimated useful life of intangible assets 12 years    
Maximum [Member] | Furniture and Equipment [Member]      
Significant Accounting Policies [Line Items]      
Estimated useful life of asset 7 years    
Maximum [Member] | Buildings and Improvements [Member]      
Significant Accounting Policies [Line Items]      
Estimated useful life of asset 50 years    
Maximum [Member] | WSP [Member]      
Significant Accounting Policies [Line Items]      
Percentage of insurance commission revenue of total revenue 1.00% 1.00% 1.00%
Maximum [Member] | CIVB Risk Management, Inc. [Member]      
Significant Accounting Policies [Line Items]      
Percentage of insurance commission revenue of total revenue 1.00% 1.00% 1.00%
Minimum [Member]      
Significant Accounting Policies [Line Items]      
Estimated useful life of intangible assets 5 years    
Minimum [Member] | Furniture and Equipment [Member]      
Significant Accounting Policies [Line Items]      
Estimated useful life of asset 3 years    
Minimum [Member] | Buildings and Improvements [Member]      
Significant Accounting Policies [Line Items]      
Estimated useful life of asset 7 years