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Summary of Significant Accounting Policies - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2017 USD ($) Segment $ / shares | Dec. 31, 2016 | Dec. 31, 2015
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 |  |
Number of operating segments | Segment | 1 |  |
ASU 2018-02 [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Cumulative effect adjustment between retained earnings and accumulated other comprehensive income | $ 199,000 |  |
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 |  |
Percentage of increase on assets due to new accounting standard updates | 1.00% |  |
Percentage of increase on liabilities due to new accounting standard updates | 1.00% |  |
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% |  |
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 |  |

X
- Definition Number of days for demand deposits with financial institutions to be considered cash and cash equivalent. + References No definition available. + Details
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- Definition Minimum balance requirement to perform detailed loan review. + References No definition available. + Details
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