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Nature of Operations and Summary of Significant Accounting Policies - Additional Information (Detail)
 12 Months Ended
Dec. 31, 2017
USD ($)
Facilities
Business
shares
Dec. 31, 2016
USD ($)
shares
Dec. 31, 2015
USD ($)
shares
Schedule Of Accounting Policies [Line Items]      
Full service facilities maintained by bank | Facilities 62    
Number of business conduct | Business 0    
Factor considered to be significant for fair value measurement 10.00%    
Accrual of interest discontinued description Principal or interest is past due 90 days or more, and the loan is not well secured or in the process of collection, or when serious doubt exists as to the collectability of a loan, the accrual of interest is discontinued.    
Commercial loans as a percentage of total loan 57.00%    
Residential real estate loans as a percentage of total loan 21.00%    
Installment loans as a percentage of total loan 18.00%    
Mortgage warehouse loans as a percentage of total loan 3.00%    
Period in which loan sold by mortgage company 30 days    
Minimum period loan held by mortgage company 90 days    
Mortgage warehousing maximum pay off period 30 days    
Costs are deferred due to the term $ 0    
Impaired loans charged off 90 days    
Status of Non-Accrual of Loan 90 days    
Impairment of Loan 30 days    
Intangibles, Gross $ 12,400,000    
Goodwill $ 119,880,000 $ 76,941,000 $ 49,600,000
Uncertain tax positions recognized 50.00%    
Shares, non-dilutive | shares 0 0 3,750
Amount available for payment of dividend $ 8,500,000    
Cash and cash equivalents maximum maturity period 1 day    
Compensation expense $ 1,700,000 $ 608,000 $ 643,000
Reclassifications effect on net income $ 0    
Buildings and Improvements [Member] | Minimum [Member]      
Schedule Of Accounting Policies [Line Items]      
Useful Life for depreciation 3 years    
Buildings and Improvements [Member] | Maximum [Member]      
Schedule Of Accounting Policies [Line Items]      
Useful Life for depreciation 40 years    
Furniture and Equipment [Member] | Minimum [Member]      
Schedule Of Accounting Policies [Line Items]      
Useful Life for depreciation 2 years    
Furniture and Equipment [Member] | Maximum [Member]      
Schedule Of Accounting Policies [Line Items]      
Useful Life for depreciation 20 years    
Accounting Standards Update 2018-02 [Member]      
Schedule Of Accounting Policies [Line Items]      
Stranded tax effects included in AOCI $ 766,000