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Summary of Significant Accounting Policies - Additional Information (Detail)
3 Months Ended 12 Months Ended
Dec. 31, 2017
USD ($)
Branches
Dec. 31, 2017
USD ($)
Branches
Property
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Summary Of Significant Accounting Policies [Line Items]        
Number of branches | Branches 172 172    
Number of ATM | Property   160    
Wholly-owned trust subsidiaries | Property   12    
Held for sale securities, percentage of purchase price as condition for sale   15.00%    
Allowance for credit losses carryover   $ 0    
Non-accrual status period   90 days    
Loans returned to accrual status, performance period   6 months    
Minimum loan balance individually tested for impairment $ 1,000,000 $ 1,000,000    
Indefinite - lived intangible assets 0 $ 0    
Period of interest rate lock commitments   60 days    
Service cost component of defined benefit plan   $ 2,578,000 $ 2,799,000 $ 3,355,000
Partnerships losses and impairment $ 300,000 $ 1,500,000    
Land Improvements [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Property, plant and equipment, Useful life   15 years    
Consumer Loan [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Period of closed end loans loans charged down to net realizable value   120 days    
Home Equity [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Period of closed end loans loans charged down to net realizable value   180 days    
Residential Real Estate Loans [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Period of closed end loans loans charged down to net realizable value   180 days    
Minimum [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Percentage of voting interest 50.00% 50.00%    
Weighted-average estimated useful lives   10 years    
Non-compete agreements, amortization period   1 year    
Minimum [Member] | Furniture and Equipment [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Property, plant and equipment, Useful life   3 years    
Minimum [Member] | Building and Building Improvements [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Property, plant and equipment, Useful life   15 years    
Maximum [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Weighted-average estimated useful lives   16 years    
Non-compete agreements, amortization period   4 years    
Maximum [Member] | Furniture and Equipment [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Property, plant and equipment, Useful life   10 years    
Maximum [Member] | Building and Building Improvements [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Property, plant and equipment, Useful life   39 years