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Fair Values of Assets and Liabilities - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Random sample percentage of securities for each quarter 100.00%    
Total gains or losses included in earnings $ (8,102,000) $ (4,329,000)  
Interest rate swaps transferred into Level 3 from Level 2 0 0  
Corporate securities transferred from Level 3 to Level 2 0 0  
Impaired loans considered to be credit risk of non-collection 100,000    
Updated appraisal requirement floor 250,000    
Other real estate owned 3,935,000 2,765,000  
Financial instrument of credit risk 156,000 198,000  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings On Assets Held At Year End 0 0  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Between Levels 1 And 2 0 0  
Provision for credit losses 12,531,000 5,087,000 $ 18,480,000
Securities available for sale 909,247,000 731,358,000  
Pooled Trust Preferred Collateralized Debt Obligations [Member]      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Total gains or losses included in earnings (8,102,000) (4,329,000)  
Interest rate swaps transferred into Level 3 from Level 2 0 0  
Corporate securities transferred from Level 3 to Level 2 0 0  
Other Assets [Member]      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Total gains or losses included in earnings 0 0  
Interest rate swaps transferred into Level 3 from Level 2 0    
Corporate securities transferred from Level 3 to Level 2 0 0  
Pooled Trust Preferred Collateralized Debt Obligations [Member]      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Securities available for sale $ 0 $ 23,646,000