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Fair Values of Assets and Liabilities - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
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 $ (4,329,000) $ (589,000)  
Interest rate swaps transferred into Level 3 from Level 2 0 536,000  
Corporate securities transferred from Level 3 to Level 2 0 536,000  
Impaired loans considered to be credit risk of non-collection 100,000    
Updated appraisal requirement floor 250,000    
Other real estate owned 2,765,000 6,805,000  
Financial instrument of credit risk 198,000 200,000  
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 5,087,000 18,480,000 $ 14,948,000
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 (4,329,000) (589,000)  
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