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Premises and Equipment - Schedule of Premises and Equipment (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Property, Plant and Equipment [Line Items]    
Property plant and equipment, gross $ 223,630 $ 207,315
Less accumulated depreciation and amortization 142,291 139,781
Total premises and equipment 81,339 67,534
Land [Member]    
Property, Plant and Equipment [Line Items]    
Property plant and equipment, gross 15,389 13,679
Buildings and improvements [Member]    
Property, Plant and Equipment [Line Items]    
Property plant and equipment, gross $ 88,386 80,364
Buildings and improvements [Member] | Minimum [Member]    
Property, Plant and Equipment [Line Items]    
Estimated Useful Life 10 years  
Buildings and improvements [Member] | Maximum [Member]    
Property, Plant and Equipment [Line Items]    
Estimated Useful Life 50 years  
Leasehold improvements [Member]    
Property, Plant and Equipment [Line Items]    
Property plant and equipment, gross $ 17,812 15,871
Leasehold improvements [Member] | Minimum [Member]    
Property, Plant and Equipment [Line Items]    
Estimated Useful Life 5 years  
Leasehold improvements [Member] | Maximum [Member]    
Property, Plant and Equipment [Line Items]    
Estimated Useful Life 40 years  
Furniture and equipment [Member]    
Property, Plant and Equipment [Line Items]    
Property plant and equipment, gross $ 64,609 61,324
Furniture and equipment [Member] | Minimum [Member]    
Property, Plant and Equipment [Line Items]    
Estimated Useful Life 3 years  
Furniture and equipment [Member] | Maximum [Member]    
Property, Plant and Equipment [Line Items]    
Estimated Useful Life 7 years  
Software [Member]    
Property, Plant and Equipment [Line Items]    
Property plant and equipment, gross $ 37,434 $ 36,077
Software [Member] | Minimum [Member]    
Property, Plant and Equipment [Line Items]    
Estimated Useful Life 3 years  
Software [Member] | Maximum [Member]    
Property, Plant and Equipment [Line Items]    
Estimated Useful Life 7 years