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Summary of Significant Accounting Policies - Narrative (Details)
12 Months Ended
Dec. 31, 2018
USD ($)
subsidiary
segment
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Property, Plant and Equipment      
Number of significant subsidiaries | subsidiary 1    
Cash on deposit with Federal Reserve Bank $ 574,400,000 $ 552,000,000  
Compensating balances with Federal Reserve Bank 28,100,000 16,100,000  
Mortgage loans held for sale 33,300,000 46,600,000  
Deferred compensation plan assets 12,100,000 12,200,000  
Deferred compensation plan liabilities 12,100,000 12,200,000  
Impairment losses 0 0 $ 200,000
Write-downs of OREO (100,000) (400,000) (600,000)
Carrying value of foreclosed OREO 2,000,000 2,700,000  
Federal Reserve Bank stock 37,500,000 10,700,000  
Federal Home Loan Bank stock 31,800,000 10,200,000  
Income tax examination, penalties $ 0 0 0
Number of operating segments | segment 1    
Advertising expense $ 3,200,000 3,500,000 2,800,000
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition 5,600,000 3,800,000 4,400,000
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation     2,200,000
Mortgage Loans in Process of Foreclosure, Amount $ 400,000    
Buildings and improvements | Minimum      
Property, Plant and Equipment      
Estimated useful lives 5 years    
Buildings and improvements | Maximum      
Property, Plant and Equipment      
Estimated useful lives 45 years    
Furniture and equipment | Minimum      
Property, Plant and Equipment      
Estimated useful lives 4 years    
Furniture and equipment | Maximum      
Property, Plant and Equipment      
Estimated useful lives 15 years    
Restricted Stock      
Property, Plant and Equipment      
Stock compensation expense $ 5,600,000 3,300,000 3,900,000
Tax benefits from stock-based compensation 200,000 600,000 1,500,000
Common Stock Including Additional Paid in Capital [Member]      
Property, Plant and Equipment      
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition   3,800,000 4,400,000
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation $ 1,100,000 $ 2,600,000 $ 2,200,000