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Shareholders' Equity (Details) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Aug. 31, 2018
Jan. 03, 2017
Jul. 11, 2016
Dec. 31, 2018
Dec. 31, 2017
Nov. 30, 2018
Dec. 31, 2016
Business Acquisition Equity Interests Issued Or Issuable [Line Items]              
Number of shares authorized to repurchase       9,000,000   5,300,000  
Share repurchase amount           $ 10.0  
Purchase of treasury stock       $ 170.2 $ 13.0    
Treasury Stock, Shares, Acquired       3,400,000 300,000    
Treasury Stock Acquired, Average Cost Per Share       $ 49.59 $ 43.83    
Common stock reissued       500,000 500,000    
Preferred stock, par value       $ 1 $ 1   $ 1.00
Non-Cumulative Perpetual Preferred Stock, Series A [Member]              
Business Acquisition Equity Interests Issued Or Issuable [Line Items]              
Issuance of preferred stock     $ 150.0        
Preferred stock, dividend rate percentage     6.25%        
Preferred stock, par value     $ 1.00        
Preferred stock liquidation preference per depositary share     25        
Preferred stock, liquidation preference per share     $ 25,000        
Business Bancshares, Inc. [Member]              
Business Acquisition Equity Interests Issued Or Issuable [Line Items]              
Stock issued for acquisition 2,000,000            
City Securities [Member]              
Business Acquisition Equity Interests Issued Or Issuable [Line Items]              
Stock issued for acquisition   200,000