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Share-Based Payment Arrangements (Narrative) (Details)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2018
USD ($)
entity
$ / shares
shares
Mar. 31, 2017
USD ($)
Options [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Intrinsic value of options exercised | $ $ 0.5 $ 3.3
Restricted and Performance Shares [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total unrecognized compensation expense | $ $ 47.5  
Weighted-average period 3 years 3 months 18 days  
Total fair value of shares vested | $ $ 0.3 $ 0.5
Shares granted | shares 54,710  
Grant date fair value per share | $ / shares $ 48.22  
Executive Management [Member] | Performance Shares [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Service period 3 years  
Executive Management [Member] | Performance Shares [Member] | Total Shareholder Return [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Shares granted | shares 26,147  
Grant date fair value per share | $ / shares $ 51.13  
Vesting performance period 3 years  
Number of peer group regional banks | entity 43  
Executive Management [Member] | Performance Shares [Member] | Core Earnings Per Share [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Shares granted | shares 26,147  
Grant date fair value per share | $ / shares $ 44.84  
Vesting performance period 2 years  
Executive Management [Member] | Tranche One [Member] | Performance Shares [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Percentage of maximum number of shares vested 200.00%