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Employee Benefit Plans (Restricted Stock Units) (Details)
$ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
annual_installment
shares
Dec. 31, 2017
USD ($)
annual_installment
shares
Dec. 31, 2016
USD ($)
annual_installment
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted average period for compensation expense recognition 6 months    
Restricted Stock Units (RSUs)      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares awarded (in shares) 467,400 642,500 640,400
Number of annual vesting installments | annual_installment 4 4 4
Period of increments vested 4 years 4 years 4 years
Compensation expense | $ $ 24.4 $ 23.5 $ 17.4
Total unrecognized compensation expense related to outstanding restricted stock units and restricted shares | $ $ 40.9    
Weighted average period for compensation expense recognition 2 years 7 months 6 days    
Restricted Stock Unit Earned As Dividend Equivalents      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares awarded (in shares) 35,071 38,986  
Director | Restricted Stock Units (RSUs)      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares awarded (in shares) 36,885 47,913 49,823
Executives and Employees | Restricted Stock Units (RSUs)      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares awarded (in shares) 395,495 555,561  
Executive Officer | Restricted Stock Units (RSUs)      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares awarded (in shares)     543,664
Executive Officer | Restricted Stock Unit Earned As Dividend Equivalents      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares awarded (in shares)     46,881