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Shareholders' Equity (Shareholders' Equity Narrative) (Details)
 12 Months Ended
Feb. 16, 2018
USD ($)
$ / shares
Oct. 13, 2017
shares
Dec. 31, 2018
USD ($)
vote
stock_class
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Sep. 06, 2011
USD ($)
Schedule of Shareholders' Equity Activity [Line Items]            
Number of classes of common stock | stock_class     3      
Preferred stock authorized (in shares)     500,000.0 500,000.0 500,000  
Preferred stock, par value (in dollars per shares) | $ / shares     $ 0.01 $ 0.01 $ 0.01  
Preferred stock issued     0 0 0  
Purchases of treasury stock | $         $ 8,800,000  
Share Price | $ / shares $ 22.18          
Employee stock ownership plan contribution | $ $ 22,300,000.0   $ 22,300,000 $ 0 $ 0  
Treasury Stock            
Schedule of Shareholders' Equity Activity [Line Items]            
Number of shares repurchased     (1,900,000) (200,000) (1,100,000)  
Purchases of treasury stock | $     $ 36,700,000 $ 3,800,000 $ 8,800,000  
Cancellation of class B treasury shares       1,200,000    
Common Class A            
Schedule of Shareholders' Equity Activity [Line Items]            
Number of votes per share | vote     1      
Number of shares repurchased     1,871,631 200,605 984,190  
Average cost per share (in dollars per share) | $ / shares     $ 19.59 $ 18.89 $ 8.96  
Purchases of treasury stock | $     $ 36,700,000 $ 3,800,000    
Remaining authorized repurchase amount | $     $ 100,000,000      
Common Class A | Treasury Stock            
Schedule of Shareholders' Equity Activity [Line Items]            
Stock repurchase program, authorized amount | $           $ 100,000,000
Common Class B            
Schedule of Shareholders' Equity Activity [Line Items]            
Number of votes per share | vote     10      
Common Class B | Treasury Stock            
Schedule of Shareholders' Equity Activity [Line Items]            
Cancellation of class B treasury shares   1,027,907        
Class B common stock converted (in shares)     284,845 136,654    
Common Class C            
Schedule of Shareholders' Equity Activity [Line Items]            
Number of votes per share | vote     10