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Shareholders' Equity (Narrative) (Details)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2017
USD ($)
vote
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2015
USD ($)
shares
Dec. 31, 2014
USD ($)
$ / shares
shares
Class of Stock [Line Items]        
Common stock, shares authorized 500,000,000      
Common stock, par value (in dollars per share) | $ / shares $ 0      
Common stock, shares issued 211,072,508 210,711,224 210,538,896 210,216,043
Common stock, shares outstanding 211,072,508 210,711,224 210,538,896 210,216,043
Common stock dividends declared | $ $ 0 $ 0 $ 0 $ 50,036
Dividends payable | $   $ 12,509 $ 12,509  
Salary period paid upon employee leaving 30 days      
Class A Shares Of Common Stock [Member]        
Class of Stock [Line Items]        
Common stock, shares authorized 420,000,000 420,000,000    
Common stock, par value (in dollars per share) | $ / shares $ 0 $ 0   $ 0
Common stock, shares issued 131,072,508 130,711,224 130,538,896 130,216,043
Common stock, shares outstanding 131,072,508 130,711,224 130,538,896 130,216,043
Shares issued during the period 361,284 172,328 322,853  
Number of votes per share (in votes) | vote 1      
Class B Shares Of Common Stock [Member]        
Class of Stock [Line Items]        
Common stock, shares authorized 80,000,000 80,000,000    
Common stock, par value (in dollars per share) | $ / shares $ 0 $ 0    
Common stock, shares issued 80,000,000 80,000,000 80,000,000 80,000,000
Common stock, shares outstanding 80,000,000 80,000,000 80,000,000 80,000,000
Number of votes per share (in votes) | vote 5