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Common Stock Narrative (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Oct. 27, 2016
Class of Stock [Line Items]          
Dividends paid   $ 101,511 $ 205,468 $ 104,628  
Amount paid for repurchase of common stock   $ 46,957 $ 83,593 $ 53,043  
Class B [Member]          
Class of Stock [Line Items]          
Number of shares authorized under share repurchase program         4,000,000
Remaining number of shares authorized to be repurchased   2,200,000      
Treasury Stock [Member]          
Class of Stock [Line Items]          
Repurchased shares   1,841,800 3,053,204 1,696,457  
SpecialDividend [Member]          
Class of Stock [Line Items]          
Dividends paid $ 102,200        
Common stock, dividends, per share, cash paid $ 1.00