Annual report pursuant to Section 13 and 15(d)

Stockholders' Equity (Common Stock) (Details)

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Stockholders' Equity (Common Stock) (Details)
$ in Thousands, shares in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
vote
shares
Dec. 31, 2016
USD ($)
shares
Dec. 31, 2015
USD ($)
shares
Class of Stock [Line Items]      
Value of stock repurchased $ 12,293 $ 58,679 $ 53,774
Class A-1 Common Stock      
Class of Stock [Line Items]      
Common stock, voting rights, number of votes | vote 1    
Stock conversion ratio 1    
Class B-1 Common Stock      
Class of Stock [Line Items]      
Common stock, voting rights, number of votes | vote 10    
Stock Buyback Program      
Class of Stock [Line Items]      
Number of shares repurchased | shares 0.2 3.5 3.0
Number of shares authorized to be repurchased $ 61,200    
Stock Buyback Program | Class A-1 Common Stock      
Class of Stock [Line Items]      
Value of stock repurchased $ 4,000 $ 55,200 $ 47,400