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Capital Stock (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 ($)
shares
Oct. 31, 2015
USD ($)
Aug. 31, 2013
USD ($)
Dec. 31, 1998
USD ($)
Capital Stock [Abstract]            
Preferred stock, authorized (in shares) 25,000,000          
Preferred stock, par value (in dollars per share) | $ / shares $ 0.001          
Common stock, shares authorized (in shares) 500,000,000 500,000,000        
Common stock, par value (in dollars per share) | $ / shares $ 0.001 $ 0.001        
Preferred stock, shares outstanding (in shares) 0 0        
Weighted Average Shares Outstanding [Abstract]            
Basic weighted-average common shares outstanding (in shares) 52,806,000 55,412,000 57,997,000      
Effect of Dilutive Securities [Abstract]            
Stock awards and options (in shares) 1,110,000 683,000 1,060,000      
Convertible note (in shares) 936,000 2,000 0      
Diluted weighted-average common shares outstanding (in shares) 54,852,000 56,097,000 59,057,000      
Repurchases of Common Stock [Abstract]            
Repurchase of Class A common stock | $ $ 71,731 $ 247,208 $ 164,094      
Amount available for repurchases | $ $ 128,000          
Stock Options [Member]            
Effect of Dilutive Securities [Abstract]            
Anti-dilutive shares excluded from calculation of diluted earnings per share (in shares) 400,000 2,000,000 1,800,000      
Common Class A [Member]            
Capital Stock [Abstract]            
Common stock, shares authorized (in shares) 500,000,000          
Common stock, par value (in dollars per share) | $ / shares $ 0.001          
Number of votes per share | Vote 1          
Number of shares of class A common stock issued after conversion of Class B common stock (in shares) 1          
Repurchases of Common Stock [Abstract]            
Shares repurchased (in shares) 1,200,000 4,500,000 3,800,000      
Authorized amount | $       $ 500,000   $ 10,000
Increase in authorized amount | $         $ 400,000  
Common Class B [Member]            
Capital Stock [Abstract]            
Common stock, shares authorized (in shares) 100,000,000          
Common stock, par value (in dollars per share) | $ / shares $ 0.001          
Common stock, shares outstanding (in shares) 0 0        
Number of votes per share | Vote 10          
Treasury Stock [Member]            
Repurchases of Common Stock [Abstract]            
Repurchase of Class A common stock | $ $ 71,731 $ 247,208 $ 164,094