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Treasury Stock - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Equity, Class of Treasury Stock [Line Items]      
Treasury stock, shares 6,405,000 6,405,000  
Treasury stock, cost $ 475,184,000 $ 475,184,000  
Class A | 2005 Share Repurchase Program      
Equity, Class of Treasury Stock [Line Items]      
Shares repurchased program expiration date Apr. 29, 2018    
Stock authorized to repurchase by board of directors $ 800,000,000    
Treasury stock, shares 3,826,947    
Shares repurchased, average price per share $ 90.23    
Treasury stock, cost $ 345,300,000    
Treasury stock, shares repurchased 0 0 0