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Stock Repurchase Program - Additional Information (Detail) - USD ($)
$ / shares in Units, shares in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jan. 31, 2015
Stock Repurchase Programs [Line Items]        
Stock repurchase program, authorized amount   $ 500,000,000    
Stock Repurchased During Period, Shares   6.8 15.6  
Stock Repurchased During Period, Value $ 200,304,000 $ 146,331,000 $ 299,949,000  
Treasury stock average price per share   $ 21.39 $ 19.20  
Cumulative repurchases, shares 5.8 22.5    
Cumulative repurchases, value $ 200,000,000 $ 446,000,000    
Common stock average price $ 34.30 $ 19.87    
Maximum        
Stock Repurchase Programs [Line Items]        
Stock repurchase program, authorized amount       $ 500,000,000