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Net Income per Share (Detail) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Numerator:        
Net income $ 107 $ 77    
Denominator:        
Shares outstanding at the beginning of the period (in shares)     59 56
1.625% Exchange (in shares) 1 0    
Denominator for basic net income per share (in shares) 60 56    
Effect of dilutive securities:        
Convertible senior notes (in shares) 1 0    
1.125% Warrants (in shares) 4 0    
Denominator for diluted net income per share (in shares) 65 56    
Net income per share:        
Basic (in dollars per share) $ 1.79 $ 1.38    
Diluted (in dollars per share) $ 1.64 $ 1.37    
Cash Convertible Senior Notes Due 2020, Warrants        
Net income per share:        
Stated percentage of warrants 1.125%      
Convertible Notes | 1.625% Convertible Notes        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Percentage of contractual interest rate 1.625%