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Net Income (Loss) per Share (Detail) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Numerator:                
Net income (loss) $ 202 $ (230) $ 309 $ (153)        
Denominator:                
Shares outstanding at the beginning of the period (in shares)         61 59 56 56
1.625% Exchange (in shares) 0 0 2 0        
Denominator for basic net income per share (in shares) 61 56 61 56        
Effect of dilutive securities:                
1.625% Convertible Notes (in shares) 1 0 0 0        
1.125% Warrants (in shares) 5 0 5 0        
Denominator for diluted net income per share (in shares) 67 56 66 56        
Net income (loss) per share:                
Basic (in dollars per share) $ 3.29 $ (4.10) $ 5.10 $ (2.74)        
Diluted (in dollars per share) $ 3.02 $ (4.10) $ 4.68 $ (2.74)        
Cash Convertible Senior Notes Due 2020, Warrants                
Potentially dilutive common shares excluded from calculations:                
Stated percentage of warrants 1.125%   1.125%          
1.125% Warrants                
Potentially dilutive common shares excluded from calculations:                
1.125% Warrants (in shares) 0 2 0 1        
Convertible Notes | 1.625% Convertible Notes                
Potentially dilutive common shares excluded from calculations:                
Percentage of contractual interest rate 1.625%   1.625%