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Net Income (Loss) Per Share (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Numerator:                        
Net income (loss) $ 201 $ 197 $ 202 $ 107 $ (262) $ (97) $ (230) $ 77 $ 707 $ (512) $ 52  
Denominator:                        
Shares outstanding at the beginning of the period (in shares)         59.3         59.3 55.8 55.1
Weighted-average number of shares issued:                        
Exchange of 1.625% Convertible Notes (in shares)                 1.4 0.1 0.0  
Conversion of 1.625% Convertible Notes (in shares)                 0.2 0.0 0.0  
Stock-based compensation (in shares)                 0.2 0.5 0.3  
Denominator for basic net income (loss) per share (in shares)                 61.1 56.4 55.4  
Effect of dilutive securities:                        
1.125% Warrants (in shares)                 4.8 0.0 0.5  
1.625% Convertible Notes (in shares)                 0.4 0.0 0.0  
Share-based compensation (in shares)                 0.3 0.0 0.3  
Denominator for diluted net (loss) income per share (in shares)                 66.6 56.4 56.2  
Net income (loss) per share:                        
Basic (in dollars per share) $ 3.24 $ 3.22 $ 3.29 $ 1.79 $ (4.59) $ (1.70) $ (4.10) $ 1.38 $ 11.57 $ (9.07) $ 0.93  
Diluted (in dollars per share) $ 3.01 $ 2.90 $ 3.02 $ 1.64 $ (4.59) $ (1.70) $ (4.10) $ 1.37 $ 10.61 $ (9.07) $ 0.92  
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]                        
Stock-based compensation (in shares)                 0.0 0.3 0.0  
1.125% Warrants                        
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]                        
Potentially dilutive common shares excluded from calculations (in shares)                 0.0 1.9 0.0  
1.625% Notes                        
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]                        
Potentially dilutive common shares excluded from calculations (in shares)                 0.0 0.4 0.0  
Stated interest rate 1.625%               1.625%      
1.125% Warrants                        
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]                        
Stated interest rate 1.125%               1.125%