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Share-Based Compensation (Options) (Details) - Service awards - Stock options - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Stock option activity        
Outstanding at the beginning of the period (in shares) 3,767,483      
Granted (in shares) 1,594,528 1,249,266 877,231  
Exercised (in shares)     0 0
Forfeited (in shares) (201,596)      
Expired (in shares) (295,990)      
Outstanding at the end of the period (in shares) 4,864,425 3,767,483    
Vested and exercisable at the end of the period (in shares) 2,212,911      
Weighted-average exercise price per share        
Outstanding at the beginning of the period (in dollars per share) $ 21.56      
Granted (in dollars per share) 8.35      
Forfeited (in dollars per share) 30.93      
Expired (in dollars per share) 60.33      
Outstanding at the end of the period (in dollars per share) 14.48 $ 21.56    
Vested and exercisable at the end of the period (in dollars per share) $ 20.88      
Additional disclosure        
Weighted-average remaining contractual term, outstanding 7 years 4 months 2 days 6 years 10 months 2 days    
Weighted-average remaining contractual term, vested and exercisable at the end of the period 5 years 10 months 6 days      
Grant date fair value (in dollars per share) $ 4.09 $ 9.18 $ 6.46  
Total grant-date fair value of awards vested $ 10 $ 6 $ 2  
Unvested stock options outstanding (in shares) 2,651,514 2,166,969