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Share-Based Compensation - Stock Option Activity (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Outstanding, beginning balance (in shares) 90 90 81  
Granted (in shares) 0 5 18  
Exercised (in shares) 0 0 0  
Expired (in shares) (23) (2) (3)  
Forfeited (in shares) (47) (3) (6)  
Outstanding, ending balance (in shares) 20 90 90 81
Options exercisable, shares underlying options (in shares) 19 65 41  
Weighted Average Exercise Price Per Share        
Outstanding, beginning balance (in usd per share) $ 1,420 $ 1,440 $ 1,650  
Granted (in usd per share) 0 594 602  
Exercised (in usd per share) 0 0 0  
Expired (in usd per share) 915 1,272 2,766  
Forfeited (in usd per share) 1,666 794 1,090  
Outstanding, ending balance (in usd per share) 1,429 1,420 1,440 $ 1,650
Options exercisable, average exercise price per share (in usd per share) $ 1,440 $ 1,656 $ 2,146  
Options outstanding, weighted average contract life 4 years 3 months 7 days 5 years 8 months 12 days 7 years 1 month 24 days 7 years 8 months 23 days
Options outstanding, aggregate intrinsic value $ 0 $ 0 $ 0 $ 1
Options exercised, aggregate intrinsic value $ 0 $ 0 $ 0  
Options exercisable, weighted average contract life 4 years 4 months 6 days 4 years 10 months 9 days 5 years 8 months 23 days  
Options exercisable, aggregate intrinsic value $ 0 $ 0 $ 0