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Stock-Based Compensation - Stock Appreciation Rights, Additional Information (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Employee Stock Ownership Plan (ESOP) Disclosures      
Stock-based compensation expense $ 123 $ 132 $ 127
SARs      
Employee Stock Ownership Plan (ESOP) Disclosures      
Weighted-average grant-date fair value per share (in usd per share) $ 6.82 $ 7.38 $ 11.33
Total grant-date fair value $ 15 $ 20 $ 26
Stock-based compensation expense $ 1    
Weighted-average period over which expense is expected to be recognized 1 year 2 months 12 days