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Stock-Based Compensation (Activity Relating to Performance Share Awards) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Compensation Expense Recorded [Line Items]      
Compensation expense $ 63 $ 71 $ 64
Performance Share Awards      
Compensation Expense Recorded [Line Items]      
Weighted grant date fair value of awards granted (in dollars per share) $ 129.68 $ 115.85 $ 105.64
Awards settled in cash $ 21 $ 19 $ 13
Awards settled in stock 53 79 39
Compensation expense $ 50 $ 60 $ 53