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Capital Stock (Schedule of Shares Repurchased) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended 18 Months Ended 31 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Jun. 30, 2019
Jan. 31, 2018
Class of Stock [Line Items]          
Total number of shares repurchased (in shares) 16,503,317 27,878,324 41,604,141    
Aggregate cost of shares repurchased $ 845 $ 1,673 $ 2,917    
Average price per share of shares repurchased (USD per share) $ 51.24 $ 60.00 $ 70.10    
July 2019 Share Repurchase Program [Member]          
Class of Stock [Line Items]          
Total number of shares repurchased (in shares) 10,100,000        
Aggregate cost of shares repurchased $ 500        
Average price per share of shares repurchased (USD per share) $ 49.29        
January 2018 Share Repurchase Program [Member]          
Class of Stock [Line Items]          
Total number of shares repurchased (in shares) 6,400,000 27,600,000   34,000,000.0  
Aggregate cost of shares repurchased $ 345 $ 1,655      
Average price per share of shares repurchased (USD per share) $ 54.36 $ 59.89   $ 58.86  
July 2015 Share Repurchase Program [Member]          
Class of Stock [Line Items]          
Total number of shares repurchased (in shares)   300,000 41,600,000   58,700,000
Aggregate cost of shares repurchased   $ 18 $ 2,917    
Average price per share of shares repurchased (USD per share)   $ 71.68 $ 70.10   $ 68.15