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Stock-Based Compensation - General Employees Stock Purchase Plan (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award Activity      
Weighted-average fair value $ 34.19 $ 41.59 $ 23.51
Allocated Share Based Compensation Expense $ 278 $ 302 $ 324
General Employees' Stock Purchase Plan (GESPP)      
Share-based Compensation Arrangement by Share-based Payment Award Activity      
Number of shares authorized 60    
Option price, percentage of market value at date of grant 85.00%    
Weighted-average fair value $ 27.14 $ 31.91 $ 30.07
Option price, discount from market value at date of grant 15.00%    
Allocated Share Based Compensation Expense $ 30 $ 30 $ 26