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Stock-Based Compensation - General Employees Stock Purchase Plan (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award Activity      
Weighted-average fair value $ 23.51 $ 22.47 $ 23.98
Allocated Share Based Compensation Expense $ 324 $ 298 $ 276
General Employees' Stock Purchase Plan (GESPP)      
Share-based Compensation Arrangement by Share-based Payment Award Activity      
Number of shares authorized 60,000,000    
Option price, percentage of market value at date of grant 85.00%    
Options granted 877,705 987,478 1,007,669
Options exercised (877,705) (987,478) (1,007,669)
Shares available for grant 26,239,152 27,116,857 28,104,335
Weighted average exercise price - Options granted $ 170.42 $ 140.06 $ 133.52
Weighted average exercise price - Options exercised 170.42 140.06 133.52
Weighted-average fair value $ 30.07 $ 24.72 $ 23.56
Option price, discount from market value at date of grant 15.00%    
Allocated Share Based Compensation Expense $ 26 $ 24 $ 24