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Restructuring - Narrative (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2020
Restructuring Cost and Reserve [Line Items]        
Restructuring costs $ 927 $ 658 $ 927  
Expected costs $ 2,500      
Percentage estimate of cumulative pretax costs that will result in cash outlays (primarily from employee separation expense) (as percent) 60.00%      
Percentage estimate of cumulative pretax costs that will be non-cash (primarily from accelerated depreciation of facilities) (as percent) 40.00%      
Forecast        
Restructuring Cost and Reserve [Line Items]        
Expected costs       $ 800