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Basis of Presentation -Narrative (Details) - USD ($)
$ in Millions
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]            
Increase in assets $ 842          
Increase in liabilities 961          
Decrease in shareholders' equity 31,764 $ 30,883 $ 30,178 $ 29,832 $ 29,662 $ 29,695
Estimated Impact of Lease Accounting Standard            
New Accounting Pronouncements or Change in Accounting Principle [Line Items]            
Increase in assets 860          
Increase in liabilities 997          
Decrease in shareholders' equity $ 40