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Investments (Fair Values by Classification) (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Fair values of available-for-sale investments by classification in the Consolidated Balance Sheets        
Cash and cash equivalents $ 11,956 $ 3,800 $ 3,000 $ 3,241
Marketable securities 17,965 37,878    
Other assets 1,648 2,119    
Total available-for-sale investments 29,418 41,169    
Total available-for-sale investments   41,318    
Available-for-sale investments [Member]        
Fair values of available-for-sale investments by classification in the Consolidated Balance Sheets        
Cash and cash equivalents 11,453 3,291    
Marketable securities 17,965 37,878    
Other assets 0 149    
Total available-for-sale investments $ 29,418      
Total available-for-sale investments   $ 41,318