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Marketable and Other Securities Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Marketable Securities [Line Items]        
Cash and cash equivalents $ 14,256 $ 15,512    
Restricted cash 3,291 2,336    
Total 17,547 17,848 $ 15,546 $ 15,160
Other assets [Member]        
Marketable Securities [Line Items]        
Cash and cash equivalents 232 $ 219    
Continuing Operations [Member]        
Marketable Securities [Line Items]        
Cash and cash equivalents 14,256      
Total 17,547   $ 14,955  
Continuing Operations [Member] | Other current assets [Member]        
Marketable Securities [Line Items]        
Restricted cash 2,713      
Continuing Operations [Member] | Other assets [Member]        
Marketable Securities [Line Items]        
Restricted cash $ 578