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Financial Instruments (Derivative) (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2016
Cash and Cash Equivalents [Line Items]        
Cash, cash equivalents, restricted cash and restricted cash equivalents at end of period $ 3,859 $ 4,447    
Restricted Cash and Cash Equivalents, Current 120 52    
Restricted Cash and Cash Equivalents, Noncurrent 127 132    
Cash, cash equivalents, restricted cash and restricted cash equivalents at beginning of year 4,106 4,631 $ 4,832 $ 4,403
Credit Derivative, Maximum Exposure, Undiscounted 23      
Bank Time Deposits [Member]        
Cash and Cash Equivalents [Line Items]        
Cash, cash equivalents, restricted cash and restricted cash equivalents at end of period $ 2,400 $ 2,900