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Fair Value Measurements (Details Textual) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Carrying/fair value of cash and cash equivalents $ 4,813 $ 6,988 $ 11,022 $ 12,785
Carrying/fair value of investments not included in cash and cash equivalents 1,130 1,426    
Carrying amount of long-term debt 30,234 32,490    
Corporate Bond Securities | Level 1        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Fair value of debt 23,245      
Other Long-term Debt | Level 2        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Fair value of debt 698      
Carrying Amount of Long-term Debt        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Carrying amount of long-term debt 23,477 26,193    
Fair Value        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Carrying amount of long-term debt $ 23,943 $ 26,627    
Maximum        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Maturity period of primarily bank time deposits, classified as cash equivalents, maximum 90 days