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Fair Value Measurements (Details 5) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Carrying Value    
Fair value and related carrying amounts of long-term debt and other long-term obligations    
Long-term debt and other long-term obligations $ 17,949 $ 19,425
Fair Value    
Fair value and related carrying amounts of long-term debt and other long-term obligations    
Long-term debt and other long-term obligations $ 19,487 $ 21,551