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Investments and Related Matters Other Segments (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Schedule of Investments [Line Items]    
Investments $ 8,364 $ 10,369
Other Segments    
Schedule of Investments [Line Items]    
Investments 846 837
Other Segments | Equity method investments    
Schedule of Investments [Line Items]    
Investments 417 430
Other Segments | Other investments (measured at fair value)    
Schedule of Investments [Line Items]    
Investments $ 429 $ 407