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Investments (Schedule of Investments) (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]    
Trading securities $ 202 $ 221
Available-for-sale investment securities 0 14
Financial investments, at fair value 202 235
Equity method investments 131 131
Equity securities $ 76 $ 152