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Cash and Marketable Securities (Schedule Of Proceeds From Sale Of Securities, Maturities Of Available-for-sale Securities And Gross Realized (Gains) Losses) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Cash and Marketable Securities [Abstract]        
Proceeds from sale of securities $ 0 $ 16 $ 0 $ 17
Maturities of securities $ 5 $ 6 $ 9 $ 15