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Fair Value Measurements (Unrealized Losses And Fair Value Of Investments With Unrealized Losses) (Details)
$ in Millions
Sep. 30, 2017
USD ($)
investment
Number of Investments | investment 62
Fair Value | $ $ 54
Corporate and asset-backed debt securities  
Number of Investments | investment 25
Fair Value | $ $ 9
United States agency debt securities  
Number of Investments | investment 15
Fair Value | $ $ 18
United States Treasury debt securities  
Number of Investments | investment 16
Fair Value | $ $ 24
Certificates of deposit  
Number of Investments | investment 6
Fair Value | $ $ 3