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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
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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

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