Fair Value Measurements (Unrealized Losses And Fair Value Of Investments With Unrealized Losses) (Details) $ in Millions	Sep. 30, 2018 USD ($) investment
Number of Investments | investment	202
Fair Value | $	$ 247
Corporate and asset-backed debt securities
Number of Investments | investment	116
Fair Value | $	$ 87
Foreign government debt securities
Number of Investments | investment	1
Fair Value | $	$ 2
United States agency debt securities
Number of Investments | investment	14
Fair Value | $	$ 24
United States Treasury debt securities
Number of Investments | investment	39
Fair Value | $	$ 86
Certificates of deposit
Number of Investments | investment	32
Fair Value | $	$ 48