Fair Value Measurements (Unrealized Losses And Fair Value Of Investments With Unrealized Losses) (Details) $ in Millions	Jun. 30, 2018 USD ($) investment
Number of Investments | investment	193
Fair Value | $	$ 243
Corporate and asset-backed debt securities
Number of Investments | investment	107
Fair Value | $	$ 76
Foreign government debt securities
Number of Investments | investment	1
Fair Value | $	$ 2
United States agency debt securities
Number of Investments | investment	16
Fair Value | $	$ 24
United States Treasury debt securities
Number of Investments | investment	27
Fair Value | $	$ 75
Certificates of deposit
Number of Investments | investment	42
Fair Value | $	$ 66