Fair Value Measurements (Unrealized Losses And Fair Value Of Investments With Unrealized Losses) (Details) $ in Millions	Mar. 31, 2017 USD ($) investment
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | investment	60
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | $	$ 34
Corporate and asset-backed debt securities
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | investment	2
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | $	$ 1
United States agency debt securities
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | investment	10
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | $	$ 8
United States treasury debt securities
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | investment	20
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | $	$ 16
Certificates of deposit
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | investment	28
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | $	$ 9