Financial Instruments (Fair Value Of Short-Term Investments By Stated Maturity Date Schedule) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Mar. 31, 2016
Financial Instruments
Short-term investments, Fair Value	$ 1,520	$ 1,341	[1]
Due in 1 year or less [Member]
Financial Instruments
Due in 1 year or less, Amortized Cost	749	571
Due in 1 year or less, Fair Value	749	571
Due in 1-2 years [Member]
Financial Instruments
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Amortized Cost Basis	415	461
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	416	462
Due in 2-3 years [Member]
Financial Instruments
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Amortized Cost Basis	295	295
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	296	296
Due in 3-4 years [Member]
Financial Instruments
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Amortized Cost Basis	59	12
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	$ 59	$ 12

[1] Derived from audited Consolidated Financial Statements.