Financial Instruments (Fair Value Of Short-Term Investments By Stated Maturity Date Schedule) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Mar. 31, 2017
Financial Instruments
Short-term investments, Fair Value	$ 2,318	$ 1,967	[1]
Short-term investments
Financial Instruments
Due in 1 year or less, Amortized Cost	1,598	1,237
Due in 1 year or less, Fair Value	1,596	1,236
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Amortized Cost Basis	725	721
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	719	719
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Amortized Cost Basis	3	12
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value	3	12
Short-term investments, Amortized Cost	2,326	1,970
Short-term investments, Fair Value	$ 2,318	$ 1,967

[1] Derived from audited Consolidated Financial Statements.