Financial Instruments (Fair Value Of Short-Term Investments) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Sep. 30, 2019	Mar. 31, 2019
Financial Instruments
Fair Value	$ 1,943	$ 737	[1]
Short-term investments
Financial Instruments
Cost or Amortized Cost	1,941	738
Gross Unrealized Gains	2	0
Gross Unrealized Losses	0	(1)
Fair Value	1,943	737
Short-term investments | Corporate bonds
Financial Instruments
Cost or Amortized Cost	744	325
Gross Unrealized Gains	2	0
Gross Unrealized Losses	0	(1)
Fair Value	746	324
Short-term investments | U.S. Treasury securities
Financial Instruments
Cost or Amortized Cost	468	153
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	468	153
Short-term investments | U.S. agency securities
Financial Instruments
Cost or Amortized Cost	11	44
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	11	44
Short-term investments | Commercial paper
Financial Instruments
Cost or Amortized Cost	355	112
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	355	112
Short-term investments | Debt Security, Government, Non-US [Member]
Financial Instruments
Cost or Amortized Cost	34	50
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	34	50
Short-term investments | Asset-backed Securities [Member]
Financial Instruments
Cost or Amortized Cost	287	53
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	287	53
Short-term investments | Certificates of Deposit [Member]
Financial Instruments
Cost or Amortized Cost	42	1
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 42	$ 1

[1] Derived from audited Consolidated Financial Statements.