Financial Instruments (Fair Value Of Short-Term Investments) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Dec. 31, 2020	Mar. 31, 2020
Financial Instruments
Fair Value	$ 1,938	$ 1,967	[1]
Short-term investments
Financial Instruments
Cost or Amortized Cost	1,935	1,970
Gross Unrealized Gains	3	5
Gross Unrealized Losses	0	(8)
Fair Value	1,938	1,967
Short-term investments | Corporate bonds
Financial Instruments
Cost or Amortized Cost	647	684
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	(4)
Fair Value	648	681
Short-term investments | U.S. Treasury securities
Financial Instruments
Cost or Amortized Cost	559	530
Gross Unrealized Gains	1	4
Gross Unrealized Losses	0	0
Fair Value	560	534
Short-term investments | U.S. agency securities
Financial Instruments
Cost or Amortized Cost	4	8
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	4	8
Short-term investments | Commercial paper
Financial Instruments
Cost or Amortized Cost	369	377
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	369	377
Short-term investments | Debt Security, Government, Non-US [Member]
Financial Instruments
Cost or Amortized Cost	72	42
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	72	42
Short-term investments | Asset-backed Securities [Member]
Financial Instruments
Cost or Amortized Cost	208	273
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	(4)
Fair Value	209	269
Short-term investments | Certificates of Deposit [Member]
Financial Instruments
Cost or Amortized Cost	76	56
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 76	$ 56

[1] Derived from audited Consolidated Financial Statements.