Financial Instruments (Fair Value Of Short-Term Investments) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Sep. 30, 2018	Mar. 31, 2018
Financial Instruments
Fair Value	$ 1,664	$ 1,073	[1]
Short-term investments
Financial Instruments
Cost or Amortized Cost	1,671	1,082
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(7)	(9)
Fair Value	1,664	1,073
Short-term investments | Corporate bonds
Financial Instruments
Cost or Amortized Cost	739	629
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3)	(5)
Fair Value	736	624
Short-term investments | U.S. Treasury securities
Financial Instruments
Cost or Amortized Cost	279	212
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2)	(2)
Fair Value	277	210
Short-term investments | U.S. agency securities
Financial Instruments
Cost or Amortized Cost	74	79
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(1)
Fair Value	73	78
Short-term investments | Commercial paper
Financial Instruments
Cost or Amortized Cost	357	109
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	357	109
Short-term investments | Debt Security, Government, Non-US [Member]
Financial Instruments
Cost or Amortized Cost	80	53
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(1)
Fair Value	79	52
Short-term investments | Asset-backed Securities [Member]
Financial Instruments
Cost or Amortized Cost	121	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	121	0
Short-term investments | Certificates of Deposit [Member]
Financial Instruments
Cost or Amortized Cost	21	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 21	$ 0

[1] Derived from audited Consolidated Financial Statements.