Financial Instruments (Fair Value Of Short-Term Investments) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Sep. 30, 2017	Mar. 31, 2017
Financial Instruments
Fair Value	$ 2,288	$ 1,967	[1]
Short-term investments
Financial Instruments
Cost or Amortized Cost	2,291	1,970
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3)	(3)
Fair Value	2,288	1,967
Short-term investments | Corporate bonds
Financial Instruments
Cost or Amortized Cost	1,235	944
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(1)
Fair Value	1,234	943
Short-term investments | U.S. Treasury securities
Financial Instruments
Cost or Amortized Cost	410	414
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(1)
Fair Value	409	413
Short-term investments | U.S. agency securities
Financial Instruments
Cost or Amortized Cost	132	152
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(1)
Fair Value	131	151
Short-term investments | Commercial paper
Financial Instruments
Cost or Amortized Cost	268	212
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	268	212
Short-term investments | Foreign Government Debt Securities [Member]
Financial Instruments
Cost or Amortized Cost	101	113
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	101	113
Short-term investments | Asset-backed Securities [Member]
Financial Instruments
Cost or Amortized Cost	119	135
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	119	135
Short-term investments | Certificates of Deposit [Member]
Financial Instruments
Cost or Amortized Cost	26	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 26	$ 0

[1] Derived from audited Consolidated Financial Statements.