(Fair Value Of Short-Term Investments) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Financial Instruments [Line Items]
Fair Value	$ 1,967	$ 1,341
Short-Term Investments [Member]
Financial Instruments [Line Items]
Cost or Amortized Cost	1,970	1,339
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(3)	0
Fair Value	1,967	1,341
Short-Term Investments [Member] | Corporate Bonds [Member]
Financial Instruments [Line Items]
Cost or Amortized Cost	944	620
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(1)	0
Fair Value	943	621
Short-Term Investments [Member] | U.S. Treasury Securities [Member]
Financial Instruments [Line Items]
Cost or Amortized Cost	414	389
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(1)	0
Fair Value	413	390
Short-Term Investments [Member] | U.S. Agency Securities [Member]
Financial Instruments [Line Items]
Cost or Amortized Cost	152	167
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	0
Fair Value	151	167
Short-Term Investments [Member] | Commercial Paper [Member]
Financial Instruments [Line Items]
Cost or Amortized Cost	212	50
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	212	50
Short-Term Investments [Member] | Foreign Government Debt Securities [Member]
Financial Instruments [Line Items]
Cost or Amortized Cost	113	113
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	113	113
Short-Term Investments [Member] | Asset-backed Securities [Member]
Financial Instruments [Line Items]
Cost or Amortized Cost	135	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 135	$ 0