Financial Instruments (Fair Value Of Short-Term Investments) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Sep. 30, 2016	Mar. 31, 2016
Financial Instruments
Fair Value	$ 1,520	$ 1,341	[1]
Short-term investments
Financial Instruments
Cost or Amortized Cost	1,518	1,339
Gross Unrealized Gains	2	2
Gross Unrealized Losses	0	0
Fair Value	1,520	1,341
Short-term investments | Corporate bonds
Financial Instruments
Cost or Amortized Cost	738	620
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	0
Fair Value	739	621
Short-term investments | U.S. Treasury securities
Financial Instruments
Cost or Amortized Cost	378	389
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	0
Fair Value	379	390
Short-term investments | U.S. agency securities
Financial Instruments
Cost or Amortized Cost	154	167
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	154	167
Short-term investments | Commercial paper
Financial Instruments
Cost or Amortized Cost	79	50
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	79	50
Short-term investments | Foreign Government Debt Securities [Member]
Financial Instruments
Cost or Amortized Cost	115	113
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	115	113
Short-term investments | Asset-backed Securities [Member]
Financial Instruments
Cost or Amortized Cost	54	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 54	$ 0

[1] Derived from audited Consolidated Financial Statements.