Financial Instruments (Fair Value Of Short-Term Investments) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Mar. 31, 2017
Financial Instruments
Fair Value	$ 2,318	$ 1,967	[1]
Short-term investments
Financial Instruments
Cost or Amortized Cost	2,326	1,970
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(8)	(3)
Fair Value	2,318	1,967
Short-term investments | Corporate bonds
Financial Instruments
Cost or Amortized Cost	1,212	944
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3)	(1)
Fair Value	1,209	943
Short-term investments | U.S. Treasury securities
Financial Instruments
Cost or Amortized Cost	448	414
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2)	(1)
Fair Value	446	413
Short-term investments | U.S. agency securities
Financial Instruments
Cost or Amortized Cost	117	152
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2)	(1)
Fair Value	115	151
Short-term investments | Commercial paper
Financial Instruments
Cost or Amortized Cost	294	212
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	294	212
Short-term investments | Foreign Government Debt Securities [Member]
Financial Instruments
Cost or Amortized Cost	98	113
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	98	113
Short-term investments | Asset-backed Securities [Member]
Financial Instruments
Cost or Amortized Cost	135	135
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	0
Fair Value	134	135
Short-term investments | Certificates of Deposit [Member]
Financial Instruments
Cost or Amortized Cost	22	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 22	$ 0

[1] Derived from audited Consolidated Financial Statements.