Financial Instruments (Fair Value Of Short-Term Investments) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Dec. 31, 2016	Mar. 31, 2016
Financial Instruments
Fair Value	$ 1,736	$ 1,341	[1]
Short-term investments
Financial Instruments
Cost or Amortized Cost	1,740	1,339
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(4)	0
Fair Value	1,736	1,341
Short-term investments | Corporate bonds
Financial Instruments
Cost or Amortized Cost	842	620
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(2)	0
Fair Value	840	621
Short-term investments | U.S. Treasury securities
Financial Instruments
Cost or Amortized Cost	400	389
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(1)	0
Fair Value	399	390
Short-term investments | U.S. agency securities
Financial Instruments
Cost or Amortized Cost	163	167
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	0
Fair Value	162	167
Short-term investments | Commercial paper
Financial Instruments
Cost or Amortized Cost	116	50
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	116	50
Short-term investments | Foreign Government Debt Securities [Member]
Financial Instruments
Cost or Amortized Cost	124	113
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	124	113
Short-term investments | Asset-backed Securities [Member]
Financial Instruments
Cost or Amortized Cost	95	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 95	$ 0

[1] Derived from audited Consolidated Financial Statements.