Financial Instruments (Fair Value Of Short-Term Investments) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Sep. 30, 2014	Mar. 31, 2014
Financial Instruments
Fair Value	$ 764	$ 583	[1]
Short-Term Investments
Financial Instruments
Cost or Amortized Cost	764	583
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	764	583
Short-Term Investments | Corporate bonds
Financial Instruments
Cost or Amortized Cost	389	279
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	389	279
Short-Term Investments | U.S. Treasury securities
Financial Instruments
Cost or Amortized Cost	165	114
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	165	114
Short-Term Investments | U.S. agency securities
Financial Instruments
Cost or Amortized Cost	141	80
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	141	80
Short-Term Investments | Commercial paper
Financial Instruments
Cost or Amortized Cost	69	110
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 69	$ 110

[1] Derived from audited consolidated financial statements.