Marketable Securities (Tables)

9 Months Ended

Dec. 31, 2011

Marketable Securities [Abstract]

Available-for-Sale Securities

	December 31, 2011 (in millions)
	Aggregate Cost Basis		Gross Unrealized Gains		Gross Unrealized Losses		Aggregate Fair Value
U.S. Treasury and agency securities	$	55	$	—	$	—	$	55
Municipal securities		2		—		—		2
Corporate debt securities		124		—		—		124
	$	181	$	—	$	—	$	181

	March 31, 2011 (in millions)
	Aggregate Cost Basis		Gross Unrealized Gains		Gross Unrealized Losses		Aggregate Fair Value
U.S. Treasury and agency securities	$	60	$	—	$	—	$	60
Municipal securities		2		—		—		2
Corporate debt securities		117		—		—		117
	$	179	$	—	$	—	$	179