Financial Instruments (Details 1) (USD $) In Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Summary of activity in Level 3 financial instruments
Balance at the beginning of the period	$ 26.8	$ 46.1	$ 27.2	$ 61.6
Redemption of auction rate securities	(0.2)	(11.4)	(0.4)	(27.5)
Change in unrealized gain (loss) included in interest and other income, net				0
Change in unrealized gain (loss) included in other comprehensive income	(1.0)	(0.8)	(1.2)	(0.2)
Balance at the end of the period	25.6	33.9	25.6	33.9
Put Option [Member]
Summary of activity in Level 3 financial instruments
Balance at the beginning of the period	0	0.9	0	1.1
Redemption of auction rate securities	0	0	0
Change in unrealized gain (loss) included in interest and other income, net		(0.9)		(1.1)
Change in unrealized gain (loss) included in other comprehensive income	0	0	0
Balance at the end of the period	0	0	0	0
Auction Rate Securities [Member]
Summary of activity in Level 3 financial instruments
Balance at the beginning of the period	26.8	45.2	27.2	60.5
Redemption of auction rate securities	(0.2)	(11.4)	(0.4)	(27.5)
Change in unrealized gain (loss) included in interest and other income, net		0.9		1.1
Change in unrealized gain (loss) included in other comprehensive income	(1.0)	(0.8)	(1.2)	(0.2)
Balance at the end of the period	$ 25.6	$ 33.9	$ 25.6	$ 33.9