Financial Instruments (Details 1) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Summary of activity in Level 3 financial instruments
Balance at the beginning of the period	$ 27.2	$ 61.6
Redemptions and sales of auction rate securities	(0.5)	(37.5)
Change in unrealized gain (loss) included in other comprehensive income (loss)	0.2	3.1
Balance at the end of the period	26.9	27.2
Put Option [Member]
Summary of activity in Level 3 financial instruments
Balance at the beginning of the period		1.1
Redemptions and sales of auction rate securities
Change in unrealized gain (loss) included in interest and other income, net		(1.1)
Change in unrealized gain (loss) included in other comprehensive income (loss)
Balance at the end of the period
Auction Rate Securities [Member]
Summary of activity in Level 3 financial instruments
Balance at the beginning of the period	27.2	60.5
Redemptions and sales of auction rate securities	(0.5)	(37.5)
Change in unrealized gain (loss) included in interest and other income, net		1.1
Change in unrealized gain (loss) included in other comprehensive income (loss)	0.2	3.1
Balance at the end of the period	$ 26.9	$ 27.2