Financial Instruments (Details 2) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012		Mar. 31, 2011
Auction Rate Securities [Member]
Measured at fair value:
Available-for-sale	$ 26.9		$ 27.2
Cash and Cash Equivalents [Member]
Measured at fair value:
Total debt and equity investments measured at fair value	95.0		563.4
Total cash, cash equivalents and investments	1,496.9		1,660.9
Cash and Cash Equivalents [Member] | Cash on hand [Member]
Measured at fair value:
Other measurement basis	632.6		412.8
Cash and Cash Equivalents [Member] | Money-market funds [Member]
Measured at fair value:
Other measurement basis	769.3		684.7
Cash and Cash Equivalents [Member] | U.S. Treasury securities [Member]
Measured at fair value:
Available-for-sale	49.9		525.0
Cash and Cash Equivalents [Member] | Certificates of deposit [Member]
Measured at fair value:
Available-for-sale	45.1		38.4
Short-term Investments [Member]
Measured at fair value:
Total debt and equity investments measured at fair value	86.1		27.8
Total cash, cash equivalents and investments	86.1		27.8
Short-term Investments [Member] | U.S. Treasury securities [Member]
Measured at fair value:
Available-for-sale	86.1		27.8
Long- term Investments [Member]
Measured at fair value:
Total debt and equity investments measured at fair value	52.6		67.8
Total cash, cash equivalents and investments	52.6		67.8
Amounts included in accumulated other comprehensive income from available-for-sale securities (pre-tax):
Unrealized losses	2.4	[1]	2.6	[1]
Long- term Investments [Member] | U.S. Treasury securities [Member]
Measured at fair value:
Available-for-sale	6.1		22.1
Long- term Investments [Member] | Auction Rate Securities [Member]
Measured at fair value:
Available-for-sale	26.9		27.2
Long- term Investments [Member] | Mutual funds [Member]
Measured at fair value:
Trading	$ 19.6		$ 18.5

[1] The unrealized losses on available-for-sale securities at March 31, 2012 and 2011 relate to the auction rate securities.