Financial Instruments (Details 2) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Mar. 31, 2011
Amounts included in accumulated other comprehensive income from available-for-sale securities (pre-tax):
Unrealized losses	$ 3.4
Auction Rate Securities [Member]
Measured at fair value:
Available-for-sale	25.9		27.2
Cash and Cash Equivalents [Member]
Measured at fair value:
Total debt and equity investments measured at fair value	55.5		563.4
Total cash, cash equivalents and investments	1,319.5		1,660.9
Amounts included in accumulated other comprehensive income from available-for-sale securities (pre-tax):
Unrealized losses	0	[1]	0	[1]
Cash and Cash Equivalents [Member] | Cash on hand [Member]
Measured at fair value:
Other measurement basis	906.9		412.8
Cash and Cash Equivalents [Member] | Money-market funds [Member]
Measured at fair value:
Other measurement basis	357.1		684.7
Cash and Cash Equivalents [Member] | U.S. Treasury securities [Member]
Measured at fair value:
Available-for-sale	0		525.0
Cash and Cash Equivalents [Member] | Certificates of deposit [Member]
Measured at fair value:
Available-for-sale	55.5		38.4
Cash and Cash Equivalents [Member] | Auction Rate Securities [Member]
Measured at fair value:
Available-for-sale	0		0
Cash and Cash Equivalents [Member] | Mutual funds [Member]
Measured at fair value:
Trading	0		0
Short-term Investments [Member]
Measured at fair value:
Total debt and equity investments measured at fair value	37.1		27.8
Total cash, cash equivalents and investments	37.1		27.8
Amounts included in accumulated other comprehensive income from available-for-sale securities (pre-tax):
Unrealized losses	0	[1]	0	[1]
Short-term Investments [Member] | Cash on hand [Member]
Measured at fair value:
Other measurement basis	0		0
Short-term Investments [Member] | Money-market funds [Member]
Measured at fair value:
Other measurement basis	0		0
Short-term Investments [Member] | U.S. Treasury securities [Member]
Measured at fair value:
Available-for-sale	37.1		27.8
Short-term Investments [Member] | Certificates of deposit [Member]
Measured at fair value:
Available-for-sale	0		0
Short-term Investments [Member] | Auction Rate Securities [Member]
Measured at fair value:
Available-for-sale	0		0
Short-term Investments [Member] | Mutual funds [Member]
Measured at fair value:
Trading	0		0
Long- term Investments [Member]
Measured at fair value:
Total debt and equity investments measured at fair value	57.4		67.8
Total cash, cash equivalents and investments	57.4		67.8
Amounts included in accumulated other comprehensive income from available-for-sale securities (pre-tax):
Unrealized losses	3.4	[1]	2.6	[1]
Long- term Investments [Member] | Cash on hand [Member]
Measured at fair value:
Other measurement basis	0		0
Long- term Investments [Member] | Money-market funds [Member]
Measured at fair value:
Other measurement basis	0		0
Long- term Investments [Member] | U.S. Treasury securities [Member]
Measured at fair value:
Available-for-sale	12.9		22.1
Long- term Investments [Member] | Certificates of deposit [Member]
Measured at fair value:
Available-for-sale	0		0
Long- term Investments [Member] | Auction Rate Securities [Member]
Measured at fair value:
Available-for-sale	25.9		27.2
Long- term Investments [Member] | Mutual funds [Member]
Measured at fair value:
Trading	$ 18.6		$ 18.5

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