Financial Instruments (Components Of Cash, Cash Equivalents And Investments) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Mar. 31, 2012
Auction Rate Securities [Member]
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Available-for-sale	$ 18.7	$ 26.9
Cash, Cash Equivalents And Investments [Member]
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total debt and equity investments measured at fair value	150.0	49.9
Total cash, cash equivalents and investments	1,057.9	1,496.9
Cash, Cash Equivalents And Investments [Member] \| United States Treasury Securities [Member]
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Available-for-sale	150.0	49.9
Cash, Cash Equivalents And Investments [Member] \| Cash On Hand [Member]
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Other measurement basis	829.7	632.6
Cash, Cash Equivalents And Investments [Member] \| Certificates Of Deposit [Member]
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Other measurement basis	52.0	45.1
Cash, Cash Equivalents And Investments [Member] \| Money-Market Funds [Member]
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Other measurement basis	26.2	769.3
Short-Term Investments [Member]
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total debt and equity investments measured at fair value	123.6	86.1
Total cash, cash equivalents and investments	123.6	86.1
Short-Term Investments [Member] \| United States Treasury Securities [Member]
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Available-for-sale	113.2	86.1
Short-Term Investments [Member] \| United States Government Agency Bonds [Member]
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Available-for-sale	5.0
Short-Term Investments [Member] \| Corporate Bonds And Commercial Paper [Member]
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Available-for-sale	5.4
Long-Term Investments [Member]
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total debt and equity investments measured at fair value	71.6	52.6
Total cash, cash equivalents and investments	71.6	52.6
Amounts included in accumulated other comprehensive income from available-for-sale securities (pre-tax):
Unrealized losses	3.0	2.4
Long-Term Investments [Member] \| United States Treasury Securities [Member]
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Available-for-sale	15.5	6.1
Long-Term Investments [Member] \| Auction Rate Securities [Member]
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Available-for-sale	18.7	26.9
Long-Term Investments [Member] \| United States Government Agency Bonds [Member]
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Available-for-sale	10.4
Long-Term Investments [Member] \| Corporate Bonds And Commercial Paper [Member]
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Available-for-sale	6.6
Long-Term Investments [Member] \| Mutual Funds [Member]
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Trading	$ 20.4	$ 19.6

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Trading Securities which consist of all investments in certain debt and equity securities that are bought and held principally for the purpose of selling them in the near term (thus held for only a short period of time) or for which an entity has elected the fair value option. Trading generally reflects active and frequent buying and selling, and trading securities are generally used with the objective of generating profits on short-term differences in price, but this category of investments may also include items which are not intended to be sold in the near term or for which the fair value option has been elected.

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