Investments - Investments in Short and Long-Term Classified as Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	$ 775.5	$ 915.3
Available-for-sale Securities, Gross Unrealized Gains	0.5	0.5
Available-for-sale Securities, Gross Unrealized Losses	(0.2)	(0.1)
Available-for-sale Securities, Fair Value	775.8	915.7
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	401.5	383.6
Available-for-sale Securities, Gross Unrealized Gains	0.3	0.3
Available-for-sale Securities, Gross Unrealized Losses	(0.2)	(0.1)
Available-for-sale Securities, Fair Value	401.6	383.8
U.S. government agencies and debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	263.8	295.8
Available-for-sale Securities, Gross Unrealized Gains	0.1	0.1
Available-for-sale Securities, Fair Value	263.9	295.9
Foreign government debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	5.0	5.0
Available-for-sale Securities, Fair Value	5.0	5.0
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	16.0	138.7
Available-for-sale Securities, Fair Value	16.0	138.7
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	89.2	92.2
Available-for-sale Securities, Gross Unrealized Gains	0.1	0.1
Available-for-sale Securities, Fair Value	$ 89.3	$ 92.3

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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This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

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