Investments (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Available-for-sale securities
Available-for-sale Securities, Amortized Cost	$ 783.6	$ 672.7
Available-for-sale Securities, Gross Unrealized Gains	0.4	0.3
Available-for-sale Securities, Gross Unrealized Losses	(0.1)	(0.3)
Available-for-sale Securities, Fair Value	783.9	672.7
Corporate debt securities [Member]
Available-for-sale securities
Available-for-sale Securities, Amortized Cost	296.3	324.8
Available-for-sale Securities, Gross Unrealized Gains	0.3	0.2
Available-for-sale Securities, Gross Unrealized Losses	(0.1)	(0.3)
Available-for-sale Securities, Fair Value	296.5	324.7
Fair values and gross unrealized losses for all available-for-sale securities in an unrealized loss position deemed to be temporary
Available-for-sale Securities, Unrealized Losses, Fair Value	118.5	164.5
Available-for-sale Securities, Unrealized Losses	(0.1)	(0.3)
U.S. Government Agencies and Debt Securities [Member]
Available-for-sale securities
Available-for-sale Securities, Amortized Cost	230.5	177.1
Available-for-sale Securities, Gross Unrealized Gains		0.1
Available-for-sale Securities, Fair Value	230.5	177.2
Municipal Bonds [Member]
Available-for-sale securities
Available-for-sale Securities, Amortized Cost		1.0
Available-for-sale Securities, Fair Value		1.0
Foreign government debt securities [Member]
Available-for-sale securities
Available-for-sale Securities, Amortized Cost	2.8	6.8
Available-for-sale Securities, Fair Value	2.8	6.8
Commercial Paper [Member]
Available-for-sale securities
Available-for-sale Securities, Amortized Cost	183.3	74.5
Available-for-sale Securities, Fair Value	183.3	74.5
Certificates of deposit [Member]
Available-for-sale securities
Available-for-sale Securities, Amortized Cost	70.7	88.5
Available-for-sale Securities, Gross Unrealized Gains	0.1
Available-for-sale Securities, Fair Value	$ 70.8	$ 88.5

This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

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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 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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