Cash, Cash Equivalents, and Investments - Maturities of Available for Sale Investments (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010
Available-for-sale and Trading Investments [Abstract]
Amortized cost due within one year	$ 631.6
Gross unrealized gains due within one year	0.4
Gross unrealized losses due within one year	0
Estimated fair value due within one year	632.0
Amortized cost due between one and five years	740.5
Gross unrealized gains due between one and five years	1.0
Gross unrealized losses due between one and five years	(0.8)
Estimated fair value due between one and five year	740.7
Total investments, amortized cost	1,372.1	999.0
Total investments, gross unrealized gains	1.4
Total investments, gross unrealized losses	0.8
Available-for-sale securities, estimated fair value	1,372.7	1,001.6
Debt Securities [Member]
Available-for-sale and Trading Investments [Abstract]
Total investments, amortized cost	1,372.1	999.0
Available-for-sale securities, estimated fair value	1,372.7	1,001.6
US Government Debt Securities [Member]
Available-for-sale and Trading Investments [Abstract]
Total investments, amortized cost	301.1	158.2
Available-for-sale securities, estimated fair value	301.0	158.4
US Government-sponsored Enterprises Debt Securities [Member]
Available-for-sale and Trading Investments [Abstract]
Total investments, amortized cost	406.3	213.8
Available-for-sale securities, estimated fair value	406.5	214.0
Foreign Government Debt Securities [Member]
Available-for-sale and Trading Investments [Abstract]
Total investments, amortized cost	0	46.8
Available-for-sale securities, estimated fair value	0	47.0
Certificates of Deposit [Member]
Available-for-sale and Trading Investments [Abstract]
Total investments, amortized cost	31.8	20.9
Available-for-sale securities, estimated fair value	31.8	21.0
Commercial Paper [Member]
Available-for-sale and Trading Investments [Abstract]
Total investments, amortized cost		9.5
Available-for-sale securities, estimated fair value		9.5
Asset-backed Securities [Member]
Available-for-sale and Trading Investments [Abstract]
Total investments, amortized cost	124.7	90.1
Available-for-sale securities, estimated fair value	124.7	90.0
Corporate Debt Securities [Member]
Available-for-sale and Trading Investments [Abstract]
Total investments, amortized cost	508.2	459.7
Available-for-sale securities, estimated fair value	$ 508.7	$ 461.7

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Gross unrealized gains due between une and five years.

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Gross Unrealized Gains Due within one year.

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Gross unrealized losses due between one and five year.

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Gross Unrealized Losses Due within one year.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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This item represents debt securities, at cost, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments, as defined), and fair value hedge accounting adjustments, if any, which are expected to mature after one year and through five years from the balance sheet date and which are categorized neither as held-to-maturity nor trading securities.

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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This item represents the fair value of debt securities which are expected to mature after one year and through five years from the balance sheet date and which are categorized neither as held-to-maturity nor trading securities.

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-Paragraph 20

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This item represents debt securities, at cost, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments, as defined), and fair value hedge accounting adjustments, if any, which are expected to mature within one year of the balance sheet date and which are categorized neither as held-to-maturity nor trading securities.

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-Name FASB Staff Position (FSP)

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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-Paragraph 20

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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-Publisher FASB

-Name Accounting Standards Codification

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This item represents the fair value of debt securities which are expected to mature within one year of the balance sheet date and which are categorized neither as held-to-maturity nor trading securities.

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-Publisher FASB

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-Number 115

-Paragraph 20

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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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 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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The increase (decrease), resulting in a gain, in the difference between the fair value and the carrying value, or in the comparative fair values, of securities held at each balance sheet date, that was included in earnings for the period.

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No definition available.

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The increase (decrease), resulting in a loss, in the difference between the fair value and the carrying value, or in the comparative fair values, of securities held at each balance sheet date, that was included in earnings for the period.

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