Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2012 M	Dec. 31, 2011
Available-for-sale securities
Less amounts classified as cash equivalents	$ (9,713)	$ (38,721)
Less amounts classified as restricted cash	(893)	(893)
Amortized Cost	185,863	196,120
Gross Unrealized Gains	8	36
Gross Unrealized Losses	(17)	(19)
Amounts classified as marketable securities	185,854	196,137
Maturity period for marketable securities
Maximum contractual maturity period (in months)	12 months
Average contractual maturity period (in months)	5
Available-for-sale securities and restricted and unrestricted cash
Available-for-sale securities
Amortized Cost	196,469	235,734
Gross Unrealized Gains	8	36
Gross Unrealized Losses	(17)	(19)
Estimated Fair Value	196,460	235,751
U.S. government securities \| Available-for-sale securities and restricted and unrestricted cash
Available-for-sale securities
Amortized Cost	61,028	66,150
Gross Unrealized Gains	5	24
Gross Unrealized Losses	(2)
Estimated Fair Value	61,031	66,174
U.S. government agencies \| Available-for-sale securities and restricted and unrestricted cash
Available-for-sale securities
Amortized Cost	82,942	93,183
Gross Unrealized Gains	2	9
Gross Unrealized Losses	(15)	(17)
Estimated Fair Value	82,929	93,175
U.S. corporate notes \| Available-for-sale securities and restricted and unrestricted cash
Available-for-sale securities
Amortized Cost	550	2,707
Gross Unrealized Losses		(2)
Estimated Fair Value	550	2,705
U.S. commercial paper \| Available-for-sale securities and restricted and unrestricted cash
Available-for-sale securities
Amortized Cost	45,736	34,973
Gross Unrealized Gains	1	3
Estimated Fair Value	45,737	34,976
Money market funds \| Available-for-sale securities and restricted and unrestricted cash
Available-for-sale securities
Amortized Cost	6,213	38,721
Estimated Fair Value	$ 6,213	$ 38,721

Represents the average maturity period for marketable securities.

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 7

-Footnote 2

-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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Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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The latest date, in a range of maturity dates, that a debt security categorized as neither trading nor held-to-maturity is scheduled to be repaid.

+ References

No definition available.

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-Subparagraph b

-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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-Glossary Available-for-Sale Securities

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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.

+ References

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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

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No definition available.

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- Definition

Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Compensating balance arrangements that do not legally restrict the withdrawal or usage of cash equivalents may be reported as cash equivalents, while legally restricted equivalents held as compensating balances against borrowing arrangements, contracts entered into with others, or company statements of intention with regard to particular equivalents are not generally reported as part of unrestricted cash equivalents.

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-SubTopic 10

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Total debt and equity financial instruments including: (1) securities held-to-maturity, (2) trading securities, and (3) securities available-for-sale which are intended to be held for less than one year or the normal operating cycle, whichever is longer.

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Cash and equivalents whose use in whole or in part is restricted for the long-term, generally by contractual agreements or regulatory requirements.

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-Publisher SEC

-Name Regulation S-X (SX)

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