Fair Value Measurements (Details) (USD $)	12 Months Ended
Dec. 31, 2011 M	Dec. 31, 2010
Fair Value Measurements
Maturity period of cash equivalents to classify as money market funds, maximum (in months)	3
Fair value of financial assets and liabilities
Debt securities	$ 72,360,000	$ 85,075,000
Fair value of debt	49,600,000
Amount by which debt carrying value exceeded estimated fair value	400,000
US government debt securities
Fair value of financial assets and liabilities
Debt securities	11,323,000	62,475,000
Corporate debt securities
Fair value of financial assets and liabilities
Debt securities	52,605,000	22,600,000
Foreign government debt securities
Fair value of financial assets and liabilities
Debt securities	8,432,000
Recurring basis \| Estimated fair value
Fair value of financial assets and liabilities
Money Market Funds	40,197,000
Total	112,557,000
Recurring basis \| Estimated fair value \| US government debt securities
Fair value of financial assets and liabilities
Debt securities	11,323,000
Recurring basis \| Estimated fair value \| Corporate debt securities
Fair value of financial assets and liabilities
Debt securities	52,605,000
Recurring basis \| Estimated fair value \| Foreign government debt securities
Fair value of financial assets and liabilities
Debt securities	8,432,000
Recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value of financial assets and liabilities
Money Market Funds	40,197,000
Total	51,520,000
Recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| US government debt securities
Fair value of financial assets and liabilities
Debt securities	11,323,000
Recurring basis \| Significant Other Observable Inputs (Level 2)
Fair value of financial assets and liabilities
Total	61,037,000
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Corporate debt securities
Fair value of financial assets and liabilities
Debt securities	52,605,000
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Foreign government debt securities
Fair value of financial assets and liabilities
Debt securities	$ 8,432,000

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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Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an 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.

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