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Fair Value Measurements (Details) (Recurring basis, USD $)
In Thousands, unless otherwise specified |
Dec. 31, 2011
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Dec. 31, 2010
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|---|---|---|
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Quoted Prices in Active markets for Identical Assets, Level 1
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| Estimated fair values of entity's financial assets | ||
| Fair value of financial assets | $ 163,501 | $ 176,851 |
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Quoted Prices in Active markets for Identical Assets, Level 1 | U.S. government securities
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| Estimated fair values of entity's financial assets | ||
| Fair value of financial assets | 66,174 | 25,976 |
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Quoted Prices in Active markets for Identical Assets, Level 1 | U.S. government agency securities
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| Estimated fair values of entity's financial assets | ||
| Fair value of financial assets | 55,901 | 24,375 |
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Quoted Prices in Active markets for Identical Assets, Level 1 | U.S. corporate notes
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| Estimated fair values of entity's financial assets | ||
| Fair value of financial assets | 2,705 | 34,695 |
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Quoted Prices in Active markets for Identical Assets, Level 1 | Money market funds
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| Estimated fair values of entity's financial assets | ||
| Fair value of financial assets | 38,721 | 91,805 |
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Significant Other Observable Inputs, Level 2
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| Estimated fair values of entity's financial assets | ||
| Fair value of financial assets | 72,250 | 127,494 |
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Significant Other Observable Inputs, Level 2 | U.S. government agency securities
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| Estimated fair values of entity's financial assets | ||
| Fair value of financial assets | 37,274 | 30,273 |
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Significant Other Observable Inputs, Level 2 | U.S. commercial paper
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| Estimated fair values of entity's financial assets | ||
| Fair value of financial assets | 34,976 | 97,221 |
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Total
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| Estimated fair values of entity's financial assets | ||
| Fair value of financial assets | 235,751 | 304,345 |
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Total | U.S. government securities
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| Estimated fair values of entity's financial assets | ||
| Fair value of financial assets | 66,174 | 25,976 |
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Total | U.S. government agency securities
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| Estimated fair values of entity's financial assets | ||
| Fair value of financial assets | 93,175 | 54,648 |
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Total | U.S. corporate notes
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| Estimated fair values of entity's financial assets | ||
| Fair value of financial assets | 2,705 | 34,695 |
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Total | U.S. commercial paper
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| Estimated fair values of entity's financial assets | ||
| Fair value of financial assets | 34,976 | 97,221 |
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Total | Money market funds
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| Estimated fair values of entity's financial assets | ||
| Fair value of financial assets | $ 38,721 | $ 91,805 |