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Fair Value Measurements (Tables) | 6 Months Ended
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Jun. 30, 2013
Fair Value Measurements |
Schedule of assets and liabilities measured on a recurring basis, at fair value | The major categories of assets and liabilities measured on a recurring basis, at fair value, as of June 30, 2013 and December 31, 2012, are as follows (in thousands):
|  | June 30, 2013 |
|  | Level 1 |  | Level 2 |  | Level 3 |  | Total |
United States treasuries |  | $ | 206,061 |  | $ | — |  | $ | — |  | $ | 206,061 |
Commercial paper |  | — |  | 4,000 |  | — |  | 4,000 |
Corporate bonds |  | — |  | 75,628 |  | — |  | 75,628 |
Government agency securities |  | — |  | 86,440 |  | — |  | 86,440 |
Total |  | $ | 206,061 |  | $ | 166,068 |  | $ | — |  | $ | 372,129 |
|  | December 31, 2012 |
|  | Level 1 |  | Level 2 |  | Level 3 |  | Total |
United States treasuries |  | $ | 278,698 |  | $ | — |  | $ | — |  | $ | 278,698 |
Government agency securities |  | — |  | 123,054 |  | — |  | 123,054 |
Total |  | $ | 278,698 |  | $ | 123,054 |  | $ | — |  | $ | 401,752 |

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- Definition Tabular disclosure of assets and liabilities, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19190-110258 + Details
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