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Investment Securities | 12 Months Ended
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Dec. 31, 2017
Investments Debt And Equity Securities [Abstract] |
Investment Securities | 3. Investment Securities Investment securities, all classified as available-for-sale, consisted of the following (in thousands):
|  | December 31, 2017 |
|  | Amortized Cost |  |  | Unrealized Gains |  |  | Unrealized Losses |  |  | Estimated Fair Value |
U.S. Treasury notes |  | $ | 32,976 |  |  | $ | — |  |  | $ | (12 | ) |  | $ | 32,964 |
Government sponsored enterprise securities |  |  | 10,082 |  |  |  | — |  |  |  | (10 | ) |  |  | 10,072 |
Corporate debt securities |  |  | 138,650 |  |  |  | 1 |  |  |  | (321 | ) |  |  | 138,330 |
Commercial paper |  |  | 90,623 |  |  |  | — |  |  |  | (65 | ) |  |  | 90,558 |
|  | $ | 272,331 |  |  | $ | 1 |  |  | $ | (408 | ) |  | $ | 271,924 |
|  | December 31, 2016 |
|  | Amortized Cost |  |  | Unrealized Gains |  |  | Unrealized Losses |  |  | Estimated Fair Value |
U.S. Treasury notes |  | $ | 82,484 |  |  | $ | 6 |  |  | $ | (3 | ) |  | $ | 82,487 |
Government sponsored enterprise securities |  |  | 73,789 |  |  |  | 1 |  |  |  | (5 | ) |  |  | 73,785 |
Corporate debt securities |  |  | 79,190 |  |  |  | — |  |  |  | (72 | ) |  |  | 79,118 |
Commercial paper |  |  | 129,861 |  |  |  | 165 |  |  |  | — |  |  |  | 130,026 |
|  | $ | 365,324 |  |  | $ | 172 |  |  | $ | (80 | ) |  | $ | 365,416 |
At each reporting date, the Company performs an evaluation of impairment to determine if any unrealized losses are other-than-temporary. Factors considered in determining whether a loss is other-than-temporary include the length of time and extent to which fair value has been less than the cost basis, the financial condition of the issuer, and the Company’s intent and ability to hold the investment until recovery of its amortized cost basis. The Company intends, and has the ability, to hold its investments in unrealized loss positions until their amortized cost basis has been recovered. Based on its evaluation, the Company determined that its unrealized losses were not other-than-temporary at December 31, 2017 and 2016. The Company has classified all of its available-for-sale investment securities, including those with maturities beyond one year, as current assets on its consolidated balance sheets based on the highly liquid nature of the investment securities and because these investment securities are considered available for use in current operations. As of December 31, 2017, the Company held $48.7 million of available-for-sale investment securities with contractual maturity dates more than one year and less than two years. No securities with similar maturity dates were held as of December 31, 2016.

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- Definition The entire disclosure for investments in certain debt and equity securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -URI http://asc.fasb.org/topic&trid=2196928 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=4738149 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2176304 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2324412 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2209399 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 954 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2289146 + Details
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