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Investments (Tables) | 6 Months Ended
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Jun. 30, 2018
Investments Debt And Equity Securities [Abstract] |
Carrying Value and Amortized Cost of Company’s Investments Summarized by Major Security Type | The carrying value and amortized cost of the Company’s investments, summarized by major security type, consisted of the following (in thousands):
|  | June 30, 2018 |
|  | Amortized Cost |  |  | Unrealized Gains |  |  | Unrealized Losses |  |  | Estimated Fair Value |
U.S. Treasury notes |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |
Government sponsored enterprise securities |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |
Corporate debt securities |  |  | 139,304 |  |  |  | — |  |  |  | (502 | ) |  |  | 138,802 |
Commercial paper |  |  | 43,879 |  |  |  | 6 |  |  |  | (14 | ) |  |  | 43,871 |
Equity securities |  |  | 3,149 |  |  |  | — |  |  |  | (247 | ) |  |  | 2,902 |
|  | $ | 186,332 |  |  | $ | 6 |  |  | $ | (763 | ) |  | $ | 185,575 |
|  | 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 |
Summary of Gross Unrealized Losses and Fair Value of Available-For-Sale Investments in Unrealized Loss Position | The following table presents gross unrealized losses and fair value for those available-for-sale investments that were in an unrealized loss position as of June 30, 2018 and December 31, 2017, aggregated by investment category and length of time that individual securities have been in a continuous loss position (in thousands):
|  | Less Than 12 Months |  |  | 12 Months or Greater |  |  | Total |
|  | Estimated Fair Value |  |  | Unrealized Losses |  |  | Estimated Fair Value |  |  | Unrealized Losses |  |  | Estimated Fair Value |  |  | Unrealized Losses |
June 30, 2018: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury notes |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |
Government sponsored enterprise securities |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |
Corporate debt securities |  |  | 136,308 |  |  |  | (502 | ) |  |  | — |  |  |  | — |  |  |  | 136,308 |  |  |  | (502 | )
Commercial paper |  |  | 24,061 |  |  |  | (14 | ) |  |  | — |  |  |  | — |  |  |  | 24,061 |  |  |  | (14 | )
Total |  | $ | 160,369 |  |  | $ | (516 | ) |  | $ | — |  |  | $ | — |  |  | $ | 160,369 |  |  | $ | (516 | )
December 31, 2017: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury notes |  | $ | 32,964 |  |  | $ | (12 | ) |  | $ | — |  |  | $ | — |  |  |  | 32,964 |  |  |  | (12 | )
Government sponsored enterprise securities |  |  | 10,072 |  |  |  | (10 | ) |  |  | — |  |  |  | — |  |  |  | 10,072 |  |  |  | (10 | )
Corporate debt securities |  |  | 129,820 |  |  |  | (321 | ) |  |  | — |  |  |  | — |  |  |  | 129,820 |  |  |  | (321 | )
Commercial paper |  |  | 90,558 |  |  |  | (65 | ) |  |  | — |  |  |  | — |  |  |  | 90,558 |  |  |  | (65 | )
Total |  | $ | 263,414 |  |  | $ | (408 | ) |  | $ | — |  |  | $ | — |  |  | $ | 263,414 |  |  | $ | (408 | )

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