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Investments (Tables) | 6 Months Ended
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Jun. 30, 2019
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, 2019 |
|  | Amortized Cost |  |  | Unrealized Gains |  |  | Unrealized Losses |  |  | Estimated Fair Value |
Corporate debt securities |  | $ | 130,372 |  |  | $ | 329 |  |  | $ | (22 | ) |  | $ | 130,679 |
Commercial paper |  |  | 176,541 |  |  |  | 235 |  |  |  | — |  |  |  | 176,776 |
|  | $ | 306,913 |  |  | $ | 564 |  |  | $ | (22 | ) |  | $ | 307,455 |
|  | December 31, 2018 |
|  | Amortized Cost |  |  | Unrealized Gains |  |  | Unrealized Losses |  |  | Estimated Fair Value |
Corporate debt securities |  | $ | 187,371 |  |  | $ | 39 |  |  | $ | (344 | ) |  | $ | 187,066 |
Commercial paper |  |  | 151,774 |  |  |  | — |  |  |  | (78 | ) |  |  | 151,696 |
|  | $ | 339,145 |  |  | $ | 39 |  |  | $ | (422 | ) |  | $ | 338,762 |
Summary of Gross Unrealized Losses and Fair Value of Available-For-Sale Investment Securities in Unrealized Loss Position | The following table presents gross unrealized losses and fair value for those available-for-sale investment securities that were in an unrealized loss position as of June 30, 2019 and December 31, 2018, aggregated by investment category and length of time that the 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, 2019: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate debt securities |  | $ | 4,000 |  |  | $ | (8 | ) |  | $ | 26,079 |  |  | $ | (14 | ) |  | $ | 30,079 |  |  | $ | (22 | )
Total |  | $ | 4,000 |  |  | $ | (8 | ) |  | $ | 26,079 |  |  | $ | (14 | ) |  | $ | 30,079 |  |  | $ | (22 | )
|  | Less Than 12 Months |  |  | 12 Months or Greater |  |  | Total |
|  | Estimated Fair Value |  |  | Unrealized Losses |  |  | Estimated Fair Value |  |  | Unrealized Losses |  |  | Estimated Fair Value |  |  | Unrealized Losses |
December 31, 2018: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate debt securities |  | $ | 91,265 |  |  | $ | (130 | ) |  | $ | 44,637 |  |  | $ | (214 | ) |  | $ | 135,902 |  |  | $ | (344 | )
Commercial paper |  |  | 151,696 |  |  |  | (78 | ) |  |  | — |  |  |  | — |  |  |  | 151,696 |  |  |  | (78 | )
Total |  | $ | 242,961 |  |  | $ | (208 | ) |  | $ | 44,637 |  |  | $ | (214 | ) |  | $ | 287,598 |  |  | $ | (422 | )

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- Definition Tabular disclosure of fair value of investments in debt and equity securities in an unrealized loss position categorized neither as held-to-maturity nor trading securities. + References No definition available. + Details
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- Definition Tabular disclosure of the reconciliation of available-for-sale securities from cost basis to fair value. + References No definition available. + Details
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