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Investments (Tables) | 12 Months Ended
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Dec. 31, 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):
|  | 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 |
Equity Securities |  |  | 3,149 |  |  |  | — |  |  |  | (1,840 | ) |  |  | 1,309 |
|  | $ | 342,294 |  |  | $ | 39 |  |  | $ | (2,262 | ) |  | $ | 340,071 |
|  | 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 December 31, 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 |
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 | )
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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- 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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