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Investment Securities (Tables) | 12 Months Ended
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Dec. 31, 2017
Investments Debt And Equity Securities [Abstract] |
Schedule of Investment Securities Classified as Available-for-Sale | 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 |

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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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