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Marketable Securities (Tables) | 9 Months Ended
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Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract] |
Summary of Available for Sale Securities | The amortized cost, gross unrealized holding gains or losses, and fair value of the Company’s marketable securities by major security type at each balance sheet date are summarized in the tables below (in thousands):
| September 30, 2021
| Amortized Cost |  | Unrealized Holding Gains |  | Unrealized Holding Losses |  | Aggregate Fair Value
Short-term marketable securities: |  |  |  |  |  |  |
U.S. government treasuries(1) | $ | 472,214 |  |  | $ | 13 |  |  | $ | (13) |  |  | $ | 472,214 |
U.S. government agency securities | 15,003 |  |  | — |  |  | — |  |  | 15,003 |
Corporate debt securities(2) | 61,187 |  |  | 5 |  |  | (10) |  |  | 61,182 |
Commercial paper | 96,223 |  |  | — |  |  | — |  |  | 96,223 |
Total short-term marketable securities | 644,627 |  |  | 18 |  |  | (23) |  |  | 644,622 |
Long-term marketable securities: |  |  |  |  |  |  |
U.S. government treasuries(3) | 299,225 |  |  | 17 |  |  | (173) |  |  | 299,069 |
Corporate debt securities(4) | 20,413 |  |  | 2 |  |  | (12) |  |  | 20,403 |
Total long-term marketable securities | 319,638 |  |  | 19 |  |  | (185) |  |  | 319,472 |
Total | $ | 964,265 |  |  | $ | 37 |  |  | $ | (208) |  |  | $ | 964,094 |
__________________________________________________ (1)Unrealized holding losses on 7 securities with an aggregate fair value of $271.8 million. (2)Unrealized holding losses on 8 securities with an aggregate fair value of $27.8 million. (3)Unrealized holding losses on 11 securities with an aggregate fair value of $251.5 million. (4)Unrealized holding losses on 5 securities with an aggregate fair value of $16.8 million.
| December 31, 2020
| Amortized Cost |  | Unrealized Holding Gains |  | Unrealized Holding Losses |  | Aggregate Fair Value
Short-term marketable securities: |  |  |  |  |  |  |
U.S. government treasuries(1) | $ | 878,906 |  |  | $ | 44 |  |  | $ | (12) |  |  | $ | 878,938 |
U.S. government agency securities(2) | 25,214 |  |  | 5 |  |  | (2) |  |  | 25,217 |
Corporate debt securities | 27,101 |  |  | 79 |  |  | — |  |  | 27,180 |
Commercial paper | 31,218 |  |  | — |  |  | — |  |  | 31,218 |
Total short-term marketable securities | 962,439 |  |  | 128 |  |  | (14) |  |  | 962,553 |
Long-term marketable securities: |  |  |  |  |  |  |
U.S. government treasuries | 2,561 |  |  | — |  |  | — |  |  | 2,561 |
Corporate debt securities(3) | 30,147 |  |  | 2 |  |  | (11) |  |  | 30,138 |
Total long-term marketable securities | 32,708 |  |  | 2 |  |  | (11) |  |  | 32,699 |
Total | $ | 995,147 |  |  | $ | 130 |  |  | $ | (25) |  |  | $ | 995,252 |
__________________________________________________ (1)Unrealized holding losses on 19 securities with an aggregate fair value of $369.9 million. (2)Unrealized holding losses on 2 securities with an aggregate fair value of $10.1 million. (3)Unrealized holding losses on 1 security with an aggregate fair value of $20.1 million.

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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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