Marketable securities consist of the following: | | | | | | | | | | | | | | | June 30, 2021 | | | | | | Gross | | Gross | | | | | | Amortized | | Unrealized | | Unrealized | | Estimated | (in thousands) | | Cost | | Gain | | Loss | | Fair Value | Corporate bonds | | $ | 214,812 | | $ | 169 | | $ | (364) | | $ | 214,617 | Total | | $ | 214,812 | | $ | 169 | | $ | (364) | | $ | 214,617 |
| | | | | | | | | | | | | | | December 31, 2020 | | | | | | Gross | | Gross | | | | | | Amortized | | Unrealized | | Unrealized | | Estimated | (in thousands) | | Cost | | Gain | | Loss | | Fair Value | Corporate bonds | | $ | 242,900 | | $ | 854 | | $ | (75) | | $ | 243,679 | Commercial paper | | | 6,997 | | | — | | | — | | | 6,997 | Total | | $ | 249,897 | | $ | 854 | | $ | (75) | | $ | 250,676 |
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