Short-term investments as of December 31, 2019 consisted of the following (in thousands): | | | | | | | | | | | | | | | Gross | | | | | | Amortized | | Unrealized | | Estimated | | | Cost | | Losses | | Fair Value | Municipal bonds and notes | | $ | 2,986 | | $ | — | | $ | 2,986 | Zero coupon bonds | | | 33,919 | | | — | | | 33,919 | Corporate bonds | | | 19,509 | | | (3) | | | 19,506 | Certificates of deposit | | | 518 | | | — | | | 518 | Total short-term investments | | $ | 56,932 | | $ | (3) | | $ | 56,929 |
Short-term investments as of June 30, 2020 consisted of the following (in thousands): | | | | | | | | | | | | | | | Gross | | | | | | Amortized | | Unrealized | | Estimated | | | Cost | | Gains | | Fair Value | Zero coupon bonds | | $ | 13,995 | | $ | 2 | | $ | 13,997 | Corporate bonds | | | 2,002 | | | — | | | 2,002 | Certificates of deposit | | | 530 | | | — | | | 530 | Total short-term investments | | $ | 16,527 | | $ | 2 | | $ | 16,529 |
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