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Cash, Cash Equivalents and Short-Term Investments (Tables) | 9 Months Ended
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Sep. 30, 2021
Cash, Cash Equivalents and Short-Term Investments |
Schedule of cash, cash equivalents and short-term investments | Cash, cash equivalents and short-term investments consisted of the following as of September 30, 2021 and December 31, 2020 (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | September 30, | ​ | December 31,
​ |  | 2021 |  | 2020
Cash | ​ | $ | 29,296 | ​ | $ | 25,053
Cash equivalents: | ​ |  | ​ | ​ | ​ | ​
Money market mutual fund | ​ |  | 14,384 | ​ | ​ | 11,820
Total cash and cash equivalents | ​ |  | 43,680 | ​ | ​ | 36,873
Short-term investments: | ​ |  | ​ | ​ | ​ | ​
U.S. Treasury notes and bills | ​ |  | 153,377 | ​ | ​ | 23,309
Mutual funds | ​ |  | 110,060 | ​ | ​ | 33,135
Corporate debt securities | ​ | ​ | 42,365 | ​ | ​ | —
Total short-term investments | ​ |  | 305,802 | ​ | ​ | 56,444
Cash, cash equivalents and short-term investments | ​ | $ | 349,482 | ​ | $ | 93,317
​
Schedule of available-for-sale debt securities | The amortized cost, gross unrealized gains, gross unrealized losses and fair value of available-for-sale investments by type of security at September 30, 2021 were as follows (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Amortized |  | Unrealized |  | Unrealized |  | ​ | ​
​ | ​ | Cost | ​ | Gains | ​ | Losses | ​ | Fair Value
U.S. Treasury notes | ​ | $ | 124,171 | ​ | $ | 13 | ​ | $ | (802) | ​ | $ | 123,382
U.S. Treasury bills | ​ | ​ | 29,987 | ​ | ​ | 8 | ​ | ​ | — | ​ | ​ | 29,995
Corporate debt securities | ​ | ​ | 42,991 | ​ | ​ | — | ​ | ​ | (626) | ​ | ​ | 42,365
Total available-for-sale investments | ​ | $ | 197,149 | ​ | $ | 21 | ​ | $ | (1,428) | ​ | $ | 195,742
​ The amortized cost, gross unrealized gains, gross unrealized losses and fair value of available-for-sale investments by type of security at December 31, 2020 were as follows (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | Unrealized |  | Unrealized |  | ​ | ​
​ | ​ | Amortized Cost | ​ | Gains | ​ | Losses | ​ | Fair Value
U.S. Treasury notes | ​ | $ | 23,179 |  | $ | 173 |  | $ | (43) | ​ | $ | 23,309
Total available-for-sale investments | ​ | $ | 23,179 |  | $ | 173 | ​ | $ | (43) | ​ | $ | 23,309
Schedule of investments based on stated contractual maturities | The following table summarizes the fair value of available-for-sale investments based on stated contractual maturities as of September 30, 2021: ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Amortized Cost |  | Fair Value
Due within one year | ​ | $ | 42,087 | ​ | $ | 42,085
Due between one and five years | ​ |  | 155,062 | ​ |  | 153,657
Total | ​ | $ | 197,149 | ​ | $ | 195,742
​ The following table summarizes the fair value of available-for-sale investments based on stated contractual maturities as of December 31, 2020: ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Amortized Cost |  | Fair Value
Due within one year | ​ | $ | 14,084 |  | $ | 14,111
Due between one and two years | ​ |  | 9,095 |  |  | 9,198
Total | ​ | $ | 23,179 |  | $ | 23,309
Schedule of unrealized gains (losses) on investments | Unrealized gains (losses) during 2021 and 2020 related to equity securities held at September 30, 2021 and 2020 are as follows (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | September 30,
​ |  | 2021 |  | 2020
Net losses recognized during the nine months ended September 30 on equity securities | ​ | $ | (308) | ​ | $ | (367)
Less: net gains (losses) recognized during the year on equity securities sold during the period | ​ |  | — | ​ | ​ | (805)
Unrealized gains (losses) recognized during the nine months on equity securities still held at September 30, 2021 and 2020 | ​ | $ | (308) | ​ | $ | 438

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