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Cash, Cash Equivalents and Short-Term Investments (Tables) | 12 Months Ended
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Dec. 31, 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 December 31, 2021 and 2020 (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Carrying Value
​ |  | 2021 |  | 2020
Cash | ​ | $ | 27,788 | ​ | ​ | 25,053
Cash equivalents: | ​ |  | ​ | ​ | ​ | ​
Money market mutual funds | ​ |  | 111,313 | ​ | ​ | 11,820
Total cash and cash equivalents | ​ |  | 139,101 | ​ | ​ | 36,873
Short-term investments: | ​ |  | ​ | ​ | ​ | ​
U.S. Treasury notes and bills | ​ |  | 223,896 | ​ | ​ | 23,309
Mutual funds | ​ |  | 110,006 | ​ | ​ | 33,135
Corporate debt securities | ​ | ​ | 155,796 | ​ | ​ | —
Total short-term investments | ​ |  | 489,698 | ​ | ​ | 56,444
Cash, cash equivalents and short-term investments | ​ | $ | 628,799 | ​ | $ | 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 debt securities by type of security at December 31, 2021 were as follows (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | ​ | ​
​ |  | Cost |  | Gains |  | Losses |  | Fair Value
U.S. Treasury notes | ​ | $ | 226,020 | ​ | $ | 12 | ​ | ​ | (2,136) | ​ | $ | 223,896
Corporate debt securities | ​ | ​ | 157,527 | ​ | ​ | — | ​ | ​ | (1,731) | ​ | ​ | 155,796
Total available-for-sale investments | ​ | $ | 383,547 | ​ | $ | 12 | ​ | $ | (3,867) | ​ | $ | 379,692
The amortized cost, gross unrealized gains, gross unrealized losses and fair value of available-for-sale debt securities by type of security at December 31, 2020 were as follows (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | ​ | ​
​ |  | 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 debt securities based on stated contractual maturities as of December 31, 2021 (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Amortized Cost |  | Fair Value
Due in less than one year | ​ | $ | 39,081 | ​ | $ | 39,035
Due between one and six years | ​ |  | 344,466 | ​ |  | 340,657
Total | ​ | $ | 383,547 | ​ | $ | 379,692
​
Schedule of unrealized gains (losses) on investments | Unrealized gains (losses) during 2021, 2020 and 2019 related to equity securities held at December 31, 2021, 2020 and 2019 are as follows (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 2021 |  | 2020 |  | 2019
Net losses recognized during the year on equity securities | ​ | $ | (1,386) | ​ | $ | (245) | ​ | $ | (102)
Less: net losses recognized during the year on equity securities sold during the year | ​ |  | — | ​ |  | (1,090) | ​ | ​ | —
Unrealized gains (losses) recognized during the year on equity securities still held at December 31, 2021, 2020 and 2019 | ​ | $ | (1,386) | ​ | $ | 845 | ​ | $ | (102)

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- Definition Tabular disclosure of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -URI http://asc.fasb.org/topic&trid=2196928 + Details
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