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Financial Instruments | 3 Months Ended
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Mar. 31, 2021
Financial Instruments |
Financial Instruments | 5. Financial Instruments The Company elected to invest a portion of its cash assets in conservative, income earning, and liquid investments. Cash equivalents and investments, all of which are classified as available-for-sale securities, consisted of the following: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | March 31, 2021 | ​ | December 31, 2020 |
​ |  | Amortized Cost |  | Gross Unrealized Gain |  | Gross Unrealized Loss |  | Estimated Fair Value |  | Amortized Cost |  | Gross Unrealized Gain |  | Gross Unrealized Loss |  | Estimated Fair Value |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | (in thousands) |
Money market deposits | ​ | $ | 24,619 | ​ | $ | — | ​ | $ | — | ​ | $ | 24,619 | ​ | $ | 28,990 | ​ | $ | — | ​ | $ | — | ​ | $ | 28,990 | ​
U.S. Treasury securities (1) | ​ |  | 492,814 | ​ | ​ | 2,784 | ​ | ​ | (60) | ​ |  | 495,538 | ​ |  | 594,252 | ​ |  | 3,512 | ​ |  | (20) | ​ |  | 597,744 | ​
Corporate bonds and notes (1) | ​ |  | 16,278 | ​ | ​ | 1 | ​ | ​ | (31) | ​ |  | 16,248 | ​ |  | 12,331 | ​ |  | 2 | ​ |  | (5) | ​ |  | 12,328 | ​
Municipal securities | ​ | ​ | 76,157 | ​ | ​ | 582 | ​ | ​ | (79) | ​ | ​ | 76,660 | ​ | ​ | 77,764 | ​ | ​ | 796 | ​ | ​ | (26) | ​ | ​ | 78,534 | ​
Total | ​ | $ | 609,868 | ​ | $ | 3,367 | ​ | $ | (170) | ​ | $ | 613,065 | ​ | $ | 713,337 | ​ | $ | 4,310 | ​ | $ | (51) | ​ | $ | 717,596 | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Classified as: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Cash equivalents (2) | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | $ | 24,619 | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | $ | 28,990 | ​
Short-term investments | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | 588,446 | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | 688,606 | ​
Total | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | $ | 613,065 | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | $ | 717,596 | ​
​
| (1) | Per the Company’s investment policy, all U.S. Treasury securities are classified as short-term investments irrespective of holding period.
| (2) | Cash equivalents includes cash sweep accounts and U.S. Treasury money market mutual funds.
​ The Company invests in U.S. Treasuries, U.S. agency and high quality municipal bonds which mature at par value and are all paying their coupons on schedule. The Company has therefore concluded there is currently no other than temporary impairment of its investments and will continue to recognize unrealized gains and losses in other comprehensive income (loss). During the three months ended March 31, 2021, the Company did not sell any investments. During the three months ended March 31, 2021, the amount of gross realized gains and realized losses upon sales of investments were insignificant. The Company uses the specific investment identification method to calculate realized gains and losses and amounts reclassified out of other comprehensive income to net income. As of March 31, 2021, the Company had 22 investments in an unrealized loss position in its portfolio. An allowance for credit losses was not necessary for the first quarter of 2021 since the fair market value for a majority of the available-for-sale securities increased as a result of a average yield rate decrease for similar securities as of March 31, 2021. The Company has assessed the unrealized loss position for available-for-sale debt securities for which an allowance for credit losses has not been recorded. The fair value for investment securities at an unrealized loss position as of March 31, 2021 was $114.1 million. The aggregate amount of unrealized losses of these securities were not significant, and the impact of the securities in a continuous loss position to the condensed consolidated statements of operations and comprehensive loss were not material as of March 31, 2021. The following table summarizes the Company’s portfolio of available-for-sale securities by contractual maturity as of March 31, 2021: ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | March 31, 2021
​ | ​ | Amortized Cost | ​ | Fair Value
​ | ​ | (in thousands)
Less than or equal to one year | ​ | $ | 250,457 | ​ | $ | 251,028
Greater than one year but less than five years | ​ | ​ | 334,792 | ​ | ​ | 337,418
Total | ​ | $ | 585,249 | ​ | $ | 588,446
​

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