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Financial Instruments (Tables) | 6 Months Ended
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Jun. 30, 2022
Financial Instruments |
Schedule of available-for-sale securities | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | June 30, 2022 | ​ | December 31, 2021
​ |  | Amortized Cost |  | Gross Unrealized Loss |  | Estimated Fair Value |  | Amortized Cost |  | Gross Unrealized Gain |  | Gross Unrealized Loss |  | Estimated Fair Value
​ | ​ | (in thousands)
Money market deposits | ​ | $ | 40,268 | ​ | $ | — | ​ | $ | 40,268 | ​ | $ | 10,041 | ​ | $ | — | ​ | $ | — | ​ | $ | 10,041
U.S. Treasury securities (1) | ​ |  | 459,509 | ​ | ​ | (12,505) | ​ |  | 447,004 | ​ |  | 689,640 | ​ |  | 1,081 | ​ |  | (2,624) | ​ |  | 688,097
Corporate bonds and notes (1) | ​ |  | 34,280 | ​ | ​ | (839) | ​ |  | 33,441 | ​ |  | 52,729 | ​ |  | — | ​ |  | (392) | ​ |  | 52,337
Municipal securities | ​ | ​ | 69,749 | ​ | ​ | (2,841) | ​ | ​ | 66,908 | ​ | ​ | 89,814 | ​ | ​ | 261 | ​ | ​ | (613) | ​ | ​ | 89,462
Total | ​ | $ | 603,806 | ​ | $ | (16,185) | ​ | $ | 587,621 | ​ | $ | 842,224 | ​ | $ | 1,342 | ​ | $ | (3,629) | ​ | $ | 839,937
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Classified as: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Cash equivalents (2) | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | 40,268 | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | 10,041
Short-term investments | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | 547,353 | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | 829,896
Total | ​ | ​ | ​ | ​ | ​ | ​ | ​ | $ | 587,621 | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | $ | 839,937
​
| (1) | Per the Company’s investment policy, all U.S. Treasury securities and debt 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.
Summarized portfolio of available-for-sale securities by contractual maturity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | June 30, 2022
​ | ​ | Amortized Cost | ​ | Fair Value
​ | ​ | (in thousands)
Less than or equal to one year | ​ | $ | 187,392 | ​ | $ | 184,947
Greater than one year but less than five years | ​ | ​ | 376,146 | ​ | ​ | 362,406
Total | ​ | $ | 563,538 | ​ | $ | 547,353

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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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