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Financial Instruments (Tables) | 3 Months Ended
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Mar. 31, 2021
Financial Instruments |
Schedule of available-for-sale securities | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | 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 | ​
Summarized portfolio of available-for-sale securities by contractual maturity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | 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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- 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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