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Marketable Securities (Tables) | 9 Months Ended
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Sep. 30, 2021
Marketable Securities |
Schedule of marketable debt securities, classified as available-for-sale | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Gross Unrealized
​ | ​ | Amortized | ​ | ​ | ​ | ​ | ​ | ​ | ​ | Fair
​ |  | Cost |  | Gains |  | Losses |  | Value
​ | ​ | (In thousands)
September 30, 2021 | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Marketable securities | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. government and municipal obligations | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Maturing in one year or less | ​ | $ | 24,291 | ​ | $ | 5 | ​ | $ | — | ​ | $ | 24,296
Maturing after one year through three years | ​ | ​ | 75,314 | ​ | ​ | 2 | ​ | ​ | (17) | ​ | ​ | 75,299
Total U.S. government and municipal obligations | ​ | $ | 99,605 | ​ | $ | 7 | ​ | $ | (17) | ​ | $ | 99,595
Corporate debt securities | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Maturing in one year or less | ​ | $ | 183,504 | ​ | $ | — | ​ | $ | (10) | ​ | $ | 183,494
Maturing after one year through three years | ​ | ​ | 67,844 | ​ | ​ | 8 | ​ | ​ | (36) | ​ | ​ | 67,816
Total corporate debt securities | ​ | $ | 251,348 | ​ | $ | 8 | ​ | $ | (46) | ​ | $ | 251,310
Total marketable securities | ​ | $ | 350,953 | ​ | $ | 15 | ​ | $ | (63) | ​ | $ | 350,905
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Gross Unrealized
​ | ​ | Amortized | ​ | ​ | ​ | ​ | ​ | ​ | ​ | Fair
​ | ​ | Cost |  | Gains |  | Losses |  | Value
​ | ​ | (In thousands)
December 31, 2020 | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Marketable securities | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. government and municipal obligations | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Maturing in one year or less | ​ | $ | 40,328 | ​ | $ | 3 | ​ | $ | (2) | ​ | $ | 40,329
Maturing after one year through three years | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | —
Total U.S. government and municipal obligations | ​ | $ | 40,328 | ​ | $ | 3 | ​ | $ | (2) | ​ | $ | 40,329
Corporate debt securities | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Maturing in one year or less | ​ | $ | 110,265 | ​ | $ | 2 | ​ | $ | (10) | ​ | $ | 110,257
Maturing after one year through three years | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | —
Total corporate debt securities | ​ | $ | 110,265 | ​ | $ | 2 | ​ | $ | (10) | ​ | $ | 110,257
Total marketable securities | ​ | $ | 150,593 | ​ | $ | 5 | ​ | $ | (12) | ​ | $ | 150,586

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