| Assets and Liabilities Measured at Fair Value on a Recurring Basis | Assets and liabilities measured at fair value on a recurring basis as of September 30, 2021 are as follows: 
 
Assets and liabilities measured at fair value on a recurring basis at December 31, 2020 are as follows:
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| (in thousands) |  | Level 1 |  | Level 2 |  | Level 3 |  | Total |  
| Assets: |  |  |  |  |  |  |  |  |  
| Cash equivalents: |  |  |  |  |  |  |  |  |  
| Money market funds |  | $ | 2,903 |  |  | $ | — |  |  | $ | — |  |  | $ | 2,903 |  |  
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| Short-term investments: |  |  |  |  |  |  |  |  |  
| Money market funds |  | 67 |  |  | — |  |  | — |  |  | 67 |  |  
| Mutual fund shares |  | 1,065 |  |  | — |  |  | — |  |  | 1,065 |  |  
| Other current assets: |  |  |  |  |  |  |  |  |  
| Derivative financial instruments |  | — |  |  | 7,133 |  |  | — |  |  | 7,133 |  |  
| Non-current assets: |  |  |  |  |  |  |  |  |  
| Money market funds |  | 3,397 |  |  | — |  |  | — |  |  | 3,397 |  |  
| Mutual fund shares |  | 17,209 |  |  | — |  |  | — |  |  | 17,209 |  |  
| Derivative financial instruments |  | — |  |  | 8,832 |  |  | — |  |  | 8,832 |  |  
| Total assets measured at fair value |  | $ | 24,641 |  |  | $ | 15,965 |  |  | $ | — |  |  | $ | 40,606 |  |  
| Liabilities: |  |  |  |  |  |  |  |  |  
| Accrued liabilities: |  |  |  |  |  |  |  |  |  
| Derivative financial instruments |  | $ | — |  |  | $ | 2,415 |  |  | $ | — |  |  | $ | 2,415 |  |  
| Other long-term liabilities: |  |  |  |  |  |  |  |  |  
| Derivative financial instruments |  | — |  |  | 466 |  |  | — |  |  | 466 |  |  
| Total liabilities measured at fair value |  | $ | — |  |  | $ | 2,881 |  |  | $ | — |  |  | $ | 2,881 |  |  
Assets and liabilities measured at fair value on a recurring basis at September 30, 2020 are as follows:
|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
| (in thousands) |  | Level 1 |  | Level 2 |  | Level 3 |  | Total |  
| Assets: |  |  |  |  |  |  |  |  |  
| Cash equivalents: |  |  |  |  |  |  |  |  |  
| Money market funds |  | $ | 119,378 |  |  | $ | — |  |  | $ | — |  |  | $ | 119,378 |  |  
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| United States government treasury bills |  | — |  |  | 234,982 |  |  | — |  |  | 234,982 |  |  
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| Short-term investments: |  |  |  |  |  |  |  |  |  
| Money market funds |  | 105 |  |  | — |  |  | — |  |  | 105 |  |  
| Mutual fund shares |  | 1,119 |  |  | — |  |  | — |  |  | 1,119 |  |  
| Other current assets: |  |  |  |  |  |  |  |  |  
| Derivative financial instruments |  | — |  |  | 2,597 |  |  | — |  |  | 2,597 |  |  
| Non-current assets: |  |  |  |  |  |  |  |  |  
| Money market funds |  | 4,059 |  |  | — |  |  | — |  |  | 4,059 |  |  
| Mutual fund shares |  | 14,657 |  |  | — |  |  | — |  |  | 14,657 |  |  
| Derivative financial instruments |  | — |  |  | 1,126 |  |  | — |  |  | 1,126 |  |  
| Total assets measured at fair value |  | $ | 139,318 |  |  | $ | 238,705 |  |  | $ | — |  |  | $ | 378,023 |  |  
| Liabilities: |  |  |  |  |  |  |  |  |  
| Accrued liabilities: |  |  |  |  |  |  |  |  |  
| Derivative financial instruments |  | $ | — |  |  | $ | 9,841 |  |  | $ | — |  |  | $ | 9,841 |  |  
| Other long-term liabilities: |  |  |  |  |  |  |  |  |  
| Derivative financial instruments |  | — |  |  | 6,590 |  |  | — |  |  | 6,590 |  |  
| Total liabilities measured at fair value |  | $ | — |  |  | $ | 16,431 |  |  | $ | — |  |  | $ | 16,431 |  |  
 
|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
| (in thousands) |  | Level 1 |  | Level 2 |  | Level 3 |  | Total |  
| Assets: |  |  |  |  |  |  |  |  |  
| Cash equivalents: |  |  |  |  |  |  |  |  |  
| Money market funds |  | $ | 56,337 |  |  | $ | — |  |  | $ | — |  |  | $ | 56,337 |  |  
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| Short-term investments: |  |  |  |  |  |  |  |  |  
| Money market funds |  | 70 |  |  | — |  |  | — |  |  | 70 |  |  
| Mutual fund shares |  | 1,025 |  |  | — |  |  | — |  |  | 1,025 |  |  
| Other current assets: |  |  |  |  |  |  |  |  |  
| Derivative financial instruments |  | — |  |  | 6,940 |  |  | — |  |  | 6,940 |  |  
| Non-current assets: |  |  |  |  |  |  |  |  |  
| Money market funds |  | 4,621 |  |  | — |  |  | — |  |  | 4,621 |  |  
| Mutual fund shares |  | 11,974 |  |  | — |  |  | — |  |  | 11,974 |  |  
| Derivative financial instruments |  | — |  |  | 3,454 |  |  | — |  |  | 3,454 |  |  
| Total assets measured at fair value |  | $ | 74,027 |  |  | $ | 10,394 |  |  | $ | — |  |  | $ | 84,421 |  |  
| Liabilities: |  |  |  |  |  |  |  |  |  
| Accrued liabilities: |  |  |  |  |  |  |  |  |  
| Derivative financial instruments |  | $ | — |  |  | $ | 2,415 |  |  | $ | — |  |  | $ | 2,415 |  |  
| Other long-term liabilities: |  |  |  |  |  |  |  |  |  
| Derivative financial instruments |  | — |  |  | 1,186 |  |  | — |  |  | 1,186 |  |  
| Total liabilities measured at fair value |  | $ | — |  |  | $ | 3,601 |  |  | $ | — |  |  | $ | 3,601 |  |  |