| Assets and Liabilities Measured at Fair Value on a Recurring Basis | Assets and liabilities measured at fair value on a recurring basis as of June 30, 2021 are as follows: 
 
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| (in thousands) |  | Level 1 |  | Level 2 |  | Level 3 |  | Total |  
| Assets: |  |  |  |  |  |  |  |  |  
| Cash equivalents: |  |  |  |  |  |  |  |  |  
| Money market funds |  | $ | 159,897 |  |  | $ | — |  |  | $ | — |  |  | $ | 159,897 |  |  
|  |  |  |  |  |  |  |  |  |  
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| Short-term investments: |  |  |  |  |  |  |  |  |  
| Money market funds |  | 162 |  |  | — |  |  | — |  |  | 162 |  |  
| Mutual fund shares |  | 976 |  |  | — |  |  | — |  |  | 976 |  |  
| Other current assets: |  |  |  |  |  |  |  |  |  
| Derivative financial instruments |  | — |  |  | 3,428 |  |  | — |  |  | 3,428 |  |  
| Non-current assets: |  |  |  |  |  |  |  |  |  
| Money market funds |  | 3,120 |  |  | — |  |  | — |  |  | 3,120 |  |  
| Mutual fund shares |  | 17,390 |  |  | — |  |  | — |  |  | 17,390 |  |  
| Derivative financial instruments |  | — |  |  | 6,772 |  |  | — |  |  | 6,772 |  |  
| Total assets measured at fair value |  | $ | 181,545 |  |  | $ | 10,200 |  |  | $ | — |  |  | $ | 191,745 |  |  
| Liabilities: |  |  |  |  |  |  |  |  |  
| Accrued liabilities: |  |  |  |  |  |  |  |  |  
| Derivative financial instruments |  | $ | — |  |  | $ | 9,181 |  |  | $ | — |  |  | $ | 9,181 |  |  
| Other long-term liabilities: |  |  |  |  |  |  |  |  |  
| Derivative financial instruments |  | — |  |  | 2,396 |  |  | — |  |  | 2,396 |  |  
| Total liabilities measured at fair value |  | $ | — |  |  | $ | 11,577 |  |  | $ | — |  |  | $ | 11,577 |  |  
Assets and liabilities measured at fair value on a recurring basis at December 31, 2020 are as follows: 
|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
| (in thousands) |  | Level 1 |  | Level 2 |  | Level 3 |  | Total |  
| Assets: |  |  |  |  |  |  |  |  |  
| Cash equivalents: |  |  |  |  |  |  |  |  |  
| Money market funds |  | $ | 119,378 |  |  | $ | — |  |  | $ | — |  |  | $ | 119,378 |  |  
|  |  |  |  |  |  |  |  |  |  
|  |  |  |  |  |  |  |  |  |  
| 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 |  |  
Assets and liabilities measured at fair value on a recurring basis at June 30, 2020 are as follows:
 
|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
| (in thousands) |  | Level 1 |  | Level 2 |  | Level 3 |  | Total |  
| Assets: |  |  |  |  |  |  |  |  |  
| Cash equivalents: |  |  |  |  |  |  |  |  |  
| Money market funds |  | $ | 151,823 |  |  | $ | — |  |  | $ | — |  |  | $ | 151,823 |  |  
|  |  |  |  |  |  |  |  |  |  
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|  |  |  |  |  |  |  |  |  |  
| Short-term investments: |  |  |  |  |  |  |  |  |  
| Money market funds |  | 51 |  |  | — |  |  | — |  |  | 51 |  |  
| Mutual fund shares |  | 924 |  |  | — |  |  | — |  |  | 924 |  |  
| Other current assets: |  |  |  |  |  |  |  |  |  
| Derivative financial instruments |  | — |  |  | 15,480 |  |  | — |  |  | 15,480 |  |  
| Non-current assets: |  |  |  |  |  |  |  |  |  
| Money market funds |  | 4,173 |  |  | — |  |  | — |  |  | 4,173 |  |  
| Mutual fund shares |  | 11,425 |  |  | — |  |  | — |  |  | 11,425 |  |  
| Derivative financial instruments |  | — |  |  | 6,775 |  |  | — |  |  | 6,775 |  |  
| Total assets measured at fair value |  | $ | 168,396 |  |  | $ | 22,255 |  |  | $ | — |  |  | $ | 190,651 |  |  
| Liabilities: |  |  |  |  |  |  |  |  |  
| Accrued liabilities: |  |  |  |  |  |  |  |  |  
| Derivative financial instruments |  | $ | — |  |  | $ | 604 |  |  | $ | — |  |  | $ | 604 |  |  
|  |  |  |  |  |  |  |  |  |  
|  |  |  |  |  |  |  |  |  |  
| Total liabilities measured at fair value |  | $ | — |  |  | $ | 604 |  |  | $ | — |  |  | $ | 604 |  |  |