| Assets and Liabilities Measured at Fair Value on a Recurring Basis | Assets and liabilities measured at fair value on a recurring basis as of 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 |  |  
|  |  |  |  |  |  |  |  |  |  
|  |  |  |  |  |  |  |  |  |  
|  |  |  |  |  |  |  |  |  |  
| 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 December 31, 2019 are as follows:  
|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
| (in thousands) |  | Level 1 |  | Level 2 |  | Level 3 |  | Total |  
| Assets: |  |  |  |  |  |  |  |  |  
| Cash equivalents: |  |  |  |  |  |  |  |  |  
| Money market funds |  | $ | 288,926 |  |  | $ | — |  |  | $ | — |  |  | $ | 288,926 |  |  
|  |  |  |  |  |  |  |  |  |  
|  |  |  |  |  |  |  |  |  |  
| United States government treasury bills |  | — |  |  | 34,928 |  |  | — |  |  | 34,928 |  |  
| Commercial paper |  | — |  |  | 33,587 |  |  | — |  |  | 33,587 |  |  
|  |  |  |  |  |  |  |  |  |  
|  |  |  |  |  |  |  |  |  |  
|  |  |  |  |  |  |  |  |  |  
|  |  |  |  |  |  |  |  |  |  
|  |  |  |  |  |  |  |  |  |  
|  |  |  |  |  |  |  |  |  |  
| Short-term investments: |  |  |  |  |  |  |  |  |  
| Mutual fund shares |  | 1,668 |  |  | — |  |  | — |  |  | 1,668 |  |  
| Other current assets: |  |  |  |  |  |  |  |  |  
| Derivative financial instruments |  | — |  |  | 14,001 |  |  | — |  |  | 14,001 |  |  
| Non-current assets: |  |  |  |  |  |  |  |  |  
| Money market funds |  | 1,792 |  |  | — |  |  | — |  |  | 1,792 |  |  
| Mutual fund shares |  | 12,172 |  |  | — |  |  | — |  |  | 12,172 |  |  
| Derivative financial instruments |  | — |  |  | 4,159 |  |  | — |  |  | 4,159 |  |  
| Total assets measured at fair value |  | $ | 304,558 |  |  | $ | 86,675 |  |  | $ | — |  |  | $ | 391,233 |  |  
| Liabilities: |  |  |  |  |  |  |  |  |  
| Accrued liabilities: |  |  |  |  |  |  |  |  |  
| Derivative financial instruments |  | $ | — |  |  | $ | 2,266 |  |  | $ | — |  |  | $ | 2,266 |  |  
| Other long-term liabilities: |  |  |  |  |  |  |  |  |  
| Derivative financial instruments |  | — |  |  | 768 |  |  | — |  |  | 768 |  |  
| Total liabilities measured at fair value |  | $ | — |  |  | $ | 3,034 |  |  | $ | — |  |  | $ | 3,034 |  |  |