| 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  | 
 | 
 
  
 |