| Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] |
| | | December 31, 2022 | |
| | | Total | | | Level 1 | | | Level 2 | | | Level 3 | |
| Money market funds | | $ | 15,462 | | | $ | 15,462 | | | $ | - | | | $ | - | |
| Certificates of deposit | | | 130,467 | | | | - | | | | 130,467 | | | | - | |
| Corporate debt securities | | | 292,586 | | | | - | | | | 292,586 | | | | - | |
| Commercial paper | | | 17,928 | | | | - | | | | 17,928 | | | | - | |
| U.S. treasuries and government agency bonds | | | 8,285 | | | | - | | | | 8,285 | | | | - | |
| Auction-rate securities backed by student-loan notes | | | 1,711 | | | | - | | | | - | | | | 1,711 | |
| Mutual funds and money market funds under deferred compensation plan | | | 43,933 | | | | 43,933 | | | | - | | | | - | |
| Total | | $ | 510,372 | | | $ | 59,395 | | | $ | 449,266 | | | $ | 1,711 | |
| | | December 31, 2021 | |
| | | Total | | | Level 1 | | | Level 2 | | | Level 3 | |
| Money market funds | | $ | 14,328 | | | $ | 14,328 | | | $ | - | | | $ | - | |
| Certificates of deposit | | | 141,478 | | | | - | | | | 141,478 | | | | - | |
| Corporate debt securities | | | 386,883 | | | | - | | | | 386,883 | | | | - | |
| U.S. treasuries and government agency bonds | | | 7,456 | | | | - | | | | 7,456 | | | | - | |
| Auction-rate securities backed by student-loan notes | | | 2,401 | | | | - | | | | - | | | | 2,401 | |
| Mutual funds and money market funds under deferred compensation plan | | | 31,786 | | | | 31,786 | | | | - | | | | - | |
| Total | | $ | 584,332 | | | $ | 46,114 | | | $ | 535,817 | | | $ | 2,401 | |
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