Note 3 - Cash, Cash Equivalents, Investments and Restricted Cash (Tables)
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12 Months Ended |
Dec. 31, 2022 |
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| Notes Tables |
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| Fair Value, by Balance Sheet Grouping [Table Text Block] |
| | | December 31, | |
| | | 2022 | | | 2021 | |
| Cash | | $ | 273,145 | | | $ | 174,937 | |
| Money market funds | | | 15,462 | | | | 14,328 | |
| Certificates of deposit | | | 130,467 | | | | 141,478 | |
| Corporate debt securities | | | 292,586 | | | | 386,883 | |
| Commercial paper | | | 17,928 | | | | - | |
| U.S. treasuries and government agency bonds | | | 8,285 | | | | 7,456 | |
| Auction-rate securities backed by student-loan notes | | | 1,711 | | | | 2,401 | |
| Total | | $ | 739,584 | | | $ | 727,483 | |
| | | December 31, | |
| | | 2022 | | | 2021 | |
| Reported as: | | | | | | | | |
| Cash and cash equivalents | | $ | 288,607 | | | $ | 189,265 | |
| Short-term investments | | | 449,266 | | | | 535,817 | |
| Investment within other long-term assets | | | 1,711 | | | | 2,401 | |
| Total | | $ | 739,584 | | | $ | 727,483 | |
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| Investments Classified by Contractual Maturity Date [Table Text Block] |
| | | Amortized Cost | | | Fair Value | |
| Due in less than 1 year | | $ | 355,492 | | | $ | 351,915 | |
| Due in 1 - 5 years | | | 101,919 | | | | 97,351 | |
| Due in greater than 5 years | | | 1,770 | | | | 1,711 | |
| Total | | $ | 459,181 | | | $ | 450,977 | |
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| Unrealized Gain (Loss) on Investments [Table Text Block] |
| | | December 31, 2022 | |
| | | Amortized Cost | | | Unrealized Gains | | | Unrealized Losses | | | Fair Value | |
| Money market funds | | $ | 15,462 | | | $ | - | | | $ | - | | | $ | 15,462 | |
| Certificates of deposit | | | 130,467 | | | | - | | | | - | | | | 130,467 | |
| Corporate debt securities | | | 300,529 | | | | 18 | | | | (7,961 | ) | | | 292,586 | |
| Commercial paper | | | 17,928 | | | | - | | | | - | | | | 17,928 | |
| U.S. treasuries and government agency bonds | | | 8,487 | | | | - | | | | (202 | ) | | | 8,285 | |
| Auction-rate securities backed by student-loan notes | | | 1,770 | | | | - | | | | (59 | ) | | | 1,711 | |
| Total | | $ | 474,643 | | | $ | 18 | | | $ | (8,222 | ) | | $ | 466,439 | |
| | | December 31, 2021 | |
| | | Amortized Cost | | | Unrealized Gains | | | Unrealized Losses | | | Fair Value | |
| Money market funds | | $ | 14,328 | | | $ | - | | | $ | - | | | $ | 14,328 | |
| Certificates of deposit | | | 141,478 | | | | - | | | | - | | | | 141,478 | |
| Corporate debt securities | | | 388,081 | | | | 570 | | | | (1,768 | ) | | | 386,883 | |
| U.S. treasuries and government agency bonds | | | 7,495 | | | | - | | | | (39 | ) | | | 7,456 | |
| Auction-rate securities backed by student-loan notes | | | 2,520 | | | | - | | | | (119 | ) | | | 2,401 | |
| Total | | $ | 553,902 | | | $ | 570 | | | $ | (1,926 | ) | | $ | 552,546 | |
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| Debt Securities, Available-for-Sale, Unrealized Loss Position, Fair Value [Table Text Block] |
| | | December 31, 2022 | |
| | | Less than 12 Months | | | Greater than 12 Months | | | Total | |
| | | Fair Value | | | Unrealized Losses | | | Fair Value | | | Unrealized Losses | | | Fair Value | | | Unrealized Losses | |
| Corporate debt securities | | $ | 72,943 | | | $ | (973 | ) | | $ | 202,074 | | | $ | (6,988 | ) | | $ | 275,017 | | | $ | (7,961 | ) |
| U.S. treasuries and government agency bonds | | | 987 | | | | (2 | ) | | | 7,298 | | | | (200 | ) | | | 8,285 | | | | (202 | ) |
| Auction-rate securities backed by student-loan notes | | | - | | | | - | | | | 1,711 | | | | (59 | ) | | | 1,711 | | | | (59 | ) |
| Total | | $ | 73,930 | | | $ | (975 | ) | | $ | 211,083 | | | $ | (7,247 | ) | | $ | 285,013 | | | $ | (8,222 | ) |
| | | December 31, 2021 | |
| | | Less than 12 Months | | | Greater than 12 Months | | | Total | |
| | | Fair Value | | | Unrealized Losses | | | Fair Value | | | Unrealized Losses | | | Fair Value | | | Unrealized Losses | |
| Corporate debt securities | | $ | 285,954 | | | $ | (1,765 | ) | | $ | 4,760 | | | $ | (3 | ) | | $ | 290,714 | | | $ | (1,768 | ) |
| U.S. treasuries and government agency bonds | | | 7,456 | | | | (39 | ) | | | - | | | | - | | | | 7,456 | | | | (39 | ) |
| Auction-rate securities backed by student-loan notes | | | - | | | | - | | | | 2,401 | | | | (119 | ) | | | 2,401 | | | | (119 | ) |
| Total | | $ | 293,410 | | | $ | (1,804 | ) | | $ | 7,161 | | | $ | (122 | ) | | $ | 300,571 | | | $ | (1,926 | ) |
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| Restrictions on Cash and Cash Equivalents [Table Text Block] |
| | | December 31, | |
| | | 2022 | | | 2021 | |
| Cash and cash equivalents | | $ | 288,607 | | | $ | 189,265 | |
| Restricted cash included in other long-term assets | | | 122 | | | | 124 | |
| Total cash, cash equivalents and restricted cash reported on the Consolidated Statements of Cash Flows | | $ | 288,729 | | | $ | 189,389 | |
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