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Note 2 - Fair Value of Financial Instruments - Cash and Cash Equivalents (Details) - USD ($)
$ in Thousands
Dec. 31, 2021
Dec. 31, 2020
Cash and cash equivalents $ 137,328 $ 74,132
Short-term marketable securities, amortization cost 268,027 281,346
Unrealized gains, short-term securities 103 557
Short-term marketable securities, unrealized loss (170) (11)
Short-term marketable securities 267,960 281,892
Long-term marketable securities, amortization cost 111,374 97,414
Unrealized gains, long-term securities 178 1,047
Long-term marketable securities, unrealized loss (354) (3)
Long-term marketable securities 111,198 98,458
Cash and cash equivalents and marketable securities, amortization cost 516,729 452,892
Unrealized gains, securities 281 1,604
Unrealized loss, securities (524) (14)
Cash and cash equivalents and short-term and long-term Marketable Securities, fair value 516,486 454,482
Commercial Paper, Not Included with Cash and Cash Equivalents [Member]    
Short-term marketable securities, amortization cost 28,869 6,147
Unrealized gains, short-term securities 101 0
Short-term marketable securities, unrealized loss (7) 0
Short-term marketable securities 28,963 6,147
Corporate Debt Securities [Member]    
Short-term marketable securities, amortization cost 3,952 24,368
Unrealized gains, short-term securities 0 170
Short-term marketable securities, unrealized loss 0 0
Short-term marketable securities 3,952 24,538
Long-term marketable securities, amortization cost 57,762 51,068
Unrealized gains, long-term securities 160 839
Long-term marketable securities, unrealized loss (182) 0
Long-term marketable securities 57,740 51,907
Asset-backed Securities [Member]    
Short-term marketable securities, amortization cost 217,160 6,263
Unrealized gains, short-term securities 2 18
Short-term marketable securities, unrealized loss (163) 0
Short-term marketable securities 216,999 6,281
Long-term marketable securities, amortization cost 14,941 38,456
Unrealized gains, long-term securities 6 160
Long-term marketable securities, unrealized loss (36) (3)
Long-term marketable securities 14,911 38,613
US Government Agencies Debt Securities [Member]    
Short-term marketable securities, amortization cost 18,046 244,568
Unrealized gains, short-term securities 0 369
Short-term marketable securities, unrealized loss 0 (11)
Short-term marketable securities 18,046 244,926
Long-term marketable securities, amortization cost 37,664 6,884
Unrealized gains, long-term securities 0 17
Long-term marketable securities, unrealized loss (136) 0
Long-term marketable securities 37,528 6,901
Foreign Government Agencies [Member]    
Long-term marketable securities, amortization cost 1,007 1,006
Unrealized gains, long-term securities 12 31
Long-term marketable securities, unrealized loss 0 0
Long-term marketable securities 1,019 1,037
Cash [Member]    
Cash and cash equivalents 61,220 33,105
Money Market Funds [Member]    
Cash and cash equivalents 75,258 38,028
Commercial Paper [Member]    
Cash and cash equivalents $ 850 $ 2,999