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Note 6 - Marketable Securities - Short-term and Long-term Marketable Securities (Details) - USD ($)
$ in Thousands
Sep. 30, 2020
Dec. 31, 2019
Amortized Cost $ 82,362 $ 48,397
Unrealized Gains 465 112
Unrealized Losses (40) (22)
Fair Value 82,787 48,487
Bank Time Deposits [Member]    
Amortized Cost 5,189 1,484
Unrealized Gains 105 22
Unrealized Losses 0 0
Fair Value 5,294 1,506
US States and Political Subdivisions Debt Securities [Member]    
Amortized Cost 17,795 10,204
Unrealized Gains 68 9
Unrealized Losses 0 (3)
Fair Value 17,863 10,210
Corporate Debt Securities [Member]    
Amortized Cost 38,876 26,944
Unrealized Gains 283 80
Unrealized Losses (25) (12)
Fair Value 39,134 27,012
US Government Agencies Debt Securities [Member]    
Amortized Cost 19,003 6,274
Unrealized Gains 9 1
Unrealized Losses (15) (7)
Fair Value 18,997 6,268
Commercial Paper, Not Included with Cash and Cash Equivalents [Member]    
Amortized Cost 1,499 3,491
Unrealized Gains 0 0
Unrealized Losses 0 0
Fair Value $ 1,499 $ 3,491