Note 4 - Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2023

Notes Tables

Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]

	December 31, 2023
	Total		Level 1		Level 2		Level 3
Money market funds	$	135,514	$	135,514	$	-	$	-
Certificates of deposit		127,123		-		127,123		-
Corporate debt securities		95,101		-		95,101		-
U.S. treasuries and government agency bonds		358,409		-		358,409		-
Auction-rate securities backed by student-loan notes		567		-		-		567
Mutual funds and money market funds under deferred compensation plan		54,836		54,836		-		-
Total	$	771,550	$	190,350	$	580,633	$	567

	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