Note 4 - Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2019

Notes Tables

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

	December 31, 2019
	Total		Level 1		Level 2		Level 3
Money market funds	$	28,100	$	28,100	$	-	$	-
Corporate debt securities		260,950		-		260,950		-
Commercial paper		1,994		-		1,994		-
U.S. treasuries and government agency bonds		19,493		-		19,493		-
Auction-rate securities backed by student-loan notes		3,138		-		-		3,138
Mutual funds and money market funds under deferred compensation plan		21,975		21,975		-		-
Total	$	335,650	$	50,075	$	282,437	$	3,138

	December 31, 2018
	Total		Level 1		Level 2		Level 3
Money market funds	$	41,135	$	41,135	$	-	$	-
Corporate debt securities		170,909		-		170,909		-
U.S. treasuries and government agency bonds		32,068		-		32,068		-
Certificates of deposit		1,600		-		1,600		-
Auction-rate securities backed by student-loan notes		3,241		-		-		3,241
Mutual funds and money market funds under deferred compensation plan		18,867		18,867		-		-
Total	$	267,820	$	60,002	$	204,577	$	3,241

Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block]

Balance at January 1, 2018	$	5,256
Change in unrealized loss included in other comprehensive income		(15	)
Sale and settlement at par		(2,000	)
Balance at December 31, 2018		3,241
Change in unrealized gain included in other comprehensive income		147
Sale and settlement at par		(250	)
Balance at December 31, 2019	$	3,138

Schedule Of Level 3 Assumptions [Table Text Block]

	December 31,
	2019			2018
Time-to-liquidity (in years)	2	-	3	2	-	3
Discount rate	4.0%	-	8.3%	4.9%	-	10.1%