Note 10 - Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2020

Notes Tables

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

	March 31, 2020
	Total		Level 1		Level 2		Level 3
Money market funds	$	10,954	$	10,954	$	-	$	-
Corporate debt securities		308,909		-		308,909		-
Commercial paper		4,943		-		4,943		-
U.S. treasuries and government agency bonds		20,534		-		20,534		-
Auction-rate securities backed by student-loan notes		3,057		-		-		3,057
Mutual funds and money market funds under deferred compensation plan		20,226		20,226		-		-
Total	$	368,623	$	31,180	$	334,386	$	3,057

	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

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

Balance at January 1, 2020	$	3,138
Change in unrealized gain included in other comprehensive income		19
Sale and settlement at par		(100	)
Balance at March 31, 2020	$	3,057

Fair Value Measurement Inputs and Valuation Techniques [Table Text Block]

	March 31, 2020 (1)				December 31, 2019
Time-to-liquidity (in years)	2	-	3	(2.5)	2	-	3
Discount rate	3.1%	-	7.3%	(5.0%)	4.0%	-	8.3%