Note 4 - Fair Value Measurement (Tables)

12 Months Ended

Dec. 31, 2018

Notes Tables

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

	Fair Value Measurement at 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 Measurement at December 31, 2017
	Total		Level 1		Level 2		Level 3
Money market funds	$	7,134	$	7,134	$	-	$	-
Corporate debt securities		203,807		-		203,807		-
U.S. treasuries and government agency bonds		13,024		-		13,024		-
Auction-rate securities backed by student-loan notes		5,256		-		-		5,256
Mutual funds and money market funds under deferred compensation plan		16,625		16,625		-		-
Total	$	245,846	$	23,759	$	216,831	$	5,256

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

Balance at January 1, 2017	$	5,354
Change in unrealized loss included in other comprehensive loss		(98	)
Balance at December 31, 2017		5,256
Sale and settlement at par		(2,000	)
Change in unrealized loss included in other comprehensive loss		(15	)
Balance at December 31, 2018	$	3,241

Schedule Of Level 3 Assumptions [Table Text Block]

	December 31,
	2018			2017
Time-to-liquidity (years)	2	-	3	2	-	3
Discount rate	4.9%	-	10.1%	4.5%	-	9.6%