Note 10 - Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2018

Notes Tables

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

	Fair Value Measurement at September 30, 2018
	Total		Level 1		Level 2		Level 3
Money market funds	$	25,235	$	25,235	$	-	$	-
Corporate debt securities		190,924		-		190,924		-
U.S. treasuries and government agency bonds		24,230		-		24,230		-
Certificates of deposit		1,600		-		1,600		-
Auction-rate securities backed by student-loan notes		5,257		-		-		5,257
Mutual funds and money market funds under deferred compensation plan		16,284		16,284		-		-
Total	$	263,530	$	41,519	$	216,754	$	5,257

	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

Schedule Of Level 3 Assumptions [Table Text Block]

	September 30,			December 31,
	2018			2017
Time-to-liquidity (years)	2	-	3	2	-	3
Discount rate	5.4%	-	10.5%	4.5%	-	9.6%