Note 10 - Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2018

Notes Tables

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

	Fair Value Measurement at June 30, 2018
	Total		Level 1		Level 2		Level 3
Money market funds	$	16,081	$	16,081	$	-	$	-
Corporate debt securities		207,461		-		207,461		-
U.S. treasuries and government agency bonds		22,967		-		22,967		-
Auction-rate securities backed by student-loan notes		5,247		-		-		5,247
Mutual funds and money market funds under deferred compensation plan		17,113		17,113		-		-
Total	$	268,869	$	33,194	$	230,428	$	5,247

	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, 2018	$	5,256
Change in unrealized loss included in other comprehensive income		(9	)
Balance at June 30, 2018	$	5,247

Schedule Of Level 3 Assumptions [Table Text Block]

	June 30,				December 31,
	2018				2017
Time-to-liquidity (years)		2	-	3		2	-	3
Discount rate		5.3%	-	10.3%		4.5%	-	9.6%