Note 4 - Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2017

Notes Tables

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

	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 under deferred compensation plan		16,625		16,625		-		-
Total	$	245,846	$	23,759	$	216,831	$	5,256

	Fair Value Measurement at December 31, 2016
	Total		Level 1		Level 2		Level 3
Money market funds	$	24,956	$	24,956	$	-	$	-
Corporate debt securities		109,644		-		109,644		-
U.S. treasuries and government agency bonds		45,877		-		45,877		-
Auction-rate securities backed by student-loan notes		5,354		-		-		5,354
Mutual funds under deferred compensation plan		12,108		12,108		-		-
Total	$	197,939	$	37,064	$	155,521	$	5,354

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

Balance at January 1, 2016	$	5,361
Change in unrealized loss included in other comprehensive loss		(7	)
Balance at December 31, 2016		5,354
Change in unrealized loss included in other comprehensive income		(98	)
Balance at December 31, 2017	$	5,256

Schedule Of Level 3 Assumptions [Table Text Block]

	December 31,
	2017				2016
Time-to-liquidity (years)		2	-	3			2
Discount rate		4.5%	-	9.6%		4.3%	-	9.3%