Note 3 - Financial Instruments Measured at Fair Value (Tables)

3 Months Ended

Mar. 31, 2018

Notes Tables

Available-for-sale Securities [Table Text Block]

	March 31, 2018
			Gross		Gross		Estimated
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses (1)		Value
Corporate debt securities (2)	$	11,313	$	-	$	9	$	11,304
U.S. Treasury securities		8,913		-		8		8,905
Bank certificates of deposit		2,800		-		1		2,799
Government-sponsored enterprise securities		992		-		-		992
Foreign government security		594		-		-		594
	$	24,612	$	-	$	18	$	24,594

	December 30, 2017
			Gross		Gross		Estimated
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses (1)		Value
Corporate debt securities (2)	$	12,784	$	1	$	6	$	12,779
U.S. treasury securities		7,935		-		4		7,931
Foreign government security		619		-		-		619
	$	21,338	$	1	$	10	$	21,329

Investments Classified by Contractual Maturity Date [Table Text Block]

	March 31, 2018				December 30, 2017
	Amortized		Estimated		Amortized		Estimated
	Cost		Fair Value		Cost		Fair Value
Due in one year or less	$	24,612	$	24,594	$	21,338	$	21,329

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

	Fair value measurements at March 31, 2018 using:
							Total estimated
	Level 1		Level 2		Level 3		fair value
Cash	$	84,773	$	-	$	-	$	84,773
U.S. Treasury securities		-		9,904		-		9,904
Corporate debt securities		-		25,330		-		25,330
Government-sponsored enterprise securities		-		992		-		992
Money market funds		-		15,338		-		15,338
Bank certificates of deposit		-		2,798		-		2,798
Foreign government security		-		595		-		595
	$	84,773	$	54,957	$	-	$	139,730

	Fair value measurements at December 30, 2017 using:
							Total estimated
	Level 1		Level 2		Level 3		fair value
Cash	$	100,850	$	-	$	-	$	100,850
Corporate debt securities		-		22,014		-		22,014
U.S. treasury securities		-		8,431		-		8,431
Government-sponsored enterprise securities		-		1,496		-		1,496
Money market funds		-		22,205		-		22,205
Foreign government security		-		619		-		619
	$	100,850	$	54,765	$	-	$	155,615