Note 5 - Financial Instruments Measured at Fair Value (Tables)

12 Months Ended

Dec. 31, 2022

Notes Tables

Unrealized Gain (Loss) on Investments [Table Text Block]

	At December 31, 2022
			Gross		Gross		Estimated
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses (1)		Value
Corporate debt securities (2)	$	59,283	$	30	$	240	$	59,073
U.S. treasury securities		34,614		1		418		34,197
Bank certificates of deposit		36,500		20		41		36,479
Asset-backed securities		12,727		10		79		12,658
Foreign government security		828		-		-		828
	$	143,952	$	61	$	778	$	143,235

	At December 25, 2021
			Gross		Gross		Estimated
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses (1)		Value
Corporate debt securities (2)	$	84,060	$	2	$	31	$	84,031
U.S. treasury securities		3,953		-		5		3,948
Bank certificates of deposit		800		-		-		800
Foreign government security		925		-		-		925
	$	89,738	$	2	$	36	$	89,704

Investments Classified by Contractual Maturity Date [Table Text Block]

	Amortized		Estimated
(in thousands)	Cost		Fair Value
Due in one year or less	$	112,956	$	112,683
Due after one year through three years		30,996		30,552
	$	143,952	$	143,235

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

	Fair value measurements at December 31, 2022 using:
							Total estimated
	Level 1		Level 2		Level 3		fair value
Cash	$	190,371	$	-	$	-	$	190,371
Corporate debt securities		-		69,753		-		69,753
Money market funds		-		40,290		-		40,290
Bank certificates of deposit		-		37,480		-		37,480
U.S. treasury securities		-		34,196		-		34,196
Asset-backed securities		-		12,658		-		12,658
Foreign government security		-		828		-		828
	$	190,371	$	195,205	$	-	$	385,576

	Fair value measurements at December 25, 2021 using:
							Total estimated
	Level 1		Level 2		Level 3		fair value
Cash	$	195,297	$	-	$	-	$	195,297
Money market funds		-		92,400		-		92,400
Corporate debt securities		-		86,535		-		86,535
U.S. treasury securities		-		3,948		-		3,948
Foreign government security		-		925		-		925
Bank certificates of deposit		-		800		-		800
	$	195,297	$	184,608	$	-	$	379,905