Note 5 - Financial Instruments Measured at Fair Value - Assets Measured at Fair Value on Recurring Basis (Details) - USD ($)		Dec. 31, 2022	Dec. 25, 2021
Short-term investments		$ 143,235,000	$ 89,704,000
Corporate Debt Securities [Member]
Short-term investments	[1]	59,073,000	84,031,000
Certificates of Deposit [Member]
Short-term investments		36,479,000	800,000
US Treasury Securities [Member]
Short-term investments		34,197,000	3,948,000
Debt Security, Government, Non-US [Member]
Short-term investments		828,000	925,000
Asset-Backed Securities [Member]
Short-term investments		12,658,000
Fair Value, Recurring [Member]
Assets, Fair Value Disclosure, Total		385,576,000	379,905,000
Fair Value, Recurring [Member] | Corporate Debt Securities [Member]
Short-term investments		69,753,000	86,535,000
Fair Value, Recurring [Member] | Certificates of Deposit [Member]
Short-term investments		37,480,000	800,000
Fair Value, Recurring [Member] | US Treasury Securities [Member]
Short-term investments		34,196,000	3,948,000
Fair Value, Recurring [Member] | Debt Security, Government, Non-US [Member]
Short-term investments		828,000	925,000
Fair Value, Recurring [Member] | Asset-Backed Securities [Member]
Short-term investments		12,658
Fair Value, Recurring [Member] | Cash [Member]
Cash and cash equivalents		190,371,000	195,297,000
Fair Value, Recurring [Member] | Money Market Funds [Member]
Cash and cash equivalents		40,290,000	92,400,000
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure, Total		190,371,000	195,297,000
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member]
Short-term investments		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Certificates of Deposit [Member]
Short-term investments		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | US Treasury Securities [Member]
Short-term investments		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Security, Government, Non-US [Member]
Short-term investments		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Asset-Backed Securities [Member]
Short-term investments		0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Cash [Member]
Cash and cash equivalents		190,371,000	195,297,000
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash and cash equivalents		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure, Total		195,205,000	184,608,000
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Short-term investments		69,753,000	86,535,000
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Certificates of Deposit [Member]
Short-term investments		37,480,000	800,000
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | US Treasury Securities [Member]
Short-term investments		34,196,000	3,948,000
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Security, Government, Non-US [Member]
Short-term investments		828,000	925,000
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Asset-Backed Securities [Member]
Short-term investments		12,658
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Cash [Member]
Cash and cash equivalents		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Cash and cash equivalents		40,290,000	92,400,000
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure, Total		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Corporate Debt Securities [Member]
Short-term investments		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Certificates of Deposit [Member]
Short-term investments		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | US Treasury Securities [Member]
Short-term investments		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Security, Government, Non-US [Member]
Short-term investments		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Asset-Backed Securities [Member]
Short-term investments		0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Cash [Member]
Cash and cash equivalents		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]
Cash and cash equivalents		$ 0	$ 0

[1] Corporate debt securities include investments in financial and other corporate institutions. No single issuer represents a significant portion of the total corporate debt securities portfolio.