Note 5 - Financial Instruments Measured at Fair Value - Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands		Mar. 26, 2022	Dec. 25, 2021
Short-term investments		$ 91,502	$ 89,704
US Treasury Securities [Member]
Short-term investments		8,645	3,948
Corporate Debt Securities [Member]
Short-term investments	[1]	76,955	84,031
Asset-Backed Securities [Member]
Short-term investments		728
Certificates of Deposit [Member]
Short-term investments		4,290	800
Debt Security, Government, Non-US [Member]
Short-term investments		884	925
Fair Value, Recurring [Member]
Assets, Fair Value Disclosure, Total		358,570	379,905
Fair Value, Recurring [Member] | US Treasury Securities [Member]
Short-term investments		8,645	3,948
Fair Value, Recurring [Member] | Corporate Debt Securities [Member]
Short-term investments		79,951	86,535
Fair Value, Recurring [Member] | Asset-Backed Securities [Member]
Short-term investments		728
Fair Value, Recurring [Member] | Certificates of Deposit [Member]
Short-term investments		4,290	800
Fair Value, Recurring [Member] | Debt Security, Government, Non-US [Member]
Short-term investments		884	925
Fair Value, Recurring [Member] | Cash [Member]
Cash and cash equivalents		156,024	195,297
Fair Value, Recurring [Member] | Money Market Funds [Member]
Cash and cash equivalents		108,048	92,400
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure, Total		156,024	195,297
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] | Corporate Debt Securities [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] | Certificates of Deposit [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] | Cash [Member]
Cash and cash equivalents		156,024	195,297
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		202,546	184,608
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | US Treasury Securities [Member]
Short-term investments		8,645	3,948
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Short-term investments		79,951	86,535
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Asset-Backed Securities [Member]
Short-term investments		728
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Certificates of Deposit [Member]
Short-term investments		4,290	800
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Security, Government, Non-US [Member]
Short-term investments		884	925
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		108,048	92,400
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] | US Treasury Securities [Member]
Short-term investments		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] | Asset-Backed Securities [Member]
Short-term investments		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] | Debt Security, Government, Non-US [Member]
Short-term investments		0	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.