Note 5 - Financial Instruments Measured at Fair Value - Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands		Jun. 25, 2022	Dec. 25, 2021
Short-term investments		$ 87,495	$ 89,704
US Treasury Securities [Member]
Short-term investments		8,617	3,948
Corporate Debt Securities [Member]
Short-term investments	[1]	63,215	84,031
Asset-Backed Securities [Member]
Short-term investments		4,103
Certificates of Deposit [Member]
Short-term investments		10,719	800
Debt Security, Government, Non-US [Member]
Short-term investments		841	925
Fair Value, Recurring [Member]
Assets, Fair Value Disclosure, Total		368,081	379,905
Fair Value, Recurring [Member] | US Treasury Securities [Member]
Short-term investments		8,618	3,948
Fair Value, Recurring [Member] | Corporate Debt Securities [Member]
Short-term investments		70,207	86,535
Fair Value, Recurring [Member] | Asset-Backed Securities [Member]
Short-term investments		4,103
Fair Value, Recurring [Member] | Certificates of Deposit [Member]
Short-term investments		10,719	800
Fair Value, Recurring [Member] | Debt Security, Government, Non-US [Member]
Short-term investments		841	925
Fair Value, Recurring [Member] | Cash [Member]
Cash and cash equivalents		221,695	195,297
Fair Value, Recurring [Member] | Money Market Funds [Member]
Cash and cash equivalents		51,898	92,400
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure, Total		221,695	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		221,695	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		146,386	184,608
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | US Treasury Securities [Member]
Short-term investments		8,618	3,948
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Short-term investments		70,207	86,535
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Asset-Backed Securities [Member]
Short-term investments		4,103
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Certificates of Deposit [Member]
Short-term investments		10,719	800
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Security, Government, Non-US [Member]
Short-term investments		841	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		51,898	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.