Note 5 - Financial Instruments Measured at Fair Value - Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands		Sep. 24, 2022	Dec. 25, 2021
Short-term investments		$ 136,983	$ 89,704
US Treasury Securities [Member]
Short-term investments		32,100	3,948
Corporate Debt Securities [Member]
Short-term investments	[1]	72,267	84,031
Asset-Backed Securities [Member]
Short-term investments		8,550
Certificates of Deposit [Member]
Short-term investments		23,283	800
Debt Security, Government, Non-US [Member]
Short-term investments		783	925
Fair Value, Recurring [Member]
Assets, Fair Value Disclosure, Total		369,381	379,905
Fair Value, Recurring [Member] | US Treasury Securities [Member]
Short-term investments		32,100	3,948
Fair Value, Recurring [Member] | Corporate Debt Securities [Member]
Short-term investments		80,744	86,535
Fair Value, Recurring [Member] | Asset-Backed Securities [Member]
Short-term investments		8,550
Fair Value, Recurring [Member] | Certificates of Deposit [Member]
Short-term investments		23,283	800
Fair Value, Recurring [Member] | Debt Security, Government, Non-US [Member]
Short-term investments		783	925
Fair Value, Recurring [Member] | Cash [Member]
Cash and cash equivalents		152,841	195,297
Fair Value, Recurring [Member] | Money Market Funds [Member]
Cash and cash equivalents		71,080	92,400
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure, Total		152,841	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		152,841	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		216,540	184,608
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | US Treasury Securities [Member]
Short-term investments		32,100	3,948
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Short-term investments		80,744	86,535
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Asset-Backed Securities [Member]
Short-term investments		8,550
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Certificates of Deposit [Member]
Short-term investments		23,283	800
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Security, Government, Non-US [Member]
Short-term investments		783	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		71,080	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.