Note 5 - Financial Instruments Measured at Fair Value - Assets Measured at Fair Value on Recurring Basis (Details) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Mar. 27, 2021	Dec. 26, 2020
Assets, Fair Value Disclosure	$ 291,045	$ 170,027
US Treasury Securities [Member]
Short-term investments	7,811	2,012
Corporate Debt Securities [Member]
Short-term investments	136,405	15,694
US Government-sponsored Enterprises Debt Securities [Member]
Short-term investments	2,000	1,998
Certificates of Deposit [Member]
Short-term investments	750	750
Debt Security, Government, Non-US [Member]
Short-term investments	960	965
Cash [Member]
Cash and cash equivalents	115,626	128,874
Money Market Funds [Member]
Cash and cash equivalents	27,493	19,734
Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure	115,626	128,874
Fair Value, Inputs, Level 1 [Member] | US Treasury Securities [Member]
Short-term investments	0	0
Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member]
Short-term investments	0	0
Fair Value, Inputs, Level 1 [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Short-term investments	0	0
Fair Value, Inputs, Level 1 [Member] | Certificates of Deposit [Member]
Short-term investments	0	0
Fair Value, Inputs, Level 1 [Member] | Debt Security, Government, Non-US [Member]
Short-term investments	0	0
Fair Value, Inputs, Level 1 [Member] | Cash [Member]
Cash and cash equivalents	115,626	128,874
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash and cash equivalents	0	0
Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure	175,419	41,153
Fair Value, Inputs, Level 2 [Member] | US Treasury Securities [Member]
Short-term investments	7,811	2,012
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Short-term investments	136,405	15,694
Fair Value, Inputs, Level 2 [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Short-term investments	2,000	1,998
Fair Value, Inputs, Level 2 [Member] | Certificates of Deposit [Member]
Short-term investments	750	750
Fair Value, Inputs, Level 2 [Member] | Debt Security, Government, Non-US [Member]
Short-term investments	960	965
Fair Value, Inputs, Level 2 [Member] | Cash [Member]
Cash and cash equivalents	0	0
Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Cash and cash equivalents	27,493	19,734
Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure	0	0
Fair Value, Inputs, Level 3 [Member] | US Treasury Securities [Member]
Short-term investments	0	0
Fair Value, Inputs, Level 3 [Member] | Corporate Debt Securities [Member]
Short-term investments	0	0
Fair Value, Inputs, Level 3 [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Short-term investments	0	0
Fair Value, Inputs, Level 3 [Member] | Certificates of Deposit [Member]
Short-term investments	0	0
Fair Value, Inputs, Level 3 [Member] | Debt Security, Government, Non-US [Member]
Short-term investments	0	0
Fair Value, Inputs, Level 3 [Member] | Cash [Member]
Cash and cash equivalents	0	0
Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]
Cash and cash equivalents	$ 0	$ 0