Note 7 - Financial Instruments Measured at Fair Value - Assets Measured at Fair Value on Recurring Basis (Details) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Dec. 26, 2020	Dec. 28, 2019
Assets, Fair Value Disclosure	$ 170,027	$ 156,098
Debt Security, Government, Non-US [Member]
Short-term investments	965	904
Corporate Debt Securities [Member]
Short-term investments	15,694
US Treasury Securities [Member]
Short-term investments	2,012
US Government-sponsored Enterprises Debt Securities [Member]
Short-term investments	1,998
Certificates of Deposit [Member]
Short-term investments	750
Cash [Member]
Cash and cash equivalents	128,874	147,523
Money Market Funds [Member]
Cash and cash equivalents	19,734	7,671
Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure	128,874	147,523
Fair Value, Inputs, Level 1 [Member] | Debt Security, Government, Non-US [Member]
Short-term investments	0	0
Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member]
Short-term investments	0
Fair Value, Inputs, Level 1 [Member] | US Treasury Securities [Member]
Short-term investments	0
Fair Value, Inputs, Level 1 [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Short-term investments	0
Fair Value, Inputs, Level 1 [Member] | Certificates of Deposit [Member]
Short-term investments	0
Fair Value, Inputs, Level 1 [Member] | Cash [Member]
Cash and cash equivalents	128,874	147,523
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	41,153	8,575
Fair Value, Inputs, Level 2 [Member] | Debt Security, Government, Non-US [Member]
Short-term investments	965	904
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Short-term investments	15,694
Fair Value, Inputs, Level 2 [Member] | US Treasury Securities [Member]
Short-term investments	2,012
Fair Value, Inputs, Level 2 [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Short-term investments	1,998
Fair Value, Inputs, Level 2 [Member] | Certificates of Deposit [Member]
Short-term investments	750
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	19,734	7,671
Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure	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] | Corporate Debt Securities [Member]
Short-term investments	0
Fair Value, Inputs, Level 3 [Member] | US Treasury Securities [Member]
Short-term investments	0
Fair Value, Inputs, Level 3 [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Short-term investments	0
Fair Value, Inputs, Level 3 [Member] | Certificates of Deposit [Member]
Short-term investments	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