Note 5 - Financial Instruments Measured at Fair Value - Assets Measured at Fair Value on Recurring Basis (Details) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Sep. 26, 2020	Dec. 28, 2019
Assets, Fair Value Disclosure	$ 170,875	$ 156,098
Debt Security, Government, Non-US [Member]
Short-term investments	949	904
Cash [Member]
Cash and cash equivalents	144,727	147,523
Money Market Funds [Member]
Cash and cash equivalents	25,199	7,671
Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure	144,727	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] | Cash [Member]
Cash and cash equivalents	144,727	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	26,148	8,575
Fair Value, Inputs, Level 2 [Member] | Debt Security, Government, Non-US [Member]
Short-term investments	949	904
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	25,199	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] | Cash [Member]
Cash and cash equivalents	0	0
Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]
Cash and cash equivalents	$ 0	$ 0