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. 28, 2019	Dec. 29, 2018
	$ 145,667	$ 165,020
Debt Security, Government, Non-US [Member]
Short-term investments	573	560
Cash [Member]
Cash and cash equivalents	132,065	144,696
Money Market Funds [Member]
Cash and cash equivalents	13,029	19,764
Fair Value, Inputs, Level 1 [Member]
	132,065	144,696
Fair Value, Inputs, Level 1 [Member] | Debt Security, Government, Non-US [Member]
Short-term investments
Fair Value, Inputs, Level 1 [Member] | Cash [Member]
Cash and cash equivalents	132,065	144,696
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash and cash equivalents
Fair Value, Inputs, Level 2 [Member]
	13,602	20,324
Fair Value, Inputs, Level 2 [Member] | Debt Security, Government, Non-US [Member]
Short-term investments	573	560
Fair Value, Inputs, Level 2 [Member] | Cash [Member]
Cash and cash equivalents
Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Cash and cash equivalents	13,029	19,764
Fair Value, Inputs, Level 3 [Member]
Fair Value, Inputs, Level 3 [Member] | Debt Security, Government, Non-US [Member]
Short-term investments
Fair Value, Inputs, Level 3 [Member] | Cash [Member]
Cash and cash equivalents
Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]
Cash and cash equivalents