Note 5 - Financial Instruments Measured at Fair Value - Effective Maturities of Short-term Investments (Details) - USD ($) $ in Thousands	Mar. 27, 2021	Dec. 26, 2020
Due in one year or less, amortized cost	$ 146,960	$ 20,668
Due in one year or less, fair value	146,966	20,669
Due after one year through three years, amortized cost	960	0
Due after one year through three years, fair value	960	0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Amortized Cost, Total	147,920	20,668
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value, Total	$ 147,926	$ 20,669