Note 5 - Financial Instruments Measured at Fair Value - Effective Maturities of Short-term Investments (Details) - USD ($) $ in Thousands	Sep. 25, 2021	Dec. 26, 2020
Due in one year or less, amortized cost	$ 73,545	$ 20,668
Due in one year or less, fair value	73,553	20,669
Due after one year through three years, amortized cost	2,467	0
Due after one year through three years, fair value	2,467	0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Amortized Cost, Total	76,012	20,668
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value, Total	$ 76,020	$ 20,669