Note 3 - Financial Instruments Measured at Fair Value - Effective Maturities of Short-term Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 30, 2017
Due in one year or less, amortized cost	$ 23,319	$ 21,338
Due in one year or less, estimated fair value	23,309	21,329
Due after one year through three years, amortized cost	556
Due after one year through three years, fair value	556
Total amortized cost	23,875	21,338
Total estimated fair value	$ 23,865	$ 21,329