Note 3 - Financial Instruments Measured at Fair Value - Effective Maturities of Short-term Investments (Details) - USD ($) $ in Thousands	Mar. 25, 2017	Dec. 31, 2016
Due in one year or less, amortized cost	$ 21,757	$ 31,372
Due in one year or less, estimated fair value	21,754	31,367
Due after one year through three years, amortized cost	615	623
Due after one year through three years, estimated fair value	615	623
Total amortized cost	22,372	31,995
Total estimated fair value	$ 22,369	$ 31,990