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Note 6 - Marketable Securities - Adjusted Cost Basis and Fair Value of Debt Securities By Contractual Maturity (Details) - USD ($)
Mar. 31, 2020
Dec. 31, 2019
Due in one year or less, adjusted cost  
Due in one year or less, fair value  
Due after one year through five years, adjusted cost  
Due after one year through five years, fair value  
Due after five years through ten years, adjusted cost  
Due after five years through ten years, fair value  
Due after ten years, adjusted cost 5,104,000  
Due after ten years, fair value 5,083,000  
Total, adjusted cost 5,104,000 $ 5,104,000
Total, fair value $ 5,083,000 $ 5,479,000