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Note 1 - Significant Accounting Policies - Fair Value of the Company's Investments by Type (Details) - USD ($)
Sep. 30, 2020
Dec. 31, 2019
Amortized Cost $ 49,789,000 $ 21,899,000
Accrued Interest 31,000 116,000
Gross Unrealized Gains 9,000 40,000
Gross Unrealized Losses 0 (1,000)
Estimated Fair Value 49,829,000 22,054,000
US Government Agencies Debt Securities [Member]    
Amortized Cost 45,722,000 10,488,000
Accrued Interest 18,000 50,000
Gross Unrealized Gains 6,000 23,000
Gross Unrealized Losses 0 0
Estimated Fair Value 45,746,000 10,561,000
Corporate Debt Securities [Member]    
Amortized Cost 590 9,742,000
Accrued Interest 4 59,000
Gross Unrealized Gains 0 10,000
Gross Unrealized Losses 0 (1,000)
Estimated Fair Value 594 9,810,000
Certificates of Deposit [Member]    
Amortized Cost 3,477,000 1,669,000
Accrued Interest 9,000 7,000
Gross Unrealized Gains 3,000 7,000
Gross Unrealized Losses 0 0
Estimated Fair Value $ 3,489,000 $ 1,683,000