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Note 1 - Significant Accounting Policies and Concentrations of Risk - Fair Value of the Company's Investments by Type (Details) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Amortized Cost | $ 21,899 | $ 99,965
Accrued Interest | 116 | 444
Gross Unrealized Gains | 40 | 19
Gross Unrealized Losses | (1) | (316)
Estimated Fair Value | 22,054 | 100,112
US Government Agencies Debt Securities [Member] |  |
Amortized Cost | 10,488 | 50,613
Accrued Interest | 50 | 176
Gross Unrealized Gains | 23 | 15
Gross Unrealized Losses |  | (131)
Estimated Fair Value | 10,561 | 50,673
Corporate Debt Securities [Member] |  |
Amortized Cost | 9,742 | 45,793
Accrued Interest | 59 | 254
Gross Unrealized Gains | 10 | 4
Gross Unrealized Losses | (1) | (171)
Estimated Fair Value | 9,810 | 45,880
Certificates of Deposit [Member] |  |
Amortized Cost | 1,669 | 3,559
Accrued Interest | 7 | 14
Gross Unrealized Gains | 7 |
Gross Unrealized Losses |  | (14)
Estimated Fair Value | $ 1,683 | $ 3,559

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