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Note 1 - Significant Accounting Policies - Fair Value of the Company's Investments by Type (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Amortized Cost | $ 96,872 | $ 105,241
Accrued Interest | 432 | 412
Gross Unrealized Gains |  | 1
Gross Unrealized Losses | (410) | (244)
Estimated Fair Value | 96,894 | 105,410
US Government Agencies Debt Securities [Member] |  |
Amortized Cost | 51,436 | 60,121
Accrued Interest | 170 | 177
Gross Unrealized Gains |  |
Gross Unrealized Losses | (188) | (122)
Estimated Fair Value | 51,418 | 60,176
Corporate Debt Securities [Member] |  |
Amortized Cost | 40,893 | 34,021
Accrued Interest | 244 | 203
Gross Unrealized Gains |  |
Gross Unrealized Losses | (207) | (108)
Estimated Fair Value | 40,930 | 34,116
Certificates of Deposit [Member] |  |
Amortized Cost | 4,543 | 11,099
Accrued Interest | 18 | 32
Gross Unrealized Gains |  | 1
Gross Unrealized Losses | (15) | (14)
Estimated Fair Value | $ 4,546 | $ 11,118

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