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Note 1 - Significant Accounting Policies - Fair Value of the Company's Investments by Type (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Amortized Cost | $ 89,236 | $ 105,241
Accrued Interest | 379 | 412
Gross Unrealized Gains |  | 1
Gross Unrealized Losses | (476) | (244)
Estimated Fair Value | 89,139 | 105,410
US Government Agencies Debt Securities [Member] |  |
Amortized Cost | 44,116 | 60,121
Accrued Interest | 148 | 177
Gross Unrealized Gains |  |
Gross Unrealized Losses | (205) | (122)
Estimated Fair Value | 44,059 | 60,176
Corporate Debt Securities [Member] |  |
Amortized Cost | 35,299 | 34,021
Accrued Interest | 201 | 203
Gross Unrealized Gains |  |
Gross Unrealized Losses | (250) | (108)
Estimated Fair Value | 35,250 | 34,116
Certificates of Deposit [Member] |  |
Amortized Cost | 9,821 | 11,099
Accrued Interest | 30 | 32
Gross Unrealized Gains |  | 1
Gross Unrealized Losses | (21) | (14)
Estimated Fair Value | $ 9,830 | $ 11,118

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