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Note 1 - Significant Accounting Policies - Fair Value of the Company's Investments by Type (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Amortized Cost | $ 87,473 | $ 105,241
Accrued Interest | 359 | 412
Gross Unrealized Gains |  | 1
Gross Unrealized Losses | (452) | (244)
Estimated Fair Value | 87,380 | 105,410
US Government Agencies Debt Securities [Member] |  |
Amortized Cost | 47,023 | 60,121
Accrued Interest | 136 | 177
Gross Unrealized Gains |  |
Gross Unrealized Losses | (204) | (122)
Estimated Fair Value | 46,955 | 60,176
Corporate Debt Securities [Member] |  |
Amortized Cost | 33,465 | 34,021
Accrued Interest | 199 | 203
Gross Unrealized Gains |  |
Gross Unrealized Losses | (226) | (108)
Estimated Fair Value | 33,438 | 34,116
Certificates of Deposit [Member] |  |
Amortized Cost | 6,985 | 11,099
Accrued Interest | 24 | 32
Gross Unrealized Gains |  | 1
Gross Unrealized Losses | (22) | (14)
Estimated Fair Value | $ 6,987 | $ 11,118

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