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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