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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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Represents the amount of accrued interest in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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