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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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