Note 2 - Cash, Cash Equivalents and Marketable Investments - Unrealized Gains and Losses Related to Marketable Investments (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Cash and cash equivalents, Amortized Cost	$ 14,784,000	$ 13,775,000	$ 11,275,000	$ 10,868,000
Cash and cash equivalents, Fair Market Value	14,784,000	13,775,000
Marketable investments, Amortized Cost	35,695,000	40,311,000
Marketable investments, Gross Unrealized Gains	10,000	9,000
Marketable investments, Gross Unrealized Losses	(13,000)	(21,000)
Marketable investments, Fair Market Value	35,692,000	40,299,000
Marketable investments, Amortized Cost	50,479,000	54,086,000
Marketable investments, Fair Market Value	50,476,000	54,074,000
US Treasury and Government [Member]
Marketable investments, Amortized Cost	5,133,000	8,403,000
Marketable investments, Gross Unrealized Gains		4,000
Marketable investments, Gross Unrealized Losses	(4,000)	(9,000)
Marketable investments, Fair Market Value	5,129,000	8,398,000
US Government Agencies Debt Securities [Member]
Marketable investments, Amortized Cost		3,918,000
Marketable investments, Gross Unrealized Gains
Marketable investments, Gross Unrealized Losses		(2,000)
Marketable investments, Fair Market Value		3,916,000
US States and Political Subdivisions Debt Securities [Member]
Marketable investments, Amortized Cost	201,000	1,325,000
Marketable investments, Gross Unrealized Gains
Marketable investments, Gross Unrealized Losses
Marketable investments, Fair Market Value	201,000	1,325,000
Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Marketable investments, Amortized Cost	15,942,000	12,299,000
Marketable investments, Gross Unrealized Gains	1,000	2,000
Marketable investments, Gross Unrealized Losses	(1,000)	(2,000)
Marketable investments, Fair Market Value	15,942,000	12,299,000
Corporate Debt Securities [Member]
Marketable investments, Amortized Cost	14,419,000	14,366,000
Marketable investments, Gross Unrealized Gains	9,000	3,000
Marketable investments, Gross Unrealized Losses	(8,000)	(8,000)
Marketable investments, Fair Market Value	14,420,000	14,361,000
Cash [Member]
Cash and cash equivalents, Amortized Cost	6,703,000	6,672,000
Cash and cash equivalents, Fair Market Value	6,703,000	6,672,000
Money Market Funds [Member]
Cash and cash equivalents, Amortized Cost	1,636,000	6,053,000
Cash and cash equivalents, Fair Market Value	1,636,000	6,053,000
Commercial Paper [Member]
Cash and cash equivalents, Amortized Cost	6,445,000	1,050,000
Cash and cash equivalents, Fair Market Value	$ 6,445,000	$ 1,050,000