Note 2 - Cash, Cash Equivalents and Marketable Investments - Unrealized Gains and Losses Related to Marketable Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Cash and cash equivalents and marketable investments, amortized cost	$ 19,519	$ 33,921
Cash and cash equivalents and marketable investments, Fair Market Value	19,520	33,921
Cash and cash equivalents and marketable investments, amortized cost, Gross Unrealized Gains	2
Cash and cash equivalents and marketable investments, Gross Unrealized Losses	(1)
Cash and cash equivalents and marketable investments, Gross Unrealized Losses	(2)
US Treasury and Government [Member]
Cash and cash equivalents and marketable investments, amortized cost	804	4,114
Cash and cash equivalents and marketable investments, Fair Market Value	805	4,114
Cash and cash equivalents and marketable investments, amortized cost, Gross Unrealized Gains	1
Cash and cash equivalents and marketable investments, Gross Unrealized Losses
Cash and cash equivalents and marketable investments, Gross Unrealized Losses	(1)
Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Cash and cash equivalents and marketable investments, amortized cost	3,941	3,491
Cash and cash equivalents and marketable investments, Fair Market Value	3,941	3,491
Cash and cash equivalents and marketable investments, amortized cost, Gross Unrealized Gains	1
Cash and cash equivalents and marketable investments, Gross Unrealized Losses	(1)
Cash and cash equivalents and marketable investments, Gross Unrealized Losses	(1)
Marketable Investments [Member]
Cash and cash equivalents and marketable investments, amortized cost	4,745	7,605
Cash and cash equivalents and marketable investments, Fair Market Value	4,746	7,605
Cash and cash equivalents and marketable investments, amortized cost, Gross Unrealized Gains	2
Cash and cash equivalents and marketable investments, Gross Unrealized Losses	(1)
Cash and cash equivalents and marketable investments, Gross Unrealized Losses	(2)
Cash and Cash Equivalents [Member]
Cash and cash equivalents and marketable investments, amortized cost	14,774	26,316
Cash and cash equivalents and marketable investments, Fair Market Value	$ 14,774	$ 26,316