Note 4 - Fair Value of Financial Instruments - Fair Value of Financial Instruments (Details) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Marketable investments:
Available-for-sale securities	$ 6,448	$ 9,523
Total assets at fair value	13,853	13,606
Fair Value, Inputs, Level 1 [Member]
Marketable investments:
Total assets at fair value	3,061	3,036
Fair Value, Inputs, Level 2 [Member]
Marketable investments:
Available-for-sale securities	6,448	9,523
Total assets at fair value	10,792	10,570
Fair Value, Inputs, Level 3 [Member]
Marketable investments:
Total assets at fair value
Money Market Funds [Member]
Cash equivalents:
Cash Equivalents	3,061	3,036
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents:
Cash Equivalents	3,061	3,036
Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Cash equivalents:
Cash Equivalents
Money Market Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Cash equivalents:
Cash Equivalents
Commercial Paper [Member]
Cash equivalents:
Cash Equivalents	4,344	1,047
Commercial Paper [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents:
Cash Equivalents
Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member]
Cash equivalents:
Cash Equivalents	4,344	1,047
Commercial Paper [Member] | Fair Value, Inputs, Level 3 [Member]
Cash equivalents:
Cash Equivalents