Note 12 - Fair Value Measurements - Fair Value, Assets Measured on a Recurring Basis (Details) - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019
Total		$ 152,960
Fair Value, Inputs, Level 1 [Member]
Total		152,960
Fair Value, Inputs, Level 2 [Member]
Total		0
Fair Value, Inputs, Level 3 [Member]
Total		0
Money Market Funds [Member]
Money market mutual funds		102,977	$ 27,644
Money market mutual funds		102,977	27,644
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Money market mutual funds		102,977	27,644
Money market mutual funds		102,977	27,644
Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Money market mutual funds		0	0
Money market mutual funds		0	0
Money Market Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Money market mutual funds		0	0
Money market mutual funds		0	$ 0
US Treasury Securities [Member]
U.S. Treasury bills	[1]	49,983
US Treasury Securities [Member] | Fair Value, Inputs, Level 1 [Member]
U.S. Treasury bills	[1]	49,983
US Treasury Securities [Member] | Fair Value, Inputs, Level 2 [Member]
U.S. Treasury bills	[1]	0
US Treasury Securities [Member] | Fair Value, Inputs, Level 3 [Member]
U.S. Treasury bills	[1]	$ 0

[1] The carrying amount of the Company's investments in U.S. Treasury bills approximate fair value due to their short-term maturities, and there have been no events or changes in circumstances that would have had a significant effect on the fair value of these securities at September 30, 2020.