Note 4 - Fair Value Measurements - Assets Measured at Fair Value on Recurring Basis (Details) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Total cash equivalents	$ 6,114	$ 6,114
Restricted cash	337	337
Total assets measured at fair value	6,451	6,451
Money Market Funds [Member]
Total cash equivalents	6,114	6,114
Commercial Paper [Member]
Total cash equivalents
Corporate Debt [Member]
Total cash equivalents
Fair Value, Inputs, Level 1 [Member]
Total cash equivalents	6,114	6,114
Restricted cash	337	337
Total assets measured at fair value	6,451	6,451
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Total cash equivalents	6,114	6,114
Fair Value, Inputs, Level 1 [Member] | Commercial Paper [Member]
Total cash equivalents
Fair Value, Inputs, Level 1 [Member] | Corporate Debt [Member]
Total cash equivalents
Fair Value, Inputs, Level 2 [Member]
Total cash equivalents
Restricted cash
Total assets measured at fair value
Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Total cash equivalents
Fair Value, Inputs, Level 2 [Member] | Commercial Paper [Member]
Total cash equivalents
Fair Value, Inputs, Level 2 [Member] | Corporate Debt [Member]
Total cash equivalents