Note 4 - Fair Value Measurements - Assets Measured at Fair Value on Recurring Basis (Details) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Total cash equivalents	$ 6,114	$ 3,620
Restricted cash	337	600
Total short-term investments		2,249
Total assets measured at fair value	6,451	6,469
Money Market Funds [Member]
Total cash equivalents	6,114	1,871
Commercial Paper [Member]
Total cash equivalents		999
Corporate Debt [Member]
Total cash equivalents		750
Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Total short-term investments		748
Corporate Debt Securities [Member]
Total short-term investments		1,501
Fair Value, Inputs, Level 1 [Member]
Total cash equivalents	6,114	1,871
Restricted cash	337	600
Total short-term investments
Total assets measured at fair value	6,451	2,471
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Total cash equivalents	6,114	1,871
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 1 [Member] | Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Total short-term investments
Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member]
Total short-term investments
Fair Value, Inputs, Level 2 [Member]
Total cash equivalents		1,749
Restricted cash
Total short-term investments		2,249
Total assets measured at fair value		3,998
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		999
Fair Value, Inputs, Level 2 [Member] | Corporate Debt [Member]
Total cash equivalents		750
Fair Value, Inputs, Level 2 [Member] | Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Total short-term investments		748
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Total short-term investments		$ 1,501