Note 11 - Stockholders' Equity - Summary of Warrant Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Stock purchase warrants exercised, number (in shares)	(1,815,012,000)	(5,155,976)
Purchase Warrants [Member]
Stock purchase warrants outstanding, number (in shares)	3,206,910	6,620,620
Shares purchasable under outstanding warrants, exercise price (in dollars per share)	$ 6.36	$ 5.41
Stock purchase warrants issued, number (in shares)		2,066,667
Stock purchase warrants issued, exercise price (in dollars per share)		$ 6.00
Stock purchase warrants exercised, number (in shares)	(1,815,012)	(5,155,976)
Stock purchase warrants exercised, exercise price (in dollars per share)	$ 6.64	$ 5.08
Stock purchase warrants expired (in shares)	(231)	(324,401)
Stock purchase warrants expired (in dollars per share)	$ 7.58	$ 5.00
Stock purchase warrants outstanding, number (in shares)	1,391,667	3,206,910
Shares purchasable under outstanding warrants, exercise price (in dollars per share)	$ 6.00	$ 6.36