Note 9 - Stock-based Compensation - Stock Option Activity (Details) - $ / shares	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Options outstanding, number of shares (in shares)	506,810
Options outstanding, weighted average exercise price (in dollars per share)	$ 6.54	$ 6.89
Granted, number of shares (in shares)	66,300
Granted, weighted average exercise price (in dollars per share)	$ 3.69
Exercised, number of shares (in shares)
Forfeited, number of shares (in shares)	24,790
Forfeited, weighted average exercise price (in dollars per share)	$ 6.10
Options outstanding, number of shares (in shares)	548,320
Options outstanding, weighted average exercise price (in dollars per share)	$ 6.54