Note 9 - Stock-based Compensation - Stock Option Activity (Details)	6 Months Ended
	Jun. 30, 2020 $ / shares shares
Options outstanding, number of shares (in shares)	506,810
Options outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 6.89
Granted, number of shares (in shares)	113,980
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 3.49
Exercised, number of shares (in shares)
Forfeited, number of shares (in shares)	(27,140)
Forfeited, weighted average exercise price (in dollars per share) | $ / shares	$ 6.30
Options outstanding, number of shares (in shares)	593,650
Options outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 6.26