Note 2 - Net Loss Per Common Share - Anti-dilutive Common Stock Equivalents Excluded from Diluted Net Loss Per Common Share (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Total common stock equivalents (in shares)	5,399,174	6,131,463	5,641,543	6,286,530
Share-based Payment Arrangement, Option [Member]
Total common stock equivalents (in shares)	4,492,414	5,299,296	4,691,424	5,562,611
Employee Stock Purchase Plan Shares [Member]
Total common stock equivalents (in shares)	15,673	51,253	20,151	32,272
Restricted Stock Units (RSUs) [Member]
Total common stock equivalents (in shares)	891,087	780,914	929,968	691,647