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		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Total common stock equivalents (in shares)	5,795,658	6,503,879	5,756,805	6,370,306
Share-based Payment Arrangement, Option [Member]
Total common stock equivalents (in shares)	4,823,917	5,721,801	4,792,022	5,696,450
Employee Stock Purchase Plan Shares [Member]
Total common stock equivalents (in shares)	26,416	29,471	23,007	27,582
Restricted Stock Units (RSUs) [Member]
Total common stock equivalents (in shares)	945,325	752,607	941,776	646,274