Note 8 - Net Loss Per Share Attributable to Common Stockholders - Common Stock Equivalents Excluded From Calculation of Net Loss Per Share (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Antidilutive Securities (in shares)	22,543,879	26,736,643	22,543,879	26,736,643
Convertible Preferred Stock [Member]
Antidilutive Securities (in shares)	21,771,032	22,112,775	21,771,032	22,112,775
Share-based Payment Arrangement, Option [Member]
Antidilutive Securities (in shares)	772,847	4,597,226	772,847	4,597,226
Employee Stock Purchase Plan [Member]
Antidilutive Securities (in shares)		26,642		26,642