Note 9 - Net Loss Per Share Attributable to Common Stockholders - Common Stock Equivalents Excluded From Calculation of Net Loss Per Share (Details) - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities (in shares)	25,429,312	26,560,001	25,429,312	26,560,001
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)	3,658,280	4,447,226	3,658,280	4,447,226
Employee Stock Purchase Plan [Member]
Antidilutive Securities (in shares)