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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities (in shares)	26,736,643	24,831,151	26,736,643	24,831,151
Convertible Preferred Stock [Member]
Antidilutive Securities (in shares)	22,112,775	22,112,775	22,112,775	22,112,775
Employee Stock Option [Member]
Antidilutive Securities (in shares)	4,597,226	2,683,912	4,597,226	2,683,912
Outstanding ESPP Shares [Member]
Antidilutive Securities (in shares)	26,642	34,464	26,642	34,464