Note 8 - Common Stock (Tables)	12 Months Ended
	Dec. 31, 2018
Notes Tables
Common Stock Reserved for Future Issuance [Table Text Block]	<div style="display: inline; font-family: times new roman; font-size: 10pt"><table cellpadding="0" cellspacing="0" style="border-collapse: collapse; min-; min-width: 700px;"> <tr style="vertical-align: bottom"> <td style="font-size: 10pt"> </td> <td style="font-size: 10pt; font-weight: bold; padding-bottom: 1pt"> </td> <td colspan="3" style="font-size: 10pt; font-weight: bold; text-align: center; border-bottom: Black 1pt solid">December 31,</td> <td style="font-size: 10pt; font-weight: bold; padding-bottom: 1pt"> </td> <td colspan="3" style="font-size: 10pt; font-weight: bold; text-align: center; border-bottom: Black 1pt solid">December 31,</td> </tr> <tr style="vertical-align: bottom"> <td style="font-size: 10pt"> </td> <td style="font-size: 10pt; font-weight: bold; padding-bottom: 1pt"> </td> <td colspan="3" style="font-size: 10pt; font-weight: bold; text-align: center; border-bottom: Black 1pt solid">2018</td> <td style="font-size: 10pt; font-weight: bold; padding-bottom: 1pt"> </td> <td colspan="3" style="font-size: 10pt; font-weight: bold; text-align: center; border-bottom: Black 1pt solid">2017</td> </tr> <tr style="vertical-align: bottom"> <td style="font-size: 10pt"> </td> <td style="font-size: 10pt"> </td> <td colspan="3" style="font-size: 10pt; text-align: center"> </td> <td> </td> <td colspan="3"> </td> </tr> <tr style="vertical-align: bottom; background-color: rgb(204,238,255)"> <td style="width: 74%; font-size: 10pt; text-align: left">Conversion of Series A Preferred Stock</td> <td style="width: 1%; font-size: 10pt"> </td> <td style="width: 1%; font-size: 10pt; text-align: left"> </td> <td style="width: 10%; font-size: 10pt; text-align: right"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;">22,112,775</div></td> <td style="width: 1%; font-size: 10pt; text-align: left"> </td> <td style="width: 1%; font-size: 10pt"> </td> <td style="width: 1%; font-size: 10pt; text-align: left"> </td> <td style="width: 10%; font-size: 10pt; text-align: right"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;">22,112,775</div></td> <td style="width: 1%; font-size: 10pt; text-align: left"> </td> </tr> <tr style="vertical-align: bottom; background-color: White"> <td style="font-size: 10pt; text-align: left">Stock-based compensation awards</td> <td style="font-size: 10pt"> </td> <td style="font-size: 10pt; text-align: left"> </td> <td style="font-size: 10pt; text-align: right"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;">5,163,957</div></td> <td style="font-size: 10pt; text-align: left"> </td> <td style="font-size: 10pt"> </td> <td style="font-size: 10pt; text-align: left"> </td> <td style="font-size: 10pt; text-align: right"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;">3,572,457</div></td> <td style="font-size: 10pt; text-align: left"> </td> </tr> <tr style="vertical-align: bottom; background-color: rgb(204,238,255)"> <td style="font-size: 10pt; text-align: left; padding-bottom: 1pt">Employee Stock Purchase Plan</td> <td style="font-size: 10pt; padding-bottom: 1pt"> </td> <td style="border-bottom: Black 1pt solid; font-size: 10pt; text-align: left"> </td> <td style="border-bottom: Black 1pt solid; font-size: 10pt; text-align: right"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;">118,120</div></td> <td style="border-bottom: Black 1pt solid; font-size: 10pt; text-align: left"> </td> <td style="font-size: 10pt; padding-bottom: 1pt"> </td> <td style="border-bottom: Black 1pt solid; font-size: 10pt; text-align: left"> </td> <td style="border-bottom: Black 1pt solid; font-size: 10pt; text-align: right"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;">192,463</div></td> <td style="border-bottom: Black 1pt solid; font-size: 10pt; text-align: left"> </td> </tr> <tr style="vertical-align: bottom; background-color: White"> <td style="font-size: 10pt; padding-bottom: 2.5pt">Total</td> <td style="font-size: 10pt; padding-bottom: 2.5pt"> </td> <td style="border-bottom: Black 2.5pt double; font-size: 10pt; text-align: left"> </td> <td style="border-bottom: Black 2.5pt double; font-size: 10pt; text-align: right"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;">27,394,852</div></td> <td style="border-bottom: Black 2.5pt double; font-size: 10pt; text-align: left"> </td> <td style="font-size: 10pt; padding-bottom: 2.5pt"> </td> <td style="border-bottom: Black 2.5pt double; font-size: 10pt; text-align: left"> </td> <td style="border-bottom: Black 2.5pt double; font-size: 10pt; text-align: right"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;">25,877,695</div></td> <td style="border-bottom: Black 2.5pt double; font-size: 10pt; text-align: left"> </td> </tr> </table></div>