Note 8 - Stock-based Compensation (Tables)

9 Months Ended

Sep. 30, 2018

Notes Tables

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

	Three Months Ended September 30,			Nine Months Ended September 30,
	2017			2017
Expected term (years)		5.6			5.9
Expected volatility		63.6	%		57.2	%
Risk-free interest rate		1.8	%		2.2	%
Dividend rate		—			—

Schedule of Share-based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions [Table Text Block]

	Three Months Ended September 30, 2017			Nine Months Ended September 30, 2017
Expected term (years)		0.5			0.5
Expected volatility		117.1	%		109.2	%
Risk-free interest rate		0.94	%		0.78	%
Dividend rate		—			—