Note 8 - Stock-based Compensation (Details Textual) - USD ($) | 3 Months Ended | 6 Months Ended |
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 |
Allocated Share-based Compensation Expense, Total | $ 627,000 | $ 935,000 | $ 1,912,000 | $ 1,916,000 |
Restricted Stock Units (RSUs) [Member] | | | | |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition | | | 1 year 36 days | |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value | | | | |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options | 700,000 | | $ 700,000 | |
2011 Equity Incentive Plan [Member] | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value | 8,000 | | 8,000 | |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options | $ 5,000,000 | | $ 5,000,000 | |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross | | | 2,465,000 | |
Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price | | | $ 2 | |
2011 Equity Incentive Plan [Member] | Certain Employees and a Contractor [Member] | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross | | | 2,464,872 | |
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period | | | 10 years | |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Fair Value | | | $ 3,100,000 | |
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate | | | 67.00% | |
2011 Equity Incentive Plan [Member] | Certain Employees and a Contractor [Member] | Minimum [Member] | | | | |
Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price | | | $ 1.80 | |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value | | | $ 1.09 | |
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term | | | 5 years 182 days | |
2011 Equity Incentive Plan [Member] | Certain Employees and a Contractor [Member] | Maximum [Member] | | | | |
Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price | | | $ 2.07 | |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value | | | $ 1.30 | |
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term | | | 6 years 40 days | |
2011 Equity Incentive Plan [Member] | Consultant [Member] | | | | |
Allocated Share-based Compensation Expense, Total | | | $ 400,000 | |
2011 Equity Incentive Plan [Member] | Employee Stock Option [Member] | | | | |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition | | | 2 years 328 days | |
2011 Equity Incentive Plan [Member] | Employee Stock Option [Member] | Certain Employees and a Contractor [Member] | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | | | 4 years | |
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate | | | 0.00% | |
2011 Equity Incentive Plan [Member] | Employee Stock Option [Member] | Certain Employees and a Contractor [Member] | Minimum [Member] | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate | | | 2.32% | |
2011 Equity Incentive Plan [Member] | Employee Stock Option [Member] | Certain Employees and a Contractor [Member] | Maximum [Member] | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate | | | 3.00% | |
The 2015 Employee Stock Purchase Plan [Member] | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized | 1,550,000 | | 1,550,000 | |
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period | | | 225,887 | |