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Note 2 - Stock-based Compensation (Details Textual)
$ / shares in Units, $ in Thousands
1 Months Ended 3 Months Ended 12 Months Ended
Aug. 31, 2013
shares
Mar. 31, 2018
USD ($)
$ / shares
shares
Mar. 31, 2017
USD ($)
$ / shares
Dec. 31, 2014
shares
Dec. 31, 2017
shares
Number of Stock-based Compensation Plans   2      
Allocated Share-based Compensation Expense, Total   $ 2,903 $ 2,462    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Total   15,717      
Employee Service Share-based Compensation Nonvested Awards Compensation Cost Expected to be Recognized For Remainder of Fiscal Year   5,475      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Expected to be Recognized Year Two   5,472      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Expected to be Recognized Year Three   3,386      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Expected to be Recognized Year Four   1,382      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Expected to be Recognized Year Five   2      
Incentive Plan [Member]          
Allocated Share-based Compensation Expense, Total   $ 2,820 $ 2,326    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value | $ / shares   $ 3.72 $ 3.75    
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate   0.00%      
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant | shares   476,000     468,000
Incentive Plan [Member] | Employee Stock Option [Member]          
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate   0.00%      
Incentive Plan [Member] | Employee Stock Option [Member] | Minimum [Member]          
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term   5 years      
Incentive Plan [Member] | Employee Stock Option [Member] | Maximum [Member]          
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term   10 years      
Incentive Plan [Member] | Employee Stock Option [Member] | Non-employee Directors [Member]          
Share-based Compensation Arrangement by Share-based Payment Award, First Vesting Period After Grant Date   1 year      
Incentive Plan [Member] | Employee Stock Option [Member] | Vest 25% Each Year Until Fully Vested [Member]          
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage   25.00%      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period   4 years      
Incentive Plan [Member] | Performance Shares [Member]          
Allocated Share-based Compensation Expense, Total   $ 0      
Incentive Plan [Member] | Performance Shares [Member] | Vest Upon Successful Completion of Specific Development Milestones [Member]          
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross | shares 1,032,000     1,250,000  
Incentive Plan [Member] | Performance Shares [Member] | Vest Upon Successful Completion of Specific Development Milestones [Member] | August 2013 [Member]          
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage   75.00%      
Share-based Compensation Arrangement by Shar-based Payment Award, Awards Unvested, Percentage   25.00%      
Incentive Plan [Member] | Performance Shares [Member] | Vest Upon Successful Completion of Specific Development Milestones [Member] | December 2014 [Member]          
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage   30.00%      
Share-based Compensation Arrangement by Shar-based Payment Award, Awards Unvested, Percentage   70.00%      
Incentive Plan [Member] | Restricted Stock Units (RSUs) [Member]          
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number, Ending Balance | shares   48,000      
Employee Stock Purchase Plan [Member]          
Allocated Share-based Compensation Expense, Total   $ 83 $ 136    
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized | shares   1,475,000      
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant | shares   277      
Percentage of Salary to Purchase Common Stock, Maximum   15.00%      
Percentage of Common Stock Shares, Beginning   85.00%      
Percentage of Common Stock Shares, Ending   85.00%      
Share-based Compensation Arrangement by Share-based Payment Award, Maximum Number of Shares Per Employee | shares   3,000      
Shar-based Compensation Arrangement by Shar-based Payment Award, Maximum Number of Shares Per Employee, Amount   $ 25      
Stock Issued During Period, Shares, Employee Stock Purchase Plans | shares   49