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Note 2 - Stock-based Compensation (Details Textual)
$ / shares in Units, shares in Thousands, $ in Thousands
1 Months Ended 3 Months Ended 6 Months Ended 12 Months Ended
Aug. 31, 2013
shares
Mar. 31, 2018
shares
Jun. 30, 2018
USD ($)
$ / shares
shares
Jun. 30, 2017
USD ($)
$ / shares
Dec. 31, 2014
shares
Dec. 31, 2017
shares
Number of Stock-based Compensation Plans     2      
Allocated Share-based Compensation Expense, Total     $ 4,987 $ 4,788    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Total     14,509      
Employee Service Share-based Compensation Nonvested Awards Compensation Cost Expected to be Recognized For Remainder of Fiscal Year     3,927      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Expected to be Recognized Year Two     5,845      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Expected to be Recognized Year Three     3,363      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Expected to be Recognized Year Four     1,372      
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     $ 4,904 $ 4,652    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value | $ / shares     $ 4.13 $ 3.74    
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     329     468
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       1,250  
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     27      
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      
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      
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