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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, 2019
USD ($)
$ / shares
shares
Mar. 31, 2018
USD ($)
$ / shares
Dec. 31, 2018
USD ($)
shares
Dec. 31, 2014
shares
Number of Stock-based Compensation Plans   2      
Share-based Payment Arrangement, Expense   $ 3,317 $ 2,903    
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total     27,594    
Employee Service Share-based Compensation Nonvested Awards Compensation Cost Expected to be Recognized For Remainder of Fiscal Year   8,033      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Expected to be Recognized Year Two   8,310      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Expected to be Recognized Year Three   6,333      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Expected to be Recognized Year Four   4,849      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Expected to be Recognized Year Five   69      
Incentive Plan [Member]          
Share-based Payment Arrangement, Expense   $ 3,166 $ 2,820    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value | $ / shares   $ 6.07 $ 3.72    
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate   0.00% 0.00%    
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant | shares   446,000   805,000  
Incentive Plan [Member] | Share-based Payment Arrangement, Option [Member]          
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate   0.00%      
Incentive Plan [Member] | Share-based Payment Arrangement, 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] | Share-based Payment Arrangement, 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] | Share-based Payment Arrangement, 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] | Share-based Payment Arrangement, 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]          
Share-based Payment Arrangement, Expense       $ 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   3      
Employee Stock Purchase Plan [Member]          
Share-based Payment Arrangement, Expense   $ 151 $ 83    
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   187,000      
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   47,000