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Note 7 - Stock-based Compensation (Details Textual)
$ / shares in Units, shares in Thousands, $ in Thousands
1 Months Ended 12 Months Ended
Aug. 31, 2013
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2015
USD ($)
$ / shares
shares
Dec. 31, 2014
shares
Number of Stock-based Compensation Plans   2      
Allocated Share-based Compensation Expense   $ 12,621 $ 8,487 $ 9,705  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross | shares   3,915      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value | $ / shares   $ 3.63      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value   $ 7,631      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Fair Value   $ 9,310 6,380 4,492  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Number | shares   13,267      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Exercise Price | $ / shares   $ 5.95      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Remaining Contractual Term   6 years 292 days      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized   $ 17,785      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Expected to be Recognized Next Year   7,607      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Expected to be Recognized Year Two   5,452      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Expected to be Recognized Year Three   3,365      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Expected to be Recognized Year Four   1,361      
Incentive Plan [Member]          
Allocated Share-based Compensation Expense   $ 12,421 $ 8,340 $ 9,485  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term   5 years 36 days      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value | $ / shares   $ 3.63 $ 2.17 $ 7.72  
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate   0.00% 0.00% 0.00%  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value   $ 1,964 $ 339 $ 10,117  
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant | shares   468 2,273 16 2,362
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] | Stock Options and Restricted Stock [Member] | Vest 50% Each Year Until Fully Vested [Member]          
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage   50.00%      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period   2 years      
Incentive Plan [Member] | Performance Shares [Member]          
Allocated Share-based Compensation 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       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 | shares   210      
Employee Stock Purchase Plan [Member]          
Allocated Share-based Compensation Expense   $ 200 $ 147 $ 220  
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   326      
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   95 75 41  
Employee Stock Ownership Plan (ESOP), Weighted Average Purchase Price of Shares Purchased | $ / shares   $ 3.61 $ 4.36 $ 8.65  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value | $ / shares   $ 2.18 $ 1.95 $ 4.93