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Note 13 - Stock-based Compensation (Details Textual) - USD ($)
$ in Thousands
1 Months Ended 12 Months Ended
Jan. 31, 2018
Dec. 31, 2011
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Common Stock, Capital Shares Reserved for Future Issuance     4,310,998    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross     0 0 0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value     $ 4,553 $ 11,312 $ 15,390
Proceeds from Stock Options Exercised     $ 719 $ 2,214 $ 5,748
Stock Issued During Period, Shares, Employee Stock Purchase Plans     283,493 171,099 285,101
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Total     $ 120,678    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition     2 years 167 days    
Employee Stock Purchase Plan [Member]          
Maximum Percentage of Aggregate Cash Compensation for Purchase of Stock Using Payroll Deduction   15.00%      
Purchase Price PerShare as Percentage of Market Value   85.00%      
Percentage of Combined Voting Power or Value of All Classes of Stock Not Eligible to Participate   5.00%      
Fair Value of Common Stock in Calendar Year per Employee Not Eligible to Participate   $ 25      
Maximum Aggregate Number of Shares of Common Stock Available for Purchase under Employee Stock Purchase Plan     2,750,000    
Restricted Stock Units (RSUs) [Member]          
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period     4 years    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period     1,639,367    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number, Ending Balance     4,051,685 3,987,552  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period     2,283,793    
Performance Shares [Member]          
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period     4 years    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period     96,580    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number, Ending Balance     129,609    
Market Value Stock Units [Member]          
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period 756,000        
Market Value Stock Units, Performance Period 3 years        
Market Value Stock Units [Member] | Minimum [Member]          
Percentage of Target Number of Shares 0.00%        
Market Value Stock Units [Member] | Maximum [Member]          
Percentage of Target Number of Shares 225.00%