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Note 12 - Equity Compensation Plans (Details Textual) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Jan. 15, 2021
Mar. 31, 2021
Mar. 31, 2020
Proceeds from Stock Options Exercised   $ 7.6 $ 19.2
Nonqualified Stock Option, Contractual Term (Year)   10 years  
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)   4 years  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares) 706,507 709,811  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term (Year)   6 years 3 months 18 days  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term (Year)   4 years 9 months 18 days  
Unrecognized Compensation Cost Related to Nonvested Share Based Compensation Arrangements   $ 128.3  
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year)   2 years 9 months 18 days  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Fair Value   $ 6.2 5.6
Performance Shares [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) 3 years 3 years  
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Per Award (in shares)   1  
Share-based Compensation Arrangement by Share-based Payment Award, Target Levels   100.00%  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) 59,144 59,144  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value   $ 3.1 2.1
Performance Shares [Member] | Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Target Levels   0.00%  
Performance Shares [Member] | Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Target Levels   200.00%  
Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) 4 years    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) 134,777 135,380  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value   $ 6.8 $ 6.1
Employee Stock Purchase Plan [Member] | UK Sharesave Plan [Member]      
Common Stock, Capital Shares Reserved for Future Issuance (in shares)   454,205  
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)   3 years  
Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Purchase Date   5.00%  
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period (in shares)   0 0
Employee Stock Purchase Plan [Member] | US ESPP [Member]      
Common Stock, Capital Shares Reserved for Future Issuance (in shares)   1,250,129  
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period (in shares)   10,137 9,451
Value of Common Stock Issued to Employees Per Share (in dollars per share)   $ 167.84 $ 132.41
The 2013 Incentive Plan [Member]      
Common Stock, Capital Shares Reserved for Future Issuance (in shares)   1,937,143