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Note 6 - Share-based Compensation (Details Textual) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Restricted Stock Units (RSUs) [Member]      
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total $ 71.4    
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year) 3 years 2 months 12 days    
Restricted Stock Units (RSUs) [Member] | Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) 3 years    
Restricted Stock Units (RSUs) [Member] | Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) 10 years    
Performance Shares [Member]      
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total $ 19.5    
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year) 2 years 6 months    
Performance Shares [Member] | Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) 2 years    
Performance Shares [Member] | Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) 10 years    
Restricted Stock Units (RSUs) and Performance Shares [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Vested $ 84.9 $ 73.0 $ 47.0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Nonvested 340.2    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value $ 48.8 $ 56.3 $ 41.1