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Note 15 - Incentive Plans (Details Textual) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Allocated Share-based Compensation Expense, Total $ 5.2 $ 9.6
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Total $ 42.3  
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition 3 years 109 days