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Note 3 - Share-based Compensation (Details Textual)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
USD ($)
shares
Sep. 30, 2019
USD ($)
shares
Restricted Stock Units (RSUs) [Member]    
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total | $ $ 68.5 $ 68.5
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition   3 years 3 months 18 days
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period | shares 225,455 225,625
Performance Shares [Member]    
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total | $ $ 16.5 $ 16.5
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition   2 years 2 months 12 days
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period | shares 75,699 127,140